**4. Conclusions**

This review presents an overview of the two main electrochemical sensing modes used with electrochemical sensors based on PPy conducting polymers and outlines the significant advances in this field. In this review, the PPy heterocyclic-based conducting polymer was chosen due to its good environmental stability, high electronic conductivity, and biocompatibility. Conducting polymers are considered good sensitive materials for the development of selective, specific, and stable sensing devices. However, having PPy alone as the conducting polymer might not give the best results for an electrochemical sensor or biosensor application. Therefore, modification of the conducting polymer is the way to enhance its properties. To get an in-depth overview of how the modification can improve electrochemical and biosensors, the two main electrochemical techniques, impedimetric and voltametric, should be utilized. From this review, it can be seen that these two techniques play an important role in order to determine the resistivity, reactivity, and sensitivity of the PPy based conducting polymers for use in electrochemical sensors and biosensors. Besides that, they can also help the understanding of the reactivity that occurs when modification of the PPy conducting polymer has been done.

**Author Contributions:** Conceptualization, N.A.D., J.I.A.R., M.M. and S.A.M.N.; writing—original draft preparation, N.A.D., J.I.A.R., M.M. and S.A.M.N.; writing—review and editing, N.A.D., J.I.A.R., M.M., V.F.K. and S.A.M.N.; visualization, N.A.D., J.I.A.R., M.M. and S.A.M.N.; supervision, S.A.M.N.; funding acquisition, V.F.K., K.K.O., N.A.M.K., W.M.Z.W.Y. and S.A.M.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** The authors are grateful to the Ministry of Education Malaysia, Development Fund F0020 for funding via UPNM/2018/CHEMDEF/ST/5.

**Conflicts of Interest:** The authors declare no conflict of interest.
