In situ characterization of low-frequency SERS features during aniline oxidation in AuNP-KI colloidal system.

While the oxidative polymerization of aniline is widely used to model the redox behavior of aromatic amines and the growth mechanism of polyaniline (PANI), systematic in-situ Raman investigations of the low-frequency region (200-600 cm-1) remain limited. In this work, time-resolved surface-enhanced Raman spectroscopy (SERS) was used to monitor ammonium persulfate (APS)-induced aniline oxidation in a potassium iodide-modified gold nanoparticle (AuNP-KI) colloidal system, with particular emphasis on the 200-600 cm-1 region. KI-dependent changes in the optical and SERS responses were used to select conditions for stable spectral acquisition. After APS addition, the bands at approximately 366, 408, 457, and 534 cm-1 exhibited distinct temporal profiles. The 366 cm-1 signal decreased rapidly, whereas the other three bands increased and subsequently approached relatively stable levels. Concurrent changes in the bands at 801, 986, 1160, and 1640 cm-1 were consistent with the evolution of aniline-rich interfacial species toward oligomeric and more oxidized PANI-like structures. This work demonstrates that low-frequency SERS fingerprints provide complementary structural evidence for monitoring aniline oxidation dynamics.