**1. Introduction**

Conducting polymers (CPs) play an important role in the design and development of electrochemical sensors. They provide the necessary electrical conductivity to transduce the occurrence of the coupling event into an analytical signal (Figure 1) [1].

CPs are conjugated polymers that possess delocalized π-bonds on the backbone of the polymers. These π-bonds assist in the migration of electrons throughout the polymeric chain. Thus, these polymers can act as conductors, semiconductors, or even superconductors. Besides that, CPs also have high electron affinity and redox activity. The general physical properties of CPs depend on the size and length of the conducting polymer, which can also be described in terms of their molecular weight [2]. There are many types of CPs such as polyaniline (PANI), polypyrrole (PPy), polyacetylene, polyparaphenylene (PPPh), polyparaphenylene vinylene (PPV), polythiophenes (PTh), polydiaminonaphthalene, polyazulene, and poly (3,4-ethylene dioxythiophene) (PEDOT) [3]. Electrically conductive polymers are a class of materials that can be fabricated to generate either transient or static electrical charges because of their physico-chemical properties [4]. Among those polymers, PPy is the most extensively used as a conducting polymer in electrochemical sensing applications because it is a heterocyclic conducting polymer, with good

**Citation:** Dzulkurnain, N.A.; Mokhtar, M.; Rashid, J.I.A.; Knight, V.F.; Wan Yunus, W.M.Z.; Ong, K.K.; Mohd Kasim, N.A.; Mohd Noor, S.A. A Review on Impedimetric and Voltammetric Analysis Based on Polypyrrole Conducting Polymers for Electrochemical Sensing Applications. *Polymers* **2021**, *13*, 2728. https:// doi.org/10.3390/polym13162728

Academic Editor: Jung-Chang Wang

Received: 7 June 2021 Accepted: 27 July 2021 Published: 15 August 2021

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environmental stability, high electronic conductivity, and biocompatibility compared to other CPs [5–8].

**Figure 1.** Schematic diagram of a conducting polymer and analyte on an electrochemical sensor.

Furthermore, PPy is easy to synthesize as a black powder chemically and electrochemically as thin films on various electrodes for both aqueous and nonaqueous media [4,9–11]. PPy has also been applied in multidisciplinary applications such as batteries, supercapacitors, coatings, etc. [12–14]. Despite these excellent properties, it also suffers from certain drawbacks, namely, poor thermal solubility and low mechanical stability [15]. These drawbacks can affect the response characteristics and sensitivity to small perturbation of the electrochemical sensor [3]. In the past two decades, scientists have applied different approaches to modifying PPy to improve its properties. These approaches include blending [16], electro-polymerization [17], interpenetrating network formation, and composite synthesis [18,19].
