Insights of Current Developments in Optics-Based-Biosensors for Analysis of Environmental Contamination and Pollution

Technical Report, 2015

26 Pages, Grade: 1



1. Introduction

2. Bio-Recognition Elements used for Bio sensing purpose
2.1 The Enzymes as Bio-Recognition Element
2.2 The Antibodies as Bio-Recognition Element
2.4. The Aptamers as Bio-Recognition Element
2.5. The Cells as Bio-Recognition Element

3. Use of Nanomaterials in Bio sensing purpose
3.1. Use of Quantum Dots in Bio sensing purpose
3.2. Use of Gold Nanoparticles in Bio sensing purpose
3.3. Use of Graphene and Graphene Oxide in Bio sensing purpose

4. OPTICS-Based Biosenesors used for the purpose of environmental analysis
4.1. Evanescent Wave Fiber Optic Biosensors
4.2. SPR Biosensors in Environmental pollution analysis
4.3. Nano-Structured Optical Biosensors in Environmental Pollution analysis

5. New Emerging Optical Biosensors for Environmental Surveillance
5.1. Optical ring resonator based biosensors
5.2. Photonic crystal biosensors

6. Optical Biosensors for the purpose of surveillance of Pollution in the Environment

7. Conclusion


1. Introduction

In the present era of industrial development, several pollutants continue to contaminate the environment which effects the health of the nature as a whole. To meet this growing need to monitor these pollutants, many advanced analytical devices are being developed to detect and control these harmful substances. To detect the various contaminants that pollute water, quantitative analysis of water samples is done by chromatographic and spectroscopic methods. Despite being highly accurate and sensitive, these methods require expensive and highly sophisticated instruments, skilled personnel for handling, operation and to carry out the complicated procedure for the preparation of the samples. These methods also fail to accomplish real-time, onsite and high frequency monitoring of the pollutants. To overcome these drawbacks, extensive research is being carried out to devise cost-effective and dynamic monitoring techniques for accurate detection of these pollutants. Biosensors, which is designed from the combination of biochemistry, biology, nanotechnology, physics and electronics, exhibits all the desired characteristics like accuracy, speed, stability, low cost, real-time remote monitoring capabilities and has helped to improvise immensely in the area of risk management approaches and environmental issues.

A biosensor is an analytical device, used for the detection of an analyte that combines a biological component with a physicochemical detector. The Optical biosensors explores light absorption, fluorescence, luminescence, Raman scattering, reflection, and refractive index which are good substitutes to conventional techniques (Figure 1). These sensors provide fast, sensitive, real-time, and high-frequency monitoring without any time- consuming sample concentration. Even Though optical biosensors have huge potential applications in the fields of environmental monitoring, food safety, drug development, biomedical research, and diagnosis, but unfortunately their use in the areas of environmental pollution control is still in the infant stages [1-3]. Huge progress has been achieved in the research and development of optical biosensors, and a lot of research papers and outstanding reviews were published in the literature. This review mainly is centered on recent development and advancements in optical biosensors, providing examples of relevance, applications and the analytical performance in environmental pollution monitoring. For bio sensing bio-recognition molecules plays a vital part and are highlighted first. The important advances in optical biosensor will then be discussed and then new developments in the field of optical biosensors for pollution control will be reviewed.

illustration not visible in this excerpt

Figure 1. Schematic diagram of an optical biosensor.

2. Bio-Recognition Elements used for Bio sensing purpose

The important and key property of a biosensor is the construction of the bio-recognition element so that it could interaction with the targets molecules.

2.1 The Enzymes as Bio-Recognition Element

Enzymes are highly selective catalysts, greatly accelerating both the rate and specificity of metabolic chemical reactions, from the digestion of food to the synthesis of DNA. Optical biosensors based on enzymes have been rigorously in research during the last few decades due to the important practical based needs of industry for the purpose of environmental control and monitoring . Immobilization of enzymes on solid substrates is very vital because of the immobilization method do have the ability to increase the lifetime and sensitivity of the biosensors in practice. The optical transducers used in case of enzyme-based biosensors can be considered as the heart for the development of a compact, self-reliant device which could be used for environmental monitoring purpose. Cholinesterase (ChE) enzymes that can be inhibited by many toxic chemicals like organophosphates, heavy metals, and toxins. As a result of which, ChE biosensors are the main area interest in the context of global monitoring of toxicity. Taking in consideration that different pollutants inhibit enzyme activity in different ways, multi-analyte detection can also be done by using enzyme sensors. Taking an example, pesticides and heavy metal ions can be detected at a time in a single sample solution with inhibition of butyrylcholine esterase by pesticides and urease by heavy metals ions [4].

