**1. Introduction**

The initial development of conducting polymers (CPs) began in 1977 by the American scientists MacDiarmid and Heeger and their Japanese colleague Shirakawa, as they discovered a highly conductive polyacetylene (PA) via chemical doping with iodine or other ionic dopants which endowed the polymer with metal-like properties, producing copper-colored films with an increased conductivity of 10 orders of magnitude (Noble Laureate in Chemistry in 2000) [1–3]. However, the instability and ease of degradation of PA by oxidative degradation was a big obstacle to find applications such as batteries or electronic devices. Since then, there have been worldwide considerable efforts in synthesizing numerous other CPs similar to those of PA with high doping level over the range from insulator to metal, such as polyphenylene (PP), polypyrrole (PPY), polythiophene (PTH), and polyaniline (PANI) [4–7].

Hitherto, however, studies on CPs have been extensively investigated in both fundamental and practical perspectives; their unique chemical and physical characteristics have been continuously discovered. These fascinating properties are derived from their π-electron conjugation system along the polymer chain, which allow

the formation of delocalized electronic states, resulting in a resonance-stabilized structure of the polymer [8]. Over a wide range of polymer-based materials, CPs are of particular interest due to their unique electrical and optical properties rivaling metals or inorganic semiconductors and still retain the attractive mechanical properties and processing advantages of polymeric materials, known as "synthetic metals" [9]. In addition to a variety of advantages, ease of preparation procedures, controlling the morphology, structural flexibility, light weight, and cost-effectiveness are included. The molecular structures of some of the prominent CPs include PANI, PPY, poly-paraphenylene (PPP), and PTH, which are shown in **Figure 1** (left) in their non-conducting (undoped) states. In the past few decades, CPs have been continuously studied for their tremendous use in electronic and optoelectronic devices. In this context, particular interest has been paid toward utilization of nanostructured CP-based sensors as high-performance signal transducers with enhanced sensing capability relative to their conventional bulk-scale materials, because of their high surface-to-volume ratios and unique electrical and physical properties [10]. In the sense of sensor applications, the distinguishing properties of CPs offer a great potential in efficient sensing systems by virtue of their unique electrical, optical, and mechanical transduction mechanisms [11, 12]. To achieve superior performances in both sensitivity and response time, critical issues include conductivity, morphological design, size control, bioprobes, and surface modification which are very crucial. In terms of conductivity change, oxidation level achieved by chemical and electrochemical doping/dedoping mechanisms could in turn fabricate a sensitive and rapid sensor response to an analyte of interest at room temperature [11, 13]. Recently, special attention has been paid to nanostructured CPs as one of the most substantial achievements in sensor technology because their high surface areas, multidimensional architectures, and easy functionalization with a variety of functional groups enable the sensing of a trace amount of a target species [14]. Further, the signal intensity of the sensor can be enhanced by controlling the shapes because of the one-directional signal pathway of the CP nanostructure.

Typical CPs, such as PPY, PANI, PTH, and poly(3,4-ethylenedioxythiophene) (PEDOT), have been extensively studied in environmental monitoring of various types of target analytes, such as volatile organic compounds (VOCs), gases, heavy metals, and biomolecules [15–17]. On exposure to analyte, their response mechanisms comprise chemical and physical interactions, including oxidation/reduction, swelling, conformational changes, charge transfer, and so on. In terms of charge

#### **Figure 1.**

*(Left) Representative of typical molecular structures of CPs and (right) illustration of doping mechanisms of PANI. The terms "LEB and ES" represent the completely reduced form of colorless PANI called "leucoemeraldine base" and the highly conducting emeraldine salt (ES) obtained by chemical reaction with protonic acids whose color is green, and the conductivity is around 15 S·cm<sup>−</sup><sup>1</sup> .*

**127**

**Figure 2.**

*Gas Sensors Based on Conducting Polymers DOI: http://dx.doi.org/10.5772/intechopen.89888*

the high performance of CP-based sensors.

**2. Synthesis of nanostructured conducting polymers**

transfer, doping and oxidation levels and conjugation length are key intrinsic factors of CPs in which the delocalization of the π-electron takes place. It is well-known that most of CPs are p-type semiconductors, and thus they feature the emergence of charge carriers (polaron and biopolaron) as oxidative doping proceeds as shown in **Figure 1** (right). Accordingly, these intrinsic factors are very useful for designing

Conducting polymers have traditionally been synthesized either by chemical or electrochemical oxidation routes of the corresponding monomers with acid or peroxide initiators resulting in insulating materials that require a post-doping process [11, 18–20]. In both cases, the overall polymerization process includes the oxidation of monomer, followed by coupling reaction of the charged monomers to produce a polymer chain. Chemical polymerization method is usually applicable for large-scale production of CP powders. In contrast, electrochemical polymerization offers an in situ one-step effective process for producing CP nanomaterials deposited onto the electrode surface as films for a sensor device, which grow along the direction of the electric field to form oriented nanostructures. The morphological structure and thickness of the CP films can be tailored by controlling the electrochemical polymerization conditions, applied potential or current density, and electrolyte. Owing to their electrical conductivity, CPs can grow

electrochemically on an electrode surface without addition of oxidizing agents.

