**2. Synthesis of nanostructured conducting polymers**

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.

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

#### **Figure 2.**

*(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).*

nanomaterials [43–48]. Depending on the monomer properties, whether it interacts electrostatically or chemically bound to the template, different CP micro- and nanostructures can be fabricated retaining the original shape of the porous template itself.

So far, the hard template approach is utilized for the synthesis of one-dimensional (1D) CP nanostructures such as nanotubes, nanorods, and nanofibers which are synthesized using anodic aluminum oxide (AAO) membranes, zeolite channels, mesoporous silica, and track-etched polycarbonate [49, 50]. Depending on the pore length and diameter of the membrane template, size and diameter of CP nanostructures can be precisely controlled because the monomers are absorbed or attached inside the pore walls, followed by chemical oxidative polymerization [51]. In addition, the wall thickness can be tuned by controlling the polymerization time and concentration of monomer. In pioneering studies to Jang et al., they proposed a facile route to synthesize PPY nanotubes with a wall thickness of a few nanometers using AAO membrane template via VDP as signal transducers [52, 53]. Park et al. reported a one-pot synthesis of Ag NPs decorated PEDOT nanotubes with high surface area and enhanced conductivity via VDP method using an AAO template and Fe(NO3)3 as an oxidant for sensing ammonia gas [54]. Furthermore, combination of electrospinning using electrospun nanofiber templates and VDP methods provided CP nanomaterials with remarkable surface areas and uniform nanostructures [55]. Kwon et al. reported the use of electrospun ultrathin poly(methyl methacrylate) (PMMA) nanofibers as a template to synthesize PEDOT after immersing with ferric chloride solution followed by VDP of EDOT monomer at controlled temperatures and pressures to yield coreshell PEDOT nanofibers [56]. However, this method suffers from the difficulties of removing the template without aggregation of the resulting CPs and is not suitable for commercial applications. Regardless, high-impact nanostructured CP composites can be prepared without removal of template. Furthermore, previous studies reported that the synthesis of different CP nanohybrids, such as metal, metal sulfides, and metal oxides/PPY nanowires, was prepared by the hard template method [57–59].

An alternative strategy known as the soft template approach has been used to effectively fabricate CP nanomaterials using templates such as surfactants, block copolymers, polyelectrolytes, and liquid crystals combined with interfacial polymerization and emulsion/dispersion polymerization [60–62]. Accordingly, 1D CP nanostructures can be tailored by varying the synthetic conditions and produced in large scale [63]. A cationic surfactant, dodecyltrimethylammonium bromide (DTAB), has been utilized to form spherical micelles reinforced with decanol in aqueous solution as a stable microemulsion for the synthesis of monodispersed PPY nanoparticles with a large quantity [64]. However, this technique requires high surfactant concentration, which is problematic in terms of cost and environmental pollution. Jang and coworkers reported the formation of PPY nanotubes and nanoparticles using a reverse micelle system (water-in-oil systems) with a molecular template, sodium bis(2 ethylhexyl) sulfosuccinate (known as AOT), and dispersion polymerization (watersoluble polymers) employing polyvinyl alcohol (PVA) in an aqueous solution, respectively [65–67]. In addition, PANI nanowire network was carefully synthesized using a cationic surfactant, hexadecyltrimethylammonium bromide and oxalic acid in aqueous solution [68]. Further, an anionic oxidant/cationic surfactant complex was used as a template to fabricate clip-like nanostructures of PPY, PANI, and PEDOT [69]. By judiciously changing the combination of surfactants, oxidizing/doping agents, pH, and temperature, infinite nanostructures with desirable morphology could be successfully fabricated. A facile method to synthesize nanostructured coreshell PPY/Ag using sodium dodecyl benzyl sulfonate (SDBS) and CTAB as templates through a redox reaction was proposed [70]. Very recently, Stejskal et al. demonstrated a cotton fabric coating of PPY and PANI nanotubes, colloidal PPY nanotubes/ nanorods, and microporous PANI cryogels obtained by the chemical polymerization

**129**

**Figure 3.**

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

group for fabricating nanotubular PPY [85].

