**3.1 Effect of pH toward photochemical reaction on PATP-adsorbed PMNMs**

In order to give a clear explanation of the effect of pH, adsorption of molecules for surface catalytic coupling reactions, and oxygen-assisted photocatalytic reactions on PMNMs, we have chosen only para-aminothiophenols (PATPs) as representative compounds. The SERS signals are highly sensitive to the pH of the electrolyte and applied potential. It is evident from the previous SERS measurements of the adsorption of PATP on the surface of rough silver and gold electrodes, reported by Hill and Wehling [31]. The property of SERS significantly varies from acidic to alkaline medium. For the case of anodic and cathodic polarization, SERS spectra of PATP show substantial variations in alkaline solution. In contrast to alkaline medium, a good reversibility behavior (in intensity of SERS) was seen in acidic medium based on changing potentials. As the applied potentials move toward the negative direction, the SERS peaks (1130, 1390, and 1440 cm<sup>−</sup><sup>1</sup> ) retained virtually in the alkaline medium. In order to describe the above character, an isomerization of aromatic and quinonoidic configurations was postulated for PATP adsorbed on the surface of metal electrodes in acidic medium. Under negative applied potential (−1.4 V vs. Ag|AgCl) in pure sodium disulfide, the quinonoidic configuration was proposed in alkaline medium to support the above hypothesis. Based on the adsorption of PATP in the nanoscale cavity between the substrate (gold) and silver nanoparticles, the isomerization and charge transfer mechanism are given concurrently to the SERS mechanism by Zhou et al. [32]. Till now, we know these changes of SERS are closely associated with the surface catalytic coupling reaction of PATP to DMAB.
