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

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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.