Enzyme based biosensor for the measurement of toluene in an aqueous solution was developed and characterized. In this case Toluene ortho-monooxygenase was being used as element of bio recognition, and an oxygen-sensitive ruthenium-based phosphorescent dye played the role of a transducer. Determination of Toluene for the purpose was based upon the enzyme-catalyzed consumption of oxygen that have the ability to change the phosphorescence intensity of the oxygen-sensitive probe [5-7]. Even though the enzyme based biosensor do have the ability to detect toluene in wastewater within a limit of detection (LOD) of 3.1 mM and a linear signal ranging up to 110 mM, but the response time is very long (1.2 h), and the activity goes on decreasing with each measurement. Huang et al. designed a fiber optic based biosensor to the determine adrenaline based on immobilization of laccase catalysis. The nanoparticles contained in laccase and the luminescent oxygen- sensing membrane were being deposited at the tip of the optical fiber. With the consumption of oxygen the enzyme laccase catalyzes the oxidation of adrenaline. The biosensor mentioned can detect adrenaline with in a range of 15 nM to 2 mM concentrations with a response time of 40 s. Immobilized enzyme is stable in nature. Enzyme-based optical biosensors provide new ways to perform remote, in-line determinations with high speed and accuracy for environmental pollution control and early warning. Although great improvement has been made in the reliability, sensitivity, selectivity and speed of response of these enzyme-based biosensors, certain limitations still exist. First, there are limited number of substrates that can be acted upon by specific enzymes; second, limited interaction between the environmental pollutants and specific enzymes; third, the enzymes fail to specifically differentiate among compounds of similar classes [8-9].

2.2 The Antibodies as Bio-Recognition Element

Use of particular interactions within antigen and antibody, the immunosensors have been considered as the novel-standard technique in environmental monitoring and clinical diagnostics. The highly certain interaction between two binding sites of an antibody with a specific target can be detected by means of a transducer (e.g.electronic,electrical,optical). As a result of which, the immunosensor provides a high repeatability, making it enable to recognize particular environmental pollutants [14-21]. Non-immunogenic environmental contaminants or pollutants having lower molecular weight (<1.2 kDa), which are called haptens, more or less becomes immune upon conjugation to carrier proteins . The Antibodies going against haptens, for example pesticides, persistent organic pollutants (POPs), and endocrine disrupting chemicals (EDCs), can be prepared by means of synthesizing the immunogens from the covalent binding of haptens into a carrier protein and then immunizing them with in animals [22].

The product related to the chemical binding of hapten to the carrier protein determines the specificity and quality of antibody which is very much important for immunoassay and it is called antigen. For the purpose of detecting the microcystin-LR (MC-LR), which is more frequent and mostly toxic hepatotoxin, the corresponding complete antigen (MC-LR-BSA) can be easily synthesized by introduction of a primary amino group in the seventh N- methyldehydroalanine residue of MC-LR. Then the resltant product aminoethyl-MC-LR can be coupled to bovine serum albumin (BSA) with glutaraldehyde. A monoclonal antibody (Clone MC8C10) against MC-LR can also be produced by means of immunizing with MC-LR-BSA [23-27]. Pollutants present in the environment with low molecular weight (molecular weight <1.2 kDa), and are much difficult to directly immobilize onto the biorecognition element sensing surface, results in antibodies that can be immobilized into preparation of the sensing surface for the immunosensors .

But, controlling the number, orientation, and the position of the antibodies in relevance to the sensor surface is a difficult task . Inadvertent disturbance on the binding site can occur if the antibody is in conjugate with the active surface of the sensor, which causes in the inevitable loss relating to the activity of the antibody. The most important point is , the use of strong acid in case of the regeneration process that reduces the capability of recognition of the immobilized antibodies after the reuse of the sensor surface, which ultimately affects the immunosensor interms of stability and reliability. The process of regeneration can be performed not more than 15 times, and the activity of the antibody tends to decrease in each cycle, resulting in inaccurate detection. To get a stable reusable sensor conjugation of the hapten-carrier-protein as bio- recognition element can be immobilized into the surface of the immunosensor . For instance, a reusable immune surface can be formed with the help of the covalent attachment of MC-LR-OVA to a self-assembled monolayer generated onto the fiber optic sensor with a heterobifunctional reagent. Process of regeneration on the surface of the sensor helps the performance of more than 110 cycles of assay devoid of any significant loss of reactivity which is less than 3% [26].

