Simulating pH‐dependent surface‐enhanced Raman spectra by density functional theory calculations.

The pH‐dependent surface‐enhanced Raman spectra of two typical surface sensor molecules p‐mercaptobenzoic acid (PMBA) and p‐mercaptopyridine (PMPY) were simulated by density functional theory calculations. First, the acid dissociation constants and individual surface Raman spectra of PMBA and PMPY were computed and compared with experimental results. It was found that acid dissociation constants calculated by the hybrid implicit‐explicit model and surface‐enhanced Raman scattering (SERS) spectra simulated by the explicit model most closely coincide with experimental results. Then, the pH‐dependent SERS spectra of PMBA and PMPY were obtained by overlaying the SERS of individual acid species and base species multiplied by their molar fractions, which were calculated from the acid dissociation constants. During the deprotonation process of PMBA, the Raman intensity from COOH stretching decreases and the Raman intensity from COO− stretching increases. During the protonation process of PMPY, the ν8a mode undergoes a significant blueshift and the relative Raman intensity of ν7a mode decreases. At last, pH calibration curves of PMBA and PMPY were obtained according to the simulated pH‐dependent Raman spectra, in which the logarithmic value of relative Raman intensity of characteristic peaks varies almost linearly with solution pH. [ABSTRACT FROM AUTHOR]