Surface-Enhanced Raman Scattering of 4-Aminobenzenethiol on Ag and Au: pH Dependence of b2-Type Bands

The surface-enhanced Raman scattering (SERS) of 4-aminobenzenehtiol (4-ABT) has seen a surge of interest recently, since its SERS spectral features are dependent not only on the kinds of SERS substrates but also on the measurement conditions. The most unusual SERS feature is the appearance of b(2)-type bands in the region 1100-1500 cm(-1), in contrast to their absence in the normal Raman spectrum, but their origin is not yet clarified. However, propositions have been made suggesting that their appearance is associated with either a charge transfer phenomenon or a surface-induced photoreaction product such as 4,4'-dimercaptoazobenzene (4,4'-DMAB). In this work, we found that the b(2)-type bands of 4-ABT are strongly affected also by the solution pH. Regardless of the excitation wavelength and kind of SERS substrates, the b(2)-type bands appeared very weakly or negligibly at acidic pHs, while they were observed very distinctly at basic pHs. For the case of 4,4'-DMAB, any such pH dependence was not observed at all in its SERS spectra. Since the pH dependence in the SERS of 4-ABT was observed reversibly, the appearance and disappearance of the b(2)-type bands must have nothing to do with formation of any surface-induced photoreaction product like 4,4'-DMAB. Consulting the pH-dependent UV-vis absorption spectra and ab initio quantum mechanical calculation, the disappearance of the b(2)-type bands at acidic pHs is presumed to be associated with the upshift of the lowest unoccupied molecular orbital level of 4-ABT caused by protonation of the amine group: the charge transfer resonance chemical enhancement will then be less likely to occur.