The conducting process in polymers is rather complex. For the conducting process, the charge carriers have to be able to move in the material. This is achieved because the conjugated backbone of the polymer provides delocalized pi-orbitals that overlap continuously to allow free movement of the electrons [35, 40]. Besides delocalized electrons, the conductive polymers can have other charge carriers, which are often produced by doping with electron acceptors or donors [37]. The concept of polarons, bipolarons and solitons has been introduced to describe the charge defects introduced by the doping [41, 42]. Oxidation of the polymer creates holes (p-doping) while reduction adds electrons (n-doping) [34]. Different methods of doping can be applied [37]. For example, through chemical doping with iodine, the conductivity of polyacetylene could be increased over seven orders of magnitudes [37, 43, 44]. This discovery was rewarded the Nobel Prize in chemistry in 2000. Electrochemical doping can also be performed by the electropolymerization of the monomers. With this method, the ions come from the electrolyte solution [34]. The advantage of this method is that a polymer film is directly created on the electrode [45]. Photodoping introduces charge defects by irradiation of light [37].

In the following, three examples of conductive polymers will be discussed because these are the most commonly used in biosensors. Table 17.1 shows the chemical structures of polypyrrole, polyaniline and PEDOT, the three conductive polymers reviewed in this section.

Polyaniline is a CP that has been known and used for a very long time. The synthesis of polyaniline, often abbreviated as PANI, is based on the oxidation of the monomer aniline. PANI has been reported to have a high conductivity (up to 103 S/m) [46, 47]. Thanks to its properties, it has found a wide range of sensing applications, inclusive in biosensors. The reader is referred to the excellent review of Dhand et al. [48]. Horseradish peroxidase (HRP) is an enzyme extensively incorporated in biosensors. It catalyzes the reduction of hydrogen peroxide (H₂O₂), a reactive oxygen species, which is produced by a wide range of enzymatic reactions [49]. Detection of H₂O₂ is highly relevant in food and pharmaceutical industry. Chairam et al. [50] developed an enzyme based amperometric biosensor for the detection of H₂O₂. HRP was immobilized in the layer of conducting polymer poly(aniline-co-o-aminobenzoic acid) via electrostatic interactions. After mixing of the enzyme in the polymer, the mixture was simply cast onto a glassy carbon electrode. A layer of the biopolymer chitosan was then coated onto the HRP-p (Ani-co-o-Aba). This layer was used as a binder for the enzyme. This sensor showed a linear range from 10 to 1,000 µM with a detection limit of 1.8 µM. This novel and fast fabrication method showed excellent promising results for highly sensitive biosensors.

The biopolymer chitosan has also been used in combination with PANI films by Srivastava et al. [51]. They developed a modified PANI-gold electrode with immobilization of cholesterol oxidase on a chitosan matrix. First, nanocomposites of PANI and gold were prepared. After complete polymerisation, the nanocomposites were incorporated in chitosan. This mixture was then spin coated onto an indium tin oxide (ITO) glass. The success of the immobilization was assessed by cyclic voltammetry measurements that showed a lower current peak caused by the electrical hindrance of the immobilized enzyme. The biosensor was optimized for the detection of cholesterol. Cholesterol is oxidized by cholesterol oxidase which produces H₂O₂, that is oxidized producing electrons that are transferred to the electrodes via the redox mediator Fe(CN)₆⁻³. The signal is a peak in current that increases with the cholesterol concentration. The biosensor was stable over the time and retained a 90 % cholesterol oxidase activity over two weeks.

Composites of gold and PANI on a glassy carbon electrode were also used by Xiang et al. [52] for the development of a glucose biosensor. They used a bienzymatic approach where glucose was oxidized thereby producing H₂O₂, which was reduced by cytochrome oxidase c, absorbed on the surface of the nanocomposites. This sensor enabled a direct electron transfer to the electrode material. The sensor has a detection limit of 0.01 mM.

A similar electrochemical based sensor was developed using nanocomposites of PANI, graphene and horse radish peroxidase on ITO glas [53]. This biosensor was able to measure very low concentration of anti-malaria drug in various pharmaceutical preparations and in different body fluids (urine, plasma and serum).

Similarly, nanocomposites of graphene and PANI were used by Ruecha et al. [54] to develop a paper based biosensor for the enzyme based detection of cholesterol and the electrochemical detection of the by-product H₂O₂ of the enzymatic reaction. Interestingly they incorporated polyvinylpyrrolidone in the modified electrode in order to improve the stabilization of the graphene concentration. Those biosensors based on composites of different materials often show a combination of the parameters of the materials and therefore offer a higher thermal stability, better conductivity and improved resistance to salts.

