Synthesis, Characterization And Application Of Metal And Metal Oxide Nanostructures

Table of Contents

Chapter 1 INTRODUCTION
1.1 Introduction
1.2 Classification of nanomaterial
1.2.1 Zero dimension (0-D) nanomaterials
1.2.2 One dimension (1-D) nanomaterials
1.2.3 Two dimension (2-D) nanomaterials
1.2.4 Three dimension (3-D) nanomaterials
1.3 Cheap copper metal nanoparticles for colorimetric sensing and catalysis
1.4 Nickel metal nanoparticles for catalysis
1.5 NiO nanostructures for non-enzymatic glucose sensing
1.6 Co3O4 nanostructures for non-enzymatic glucose sensing
1.7 Problem Statement
1.8 Significance and Scope of research
1.9 Aims and Objectives

Chapter 2 LITERATURE REVIEW
2.1 Synthesis of copper nanoparticles
2.2 Synthesis of nickel nanoparticles
2.3 Synthesis of metal oxide nanostructures
2.4 Metal nanoparticles as optical sensors
2.5 Application of metal nanoparticles in catalysis
2.6 Application of metal oxide in electrochemical sensor

Chapter 3 MATERIAL AND METHODS
3.1 L-cysteine protected copper nanoparticles as colorimetric sensor for mercuric ions
3.1.1 Materials and reagents
3.1.2 Instrumentation
3.1.3 Synthesis of L-cysteine functionalized Cu nanoparticles
3.1.4 Colorimetric sensing of Hg2+ ions by cyst-Cu NPs
3.1.5 Use of Cyst-Cu NPs as sensor for assay of Hg2+ions in real water samples
3.2 Synthesis of air stable copper nanoparticles and their use in catalysis
3.2.1 Materials
3.2.2 Method of Synthesis of Cu NPs
3.2.3 Instrumentation and sample preparations
3.2.4 Catalytic test for reductive degradation of dye
3.3 Catalytic reductive degradation of methyl orange using air resilient copper nanostructures
3.3.1 Materials
3.3.2 Preparation of copper nanostructures
3.3.3 Characterization
3.3.4 Catalytic test for reductive degradation of dye
3.4 Catalytic degradation of imidacloprid using l-serine capped nickel nanoparticles
3.4.1 Material and methods
3.4.2 Synthesis of L-Serine capped Ni NPs
3.4.3 Instrumentation
3.4.4 Catalytic degradation of imidacloprid
3.5 Electrochemical sensing of glucose based on novel hedgehog-like NiO nanostructures
3.5.1 Materials
3.5.2 The Synthesis of novel NiO nanostructures by hydrothermal method
3.5.3 The characterization of NiO nanostructures and electrochemical measurement
3.5.4 The modification of glassy carbon electrode (GCE) with hedgehog-like NiO nanostructures
3.5.5 Electroanalysis
3.5.6 Glucose determination in real samples
3.6 Development of sensitive non-enzymatic glucose sensor using complex nanostructures of cobalt oxide
3.6.1 Materials
3.6.2 The synthesis of cobalt oxide nanostructures by hydrothermal growth method
3.6.3 Characterization of cobalt oxide nanostructures and their electrochemical measurement
3.6.4 Modification of glassy carbon electrode (GCE)
3.6.5 Electroanalysis
3.6.6 Glucose determination in human blood serum samples

Chapter 4 RESULTS AND DISCUSSIONS
PART I
4.1 L-cysteine protected copper nanoparticles as colorimetric sensor for mercuric ions
4.1.1 Spectroscopic characterization of functionalized Cu NPs
4.1.2 Fourier transforms infrared spectroscopy (FTIR)
4.1.3 X-ray powdered diffractometry (XRD)
4.1.4 Colorimetric sensing of mercuric ion
4.1.5 Evaluation of sensing mechanism using TEM analysis
4.1.6 Analytical Performance of colorimetric assay
4.1.7 Selectivity of sensor
4.1.8 Application of developed sensor to real water samples
PART II
4.2 Surfactant protected copper nanoparticles for the catalytic reduction of dyes
4.2.1 Optical characterization
4.2.2 Effect of reducing agent on SPR peak position
4.2.3 Effect of surfactant on SPR peak position
4.2.4 Effect of precursor salt on SPR peak position
4.2.5 Effect of pH
4.2.6 Mechanism of formation
4.2.7 Fourier transform infrared spectroscopy
4.2.8 Atomic force microscopy
4.2.9 Transmission electron microscopy
4.2.10 X-ray diffraction
4.2.11 Catalytic activity
4.2.12 Kinetic study
4.2.13 Recovery and reuse of Cu NPs
PART III
4.3 Competitive catalytic reduction of methylene blue (MB) using copper nanoparticles and nanorods: Influence of morphology and structural features on catalytic potential Cu nanostructures
4.3.1 Optical Characterization
4.3.2 Fourier transform infrared spectroscopy (FTIR).
4.3.3 Atomic force microscopy (AFM)
4.3.4 X-ray powder diffraction (XRD)
4.3.5 Growth mechanism for copper nanostructures
4.3.6 Catalytic evaluation of copper nanostructures for degradation
4.3.7 Kinetic evaluation of degradation reaction
4.3.8 Energetic evaluation of degradation reaction
4.3.9 Reductive degradation of real dyeing waste water samples
PART IV
4.4 Nickel nanoparticles as an efficient catalyst for reductive degradation of pesticides
4.4.1 Optical characterization of L-serine capped Ni nanoparticles
4.4.2 Morphological characterization of L-serine capped Ni NPs
4.4.3 Catalytic evaluation of L-serine capped Ni NPs
4.4.4 Recyclability of Catalyst
PART V
4.5 2-D NiO nanostructures for the electrochemical determination of glucose
4.5.1 The characterization of hedgehog-like NiO nanostructures
4.5.2 The measurement of non-enzymatic glucose sensing response of the complex nanostructures of NiO
4.5.3 The selectivity, reproducibility and stability
4.5.4 The detection of glucose by complex nanostructures of NiO modified glassy carbon electrode from real samples
PART VI
4.6 Temperature controlled growth of cobalt oxide nanostructures for glucose electrochemical sensor
4.6.1 Characterization of Co3O4 nano disc
4.6.2 Growth mechanism for Co3O4 NDs
4.6.3 Electrochemical sensing of glucose
4.6.4 Analytical Performance of Co3O4 NDs -GCE
4.6.5 Reproducibility study
4.6.6 Use of Co3O4NDs -GCE for the detection of glucose in the real samples

Chapter 5 CONCLUSION
5.1 Conclusion
5.2 Socio-Economic Impact

References

List of Publications

Re-prints of published paper

Dedication

This thesis is dedicated to my loving mother Associate Professor Shahida Praveen whose encouragement, support, and dedication fuelled my passion to become a successful scientist.

Acknowledgements

I would deeply acknowledge my supervisors Prof. Dr. Syed Tufail Hussain Sherazi, Prof. Dr. Sirajuddin and Dr. Najma Memon for believing in me and considering me as a part of their family. They not only guided me during the entire period but also supported me in my rough time as family members are expected to do. I am especially thankful to Dr. Sirajuddin for letting me learn from his extraordinary knowledge of electrochemistry. Dr. Sirajuddin has always been my source of motivation and reason behind my knowledge regarding electro-chemical sensor system

Razium Ali Soomro

LIST OF FIGURES

Figure 1.1.1 A diagram generalizing the entire dimensional classification of nanostructures

Figure 4.1.1 Visible spectral profiles for optimization of various reaction parameters (a) precursor salt (b) reducing agent (c) capping agent (d) pH of colloidal Cyst-Cu NPs and (e) stability profile for cyst-Cu NPs

Figure 4.1.2 FTIR spectrum of (a) pure L-cysteine (b) L-cysteine functionalized-Cu NPs

Figure 4.1.3 XRD patterns (a) cyst-Cu NPs (b) Hg2+ induced aggregated particles

Figure 4.1.4 Visible Spectral Profile (a) Cyst-Cu NPs (b) Cyst-Cu NPs+Hg2+

Figure 4.1.5 TEM micrographs of Cyst-Cu NPs (a) low resolution (b) high resolution (c) Cyst-Cu NPs+3.5 µM Hg2+ (d) Cyst-Cu NPs+4.0 µM Hg2+

Figure 4.1.6 Hg2+ induced aggregation of Cyst-Cu NPs

Figure 4.1.7 XRD patterns of Cyst-Cu NPs recorded with different concentration of mercuric ion (a) 1.0 µM (b) 2.0 µM (c) 3.0 µM of Hg+2 ions

Figure 4.1.8 Zeta potential measurement for (a) Cyst-Cu NPs (b) Hg+2 add Cyst-Cu NPs

Figure 4.1. 9 (a) Decline in the LSPR abs with increasing concentration of Hg2+ (b) Linear regression plot between ΔA (LSPR) and Hg2+ concentrations

Figure 4.1.10 (a) Visible spectral profile demonstrating selectivity of cyst-Cu NPs for Hg2+ (b) Bar diagram exhibiting magnitude of change ΔA (LSPR) for various cations

Figure 4.2.1 SPR variation of Cu NPs, (a) at different time intervals during the reaction and corresponding color change in colloidal sol, shown in inset photograph (b) at different concentration of reducing agent and corresponding color of Cu NPs (inset photograph) (c) with surfactant concentration (d) with precursor salt concentration and (e) with variation in pH of colloidal Cu NPs

Figure 4.2.2 Mechanism for formation of spherical Cu NPs in aqueous SDS medium

Figure 4.2.3 FTIR spectra, (a) standard SDS surfactant and (b) SDS capped Cu NPs

Figure 4.2.4 A typical AFM image of Cu NPs, (a) showing well dispersed and size heterogeneous NPs and (b) 3D topographical map of Cu NPs showing dents and surface irregularities

Figure 4.2.5 TEM image of SDS capped Cu NPs showing well distributed NPs with spherical shapes

Figure 4.2.6 XRD patterns of the powder samples (a) collected from brick red solution of Cu NPs and (b) collected from greenish suspension of Cu2O

Figure 4.2.7 UV-Vis spectra, (a) 100 μM of EB and 500 μL 0.01 M NaBH4 in absence of nanocatalysts and (b) reduction of 100 µM of EB mixed with 500 μL 10 mM NaBH4 by use of 0.1 mg powder of nanocatalyst (i.e. Cu NPs)

Figure 4.2.8 The amounts remaining of EB, (a) illustrating an exponential decay of the dye and (b) the first order kinetics followed by the reduction/degradation reaction

Figure 4.2.9 Recovery of SDS capped Cu NPs three times to carry out reductive degradation of EB dye with negligible loss of catalytic efficiency

Figure 4.3.1 SPR bands of (a) SDS capped Cu NPs and (b) CTAB capped Cu NRds

Figure 4.3.2 FTIR spectra of (a) standard CTAB, (b) CTAB capped Cu NPs, (c) standard SDS, and (d) SDS capped Cu NRds

Figure 4.3.3 AFM images of Cu nanostructures: (a) typical medium scale AFM image (0.9 × 0.9 𝜇m), (b) topographical map of the SDS capped Cu NPs, (c) typical medium scale AFM image (0.9 × 0.9 𝜇m), and (d) topographical map of the CTAB capped Cu NRds

Figure 4.3.4 SEM images for (a) SDS capped Cu NPs and CTAB capped Cu NRds

Figure 4.3.5 XRD patterns of (a) Cu NPs and (b) Cu NRds

Figure 4.3.6 Formation of Cu NPs in SDS rich aqueous medium

Figure 4.3.7 Formation of Cu NRds in CTAB rich aqueous medium

Figure 4.3.8 UV-Vis spectral profiles for (a) un-catalysed reduction of 100 𝜇M (MO) with 00𝜇L 0.01M (NaBH4) and (b) and (c) catalysed reductive degradation of MO in a similar sample environment with SDS capped Cu NPs and CTAB capped Cu NRds, respectively

Figure 4.3.9 Linear regression plot showing pseudo first-order kinetics for the Cu nanostructure catalysed reductive degradation of MO with SDS capped Cu NPs and CTAB capped NRds

Figure 4.3.10 Linear regression for Arrhenius equation with estimation of corresponding activation energy for surfactant capped Cu NPs and NRds

Figure 4.3.11 UV-Vis spectra for the reductive degradation of real samples (a, b, c) un-catalyzed reduction of 10 μl real sample diluted up to 03 mL in presence of 100 mM (NaBH4) reductant; (d, e, f) catalyzed reductive degradation of similar samples with 0.5 mg of Cu NPs

Figure 4.4.1 UV-Vis spectral profiles for optimization of various reaction parameters (a) precursor salt (b) capping agent (c) pH of colloidal Ser-Ni NPs and (d) stability of as-synthesized Ser-Ni NPs

Figure 4.4.2 TEM images of freshly formed L-serine capped Ni NPs with an inset size distribution histogram

Figure 4.4.3 XRD spectra for Ser-Ni NPs

Figure 4.4.4 XPS spectra for the binding energy of Ser-Ni NPs

Figure 4.4.5 FTIR spectrum of (black) pure L-serine (red) and Ser-Ni NPs

Figure 4.4.6 Schematic representation of formation of L-serine capped Ni NPs

Figure 4.4.7 UV-Vis spectra, (a) un-catalysed reductive degradation of 100 mg L-1 (IM) in presence 0.12 M NaBH4 (b) catalysed reductive degradation of IM in a similar sample environment with 0.1 mg Ser-Ni NPs and (c) linear regression for pseudo first order kinetics for the reductive degradation of IM

Figure 4.4.8 Bar graph showing the efficiency of recycled Ser-Ni NPs for the reduction degradation of IM after five consecutive cycles

Figure 4.5.1 XRD diffraction pattern of hedgehog-like NiO nanostructures

Figure 4.5.2 XPS spectra of the NiO hedgehog-like NiO nanostructures: slow scan spectra of (a) full survey scan spectrum (b) Ni 2p and (c) O 1s

Figure 4.5.3 SEM image of the hedgehog-like NiO nanostructures (a, b) low resolution pictures showing high distribution view (c, d) high resolution pictures with tapering features of individual nanostructure

Figure 4.5.4 Cyclic voltammograms recorded in 1.0 mM glucose solution (Black) NiO HSs-Nafion/GCE (Blue) GCE modified with NiO structures synthesized in the absence of L-cysteine

Figure 4.5.5 FT-IR spectra of hedgehog-like NiO nanostructures (a) before annealing (b) after annealing of precursors

Figure 4.5.6 Schematic illustration of the formation of hedgehog-like NiO nanostructures

Figure 4.5.7 Cyclic voltammogram (CV) profile of bare GCE (a) in the absence of glucose (b) in the presence of 1.0 mM glucose, NiO HSS-Nafion/GCE (c) in the absence of glucose and (d) in the presence of 1.0 mM glucose

Figure 4.5.8 Cyclic voltammograms of NiO HSS-Nafion/GCE at different scan rates from 0.05 mV/s to 1.0 mV/s in 0.1 M NaOH with 1.0 mM glucose. Inset: plot of anode and cathode current vs. the scan rate (c) Ip responses with successive increase of electrolytic volume (0.1 M NaOH) from 1.0 to 10.0 mL (d) Ip responses with successive increase of NiO deposition volume from 05 to 20 µL at 0.53 V for NiO HSS-Nafion/GCE

Figure 4.5.9 (a) Cyclic voltammograms responses for NiO HSS-Nafion/GCE with successive addition of glucose from 0.1 to 5.0 mM at 0.53 V (b) the corresponding calibration curve

Figure 4.5. 10 Cyclic voltammograms recorded for (a) negative or positive influence of electro active interferents (b) reproducibility of electrode with inset bar graph depicting <1.0%RSD value for 10 electrode prepared in same manner (c) stable response of electrode from 1 week up to 6 months with inset bar graphs showing 95% retention of its initial current response

Figure 4.6. 1 SEM images of the Co3O4 NDs (a, b) low magnification images with showing density of NDs (c, d) high magnification images of individual discs

Figure 4.6. 2 XRD patternsofCo3O4 NDs

Figure 4.6. 3 FTIR specturmofas-synthesizedCo3O4 NDs

Figure 4.6. 4 XPS spectra of the Co3O4 NDs: slow scan spectra of (a) Co2p, (b) O1s and (c) full survey scan spectrum

Figure 4.6. 5 Growth mechanisms of Co3O4 NDs in aqueous solution

Figure 4.6. 6 Cyclic voltammograms (CVs) of bare GCE (a) in the absence of glucose (b) in the presence of 0.5 mM glucose, Co3O4-GCE (c) in the absence of glucose and (d) in the presence of 0.5 mM glucose

Figure 4.6. 7 Cyclic voltammograms (CVs) of the Co3O4-GCE recorded for 0.5 mM glucose (a) in 0.1M NaOH solution at different scan rates (inner to outer):0.05–1.0Vs-1 (b) in various volumes of 0.1M NaOH solution (c) with various deposition volume in range of 5 to 15 µL of Co3O4 NDs

Figure 4.6. 8 Interference study showing negative or positive influence of electro active interferences (uric acid, ascorbic acid and dopamine). The inset shows the histograms for each interferent along with control experiment carried out with Co3O4-GCE in presence of 0.5 mM glucose and 9 mL of 0.1 M NaOH solution

Figure 4.6. 9 CV voltammograms of 0.5–4.5 mM glucose under optimized conditions (b) respective linear regression plot between current (Ip) and glucose concentrations

Figure 4.6. 10 08 consecutive CV votammograms of Co3O4-GCE for 0.5 mM glucose solution mixed with optimized 0.1 M NaOH solution at 0.05mVs−

LIST OF TABLES

Table 4.1.1 Comparison of L-cysteine functionalized Cu NPs as a colorimetric sensor for the detection of Hg2+ with previously reported methods

Table 4.1.2 Determination of mercuric ions in real water samples collected from Sindh River near Kotri Barrage, Jamshoro, Pakistan

Table 4.3.1 Comparative results obtained for MO degradation and decolorization by various methods

Table 4.5.1 Comparison of different non enzymatic glucose sensors in terms of detection limit, linear range and sensitivity

Table 4.5.2 Determination of glucose level in real blood serum samples

Table 4.6.1 Comparison of Co3O4 NDs based electrochemical sensor with various enzyme free glucose sensor systems

Table 4.6.2 Determination of glucose level in real blood serum samples.

LIST OF ABBREVIATIONS

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Abstract

Scientific interest in nanostructures of various shapes and morphologies is growing proportionally. Besides the distinct size characteristic, properties of nanostructures vary widely with alteration of structural and morphological features. These shape-dependent nanostructures hold an impressive potential for to be used in numerous scientific technologies with promising applications in areas like optics, electronics, magnetism, catalysis and sensor.

This thesis compiles, studies carried out for the synthesis of metal nanoparticles like copper (Cu), nickel (Ni) and one dimensional metal oxide nanostructures such as nickel oxide (NiO) and cobalt oxide (Co3O4). Metal nanoparticles particularly, gold (Au), silver (Ag) and copper (Cu) have drawn tremendous attention over other metals due to their most unique and fascinating property known as Localized Surface Plasmon Resonance (LSPR). The first study demonstrates a novel, simple and efficient protocol for the synthesis of copper nanoparticles in aqueous solution using L-cysteine as capping or protecting agent. The as-prepared Cyst-Cu NPs were tested as colorimetric sensor for the determining mercuric (Hg2+) ions in aqueous system. Cyst-Cu NPs demonstrated very sensitive and selective colorimetric detection of Hg2+ ions in the range of 0.5 × 10-6–3.5 × 10-6 mol L-1 based on decrease in LSPR intensity as monitored by a UV–Vis spectrophotometer. The developed sensor is simple, economic compared to those based on precious metal nanoparticles and sensitive to detect Hg2+ ions with detection limit down to 4.3 × 10-8 mol L-1. The sensor developed in this work has a high potential for rapid and on-site detection of Hg2+ ions. The sensor was successfully applied for assessment of Hg2+ ions in real water samples collected from various locations of the Sindh River. To further elaborate the morphological and structural influences of copper nanostructures on its catalytic properties, the second and third studies are dedicated towards the synthesis and competitive catalytic evaluation of Cu nanoparticle (Cu NPs) and Cu nanorods (Cu NRds). The synthesis was achieved using various surfactant as effective capping agent with sodium borohydride as effective reductant in wet-chemical reduction process. The as-synthesized Cu nanostructures were studied for their catalytic potential against common toxin known as Eosin B (EB) dye followed by a competitive experiment between Cu NPs and Cu NRds for reductive degradation of M.O dye that would allow understanding the shape dependency of the heterogeneous catalysis. This shape dependent catalytic efficiency was further evaluated from activation energy (EA) of reductive degradation reaction. The more efficient Cu NPs were further employed for reductive degradation of real waste water samples containing dyes collected from the drain of different local textile industries situated in Hyderabad region, Pakistan.

In the fourth study, a simple and efficient approach was adopted for the synthesis of pure metallic nickel nanoparticle using L-serine as a capping agent (Ser-Ni NPs). The as-synthesised Ser-Ni NPs were noted to be fine, spherical, uniformly distributed with average size of 10 ±2.5 nm. In, addition they were determined to be extremely air durable with life time of more than one month. The as-synthesized Ser-Ni NPs were known to exhibited excellent potential for heterogeneous catalytic reductive degradation of imidacloprid pesticide (IM) in aqueous solution with corresponding rate constant (k) of 0.103 s-1. Furthermore, Ser-Ni NPs catalyst exhibited excellent recyclability with negligible catalytic poisoning, suggesting its high capability for reusability.

The fourth and fifth part of the thesis is dedicated to the metal oxide nanostructures. The studies comprises synthesis of novel nickel oxide (NiO) and cobalt oxide (CO3O4) nanostructures using simple hydrothermal strategy. NiO nanostructures were obtained using L-cysteine as an effective bio-compatible template which enabled formation of hedgehog-like structural features. The prepared NiO nanostructures were applied for the design and engineering of non-enzymatic glucose sensor in an alkaline medium. The electrode material for glucose sensing demonstrated extremely high electrochemical response with high sensitivity (1052.8 µA mM-1 cm-2), low detection limit (LOD) (1.2 µM) (S/N=3), high selectivity, wide linear range (0.1-5.0 µM)(R2=0.9982) and the outstanding reproducibility. In a similar way Co3O4 which were produced without the application of any template were also known to exhibit nanodisc like morphology with the size dimension in range of 300–500 nm. The obtained morphological features were evaluated for their electrochemical potential towards oxidation of glucose which enabled development of sensitive (27.33 mA mM-1 cm-2), and stable enzyme free glucose sensor. The developed sensor demonstrated excellent linearity (R2 = 0.9995), wide detection range (0.5–5.0 mM), lower detection limit (0.8 mM) and extreme selectivity towards glucose in the presence of common interferents like dopamine (DP), ascorbic acid (AA) and uric acid (UA). The sensor systems were also test for real blood glucose sample where excellent performances suggested their analytical capability.

Chapter 1 INTRODUCTION

1.1 Introduction

The practice of nanotechnology for the production of nanoscale materials with diverse and superior phyiso-chemical characteristics has enabled multitude applications in research and practical fields. The innovative usage of nanomaterials in various fields such as material science, optics, electronics, energy, environment, healthcare, sensor and biosensor has encouraged nanotechnology to become a leading science of this millennium. Nanotechnology is basically a combination of sciences, and it is this convergence with different disciplines like chemistry, physics, biotechnology, engineering and information technology that holds the promise for a better and improved future for human kind.

Whatever has been accomplished in nanotechnology is just a leading edge of a greater wave waiting to be explored. However, the full potential of this technology can be utilized by understanding how nanomaterials differ from their conventional shapes and size. How they can be synthesised in diverse shapes, sizes and what potential application are affected by such changes. The general construction methods utilize two different approaches: top-down and bottom up approach.

Top-down method uses the bulk structures, for to produce nanostructures. Basically, it moves from large to small by externally controlling the processing parameters. Few examples include lithographic techniques, plasma arc method and ball milling. Although efficient, top-down approach suffers the drastic drawback of not being cheap to produce nanoscale material at preparative scale. Besides this, the formation of un-even surface with imperfect structures as a consequence of no control over morphology makes this method not very attractive. In contrast, bottom-up strategy is much more reliable and efficient. This approach allows control of morphology and structures using simple internal (experimental) parameters like, concentration, pH and temperature. This as a consequence enables miniaturization of components at atomic level (formation of ions) leading to a process of self-assembly which further is responsible for formation of nanostructures. This self-assembly is governed by the forces like electrostatic, ionic interaction and van der Waals. Typical examples include wet-chemical reduction, hydrothermal and sol-gel method. Since bottom up approach enables the nanotechnologist the control over the shape, size and morphology via number of operation parameters, this method allows generation of uniform, homogenous and pure nanostructures compared to top-down approach.

