Application of conducting polymers to biosensors

Abstract

Recently, conducting polymers have attracted much interest in the development of biosensors. The electrically conducting polymers are known to possess numerous features, which allow them to act as excellent materials for immobilization of biomolecules and rapid electron transfer for the fabrication of efficient biosensors. In the present review an attempt has been made to describe the salient features of conducting polymers and their wide applications in health care, food industries, environmental monitoring etc.

Introduction

Polymers are being discarded for their traditional roles as electric insulators to literally take charge as conductors with a range of novel applications. Scientists from many disciplines are now combining expertise to study organic solids that exhibit remarkable conducting properties. A large number of organic compounds, which effectively transport charge are roughly divided into three groups i.e. charge transfer complexes/ion radical salts, organometallic species and conjugated organic polymers. A new class of polymers known as intrinsically conducting polymers or electroactive conjugated polymers has recently emerged. Such materials exhibit interesting electrical and optical properties previously found only in inorganic systems. Electronically conducting polymers differ from all the familiar inorganic crystalline semiconductors e.g. silicon in two important features that polymers are molecular in nature and lack long range order (Duke and Schein, 1980). A key requirement for a polymer to become intrinsically electrically conducting is that there should be an overlap of molecular orbitals to allow the formation of delocalized molecular wave function. Besides this, molecular orbitals must be partially filled so that there is a free movement of electrons throughout the lattice (Bloor and Movaghar, 1983).

Conducting polymers contain (π-electron backbone responsible for their unusual electronic properties such as electrical conductivity, low energy optical transitions, low ionization potential and high electron affinity. This extended (π-conjugated system of the conducting polymers have single and double bonds alternating along the polymer chain. The higher values of the electrical conductivity obtained in such organic polymers have led to the name ‘synthetic metals’. Many applications of conducting polymers including analytical chemistry and biosensing devices have been reviewed by various researchers (Trojanowicz and vel Krawczyk, 1995, Situmorang et al., 1998, Schuhmann, 1995, Wring and Hart, 1992, Guiseppi-Elie et al., 1997). They have widened the possibility of modification of surface of conventional electrodes providing new and interesting properties. They were applied in electrocatalysis, membrane separations and chromatography. They also create new technological possibilities in design of chemical and biochemical sensors (Trojanowicz and vel Krawczyk, 1995, Bidan, 1992, Bartlett and Cooper, 1993).

Research on conducting polymers intensified soon after the discovery of poly(sulphur nitride) [(SN)x] in 1975 which becomes superconducting at low temperatures (Greene et al., 1975). Although, conducting polymer complexes in the form of tetracyano and tetraoxalato-platinates, the Krogman salts charge transfer complexes (Minot and Peristein, 1971) had been known earlier, the significance lies in the rediscovery of PA in 1977 (initially discovered by Shirakawa et al., 1977 using a Ziegler Natta type polymerization catalyst) by MacDiarmid and Heeger, University of Pennsylvania. They were able to enhance the electrical conductivity of PA (10⁻⁹ S cm⁻¹) 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. This has generated renewed interest of the scientific community towards the study and discovery of new conducting polymeric systems.

Poly-paraphenylene was synthesized by Ivory et al. (1979). It forms highly conducting charge transfer complexes with both n and p type dopants. Doping with AsF₅ increases its conductivity to its values from 10⁻⁵ to 500 cm⁻¹. Theoretical models and electron spin resonance measurements indicate that the charge transport in PPP is a polaron/bipolaron. PPS was the first non-rigid, but not fully carbon backbone linked conducting polymer. Its discovery was particularly exciting, since its property of solution processability opened the door for potentially obtaining commercially viable conducting plastics (Rabolt et al., 1980).

Amongst polyheterocyclines, polypyrrole (PPY) has been investigated the most. The electrochemical oxidation of pyrrole in aqueous H₂SO₄ can be carried out on platinum electrode. The product is a conducting polymer known as ‘Pyrrole Black’ Kanazawa et al. (1979) produced coherent films of PPY with a conductivity of 100 Scm⁻¹ and exhibited excellent air stability. But the main hindrance of its processability is in its insolubility in any organic solvents.

