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.

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The adsorption of both (L-and D-) enantiomeric forms of cysteine on the silver electrode surface was studied by surface-enhanced Raman scattering spectroscopy (SERS) as a function of electrode potential and pH value of the solution. It was demonstrated that at potentials more positive than -0.7 V (for pH 3) or -0.8 V (for pH 2 or lower), in acidic environment L-cysteine molecules are adsorbed mainly as P H (gauche) conformer, in zwitterionic form with the COO -groups close to the surface. At more negative potentials, NH 3 + groups deprotonate at the surface with simultaneous weakening of the interaction of the carboxylic groups with the surface. Spectroscopic evidence for at least partial protonation of the COO -groups at strongly acidic solutions was given by observing the CdO stretching band at frequency lowered by about 30 cm -1 in comparison with that observed for crystalline cysteine hydrochloride. It points to the considerable enhancement of the strength of hydrogen bonds and may be ascribed to the formation of cyclic L-cysteine dimers at the electrode surface. In neutral and alkaline solutions, adsorbed L-cysteine molecules have deprotonated amino groups at wide potential range. Similar spectroelectrochemical experiments were performed for D-cysteine and for a racemic mixture of D,L-cysteine. As expected, results for D-cysteine were similar to those for L-cysteine. However, for racemic mixture at acidic pH, the spectral effects corresponding to potential-induced transition from adsorbed zwitterions to neutral molecule were considerably smaller. This effect was discussed in terms of stereoselective dimerization of cysteine molecule at the electrode surface.