provided nucleation sites for Ag+

**3. Gas sensors based on CPs**

of pyrrole in the presence of a structure-directing dye, methyl orange (MO), as a starting template, and poly(*N*-vinypyrrolidone) (PVP), respectively [71–78]. The interaction between starting materials and dye was expected to produce a template, which is further used for the growth of CP nanotubes. After the addition of oxidant, MO itself in its acid form has limited solubility which may serve as a starting template and the growth of nanotubes may proceed beyond the template [79, 80]. Also, the partial solubilization of MO-FeCl3 template in the presence of CTAB was an effective way to fabricate PPY nanotubes having smaller diameter by reactive self-degrade template method [81, 82]. In addition, ionic liquid template-assisted synthesis of PANI/AgCl and PPY/Ag nanocomposites has recently been conducted by the direct oxidation of pyrrole by silver cations from silver bis(trifluoromethanesulfonyl)imide (AgTf2N), using 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMImTf2N) as solvent and template [83, 84]. Moreover, dual-template approach involving an AAO template and surfactants was also applied by another research

Template-free approach is a quite facile and a straightforward method for producing CP nanomaterials with large quantity without using specific sacrificial templates and post−/pretreatment procedures; however extensive efforts are needed to design building blocks that can spontaneously self-assemble into nanostructures under certain conditions without addition of artificial templates. PANI nanofibers prepared from template-free synthesis have been considered as an interesting sensing material since the pioneering study by Huang et al. [86]. Several types of templatefree methods, such as chemical, electrochemical, dispersion, and aqueous/organic interfacial polymerization, resulting in various CP micro−/nanostructures, including nanotubes, nanofibers, hollow nanoparticles, core-shell nanoparticles, and multidimensional nanotubes, were extensively reported [87–96]. In addition, a template-free site-specific electrochemical method was developed for the fabrication of PPY, PANI, and PEDOT nanowires on microelectrode junctions [97, 98]. Moreover, a bottom-up approach was applied for the fabricating of PPY/Ag core-shell nanoparticles with an average core diameter of 36 nm and a shell thickness of 13 nm via a simple one-pot synthesis using a starch [99]. In this process, the OH<sup>−</sup> groups of the soluble starch

Simultaneously, pyrrole monomers were oxidized to form radical cations that have led to the generation of PPY short chains which are further oxidized by the silver

Chemical sensor is composed of a sensitive material to a particular analyte (molecular recognition) and a transducer, which transforms the concentrations of an analyte into other detectable physical signals, such as current, absorbance, or mass (**Figure 3**). Depending on signal transduction, gas sensor devices based on CPs have been classified by IUPAC [100]. Sensors based on chemical modulation of electronic, optical, or mechanical transduction mechanisms of CPs will be

*Illustration of a chemical sensor. (modified and adapted with permission from Ref. [103]. Copyright 2008, MDPI).*

nanoseed active sites to finally produce PPY core-shell nanostructures.

discussed in detail in light of the gas sensing applications.

that were readily reduced by the pyrrole monomer.

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

*Gas Sensors*

nanomaterials [43–48]. Depending on the monomer properties, whether it interacts electrostatically or chemically bound to the template, different CP micro- and nanostructures can be fabricated retaining the original shape of the porous template itself. So far, the hard template approach is utilized for the synthesis of one-dimensional