1.2 Classification of nanomaterial

Nanomaterials are classified based on their applications and philosophy associated with them. From the perspective of literature nanomaterials are subdivided into categories based on their macroscopic dimension i.e. 0, 1, 2 or 3 dimension. In accordance to this subdivision, such materials must have at least one of their dimensions in a nanometric range (within 100 nm). Typical examples for such nanomaterials are as follows:

1.2.1 Zero dimension (0-D) nanomaterials

Zero dimension nanomaterials are materials with all dimension are within nanoscale range. Such materials include nanoclusters and nanodispersions. We can understand the concept of zero dimensions from the point of the matrix in which material is present. 0-D nanomaterials include quantum dots and nanoparticles which are present in their matrix as in an isolated form. These nanomaterials have their size confined in all-direction. From the perspective of quantum theory, stronger the confinement greater the rigid boundary conditions over the charge carriers wave function within the nanomaterials and larger the surface to volume ratio.

1.2.2 One dimension (1-D) nanomaterials

One dimension nanomaterials are those materials that have their one dimension out of nanomateric range. Such materials include nanowires, nanotubes, nanofibers, nanobelts and nanoribbons. From the perspective of quantum theory, these materials are confined from two directions and extensively used the area of electronic, catalysis and optics.

1.2.3 Two dimension (2-D) nanomaterials

These are materials having at least two dimensions outside the nanometric range. Such materials include nanofilms, coating and surfaces. Such materials are confined to nanoscale from one direction .i.e. the thickness of surface, coating and films is within nanoscale (< 100 nm).

1.2.4 Three dimension (3-D) nanomaterials

These materials are those, which are not confined to nanoscale range from any dimension. Hence, we can say that bulk materials who have their all dimension above >100 nm. It is very important to distinguish 3-D from 0-D materials. We can say that 3-D materials are a compact (bulk) materials with their entire volume filled with nano size grains (nanoparticles, nanowires, nanosheets). The major difference in 3-D and 0-D is the absence of free surface and presence of only one grain interface in 3-D relative to 0-D materials. This definition also distinguishes the 3-D materials from nanocrystalline powders available with various degrees of agglomeration and consists of uniform particles of homogenous size. Figure 1.1.1 shows a diagram generalizing the entire dimensional classification of nanostructures.

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Figure 1.1.1 A diagram generalizing the entire dimensional classification of nanostructures

1.3 Cheap copper metal nanoparticles for colorimetric sensing and catalysis

Recently, considerable scientific interest has been focused on the fabrication and application of metal nanoparticles (NPs). The diversity of metal NPs in shapes and diminutive sizes has allowed researchers to explore their captivating applications in fields like catalysis, electronics, sensors, optical devices and biotechnology. Metal nanoparticles particularly, gold (Au), silver (Ag) and copper (Cu) have drawn tremendous attention over other metals due to their most unique and fascinating property known as Localized Surface Plasmon Resonance (LSPR) (S. Zhu et al., 2008).

When the size of metal particles diminishes to nano scale regime, a strong UV-Vis excitation band is observed due to collective oscillation of conducting band electron at metal surface. Unlike the customary UV-Vis band, this LSPR of metal nanoparticles is the characteristics of nano scale size and demonstrate extreme sensitivity towards the shape, size, and composition of metal under study (Kazuma et al., 2014). Noble metal NPs like Au, Ag and Cu within the size range of 10- 60 nm exhibit LSPR band around 520, 400 and 570 nm, with magnificent purple, yellow, and red colloidal colour, respectively (Eustis et al., 2006; Smitha et al., 2008; S. Sun et al., 2011; S.-H. Wu et al., 2004).

In particular, LSPR property of a metal nanoparticle strongly resides on the inter particle distance between the aggregated nanoparticles, thus allowing LSPR based metal absorption to be used as an analytical tool for optical and calorimetric sensing of various chemical species with low cost, simplicity and sensitivity (Sung et al., 2013). Recently, considerable scientific interest has been focused on the fabrication and application of metal nanoparticles (NPs). The diversity of metal NPs in shapes and diminutive sizes has allowed researcher to explore their captivating applications in fields like catalysis, electronics, sensors, optical devices and biotechnology.

At present much of research attention concerning the preparation and application of plasmonic nanostructure is limited to those of precious metals like Au and Ag. However, the high cost margin and difficult availability of these metals restricts the development of economic, on site assessment as feasible calorimetric probes, besides their usage in large volume production (Farhadi et al., 2012). In such connection, copper with LSPR band residing within the visible region of electromagnetic spectrum, lower cost and easy availability compared to Au and Ag can serves as a suitable alternative. In context with the application of LSPR based materials, sensitive and selective chemo sensors broadly used for the determination of heavy metal ions have been considerably attended in order to avoid their cytotoxicity effects. Among such metals, mercury is considered as one of the most important toxic metal ion for environment and humans because of its wide distribution in various metallic and organic and inorganic forms that are found in air, water and soil (de Silva et al., 2000; F.A. Cotton, 1999.; Tchounwou et al., 2003).

Mercuric ion (Hg2+), due to its high water solubility is considered as most stable form of inorganic mercury, which can easily exists in surface waters and can cause several health related issues including damage to brain, kidneys, and nervous system (Clarkson et al., 2003; Yong Wang et al., 2010). The various classical instrument based determination methods include atomic absorption/emission spectrometry (AAS/AES) (de Castro Maciel et al., 2003; Suddendorf et al., 1981; Q. Yang et al., 2005), inductively coupled plasma mass spectrometry (ICPMS) (Fong et al., 2007; Karunasagar et al., 1998), high-performance liquid chromatography (HPLC) (Ichinoki et al., 2004), atomic fluorescence spectrometry (AFS) (Nevado et al., 2005), flame photometry and ion selective electrode (ISE) (Kuswandi et al., 2007). These techniques although powerful and efficient for heavy metal ion determination, suffer from inadequacies like highly expensive instrumentation, time consuming sample preparations and inability to be used as an on-site monitoring feasible techniques. In comparison, colorimetric based methods can provide rapid naked eye detection, appropriate on site assessment capability, real time analysis of target metal ion due to simple configuration and portability to be used on site (A. Fan et al., 2010).

To date, Au and Ag nanoparticles have been extensively employed as colorimetric detectors for heavy metal ions in the aqueous phase due to their excellent LSPR characteristics allowing easy visualization of colour change(Sung et al., 2013). (Solanki et al., 2010) reported a simple colorimetric detection method for Hg2+ and Pb2+ ions based metal ions-peptide complex inducing the aggregation of Au NPs. Similarly (Bothra et al., 2013) developed p-phenylenediamine (p-PDA) functionalized Ag NPs for the sensitive calorimetric determination of Hg2+ and Fe2+ in aqueous medium. (D. Liu et al., 2010) showed highly sensitive and selective calorimetric detection of Hg2+ ion based on the aggregation of quaternary ammonium group-capped gold nanoparticles caused by abstraction of thiols from the surface of gold nanoparticles.

In contrast calorimetric determination based chemical reaction between green synthesized Ag NPs and Hg2+ ions have also been reported by (Farhadi et al., 2012).

The protection of some cost effective metal nanoparticles via their stabilization against oxidation is the need of the day to develop cheaper and still efficient products for various tasks. In previous work, our group fabricated stable nickel nanoparticles using L-threonine (Kalwar et al., 2013) as protecting agents and used them in the field of catalysis. Most of the unique properties shown by metal nanostructures are a consequence of their nano size scale regime. However, recently it has been found that properties of nano materials are also influenced by their shape for example (C.J. Murphy, 2002) reported shape dependent SPR peaks of silver and gold nanoparticles. Similarly anisotropic behavior in optical properties of copper nanorods has also been reported by (Henglein, 1989). Thus special shaped nano composite materials are the focus of present day scientific research. Although variety of metal nanostructures are being employed for numerous application in various fields the use of nano size metal structures as a heterogeneous recyclable catalyst in different environmental problems associated with hazardous wastes and toxic water pollutants pollution is the need of the day.

Textile industry and its wastewater have been increasing proportionally, making it one of the main sources of severe water pollution worldwide (K.-S. Wang et al., 2010). In particular, dyes comprise a major section of industrial waste water effluents as they are released in abundance, up to 50% of dyes may be lost directly into waterways due to inefficient and uneconomic dyeing techniques. The release of such chemicals in aquatic system is of environmental concern due to their carcinogenic, persistent and recalcitrant nature (He et al., 2012b).

Dyes released in waste water may also undergo incomplete anaerobic degradation, inducing additional toxicity caused by mutagenic end products. Besides this coloring decreases sunlight penetration and oxygen dissolution in water which is also considerable threat to aquatic eco-system (Rauf et al., 2011). In order to cope with increasingly strict legislations and regulations concerning waste water management, the associated industries are required to find a green, efficient and economically viable wastewater treatment method. Some of water treatment methods include adsorption, biological, coagulation routes and ozonation (Z. Sun, 2002). However, these methods are time consuming, expensive, inefficient and result in secondary pollution with overall increase in method cost because of extra disposal procedures. Advanced oxidation processes (AOPs), employing metal oxides such as TiO2 and ZnO are technically much feasible degradation process but requirement for short wavelength light source with difficulties like low quantum yields restraints its widespread acceptance as an efficient and practical remediation process (Bokare et al., 2008; Maa et al., 2002; 2003 ).

Recently, multistep processes for significant advancement in practical applications regarding dye degradation has been reported (Xiong et al., 2001). Among such processes reductive degradation of organic dyes with metal nanostructures is a convenient degradation model system which is not only viable in terms of efficiency and costliness but also greener as it provides biodegradable end products like aromatic amine, which are readily and easily degraded by micro-organisms(Appleton, 1996) (Hassan et al., 2011) reported the catalytic reduction of mixture of dyes, using gold nanoparticles within 15 s of reaction time. Similarly (Lang et al., 2008) used Pd nanoparticles with good catalytic properties in the degradation of azo dyes. (Sirajuddin et al., 2011b) reported reductive degradation of methylene blue with cysteine capped gold nanoparticles in just 5 min of reaction time(Ai et al., 2013) used alginate hydro gel (AH) capped silver nanoparticles for the heterogeneous catalytic reduction of 4-nitrophenol by NaBH4 in aqueous solution and (Sau et al., 2001) addressed catalysis of eosin reduction in the presence of NaBH4 with Triton X-100 coated gold nanoparticles.

From all previous reports, it is clear that most of relevant recent studies carried in the field of catalysis have focused on the use of transition metals like silver, gold and platinum with very less understanding of the shape selective catalytic performance of NPs. In addition, the high cost, and troublesome availability of these noble metals restricts their applications in larger volume production. In such perspective cheap metal like copper can serve as a suitable alternative with its low cost, easy availability, higher electronic and thermal conductivity, compared to traditional noble metals like silver gold and platinum (Dang et al., 2011 ).

Interestingly some of the properties of special shaped copper nanoparticles are much greater than bulk copper, thus synthesis of special shaped copper nanoparticles is much focused. However it is very difficult to fabricate copper nanomaterials in aqueous solution because of its easy oxidizing capability (Dang et al., 2011a). Until now tremendous efforts are carried out for the preparation of pure metallic nano sized copper. (C. Petit, 1993) have used the microemulsion based method to stabilize metallic copper nanoparticles. (X. Cao, 2003) synthesized copper nanorods in high yield using a composite template of polyethylene glycol (PEG) and cetyltrimethylammonium bromide (CTAB). (Joshi et al., 1998a) used oxygen free atmosphere to prevent oxidation of copper nanoparticles in aqueous medium. Similarly (Park et al., 2007) addressed the use of polymer like PVP for the prevention of oxidation and aggregation of copper nanoparticles. However, most of these recent strategies employ oxygen free environment and heavy molecular weight polymers as protecting agent, limiting the usage of copper nanoparticles in aqueous mediums with restrained catalytic efficiency as heavy polymer molecules block the active sites of the catalyst.

Although successes synthesis of copper nanoparticles have been carried via numerous routes (Rafique, 2012), very less is known about the mechanism concerning control of shape and size at nanoscale level with the basic understanding of shape dependent performance of copper nano structures as a suitable catalyst.

1.4 Nickel metal nanoparticles for catalysis

The field of nanotechnology has made tremendous progress over the last two decades, in a way highly respected by economic and society (Magaye et al., 2014). Metal nanoparticles are particularly interesting because of the ease with which they can be synthesized and altered chemically (A. Kumar et al., 2013). At present, nanoscale metal like Ag, Au, Cu and Ni are subject of extensive research due to their unique properties and diverse technological application ranging from chemical sensor to catalysis (Bothra et al., 2013). Along their extensive application in various fields, there is a growing interest in the catalytic properties of such nano scale transition metals (Nelson et al., 2013). The high surface area to volume ratio of solid supported metal nanoparticles and nature of metal is highly responsible for its catalytic behavior and thus, can be exploited in many environmentally and industrially significant chemical reactions(Janos et al., 2014).

From the catalytic perspective, nickel metal nanoparticles have gained significant attraction based on their inexpensive nature, requirement of mild reaction condition for high product yield and shorter reaction times compared to conventional catalyst (Gao et al., 2013). Thus, based on special physical and chemical properties along with the inexpensive nature of Ni nanoparticles they are regarded as a suitable substitute for the transitional noble catalysts (C. Chen et al., 2014). However, beside the efficient usage, designing a feasible, economical, easy and effective synthesis protocol is also an important task to achieve. In particular for, nickel nanoparticles which are prone to oxidation in an aqueous environment along with their tendency to grow into larger aggregates due to fusion of un-capped particles (Kalwar et al., 2011). In order to overcome these restrictions, various strategies have been proposed which include the usage of non-aqueous solvent system, application of inert atmosphere and suitable stabilizing agents like surfactants and polymers (Z. G. Wu et al., 2010). Presently employed synthesis method include Ball milling, electrodeposition, thermal plasma, polyol process, chemical vapor deposition (CVD), decomposition of organometallic precursors, chemical reduction in the liquid phase and many other methods (de Caro et al., 1997; Degen et al., 1999; Koltypin et al., 1999; Steigerwald et al., 1988; H. T. Zhang et al., 2006).

Chemical reduction is most commodiously employed because of its economic feasibility and ability to manipulate the resultant nanocrystal with simple reaction controlling parameters like pH, concentration, and temperature(R. A. Soomro et al., 2014). Despite the extensive usage of metal nanoparticles in numerous fields, the capability of such materials as a heterogeneous recyclable catalyst in environmental problems associated with toxic water pollutants need to be extensively explored. The constant global rise in genetically engineered crops has led to an overall increase in the usage of pesticides (Benbrook, 2012). The extensive usage have resulted in accumulation of such toxic pollutant in environment which is hazardous both for environment and human health (Wilson et al., 2001) In particular imidacloprid, which because of its low cost, high water solubility (0.58 g/L) and stability accounts for 11–15% of the total insecticide in the world market (Böttger et al., 2013). It is extensively being used in under developing countries like Algeria, Colombia, and Pakistan(Schreinemachers et al., 2012). The application rate of such toxic pesticide in pest control is increasing 25% per year particularly for Pakistan (Tariq et al., 2004). Thus, due to such consideration there is a growing need for simple, efficient yet an in expensive remediation system for removal of such toxic materials.

Presently used pesticide removal methodologies include AOP and fenton processes. Advance oxidation process (AOP) utilizing metal oxides like TiO2 and ZnO are technically much feasible remediation process besides their urge for short wavelength light source and problems like low quantum yields which inhibits their widespread application as an efficient and practical remediation process(Bokare et al., 2008; 2010; Maa et al., 2002; 2003 ). Fenton process on the other hand suffers from the drawbacks like continuous loss of the catalyst in the effluent and requirement of acidification of the initial solution to maintain optimum operating pH around 3(Kouraichi et al., 2015). Contrary to these methods, multistep processes for significant advancement in practical applications regarding toxic pollutant degradation are gaining greater attention (Xiong & Karlsson, 2001). Among such processes, reductive degradation of toxic pollutant catalysed by metal nanoparticles is a convenient degradation model. This process besides being efficient is also highly economical as it employs cheap, stable, easily synthesized transition metal nanoparticles as catalyst.

1.5 NiO nanostructures for non-enzymatic glucose sensing

The quantification and monitoring of glucose level have been highly studied because of its wide range of applications in biotechnology, clinical diagnostics, and food industry (Noh et al., 2012; Safavi et al., 2009). Today, the trend towards the fabrication and engineering of non-enzymatic glucose sensors is highly increased due to their over edge advantages compared to the enzymatic based glucose biosensors (Si et al., 2013). In this context, the extensive research has been carried out for the synthesis of various nanostructured materials to be used as an electrode material for the development of non-enzymatic glucose sensors.

At the present noble metals like Au, palladium (Pd), platinum (Pt), their alloys (Pt-Au, Pt-Pd), transition metals like Ni, Cu and their oxide nanostructures such as (NiO, CuO, Cu2O, Co3O4) (W. Chen et al., 2012; R. Khan et al., 2014; M. Li, X. Bo, Y. Zhang, et al., 2014; J. Lu et al., 2008; Ma et al., 2009; Si et al., 2013; X. Zhang et al., 2010; X. Zhou et al., 2012) are most extensively applied for the estimation of enzyme free sensing of glucose. Among these materials, nickel oxide (NiO) has been widely used for the different applications in addition to the non-enzymatic glucose sensors (M. Li, X. Bo, Z. Mu, et al., 2014). Different nanostructures of NiO have been used in the design of non-enzymatic glucose sensors such as nanofibers (F. Cao et al., 2011; Y. Ding, Y. Liu, L. Zhang, et al., 2011), nanoparticles(Kaneko et al., 2011; W.-D. Zhang et al., 2010), nanoflake arrays (G. Wang et al., 2012) and nanocomposites (S.-J. Li et al., 2014). Recently, NiO hollow spheres have also been reported and demonstrated outstanding performance in various fields including supercapacitor (C.-Y. Cao et al., 2011; S. Ding et al., 2011), photocatalyst (Song et al., 2008), lithium ion batteries(X. H. Huang et al., 2010; L. Liu et al., 2009), gas sensing (L. Wang et al., 2012) and glucose sensing (Ci et al., 2014), which are associated with large pore size and high surface area. Despite the excellent stability and outstanding electro catalytic activity of the nickel oxide nanostructures, the application of unique shaped nanostructures in the development of promising sensor for the biomolecules detection is still a challenging assignment and there is high demand for the exploration to obtain much efficient and economical biosensors (J. Y. Kim et al., 2014).

The nickel oxide nanostructures have been synthesized by various growth methods such as sol-gel(Talebian et al., 2014), surfactant assisted growth (Denayer et al., 2014; Y.-d. Wang et al., 2002), thermal decomposition (Tang et al., 2014; Xiang et al., 2002) and polymer matrix template synthesis (Deki et al., 2003). However, most of these methods suffer the disadvantages of obtaining uniform size, well defined morphology and satisfactory product yield. Besides these growth techniques, hydrothermal method is very well known and suitable at large scale because of its several advantageous features such as low temperature, cost effectiveness, simplicity and high product yield. Currently, the use of bio-templates for the synthesis of metal oxides to achieve unique morphological nanostructures has received significant attraction (Luo et al., 2011). The bio-templates based on their strong potential to control the dimension and morphology of nanomaterials allow fabrication of complex nanostructures with greater precision and reproducibility.

1.6 Co3O4 nanostructures for non-enzymatic glucose sensing

Recently, major scientific interest targeting fabrication of nanostructures with unique shape and diminutive size is rapidly increasing in regards to their intimating properties compared to the bulk counter parts (Abdulla-Al-Mamun et al., 2009; Das et al., 2011; Mallick et al., 2005). The capability to fabricate nanostructures with variety of shape and sizes enables exploration of their fascination properties in fields like catalysis, electronics and sensors. Although most of the properties shown by these nanostructures are a consequence of their nanoscale regime however, it has also known for the fact that such properties are also largely shape dependent.

For example (Solla-Gullon et al., 2011) showed the importance of shape-controlled Pt, Au and Pd nanoparticles in electroctalytic redox reactions such as reduction of O2 and CO, electroxidation of methanol, ethanol and formic acid. Similarly (Sharifi et al., 2014) showed shape-dependent electron transfer kinetics and catalytic activity of nickel oxide nanoparticles immobilized onto DNA modified electrode. Barakat et al., 2013 demonstrated influence of the nanofibrous morphology on the catalytic activity of NiO nanostructures and Zhang et al., 2014 reported flower-like zinc oxide (ZnO) nanostructure as the electrode material for sensitive detection of lead (Pb2+) ions in human serum and water samples.

At the present, the quantification of glucose is an essential task to be carried in biotechnology, clinical laboratories and food industry. Hence, there is an increasing demand for reliable, stable, portable and yet inexpensive sensor for routine glucose analysis (Y. Lin et al., 2003; A. Sun et al., 2012; Yuan et al., 2013). Beside the existence of several glucose sensing techniques like fluorescent and colorimetric assays (Biju et al., 2001; R. Soomro et al., 2014), the electrochemical approach is one of the most versatile route available for the development of simple and cost effective sensors because of its advantages like enhanced sensitivity, simplicity, and easy miniaturization (P. Wu et al., 2010; M. Yang et al., 2006; Ye et al., 2004).

The conventional glucose biosensors are based on the immobilized glucose oxidase (GO), which catalyses the oxidation of glucose in the vicinity of O2 to generate hydrogen peroxide and results in highly sensitive and selective glucose response (S. Liu et al., 2011; Shan et al., 2010; B. Wu et al., 2004). Although, GO based biosensors are sensitive and selective, but these biosensors suffer from limitations like complex and multistep immobilization procedures, thermal and chemical instability and expensive fabrication processes. Henceforth, restricting the wide scope application of GO based sensor. To surmount such drawbacks, considerable interest is being paid in the development of non-enzymatic glucose sensors (A. Sun et al., 2012).

In the recent past years, several non-enzymatic glucose sensors have been fabricated using various noble metals including platinum Pt, Au, Ag and alloy-metals (Ying Li et al., 2007; Xiao et al., 2009; Zhong et al., 2005). However, the difficult availability of precious metals along with higher cost, smaller air durability and toxic nature they are considered unsuitable candidates for poor resource based situations. At the same time different metal oxides like NiO, CuO, manganese oxide (MnO2), and Co3O4 have demonstrated excellent performance in the enzyme free quantification of glucose (J. Chen et al., 2008; Y. Ding et al., 2010a; Jiang et al., 2010; Y. Zhang et al., 2011). Among the various kind of transition metal oxides, cobalt oxide nanostructures are attractive due to their unique properties and polymorphs such as cobaltous oxide (CoO), cobaltic oxide (Co2O3) and cobaltosic oxide (Co3O4) (Shinde et al., 2006). Such nanostructures are of high interest due their wide spreading applications in catalytic oxidation of different molecules.

In addition, the biocompatibility of cobalt oxide nanostructures widens its application for immobilization of biomolecules such as FAD and haemoglobin (Salimi et al., 2012; Salimi et al., 2008). Although number of different morphologies of cobalt oxide including nanofibers (Yu Ding et al., 2010; Guo et al., 2013), and nanorods (Kung et al., 2011), nanotubes,(X. W. Lou et al., 2008) nanobelts, (Yu Wang et al., 2010) nanocubes,(M. Wang et al., 2011) nanosheets,(L. Chen et al., 2010; X. Wang et al., 2011) and hexagonal discs (Jing Yang et al., 2010) have been synthesized via various routes, but still very less morphologies have been explored for their potential applications in electrochemical sensing of various biologically important chemicals.

1.7 Problem Statement

The increasing developments in the nanotechnology has advanced the production of functionalized material. Such materials have demonstrated potential capabilities in areas such as optics and electrochemical sensors. The sensing capability of such materials is highly influenced by the shape, dimension and morphology of the structures. Besides this the associated functionality is also known to play an important role. In this regard, usage of templates have proven to be efficient. However, the choice of which template to use depends on the growth directing and controlling potential of the selected template. Thus, this thesis explores the potential capability of bio-compatible templates particularly, amino acids as effective growth modifiers and functionalizing agent for the metals and metal oxides nanostructures. Various amino acids have been utilized as capping agents, functionalizing agents and as templates to elaborate their ability to function as growth controller, active moiety and growth director for metal and metal oxides nanostructures respectively.

