## 3 Results and Discussion
### Comparison of Sensing Performance  
The changes in the real-time responsive resistance were measured for different concentrations of gases at 25 \({}^{\circ}\)C. Figure 7a shows the sensing performance of the electrodes toward NH\({}_{3}\) gas, which is one of the reducing gases, with respect to the pH. The NH\({}_{3}\) molecules generate electrons, owing to the reduction characteristics during detection. These electrons are transferred to the PPyNPs, thereby reducing the amount of the charge carriers (hole) in the polymer chain. Therefore, the resistance of the sensor electrode increases, owing to the p-type semiconductor characteristics of the polymer. The sensor electrodes exhibit being highly sensitive toward NH\({}_{3}\) down to 10 ppb due to the uniformly decorated nanoparticles on the IDA electrode. However, the response amounts of the electrodes display increase with reducing pH of solution. The more protonated PPyNPs (treated at pH 1) have more holes in the chain structure, indicating a higher degree to change in external electron flow. On the other hand, the electrode that was treated at pH 13 shows the smallest amount of resistance change, even though it has the smallest diameter. Thus, a reduction in the amount of holes, rather than a reduction in the surface area, has a greater effect on response change. Figure 7b shows the recovery times of the electrodes, exhibiting a decrease from 59 s (pH 13) to 19 s (pH 1). The numerous charge carriers (hole) at pH 1 induce the rapid transfer of electrons from the analyte and reduce the response time to <20 s. However, the recovery times of the electrodes exhibit similar values, i.e., 66 s for pH 13, 61 s for pH 7, and 49 s for pH 1, because the detachment of the target molecule is related to the applied external energy rather than to the charge carriers and number of active sites (Figure 7c). Moreover, the sensing performance with decreasing NH\({}_{3}\) concentration was also measured to confirm the reproducibility of the electrode. Figure 7d shows that the sensing ability of the electrode remains constant, regardless of the direction of change in NH\({}_{3}\) concentration. In addition, we also compared the response and recovery times of the electrode with the direction of concentration changes. The response time of the sensor electrode is similar (19 s) in both cases, as shown in Figure 7e. However, the recovery time for backward direction (60 s) presents a little longer than forward direction (49 s), due to exposure to large amount of NH\({}_{3}\) molecules during detection (Figure 7f). Therefore, PPy-based sensor electrodes have good reproducibility for environmental changes. Additionally, methanol (MeOH) was used to determine the sensing ability for an oxidizing gas (opposite charge-transfer direction) (Figure 8a). The electrode oxidized at pH 1 is superior to other electrodes for the same reason, as observed in the case of NH\({}_{3}\). Figure 8b shows the variation in the normalized resistance of the electrodes as a function of NH\({}_{3}\) and MeOH gas concentrations. The linear-behavior range is larger for the electrode oxidized at pH 1 (1 ppb-10\({}^{2}\) ppm for NH\({}_{3}\) and 0.1-10\({}^{3}\) ppm for MeOH) than for the other electrodes. Therefore, the sensor electrode oxidized at pH 1 can be effectively utilized as a signal transducer for detecting NH\({}_{3}\) and MeOH gases at various concentrations. Additionally, outstanding cycle stability is required for use of the electrode materials in practical sensor devices. Figure 8c shows the change in the resistance of the electrode at pH 1 upon periodic exposure to 10 ppb NH\({}_{3}\) gas. During the five repetitive on-off tests, the resistance changes remain relatively constant, without a delay in the response or recovery. Furthermore, the sensor electrodes maintain their sensing ability during exposure to 0.1 ppm of NH\({}_{3}\) over 15 days (Figure 9). Figure 7: Reversible and reproducible responses are measured at a constant current (10\({}^{-6}\) A) for PPy-based electrodes.