SURFACE-ENHANCED RAMAN-SPECTROSCOPY OF P-AMINOBENZOIC ACID WITH EXCITATION IN THE VISIBLE AND NEAR-INFRARED SPECTRAL REGION

Surface-enhanced Raman spectroscopy (SERS) of p-aminobenzoic acid (PABA) in yellow silver colloids was studied with excitation in the visible as well as in the near infrared spectral region by means of a conventional scanning spectroscopic technique. Two kinds of adsorbed surface species, one binding through the carboxylate and another through the amine group to the colloidal silver surface, were deduced from the spectral changes before and after addition of sodium chloride solution. It was found that the species adsorbed through the amine group was probably desorbed by competitive adsorption of chloride ions. The enhancement factor in the near infrared is larger than in the visible spectral region and the optimum conditions for near infrared (NIR)-SERS were found different from those for SERS with visible light excitation. Several enhancement mechanisms may contribute simultaneously to the NIR-SERS of molecules in silver colloids, two of which are presented and discussed in detail in this paper.