1.8 Significance and Scope of research

The application of amino acids as an effective template and functionalizing agent for metal and metal oxides nanostructures can allow development of new structures with enormous potential capability to be used in areas where morphology plays a key role such as catalyst, optics and electrochemical sensors. The exploration of amino acids for the said purpose can provide a step forward in the development of novel and effective materials with enhanced and robust properties compared to their template free counter parts. Besides this, the usage of amino acids as templates would not only provide bio-compatibility to the system but would also impart reproducibility in the as-synthesized nanostructure which is highly crucial when it comes to sensor development. The scope of the produced work encompasses a broad range of analytical applications which include catalysis, colorimetric and electrochemical sensors. The elaborate exploration of various amino acids in encouraging synthesis of nanostructures (metal/metal oxides) for the said applications would not only promote the development of novel nanostructure but also provide a new path way towards the understanding of how importance the shape of nanomaterial is in many processes such as electro-catalysis and optical sensors.

1.9 Aims and Objectives

The above discussed literature highlights the usage of cheap metal like copper and nickel in applications like optical sensors and catalysis. In addition, applications of metal oxides like NiO and Co3O4 for development of enzyme free glucose sensing. The major aims and objectives of this thesis can be divided three categories.

i. Synthesizing cheap metal and metal oxide nanostructures using bottom-up approach

This thesis focuses on the use of green amino acids like L-cysteine for the production of cheap nanoparticles of copper. Although, copper is very likely to oxidize in aqueous environment and hence very difficult to be utilized as optical probe for calorimetric sensing, we have dealt with this problem by functionalizing copper nanoparticles with thiol containing amino acid, which has a very high affinity for copper metal. This enabled dual rewards as Cu NPs being stable and functionalized specifically to be employed as optical probe for Hg2+. This thesis also discusses the problem to obtain high reproducible nanostructures of NiO. The issue was addressed by again using green amino acids during the hydrothermal growth of metal oxide. The presence of amino acid functioned as an effective bio-template controlling growth and direction of structures. This strategy not only allowed formation of entirely novel, reproducible nanostructures but also opened up a new path way to explore the potential electrochemical application of these nanostructures.

ii. Developing novel and economic optical sensor for toxic metals

For the first time, this thesis discusses the usage of functionalized copper nanoparticles to be applied as an optical probe to determine the concentration of mercuric ion in aqueous medium. LSPR property of Cu NPs was heavily explored in this along with highlighting the underneath mechanism involved in the sensing phenomena.

iii. Usage of metal nanoparticles for catalysis

This thesis further stretches the application of copper nanoparticles in catalytic removal of organic dyes from the coloured waste water of industries. It also discusses the shape dependent catalytic activity of copper nanostructures highlighting how such nanostructures are formed and why their catalytic behaviours varies with change in structural features. In addition, this thesis also discusses, the first time usage of nickel nanoparticles for the reductive degradation of a toxic and extensively used pesticide (imidacloprid) in Pakistan.

iv. Developing cheap, reliable and stable electrochemical glucose sensors

This thesis explores the capability of novel, stable and reproducible metal oxide nanostructures (NiO and Co3O4) to oxide glucose in an alkaline medium. This enabled development of highly sensitive, reliable, selective non-enzymatic glucose sensor with their application for real blood samples.

Chapter 2 LITERATURE REVIEW

2.1 Synthesis of copper nanoparticles

Metallic nanoparticles are of great interest due to their excellent chemical, physical and catalytic properties (Dang et al., 2011 ). Among the metal nanoparticles, Cu NPs have received considerable public interest, which may be due to their good optical, electrical and thermal properties. Cu NPs were assumed cost-effective as compared to noble metals like Ag, Au and Pt. Hence, they are potentially applied in the fields of catalysis (P. Singh et al., 2008), cooling fluids (H.-S. Kim et al., 2009) and conductive inks (Youngil Lee, 2008). It is evident from the previously reported studies that synthesis of Cu NPs is equally significant to their applications in various fields. Various methods like radiation (Jushi et al., 1998), micro-emulsion (Solanki et al., 2010), thermal decomposition (Y. H. Kim et al., 2006), and aqueous chemical reduction (Q.-m. Liu et al., 2012) have been applied for the synthesis of Cu NPs. The aqueous reduction method is widely selected for the synthesis of Cu NPs because it is robust, cost effective, efficient in yield, and requires limited equipment. It effective provides control on the size and distribution of particles using simple reaction parameters, such as concentration of precursor salt, capping agent and pH of solution (Athawale et al., 2005; Coussy et al., 2005; Q.-m. Liu et al., 2008; H.-t. Zhu et al., 2004). However, Cu NPs in aqueous environment are largely prone to oxidation. The inevitably generation of surface oxide layers (Cu-oxides) is thermodynamically considered more stable as compared to pure Cu metal. Moreover, it was also noticed that copper particles form aggregates without proper protection. Such problems like aggregation and oxidation can be solved by employing various protecting agents, such as polymers (Giuffrida et al., 2008; H. X. Zhang et al., 2009) and organic ligands (X. Zhang, H. Yin, X. Cheng, H. Hu, et al., 2006; X. Zhang, H. Yin, X. Cheng, Z. Jiang, et al., 2006).

The synthesis of Cu NPs in an aqueous solution under nitrogen environment has been reported by some researchers (Joshi et al., 1998b). While, formation of Cu NPs in non-aqueous environment using PVP polymer as a capping agent has also been suggested (Park et al., 2007). Some other methods (Solanki et al., 2010; Q.-l. Zhang et al., 2010) have also been proposed for the synthesis of Cu NPs using bis (ethylhexyl) hydrogen phosphate (HDEHP) in organic phase to prevent oxidation and KHB4 as reducing agent and PEG-4000 as a protecting agent respectively. Most of the aforementioned procedures employ an oxygen-free atmosphere and heavy molecular weight polymers as protecting agent to prevent oxidation and aggregation of Cu NPs. Hence, it limits the use Cu NPs in an open environment and restrains the catalytic activity as most of active sites of the particles are blocked by heavy polymer chains.

Vaseem M and Lee KM (Vaseem et al., 2011), have reported the synthesis of copper nanoparticles employing CTAB as a stabilizing agent and N2H2 as reducing agent, where production of N2 by-product from hydrazine avoided the use of protecting gases. Similarly copper nanoparticles have been prepared with the application of polymer like PVP and NaH2PO2 without using any protective gas (D. Lai et al., 2013). However, synthetic procedure was time dependent and took a long time of seventeen hours with heat requirement. Zhang D, Yang H (D. Zhang et al. 2013) prepared copper nanoparticles employing gelatin as capping agent. Since gelatin is less soluble in water at room temperature, it would be difficult to re-disperse the particles in water.

2.2 Synthesis of nickel nanoparticles

Various methods have demonstrated excellent and important synthesis protocol for achieving small and uniform nickel nanoparitcles. In a study, the simultaneous usage of ethylene glycol as both the solvent and reducing agent for dodecylamine (DDA) and polyvinyl pyrrolidone (PVP) protected nickel nanoparticles has been studied (D. Li et al., 2006). Where DDA and PVP ratio to metal ion was considered as a crucial parameter for controlling the particle size.

The synthesis of flower shaped Ni nanostructures, as-assembled and stabilized via the usage of PVP have also been reported (Xu et al., 2008). The important role of trivial amount of Na2CO3 was also discussed, followed by application of hydrazine for the reduction of Ni ions to form such nanostructures from monomorphic Ni NPs. In contrast, Gérardin, C and Kostadinova, D (Gérardin et al., 2005) reported production of colloidal Ni NPs by controlled hydroxylation of Ni ions which were complexed with cirate anions. The paper presented formation of various compositional architectures upon reduction of the intercalated nickel complex.

The formation of pure nickel nanoparticles via hydrazine reduction was demonstrated by Roselina NRN (Roselina et al., 2013). Whereas, the synthesis of stable Ni NPs using wet chemical methodology with hexadecylamine (HDA) and trioctylphosphine oxide (TOPO) as protecting agents have also been reported (Hou et al., 2005). Similarly, Roselina NRN (Roselina et al., 2013) demonstrated the formation of nickel nanostructures via chemical reduction employing hydrazine and ethylene glycol as the reducing and stabilizing agents respectively. Recently, Tontini, G. Koch and Jr, A. (Tontini et al., 2015) reported the synthesis of temperature controlled loose nickel micro-urchins using a chemical reduction route. From the present literature perspective, it is clear that the development of a feasible synthetic method for NPs is important to support their use in various practical applications. This applies particularly to Ni NPs which are very much vulnerable towards oxidation. The extent of this surface oxidation increases with the decrease in average particle diameter, which as a consequence is responsible for diminished catalytic performance. Tremendous efforts were carried out for the preparation of pure metallic nano-sized Ni NPs through various synthesis strategies. Kumar, A and Saxena, A (A. Kumar et al., 2013) reported the controlled synthesis of Ni NPs using water and oil microemulsions. In contrast, Wu and Chen (S.-H. Wu et al., 2003) reported the formation of spherical Ni NPs by the hydrazine reduction of nickel chloride in ethylene glycol solvent and recently Roy and Bhattacharya (Roy et al., 2014) addressed the use of polymers to produce air-durable Ni NPs. However, all such strategies either include the application of inert atmosphere, toxic reducing agents or dense molecular weight polymers as capping/protecting agents for stability this as a consequence, restricts the large volume production and application of Ni NPs in various catalytic reactions.

2.3 Synthesis of metal oxide nanostructures

Currently considerable interest in nano-crystalline oxide materials exists owing to their unusual properties. Decreasing particle size and fabricating unique morphologies results in some remarkable electronic phenomena’s. For example Li, C and Liu, Y (C. Li et al., 2008) reported the fabrication of NiO hollow nanospheres and its electro-catalytic potential towards amperometric sensing. Similarly Umar, A and Rahman, M (Umar et al., 2009) prepared polyhedral nanocages of MgO, which were determined to be an effective electrode material in bio-sensing of glucose and very recently, Chen, T and Li, X (T. Chen et al., 2014) prepared novel Co3O4/ PbO2 core-shell nanorods arrays and examined their electrochemical capability towards enzyme free glucose determination.

Vassilyev, Y and B. Khazova (Vassilyev et al., 1985) discussed the kinetics and mechanism of glucose electro-oxidation on different electrode-catalysts. Similarly Ding, Yu and Wang, Ying (Y. Ding et al., 2010b) utilized Co3O4 nanofibers fabricated via electrospun technique and used them for sensitive glucose sensing. RuO2 were utilized by Lyons, M. E. G and Fitzgerald, C. A (Lyons et al., 1994). The study proposed that the surface bound oxyruthenium groups are the active catalytic species responsible for oxidation of glucose.

Chen, J and Zhang, W (J. Chen et al., 2008) demonstrated the usage of MnO2/MWNTs nanocomposite for non-enzymatic glucose sensing. The MnO2 were coated over the vertically aligned MWNTs by using electrodeposition technique. From these metal oxides, NiO nanostructures particularly, are believed to exhibit excellent electrocatalytic response besides the simplicity offered in their growth procedures (Elumalai et al., 2009). Various morphologies of nickel oxide (NiO) have been widely used for the different applications in addition to the non-enzymatic glucose sensors (M. Li, X. Bo, Z. Mu, et al., 2014). Different nanostructures of NiO have been used in the design of non-enzymatic glucose sensors. For example Cao, C-Y, Guo (C.-Y. Cao et al., 2011) demonstrated formation of flower shaped NiO nanostructures via microwave-assisted gas/liquid interfacial method. These nanostructures were found very productive to be used as super-capacitor electrodes. Ding, Y and Liu, Y (Y. Ding, Y. Liu, J. Parisi, et al., 2011) published report preparation of NiO–Au hybrid nanobelts. Similarly Kaneko, S and Ito, T (Kaneko et al., 2011) reported the synthesis of NiO nanoparticles synthesized via pulse laser deposition technique. Free standing nanoflakes were synthesized by Wang, G and Lu, X (G. Wang et al., 2012) using low temperature hydrothermal growth method. NiO nanoparticles and platinum nanoparticles over reduced graphene oxide for electrochemical determination of glucose have also been reported (S.-J. Li et al., 2014).

Recently, Song and Gao (Song & Gao, 2008) reprted the formation of NiO hollow spheres and demonstrated outstanding performance in photo-catalyst. Huang, C.-C and Chang, H.-T (X. H. Huang et al., 2010) synthesized hollow microspheres of NiO as anode materials for lithium-ion batteries. These hollow spheres were synthesized via heating NiCl2/resorcinol-formaldehyde (RF) gel in argon at about 700 °C for at least 2 h, and later in oxygen at about 700 °C for at least 2 h.(Ci et al., 2014) demonstrated the formation of nickel oxide hollow microsphere for non-enzyme glucose sensing solvothermal method using mixed solvents like ethanol and water in the presence of sodium dodecyl sulphate (SDS). These nanostructures were further used for non-enzymatic glucose sensing.

2.4 Metal nanoparticles as optical sensors

At present much of research attention regarding application of Plasmonic nanostructures is only concerned to noble metals like Au and Ag. Since Au NPs are among the most stable particles based on their chemical inertness, such nanoparticles can be easily and efficiently utilized for calorimetric sensing applications (M. Chen et al., 2004; Marx, 2004). In contrast silver, although less stable demonstrates higher molar extension coefficient compared to gold nanoparticles. This particular property enables silver to exhibit better visibility and hence greater sensitivity. Lim, D.-K and Chen, Z.(Lim et al., 2008 and.Z. Chen et al., 2014) demonstrated the capability of functionalized Au NPs for the calorimetric sensing of potassium ions. The group has successful to detect the potassium ion in aqueous solution selectively up to 5 nM. They used thiolated aptamers for effective functionalization of Au NPs, where potassium ions bind to the G-rich nucleci acids resulting into G4 type structures and simultaneous aggregation of Au NPs.

The first Ca2+ detector was developed using lactose-functionalised AuNPs, where addition of Ca2+ induced the interaction between the carbohydrate moieties leading to colorimetric changes because of nanoparticle aggregation (Oliveira et al., 2015) . In connection to this method two more sensitive methods have been published recently. These assays relied on calsequestrin (CSQ) and cytidine triphosphate (CTP) functionalization of Au NPs Kim, S and Zhang, J (S. Kim et al., 2011; S. Kim et al., 2009).(J. Zhang et al., 2011) further developed calorimetric assays for the alkaline earth metal ions. The reported protocol used 2-mercaptosuccinic acid (MSA)-functionalised Au NPs which enabled simultaneous sensing of calcium, strontium and barium metal ions.

From the perspective of toxicity of Cr6+ Lai, Y.-J and Tseng, W.-L (Y.-J. Lai et al., 2011) demonstrated excellent analytical sensing via 5-thio-2-nitrobenzoic acid (TNBA) capped Au NPs. The protocol was assisted by the usage of vitamin C, which allowed rapid reduction of Cr6+ to Cr3+, simultaneously aggregating Au NPs because of high affiliation of Cr+3 towards functional moieties of TNBA. Similar affinity approach was also utilized by Tan, F and Liu, X (Tan et al., 2011), where strong affinity between Cr6+ and thiol containing amino acids such as cysteine and glutathione was utilized for development of colorimetric probe. Similarly Zhao, L and Jin, Y. (Zhao et al., 2012) published report for selective sensing of Cr3+ utilizing dithiocarbamate-modified N-benzyl-4-(pyridine-4-ylmethyl) aniline compound (BP-DCT) capped Au NPs.

In regard to detection of Hg2+in aqueous environment Fernández-Lodeiro and J., Núñez (Fernández-Lodeiro et al., 2013) described sensitive sensing of Hg2+ using fluorescence in associated Au NPs. The presence of Hg2+ in such environment lead to change in colour from red to blue and enable exhibition of new band around 635 nm. The protocol was based on the chelate formation supported via Hg2+ and carboxylate moiety in fluorescein group.

Anti-aggregation approach has also been found productive for sensing of Hg2+ ions. Li, Y and Wu, P (Yan Li et al., 2011) reported calorimetric sensing based on anti-aggregation of 4,4’-dipyridyl when Hg2+ was introduced within the Au NPs. Similar approach was adopted by Yang, X and Liu, H (X. Yang et al., 2011)who reported anti-aggregation of pyridine in presence of Hg2+ions Lou, T and Chen, L (T. Lou et al., 2012) described similar approach using thymine anti-aggregation agent and Zhou, Y and Dong, H (Y. Zhou et al., 2014) presented the similar approach using 4-mercaptophenylboronic acid.

Silver nanoparticles have also been extensively used for calorimetric sensing. Ravindran, A and Elavarasi, M (Ravindran et al., 2012) used citrate capped Ag NPs for detection of Cr3+ ions using chelation methodology. Similarly, Wu, X and Xing, W.(Xiaoyan Wu et al., 2013) reported the dual usage of ascorbic acid (AA) as capping and reducing agent for Ag NPs. These particles were further explored for their calorimetric sensing of Cr3+. However the detection limit was not even close to that reported by Yao, Y and Tian, D (Yao et al., 2010) demonstrated the usage of triazole-carboxyl functionalized Ag NPs for the selective and sensitive sensing of Co2+ ions in aqueous solution. Similarly, calorimetric sensor for determination of Ni2+ ions was reported by Shang, Y and Wu, F. (Shang et al., 2012). These nanoparticles were synthesized by wet-chemical reduction method employing sodium borohydride (NaBH4) and glutathione (GSH) and N-acetyl-L-cysteine (NAC) as reducing and capping agent respectively. When it comes to sensing mercury, Ag NPs associated sensor work basically using the redox chemical reaction between Ag and Hg2+ ions. Fan, Y and Wang, G (Y. Fan et al., 2009) (G.-L. Wang et al., 2012) demonstrated the sensing of Hg2+ ion in an aqueous environment based on the formation of amalgam particles after the redox reaction between the two species. The formation of amalagum results in a slight shift in the surface Plasmon’s of Ag NPs which was accounted for the change in concentration of Hg2+ion. Similarly Ramesh, G and Radhakrishnan, T (Ramesh et al., 2011) reported formation of Ag NPs associated with poly (vinyl alcohol) (PVA) thin film for the sensing of Hg2+ ions. The analytical advantage of the proposed method was the simultaneous detection of all three oxidation states (+2, +1 and 0) of mercury. Although very efficient but the high cost margin and hence difficult availability of gold and silver metals restricts the development of economic, on site assessment as feasible calorimetric probes, besides their usage in large volume production (Farhadi et al., 2012). In such connection, copper with LSPR band residing within the visible region of electromagnetic spectrum, lower cost and easy availability compared to Au and Ag can serves as a suitable alternative. Recently, Hatamie, A and Zargar, B (Hatamie et al., 2014) have reported the use LSPR of copper nanoparticles for selective and sensitive colorimetric detection of sulfide ion (S-2) to micro molar concentration. However, since copper nanoparticles were obtained via CTAB assisted reduction route in capped bottle, poor efficiency of CTAB would still result in oxide formation in open air. Unfortunately, very less attention has been paid towards the fabrication of pure copper nanoparticles for plasmonic application.

2.5 Application of metal nanoparticles in catalysis

Catalysis is an important field of work because of its gigantic impact over the world economy, as more than 90% of chemical producing processes utilize catalysis(Cuenya, 2010). Shiraishi, Y and Toshima, N (Shiraishi et al., 1999, 2000) demonstrated the excellent capability of Ag NPs for the catalytic conversion of ethylene to ethylene oxide. In similar content, Kundu, S and Mandal, M (Kundu et al., 2004), demonstrated the potential of Ag NPs for reduction of nitro-group containing compounds and Alvarez-Ros, M and Sanchez-Cortes (Alvarez-Ros et al., 2003) reported the catalytic efficiency of Ag NPs for degradation of phenolic compounds. Similarly Bhaduri, G. A and Siller, L.(Bhaduri et al., 2013) demonstrated the application of nickel nanoparticles as a catalyst for reversible hydration of CO2 for the mineralization carbon capture and storage. Rathore, P. S and Patidar, R (Rathore et al., 2015) demonstrated the synthesis of magnetically separable core–shell iron oxide at nickel (IO@Ni) via reduction of Ni2+ ions in presence of iron oxide. The protocol employed starch as an effective capping agent. These NPs were found to possess excellent activity for hydrogenation of aromatic nitro compounds using water as greener solvent. Metin, Ö and Mazumder, V (Metin et al., 2010) reported the ketjen carbon supported Ni NPs for the catalytic application of hydrogen production via hydrolysis of ammonia-borane (H3NBH3) complex. Kaur, R and Giordano, C (Kaur et al., 2014) described the application of copper nanoparticles aqueous, synthesized through the usage of copper-surfactant complex for the reduction of nitrophenol under atmospheric conditions. Decan, M. R and Impellizzeri, S (Decan et al., 2014) recently reported the usage of copper nanoparticles for the click catalysis. Mondal, P and Sinha, A (Mondal et al., 2013) reported the application of a graphene based composite of copper nanoparticles (Cu-G) as an efficient catalyst for N-arylation and O-arylation. The protocol allowed production of diaryl ethers, under mild conditions with considerable yield and excellent selectivities.

2.6 Application of metal oxide in electrochemical sensor

To date, various active materials have been investigated for electrochemical systems as anode or cathode materials. Among all them, transition metal oxides have been found most suitable candidates because of enhanced and unique electrochemical properties.

Yang, J and Yu (J. Yang et al., 2013) reported the application of nanocomposite of chitosan-reduced graphene oxide-nickel nanoparticles (CS-RGO-NiNPs) deposited onto a screen-printed electrode (SPE) for the enzyme free glucose sensing in alkaline medium. The sensor was found very selective and sensitive (318.4 mA mM-1 cm-2) for detection of glucose. The protocol allowed development of a micro-fluid device, which was then further used for real time determination of glucose in human urine samples. Similarly, Shan, C and Yang, H (Shan et al., 2010) produced application of thin films of chitosan incorporating nanocomposite of graphene and gold nanoparticles for electrochemical determination of glucose. The composite materials thin film was used for the modification of glassy carbon electrode which allowed sensitivity of 99.5 µA mM-1 cm-2 along with good selectivity and reproducibility.

Luo, S and Su, F.(Luo et al., 2011) discussed new protocol for the fabrication of nanofibers. These nanofiblers were made of TiO2 nanotube (TiO2 NT) arrays modified using cupric oxide (CuO). These nanofibers demonstrated excellent sensing performance with sensitivity of 79.79 µA mM-1 cm-2. Further developed sensor was known to exhibit extremely selective in the presence of chloride ions. Abdel Hameed, R. M (Abdel Hameed, 2013) utilized nickel nanoparticles deposited over carbon Vulcan XC-72R for development of sensitive enzyme less sensor. The deposition was carried using microwave irradiation technique the sensor possessed sensitivity of 1349.7 μA mM-1 cm-2. Moreover, the parameter of microwave such as irradiation time and mode was found to affect the obtained morphology of nickel nanoparticles. Lv, W and Jin, F.-M (Lv et al., 2012) presented potential application of graphene nanosheet/NiO hybrid (GNS/NiO) for selective enzyme less sensor. The report describes the development of sensor based on DNA dispersed GNS/ NiO suspension deposited over glassy carbon electrode (GCE). The authors argue that the ss-DNA has a greater dispersion capability for GNS/NiO hybrid materials; as a consequence water-stable dispersible hybrids of GNS/NiO/DNA can be obtained.

Karuppiah, C and Palanisamy, S (Karuppiah et al., 2014) showed the performance of enzymatic glucose sensing based on immobilization of graphene (GF) and cobalt oxide nanoparticles (Co3O4-NPs) modified electrode. The composite material showed excellent capability towards oxidation of glucose with a sensitivity of 13.52 μA mM-1 cm-2. Khun, K and Ibupoto, Z. H.(Khun et al., 2015) developed a sensor system based on Au substrate composed of highly dense cobalt oxide (Co3O4) nanowires. The nanowires were obtained via hydrothermal method employing ethylene glycol as an effective template. The electrode system was further used for sensitive (4.58 μA mM-1 cm-2) and selective determination of glucose. Han, L and Yang, D.-P (Han et al., 2015) further reported the formation of novel 3D hierarchical porous cobalt oxide (Co3O4) using leaf-templated methodology. The authors described the excellent glucose sensing properties of such structures as a consequence of high specific surface area associated with the porous structures. The protocol was known to exhibit a sensitivity of 389.7 μA mM-1 cm-2.