Tamer et al. [55] worked on the optimization of the fabrication of the composites PANI/gold nanorod for glucose detection. In this biosensor, the enzyme GOx was covalently bound to the electrode via the linker glutaraldehyde. This matrix offered a large surface to functionalize.

PANI has also been shown to be usable in combination with whole bacterial cells as bioelement. Anu Prathap et al. [56] developed a biosensor where recombinant E. coli bacteria were immobilized by adsorption into a PANI film. The bacteria had been previously transformed with the gene linA2, coding for an enzyme involved in the first step of the degradation of pesticide lindane. Using recombinant E. coli bacteria assured a higher robustness and better performances to the sensor compared to the wild type which has a slow degradation rate of lindane. The sensor was proved to be functional during two weeks if stored at 4 °C. It could detect lindane in the part per trillion concentrations. It was highly sensitive and selective since it could detect the three isomers of lindane but showed no response for two other common pesticides. The advantage of using a whole cell as bioelement is to overcome the lack of synthetic bioelement for certain targets and provide the sensor with a high specificity. The detection was based on the change of conductivity induced in the PANI film after production of HCl by the enzymatic reaction. Amperometric measurements were used to register the increase in current upon production of HCl.

PANI was also used in combination with carbon nanotubes to fabricate a sensor with cytochrome oxidase as bioelement [57]. This system could be used for different kinds of sensing events based on the enzymatic reduction of H₂O₂. The sensitivity of a biosensor based on a PANI film on a gold electrode was improved by using a composite of PANI with polypyrrole nanoparticles [58]. The sensor was used to detect glutamate in food products with an enzymatic reaction of the covalently bound glutamate oxidase.

Nanowires based biosensors offer a high sensitivity. Lee et al. [59] developed a biosensor based on a single PANI nanowire that was functionalized via the EDC and N-Hydroxysuccinimide (NHS) (EDC/NHS) chemistry with antibodies against immunoglobulin G (IgG) and Myo, two cardiac biomarkers. The nanowire was fabricated using the electrochemical growth method between two gold electrodes. The binding of either IgG or Myo was recorded by the increase of conductance of the wire. The PANI biosensor was highly selective and sensitive to its target with detection limits of 3 ng/mL for IgG and 1.4 ng/mL for Myo.

Polypyrrole (PPy) is a conducting polymer that has attracted high interest in the biosensor field. PPy forms a yellow film that that becomes darker upon polymerization. PPy is indeed biocompatible, enabling the CP for the application in the medical biosensor field and shows a high conductivity [42]. It is worth mentioning that PPy has recently found application in the drug delivery field [60]. The synthesis of PPy has been extensively studied and can be done chemically or by electropolymerization. The way that the synthesis is performed has a significant effect on the properties of the PPy film [42, 61]. High conductive polypyrrole can reach a conductivity of up to 200 S/cm. Doped PPy films are relatively stable in air. The degradation of polypyrrole films starts with the loss of dopants, followed by the degradation of the polymer backbone [62]. Polypyrrole films have been described as being planar chains along the direction of the film layer.

Xu et al. [63] recently presented a biosensor for thrombin sensing based on polypyrrole electrodes prepared by electropolymerization on platinum electrodes. They used pyrrole-nitrilotriacetic acid (pyrrole-NTA) monomers that were polymerized and complexed with Cu²⁺. As bioelement, they immobilized a histidine labelled aptamer on the polypyrrole film through the histidine-NTA-Cu²⁺ interaction. The surface of this biosensor was shown to be non-adherent to proteins. The change of impedance was measured upon binding of the target to the aptamer. Thrombin was detected in the picomolar range (4.4 × 10⁻¹² mol/L). Detection of the E. coli strain K12 was achieved by a DNA-based biosensor [64]. Short DNA sequences (25 base pairs) called hybridization probes were designed complementary to the sequence of the gene D-glucoronidase from E. coli. The DNA sequences were incorporated in the bulk of pyrrole that was electropolymerized onto a platinum electrode. The hybridization was measured by cyclic voltammetry with a clear difference between the amplitudes of the oxidation peaks resulting from the hybridization of a complementary sequence or from a non-complimentary sequence. The response of the biosensor could be obtained within 15–20 min which was much faster than any detection system involving culturing of the bacteria. Glucose biosensors have also been extensively investigated. They mainly use an enzyme-based system with GOx as bioelement and electrochemical detection. Olea et al. [65, 66] developed a new way to immobilize the enzyme GOx onto the biosensor surface. GOx was encapsulated in a multilamellar lipid vesicle before being incorporated into the polymer film. The comparison between free enzyme and encapsulated enzyme showed that the sensitivity was improved fivefold and that the stability of the biosensor was increased as well [67].