2.4. The Aptamers as Bio-Recognition Element

Aptamers are single-stranded DNA or RNA (ssDNA or ssRNA) molecules that can bind to pre-selected targets including proteins and peptides with high affinity and specificity. These molecules can assume a variety of shapes due to their propensity to form helices and single-stranded loops, explaining their versatility in binding to diverse targets. They are used as sensors, and therapeutic tools, and to regulate cellular processes, as well as to guide drugs to their specific cellular targets [29-30]. Contrary to the actual genetic material, their specificity and characteristics are not directly determined by their primary sequence, but instead by their tertiary structure. Interaction between the aptamer and the target is inclusive of the following, structure compatibility, stacking of aromatic rings, electrostatic and van der Waals interactions, hydrogen bonding, or a combination of all these effects. Because of the unique character of the aptamers they can be treated as useful alternative to a ntibodies as sensing elements [31]. Synthesis of Aptamers can easily be done chemically, and it does not require any complicated and expensive purification steps, resulting in elimination of the batch-to-batch variation found while using antibodies. Moreover, aptamers can undergo further modification through chemical synthesis for the purpose of enhancing the stability, affinity, and specificity of the molecules, also aptamers are more stable, and more resistant to denaturation and degradation than antibodies [32] . DNA/RNA aptamers are mainly designed for POPs, EDCs, organophosphorus pesticides biotoxins, and pathogenic microorganisms. Aptamers have become increasingly important bioassay materials for the purpose of environmental detection aptamers can play a vital role as bioassay material . According to a research literature reusable evanescent wave aptamer-based biosensor was developed for rapid, sensitive and highly selective detection of 17- b -estradiol, a natural endocrine based disrupting compound (EDC) having very high estrogenic activity. The b -Estradiol 6-(O-carboxymethyl)oxime-BSA was covalently immobilized into the surface of a optical fiber sensor. The dose-response curve of 17- b - estradiol was then developed with in a detection limit of 2.1 nM [33-39].

High specificity and selectivity of the optical biosensor were being exhibited by examining its response to a number of interfering endocrine-disrupting compounds. The Potential interference of real environmental sample matrices was examined using spiked samples in several wastewater effluents. The particular system described above can be used for on-site and real-time monitoring of 17- b -estradiol in wastewater treatment effluents or water bodies. There are various DNA aptamer fluorescence-based bio sensors which have been designed for the purpose of detection of Hg2+, Pb2+, and other trace of pollutants [46-49]. Kim et al. designed a DNA aptamer for detection of arsenic that can bind to arsenate [(As(V)] and arsenite [As(III)] with a dissociation constant of 4 and 8 nM, respectively . The use of the aptamer, a resonance scattering (RS)-based biosensor for the ultrasensitive detection of As(III) in aqueous solution via aggregation of gold nanoparticles (AuNPs) by the special interactions between arsenic-binding aptamer, target and cationic surfactant was being developed. The results shows that the variations of absorbance and RS intensity were exponentially related to the concentration of As(III) in the range from 1 to 1450 ppb, with the detection limit of 0.5 ppb for colorimetric assay and 0.78 ppb for RS assay [38].

There was another report on aptamer-based fluorescent biosensor for the highly selective and sensitive detection of Pb2+ and Hg2+ using a G-rich ssDNAs , which was labeled with the donor FAM at one end and the quencher DABCYL at the other end. The mentioned aptamer is in random-coil structure that gets converted into a G-quartet structure and a hairpin-like structure with the binding of Pb2+ and Hg2+, resulting moving of the fluorophore closer to the quencher and resulting in the decrease of the intensity of the fluorescence. The limits of detection of Pb2+ and Hg2+ ions are in the range of 0.2 nM and 4.0 nM, respectively. Though a variety of aptamers are under test for the detection of contaminants in real water samples [21].

2.5. The Cells as Bio-Recognition Element

The Whole cells are the best indicators of toxic components. Various microbial-based optical biosensors have been designed to detect toxicity and contaminants by monitoring the bio luminesce of light production or fluorescence . Olaniran et al. designed a whole- cell based bacterial biosensors for the fast and effective monitoring of heavy metals and inorganic contaminant in water . By the Use of Escherichia coli, the optical biosensors were found to be highly sensitive towards toxicity of wastewater effluents. The Bioluminescence increases with the increase in concentration of heavy metals and inorganic contaminants in water with a correlation coefficient of (r2) as high as 0.875 and 0.921, respectively. These bacterial biosensors can achieve the fast, sensitive and cost effective detection of contaminants in water [22-26].