Zhang, J and Gao, W (J. Zhang et al., 2015) developed a highly sensitive electrochemical sensor for hydrazine in aqueous environment. The sensor system relied on the catalytic properties of Porous Co3O4 NWs, which were fabricated via interconnected nanorods leading to formation of 3D porous morphology. The sensor exhibited an excellent selectivity and sensitivity (28.63 µA mM-1). Wang, S and Xu, X (S. Wang et al., 2015) demonstrated the effective sensing of hydrazine using electrode incorporating porous CuO nanobelts on Cu nanorods as a composite material. The electrode showed promising results with a sensitivity of 41.17 mA mM-1 cm-2.

Kumar, S and Bhanjana, G (S. Kumar et al., 2015) in similar way demonstrated the catalytic potential of ZnO material for electrochemical sensing of hydrazine. The ZnO employed possessed cones shaped morphological features and were fabricated using a low-temperature solution method. The developed sensor exhibited a sensitivity of 50×104 µA mM-1 cm-2.

Chapter 3 MATERIAL AND METHODS

3.1 L-cysteine protected copper nanoparticles as colorimetric sensor for mercuric ions

3.1.1 Materials and reagents

All chemicals used were of analytical grade and used without further purification by employing pure Milli-Q water as the preparatory medium. CuCl2.5H2O (97%), hydrazine, hydrate N2H4. H2O, (80%), were obtained from E. Merck Germany, L-cysteine (C3H7NO2S), from Fluka Chemika Germany. NaOH (98%), HCl (37%), and all different cations, in the form of salts including (MgCl2·6H2O, Ca (NO3)2·4H2O, Zn (NO3)2·6H2O, Co (NO3) 6H2O, Ni (NO3)2·6H2O, Mn (NO3)2·4H2O, Pb (NO2) and HgSO4·H2O and were purchased from Sigma-Aldrich chemical company, Germany. The stock solutions of metal ions were prepared by dissolving a known amount (per mg) of salts in pure Milli-Q water and all experiments were performed at ambient temperature (25 ±2oC).

3.1.2 Instrumentation

UV–Visible spectroscopy (Lambda 35 of PerkinElmer) was used for tracing LSPR of Cyst-Cu NPs and performing colorimetric assay measurements within the spectral range of 400-800 nm. Surface functionalization of Cu NPs with L-cysteine was observed by Fourier transform infrared (FTIR) spectroscopy (Nicolet 5700 of Thermo) using KBr pelleting method. Solid sample were obtained, after parching colloidal dispersion of Cyst-Cu NPs under nitrogen atmosphere. Morphological characterization for size distribution and shape homogeneity of colorimetric sensor (Cyst-Cu NPs) before and after the colorimetric assay was performed using TEM (Jeol JEM 1200 EX MKI). Similarly changes in phase composition of the colorimetric sensor were studied using XRD model D-8 of Bruker. Photographs of the colloidal Cyst-Cu NPs used for visual colorimetric detection of Hg2+ were recorded by using a digital camera.

3.1.3 Synthesis of L-cysteine functionalized Cu nanoparticles

The synthesis of Cyst-Cu NPs was achieved in a capped test tube of 10 mL volume capacity. In a typical synthetic procedure, 300 µL of 0.003 M solution of CuCl2 was diluted with de-ionized water up to 08 mL taken in a 10 mL test tube. To this was added 20 µL of 0.01 M L-cysteine solution which turned the colour of mixture from light blue to colourless. This was followed by addition of 1000 µL of 0.1 M hydrazine monohydrate (N2H4.H2O) for reduction of metal ions during which the solution mixture changed from colourless to red within 60 min of reaction time. The pH of entire mixture was maintained at 6.5 in accordance to pka values of L-cysteine to get hold of smallest particle size.

The change in colour indicated the formation of Cu NPs and the resulting product was used for colorimetric assay. The use of hydrazine monohydrate as mild reducing agent allowed preserving homogeneity of synthesized nanoparticles with generation of an inert atmosphere, making this protocol independent from the use of protective gas and hence tedious strategies to avoid oxidation.

3.1.4 Colorimetric sensing of Hg2+ ions by cyst-Cu NPs

The colorimetric detection of aqueous Hg2+ ions was performed at room temperature. Briefly, 10 μL of Hg2+ ions with different concentrations ranging from 0.5-3.5 µM were added to 3 mL of Cyst-Cu NPs solution. These solutions were mixed slowly for 2-3 min followed by transfer of small portion into a 1 cm quartz cell to record absorbance. Absorbance was measured at LSPR wavelength of 565 nm, against a blank reagent. ΔA (LSPR) was used as the analytical signal and color changes from light red to pale yellow taken as a naked eye colorimetric response of system. The corresponding color changes were recorded with an 18 mega pixel digital camera after mixing the solution for 2 min.

3.1.5 Use of Cyst-Cu NPs as sensor for assay of Hg2+ions in real water samples

Three samples were collected from various localities of river Sindh near Kotri barrage. The samples were filtered, and two times diluted before performing analytical assay. These samples were treated in the same way as true in case of Hg2+ ions detection in standard using Cyst-Cu NPs as colorimetric sensor.

3.2 Synthesis of air stable copper nanoparticles and their use in catalysis

3.2.1 Materials

For the present work, we used analytical grade chemicals such as copper chloride pentahydrate, sodium dodecylsulphate (SDS) (98%), ascorbic acid (Vitamin-C) (98%) purchased from E. Merck and sodium borohydride (NaBH4)(98%), sodium hydroxide, hydrochloric acid obtained from Sigma-Aldrich, Germany. All chemicals were used as-received without further purification.

3.2.2 Method of Synthesis of Cu NPs

In a typical synthetic procedure Cu NPs were obtained via a modified wet chemical reduction route. This new procedure for Cu NPs fabrication was a simple process. 0.2 mL of 0.03 M CuCl2 5H2O solution was diluted up to 5 mL with deionized water and got a blue color solution. Then, (1.0 mL of 1.0 M SDS) and (0.1 mL of 0.1 M Vitamin C) were mixed with the blue colored solution followed by further addition of the reducing agent (0.3 mL of 0.01 M NaBH4), the solution was further diluted with deionized water and finally a 10 mL solution was obtained. With the passage of time, the color of the solution gradually changed from faint yellow to brick red with a number of intermediate stages. The appearance of the yellow/orange color indicated that the reduction reaction had initiated. The mechanism for reduction of Cu2+ to zero-valant Cu particles with NaBH4 is given in the following reaction:

The appearance of yellow color followed by orange color indicated the formation of fine nanoscale Cu particles resulting from borohydride-assisted reduction. The reaction was allowed to proceed for 15 min in ambient atmosphere to ensure complete reduction and capping of size-homogeneous Cu NPs. Various optimization studies were performed to investigate the size and shapes of Cu NPs.

3.2.3 Instrumentation and sample preparations

UV-VIS spectroscopy, path length of 1.0 cm, spectral range 200-800 nm, and scan rate 1920 nm/s was used for preliminary estimation of Cu NPs synthesis. FTIR spectra were recorded for centrifuged and nitrogen dried samples of Cu NPs in solid state KBr discs. FTIR spectra provided information about the binding interactions of SDS and Vitamin-C with nanosized zerovalant copper particles. Morphological study of the products was carried out with AFM (Agilent 5500), USA image analysis, for which 250 μg/mL nanocluster solutions were centrifuged for 1 min and sonicated (KQ 500-DE) for 30 min. Then 30 μL aliquots were extracted and deposited on freshly cleaved mica surfaces for AFM analysis.

TEM images were recorded to confirm size distribution and shape homogeneity of newly synthesized Cu NPs. Samples were prepared by taking small quantities of SDS-capped Cu NPs separated by centrifugation from aqueous solution and re-dispersed in ethanol. These nanoparticles were then mounted on carbon-coated copper grids by dip coating method and vacuum dried in a desiccator for 25 min to ensure complete removal of solvent. XRD was carried for phase confirmation and degree of crystallinity of the products. A suitable quantity of Cu NPs sample for XRD analysis was prepared by drying sufficient Cu NPs solution under nitrogen to avoid oxidation. The product thus obtained was washed thoroughly with deionized water and acetone to remove any impurities and un-reacted surfactant and then dried in a pre-heated oven at 100 °C.

3.2.4 Catalytic test for reductive degradation of dye

The catalytic performance of newly synthesized SDS-capped Cu NPs was investigated for reductive degradation of EB dye. The test was performed before and after the addition of Cu NPs catalyst to 100 µM EB with 10 mM NaBH4 to check the reduction/degradation process. The test was carried out in an aqueous solution in quartz cells. UV-Visible spectral changes were recorded to monitor reductive degradation of EB. To investigate the catalytic performance, Cu NPs were first deposited onto pre-weighed glass cover slips, and then dried under inert atmosphere to ensure complete adherence of Cu NPs on the glass surface. The glass cover slips were re-weighed after the deposition of Cu NPs to determine the mass of catalyst. These glass cover slips having the deposit of Cu NPs were placed inside the quartz cell, which already contained the mixture of targeted dye and NaBH4 in aqueous medium.

3.3 Catalytic reductive degradation of methyl orange using air resilient copper nanostructures

3.3.1 Materials

All chemicals used were of analytical grade and were used without further purification by employing pure Milli-Q water as the preparatory medium. Copper (II) chloride, sodium dodecyl sulfate, cetyltrimethylammonium bromide (CTAB (98%)), vitamin-C and MO (C14H14N3NaO3S (99%)) were purchased from E. Merck and sodium borohydride, sodium hydroxide, and hydrochloric acid were from Sigma-Aldrich.

3.3.2 Preparation of copper nanostructures

Surfactant capped copper nanostructures were synthesized via an aqueous reduction route. SDS capped copper nanoparticles (Cu NPs) were prepared as mentioned in our previous work (R.A Soomro et al., 2014). However, slight changes in the optimum amount of precursor ingredients were made to obtain best nano size particles with maximum catalytic performance. In the typical synthesis of copper nanoparticles, optimized amounts of ingredients were used. 0.5 mLof 0.02 M CuCl2.5H2O was taken in a test tube. To this was added 10 𝜇L of 1.0M SDS followed by the addition of 0.1 mL of 0.1 M ascorbic acid. The mixture volume was adjusted to 9 mL with de-ionized water and finally 0.3mL of 0.01M NaBH4 was added slowly. To ensure complete reduction and capping of copper nuclei after reduction, reaction was allowed to proceed for about 15 min. With similar methodology, CTAB capped copper nanorods (Cu NRds) were also synthesized by taking 0.1 mL of 0.03 M copper (II) chloride solution in a similar test tube followed by addition of 30 𝜇L of 0.1M cationic surfactant CTAB. 0.1 mL of 0.1M ascorbic acid was added as a quenching agent and the mixture was diluted with de-ionized water up to 9.0 mL. Reduction was carried out by the addition of 0.3 mL of 0.01M NaBH4 slowly down the walls of the test tube. This process was repeated several times to obtain stable surfactant capped copper nanostructures, which were further used for characterization and application.

The proposed ionic mechanism for reduction of Cu2+ to Cu0 with sodium borohydride is given below:

3.3.3 Characterization

UV-Vis spectroscopy was used for tracing SPR of surfactant capped copper nanostructure. Surface interaction study between surfactants and copper nanostructures was carried out with FTIR using the KBr pelleting method, and solid samples were obtained, after parching colloidal dispersions of as-synthesized Cu nanostructures under nitrogen atmospheres. Morphological characterization with size determination was performed using atomic force microscopy (AFM) (Agilent 5500), USA and scanning electron microscopy (SEM) (Jeol, Japan) on freshly cleaved mica and carbon tape surface, respectively. Phase purity and crystalline patterns for Cu nanostructures were studied using XRD.

3.3.4 Catalytic test for reductive degradation of dye

In a representative degradation experiment, 1 mg of SDS capped Cu NPs or CTAB capped Cu NRds was deposited on pre-weighted glass cover slips and dried under nitrogenous atmospheres for complete adhesion to the surface. These cover slips with specific amounts of copper nanostructures were further used in heterogeneous reductive degradation of model azo dye (MO) and real dyeing waste water samples. For an un-catalyzed reaction, an aqueous solution of 100 mM (MO) was taken in a 4 mL capacity quartz cuvette along with 10 mM of (NaBH4) reducing agent. The reaction mixture was studied for some time with an UV-Vis spectrophotometer at room temperature and atmospheric pressure. In a similar manner for catalyzed reaction, glass cover slips with appropriate amounts of surfactant capped Cu nanostructures were placed inside the sample container, already containing model dye and reductant (NaBH4) solution. The catalysed reaction was followed by measuring the time-dependent fall in absorbance (Abs) at 556 nm. The kinetic study of the reductive degradation procedure was performed at constant temperatures and atmospheric pressure with the calculation of activation energy for nanostructures. Degradation of real dyeing waste water samples was carried out with the most efficient copper nanostructure (SDS capped Cu NPs). The reductive degradation experiment used the optimum amount of SDS capped Cu NPs (0.5mg), previously deposited on a clean glass cover slip. This specific amount was further introduced in a mixture containing 10 µL of real waste water sample and 10mMreductant (NaBH4), diluted up to the total volume of 3.5 mL with de-ionized water.

3.4 Catalytic degradation of imidacloprid using l-serine capped nickel nanoparticles

3.4.1 Material and methods

The chemical reagents like NiCl2.6H2O (97%), N2H4 (99%), L-serine (99%), pellets of NaOH and HCl were all purchased from E. Merk. In a typical synthetic procedure standard stock solutions of each compound with desired molar concentrations were prepared in de-ionized water. All the chemical reagents were pure analytical grade and used without further treatment.

3.4.2 Synthesis of L-Serine capped Ni NPs

Synthesis of Ser-Ni NPs with homogeneous size and morphology was carried using wet chemical reduction route. In a typical experiment 60 µL of NiCl2 6H2O (0.033 M) in a 10 mL volumetric flask, was homogenized with of 50 µL L-serine (0.033 M) and 100 L of NaOH (0.1 M). After complete homogenization, 60 µL of N2H4 (0.2 M) was slowly poured down the neck of flask followed by dilution up to mark with de-ionized water. The light green colour of mixed solution gradually changed to black transparent after the addition of N2H4. This indicated the reduction of nickel ions to zero valent nickel atoms as proposed by several other studies. The formation of L-serine capped Ni NPs was further monitored and investigated for various experimental parameters by tracing its SPR (surface Plasmon resonance) using UV-Vis spectrometer. The choice of using hydrazine as a mild reducing agent enabled generation of inert atmosphere and preserving homogeneity of synthesized Ni NPs. This allowed the proposed protocol to be free from the usage of protective gases and tedious strategies to resist oxidation of Ser-Ni NPs.

3.4.3 Instrumentation

UV-VIS spectrometer equipped with cell of 1.0 cm2 path length was used to monitor UV-Vis spectra during optimization studies of various parameters. TEM operated at 120 kV upon magnifications of 500k was used to record micrographs of Ni NPs by dip coating of sample on carbon coated copper grid, and drying under N2 atmosphere for about half hour before TEM imagining. XRD with CuKα radiation (λ= 1.54050 Å) and (2θ) scale range of 30 to 80° was used for phase composition, XPS with a monochromatic Al (Kα) X-ray source using photons with frequency (hυ= 1486.6 eV) and ultrahigh vacuum with a base pressure of 10-10 mbar was used for surface chemical analysis. FTIR spectra of solid state L-serine (pure standard) and the Ser-Ni NPs were obtained through KBr disc method.

3.4.4 Catalytic degradation of imidacloprid

In a typical catalytic experiment, 0.1 mg of Ser-Ni NPs were casted on pre-weight glass cover slips and dried under atmospheric condition to ensure complete adherence of NPs to surface. These modified glass cover slips were then further employed for the heterogeneous catalytic degradation of imidacloprid. For the un-catalysed reaction an aqueous solution of 100 mg L−1 (IM) along 0.12 M NaBH4 (reducing agent) were taken in a 4 mL capacity quartz cuvette and the catalytic reductive degradation reaction was monitored using a UV-Vis spectrometer. Such concentration of imidacloprid represents the maximum levels determined in contaminated effluent. Similarly for catalysed reaction, modified glass cover slips were introduced in the sample container previously containing IM and reductant (NaBH4) solution. The course of catalysed degradation was followed by monitoring fall in absorbance (Abs) at 556 nm wavelength.

3.5 Electrochemical sensing of glucose based on novel hedgehog-like NiO nanostructures

3.5.1 Materials

Analytical grade nickel nitrate hexahydrate (Ni(NO3)2.6H2O), L-cysteine (C3H7NO2S), 33% ammonia (NH3), D-glucose (C6H12O6), dopamine (C8H11NO2), ascorbic acid (C6H8O6), uric acid (C5H4N4O3) and NaOH were obtained from Sigma Aldrich, Germany. 1.0% nafion was prepared in isopropanol (Merck), while other stock solutions were prepared using Milli-Q water.

3.5.2 The Synthesis of novel NiO nanostructures by hydrothermal method

The hydrothermal method was used for the synthesis of NiO nanostructures using L-cysteine as bio template. In the experiment 100 mM of (Ni(NO3)2.6H2O), 1 gram of L-cysteine (C3H7NO2S) were dissolved in 100 mL of deionized water and 5 mL of 33% (NH3) solution was also added. After that the beaker was well sealed with aluminium foil and growth solution was left in preheated electric oven at the temperature of 95°C for 4-6 hours. After the completion of growth time, (Ni (OH)2) nanomaterial were collected, dried at room temperature and annealed at 450°C for 2-3 hours in electric furnace in order to get the pure phase of NiO.

3.5.3 The characterization of NiO nanostructures and electrochemical measurement

FEG-SEM, XRD, XPS and FTIR techniques were employed for the morphological, crystallinity, chemical composition and chemical bonding studies of the as-synthesized hedgehog-like NiO nanostructures respectively. The electrochemical measurements were performed using Bi-potentiostate of CH-instruments model 760E USA.

3.5.4 The modification of glassy carbon electrode (GCE) with hedgehog-like NiO nanostructures

Before the surface modification, glassy carbon electrode (GCE, diameter 3 mm) was polished with 1 µm and 0.05 µm alumina paste respectively, and then washed with the deionized water. Then electrode was sonicated in ethanol, after that washed with deionized water, dried at room temperature, and finally ready for the modification. Then 5 µL of synthesized NiO nanostructures dispersed in ethanol (5 mg/ mL) were drop casted on the polished GCE followed by 5 µL drops of 1% nafion solution (1 wt% in isopropanol) in order to adherence more stronger and resist leaching of the material. Then the modified electrode (NiO HHs-Nafion/GCE) was dipped in the deionized water for 1 h to wet the Nafion layer before use. The finally the obtained NiO HHs-Nafion/GCE was applied as a working electrode in all experiments performed for the enzyme free glucose sensing.

3.5.5 Electroanalysis

Cyclic voltammetry (CV) was designated as a primary mode for the development of non-enzymatic glucose sensor. All the electrochemical measurements were carried in electrochemical cell system consisted of a reference (silver-silver chloride wire (Ag/AgCl)), a counter (platinum (Pt)) and NiO HHs-Nafion/GCE taken as working electrodes respectively. All the measurements were carried at room temperature and pressure. The overall optimized conditions obtained were: 8 mL of 0.1 M NaOH electrolytic solution, initial potential range, 0 to 0.7 (V), scan rate, 0.05 (V/s), quiet time, 5 (s), sensitivity, 1×10-6 (A/V).

3.5.6 Glucose determination in real samples

The determination of glucose concentration in the human blood samples was done using the NiO HHs-Nafion/GCE modified electrode. The blood samples were collected from known individuals and classified as random and fasting blood samples. The blood sera were collected and separated using high speed centrifugation (30 min) followed by filtration and proper diluted with 0.1 M NaOH. The samples were analyzed for the estimation of glucose concentrations using similar methodology as mentioned in section (3.5.5). The obtained results were further authenticated by comparing them with those obtained by using a high quality commercially available glucometer.

3.6 Development of sensitive non-enzymatic glucose sensor using complex nanostructures of cobalt oxide

3.6.1 Materials

Analytical grade cobalt chloride hexahydrate (CoCl2·6H2O), 33 % of ammonia (NH3), D-glucose, dopamine, ascorbic acid, uric acid and sodium hydroxide were purchased from Sigma Aldrich. Stock solution of 1.0% nafion was prepared in isopropanol obtained from (Merck).

3.6.2 The synthesis of cobalt oxide nanostructures by hydrothermal growth method

The cobalt oxide nanostructures were synthesized by low temperature aqueous chemical growth technique. Cobalt chloride hexahydrate (0.1M) and 5 mL of 33% ammonia were mixed to react in 100 mL of deionized water and continuously stirred for 30 minutes. After complete homogenization, the growth solution was kept in a preheated electric oven at 95 °C for the period of 5-6 hours. After completion of growth, the precursor material was collected by filtration, followed by annealing at 450 °C for 3-4 hours in an electrical furnace to obtain pure phase cobalt oxide nanostructures.

3.6.3 Characterization of cobalt oxide nanostructures and their electrochemical measurement

FEG-SEM was used for the investigation of morphological getup of Co3O4 nanostructures and crystalline phase was studied by XRD technique. Phase structural changes and interactions were confirmed using FTIR spectroscopy using KBr pelleting method. The chemical composition was studied by XPS using ultrahigh vacuum and a base pressure of 10-10 mbar. The chamber was equipped with a monochromatic Al (Kα) X-ray source using photons having a frequency of 1486.6 eV. The voltammetric studies were performed using Bi-potentiostate model 760 USA.

3.6.4 Modification of glassy carbon electrode (GCE)

Glassy carbon electrode (GCE) (3 mm) was cleaned by polishing it with 0.05 µm aqueous alumina paste followed by sonication in milli-Q water and ethanol for 1 min respectively. The polished GCE was modified via drop casting 5 µL methanolic dispersion (5 mg/mL) of as-synthesized Co3O4 NDs followed by drying under room temperature. The as-modified GCE was further treated with 2 µL drop of 1% nafion and dried under hot air to ensure complete adherence of nanostructures onto the surface of electrode. The finally obtained Co3O4NDs-GCE was employed as an efficient working electrode during the electrochemical measurements of glucose.

3.6.5 Electroanalysis

CV was selected as a primary mode for the development of non-enzymatic glucose sensor. The electrochemical measurements were performed in a cell system consisting of a silver-silver chloride (Ag/AgCl) electrode as reference, platinum (Pt) wire as counter and Co3O4NDs-GCE as a working electrode. All the experiments were performed at room temperature and pressure with the following resultant optimized conditions: 9 mL of 0.1 M NaOH electrolytic solution, potential range, 0 to 0.7 (V), scan rate, 0.05 (V/s), quiet time, 05 (s) and sensitivity, 1×10-6(A/V).

3.6.6 Glucose determination in human blood serum samples

Glucose levels of human blood samples were analyzed using the Co3O4NDs-GCE, and the results were validated by comparing with those obtained via a commercial glucometer. The blood samples were collected from few volunteers, followed by high speed centrifuging of the each sample for 35 min to separate corresponded blood serum. The separated blood sera were filtered and properly diluted with 0.1M NaOH followed by glucose determination using similar methodology and reaction condition as mentioned above in section (3.6.5).

Chapter 4 RESULTS AND DISCUSSIONS

PART I

Annotations : The entire study presented in part 1 is published with the following reference:

Soomro, R. A., Nafady, A., Sirajuddin, Memon, N., Sherazi, T. H., & Kalwar, N. H. (2014). L-cysteine protected copper nanoparticles as colorimetric sensor for mercuric ions. Talanta, 130, 415-422.

4.1 L-cysteine protected copper nanoparticles as colorimetric sensor for mercuric ions

This work discusses the usage of L-cysteine functionalized copper nanoparticles for the colorimetric determination of mercuric ions in aqueous phase.

