*3.5. PPy Based Voltammetric Sensor*

In order to develop sensitive detection instruments, different signal amplification strategies also can be utilized to improve the sensitivity of the sensors used. Ma et al. designed an ultrasensitive and specific electrochemical biosensor for the determination of miRNAs based on SA-PPy/AuNPs. It was analyzed using the SWV technique with a dual signal amplification strategy involving the catalytic hairpin assembly (CHA) reaction and Cu2+/Fe3+ catalytic reaction [58]. They found that, with the usage of this dual signal amplification strategy and a sensitive analyzer technique, which was SWV, the LOD value was significantly improved and could be as low as 0.34 fM compared to other research work [78–81]. Table 1 shows the comparison of different methods and signal amplification strategies with miRNA-21 as the target. The CHA reaction was triggered and numerous double helix-DNA were formed in the presence of the target miRNA. Furthermore, the intercalated planar Cu (II) complex could bind with the double-helix DNA, which was employed as a signal amplification molecule in the presence of Fe3+. After the dual signal amplification, the reduction of the Cu (II) complex was recorded to represent miRNA levels.

**Table 1.** Comparison of different methods and signal amplification strategies with miRNA-21 as the target.


Table 2 summarizes the electrochemical sensing modes of PPy based electrochemical sensors reviewed in this paper with their possible applications.


**Table 2.** Summary of electrochemical sensing modes of PPy-based electrochemical sensors with their applications.