Another research reported that an integrated fluorescence-based sensor for pH and oxygen [54], in which bacterial respiration was observed with the decrease in the oxygen partial pressure of the closed system and also with the decrease in pH value. Then the detection of inhibitory effect of toxic metal ions on the cellular operation of E. coli and Pseudomonas putida was achived. Amaro et al. reported a whole-cell biosensor for the detection of heavy metals based on metallothionein promoters from Tetrahymena thermophila . Two gene constructed with the help of Tetrahymena thermophila MTT1 and MTT5 metallothionein promoters linked with the eukaryotic luciferase gene, was being considerd as a reporter. The particular biosensor appeared to be the most sensitive eukaryotic metal biosensor among all other reported cell biosensors.The use of bioluminescent caused the bacteria immobilization in an alginate matrix on the bottom of the wells in a 87-well microplate, Eltzov et al. designed a fiber-optic biosensor for monitoring the toxicity of air . The Bioluminescence was being suppressed when the biosensor was exposed to toxic elements present in air Chloroform and that could be detected by this method with a LOD of 5.8 ppb [18].

3. Use of Nanomaterials in Bio sensing purpose

Nanomaterials exhibit unique size-tunable and shape-dependent physicochemical properties and have numerous possible applications in biosensors. The integration of nanomaterials and functional biological molecules (e.g., antibodies, nucleic acids, peptides) opens a new era in the optical biosensor field.

3.1. Use of Quantum Dots in Bio sensing purpose

Quantum dots (QDs), light-emitting semiconductor nanorystals, have been increasing used as biomolecular detection tools because of their unique optical properties, which conferred advantages over traditional fluorophores such as organic dyes. QDs have found applications ranging frombioanalytical assays, to live cell imaging, fixed cell and tissue labeling, and biosensors. The narrow, size-tuned, and symmetric emission spectra of QDs have made them excellent donors for fluorescence resonance energy transfer (FRET) sensors. Moreover, the overlap between the emission spectra of the donor and acceptor is reduced, and the cross-talk in such FRET pairs is circumvented . The broad excitation spectra of QDs facilitate excitation at a single wavelength far removed (>100 nm) from strategy employing water-soluble long lifetime fluorescence quantum dots and GNPs was used to detect trace Hg2+ ions in aqueous solution. The sensing system exhibits the detection limits their respective emissions, allowing QDs to be used in multiplex assays with single excitation sources. Using covalent or non-covalent linking approaches, the surface modification of QDs with antibodies, aptamers, and peptides are the most developed and widespread detection bioprobes. The long-term photostability, superior brightness, and good chemical stability of QDs enable them to greatly improve bioassay sensitivities and limits [61-63].However, controlling the number of antibodies (or aptamers) per QD as well as their orientation and position relative to the QD is difficult. Given the possibility of the inadvertent disruption of the binding site when QD conjugates with the antibody, the activity loss of the antibody is inevitable [60,64]. Additionally, antibodies usually need to be cryopreserved, but QDs cannot be frozen, thus making the storage of the QD-antibody a major obstacle to its practical application A carrier-protein-hapten-coupled QD nanobioprobe protocol has been developed to perform rapid and sensitive detection of small targets in environmental samples [31-34] .

The determination of 2,4-D in aqueous media was performed by grafting haptens-BSA conjugate on QDs and using the resulting material as a nano-bioprobe for 2,4-D biosensing. Samples containing different concentrations of 2,4-D were mixed with a given concentration of QD immunoprobe and fluorescence-labeled antibody, after which they were competitively detected by the all-fiber microfluidic biosensing platform. A higher concentration of 2,4-D resulted in less fluorescence-labeled anti-2,4-D antibody bound to the QD immunoprobe surface and consequently, lower fluorescence signal . The quantification of 2,4-D over concentration ranges from 0.5 nM to 3 M with a LOD of 0.5 nM. The method combined the merits of specific and stable binding interactions between environmental pollutants and its specific antibody, as well asthe excellent photophysical properties of QDs. The proposed immunosensor had the following unique advantages: first, QD-BSA- haptens conjugates used as recognition elements prevent compromise among the binding properties of the immobilized biomolecules (e.g., antibodies and enzymes); second, the binding sites of QD-BSA-haptens avoided steric hindrance and retained their high activity for their antibody; third, the structure of the QD-BSA-haptens conjugate was more stable in complex environmental samples than typical biorecognition molecules (e.g., antibody and enzymes). The FRET efficiency was higher because of the more abundant acceptor dyes bound to one QD surface, which conferred the QD-FRET assay with high sensitivity. This will provide a universal approach using a QD-bioconjugate as a nano-bioprobe to construct practical FRET-based immunoassay of various small molecules and other applications [32-33].