(1D) CP nanostructures such as nanotubes, nanorods, and nanofibers which are synthesized using anodic aluminum oxide (AAO) membranes, zeolite channels, mesoporous silica, and track-etched polycarbonate [49, 50]. Depending on the pore length and diameter of the membrane template, size and diameter of CP nanostructures can be precisely controlled because the monomers are absorbed or attached inside the pore walls, followed by chemical oxidative polymerization [51]. In addition, the wall thickness can be tuned by controlling the polymerization time and concentration of monomer. In pioneering studies to Jang et al., they proposed a facile route to synthesize PPY nanotubes with a wall thickness of a few nanometers using AAO membrane template via VDP as signal transducers [52, 53]. Park et al. reported a one-pot synthesis of Ag NPs decorated PEDOT nanotubes with high surface area and enhanced conductivity via VDP method using an AAO template and Fe(NO3)3 as an oxidant for sensing ammonia gas [54]. Furthermore, combination of electrospinning using electrospun nanofiber templates and VDP methods provided CP nanomaterials with remarkable surface areas and uniform nanostructures [55]. Kwon et al. reported the use of electrospun ultrathin poly(methyl methacrylate) (PMMA) nanofibers as a template to synthesize PEDOT after immersing with ferric chloride solution followed by VDP of EDOT monomer at controlled temperatures and pressures to yield coreshell PEDOT nanofibers [56]. However, this method suffers from the difficulties of removing the template without aggregation of the resulting CPs and is not suitable for commercial applications. Regardless, high-impact nanostructured CP composites can be prepared without removal of template. Furthermore, previous studies reported that the synthesis of different CP nanohybrids, such as metal, metal sulfides, and metal oxides/PPY nanowires, was prepared by the hard template method [57–59]. An alternative strategy known as the soft template approach has been used to effectively fabricate CP nanomaterials using templates such as surfactants, block copolymers, polyelectrolytes, and liquid crystals combined with interfacial polymerization and emulsion/dispersion polymerization [60–62]. Accordingly, 1D CP nanostructures can be tailored by varying the synthetic conditions and produced in large scale [63]. A cationic surfactant, dodecyltrimethylammonium bromide (DTAB), has been utilized to form spherical micelles reinforced with decanol in aqueous solution as a stable microemulsion for the synthesis of monodispersed PPY nanoparticles with a large quantity [64]. However, this technique requires high surfactant concentration, which is problematic in terms of cost and environmental pollution. Jang and coworkers reported the formation of PPY nanotubes and nanoparticles using a reverse micelle system (water-in-oil systems) with a molecular template, sodium bis(2 ethylhexyl) sulfosuccinate (known as AOT), and dispersion polymerization (watersoluble polymers) employing polyvinyl alcohol (PVA) in an aqueous solution, respectively [65–67]. In addition, PANI nanowire network was carefully synthesized using a cationic surfactant, hexadecyltrimethylammonium bromide and oxalic acid in aqueous solution [68]. Further, an anionic oxidant/cationic surfactant complex was used as a template to fabricate clip-like nanostructures of PPY, PANI, and PEDOT [69]. By judiciously changing the combination of surfactants, oxidizing/doping agents, pH, and temperature, infinite nanostructures with desirable morphology could be successfully fabricated. A facile method to synthesize nanostructured coreshell PPY/Ag using sodium dodecyl benzyl sulfonate (SDBS) and CTAB as templates through a redox reaction was proposed [70]. Very recently, Stejskal et al. demonstrated a cotton fabric coating of PPY and PANI nanotubes, colloidal PPY nanotubes/ nanorods, and microporous PANI cryogels obtained by the chemical polymerization

**128**

of pyrrole in the presence of a structure-directing dye, methyl orange (MO), as a starting template, and poly(*N*-vinypyrrolidone) (PVP), respectively [71–78]. The interaction between starting materials and dye was expected to produce a template, which is further used for the growth of CP nanotubes. After the addition of oxidant, MO itself in its acid form has limited solubility which may serve as a starting template and the growth of nanotubes may proceed beyond the template [79, 80]. Also, the partial solubilization of MO-FeCl3 template in the presence of CTAB was an effective way to fabricate PPY nanotubes having smaller diameter by reactive self-degrade template method [81, 82]. In addition, ionic liquid template-assisted synthesis of PANI/AgCl and PPY/Ag nanocomposites has recently been conducted by the direct oxidation of pyrrole by silver cations from silver bis(trifluoromethanesulfonyl)imide (AgTf2N), using 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMImTf2N) as solvent and template [83, 84]. Moreover, dual-template approach involving an AAO template and surfactants was also applied by another research group for fabricating nanotubular PPY [85].

Template-free approach is a quite facile and a straightforward method for producing CP nanomaterials with large quantity without using specific sacrificial templates and post−/pretreatment procedures; however extensive efforts are needed to design building blocks that can spontaneously self-assemble into nanostructures under certain conditions without addition of artificial templates. PANI nanofibers prepared from template-free synthesis have been considered as an interesting sensing material since the pioneering study by Huang et al. [86]. Several types of templatefree methods, such as chemical, electrochemical, dispersion, and aqueous/organic interfacial polymerization, resulting in various CP micro−/nanostructures, including nanotubes, nanofibers, hollow nanoparticles, core-shell nanoparticles, and multidimensional nanotubes, were extensively reported [87–96]. In addition, a template-free site-specific electrochemical method was developed for the fabrication of PPY, PANI, and PEDOT nanowires on microelectrode junctions [97, 98]. Moreover, a bottom-up approach was applied for the fabricating of PPY/Ag core-shell nanoparticles with an average core diameter of 36 nm and a shell thickness of 13 nm via a simple one-pot synthesis using a starch [99]. In this process, the OH<sup>−</sup> groups of the soluble starch provided nucleation sites for Ag+ that were readily reduced by the pyrrole monomer. Simultaneously, pyrrole monomers were oxidized to form radical cations that have led to the generation of PPY short chains which are further oxidized by the silver nanoseed active sites to finally produce PPY core-shell nanostructures.
