# Electrochemical Biosensors Based on Conducting Polymers: A Review
## 1 Introduction  
Conducting polymers have attracted much interest since [PERSON] et al. demonstrated in 1977 that halogen doping of polyacetylene strongly increased its conductivity [1]. Thanks to this revolutionary research, [PERSON], [PERSON], and [PERSON] were awarded the Nobel Prize in Chemistry in 2000, and opened the way to the development of other conducting polymers combining properties of organic polymers and electronic properties of semiconductors. Another major breakthrough in this field was achieved by [PERSON] et al., who reported the electrodeposition of highly conductive, stable and processable polypyrrole films [2; 3; 4]. Following these pioneering studies, numerous conducting polymers have been prepared and used in various applications, such as polyacetylene, polypyrrole (PPy), polyaniline (PANI), polycarbonate, polythiophene (PTh), poly(3,4-ethylenedioxythiophene) (PEDOT), polyphenylene, poly(phenylene vinylene), and polyfluorene (Table 1). All these organic polymers are characterized by alternating single (\(\sigma\)) and double (\(\pi\)) bonds and by the presence of \(\pi\) electrons delocalized across their entire conjugated structure, thus resulting in polymers which can be easily oxidized or reduced [5]. This doping, that can be performed upon oxidation (p-doping) or reduction (n-doping), increases significantly the conductivity of the polymers since this conductivity can vary from less than \(10^{-6}\) S/cm in the neutral state [5] to more than \(10^{5}\) S/cm in the doped state [6; 7]. The conductivity of the polymers is also dependent on a number of factors including the nature and concentration of the dopant [8; 9; 10], temperature [11; 12; 13], swelling/deswelling [14], polymer morphology [8], pH and applied potential [15], and polymer chain length [16]. For most heterocyclic polymers, such as PPy [17] or PTh [18], the mechanism of conduction corresponds to a p-doping and starts with the removal of one electron from the initial monomer leading to the formation of an unstableradical cation (named polaron). Then, a second electron is removed from another monomer or from an oligomer, leading to the formation of a dication (named bipolaron) [19]. Under an applied electric field, these polarons and bipolarons serve as charge carriers which are delocalized over the polymer chains and their movement along polymer chains produces electronic conductivity [20].  
Conducting polymers have become an important class of materials since they combine some useful properties of organic polymers (such as strength, plasticity, flexibility, toughness or elasticity) with unusual electronic [5], optical [21; 22] and thermoelectric [23; 24] properties due to the charge mobility along the \(\pi\) electron polymer chains. These unique properties explain the use of conducting polymers in a wide variety of applications including energy storage with rechargeable batteries [25; 26] and supercapacitors [27; 28], photovoltaics with solar cells [29; 30; 31; 32], light-emitting diodes [33; 34], electrocatalysis [35], anti-corrosion [36; 37] or electrochromic applications such as electrochromic displays [38; 39] or rearview mirrors and smart windows [40; 41].