Surface-enhanced Raman scattering of 3-mercaptopropionic acid adsorbed on a colloidal silver surface

Chemisorption of 3-mercaptopropionic acid (3MPA) on a colloidal silver surface was investigated using surface-enhanced Raman scattering (SERS). The nu(SH) vibration has never been observed in the SERS of 3MPA recorded over a wide range of pH. That behaviour enables us to deduce that the -SH group is deprotonated and links to the metal forming an Ag - S bond but, in contrast, the carboxylic or carboxylate groups do not link to the metal. The carboxylic group is preserved even at pH values at which they should be deprotonated and only under rather basic conditions (pH greater than or equal to 8) do the carboxylic groups dissociate. Consequently, the behaviour of 3MPA adsorbed on colloidal silver is analogous to that observed by Kudelski on a silver electrode. Finally, it is observed in all the SERS spectra recorded at any pH that it is the T-conformer, defined around the C(2) - C(3) bond, which adsorbs preferentially. Copyright (C) 2004 John Wiley Sons, Ltd.