Conductive Polymers - Applications for Electronic Devices and Sensors

Conductive polymers have seen great advancement during the last two decades. In general a half filled valence band, formed from a continuous delocalized π –system, is an ideal condition for conduction of electricity. However, a π –conjugated polymer can lower its energy more efficiently by bond alteration (alternating single and double bonds) and can introduce a band width of 1.5 eV making it a high energy gap semiconductor.

Conducting polymers are extensively studied due to their remarkable electronic properties such as electrical conductivity, low energy optical transitions, low ionization potential, and high electron affinity. Their fundamental spectroelectrochemistry, and the applications to solar cells, biosensors, and biofuelcells have been extensively explored by our laboratory during the last decade. Some important contributions are highlighted below.

Fundamental Spectroelectrochemistry of Conductive Polymers

The spectrochemical properties of conducting polymers provide information about their electronic properties and this helps in the construction of conducting polymer biosensors. One of example is a newly synthesized 1-(3-pyridinyl)-2,5-di (2-thienyl)-1H-pyrrole (PTPy) and 1-(1,10-phenanthrolinyl)-2,5-di(2-thienyl)-1H-pyrrole (PhenTPy) bearing 2,5-di(2-thienyl)-1H-pyrrole derivatives monomer that is electrochemically polymerized to conductive PTPy and PhenTPy from a solution of dichloromethane in our laboratory (see fig. 1).

The spectroelectrochemical data for the poly-PTPy clearly distinguished the neutral, polaron states and bipolaron states of the absorption bands at 430 nm, 520 nm, and 836 nm, respectively. In the case of PhenTPy, the absorption bands at 451, 544, and 836 nm correspond to the neutral, polaron, and bipolaron formations, respectively. The band gap energies of poly-PTPy and poly-PhenTPy were 2.1 and 1.85 eV, respectively, and therefore, can be used in electron transporting materials.

The poly-PTPy and poly-PhenTPy film exhibits a good switching time (about 1.0 s) and good stability. Based on these properties, poly-PTPy and poly-PhenTPy are promising candidates for both electrochromic devices and the construction of conducting polymer based biosensors [1]. The electrochemical and spectroelectrochemical characterizations of these films reveal that the conducting polymers are highly electroactive and robust in terms of electrochromics.

Fig. 1. (A) The synthetic route to PTPy and PhenTPy. (B-a) UV-vis absorption and (B-b) photoluminescence (PL) spectra of PTPy and PhenTPy in dichloromethane solution.

Conductive Polymers in Solar Cells

Silicon-based solar cells are commercially available due to their high solar-to-electric energy conversion efficiency. However, several disadvantages, such as, heavy weight, high cost, and lack of flexibility necessitated the development of organic photovoltaic cells and dye-sensitized solar cells (DSSCs) [2]. DSSCs have a maximum energy conversion efficiency of ∼10% and hence, conducting polymers are used as electron transfer mediators or photoreceptors to increase the efficiency. Poly(terthiophene) derivative-sensitized solar cells have been successfully fabricated in our laboratory. Conducting polymer layers were formed on the TiO₂ electrode by electropolymerization of the monomers in a 0.1M tetrabutylammonium perchlorate/CH₂Cl₂ solution using the potential cycling method. The band gap energies of poly 5,2’:5’,2”-terthiophene-3’-carboxylic acid (poly-TTCA), poly 3’-cyano-5,2’:5’,2”-terthiophene (poly –CTT), and poly 3’,4-diamino-2,2’:5’,2”-terthiophene (poly-DATT) are in the range of 1.93-2.10 eV. The band gap energies are similar, however, the carboxylic group containing poly-TTCA exhibits a higher energy conversion efficiency than that of poly-CTT and poly-DATT. In the case of poly-TTCA, the anchoring group, -COOH enhances the adsorption of the dye onto the TiO₂ layer effectively and strongly through the formation of C-O-Ti bonds, which improve the transfer of the electrons from poly-TTCA to TiO₂. Furthermore, poly-TTCA exhibits relatively fast electron transfer at theTiO₂/poly-TTCA/electrolyte interface, which leads to a greater energy conversion efficiency. The poly(terthiophene) derivative bearing carboxylic acid groups are the most efficient photosensitizer, and it is a possible alternative material to the Ruthenium complexes. The maximum energy conversion efficiency of the poly-TTCA solar cell is 2.3% under an AM 1.5 solar simulated light irradiation of 100mW.cm⁻² [3].

Recently poly-TTBA has been synthesized by introduction of a benzene ring at the para position into poly-TTCA which enhances the energy conversation efficiency of polymer dye solar cell by about 3.9% [4]. The LUMO orbital of TTBA, electrons move towards the acceptor moiety from therthiophene moiety through a benzene ring compared with TTCA, which is located at the junction between terthiophene and acceptor moiety (see fig. 2). This means that the modification of the polymer structure to be efficient for the electron transfer with small steric hinderance and enhanced conjugation structure.