PTH shows remarkable stability of both oxidized (p-doped) conducting form and its neutral (undoped) insulating form in both air and water. It shows high doping level upto 50% which may be attributed to its partially crystalline nature that has been confirmed by X-ray photoelectron spectroscopy studies (Tourillon and Garnier, 1983). Many other conducting polymers such as polyfuran, polyindole, polycarbazole, polyaniline etc. have also been synthesized. Structures of some typical conducting polymers have been shown in Fig. 1

The mechanism of conduction in such polymers is very complex since such a material exhibits conductivity across a range of about fifteen orders of magnitude and many involve different mechanisms within different regimes. Conducting polymers show enhanced electrical conductivity by several orders of magnitude of doping. The concept of solitons, polarons and bipolarons has been used to explain the electronic phenomena in these systems (Heeger, 1986). Conductivity in conducting polymers is influenced by a variety of factors including polaron length, the conjugation length, the overall chain length and by the charge transfer to adjacent molecules (Kroschwitz, 1988). These are explained by large number of models based on intersoliton hopping, hopping between localized states assisted by lattice vibrations, intra-chain hopping of bipolarons, variable range hopping in 3-dimensions and charging energy limited tunneling between conducting domains.

Various methods are available for the synthesis of conducting polymers. However, the most widely used technique is the oxidative coupling involving the oxidation of monomers to form a cation radical followed by coupling to form di-cations and the repetition leads to the polymer. Electrochemical synthesis is rapidly becoming the preferred general method for preparing electrically conducting polymers because of its simplicity and reproducibility. The advantage of electrochemical polymerization is that the reactions can be carried out at room temperature. By varying either the potential or current with time the thickness of the film can be controlled.

Electrochemical polymerization of conducting polymers is generally employed by: (1) constant current or galvanostatic; (2) constant potential or potentiostatic; (3) potential scanning/cycling or sweeping methods. Standard electrochemical technique which employs a divided cell containing a working electrode, a counter electrode and a reference electrode generally produces the best films. The commonly used anodes are chromium, gold, nickel, palladium, titanium, platinum and indium-tin oxide coated glass plates. Semi-conducting materials such as n-doped silicon (Noufi et al., 1981), gallium arsenide (Noufi et al., 1980), cadmium sulphide and semi-metal graphite (Bull et al., 1983) are also used for the growth of polymer films. Electrochemical synthesis can be used to prepare free standing, homogeneous and self doped films. Besides this, it is possible to obtain copolymers and graft copolymers. Polyazulene, polythiophene, polyaniline, polycarbazole and several other polymers have been synthesized using this approach.

The increasing number of academic, governmental and industrial laboratories throughout the world involved in basic research and assessment of possible applications of conducting polymers show that this area is interdisciplinary in nature. These conducting organic molecular electronic materials have attracted much attention largely because of their many projected applications in solar cells, light weight batteries, electrochromic devices, sensors and molecular electronic devices.

Polymeric heterojunctions, solar cells have been fabricated by electrochemical deposition of PPY on n-silicon (Audebert and Bidan, 1985). Many conducting polymers such as polyacetylene, polythiophene, polyindole, polypyrrole, polyaniline etc. have been reported as electrode materials for rechargeable batteries (Saraswathi et al., 1999, Kawai et al., 1990, Santhanam and Gupta, 1993). It has been reported (Gazard et al., 1986) that the conducting polyheterocycles are good candidates for electrochromic displays and thermal smart windows. Scientists have used PPY films in a neurotransmitter as a drug release system into the brain (Zinger and Miller, 1984). The potential for conducting polymers in the area of electronics and photonics (non-linear optics) is enormous and has been used to fabricate diodes, capacitors, field-effect transistors (FET) and printed circuit boards. Polyaniline PANI is being used by Hitachi-Maxell for anti-static coating of 4 MB barium ferrite floppy disk (Friend, 1993).

In analytical chemistry the problem of selectivity, particularly at the low analyte concentrations and in the presence of interfering substances is of paramount importance. The development of sensors, which are highly selective and easy to handle opens the door to the problem in analysis. Conducting polymers have enough scope for the development of various sensors. A chemical or biosensor based on conducting polymers, also rely on sensible changes in the optical and electrical properties of these materials. The industrial applications of biochemical and morphological processes in fields such as production of pharmaceuticals, food manufacturing, waste water treatment and energy production is on increase. This has led to the development of biosensors. Biosensors have found promising applications in various fields such as biotechnology, food and agriculture product processing, health care, medicine and pollution monitoring.