# The First Stages of Chemical and Electrochemical Aniline Oxidation--Spectroscopic Comparative Study
## 3 Results
### Raman SEC of the Aniline Oxidation at pH 7  
Electrochemical oxidation of aniline was followed in situ by SERS SEC. The voltamperogram of the aniline oxidation at pH 7 (Figure 8a) shows only one oxidation peak at ca 0.4 V, and an oxidation ramp of aniline to cation radical, while the electrochemical oxidation of aniline at pH 1 shows only the aniline oxidation ramp (Figure 8b). In the reduction phase, the pH 1 sample displays the weak reduction peaks of PANI at ca 0.45, 0.25 and \(-\)0.15 V, while the pH 7 reduction is flat, probably as the electrode is passivated by the AO layer. The in-situ SERS SEC of aniline oxidation at pH 1 (Figure 9) was thoroughly described elsewhere [18]. For the purpose of this paper, this experiment is used as a reference of linear AO redox behavior. The Raman spectra of the products of the electrochemical aniline oxidation at pH 7 (Figure 10) do not change much with potential; they mainly flatten due to increasing fluorescence typical for the non-protonated AO [17]. We will focus on the spectra excited with the 633 nm line, as it should lead to surface enhanced resonance Raman scattering (SERRS) enhancement of the charged phenazine-like  
Figure 6: Raman spectra of gold SERS electrodes removed from the reaction mixture of starting pH 1 at various times. Excitation 785 nm (**a**) and 633 nm (**b**). Figure 7: Raman spectra of gold SERS electrodes removed from the reaction mixture of starting pH 1 and 7 at adsorption stage (2 resp. 5 s after mixing) and at higher oligomer stage (1.5 resp. 2 min after mixing). Excitation 633 nm. oligomers [28], while the 785 nm excitation line resonantly enhances the spectral features of the linear oligomer fraction. The small changes are visualized with potential dependency plots (Figure 11). The ring stretching mode shifts gradually from 1592 cm\({}^{-1}\) (benzenoid ring (Table 3)) to 1598 cm\({}^{-1}\) (charged semiquinonoid ring) (Figure 11a), implying gradual oxidation of the material deposited onto the electrode. The bands around 1170 cm\({}^{-1}\) (C-H deformation on a benzenoid or semiquinonoid ring in a linear structure), 1592-1598 cm\({}^{-1}\) (ring stretching of a semiquinonoid ring in a linear structure) and 1626 cm\({}^{-1}\) (ring stretching of a benzenoid ring located next to a quinonoid ring in a linear structure) reach maximum intensity around 0.4 V--the maximum of the oxidation peak (Figure 11b). These bands correspond to the emeraldine form of protonated PANI. The intensity of the bands at 1348 cm\({}^{-1}\) (C-H bending on a benzenoid ring in a PANI base) and 1565 cm\({}^{-1}\) (ring stretching of a benzenoid ring in a PANI base) increase at potentials above 0.4 V (Figure 11c), implying the formation of the oxidized but neutral PANI base. Figure 8: Electrochemical oxidation of aniline in aqueous medium of pH (**a**) 7 and (**b**) 1. Red marks separate potential ranges connected with one Raman spectrum. Figure 9: Raman spectra of gold SERS electrodes during the electrochemical oxidation of aniline in aqueous medium of pH 1 (excitation 785 nm (**a**) and 633 nm (**b**)). The bands at 1404 cm\({}^{-1}\) (quinonoid ring stretching of PANI base) and 1464 cm\({}^{-1}\) (C=N stretching) have minimum intensity around 0.4 V (Figure 11d). This is probably the result of the higher intensity of other bands around 0.4 V.  
From the in-situ aniline oxidation at pH 7 we can conclude that a layer of a mixture of AO structures is formed on the electrode. The linear fraction, that is electroactive, is responsible for the changes observed in the Raman spectra. The high background is probably due to the _ortho_-linked AOs that tend to display fluorescence if not protonated [17]. Figure 11: Potential dependencies of the position of the ring-stretching mode around 1600 cm\({}^{-1}\) (**a**) and normalized band intensities of several changing bands during the electrochemical oxidation of aniline in aqueous medium of pH 7 (**b–d**). Excitation 633 nm.