4.1.1 Spectroscopic characterization of functionalized Cu NPs

In recent years, several studies have shown that optical properties of metal nanoparticles depend upon the geometry and size; thus the optical response of metal nanoparticles (NPs) can be tuned to control shape and size of metal nanostructure (Abdulla-Al-Mamun et al., 2009; Ghodselahi et al., 2009; Noguez, 2007). Since surface Plasmon modes of metallic nanoparticles like Au, Ag and Cu reside within the optical region of electromagnetic spectrum (F. Chen et al., 2011; Ghosh et al., 2007), optical spectroscopy was used as a primary tool for investigation of Cyst-Cu nanoparticles. Visible spectral profiles were recorded for optimization of various reaction parameters like concentration of copper chloride, L-cysteine, hydrazine, and pH of solution. The most stable and small sized Cyst-Cu NPs (with blue shifted spectrum) were obtained by taking appropriate concentrations of the precursor salt solutions. Experimental results provided in Figure 4.1.1(a) show an increase in absorbance parallel to very slight shift in LSPR band wavelength with greater amount of Cu ions (0.1-0.8 mL of 0.003 M). However, with higher amount of precursor Cu ions > 0.8 mL, precipitation occurred with disappearance of LSPR band. The color intensity of Cyst-Cu NPs was noted to vary in order with concentration of Cu ions as shown in the inset digital photograph. This increase in LSPR absorbance may be defined as the consequence of increased nucleation rate with greater Cu (II) ions available in solution generating smaller nanoparticles as reported elsewhere (De et al., 2013b; Park et al., 2007).

Visible Spectral Profile, (Figure 4.1.1 (b) based on optimization of reducing agent (50-200 µL) of 0.001 M) suggests reduced particle size as evident from the shift in LSPR wavelength from 577 nm to shorter wavelength of 567 nm. This trend in spectral profile is owing to diminished number of solute per growing particle as number of nuclei produced per single nucleation period increases (De & Mandal, 2013b). It is interesting to note that visible spectral profile (Figure 4.1.1(c) based on increasing concentration of functionalizing or protecting agent (L-cysteine), shows change in LSPR band shape and a blue shift from 567 nm to 565 nm with increasing concentration of L-cysteine. Decreased broadening of LSPR band may be ascribed to the uniform particles size (homogenous) distribution within the sample solution. In general, an increase in concentration of the bio-molecule (L-cysteine) as capping agent enhances the formation of metallic Cu NPs along with reduction in size, as has been previously observed in the case of gold NPs (Zhong et al., 2005).

The key factor for effective functionalization and greater stability of NPs depends on the pH of the solution. Effect of pH on the colloidal stability of the Cyst-Cu NPs was shown by changes in LSPR shape and size registered in the pH range of 4.7 to 7.7 along with prevention of strong basic medium to resist the formation of Cu oxide (Thi My Dung, Thi Tuyet Thu, et al., 2011). It is known that the exact position of the LSPR band depends on variety of factors including size of particles, shape, capping agent, composition and aggregation state of the particles assemblies (Guajardo-Pacheco et al., 2010b). However, in this case variation in the pH of solution might have affected the protonation or de-protonation of the acidic group in the L-cysteine structure resulting in the observed changes in LSPR band. Figure 4.1.1 (d) portrays that increase in pH value from 4.7-7.7 results in altering the shape of LSPR band from broad to narrow with slight shift in wavelength. The color of colloidal sol also darkens at lower pH as depicted by digital photograph in the inset Figure. Based on these experimental results pH 6.5 was selected as optimum working pH for Cyst-Cu NPs. The blue shift in LSPR band position > pH 5 (above the iso-electric pH 5.07) confirms the loss of dissociable proton from ammonium moiety of L-cysteine structure as a consequence of which electrostatic repulsions between the negatively charged (-COO−) group of the surface-binded L-cysteine molecules exist and the formation of aggregates is hindered (Csapó et al., 2012; Mocanu et al., 2009). Also, at lower pH values hydrogen bonding between protonated (-COO-) and (-NH2) groups accounts for the broadening of Cyst-Cu NPs LSPR band(Csapó et al., 2012). Visible spectral profile in connection to the stability of Cyst-Cu NPs with time was recorded and is presented in Figure 4.1.1 (e). No change in wavelength and color of colloidal sol was observed even after several weeks of synthesis. Previous research has shown that Cu NPs does not change in inert atmosphere. In the presence of oxygen dissolved in the aqueous solution, the Cu NPs concentration and the intensity of the LSPR peak gradually decreased as a result of oxidation (Hatamie et al., 2014). In the present study, however, absorption intensity remained unchanged for more than a month after formation and no precipitation and oxidation occurred. This indicates that the Cu NPs are very stable in aqueous solution in the presence of cysteine and hydrazine in a capped test tube.

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Figure 4.1.1Visible spectral profiles for optimization of various reaction parameters (a) precursor salt (b) reducing agent (c) capping agent (d) pH of colloidal Cyst-Cu NPs and (e) stability profile for cyst-Cu NPs

4.1.2 Fourier transforms infrared spectroscopy (FTIR)

Interaction of Cu metal surface with L-cysteine molecules was studied using FTIR spectroscopy. L-cysteine being an amino acid exist as a zwitterion and exhibits bands corresponding to very broad (-NH3+) stretching, asymmetric/symmetric (-NH) bending, and asymmetric/symmetric carboxylate ion (-COO−) stretching as characteristic vibrations for both primary amine and carboxylate salt. Figure 4.1.2(a, b) shows the FTIR spectrum of pure L-cysteine and Cyst-Cu NPs respectively. The characteristic bands in Figure 4.1.2(a) can be labeled as: asymmetric and symmetric stretching of (COO−) at 1630 and 1390 cm−1 respectively, (-NH) bending vibration at 1530 cm−1 and the very broad band of (-NH3+) stretching observed in the 3000–3500 cm−1 range. In addition, to these a weak band attributed to (–SH) group of cysteine molecule was observed near 2500 cm−1. These results are in good agreement with IR study of a typical amino acid (Barth, 2007; Sirajuddin et al., 2011a; Sudipa et al., 2006). The FTIR spectrum of Cyst-Cu NPs (Figure 4.1.2(b)) illustrates nearly similar vibrational bands however the band due to (-SH) as true in case of Figure 4.1.2(a) was not observed in this spectrum which confirms the S-Cu interaction. This observation is further strengthened when one considers the shift in the vibration frequency of carbonyl (COO−) stretching from 1630 cm-1 to 1618 cm-1 in the spectrum of Cyst-Cu NPs, indicating interaction between acid group and other cysteine monomers possibly via hydrogen bonding (M. M. Khan et al., 2012). In addition, slight shifts in frequencies for other groups in spectrum of Cyst-Cu NPs may be explained on the basis of change in dipole as a consequence of L-cysteine binding on high electron dense metal surface (Cai et al., 2006).

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Figure 4.1.2 FTIR spectrum of (a) pure L-cysteine (b) L-cysteine functionalized-Cu NPs

4.1.3 X-ray powdered diffractometry (XRD)

The X-ray diffractograms were recorded for phase composition of both L-cysteine functionalized copper nanoparticles and aggregated product collected at the end of colorimetric assay. Figure 1.3 (a) represents the XRD patterns for pure Cyst-Cu NPs with characteristic Miller indices (111), (200) and (220) lattice planes indexed for face centered cubic structure (FCC) observed at 40.9o, 47.8o and 75.3o. The data in Figure 4.1.3 (a) is referred to pure Cu NPs with no peaks indexed to copper oxide. However, final product (Figure 4.1.3(b)) obtained after exposing Cyst-Cu NPs to excess of Hg2+ ions is presented as Cu2O with indices 29.5°, 36.4°, 42.2°, 61.3°, 73.5° and 77.3° corresponding to (110), (111), (200), (220),(311) and (222) crystal planes. The formation of Cu2O was expected, as the capping molecules exile the surface and oxidation would occurs which is evident by the gradual decline in the intensity of the LSPR band of Cyst-Cu NPs. The obtained results are in good agreement for both Cu and its oxide with previous studies (D. Lai et al., 2013; Nowak et al., 2014; Panigrahi et al., 2006; Qiu et al., 1999)

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Figure 4.1.3 XRD patterns (a) cyst-Cu NPs (b) Hg2+ induced aggregated particles

4.1.4 Colorimetric sensing of mercuric ion

Typically, the colloidal sol of Cu NPs exhibits brick red color owing to its LSPR band position around 400-600 nm (Hatamie et al., 2014). The visible spectrum of the Cyst-Cu NPs is presented in Figure 4.1.4(a). As can be seen, the characteristic LSPR band resides at 565 nm with narrow band shape, confirming the crystalline nature and uniform particle distribution within aqueous medium. Similar to SPR which is sensitive to bulk refractive index changes near metal particle surface causing a shift in resonance angle, LSPR is simply measure of an excitation (absorption + scattering) peak shift or change. However in comparison to SPR, the LSPR is much more sensitive towards refractive index changes in the close vicinity of the NP surface due to closely confined optical near field effect thus, allowing it to facilitate real time and on-site analyses (Kazuma & Tatsuma, 2014).

In case of interaction of Cyst-Cu NPs with Hg2+ ions , a colorimetric change from brick red (Figure 4.1.4(a)) to pale yellow (Figure 4.1.4(b) corresponding to change in the spectral profile of Cyst-Cu NPs from narrow, blue shifted band to broader and red shifted band with decreased intensity was observed as illustrated in Figure 4.1.4. The change in spectral profile after exposure to Hg2+ can be explained on the basis of aggregation of particles within the aqueous solution.

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Figure 4.1.4 Visible Spectral Profile (a) Cyst-Cu NPs (b) Cyst-Cu NPs+Hg2+

4.1.5 Evaluation of sensing mechanism using TEM analysis

The visual insight regarding morphological changes based on non-aggregated and aggregated forms of Cu NPs was obtained by TEM imaging technique. It was observed that in the absence of Hg2+, the Cyst-Cu NPs formed were spherical and well dispersed with an average particle size of 34 ±2.1 nm ranging from 10 to 108 nm sizes as depicted in Figure 4.1.5(a, b). These morphological features are in good agreement with the LSPR band of Cyst-Cu NPs observed in Figure 4.1.4(a). The present scenario clearly reflects that the interaction between the bio molecules (L-cysteine) and surface of Cu NPs was sufficient enough for the formation of stable, spherical nanoparticles. In contrast to this TEM images in Figure 4.1.5(c, d) obtained at same scale size after the interaction of Cyst-Cu NPs with Hg2+ ions (3.5 µM and 4.0 µM) illustrates the formation of very large aggregates > 100 nm, with high degree of twining and fusion. Generally, face-centered cubic (FCC) structured metallic nanocrystals have a tendency to nucleate and grow into twinned particles with their surfaces surrounded by lowest energy facets (Thi My Dung, Thi Tuyet Thu, et al., 2011). Thus, such growth accounts for the decrease intensity and change in LSPR shape of Cyst-Cu NPs.

Based on these images, a mechanism for aggregation of particles is presented in Figure 4.1.6 We believe that the change in LSPR shape and size is a consequence of detachment of L-cysteine molecules from the surface of Cyst-Cu NPs, allowing well dispersed small nanoparticles to grow into larger aggregates due to fusion of un-capped particles, which are incapable of producing surface Plasmon’s according to the Mie theory (Vasimalai et al., 2012). In the presence of oxygen dissolved in the aqueous solution, the Cu NPs concentration and the intensity of the LSPR band gradually decreased as a result of oxidation(Vasimalai et al., 2012; S.-H. Wu & Chen, 2004). The hypothesis is further supported by the increased peak intensities indexed to Cu2O in the XRD patterns obtained at different concentration of mercuric ion (Figure 4.1.7. These results indicate gradual oxidation of Cyst-Cu NPs with increasing concentration of Hg2+ ions. Thus, excess of Hg2+ ion would provide very little or no opportunity to L-cysteine to bind with the metallic surface of Cu, owing to its high thiophilicity compared with other metal cations leading to complete conversion of metallic copper to copper oxide in aqueous solution which was confirmed from XRD pattern shown in Figure 4.1.7(b).In addition decrease in zeta potential measurement (Figure 4.1.8 (a, b) of well-dispersed Cyst-Cu NPs from about -23 mV to -13 mV after Hg2+-induced aggregation further strengthens our hypothesis of Hg2+-induced loss of ligands from the surface of Cu NPs.

More experiment showed that the decline in the intensity of LSPR band (or rA) is proportional to the Hg2+ concentration. So the present work represent novel, simple, selective, economic and fast colorimetric method for determination of Hg2+ in aqueous solution using L-cysteine functionalized Cu NPs.

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Figure 4.1.5 TEM micrographs of Cyst-Cu NPs (a) low resolution (b) high resolution (c) Cyst-Cu NPs+3.5 µM Hg2+ (d) Cyst-Cu NPs+4.0 µM Hg2+

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Figure 4.1.6Hg2+ induced aggregation of Cyst-Cu NPs

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Figure 4.1.7 XRD patterns of Cyst-Cu NPs recorded with different concentration of mercuric ion (a) 1.0 µM (b) 2.0 µM (c) 3.0 µM of Hg+2 ions

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Figure 4.1.8 Zeta potential measurement for (a) Cyst-Cu NPs (b) Hg2+ add Cyst-Cu NPs

4.1.6 Analytical Performance of colorimetric assay

Quantitative response based on colorimetric assay was obtained by monitoring the changes in the ΔA (LSPR) band of Cyst-Cu NPs in the spectral range of 400-800 nm (Figure 4.1.9(a)) upon addition of Hg2+ ions. The useful analytical data was obtained by Linear regression analysis of ΔA (LSPR) at 565 nm plotted against Hg2+ concentrations. The method showed good linearity in the calibration range 0.5×10−6 to 3.5 ×10−6 mol L-1 of Hg2+ ions (Figure 4.1.9(b)). The correlation of determination (R2) was 0.9882, with limit of detection and quantification calculated to be 4.30×10−8 and 0.1 ×10−6 mol L-1 respectively. The LOD and LOQ were determined from three times the standard deviation of the blank signal (3×∂/slope), and ten times the standard deviation of blank signal (10×∂/slope) respectively.

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Figure 4.1. 9(a) Decline in the LSPR abs with increasing concentration of Hg2+ (b) Linear regression plot between ΔA (LSPR) and Hg2+ concentrations

Table 4.1.1 presents the comparative data of limit of detection by various types of Ag and Au based sensor regarding the detection of Hg2+ ions. The table shows that the newly developed Cyst-Cu NPs based colorimetric sensor for Hg2+ ions in our case produced better results than several of the reported sensors with the best advantage of high economy.

Table 4.1.1Comparison of L-cysteine functionalized Cu NPs as a colorimetric sensor for the detection of Hg2+ with previously reported methods.

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4.1.7 Selectivity of sensor

To test the selectivity of Cyst-Cu NPs for other metal ions, we investigated the colorimetric response in the presence of various metal ions including Pb2+, Ca2+, Co2+, Zn2+Mg2+, Ni2+, Ag+, and Cd2+ ions at the concentration of 10 µM. These metals were added into 3 mL of Cyst-Cu NPs solution under the same conditions as true for Hg2+ ions. The selectivity of sensor can be visualized with naked eye as represented by the inset picture in Figure 4.1.10(a); upon interaction of freshly prepared Cyst-Cu NPs with various metal ions, the only solution containing Hg2+ ions shows changes from red to pale yellow, while the effect of the other metal ions on the color and LSPR band of Cyst-Cu NPs solution is negligible, indicating that the developed sensor has very high selectivity toward Hg2+ ions. Figure 4.1.10 (b) represents the bar diagram for selectivity of the sensor. Slight variation in the LSPR of Cyst-Cu NPs upon addition of metal ions could be ascribed as a result of slight change in local refractive index at the interface between Cyst-Cu NPs and solution. However, increased intensities and narrowness in LSPR of Cyst-Cu NPs for some metal cations can be described as consequence of non-uniform absorption and variable sizes of these ions, leading to a layer surrounding the copper nanoparticles and changing dielectric medium near nanoparticles (Yunus, 2011)

The excellent selectivity of this model can be ascribed as a consequence of higher stability constants (Log Kf) of Hg (cysteine) ca. 43.5 compared to other metal cations like Pb2+, Co2+, Mg2+, Ni2+, Cd2+, Ag+ and Zn2+ ions are 12, 16, 4, 19, 7.83, 12.7, 18 respectively (Patel et al., 2002).

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Figure 4.1.10 (a) Visible spectral profile demonstrating selectivity of cyst-Cu NPs for Hg2+ (b) Bar diagram exhibiting magnitude of change ΔA (LSPR) for various cations

4.1.8 Application of developed sensor to real water samples

Samples of water from various localities of river Sindh near Kotri barrage were subjected for Hg2+ ions determination using the developed sensor based on LSPR of Cyst-Cu NPs. The samples were treated after proper dilution in order to bring the final Hg2+ ions concentration in the linear range of the sensor. The linear relation in Figure 4.1.9(b) was used for the determination of mercury ion content in river water samples. According to final results, the Hg2+ ions in the river sample was found to be in range between 5.9 µM to 6.3 µM as presented in table 4.1.2. The result shows very high mercury content, above the EPA criterion of 0.002 mg/L (equivalent to 0.009 µM or 2ppb) set to protect public health (Driscoll et al., 2007).

Table 4.1.2 Determination of mercuric ions in real water samples collected from Sindh River near Kotri Barrage, Jamshoro, Pakistan.

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Conditions: 3mL Cyst-Cu NPs solution, 10 µl real sample (2 times diluted), no. of replications =3 and reaction time: 2–3 min

PART II

Annotations : The entire study presented in part II is published online with the following reference:

Soomro, R. A., Sherazi, T. H., Memon, N., Shah, MR., Kalwar, N.H., Hallam, K. R., & Shah, A. (2014). Synthesis of air stable copper nanoparticles and their use in catalysis. Advanced Materials Letters, 5(4), 191-198.

4.2 Surfactant protected copper nanoparticles for the catalytic reduction of dyes

This work describes the catalytic application of SDS capped copper nanoparticles for the reduction of Eosin B (EB) dye.

4.2.1 Optical characterization

The collective oscillations of conduction electrons at the surface of nanosized metal particles absorb visible electromagnetic waves, the phenomenon is known as SPR (Dang et al., 2011 ). This effect can be used to estimate the formation of nanoscale metal particles in the solution medium through simple UV-Vis spectrophotometry. It is worth mentioning that SPR of metal nanoparticles is greatly a size-dependent phenomenon. The electron scattering enhancement at the surfaces of nanoparticles increase bandwidth and decrease the particle size. Hence, variations in bandwidth and shifts in resonance are very important parameters in characterizing the nanosized regime metal particles (Dang et al., 2011 ). During the synthesis of SDS-stabilized Cu NPs in aqueous solution, the UV-Visible spectra of selected samples were recorded at different time intervals, immediately after the addition of NaBH4 and then after 10 and 15 minutes, as depicted in Figure 4.2.1 (a). A yellow/orange color appeared immediately after the addition of NaBH4 with the absence of SPR, shown by dashed line in Figure 4.2.1 (a).

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Figure 4.2.1 SPR variation of Cu NPs, (a) at different time intervals during the reaction and corresponding color change in colloidal sol, shown in inset photograph (b) at different concentration of reducing agent and corresponding color of Cu NPs (inset photograph) (c) with surfactant concentration (d) with precursor salt concentration and (e) with variation in pH of colloidal Cu NPs.

However, a broad SPR band was generated by the same sample within 10 min of reaction time with maximum absorption around 589 nm, resulting in a purple color of the solution. A brick red color appeared after 15 min together with a highly sharp SPR peak having maximum absorption at 569 nm. These experimental investigations were found to be in very good agreement with the results already presented in the literature (Guajardo-Pacheco et al., 2010a). Moreover, UV-Vis spectral analysis was carried out for a number of samples prepared in the aqueous medium to optimize parameters such as concentration of precursor salt (CuCl2•5H2O), concentration of reducing agent (NaBH4), effect of pH and reaction time, as depicted in Figure 4.2.1(b-e).It is obvious that the copper particles with diameters below 4 nm exhibit strong broadening of the Plasmon band (hossain et al., 2012). It is also evident that the small increase in absorbance (550-600 nm) turn yellow/orange color to red, which indicates the formation of nanoclustor of Zero valent Cu under the influence of reduction reaction (Guajardo-Pacheco et al., 2010a). The reaction was allowed to continue in air. The solution was kept under an ambient atmosphere for several days to investigate possible oxidation of the formed Cu NPs. Several color tinges appeared with the passage of time and finally the solution was turned into dark green color on the 21st day, indicating the formation of cuprous oxide which was then confirmed by XRD analysis.

4.2.2 Effect of reducing agent on SPR peak position

The formation of Cu NPs was confirmed by the generation SPR with a maximum absorbance at 586 nm. An excess of NaBH4 was required to produce size homogeneity and well-dispersed Cu NPs in colloidal form. Shifts in the SPR band, along with color change, were observed by varying the amount of reducing agent (0.01 M NaBH4) in the volume range of 50-350 µL; the resulting UV-Vis spectra are shown in Figure 4.2.1(b). A blue shift with gradual increase in the absorbance from 597 nm to 569 nm was seen due to the electrostatic interactions between Zero valent Cu NPs (De & Mandal, 2013b). There was no clear SPR signal from the yellow colored Cu solution. But a broad and clear peak at 568 nm was observed when the color changed from yellow to orange and also when the reaction mixture gives a brown color (digital photographs inset in Figure 4.2.1(b). This probably confirmed the onset of particle formation. Moreover, with increasing the concentration of reducing agent higher nucleation rates resulted and consequently generated greater number of nanoparticles. Hence, the SPR peak narrowed with increase in absorbance; the phenomenon was also supported by previous studies (De & Mandal, 2013b). It was observed that the color changed from light yellow to orange and finally turned brick red, where maximum absorbance was noticed at 569 nm.

4.2.3 Effect of surfactant on SPR peak position

The UV-Vis spectral profile generated for SDS-capped Cu NPs revealed that gradual increase in the concentration of surfactant occurred by varying the volume of 1.0 M SDS from 0.1-0.6 mL. This increase resulted in a blue shift in the SPR position from 576-569 nm and confirms the generation of smaller nano-particles as shown in Figure 4.2.1(c). Further increase in the amount of SDS generated a red shift from 569-571 nm indicating the growth of Cu NPs. The red shift in SPR led to gradual increase in the nano-particles size and strong inter-micelle interactions. The higher concentration of surfactant molecules also resulted into potential agglomeration of many small particles of the nanosized regime copper and formed larger spherical particles.

4.2.4 Effect of precursor salt on SPR peak position

UV-Vis spectral lines were recorded for the shift in SPR peak position with variation in the amount (0.05-0.8 mL) of precursor salt, as shown in Figure 4.2.1(d). A blue shift in the wavelength from 577 nm to 569 nm was observed with increase in the amount of precursor salt (0.05-0.3 mL of 0.03 M CuCl25H2O). This shift can be explained on the basis of increased nucleation rate due to greater amount of Cu(II) ions and generation of smaller nanoparticles in the solution. However, with further increase in the amount of precursor ions, from 0.4 to 0.8 mL, a red shift in the SPR was observed from 569 nm to 658 nm. This may be due to collision between small nanoparticles, which lead to particle growth (Dang et al., 2011 ). The inset digital photo graph in Figure 4.2.1(d) clearly shows the change in color with the size of Cu NPs. A brick red color was noted for the optimal amount of precursor salt producing greatest number of Cu NPs in aqueous medium.

4.2.5 Effect of pH

Solution pH greatly affects the SPR of Cu NPs. The influence of pH on the progress of reduction reaction has also been reported by various scientists (Huanga et al., 2010). The pH was studied in the range from 2 to 7 with the drop wise addition of 0.1 M NaOH and HCl solutions. Strongly basic medium was avoided to resist the formation of copper oxides (Hossain et al., 2012). The UV-Vis spectral profiles of selected colloidal Cu NPs solutions, prepared under the same experimental conditions except for the pH, are presented in Figure 4.2.1(e). The surface Plasmon band for each solution can be seen, except at pH 2 and pH 3. This likely indicated generation of very small particles at such low pH. Plasmon resonance was clearly visible for pH from 4 to7 with measured values of 569 nm, 564 nm, 566 nm and 567 nm respectively. The maximum blue shift in SPR peak position at pH 5 could be attributed to the decrease in particle size compared to nanoparticles prepared at other pH values. The exact position of the SPR band depends on a variety of factors, including size and shape of the particles, nature of the solvent and capping agent (Guajardo-Pacheco et al., 2010a). In this case, variations in the pH of solution have resulted in different arrangements of the capping molecules around the Cu NPs, which affected the shift in SPR.

4.2.6 Mechanism of formation

The mechanism for formation of spherical Cu NPs using SDS as the stabilizing and capping agent in aqueous solution is illustrated in Figure 4.2.2. Above the critical micelle concentration (CMC), the aqueous system was densely populated with SDS surfactant micelles. Initially, a large population of copper precursor ions (Cu2+) gathers at the micelle head groups due to electrostatic attraction between the negative surfactant head group and the positively charged copper ions, Figure 4.2.2 (a).

