*1.1. Synthesis and Preparation of Polypyrrole*

PPy is commonly synthesized through chemical or electrochemical oxidation of pyrrole monomer using oxidant agents through a conjugated bond system with the polymer backbone. However, in the presence of only PPy, it may suffer from a certain drawback as mentioned above, and this will restrict the device's application. Therefore, approaches such as blending, electro-polymerization, interpenetrating network formation, and composite synthesis have been done to enhance the PPy properties, thus improving the device's performance.

Hosseini and Entezami, 2003, prepared the polymer by blending PPy with polyvinyl acetate (PVAc), polystyrene (PS), and polyvinyl chloride (PVC) using a chemical method to produce flexible and free-standing blended polymer films. The sensing abilities of these films towards toxic gases and vapors were investigated and it was discovered that the PPy blended film had improved mechanical strength and was also able to exhibit greater environmental stability. Besides that, it was also found the sensoric properties of the PPy blends towards toxic gases and vapors against hydrogen halides, hydrogen cyanide, and halomethyl compounds were better than non-blended PPys. Therefore, it can be surmised from their findings that PPy blends are good candidates for sensing toxic gases and vapors [16].

Meanwhile, Song et al., 2019, prepared three-dimensional graphene oxide with an interconnected porous polypyrrole (pGO/PPy) nanostructure-based actuator through electro-polymerization and sonication. This configuration allowed the actuator to adsorb trace cadmium by the carboxyl functional group in the GO and also was able to widen the electrode detection range of the PPy which was densely covered with gold substrate. From the results obtained, they suggest that the pGO/PPy was a promising material that had an ability to enhance the pre-concentration factors, enrich the potential window and

greatly increase the sensitivity of the cadmium sensor. The cadmium detection in the presence of interference ions showed good selectivity using this pGO/PPy nanostructure based actuator. Besides which, the nanostructure also achieved a wider linear range and a lower limit of detection (LOD). Moreover, this method could be developed into a low cost, portable and reliable sensor that is both sensitive and selective towards cadmium in aqueous systems and could potentially facilitate detection of other heavy metals such as lead, mercury and copper [17].

Hassanein et al., 2017, fabricated biosensors based on chitosan-ZnO/Polypyrrole nanocomposites. The sensor was prepared using the oxidative polymerization of pyrrole monomer with (NH4)2S2O8 as the oxidant and followed by mixing with chitosan–zinc oxide composites. The conductive polymers and oxide nanoparticles (organic–inorganic nanocomposite materials) have been previously widely used because of the novel properties of this nanocomposite which can be attributed to the successful blending of the individual characteristics of the parent constituents into a single material. The advantages of the oxide nanoparticles are in their ability to modify their chemical, mechanical, electrical, structural, morphological, and optical properties under specific circumstances. Moreover, these nanostructure materials have a larger percentage of surface atoms available which possess high reactivity. From the results, it was found that there was a significant improvement in electrical conductivity from the cyclic voltammetry measurements of the K3(Fe(CN)6) sample. A large enhancement of the stripping of peak current compared to bare CPE was identified using the square-wave adsorptive anodic stripping voltammetry method. Consequently, the proposed material proved to possess suitable ability as sensing materials in biosensor applications [18].

In other work done by Tlili et al., 2005, they reported the technique of interpenetrating network formation, where they immobilized DNA probes bearing amine groups by covalent grafting on a supporting polypyrrole matrix functionalized with activated ester groups. The immobilization step played an important role in determining the overall performance of the biosensor. In order to achieve high sensitivity and selectivity, it required minimization of non-specific adsorption and stability of the immobilization. Polypyrrole (PPy) conducting polymer was chosen in their study because of its biocompatibility, high hydrophilic character combined with high stability in water and facile incorporation of many counter ions which make it highly suitable as an interface for grafting DNA probes onto a micro-sized surface. From the results, it was discovered that the large surface area obtained by using porous polypyrrole leads to an increase in the density of the immobilized DNA probes, which then helps to monitor more easily the DNA hybridization reaction [19].

In yet another study, Hsu et al., 2014, used the electropolymerization method to incorporate chloro(protoporphyrinato) iron(III) (hemin), polypyrrole (PPy), and silver (Ag) in order to achieve sufficient sensitivity for an environmental dissolved oxygen (DO) sensor. The electropolymerization method provides a strong adhesive bond at the substrate/hemin interface and allows for an increased concentration of hemin. However, due to their poor current collection capacity, electropolymerized films with higher hemin loading do not instead produce proportionally higher current or increased sensitivity. Therefore, co-electropolymerizing hemin and pyrrole to fabricate a sensing electrode for dissolved oxygen sensing applications is one of the better methods for solving this lower sensitivity problem. Thus, this sensor is able to be manufactured at a lower cost and with longer lifespan. In addition, since it is a solid state sensor, it has the potential to be miniaturized and integrated within a micro-fabricated reference electrode to form a complete sensing system at a very low cost [20].
