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

Abstract

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.

A gold nanopopcorn attached single-walled carbon nanotube hybrid for rapid detection and killing of bacteria

“…The Raman spectrum of 4-ATP has bands at 464 cm −1 δ(CS), 890 cm −1 associated with π(CH) + δ(SH), 1089 cm −1 v(CS), 1439 cm −1 v(CC) + δ(CH) and 1595 cm −1 v(CC). 50–51 Hence the observed bands in Figure 3A can be interpreted to originate with the numerous phenyl rings used as starting materials during the modification. The shifts in the peak frequencies in Figures 3(A) and (B) (b) provide evidence for significant chemical bonding.…”

“…In Figure 3A, the new bands at 480 cm −1 δ(CS), ~1080 cm −1 v(CS), 1145 cm −1 δ(CH) and 1390 cm −1 v(CC) + δ(CH), are due to chemical modification. 50–51 These bands are absent in the Raman spectrum of pristine SWCNTs, (Figure. 3(B) (a)).…”

Role of Surface Adsorption in the Surface-Enhanced Raman Scattering and Electrochemical Detection of Neurotransmitters

“…1, 7-9 Examining the SERS signal of these systems with respect to pH changes 3, 10-12 , electrochemical potential 10, 13 , and temperature 3 allows for the elucidation of the analyte-substrate interaction. Molecules adsorbed at surfaces have been used to evaluate the distance dependence of the SERS effect.…”

Direct evidence for conversion of p‐aminothiophenol to p,p'‐dimercaptoazobenzene by in situ reduplicative surface‐enhanced Raman scattering measurements

“…Especially to the effect of solution pH, it is found that the SERS spectra of DMAB were not subjected to change upon variation of pH. But the SERS spectra of PATP varied in different pHs, and the pH dependence was reversible . Thus, they concluded that the appearance and disappearance of the b 2 modes had nothing to do with formation of DMAB, but associated with the chemical enhancement in the SERS of PATP.…”