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Figure 4.2.2 Proposed mechanism for formation of spherical Cu NPs in aqueous SDS medium.

With the addition of reducing agent (NaBH4), reduction occurs through electron transfer from borohydride anions to copper ions followed by nucleation of copper atoms within the micelle network. These nuclei tend to connect via magnetic dipole interactions resulting in the formation of spherical nanoparticles. The strong micellular effect resists the directional growth of nanoparticles to any other morphology, as explored in Figure 4.2.2 (b-d). Conversely, the appearance of aggregated nanoparticles in some regions of TEM images may be due to attractive inter-micellular interactions. Such interactions promote micelle growth, and potential agglomeration of micelles to larger spherical particles, as presented in Figure 4.2.2 (e).

4.2.7 Fourier transform infrared spectroscopy

The surface binding interaction study of SDS with Cu NPs was carried out by recording FTIR spectra in the range of 4000-400 cm−1, as shown in Figure 4.2.3.

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Figure 4.2.3FTIR spectra, (a) standard SDS surfactant and (b) SDS capped Cu NPs.

The characteristic bands of pure SDS can be divided into two regions, two absorption bands in the range of 2950-2850 cm–1, attributed to the aliphatic group (tail group) and another band at 1226 cm–1, attributed to sulfonic acid (head group) of SDS molecules (Taffarel et al., 2010). Moreover, the 1226 cm–1 band in pure SDS was blue-shifted to 1234 cm–1 in capped Cu NPs. It showed that the capping was due to negatively charged head group moieties. This also reinforces the above mechanism (Figure 4.2.2) for formation of Cu NPs. The absence of others prominent bands around 623 cm-1, 588 cm-1, 534 cm-1 and 480 cm-1, excluded the possibilities of copper oxides existence, such as Cu2O and CuO impurities (M. Kooti et al., 2010; Tian et al., 2012).

4.2.8 Atomic force microscopy

AFM is an important technique for studying the morphology of nanoparticles. Tapping mode AFM imaging was applied to study the SDS-capped Cu NPs. Figure 4.2.4(a) shows a typical medium-scale AFM image (0.9 μm × 0.9 μm) of the SDS-capped Cu NPs, whereas, Figure 4.2.4 (b) presents a topographical view of the sample.

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Figure 4.2.4 A typical AFM image of Cu NPs, (a) showing well dispersed and size heterogeneous NPs and (b) 3D topographical map of Cu NPs showing dents and surface irregularities.

The topographic map revealed that the nanoparticles were rich in dents and irregularities on their surfaces. The rough surfaces provide a greater number of active sites and comparatively possess greater surface area than smooth ones; such particles can play a better role in the field of catalysis. Results obtained with AFM identified that newly synthesized Cu NPs possessed an average size of 14 nm. The results acquired for size elucidation using AFM and TEM image analysis were in good agreement.

4.2.9 Transmission electron microscopy

High resolution TEM was used to examine the size and shape of synthesized products. The SDS micelle network was applied as capping agent to get the controlled formation of spherical Cu NPs. This hypothesis was confirmed from TEM image analysis of a selected sample of Cu NPs, as shown in Figure 4.2.5. This image confirmed that synthesized Cu NPs were spherical in shape and grown with well-defined morphology.

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Figure 4.2.5 TEM image of SDS capped Cu NPs showing well distributed NPs with spherical shapes.

The diameters of these nanoparticles were found to lie in the range between 8 nm to 25 nm, with a mean of 14 nm. In some regions, agglomeration of small nanoparticles was observed which might be caused by interactions of surfactant chains of SDS molecules with each other. The size of Cu NPs was confirmed with TEM and AFM analyses.

4.2.10 X-ray diffraction

The phase composition of crystal structures of the synthesized products, collected from brick red and greenish suspensions of Cu NPs, were analyzed by XRD, as shown in Figure 4.2.6.

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Figure 4.2.6 XRD patterns of the powder samples (a) collected from brick red solution of Cu NPs and (b) collected from greenish suspension of Cu2O.

The diffraction data presented in Figure 4.2.6 (a) inferred the formation of pure crystalline metallic phase Cu NPs with face centered cubic (FCC) structures having characteristic peaks indexed to (111), (200) and (220) at corresponding 2 theta value of 43.8º, 51.0º and 75.6º respectively.On the other hand, XRD peaks shown in Figure 4.2.6(b) for greenish suspension obtained after 21 days of the synthesis were indexed to cuprous oxide (Cu2O) with characteristic diffraction indices (111), (200), (220) and (311) at 2 theta value of 34.6º, 41.2º, 62.3º and 73.2º respectively. These results were found in a close relation with the reported work (Tian et al., 2012).

4.2.11 Catalytic activity

The catalytic activity of SDS-capped Cu NPs was monitored by using EB dye as a test compound. The progression of the catalytic degradation of EB dye can easily be examined by decrease in optical density at 515 nm, as shown in Figure 4.2.7 (a). UV-Vis spectra of EB (100 μM) and NaBH4 (10 mM) mixtures in the absence of Cu NPs showed only a small increase of reductive degradation (up to 14.1%) with time, as shown in Figure 4.2.7 (a). The application of others metals nano-particles for the degradation of dyes have also been reported in literature (hossain et al., 2012; ZJ, 2005). However, the reductive degradation observed after addition of Cu NPs catalyst in the same sample solutions. The process of degradation of EB dye was completed within 20 sec, as shown in Figure 4.2.7 (b). It was also observed that the reaction rate of EB degradation with Cu NPs was enhanced 30 times with 100 % degradation efficiency when compared with the results of the control test.

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Figure 4.2.7 UV-Vis spectra, (a) 100 μM of EB and 500 μL 0.01 M NaBH4 in absence of nanocatalysts and (b) reduction of 100 µM of EB mixed with 500 μL 10 mM NaBH4 by use of 0.1 mg powder of nanocatalyst (Cu NPs).

The undertaken study explores the extraordinary catalytic behavior of Cu NPs for the degradation of EB dye.The high catalytic activity of Cu NPs may be due to the spherical structure, greater surface active sites from all three dimensions, and rough surface morphology as evident by AFM results. Moreover, well dispersed nature of Cu NPs have the merit of less aggregation providing high surface area per unit volume during the catalytic process.

4.2.12 Kinetic study

UV-Vis spectral results (Figure 4.2.8) were used to study the kinetic mechanism of the catalytic reaction. The experiment was carried out by adding 0.1 mg of Cu NPs into 100 µM of EB and 10 mM of NaBH4 in an aqueous medium. The degradation rate of EB was monitored at a maximum wavelength of 515 nm, in the visible region and was found to follow first order kinetics as given by the following equation:

The first order rate constant (k) for degradation of EB was found to be 0.240s-1 as shown in Figure 4.2.8 (b).

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Figure 4.2.8 The amounts remaining of EB, (a) illustrating an exponential decay of the dye and (b) the first order kinetics followed by the reduction/degradation reaction.

4.2.13 Recovery and reuse of Cu NPs

The newly devised method for synthesis and use of SDS-capped Cu NPs for catalytic reductive degradation of dye, demonstrates simple and effortless recyclability of the nanocatalyst. A bar graph demonstrating the high catalytic efficiency of Cu NPs with triplicate use is shown in Figure 4.2.9.

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Figure 4.2.9 % Efficiency of SDS capped Cu NPs three times to carry out reductive degradation of EB dye with negligible loss of catalytic efficiency.

Cu NPs adhered to glass cover slips were used for catalysis and were easily recovered and reused after washing with sufficient quantity of de-ionized water and drying under inert atmosphere. Negligible loss of activity was observed during each recycling.

PART III

Annotations : The entire study presented in part 3 is published with the following reference:

R .A. Soomro, Nafady, A., Sirajuddin, Sherazi, S. T. H., Kalwar, N. H., Shah, M. R., et al. (2015). Catalytic Reductive Degradation of Methyl Orange Using Air Resilient Copper Nanostructures. Journal of Nanomaterials, 2015, 12.

4.3 Competitive catalytic reduction of methylene blue (MB) using copper nanoparticles and nanorods: Influence of morphology and structural features on catalytic potential Cu nanostructures

The study explores the two distinct morphologies of copper nanomaterials i.e. nanoparticles and nanorods for the competitive catalytic reduction of MB dye.

4.3.1 Optical Characterization

Recent advances have allowed metals to be structured on the nanoscale by engineering the surface plasmon modes of metallic nanostructures (Chen et al., 2011). Surface Plasmons are collective excitations of the electrons at the interface of a metal surface, resulting in absorption of electromagnetic waves (Ghodselahi et al., 2009). This interesting phenomenon dominates when particle size is broughtwithin the nanoscale regime. In recent years, several studies have shown that optical properties of metal nanoparticles depend on the geometry and size; thus, the optical response of metal nanoparticles (NPs) can be tuned to control shape and size of metal nanostructure (Ghodselahi et al., 2009; Noguez, 2007). Since surface plasmon modes of metallic nanoparticles, like Au, Ag and Cu reside within the optical region of the electromagnetic spectrum (Chen et al., 2011; Ghosh & Pal, 2007), optical spectroscopy can be used as a primary tool for the investigation of such nanoparticles. UV-Vis spectral profiles for Cu NPs were recorded with time. The SPR band observed at 569 nm shows the completely reduced (rich red) colloidal sol as depicted in Figure 4.3.1(a). The absorption spectra have been simulated for spherical copper particles by various earlier reports (De et al., 2013a;Dung and Thu, 2011; Vaseem et al., 2011). As our absorption spectrum is in close resemblance, we conclude that copper nanoparticles were formed in this study. Similarly, the SPR band for Cu NRds (dark reddish brown) was recorded at 545 nm (Figure 4.3.1 (b)) with an increase in band width compared to the narrow SPR band of Cu NPs; it is well established that SPR band position and width are highly influenced by particle shape and size (Sharma et al., 2009). Therefore, the PWHM (peak width half-maximum) was calculated for both as-synthesized copper nanoparticles and nanorods. The PWHM value obtained for Cu NPs (70) of 100 nm indicates uniform distribution of particle size. However, for Cu NRds (118) the value is above 100 nm, which demonstrates large variance in structural size resulting in broad SPR band widths. In this case, increased band width may be the result of considerable interactions between nanoparticles from higher order multipoles and distribution of depolarization as nanoparticles assemble in the form of rod-shaped structures during nucleation. It is interesting to note that the observed colour of the Cu NRds (reddish brown) can also be described as a range/mixture of colours depending on the variance in size of nanorods.

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Figure 4.3.1 SPR bands of (Black) SDS capped Cu NPs and (Red) CTAB capped Cu NRds.

4.3.2 Fourier transform infrared spectroscopy (FTIR).

FTIR spectroscopy can provide vital information concerning surface interactions. The FTIR spectrum of nanoparticles differs considerably from that of the bulk counterpart (Eastman et al., 1993). In the case of nanoparticles, the surface to volume ratio is very high when compared to bulk form; thus, the number of atoms that constitute the surface can influence the vibration spectra of nanoparticles (Eastman et al., 1993; Yang et al., 1994). In order to understand the basics of surface interaction/capping between copper nanostructures and surfactants, FTIR spectra were recorder in the range of 4000–400 cm-1. Figure 4.3.2 shows FTIR spectra of SDS capped Cu NPs and CTAB capped Cu NRds; the FTIR spectra of pure surfactants are also present for comparison. In the case of SDS capped Cu NPs Figure 4.3.2(c and d), the characteristic vibrational bands of pure SDS can be divided into two regions, concerning its hydrophobic and hydrophilic nature. Twin absorption peaks in the range of 2950–2850 cm-1 are attributed to aliphatic group (tail group) and another band at 1226 cm-1 is attributed to sulfonic acid (head group) of the surfactant (Taffarel & Rubio, 2010). Blue shift in the characteristic absorption band of the sulfonic acid group from1226cm-1 in pure SDS to 1234 cm-1 (Figure 4.3.2(d)) suggests that the copper nanoparticles were capped by the head group moiety. Furthermore, the absence of the characteristic bands around 623, 588, 534, and 480 cm-1 excludes the possibility of Cu2O and CuO impurities, respectively (Tian et al., 2012; Usman et al., 2012). In the case of CTAB capped Cu NRds Figure 4.2.3(a and b), we understand that intensive vibration bands of CTAB can also be categorized into two different regions, such as the bands associated with methylene tails of surfactant molecules and bands which are associated with alkyl ammonium head groups (Sui et al., 2006).

A characteristic peak around 3018 cm-1 as shown in Figure 4.3.2(a) can be assigned to the symmetric stretching mode of the trimethylammonium head group (CH3)3N+ of the surfactant molecules and the most intensive peaks around 2917 and 2847 cm-1 are associated with asymmetric and symmetric stretching vibration modes of the methylene group. A slight shift in the frequency of band associated the head group from 3018 cm-1 to 3025 cm-1 and (Figure 4.3.2(b)) suggests that the growth of Cu NRds is restricted via interaction of the head group [(CH3)3N+] with the surface of copper. However, the difference in critical micelle concentration (CMC) of the two surfactants and the nature of interaction allowed the directional growth of particles towards self-assembled nanorods in aqueous CTAB medium.

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Figure 4.3.2 FTIR spectra of (a) standard CTAB, (b) CTAB capped Cu NPs, (c) standard SDS, and (d) SDS capped Cu NRds.

4.3.3 Atomic force microscopy (AFM)

Surface morphology plays an important role in the field of catalysis. Atomic force microscopy (AFM) is a powerful technique that can provide direct spatial mapping of surface morphology with nanometer resolution. It requires no specific sample preparation procedures and is easy to interpret and allows for the study of morphological characteristics of samples ina non-distractive way. The tapping modes of AFM imagining were developed especially for studying both SDS capped Cu NPs and CTAB capped Cu NRds. Figure 4.3.3 (a) shows a typical medium scale AFM image (0.9 𝜇m × 0.9 𝜇m) of the SDS capped Cu NPs with spherical and uniform shape, whereas a topographical map of nanoparticles is presented in Figure 4.3.3 (b) where rough surface morphology with dents and irregularities are indicated by highlighted regions. Such rough surfaces have greater number of active sites, which provide greater number of contact points for catalysis (Chaudhari et al., 2005; Lee et al., 1997). SEM analysis was carried out to get further insight and determine the exact particle size of Cu NPs. Figure 4.3.4 at shows the SEM image with high distribution of as-synthesized Cu NPs. It can be seen that most nanoparticles are highly dispersed with spherical shape morphology. The average particle diameter calculated from SEM analysis was about 35 ± 2.8 nm in the scale range of 15–40 nm. In a similar pattern, Cu NRds were also characterized for morphology. Figure 4.3.3 (c) shows a medium scale AFM image of CTAB capped Cu NRds where high surface roughness is evident. The SEM image of as-synthesized Cu NRds is presented in Figure 4.3.4 (b). It can be seen that the formed nanorods are very well dispersed with negligible aggregation. We assume that considerable interaction between small nanodots during nucleation mediated by the surfactant resulted in a self-assembly of particles towards rod-shaped structures. This issue is further discussed later in Section (4.3.5). The average width of nanorods estimated from SEM analysis was determined to be 65 ± 3.8 nm with an average aspect ratio of 9.5. Such a high aspect ratio along with irregular surface topography as depicted in Figure 4.3.3 (c) allows nanorods to create a network of surfaces in the reaction medium leading to increased physical contacts between catalyst and reactants molecules.

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Figure 4.3.3 AFM images of Cu nanostructures: (a) typical medium scale AFM image (0.9 × 0.9 𝜇m), (b) topographical map of the SDS capped Cu NPs, (c) typical medium scale AFM image (0.9 × 0.9 𝜇m), and (d) topographical map of the CTAB capped Cu NRds.

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Figure 4.3.4 SEM images for (a) SDS capped Cu NPs and CTAB capped Cu NRds

4.3.4 X-ray powder diffraction (XRD)

The X-ray diffractograms of surfactant capped Cu NPs and NRds are shown in Figures 4.3.5(a) and 4(b), respectively. The characteristic Miller indices (1 1 1), (2 2 0), and (2 2 0) lattice planes were observed for both Cu NPs and Cu NRds. The data refer to pure copper metal with face centered cubic structure (FCC). No characteristic peaks indexed to copper oxide were observed, indicating the phase purity of copper metal. The results obtained are in strong correlation with previous reports (M Kooti et al., 2010). However, differences in intensity and broadness in corresponding peaks were evident. The measured intensity ratio of diffraction peaks indexed as (1 1 1) and (2 0 0) between Cu NPs and Cu NRds was 1.96 and 1.72 respectively. This increased ratio for (1 1 1) planes refers to the exposed facets along the crystal surface of copper nanoparticles and relatively strong diffraction intensity compared to Cu NRds. This may be a consequence of an isotropic growth of particular planes during the nucleation step, which has been manifested from their particle and rod-like structural shapes. In addition, increased broadness in XRD peak widths of Cu NPs relative to Cu NRds suggests smaller grain size, respectively.

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Figure 4.3.5XRD patterns of (a) Cu NPs and (b) Cu NRds.

4.3.5 Growth mechanism for copper nanostructures

Experimental studies were carried out for [Cu] and [SDS] at 1:1 ratio which resulted in stable blood red colored copper nanoparticles with an SPR band at 569 nm. The mechanism of formation for SDS capped Cu NPs is explained in Figure 4.3.6. At concentration of 1 mM which is approximately eight times higher than CMC (8 × 10-3 M) of SDS surfactant aqueous medium is rich in SDS micelles; thus, a large population of copper ions are gathered at the negatively charged head group of the surfactant as a result of electrostatic attraction between oppositely charged copper ion and surfactant head group as shown in Figure 4.3.6(a). As the electron transfer starts with the abrupt addition of reducing agent (NaBH4), explosive nucleation occurs, consuming most of the precursor ions and aggregation of small metal nuclei at the very instant as a consequence of strong interaction between their magnetic dipoles. However, due to the presence of a dense micelle network and strong interaction between oppositely charged groups, most of the nucleation occurs within the SDS micelles which restrict the growth of particles by adsorbing onto the surface. This adsorption of surfactant around nanoparticles results in an overall decrease in grain boundary energy, which is highly related to surface energy. Thus, decreasing the grain boundary energy would result in a decrease in driving force for particle growth (Figure 4.3.6(b–d). AFM and SEM studies indicate the formation of spherical copper nanoparticles at [Cu2+]:[SDS] having 1:1 ratio as shown in Figure 4.3.3 (a).The results are in contrast to those obtained with CTAB at a similar ratio [Cu2+]:[CTAB]=1:1, where copper nanorods are obtained. The formation of copper nanorods can be explained as presented in Figure 4.3.7. We know that CTAB is a cationic surfactant; thus, at concentrations above CMC, the precursor ions are mostly located in the micelles head group due to the presence of the counter ion Br- (Figure 4.3.7(a) not within the micelle network as proposed in Figure 4.3.7.With the introduction of reducing agent (NaBH4), subsequent unidirectional nucleation occurs as one side of the particles is no longer free due to the presence of micelle (Figure 4.3.7(b and c). This restriction results in unidirectional growth of particles along the specific facets that are exposed to water and ultimately leading to the formation of rod-like structures from the self-assembly of small nanoparticles (Figure 4.3.7(d)). The growth mechanism is in correspondence with a recently published report on copper nanorods (Ghosh & Pal, 2007). Based on an aforementioned mechanism, we conclude that copper nanoparticles and nanorods are both formed by a similar process, that is, surfactant directed growth. However, differences in directional growth of nanostructures arise because of difference in nature, charge, and micelle size. In addition, we also argue that the formation of copper nanostructures is through a template free route with surfactants used as stabilizers and growth directors. It is quite unlikely that rod like CTAB micelles and SDS associated rod-like structures may form in the absence of additives like sodium salicylate and aluminium nitrate as it is evident from the previously published reports (Hu et al., 1999; Watzky et al., 1997).

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Figure 4.3.6 Formation of Cu NPs in SDS rich aqueous medium

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Figure 4.3.7 Formation of Cu NRds in CTAB rich aqueous medium

4.3.6 Catalytic evaluation of copper nanostructures for degradation

The catalytic performance for both SDS capped Cu NPs and CTAB capped NRds was monitored, taking MO dye as a model compound for organic azo dyes. The progression of the reductive degradation of MO can be easily studied by following the decline in time-dependent absorbance at 550 nm as shown in Figure 4.3.8. The un-catalysed reaction (Figure 4.3.8(a)) was carried out to assess the capability of reductant NaBH4 (10mM) alone with MO(100 𝜇M),which showed only a small percentage of degradation (up to 8.5%) with time. In contrast, catalysed reaction carried out with surfactant capped copper nanostructures, that is, Cu NPs and CuNRds, in a similar sample solution suggested the complete reductive degradation of MO dye (100%) within 60 and 180 s of reaction time, respectively (Figure 4.3.8(b) and Figure 4.3.8(c)). The reaction rate for MO degradation with copper NPs and Cu NRds was enhanced 11.2 and 7.5 times, respectively, compared with the results of the control experiment. The rate of reaction for the heterogeneous catalysis is best described by the Langmuir–Hinshelwood (L–H) model (Houas et al., 2001b), which has the following mathematical formula (Z. Sun et al., 2002a):

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Where kL-H is the reaction rate constant, kad is the adsorption coefficient of dye on catalyst, and C is the variable concentration at any time t. Since for pseudo-first order reaction the value of k ad C is very small as compared to 1 in the denominator of above equation. So integrating the above mentioned equation for simplification, we obtain

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Here C0 is the initial concentration and [illustration not visible in this excerpt] is the pseudo-first-order reaction rate constant.

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Figure 4.3.8 UV-Vis spectral profiles for (a) un-catalysed reduction of 100 𝜇M (MO) with 00𝜇L 0.01M (NaBH4) and (b) and (c) catalysed reductive degradation of MO in a similar sample environment with SDS capped Cu NPs and CTAB capped Cu NRds, respectively

4.3.7 Kinetic evaluation of degradation reaction

Figure 4.3.9 shows the plot with linear relationship of natural logarithm of ratio of initial concentration of MO and relative remaining concentration after reductive degradation versus the corresponding reaction time (s). Linear regression analysis was used to evaluate the reaction rate constants for the reductive degradation of MO by surfactant capped copper nanostructures. Rate constant 𝑘 was found to be 0.056 ± 0.001 and 0.036 ± 0.0015 s-1 for the corresponding catalytic reductive degradation of MO by Cu NPs and Cu NRds, respectively. The 𝑅2 values clearly suggest that the removal of MO seems to fit pseudo first-order kinetics. Differences in catalytic performance between SDS capped Cu NPs and CTAB capped Cu NRds can also be explained based on the difference in the nanostructure-support contact area that is dependent on the particles shape and size. It is known that many catalytic processes occur at the perimeter interface around the nanoparticles where the fraction of step sites increases significantly with decreasing particle size (Bratlie et al., 2007). Here, Cu NPs were found to have degraded MO 1.5 times faster than Cu NRds; thus, such enhancement in reaction rate is a function of two major factors: number of surface atomsper nanoparticle and activation energy. Comparatively, the larger numbers of surface atoms of Cu NPs (323025) than of Cu NRds (121457) would provide greater numbers of low coordination sites (sharp corners and edges) over the surface of the nanocatalyst. In contrast, for Cu NRds, it can be understood that particles at connecting interfaces of rods are much less exposed to the surface resulting in decreased numbers of low coordinated sites compared to the independent spherical Cu nanoparticles, which have all sites exposed as surface and available for coordination. Also, the larger size of Cu NRds provides low surface coverage per unit volume in the reaction mixture, whereas for Cu NPs their smaller size and homogenous distribution provides increased numbers of contact sites for the reactant molecules per unit volume within the reaction medium.

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Figure 4.3.9 Linear regression plot showing pseudo first-order kinetics for the Cu nanostructure catalysed reductive degradation of MO with SDS capped Cu NPs and CTAB capped NRds.