*(a and b) STEM of PANI salt and PANI base prepared with CSA, respectively. (c and d) SEM images of nanotubular PANI and PANI/Ag nanoparticles, respectively, prepared using chemical oxidative polymerization in acetic acid (adapted with permission from Ref. [37, 38] Copyright 2009, Elsevier).*

Great efforts have been devoted toward the preparation of CP nanomaterials for the fabrication of miniaturized novel flexible sensor platforms that enable portability and high-density arrays because of using small sample amounts, which offer excellent prospects in sensor nanotechnology for advanced detection systems [21]. Recent studies have demonstrated the synthesis of nanostructured CPs with controlled shape and size, which ranged from lithographic techniques to chemical methods [22–33]. Stejskal and coworkers demonstrated synthesizing PANI nanostructures and its derivatives by the chemical oxidative polymerization in water [34–36]. Further, Ayad et al. reported the synthesis of PANI and PPY nanotubes, nanorods, nanoflowers, nanoflakes, and nanocomposites via chemical oxidative polymerization using diluted aqueous camphor sulfonic acid (CSA) and acetic acid solutions as shown in **Figure 2** [37, 38]. Besides, the incorporation of metals/metal oxide NPs, graphene, or carbon nanotubes (CNTs) into nanostructured PANI and PPY has been recently reported as a way of increasing the CP electrochemical and electrocatalytic activities and sensing capabilities [39–42]. Recently, research studies have focused on the development of templating approaches such as hard template, soft template, and template-free synthesis as an aid template in combination with other polymerization methods, like dispersion polymerization, interfacial polymerization, vapor deposition polymerization (VDP), and electrochemical polymerization for the synthesis of well-defined CP

#### *Gas Sensors Based on Conducting Polymers DOI: http://dx.doi.org/10.5772/intechopen.89888*

*Gas Sensors*

the formation of delocalized electronic states, resulting in a resonance-stabilized structure of the polymer [8]. Over a wide range of polymer-based materials, CPs are of particular interest due to their unique electrical and optical properties rivaling metals or inorganic semiconductors and still retain the attractive mechanical properties and processing advantages of polymeric materials, known as "synthetic metals" [9]. In addition to a variety of advantages, ease of preparation procedures, controlling the morphology, structural flexibility, light weight, and cost-effectiveness are included. The molecular structures of some of the prominent CPs include PANI, PPY, poly-paraphenylene (PPP), and PTH, which are shown in **Figure 1** (left) in their non-conducting (undoped) states. In the past few decades, CPs have been continuously studied for their tremendous use in electronic and optoelectronic devices. In this context, particular interest has been paid toward utilization of nanostructured CP-based sensors as high-performance signal transducers with enhanced sensing capability relative to their conventional bulk-scale materials, because of their high surface-to-volume ratios and unique electrical and physical properties [10]. In the sense of sensor applications, the distinguishing properties of CPs offer a great potential in efficient sensing systems by virtue of their unique electrical, optical, and mechanical transduction mechanisms [11, 12]. To achieve superior performances in both sensitivity and response time, critical issues include conductivity, morphological design, size control, bioprobes, and surface modification which are very crucial. In terms of conductivity change, oxidation level achieved by chemical and electrochemical doping/dedoping mechanisms could in turn fabricate a sensitive and rapid sensor response to an analyte of interest at room temperature [11, 13]. Recently, special attention has been paid to nanostructured CPs as one of the most substantial achievements in sensor technology because their high surface areas, multidimensional architectures, and easy functionalization with a variety of functional groups enable the sensing of a trace amount of a target species [14]. Further, the signal intensity of the sensor can be enhanced by controlling the shapes

because of the one-directional signal pathway of the CP nanostructure.

Typical CPs, such as PPY, PANI, PTH, and poly(3,4-ethylenedioxythiophene) (PEDOT), have been extensively studied in environmental monitoring of various types of target analytes, such as volatile organic compounds (VOCs), gases, heavy metals, and biomolecules [15–17]. On exposure to analyte, their response mechanisms comprise chemical and physical interactions, including oxidation/reduction, swelling, conformational changes, charge transfer, and so on. In terms of charge

*(Left) Representative of typical molecular structures of CPs and (right) illustration of doping mechanisms of PANI. The terms "LEB and ES" represent the completely reduced form of colorless PANI called "leucoemeraldine base" and the highly conducting emeraldine salt (ES) obtained by chemical reaction with* 

*.*

*protonic acids whose color is green, and the conductivity is around 15 S·cm<sup>−</sup><sup>1</sup>*

**126**

**Figure 1.**

transfer, doping and oxidation levels and conjugation length are key intrinsic factors of CPs in which the delocalization of the π-electron takes place. It is well-known that most of CPs are p-type semiconductors, and thus they feature the emergence of charge carriers (polaron and biopolaron) as oxidative doping proceeds as shown in **Figure 1** (right). Accordingly, these intrinsic factors are very useful for designing the high performance of CP-based sensors.