3.2. Use of Gold Nanoparticles in Bio sensing purpose

Gold nanoparticles (GNPs) with controlled geometrical, optical, and surface chemical properties have great potential applications in environmental and medical detection. GNPs can be easily modified with biomolecules. GNP-based optical biosensors commonly utilize fluorescence quenching through FRET or a visible color change attributed to the aggregation of AuNPs of appropriate sizes . GNP-based optical sensors have been used to detect environmental pollutants including heavy metals, toxins, and other pollutants . Heavy metal contaminations have greatly attracted public attention worldwide because of their serious negative health effects. Liu et al. used quaternary ammonium-functionalized GNPs to devise a colorimetric sensor for Hg2+ detection with the abstraction of GNP stabilizing thiols by Hg2+ inducing aggregation. An AuNP-rhodamine 6G-based fluorescent sensor was used to detect Hg2+, which had a LOD of 0.012 ppb . A T-Hg2+-T structure was used to develop a detection method of aqueous Hg2+ with a LOD of 50 nM , in which specific interaction of Hg2+ with thymine residues from two AuNPs induces the aggregation process and corresponding color change. Hg2+ and Ag+ could simultaneously be detected using FRET. However, this method was insufficiently sensitive for Hg2+ or Ag+ ion detection. Darbha et al. developed a AuNP-based sensor for the rapid, easy, and reliable detection of Hg2+ ions in aqueous solutions [71], which, through non-linear optical properties, had a LOD of 5 ppb (ng/mL). QDs have been utilized for FRET-based AuNPs assays for detection of environmental pollutants. An inhibition assay for identification of Pb2+ was developed based on the modulation in FRET efficiency between QDs and GNPs with a detection limit of 30 ppb of Pb2+ . The positively charged QDs form FRET donoracceptor assemblies with negatively charged GNPs by electrostatic interaction. The presence of Pb2+ aggregates AuNPs via an ion-templated chelation and inhibits the FRET Process [37-41]. A time-gated fluorescence resonance energy transfer (TGFRET) sensing of 0.49 nM in buffer and 0.87 nM in tap water samples .

3.3. Use of Graphene and Graphene Oxide in Bio sensing purpose

Fluorescent graphene-based materials have attracted great interest in the recent years due to their excellent chemical inertness, biocompatibility and low toxicity which makes them an appropriate choice for the detection of special targets. The range of graphene-based FRET biosensors' targets extends from DNA to ions, molecules and proteins through the integration of the functional biomolecules. Traditionally, the chemically derived graphene oxide (GO) has served as a predecessor for graphene but has drawn attention of many researchers for its own unique characteristics. The intrinsic and tunable fluorescence of GO can pioneer interesting and novel optical applications. A fluorescence sensor was reported for the detection of Ag(I) ions based on the target-induced conformational change of a silver-specific cytosine-rich oligonucleotide (SSO) and the interactions between the fluorogenic SSO probe and graphene oxide . Lee et al. used a platform based on chemiluminescence resonance energy transfer (CRET) between graphene nanosheets and chemiluminescent donors for homogeneous immunoassay of C-reactive protein (CRP). This 4. graphene-based CRET platform has a LOD of 1.6 ng/mL [41-43].

Liu et al. developed a homogeneous competitive fluorescence-based immunoassay for rapid and sensitive detection of microcystin-LR (MC-LR) based on the assembly of colloidal grapheme and MC-LR-DNA conjugates [78]. The MC-LR-DNA fluorescence probe was quickly adsorbed onto the graphene surface through the strong noncovalent - stacking interactions and can be effectively quenched through FRET. The competitive binding of anti-MC-LR antibody with MC-LR-DNA destroyed the graphene/MC-LR-DNA interaction, thus resulting in the restoration of fluorescence [43-44].


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Insights of Current Developments in Optics-Based-Biosensors for Analysis of Environmental Contamination and Pollution
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Anupam Das (Author)Ie Mei Bhattacharyya (Author), 2015, Insights of Current Developments in Optics-Based-Biosensors for Analysis of Environmental Contamination and Pollution, Munich, GRIN Verlag,


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