*2.5. Impedimetric PPy Based Electrochemical Sensors and Biosensors*

In most EIS studies that involve electrochemical sensors, the use of an electroactive probe/indicator has been employed to evaluate an electrode's signal activity. The advantage of using an electroactive probe is that it can serve as a reference point for impedance studies. For example, Arabali et al. fabricated an amplified tramadol electrochemical sensor based on the surface modification of pencil graphite electrode (PGE) by CuO nanoparticle (CuO-NPs) and polypyrrole (PPy). From the impedance results obtained, it confirmed the modification of PGE with PPy and PPy + CuO-NPs was able to improve the electrical conductivity of the sensor and exhibited a highly sensitive electroanalytical sensor for the determination of tramadol [44]. Furthermore, in humidity sensor applications, impedance analysis is commonly used to study the effect on a conducting polymer composite with various different compositions on the percentage of relative humidity measurement, RH%. Su et al. reported that the addition of PPy-Ag into pristine SnO2 helped to reduce the impedance resistance, thereby increasing its sensitivity especially at low RH%. The study also proved that the impedance decreased as the amount of PPy-Ag increased. The addition of PPy-Ag helped to increase the mobility of solvated ions inside the system [45]. Meanwhile, Jlassi et al. reported that their findings did not follow this trend. They studied the effect of a combination PPy-Ag with modified halloysite nanoclay (HNT) films on RH% sensitivity at three different wt.% (0.25 wt.%, 0.5 wt.%, and 1 wt.%) amounts. The best finding was achieved using PPy-Ag 0.5 wt.% film due to its hydrophilic behavior. They

found that the impedance decreased as the humidity level increased. This is because the higher the humidity level, the higher the amount of moisture absorbed onto the PPy-Ag film, thereby increasing the conductivity of the film [46].