Fig. 2. (A) Schematic of a solar cell composed of the pTTBA dye polymerized on TiO₂ and I₃^-/I^- electrolyte. (B) Frontier molecular orbital of the HOMO and LUMO levels of (a) TTBA, and (b) TTCA

Conductive Polymers in Biosensors

Biosensors based on conductive polymers have been used to detect an inducible nitric oxide synthase [5], peroxynitrite [6], superoxide [7], NADH [8], thrombin [9], DNA [10, 11], glutamate [12], heavy metal ions [13], etc (see fig. 3). For example, nitric oxide (NO) or superoxide can be detected with a microbiosensor based on an enzyme or a heme protein and phospolipids alternatively immobilized onto a functionalized-conducting polymer. 5,2′:5,2″-terthiophene-3′-carboxylic acid (poly-TTCA) provides a biomembrane environment for the in vivo measurement of NO or superoxide release stimulated by an abuse drug cocaine. A nitric oxide (NO) microbiosensor based on cytochrome c (cyt c), a heme protein immobilized onto a functionalized-conducting polymer (poly-TTCA) layer has been fabricated for the in vivo measurement of NO release stimulated by an abuse drug cocaine. Based on the direct electron transfer of cyt c, determination of NO with the cyt c-bonded poly-TTCA electrode was studied using cyclic voltammetry and chronoamperometry. The interference of the sensor by NO by foreign species such as oxygen and hydrogen peroxide were minimized by covering a nafion film on the modified electrode surface. The concentrations of NO levels fluctuated by acute and repeated injections of cocaine were determined. The high sensitivity of the microbiosensor in monitoring NO concentrations in the in vivo intact brain of rats indicates that this system is very useful in clinical research and particularly in monitoring the effect of abuse drugs [14].

Fig. 3. The example of DNA and protein sensor probe with conductive polymer

The complexation between polyTTCA and Mn(II) through the formation of the Mn-O bond was used as a biosensor specifically to detect bilirubin through the mediated electron transfer by the Mn(II) ion. The bilirubin sensor exhibited good stability and fast response time (<5 s). The applicability of this sensor has been demonstrated to monitor bilirubin in a human serum sample [15]. Conductive polymers are also known for their ability to be compatible with biological molecules and cells, thus allowing their applicability in in vitro and in vivo biosensor fabrication. Moreover, the polymer itself can be modified to bind biomolecules to a biosensor probe. This aspect has been demonstrated by us in the studies on detection of NADH. A biomimetic layer composed of cyt c, lipids [1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) and cardiolipin], and ubiquinone was utilized for the detection of NADH. For this purpose, firstly DOPE is chemically bonded onto the conducting polymer film. The developed system selectively detects NADPH into the HT-29 cell lines [8].

This study is very useful in understanding the critical role of NADH in the multiple biological processes, including energy metabolism, mitochondrial function, biosynthesis, gene expression, calcium homeostasis, cell death, aging, and carcinogenesis.

Similarly, an in vivo microbiosensor is designed for the detection of glutamate released by cocaine stimulation in the extracellular fluids by using glutamate oxidase (GlOx) immobilized onto the conducting polymer layer. Glutamate is the most important excitatory neurotransmitter that plays a major role in the mammalian central nervous system and its high concentration in the extracellular fluid caused by excessive release may play a major neurotoxic role in a wide range of neurological disorders. Thus, this in vivo microbiosensor is an effective tool for monitoring the change in extracellular glutamate levels in the human body system [12].

Recently, another in vivo microbiosensor based on glutathione reductase (GR) and β-nicotinamide adenine dinucleotide phosphate (NADPH) has been studied through their immobilization on the nanocomposite conducting polymer films [16]. The microbiosensor is used for glutathione disulfide (GSSG) detection in a rat liver as a model organ for GSSG detection (see fig. 4). The purpose of this study is to monitor and investigate the oxidative stress since high concentration of GSSG in the rat liver is a biomarker of oxidative stress. The results obtained are very impressive implying a promising approach for a GSSG biosensor in clinical diagnostics and oxidative stress monitoring. Thus, we believe that these systems can be very useful as a point of care medical device in clinical laboratories.

Fig. 4. Schematic preparation of the in vivo glutathione biosensors.

Conductive Polymers in Biofuel Cells

Conventionalfuel cells use metals like platinum and nickel as catalysts, whereas, the enzymatic biofuel cells use enzymes obtained from living cells. However, the primary requirement is that the enzymes which allow the fuel cell to operate must be stably immobilized as the anode and cathode. Our laboratory utilized polyTTCA and poly-Fe(III)-[N,N’-bis[4-(5,2’:5’,2”-terthien-3’-yl)salicyliden]-1,2-ethanediamine] (polyFeTSED) which were electrochemically polymerized on an Au surface for use, as mediators and catalysts for a biofuel cell [17]. The enzymes [glucose oxidase (GOx) or horseradish peroxidase (HRP)] were immobilized onto the conducting polymer layer through covalent bond formation, which allowed the direct electron transfer processes of the enzymes. The anode with immobilized GOx and the cathode with immobilized HRP were used as model enzyme systems in biofuel cells for glucose and H₂O₂ detection, respectively (see fig. 5).

The copolymer of polyFeTSED complex with polyTTCA revealed a catalytic activity for the electrochemical reduction of H₂O₂ and resulted in approximately a seven fold increase in the power density of the biofuel cell over that of polyTTCA itself. Moreover, the conjugated polymers extended the lifetime of biofuel cell to 4 months through the stabilization of immobilized enzymes. The biofuel cell operated in a solution containing glucose and anode-produced H₂O₂ generated an open-circuit voltage of approximately 366.0 mV, while the maximum electrical power density extracted from the cell was 5.12 µW cm⁻² at an external optimal load of 25.0 kΩ. The biofuel cell electrodes fabricated in this study had a wide range of potential applications as substrates for various potential bio- and chemical sensors and bio-devices.

Fig. 5. (A) Schematic diagram of a biofuel cell with a conducting polymer catalyst. (B) CVs for electropolymerization using (a) TTCA, (b) FeTSED, and (C) TTCA + FeTSED (1:1 mole ratio) monomers. AFM images for (d) bare, (e) polyTTCA, and (f) poly(TTCA–FeTSED) coated on the highly ordered pyrolytic graphite electrodes.

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