4.3.8 Energetic evaluation of degradation reaction

The shape effect of copper nanostructures on the activation energy of reductive degradation of MO with NaBH4 was evaluated via catalytic experiment conducted as a function of three different temperatures (35°C, 45°C, and 50°C) for both SDS capped Cu NPs and CTAB capped Cu NRds. For each experiment, absorption spectra in the range of 400 to 700 nm were recorded at different time intervals. The effective rate constant values for both Cu NPs and Cu NRds were evaluated as a function of temperature as follows: Cu NPs: 0.14 ±0.02, 0.25 ±0.01, and 0.28 ±0.05 s-1 and Cu NRds: 0.10 ±0.01, 0.21 ±0.02, and 0.27 ±0.01 s-1 at 30°C, 40°C, and 55°C, respectively. The obtained values were used in the following linear form of the Arrhenius equation to estimate apparent activation energy:

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where 𝐸𝑎 is activation energy, 𝑇 is the absolute temperature, and is the universal gas constant. A linear plot of ln𝑘 versus 1/𝑇 was obtained for degradation carried out with SDS capped Cu NPs and CTAB capped NRds and the value of the apparent activation energy was estimated from the linear regression as shown in Figure 4.3.10. The activation energy obtained for the reaction carried out with Cu NPs (21 ±1.0 kJmol-1) is much smaller compared to 𝐸𝑎 value obtained for the reaction carried out with Cu NRds (33 ±1.2 kJ mol-1). The significantly lower apparent activation energies obtained with SDS capped Cu NPs then a CTAB capped NRds may be attributed to the rough surface morphology of spherical copper nanoparticles that offer higher numbers of low coordination sites from all three dimensions belonging to the nanosize regime. Many studies have shown that the ratio of corner and edge atoms increases with the decrease ofcrystal size (Greegor et al., 1980; Ladas, 1986; Van Hardeveld et al., 1969). At the nanoscale, edge and corner atoms exhibit open coordination sites thatmay result in significantly different bond enthalpies and desorption energies compared to macrostructures. In contrast, Cu NRds have large sizesand lower surface coordination sites as indicated from theirsmaller number of surface atoms per nanoparticle. Thus, variation in surface morphology, when shape of particles changes from spherical to rod, is responsible for changes in activation energy of the overall system. Some literature data on decolorization of MO dye by different methods, comparative with that obtained in this paper, are summarized in Table 4.3.1. It is clear that all the parameters tested for the catalytic system used in this paper are more effective than those of the previously reported methods. Small amounts of catalyst (1 mg surfactant capped Cu nanostructures), 100 mM reductant NaBH4, low activation energy (𝐸𝑎 = 21 ±1.0 and 33 ±1.2 kJ mol-1), with advanced reductive degradation achieved in just 80 and 120s for Cu NPs and Cu NRds, respectively, at room temperature and pressure provide a clear edge over reports listed in the literature.

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Figure 4.3.10 Linear regression for Arrhenius equation with estimation of corresponding activation energy for surfactant capped Cu NPs and NRds.

Table 4.3.1 Comparison of results obtained for MO degradation and decolourization by various methods.

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4.3.9 Reductive degradation of real dyeing waste water samples

The universality of surfactant capped copper nanostructures as a heterogeneous catalyst for dye degradation was examined by degrading real waste water dye containing samples. However, the degradation was carried out only with SDS capped Cu NPs because of their higher efficiency compared to CTAB capped Cu NRds. Real waste water samples were collected from drains of three different local textile industries of Hyderabad region. Catalytic degradation was performed with a similar methodology as mentioned above, with optimized weight of CuNPs (0.5 mg) and 0.5mL of 100mM (NaBH4) reductant and 10µL of real sample diluted up to 0.3 mL with deionized water was used for the study of real environmental samples. Figure 4.3.11 shows UV-Vis spectra for the reductive degradation of real samples. Complete degradation was observed in very short reaction time for each sample irrespective of their chemical nature and colour intensities, indicating the high efficiency and comprehensive nature of Cu NPs as a catalyst. Figure 4.3.11 (a, b and c) represents the spectra for an un-catalyzed control test with very small decrease in the absorbance with reaction time. In contrast, catalyzed tests in Figures 4.3.11 (d, e, and f) show excellent reductive degradation (100%) with time. Small differences between the reaction rates can be explained on the basis of structural difference and concentration of various dye molecules present in each real sample.

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Figure 4.3.11 UV-Vis spectra for the reductive degradation of real samples (a, b, c) un-catalyzed reduction of 10 μL real sample diluted up to 03 mL in presence of 100 mM (NaBH4) reductant; (d, e, f) catalyzed reductive degradation of similar samples with 0.5 mg of Cu NPs.

PART IV

Annotations : The entire study presented in part 4 is published online with the following reference:

Kalwar, N.H., ANafady, A., Sirajuddin, Sherazi, S.T.H., Soomro R.A., Hallam K.R., Khaskheli A.R. and Jamali, A.A., "Catalytic Degradation of Imidacloprid Using L-Serine Capped Nickel Nanoparticles." Materials Express 5, no. 2 (2015): 121-128.

4.4 Nickel nanoparticles as an efficient catalyst for reductive degradation of pesticides

This study explores the potential capability of L-serine capped Ni nanoparticles for the catalytic reduction of imidacloprid pesticide in aqueous medium.

4.4.1 Optical characterization of L-serine capped Ni nanoparticles

Optical Spectroscopy was used as a primary tool for fine tuning Ni NPs SPR band. Figure 4.4.1 (a) shows an upsurge in absorbance parallel to slight blue shift in SPR position with increasing volume of Ni ions (05-60 µL of 0.003 M). This may be attributed to increased nucleation rate with greater amount of Ni ions per unit volume of solution which as a consequence allows formation of smaller nanoparticles (De & Mandal, 2013b; Park et al., 2007). However, with volume higher than optimised (60 µL) a red shift in SPR position was recorded which might be as a result of collision between small nanoparticles leading to increase in particle size (Dang et al., 2011 ). Figure 4.4.1 (b) represents the variation in SPR band with volume of capping agent (L-serine) in range from 0.5 to 60 µL. It can be seen that L-serine has markedly influenced both the absorbance and the shape of SPR band. Since the band shape depicts the homogeneity in the particle size, it is clear from the spectral profile that as the volume of L-serine reaches up to 50 µL, SPR bandwidth gradually declines with blue shift in wavelength. Further increase in volume (> 50 µL) resulted in SPR red shift. The observed phenomena can be ascribed to protection of smaller nanoparticles at the starting of reaction with higher concentration of capping agent as observed for gold NPs in previous study(Zhong et al., 2005). The pH of solution is an important factor that governs the stability of amino acid capped nanoparticles. Figure 4.4.1 (c) shows the variation in Ni NPs SPR band with pH recorded in range from 2 to 8. Strong basic medium were avoided to resist the oxide formation(Dang et al., 2011b). Although the exact position and shape of SPR band is a subject of various factors ranging from particle size to nature of capping agents, in this case change in pH might be responsible for protonation and deprotonation of functional moieties in L-serine. As can be seen in spectral profile, change in pH from 2 to 6 has resulted in decreased broadness and considerable blue shift in SPR band wavelength. This change in SPR with pH can be understood in terms of greater tendency of capping for amino acid molecules near isoelectric pH (5.68)(Quesada-Moreno et al., 2013), which allows formation of smaller, and highly stable Ni NPs with a narrow size distribution. The stability of as-synthesized Ni NPs was evaluated by monitoring the change in SPR band of sample during 50 days of storage in ambient air condition. The recorded spectral profile is presented in Figure 4.4.1 (d). As evident from the unchanged SPR position and absorbance, it can be observed that the as-synthesized Ni NPs are highly stable in aqueous solution when L-serine and hydrazine are employed as capping and reducing agent respectively.

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Figure 4.4.1 UV-Vis spectral profiles for optimization of various reaction parameters (a) precursor salt (b) capping agent (c) pH of colloidal Ser-Ni NPs and (d) stability of as-synthesized Ser-Ni NPs.

4.4.2 Morphological characterization of L-serine capped Ni NPs

The morphological assessment, phase composition and purity of prepared material were evaluated using TEM, XRD, XPS and FTIR techniques. Figure 4.4.2 shows typical TEM image of L-serine capped Ni NPs. It can be seen that the obtained nanoparticles are spherical in shape with a uniform distribution. From the obtained histogram presented as an inset of Figure 4.3.3(a), it is clear that size of NPs ranges from 5 to 90 nm with an average size of 10 ±2.5 nm.

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Figure 4.4.2 TEM images of freshly formed L-serine capped Ni NPs with an inset size distribution histogram.

XRD pattern of synthesized Ni NPs is shown in Figure 4.4.3. The pattern consists of sharp peaks indexed to crystal planes (1 1 1), (2 0 0) and (2 2 0) of pure face centred cubic structure of Ni metal. No characteristic peaks indexed to oxide were recorded which confirmed the formation of Ni (0) nanoparticles. The recorded pattern is a complete match with standard Joint Committee on Powder Diffraction Standards (JCPDS) card No. 04-0850 reflecting the phase purity of prepared nanoparticles(Xiaozhong Wu et al., 2012).

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Figure 4.4.3 XRD spectra for Ser-Ni NPs

XRD data was further endorsed via XPS analysis as presented in Figure 4.4.4. The XPS analysis was carried under argon atmosphere prior to their utilization in catalytic reaction. The single peak at 852.2 eV corresponds to Ni 2p3/2 level of Ni (0) atoms(Alonso et al., 2010). The absence of satellites concerning O1s suggests the prepared nanoparticles are purely metallic nickel.

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Figure 4.4.4 XPS spectra for the binding energy of Ser-Ni NPs

Interaction of Ni metal surface with L-serine molecules was studied using FTIR spectroscopy. The FTIR spectrum of standard L-serine and Ser-Ni NPs is presented in Figure 4.4.5. Comparison of the obtained FTIR bands clearly reveals the interaction between Ni metal surface and L-serine is via NH3+ and Coo- groups. At the selected pH of (6.0) most of L-serine molecules exits as an zwitterionic form(Quesada-Moreno et al., 2013; Zafarani-Moattar et al., 2014) and hence slight blue shift in vibration frequencies from 1468 and 1602 to 1477 and 1627 cm-1 respectively was noted. The resultant FTIR bands were in good agreement with several other studies(Quesada-Moreno et al., 2013).

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Figure 4.4.5 FTIR spectrum of (black) pure L-serine (red) and Ser-Ni NPs

Based on the experimental studies a plausible schematic diagram reflecting capping of L-serine molecules is presented in Figure 4.4.6.

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Figure 4.4.6 Schematic representation of formation of L-serine capped Ni NPs

4.4.3 Catalytic evaluation of L-serine capped Ni NPs

The progress in reductive degradation of IM was monitored by perusing the decline in time dependent absorbance at 270 nm (λmax of IM) (Hong et al., 2013). For an un-catalyzed reaction was initially performed to access the capability of reductant (NaBH4) along with IM. The corresponding spectral profile is presented in Figure 4.4.7 (a). It is clear that very small degradation (1.8%) occurred during 25 min. However, catalyzed reaction carried with similar sample solution with (0.1 mg) of Ser-Ni NPs (Figure 4.4.7 (b)) has achieved complete degradation of IM (100%) just within the 70 s of reaction time. In the presence of catalyst the reaction rate is noted to enhance 21.4 times compared to un-catalyzed reaction. The kinetics of heterogeneous catalysis is best pronounced in terms of Langmuir-Hinshelwood (L–H) model (Houas et al., 2001a), with following mathematical expression (Sun et al., 2002b):

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Where kL-H denotes reaction rate constant, kad represents adsorption coefficient of pesticide on catalyst, and C is the variable concentration during any time t. Since the value of k ad C for pseudo first order reaction is negligible compared to 1 in denominator, thus integrating the above equation gives:

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Here C0 denotes initial concentration and [illustration not visible in this excerpt] represents pseudo-first order reaction rate constant.

Figure 4.4.7 (c) shows the scatter plot between the natural logarithm of ratio of initial concentration of IM and the relative remaining concentration after degradation and the corresponding reaction time (s). Linear regression analysis depicted the reaction rate constant (k) determined as 0.10 s-1 and corresponding R2 value of 0.994 confirmed the kinetics of the degradation to be pseudo first-order in nature. Generally, the degradation of IM using Ni NPs in presence of NaBH4 is an adsorption–reduction–desorption phenomenon. At the initial stage, the IM molecules and BH4-1 ions would get adsorbed onto the surface of the Ni NPs where reductive degradation takes place via electron transfer process between the IM and BH4-1. The catalyst (Ni NPs) is believed to serve as an electron transfer relay in this situation which concurrently accelerates the electron transfer rate and decreases the activation energy of reaction. . Such rapid electron transfer sponsored by the large surface area of as-synthesized Ni NPs leads to complete degradation of IM molecules simultaneously with the recovery of the catalyst as the degraded molecules leave the surface of Ni NPs and diffuses into solution.

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Figure 4.4.7 UV-Vis spectra, (a) un-catalyzed reductive degradation of 100 mg L-1 (IM) in presence 0.12 M NaBH4 (b) catalyzed reductive degradation of IM in a similar sample environment with 0.1 mg Ser-Ni NPs and (c) linear regression for pseudo first order kinetics for the reductive degradation of IM

4.4.4 Recyclability of Catalyst

The efficiency of Ni NPs as catalyst for multiple time reusability was also investigated. The used catalyst was easily recovered from the catalytic system and washed heavily with de-ionized water and ethanol to remove external impurities. The cleaned catalyst was then reused 6 times for degradation of IM in a similar method as mentioned in section (4.4.3). The catalytic efficiency of recovered and reused catalyst was calculated and is presented as bar graph in Figure 4.4.8. The apparent negligible loss in efficiency during each run demonstrates the capability of catalyst for reusability. Thus this work promises a cheap, environmentally sound and effective model for pesticide removal for poor resource based situations.

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Figure 4.4.8 Bar graph showing the efficiency of recycled Ser-Ni NPs for the reduction degradation of IM after five consecutive cycles

PART V

Annotations : The entire study presented in part 5 is published online with the following reference:

Soomro, R.A., Ibupoto, Z.H., Sirajuddin, Sherazi, S.T.H., Abro, M.I., Willander, M. "Electrochemical Sensing of Glucose Based on Novel Hedgehog-Like NiO Nanostructures." Sensors and Actuators B: Chemical 209, no. 0 (2015): 966-974.

4.5 2-D NiO nanostructures for the electrochemical determination of glucose

This study describes the synthesis of 2-D NiO nanostructures using amino acid as effective bio-compatible template. The as-synthesised NiO nanostructures were further used as electrode modifying materials and their applicability was evaluated for the determination of glucose.

4.5.1 The characterization of hedgehog-like NiO nanostructures

The L-cysteine directed NiO nanostructures were synthesized by low temperature aqueous chemical growth method. Figure 4.5.1illustrates the recorded XRD diffraction pattern of hedgehog-like NiO nanostructures. It can be seen that the as-synthesized NiO nanostructures exhibit three dominant diffraction peaks at 2q of 43.16, 62.77, and 75.30, which are assigned to (200), (220),and (311) crystal planes of face-centred cubic (FCC) NiO nanostructures as depicted by standard Joint Committee on Powder Diffraction Standards (JCPDS No. 71-1179)(Deki et al., 2003; Y. Wang et al., 2005). The XRD data suggest the prepared sample is purely composed of nanocrystalline NiO material with no any peak belonging to the hydroxide phase of nickel.

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Figure 4.5.1 XRD diffraction pattern of hedgehog-like NiO nanostructures

XRD data was further strengthened by the XPS analysis carried to access the chemical composition of the synthesized product. Figure 4.5.2 shows the wide scan spectrum of NiO nanostructures. The compositional purity can be observed from obtained peaks that belong to purely Ni and O. In addition, inset figures show Ni 2p and O 1s spectral profile, with Ni 2p1/2 and Ni 2p3/2 peaks positioned at 873.0 and 885.9 eV. The major peak of O 1s is positioned at 529.8 eV, attributed to the lattice oxygen of pure NiO(Chigane et al., 1998; Grosvenor et al., 2006).

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Figure 4.5.2 XPS spectra of the NiO hedgehog-like NiO nanostructures: slow scan spectra of (a) full survey scan spectrum (b) Ni 2p and (c) O 1s

SEM was used for the morphological assessment of obtained NiO nanostructures as depicted in Figure 4.5.3. The investigation revealed formation of spherical hedgehog-like NiO nanostructures in morphology with hedgehog surface architecture over NiO nanostructure in a dimension range of 100 to 200 nm. The low and high magnification SEM images are shown in Figure 4.5.3(a, b, c, d) where high density of uniformly shaped hedgehog-like NiO structures can be viewed. The high resolution images clearly show the tapering surface features where the hedgehog distribution over the entire nanostructures surface can be seen. These extremely fascinating surface features allow NiO to possess higher active sites compared to other morphologies as a consequence high electro catalytic response was obtained. Thus, to prove the significant role of L-cysteine in formation of hedgehog morphology, NiO structures were prepared with similar sample methodology as presented in section (3.5.2), but in the absence of L-cysteine. The prepared NiO material was then further employed for modification of GCE in a way similar as mentioned in section (3.5.4) and used for the enzyme free sensing of glucose. The corresponding CV measurement for both the modified electrodes is presented in Figure 4.5.4. It is noteworthy that the GCE modified with NiO structures grown in the absence of L-cysteine clearly show poor electrochemical properties compared to NiO HHs-Nafion/GCE. However, NiO HHs-Nafion/GCE showed excellent and prominent both the oxidation and reduction peaks and this explores the superior response of NiO nanostructures prepared with growth directing agent L-cysteine. Thus, it is safe to say that the unique morphological features of L-cysteine directed NiO nanostructures are responsible for the enhanced electrochemical response and henceforth, L-cysteine plays a crucial role in producing hedgehog-like NiO nanostructures.

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Figure 4.5.3 SEM image of the hedgehog-like NiO nanostructures (a, b) low resolution pictures showing high distribution view (c, d) high resolution pictures with tapering features of individual nanostructure

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Figure 4.5.4 Cyclic voltammograms recorded for 1.0 mM glucose solution (Black) NiO HSs-Nafion/GCE (Blue) GCE modified with NiO structures synthesized in the absence of L-cysteine

In order to get the better understanding of formation mechanism of such nanostructures FTIR spectrum was recorded before and after the calcination of the synthesized product critical for the formation respectively as shown in Figure 4.5.5. Major bands in FTIR spectrum of L-cysteine contained precursor as observed at 3486, 3240, 1577, 1383, 1128, 560 cm−1(Dong Xie, 2013). The broad bands at 3486 and 3240 cm−1 were attributed to the N–H stretching from amino groups and C–H stretching vibrations of cysteine molecules, respectively. The band at 1577 cm−1 can be attributed to the COO− asymmetric bending mode of acidic group of amino acid whereas, the two bands around 1389 and 1128 cm−1 can be attributed to CH2 and C–N vibrational modes, respectively. The absorption at 560 cm−1 is due to the Ni–OH stretching vibration for Ni(OH)2. The band at 450 cm-1 observed after the hydrothermal treatment of precursor is due to Ni-O stretching vibration. The comparison of spectrums clearly demonstrates a decline in the band intensities related to the hydrophobic groups (CH2-) of L-cysteine(Saghatforoush et al., 2012). Thus suggesting the growth assistance via hydrophobic interactions between –CH2 group of L-cysteine and Ni (OH)2.

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Figure 4.5.5 FTIR spectra of hedgehog-like NiO nanostructures (a) before annealing (b) after annealing of precursors

Based on SEM, FTIR information a plausible growth mechanism is presented in the Figure 4.5.6. At the initial stage Ni2+ ion coordinate with NH3.H2O to form [Ni (NH3) x]2+ complex followed by hydrolysis to form Ni(OH)2 nuclei as explained by following equations.

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These nuclei owing to their classic anisotropic growth are directed to form nanoplates, however the presence of L-cysteine molecules which are loosely bonded with nuclei allow spontaneous aggregation based on amino acids via hydrophilic interaction. With time these small aggregate slowly and continuously grow in spheres enclosed with L-cysteine molecules. In addition, some small Ni(OH)2 aggregates within the vicinity of these grown spheres reside over them to eliminate the interface and reduce their surface energy as a consequence of which hedgehog surface features are formed onto the spheres.

Upon annealing, these precursors convert to hedgehog-like NiO nanostructures while H2O, CO2 and SO2 are formed as a result of nickel hydroxide and amino acid decomposition.

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Figure 4.5.6 Schematic illustration of the formation of hedgehog-like NiO nanostructures

4.5.2 The measurement of non-enzymatic glucose sensing response of the complex nanostructures of NiO

To measure and investigate the non-enzymatic glucose sensing response of the hedgehog-like NiO nanostructures by cyclic voltammetry (CV) was selected as an effective mode for measurements. Figure 4.5.7 shows the CV response of bare and modified glassy carbon electrode both in the absence and presence of 1 mM glucose at a scan rate of 0.05 Vs-1. The observation clearly shows that bare glassy carbon electrode has no specific peak for the oxidation in the absence or the presence of glucose molecules. However, the NiO HHs-Nafion/GCE experienced a pair of oxidation and reduction peaks which could be assigned to the Ni(II)/(III). In the presence of 1 mM glucose solution, NiO HHs-Nafion/GCE has shown dominant oxidation and reduction peaks which demonstrates that the prepared hedgehog-like NiO nanostructures have high potential for the oxidation of glucose in 0.1 M NaOH solution. The electrochemical response of fabricated electrode is according to the published non-enzymatic glucose sensors using NiO nanostructures (Ding et al., 2011; Lu et al., 2013). The sensing mechanism of proposed glucose sensor by using the NiO nanostructures could be explained as: the Ni2+ is electro oxidized to Ni3+ in NaOH solution and at this event, the release of electron is showing the enhanced anodic peak. At the same time glucose is oxidized to gluconic acid by Ni3+ which further reduced to Ni2+(Chigane & Ishikawa, 1998).

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Figure 4.5.7 Cyclic voltammogram (CV) profile of bare GCE (a) in the absence of glucose (b) in the presence of 1.0 mM glucose, NiO HSS-Nafion/GCE (c) in the absence of glucose and (d) in the presence of 1.0 mM glucose

To obtain the best quality of developed sensor, optimization of crucial parameters were carried for the improvement of peak current (Ip) in 1.0 mM glucose using NiO HHs-Nafion/GCE. Figure 4.5.8(a) represents the effect of scan rate values on the corresponding peak current response of NiO HHs-Nafion/GCE in CV mode. Successive increased anodic peak current along with the increase of the scan rate in the range from 0.05-1.0 V/s can be clearly observed. In addition linear proportionality between peak current and square root of scan rate was plotted as shown in inset of Figure 4.5.8(a) which suggests the electrochemical reaction is diffusion controlled with high superiority of oxidation process over the opposed reduction process. The response of NiO HHs-Nafion/GCE with the different electrolytic volumes was also examined and is shown in Figure 4.5.8(b). The maximum increase in Ip value was observed with 09 mL of electrolytic volume; however other volumes either resulted in decline of Ip values or shifted potential towards lower values. The effect of hedgehog-like NiO nanostructures deposition volume on the response of electrode was also examined and is presented in Figure 4.5.8(c). Different volumes including 5, 10, and 15 µL were used for the modification of GCE in a similar manner as mentioned in section (3.5.4). NiO HHs-Nafion/GCE showed increased Ip responses with successive increase of nanostructures deposition volume. At 05 µL casting volume the NiO HHs-Nafion/GCE was noted to produce the most stable response without any decline in the Ip, however at much higher volumes (> 05 µL) a decline in the Ip was observed with time which may be attributed to the depletion of nanostructure material from the surface of electrode.

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Figure 4.5.8 Cyclic voltammograms of NiO HSS-Nafion/GCE at different scan rates from 0.05 mV/s to 1.0 mV/s in 0.1 M NaOH with 1.0 mM glucose. Inset: plot of anode and cathode current vs. the scan rate (c) Ip responses with successive increase of electrolytic volume (0.1 M NaOH) from 1.0 to 10.0 mL (d) Ip responses with successive increase of NiO deposition volume from 05 to 20 µL at 0.53 V for NiO HSS-Nafion/GCE.

The quantitative response of the NiO HHs-Nafion/GCE based on increasing oxidation peak with respect to increasing glucose concentration is shown in Figure 4.5.9 (a). The important analytical parameters were calculated by linear regression analysis (Fig 4.5.9(b)) of Ip vs glucose concentration at 0.49 V in range between 0.1 to 5.0 mM. The developed sensor demonstrated the excellent linearity with the correlation of determination (R2) as 0.9953 and high sensitivity of 1052.8 µA mM-1 cm-2 was obtained by dividing the slope of the linear regression equation with the electrode surface area. The LOD and limit of quantification (LOQ) were determined to be 1.2 (S/N = 3) and 4.2µM respectively.

