## 3 Results and Discussion  
### Transformation of Polymer Chain Structure  
Uniform PPyNPs (ca. diameter of 60 nm) as a sensor transducer material were synthesized via a modified monodisperse method [30]. Here, polyvinyl alcohol (PVA) and ferric chloride (FeCl\({}_{3}\)) were dissolved in deionized water to form the PVA/iron cation (Fe\({}^{3+}\)) complex that acted as the oxidizing and forming agent during the polymerization reaction. Subsequently, the pyrrole monomer was introduced into the aqueous PVA/Fe\({}^{3+}\) complex solution with vigorous stirring to allow for the formation of PPyNPs at the reactive sites, where the pyrrole monomer contacted the PVA/Fe\({}^{3+}\) complex. The charge carrier density and mobility of the PPy chain change reversibly with different protonation states, as shown in Figure 1. The expanded state of the polymer chain is formed upon protonation, while the contracted state is formed upon deprotonation. The PPyNPs were dispersed in aqueous solutions with pH values ranging from pH 1 to 13 to achieve different chemical states of the polymer chains. The concentrations of the charge carriers in the polymer chain were determined while using UV-vis-NIR spectroscopy. UV-vis-NIR spectra were collected to examine the types and transition of the polymer chain at various pH values (Figure 2). The polymer nanoparticles at pH 1 show the highest absorption intensity of the band above 1000 nm and around the 450 nm region because of the high carrier density in the polymer chain, which causes a reduction in the band gap, as shown in Figure 2a. An increase in pH results in the reduction of the band intensity above 1000 nm and a blue shift of the medium intensity around 450 nm. These spectral changes show that a high pH results in the loss of protons (reduction) from the polymer chain [33]. Thus, the polymer chains transform from expanded to contracted states with an increase in pH. Figure 2b also shows the maximum absorption at different wavelengths (300, 450, and 1000 nm). The absorption of the bands in the high-NIR region (1000 nm) continuously decreases as the chain structure changes from an expanded to contracted state. In addition, the intensity of absorption at 450 nm (visible) shifts to a low wavelength with an increase in intensity at 300 nm (UV) due to the same reason. Raman spectroscopy was performed to characterize the different chemical states of the PPyNPs. Figure 3a shows the Raman spectra at different pH values (pH 1-13). The bands at 1049 and 1332 cm\({}^{-1}\) corresponded to the in-plane C-H and C-C stretching vibrations, respectively. Moreover, the strong  
Figure 1: Chemical structures of polypyrrole (PPy) in different chemical states. Figure 2: (**a**) Ultra-violet visible and near infra-red (UV-vis-NIR) absorption spectra of polypyrrole nanoparticles (PPyNPs) at different pH values. (**b**) Normalized absorption spectra of PPyNPs with a change in pH at different wavelengths (black for 300 nm, red for 450 nm, and blue for 1000 nm). bands at 1421 and 1591 cm\({}^{-1}\) are attributed to the C=C symmetric and C=C asymmetric vibrations, respectively. These bands shift to 1415 and 1562 cm\({}^{-1}\), respectively, with an increase in pH indicating the transformation of the polymer chains from quinoid to benzenoid structures. Specifically, the positively doped species are located at the geometrical defects and form the quinoid structure. Therefore, high pH decreases the amount of (bi)polarons and changes the polymer chain structures to quinoid structures [34; 35]. Furthermore, high-resolution N 1s X-ray photoelectron spectroscopy (XPS) was employed to confirm the change in the chemical state of the PPy chain at different pH values (Figure 3b-f). The peaks at ca. 400 eV (related to the neutral amine N of the pyrrole unit (-NH-)) and ca. 398 eV (attributed to imine (-N=)) are observed in all cases [36]. For the nanoparticles at pH 1 and 4, two types of positive nitrogen peaks, including the N peaks at 403.0 eV (=NH\({}^{+}\)-) and 401.4 eV (-NH\({}^{+}\)-), are observed.