pH-Response Mechanism of p-Aminobenzenethiol on Ag Nanoparticles Revealed By Two-Dimensional Correlation Surface-Enhanced Raman Scattering Spectroscopy

The existence of pH-dependent surface-enhanced Raman scattering (SERS) of p-aminobenzenethiol (PATP) on Ag nanoparticles has been confirmed by numerous studies, but its mechanism still remains to be clarified. Discussion of the mechanism is at a standstill because of the lack of a systematic investigation of the process behind the pH-induced variation of the PATP behavior. Two-dimensional correlation spectroscopy is one of the most powerful and versatile spectral analysis methods for investigating perturbation-induced variations in dynamic data. Herein, we have analyzed the pH-dependent behavior of PATP using a static buffer solution with pH ranging from 3.0 to 2.0. The order of the variations in the different vibrational intensities was carefully investigated based on 2D correlation SERS spectroscopy. These results have demonstrated that the very first step of the pH-response process involves protonation of the amine group. The pH-response mechanism revealed is an important new component to our understanding of the origin of the b2-type bands of PATP.

Temperature effect on the SERS signature of p-aminothiophenol: a new evidence for the production of p,p'-dimercaptoazobenzene on metallic nanostructures

We investigated the effect of the temperature of the plasmonic substrate on the surface enhanced Raman scattering of p-aminothiophenol adsorbed onto a particulate film of gold nanoparticles-decorated polystyrene nanospheres. The results demonstrated that temperature induces important modifications in the overall spectral signature of the charge transfer bands, which are consistent with the generation of a new molecular species. Moreover, the analysis of the shape of the vibrational band at 1078 cm(-1) assigned to the >C-S mode, which is enhanced by electromagnetic mechanism, reveals the co-existence of two distinct bands at 1071 and 1078 cm(-1), assignable to p,p'-dimercaptoazobenzene and p-aminothiophenol, respectively.

Catalytic processes monitored at the nanoscale with tip-enhanced Raman spectroscopy

Heterogeneous catalysts play a pivotal role in the chemical industry, but acquiring molecular insights into functioning catalysts remains a significant challenge. Recent advances in micro-spectroscopic approaches have allowed spatiotemporal information to be obtained on the dynamics of single active sites and the diffusion of single molecules. However, these methods lack nanometre-scale spatial resolution and/or require the use of fluorescent labels. Here, we show that time-resolved tip-enhanced Raman spectroscopy can monitor photocatalytic reactions at the nanoscale. We use a silver-coated atomic force microscope tip to both enhance the Raman signal and to act as the catalyst. The tip is placed in contact with a self-assembled monolayer of p-nitrothiophenol molecules adsorbed on gold nanoplates. A photocatalytic reduction process is induced at the apex of the tip with green laser light, while red laser light is used to monitor the transformation process during the reaction. This dual-wavelength approach can also be used to observe other molecular effects such as monolayer diffusion.