Surface-enhanced Raman scattering (SERS) of 4-mercaptobenzoic acid on silver and gold substrates

Surface-enhanced Raman scattering (SERS) spectra were recorded for p-mercaptobenzoic acid (PMBA) monolayers on Ag and Au surfaces. As evidenced by the relative intensities of the bands corresponding to the symmetric stretching COO- vibration and to the out-of-plane vibration of the phenyl ring, the molecules are at least tilted with respect to the surface and simultaneously bonded through the sulfur atom and COO- groups. This situation takes place when modification is carried out from aqueous solutions at natural or alkaline pH and at low concentrations of monomer. At higher concentration of the monomer (>10(-3)) and/or at acidic pH, carboxylic groups are protonated and PMBA molecules adopt a more vertical configuration. It was also found that rough Ag and Au surfaces that are applied in SERS experiments promote partial decarboxylation of PMBA. Copyright (C) 2002 John Wiley Sons, Ltd.