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Figure 4.5.9(a) Cyclic voltammograms response for NiO HSS-Nafion/GCE with successive addition of glucose from 0.1 to 5.0 mM at 0.53 V (b) the corresponding calibration curve

The measured electrochemical performance of the presented sensor electrode was compared with the other NiO-based non-enzymatic glucose sensors. As shown in Table 4.5.1, it is clear that the presented electrode possesses high sensitivity and a low detection limit which can be attributed to the direct electron transfer between the NiO and the electrode at the NiO HHs-Nafion/GCE modified electrode without the employment of any immobilization matrix.

Table 4.5.1 Comparison of different non enzymatic glucose sensors in terms of detection limit, linear range and sensitivity.

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4.5.3 The selectivity, reproducibility and stability

The selectivity is the backbone parameter for evaluating the performance of non-enzymatic glucose sensors in the presence of known interferents. During the quantification of glucose level the interferents include uric acid (UA); dopamine (DP) and ascorbic acid (AA) are found in human blood serum. The normal physiological level of glucose is about 3–8 mM(Hameed, 2013), which is much higher than the concentrations of interfering species like UA (0.1 mM) and AA (0.1 mM). The interfering effect of 0.1 mM UA, DP and AA compared to 1.0 mM glucose at the potential of 0.53 V were determined as shown in Figure 4.5.10 (a). No obvious current response observed with the addition of 0.1 mM UA, DP and AA suggesting high selective nature of the developed glucose sensor. The reproducibility and stability of the developed glucose sensor was studied by preparing the 10 electrodes in the similar manner as described in section (3.5.4) for measuring the anodic peak current. The measured relative standard deviation (RSD) value for measuring anodic peak currents was ˂ 1.0% that is indicating high reproducibility of developed glucose sensor as shown in Figure 4.5.10 (b). The long term stability of developed sensor was evaluated by storing the electrode in the ambient air conditions and by measuring the electrode response in 1 mM glucose solution after 1 week to 6 months. The electrode retained 95% of its initial current response (Figure 4.5.10 (c)) during that period (used more than 24 times) of the storage.

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Figure 4.5.10 Cyclic voltammograms recorded for (a) negative or positive influence of electro active interferents (b) reproducibility of electrode with inset bar graph depicting <1.0%RSD value for 10 electrode prepared in same manner (c) stable response of electrode from 1 week up to 6 months with inset bar graphs showing 95% retention of its initial current response

4.5.4 The detection of glucose by complex nanostructures of NiO modified glassy carbon electrode from real samples

The NiO HHs-Nafion/GCE was additionally employed for real sample analysis to highlight the universal nature of the developed glucose sensor. For this experiment, blood samples of some known persons were obtained and blood serum was separated by high speed centrifuging. The samples were two fold diluted in the alkaline medium and the estimation of blood glucose was worked out using the linear regression mentioned in section (4.5.2). The glucose concentrations were determined in range between 4.8 to 5.9 mM and 4.5 to 4.9 mM for random and fasting sugar respectively. The results were confirmed by comparing them with those obtained by a commercial glucometer. The relative standard deviation of < 1.0% indicates close correlation between the obtained results. Table 4.5.2 summarizes the glucose level for real blood samples obtained using both devices. This investigation revealed that the proposed glucose sensor based on the hedgehog-like NiO nanostructures works well for the non-enzymatic sensing of glucose and can be used for the routine analysis of glucose level.

Table 4.5.2 Determination of glucose level in real blood serum samples

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Conditions NiO HHs-Nafion/GCE, 08 mL 0.1M NaOH, 2 times diluted 10 µl filtered real sample, no. of replications =3

PART VI

Annotations : The entire study presented in part 6 is published online with the following reference:

Soomro, R. A., Nafady, A., Ibupoto, Z.H., Sirajuddin, Sherazi, S.T H., Willander, M., Abro, M.I., “Development of Sensitive Non-Enzymatic Glucose Sensor Using Complex Nanostructures of Cobalt Oxide." Materials Science in Semiconductor Processing 34, no. 0 (2015): 373-381.

4.6 Temperature controlled growth of cobalt oxide nanostructures for glucose electrochemical sensor

This study describes the template free growth of cobalt oxide nanostructures using simple hydrothermal route. The study explores the potential capability of the grown nanostructures for catalytic oxidation of glucose in alkaline medium.

4.6.1 Characterization of Co3O4 nano disc

The morphological features, phase purity and composition, of the Co3O4 NDs obtained by the hydrothermal method were investigated systematically. Figure 4.6.1 shows distinctive SEM images of Co3O4 NDs recorded at different magnifications. It can be seen that resulting morphology of Co3O4 looks like nanodiscs with average thickness of 200 to 300 nm. Figure 4.6.1 (a, b) shows large scale image with very high density of NDs uniformly distributed over the entire area. The tapering feature of individual NDs can be viewed in Figure 4.6.1 (c, d). It is obvious that NDs are composed of more than single hexagonal plate with very rough morphological features having dents and irregularities. Such rough surfaces possess large number of active sites, which provide several contact points for catalysis ( Dung & Tuyet et al., 2011). Formation mechanism for Co3O4 NDs could be due to high concentration of ammonia which produced mass production of cobalt hydroxide (Co (OH)2) at the cost of large amount of hydroxide ions in growth solution as explained in section(4.6.2).

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Figure 4.6.1 SEM images of the Co3O4 NDs (a, b) low magnification images with showing density of NDs (c, d) high magnification images of individual discs

The crystalline nature and purity of Co3O4 NDs was investigated by XRD. The characteristic XRD patterns are presented in Figure 4.6.2. The obtained patterns are well matched with standard Joint Committee on Powder Diffraction Standards (JCPDS) card No, which indicates that the product is purely cubic phase Co3O4. In addition, the sharp peaks, obtained in spectrum suggest the formation of highly crystalline material. The diffraction peaks at 2q are corresponded to crystal planes (111), (220), (331), (400), (511), and (440).

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Figure 4.6.2 XRD patternsofCo3O4 NDs

In order to further support the formation of NDs and interaction between Co and O atoms, FTIR spectra for both precursor Co(OH)2 and Co3O4 NDs were recorded (Figure 4.6.3). A broad band observed in the FTIR spectrum of Co(OH)2 at about 3575 cm-1 is attributed to characteristic of the stretching vibration of the interlayer water molecules and of hydroxyl groups hydrogen-bonded to H2O (Figure 4.6.3(b)) (Yang et al., 2012). In addition, the band at 1633 and 490 cm-1 reflects the bending mode of water molecules and Co–OH stretching vibrations respectively (Liu et al., 2005; Zhu et al., 2002). In contrast, the strong distinct peaks at 574 and 665 cm−1 observed in the spectrum of Co3O4 NDs shown in Figure 4.6.3(a) corresponds to stretching vibrations of the metal oxygen bonds, which depicts the formation of the metal oxide (Liu et al., 2012; Xia et al., 2012). Thus, FTIR study further strengthens the XRD results.

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Figure 4.6.3 FTIR spectrum of synthesized Co3O4 NDs

XPS measurement was carried out for further investigation of the chemical composition of the synthesized product. In the XPS spectrum carbon was used as a reference for optimization the binding energy in analysis the cobalt oxide nanostructures Figure 4.6.4(a) represents the Co 2p spectra, with two peaks positioned at 780.2 and 795.5 eV, attributed to the Co 2p3/2 and Co 2p1/2 respectively. The gap between the peaks is about 15.1 eV (spin orbit splitting), which is in accordance to the standard Co3O4 spectra (Ding et al., 2010a; Sung et al., 2013). The major peak of O 1s is presented in Figure 4.6.4(b).The peak is positioned at 530.1 eV, attributed to the lattice oxygen of Co3O4 (Shinde et al., 2006). Besides this the compositional purity can be viewed from the wide scan spectrum shown in Figure 4.6.4(c) and it can be seen that the prepared material is only composed of Co and O. The results of XRD, FTIR and XPS are in close correspondence, suggesting, the prepared material is purely composed of Co3O4.

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Figure 4.6.4 XPS spectra of the Co3O4 NDs: slow scan spectra of (a) Co2p, (b) O1s and (c) full survey scan spectrum

4.6.2 Growth mechanism for Co3O4 NDs

The growth mechanism for Co3O4 NDs can be understood in terms of following equations:

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At the initial stage relatively higher concentration of NH3 leads to the formation of highly stable hexaamminec obalt(III) ion ([Co (NH3)6]2+). As the reaction proceeds evaporation of NH3 results in gradual dissociation of ion complex concurrently resulting equilibrium shift towards right side in equation (I) and increased number of hydrated cobalt ion in system. This evaporation of NH3 also shifts the equilibrium of equation first towards right hand side, however due to power difference between concentration of NH3 in equation two and three very small effect on OH concentration would have been resulted, thus based on overall ion concentration, the formation of Co (OH)2 is promoted as shown by following equation. The presence of the cobalt hydroxide was confirmed by presence of stretching band in the region of 490–540 cm-1 assigned to and Co–OH vibration shown in Figure 4.6.5. In the final step, Co3O4 NDs were obtained via annealing of Co (OH)2 precursor material as expressed by above mentioned equations.

Based on detailed morphological and structural information schematic diagram representing growth of Co3O4 NDs in aqueous solution is presented in Figure 4.6.5.

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Figure 4.6.5 Proposed growth mechanisms of Co3O4 NDs in aqueous solution

4.6.3 Electrochemical sensing of glucose

Sensing application of Co3O4-NDs was demonstrated by constructing a non-enzymatic glucose sensor. The electrochemical response of the bare GCE and Co3O4NDs-GCE in the absence and presence of glucose was monitored in 0.1 M NaOH solution at a scan rate of 0.05 (Vs-1) using CV technique. The measured voltammetric responses are shown in Figure 4.6.6. It can be viewed from Figure 4.6.6(a, b) that the response of bare GCE in the absence and presence of glucose is negligible and no specific oxidation peak for glucose is observed in either case. However, the response of Co3O4NDS-GCE is shown in Figure 4.6.6(c) and prominent redox peaks at 0.49 (oxidation) and 0.47 (reduction) respectively were found which reflect the sensing response of the modified GCE for glucose detection due to excellent electro catalytic behavior of Co3O4 NDs. Figure 4.6.6(d) represents the response of Co3O4-GCE in the presence of 0.5mM glucose solution justifying its catalytic behavior with clear sensing.

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Figure 4.6.6 Cyclic voltammograms (CVs) of bare GCE (a) in the absence of glucose (b) in the presence of 0.5 mM glucose, Co3O4-GCE (c) in the absence of glucose and (d) in the presence of 0.5 mM glucose

In order to improve the quality of developed sensor to highest possible standards essential electrochemical parameters were thus optimized for step wise modification of peak current (Ip) in 0.5 mM glucose using Co3O4NDs-GCE . Figure 4.6.7shows CV profiles of 0.5 mM glucose solution under various optimized parameters. Effect of scan rate on the redox response of the Co3O4NDs-GCE was studied and response of electrode was measured at different scan rates in similar concentration (Figure 4.6.7(a)). The measured response declares the successive increase in the anodic peak with higher scan rate due to enhanced catalytic property of cobalt oxide nanostructures. This study further suggests that the fabricated sensor is well suited for as oxidative catalytic properties as compared to its reductive sensing in case of glucose. The linear proportionality between peak current densities (oxidation and reduction) and square root of scan rate as shown in inset of Figure 4.6.7(a) implied that the surface oxidation reaction is a diffusion controlled process.

Figure 4.6.7(b) shows the effect of electrolyte volume on the sensing response of Co3O4NDs-GCE and the optimum response was found at 9 mL of electrolyte. This study describes the favorable oxidation of glucose at higher volume of electrolyte due to improved catalytic potential of Co3O4NDs. The effect of drop casting of Co3O4 NDs on the response of modified GCE was also studied and measured respective scans are shown in Figure 4.6.7(c). The response was obtained by varying deposition volume in range of 5 to 15 µL for modification of GCE in a similar manner as mentioned in section (3.6.4). An increased Ip response can be seen with successive increase of Co3O4 NDs casting volume; however 5 µL produced most stable response as compared to higher volume (> 05 µL of NDs). It was noticed that in case of higher volume casting of 10 µL and 15 µL volume of NDs on GCE, the repetitive Ip responses were declined (not shown here) which indicated electrode unsuitable stable nature indicative of NDs leaching from surface of electrode.

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Figure 4.6.7Cyclic voltammograms (CVs) of the Co3O4-GCE recorded for 0.5 mM glucose (a) in 0.1M NaOH solution at different scan rates (inner to outer):0.05–1.0Vs-1 (b) in various volumes of 0.1M NaOH solution (c) with various deposition volumes in range of 5 to 15 µL of Co3O4 NDs

Selectivity of cobalt oxide modified GCE was determined by measuring its response in the presence of common interferents including ascorbic acid, uric acid and dopamine during the sensing of glucose. Since the physiological concentration of glucose is 10 times higher than the dopamine, uric acid and ascorbic acid Figure 4.6.8 shows the response of Co3O4NDs -GCE in the presence of interferents and negligible change in the anodic peak was observed which indicates the high selectivity of the modified electrode and hence justifies its use in the real physiological samples like blood serum.

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Figure 4.6.8 Interference study showing negative or positive influence of electro-active interferences (uric acid, ascorbic acid and dopamine); the inset shows the histograms for each interferent along with control experiment carried out with Co3O4-GCE in presence of 0.5 mM glucose and 9 mL of 0.1 M NaOH solution

4.6.4 Analytical Performance of Co3O4 NDs -GCE

Quantitative response based on increasing oxidation current with glucose concentration was obtained by monitoring the changes in the Ip at 0.47 V for the glucose concentration in the potential range of 0 to 0.7 V (Figure 4.6.9(a)). The useful analytical data was obtained by linear regression analysis of Ip at 0.47 V plotted against varying glucose concentration in range from 0.5 to 4.5 mM. Although the developed sensor demonstrates excellent performance outside the selected calibration range but to certain the working potential in physiological glucose range and for elegant presentation of data such range was considered best suitable. The method showed excellent linearity in the selected calibration range, of glucose (Figure 4.6.9(b)) with a slope of 1.9131 μA mM-1 and a correlation coefficient of 0.9953.The sensitivity of the Co3O4NDs-GCE sensor is calculated as 27.33 µA mM-1 cm-2 by dividing the slope of the linear regression equation by the electrode surface area. The Limit of detection (LOD) and limit of quantification (LOQ) calculated to be 0.8 and 2.6µM respectively. The LOD and LOQ were determined from three times the standard deviation of the blank signal (3×∂/slope), and ten times the standard deviation of blank signal (10×∂/slope) respectively.

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Figure 4.6.9 CV voltammograms of 0.5–4.5 mM glucose under optimized conditions (b) respective linear regression plot between current (Ip) and glucose concentrations

Table 4.6.1 presents the comparative data of limit of detection by various types of electrode based sensor regarding the non-enzymatic detection of glucose. The table shows that the newly developed Co3O4NDs-GCE based sensor for glucose in our case produced better results than the reported sensors in similar field with the best advantage of high sensitivity and low costliness.

Table 4.6.1 Comparison of Co3O4 NDs based electrochemical sensor with various enzyme free glucose sensor systems.

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4.6.5 Reproducibility study

The newly developed sensor was tested for its reproducibility by taking 08 repetitive measurements in 0.5 mM glucose solution (Figure 4.6.10). A relative standard deviation of 1.2% was found which signifies excellent performance of the sensor in terms of reproducibility and hence higher stability. The results suggest no passivity of Co3O4NDs -GCE in the presence of glucose.

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Figure 4.6.10 Consecutive CV votammograms of Co3O4-GCE for 0.5 mM glucose solution mixed with optimized 0.1 M NaOH solution at 0.05mVs−1

4.6.6 Use of Co3O4NDs -GCE for the detection of glucose in the real samples

The Co3O4NDs-GCE was successfully employed for the detection of glucose in real samples. The shape and position of the blood glucose oxidation peak was similar to that of chemical glucose (not shown here). The blood samples of four persons were collected and blood serum was separated for random and fasting sugar tests. The samples were treated after proper dilution in order to bring the final glucose concentration in the linear working range of the sensor. The linear relation in Figure 4.6.9 was used for the determination of glucose contents in blood serum. According to data, the glucose concentration in the blood samples was found between in the range of 4.9 to 5.7 and 4.7 to 4.8 mM for random and fasting sugar respectively. The measured response of Co3O4NDs-GCE was compared with a sophisticated commercial glucometer. The measured glucose concentration by developed sensor was in good agreement with the standard glucometer for the quantification of glucose. The results for determination of glucose by both devices are summarized in table 4.6.2. All the results by fabricated biosensor indicate its highly selectivity and sensitivity, stable responses, cost effective nature and facile fabrication. The developed biosensor was observed for its life time and was found stable over studied three months with relative standard deviation of 2.5%.

Table 4.6.2Determination of glucose level in real blood serum samples

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Conditions: Co2O3 NDs-GCE, 09 mL 0.1M NaOH, 10 µl filtered real sample (2 times diluted), no. of replications =3

Chapter 5 CONCLUSION

5.1 Conclusion

In conclusion, this thesis compiles a set of studies related synthesis, characterization and application of metal and metal oxide nanostructures. In the first part, a simple, onsite appraisal suitable, selective and sensitive detection method has been developed using L-cysteine functionalized copper NPs as a colorimetric probe that allows colorimetric determination of Hg2+. It is the first time to employ L-cysteine as a greener protecting/ functionalizing agent for copper nanoparticles. Experimentation is performed at room temperature without the application of any inert gas or organic solvent making this approach simple, cost effective and efficient route for stable copper nanoparticles in aqueous medium. This assay relies on Hg2+ based aggregation (fusion) of pure metallic copper nanoparticles into copper oxide, allowing rapid qualitative and quantitative monitoring. Compared with previous aggregation based methodologies which employ precious metals like gold and silver, this assay is based on the usage of cheaper, easily available copper metal, making this cost effective for resource poor settings and easy to implement. Further, second part deals with oxidation resistant and stable Cu NPs with narrow and homogenous size distributions synthesized in aqueous medium without employing any protective gas. The Cu NPs were synthesized using sodium borohydride as a reducing agent, SDS as a capping agent and ascorbic acid (natural vitamin C) was used in capacity of supporting reagent with antioxidant activity. These nanoparticles were employed as recyclable heterogeneous catalysts for ultra-fast reductive degradation of EB dye, which to the best of our knowledge has not been addressed previously. Moreover, the study suggests that the synthesis route is free from requirements like high energy, extended preparation time or special equipment. The prepared nanocatalysts can easily be recovered and reused without any significant decline in the efficiency.

In the third part of the thesis, it is concluded that surfactant capped Cu nanoparticles and nanorods can be efficiently synthesized in an aqueous medium via a surfactant assisted wet-chemical reduction route. Another aspect of this study highlights the basic mechanism for shape variation of the copper nanostructures in aqueous surfactant medium and depicts the formation to be based on concentration [surfactant] to [copper ion] ratio, nature of surfactant, micelle size, and alignments of the initially formed particles. Shape and size dependent catalytic activity of copper nanostructures is also evaluated by degrading methyl orange in the presence ofNaBH4 used as a reductant under ambient reaction conditions. Comparison has been made to show their different catalytic performance in terms of kinetic and thermodynamic parameters. Copper nanoparticles were found to be a highly efficient catalyst, as compared to copper nanorods, because of their smaller work function and high number of surface atoms. Lastly, the universal nature of copper nanostructures as a catalyst was demonstrated by efficiently degrading real dyeing waste water samples with copper nanoparticles, collected from drainage of local industries, situated in Hyderabad region, Pakistan. The study could be extended to all types of reductive degradation of other dyes as well as other pollutants in waste water research. In regard to catalytic activity, the fourth part of this thesis deals with synthesis of Ni nanoparticles and its catalytic activity. The study demonstrates a simple, cost effective and efficient route towards small, spherical, highly dispersed Ni nanoparticles. The as-synthesized Ni NPs were obtained via assistance of L-serine (amino acid) utilized as a greener protecting/capping agent and hydrazine as reducing agent. The application of hydrazine provided sufficient N2 atmosphere to resist oxidation of Ni NPs and avoid use of any external inert gas source. Furthermore, these stable Ser-Ni NPs were used as simpler, easily implemented and highly efficient heterogeneous catalyst for the reductive degradation of imidacloprid. The newly developed catalyst demonstrated excellent catalytic reductive proficiency for complete degradation of standard IM with the degradation kinetics following pseudo first order and rate constant of 0.104 s-1. The extremely high reaction rate and excellent reusability with negligible catalytic poisoning makes this approach highly economic for poor resource based situations. In addition, the study could be extended to degradation of all types of other pesticides as well as other pollutants in waste water research.

In the fifth part, we discuss the development of novel non-enzymatic glucose sensors based on hedgehog-like NiO nanostructures. The unique nanostructures were prepared via L-cysteine assisted hydrothermal method. The present study also highlights the basic growth mechanism of hedgehog-like NiO nanostructures suggesting L-cysteine crucial role in the formation of such unique shaped nanostructures. The as-fabricated hedgehog-like NiO nanostructures exhibited excellent catalytic performance for the oxidation of glucose and thus allowed the development of very stable, simple, easily implemented, highly sensitive, extremely selective, robust and its high economic feasibility for the enzyme free sensing of glucose. Moreover, the successful application of sensor in the real blood samples with sensitive glucose detection proves its feasibility for the routine analysis of glucose level from clinical, food industry, biotechnology and real blood samples. Furthermore, the sixth part of the thesis is dedicated to Co3O4 nanostructures synthesised using aqueous hydrothermal growth method without the application of any hard template or growth directing agent. The described approach is facile, simple, and cost effective route for preparation of stable Co3O4 NDs in aqueous medium. The as-synthesized Co3O4 NDs possessed excellent surface features which encouraged the development of highly selective and sensitive enzyme free glucose sensor. Furthermore, the successful application of the developed sensor for real blood glucose determination proves its feasibility for real analytical applications.

5.2 Socio-Economic Impact

To explore the potential application of very cheap, easily available and reliable metal nanoparticles (Cu NPs) for colorimetric sensing of mercuric ions (Hg2+) in aqueous systems. To provide a step forward in developing reliable sensor system to be efficiently used in resource deficient countries like Pakistan, India and Bangladesh. Such sensor system not only covers the economic aspect but also provides a route to develop more advanced portable, fast and simple sensor system to detect presence of Hg2+ ions with simple change of color.

To provide efficient and cheap catalysts for removing colored compounds from industrial waste effluents. The usage of copper nanostructures will provide an alternative approach for catalytic removal of toxic compared to presently used noble metal (Pt and Pd) in the same area. The catalytic efficiency of copper nanostructures is such that it consumes smaller amount of energy, less time (fast), is less passive and much cheaper than noble metals.

A step forward, in understanding the formation of novel metal oxide nanostructures using bio-compatible templates. The understanding of how and why electro-chemical behavior of materials varies with changes in structural, morphological and dimensional features and how, various bio-compatible templates affect the growth mechanisms. The studies provided a critical approach in describing which bio-compatible templates best suits metal oxide formation and which novel structures exhibits enhance chemical properties.

Development of sensor systems based on cheap, easily available, reliable metal oxide nanostructures with greater sensitivity, selectivity and stability for enzyme free glucose sensing. An insight, to understand how metal oxide work and how their electro-chemical properties responds to organic molecules under study.

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List of Publications

1. Soomro, R.A., et al., Catalytic Reductive Degradation of Methyl Orange Using Air Resilient Copper Nanostructures. Journal of Nanomaterials, 2015. 2015: p. 12.

2. Soomro, R.A., et al., Development of sensitive non-enzymatic glucose sensor using complex nanostructures of cobalt oxide. Materials Science in Semiconductor Processing, 2015. 34 (0): p. 373-381.

3. Soomro, R.A., et al., Electrochemical sensing of glucose based on novel hedgehog-like NiO nanostructures. Sensors and Actuators B: Chemical, 2015. 209 (0): p. 966-974.

4. Soomro, R., et al., Controlled synthesis and electrochemical application of skein-shaped NiO nanostructures. Journal of Solid State Electrochemistry, 2015. 19 (3): p. 913-922.

5. Kalwar, N.H., et al., Catalytic degradation of imidacloprid using L-serine capped nickel nanoparticles. Materials Express, 2015. 5 (2): p. 121-128.

6. Kalwar, N.H., et al., Fabrication of small l-threonine capped nickel nanoparticles and their catalytic application. Applied Catalysis A: General, 2013. 453 (0): p. 54-59.

Re-prints of published paper

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