Prospects of conducting polymers in biosensors

Abstract

Applications of conducting polymers to biosensors have recently aroused much interest. This is because these molecular electronic materials offer control of different parameters such as polymer layer thickness, electrical properties and bio-reagent loading, etc. Moreover, conducting polymer based biosensors are likely to cater to the pressing requirements such as biocompatibility, possibility of in vivo sensing, continuous monitoring of drugs or metabolites, multi-parametric assays, miniaturization and high information density. This paper deals with the emerging trends in conducting polymer based biosensors during the last about 5 years.

Introduction

Biosensors have recently attracted much interest. This is because these interesting bio-devices have been shown to have applications in clinical diagnostics, environmental monitoring, food freshness and bioprocess monitoring [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95], [96], [97], [98], [99], [100], [101], [102], [103], [104], [105], [106], [107], [108], [109], [110], [111], [112], [113], [114], [115], [116], [117], [118], [119], [120], [121], [122], [123], [124], [125], [126], [127], [128], [129], [130], [131], [132], [133], [134], [135], [136], [137], [138], [139], [140], [141], [142], [143], [144], [145], [146], [147], [148], [149], [150], [151], [152], [153], [154], [155], [156], [157], [158], [159], [160], [161], [162], [163], [164], [165], [166], [167], [168], [169], [170], [171], [172], [173], [174], [175], [176], [177], [178], [179], [180], [181], [182], [183], [184], [185], [186], [187], [188], [189], [190], [191], [192], [193], [194], [195], [196], [197], [198], [199], [200], [201], [202], [203], [204], [205], [206], [207], [208], [209], [210], [211], [212], [213], [214], [215], [216], [217], [218], [219], [220], [221], [222], [223], [224], [225], [226], [227], [228], [229], [230], [231], [232], [233], [234], [235], [236], [237], [238], [239], [240], [241], [242], [243], [244], [245], [246], [247], [248], [249]. A number of materials such as polymers, sol–gels and conducting polymers have been used to improve the stability of the biomolecules used in the fabrication of the desired biosensors. In this context, polymers have become the materials of choice for recent technological advances in biotechnology. Initially polymers were thought to be insulators and were utilized in various electrical and electronic devices. The discovery of poly (sulphur nitride) [(SN)_x] which becomes superconducting at low temperatures led to the renewed interest in polymers [10]. The electrical conductivity of [(SN)_x] can be enhanced by several orders, i.e. 10⁵ S cm⁻¹ by simple doping with oxidizing agents, e.g. I₂, AsF₅, NOPF₆ (p-doping) or reducing agents (n-doping), e.g. sodium napthalide. Subsequently, a new class of polymers such as poly-para-phenylene (PPP), polyphenylene sulphide (PPS), polythiophenes and polypyrroles (PPy) were reported [11], [12], [13]. Diaz et al. produced coherent films of PPy with conductivity of 100 S cm⁻¹ and this conducting polymer exhibits excellent air stability [14]. Chen et al. have estimated the concentration of cytochrome C using electrochemically prepared conducting polymers based on ferrocene substituted thiophene and terthiophene [44]. Many other conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT), polyfuran, polyindole, polycarbazole, polyaniline, etc. have been synthesized and studied extensively [12], [13], [14], [15], [16], [45].

Compared to saturated polymers, conducting polymers have different electronic structures. Chemical bonding in conducting polymers provides one unpaired electron, i.e. π electron per carbon atom in the backbone of the polymer. Carbon atoms are in sp²p_z configuration in π bonding and orbitals of successive carbon atoms overlap providing delocalization of electrons along the backbone of polymer [12]. This delocalization provides the charge mobility along the backbone of the polymer chain and induces unusual properties such as electrical conductivity, low ionization potential, low energy optical transitions and high electron affinity. The π bonds in conjugated polymers are highly susceptible to chemical or electrochemical oxidation or reduction. The origin of electrical conduction in conducting polymers has been ascribed to the formation of non-linear defects such as solitons, polarons or bipolarons formed either during doping or polymerization of a monomer [19], [46], [47], [57]. The conductive and semiconducting properties of these polymers make them an important class of materials for a wide range of electronic, optoelectronic and biotechnological applications such as in rechargeable batteries, molecular electronics, electronic displays, solar cells, ion exchange membrane in fuel cells, diodes, capacitors, field-effect-transistors, printed circuit boards, chemical sensors, drug release systems and biosensors, etc. [14], [15], [16], [17]. It is being projected that conducting polymers can be used to transport small electronic signals in the body, i.e. act as artificial nerves. Perhaps modifications to the brain may eventually be contemplated. Scientists have used films in a neurotransmitter as a drug release system into the brain [18].

Conducting polymers have emerged as potential candidates for biosensors. Gerard et al. have reviewed the literature on applications of conducting polymers to biosensors [57]. Geetha et al. [77] have discussed the applications of conducting polypyrrole to drug delivery. Andreescu and Sadik have reviewed the challenges and trends in biosensors for environmental and clinical monitoring [78]. Cosnier [79], [80] has discussed the analytical applications of affinity biosensors based on electropolymerized films. Ramaniviciene and Ramanavicius have reported an interesting overview on the potential use of conducting polymers as electrochemical based affinity biosensors [81]. Malinauskas et al. have reviewed the electrochemical aspects of conducting polymer-based nano-structured materials for application to super-capacitors, energy conversion systems, batteries and sensors [82]. Wanekaya et al. have reviewed recent advances in biosensors based on one-dimensional (1-D) nanostructures [83]. Ming-Hung Lee et al. have highlighted the current developments of DNA-based bioanalytical microsystesm for point-of-care diagnostics [84]. Adhikari and Majumdar have discussed the role of non-conducting and intrinsically conducting polymers in sensor devices [56]. Terry et al. [86] have assessed the future and current trends of biosensors in food industry. Drummond et al. have discussed numerous approaches to electrochemical detection based on modified electrodes, electrochemical amplifications with nanoparticles and electrochemical devices using DNA-mediated charge transport chemistry and electrochemistry of DNA-specific redox reporters [87]. Habermuller et al. have reported on the various electron-transfer mechanisms operating in amperometric biosensors [88]. Kerman et al. have predicted that electrochemical DNA biosensors with suitable microfabrication techniques are likely to be increasingly popular in the near future [89]. The present paper focuses on the prospective applications of conducting polymers in biosensors.