An Introduction to Molecular Recognition Approach. Tool for Environmental Analysis

Introduction

In the past few decades, an increase in the number of various chemical species (gases, anions, cations, organic and inorganic compounds) has resulted in the degradation of the natural environment.1-3 Many chemical species in different environmental sub-systems are responsible for various detrimental effects, including degradation of water and air.4-7 These detrimental effects continue to accelerate due to urbanization and fossil fuel derived energy generation.8-10 Thus, there is a growing need to monitor the environment and the sources of contaminants to the environment, in order to control pollution and prevent its rise in the future.11-13

Development of new and convenient approaches to monitor environmental pollution and understanding of environmental processes is imperative.14-16 In this direction, advances in method development for environmentally significant analytes is vital.17-19 Molecular recognition presents one such advancement via analytical tools in the form of molecular receptors.20-26 Such approaches provide rapid and naked-eye based signaling response. Most importantly, they are easy to use and provide a cost-effective means of analysis.27-29

Molecular recognition and receptor approach

Molecular recognition events yield information about environmental analytes at molecular level through specific binding of substrate by a molecular receptor. Such binding is regulated by geometrical and electronic complementarity between receptor and analyte.30-33 The receptor can be defined as a molecular entity of abiotic origin that interacts with the analyte (anion, cation or a neutral molecule) and offers a unique response (Figure 1), with a simultaneous signal transduction in the form of photophysical or redox signaling.

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Figure 1 Presentation of molecular receptor approach.

Usually, molecular recognition events employ two different strategies to identify molecules, depending upon the mode of binding between the receptor (usually a big molecule) and the analyte (usually a minute structure).34 These two strategies have been categorized as chemosensing and chemodosimetry. On one hand, chemosensors utilize interaction of target analyte with the receptor through a non-covalent interaction in order to yield a measurable optical signal with real-time response, usually within seconds. Such basis ofürecognition offers them with the advantages ofüreusability or recyclability. On the other hand, chemodosimetry involves making and breaking of bonds, and events are generally slow and irreversible.35

Chemosensensing

Chemosensing utilizes reversible interactions via a binding-site signaling sub-unit or through displacement approach.36 In binding-site signaling approach, a linker (usually aliphatic chain) separates signaling unit (chromophore or a fluorophore) from the target site of the analyte.36-39 This can further be tailored by making receptor site itself a part of signaling unit, so as to generate effective read-out behavior and hence achieve high sensitivity (Figure 2). The approach operates through hydrogen-bond donation from receptor to analyte or proton transfer signaling. In addition to this, such receptors also involve halogen bonding, pi -interactions and hydrophobic interactions.40

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Figure 2 Analyte (A), approaching towards the receptor site, in binding site-signalling approach.

In the binding site-signaling approach, Liu et al. reported ratiometric fluoride anion recognition via a proton transfer signaling (PTS) with 4-benzoylamido-N-butyl-1, 8-napthalimide receptor 1.1 (Figure 3).41 Interaction of fluoride atäthe receptor site concentrated negative charge on the amide nitrogen and caused a red-shift in the absorption spectra. Similarly, measurable changes were observed in the emission properties of the fluorophore. These events were observed in the form of striking colourless to yellow and blue to orange emission signals. Similarly, Mukhopadhyay et al. produced panchromatic recognition for CN- anion and (Cu2+/Fe2+) cations by electron transfer between analyte and electron deficient naphthalene diimides 1.2 (Figure 3).42 The existence of persistent anion as well as cation radicals were evaluated through absorption (UV-Vis-NIR) and emission spectroscopy. The corresponding recognition events were noticed via both chromogenic display as well as fluorogenic quenching in the solution.

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Figure 3 Molecular recognition through binding site-signalling approach. 1.1 receptor demonstrates proton transfer signalling (PTS) for fluoride, while as 1. 2 involves electron transfer phenomenon.

In the displacement approach both the reporter as well as binding site are free from any covalent linkage, and they become an ensemble through formation of a coordination complex (Figure 4).43 The approach usually involves competition between two differentätypes of guests for the ensemble formation. Here, initially existing complex upon impact of target species, dislodges and forms a new complex with the incoming guest. The corresponding displacement reaction is marked by signal transduction. The most common interactions between receptor and analyte involve hydrogen bonding, electrostatic forces, and coordination bond with metal centers or electron transfer phenomenon. The interaction is further controlled by the geometry of the guest, charge, hydrophobicity and solvent system under operation. In displacement approach, Fabbrizzi et al. reported an ensemble of copper (II) and coumarin fluorophore (F) 1.3 (Figure 4), for HCO3- recognition. The complexation initially shows fluorescence quenching. However presence of analyte with Y-shaped bite (carbonate, acetate) replaces the fluorophore and in turn promotes fluorescence enhancement for detection of analyte in aqueous media.44

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Figure 4 Molecular recognition through displacement approach. Here A refers to target analyte and F the initially bound guest.

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