Catalytic modification of gallic acid on a silver surface studied by surface-enhanced Raman spectroscopy

We studied the chemical changes undergone by gallic acid (GA) when adsorbed on a colloidal silver surface by means of surface-enhanced Raman spectroscopy (SERS). The application of the SERS technique allows a remarkable Raman sensitivity enhancement and the strong fluorescence quenching needed to characterize the highly colored products of GA oxidative condensation. We found that GA can undergo a ring opening, a polymerization of aromatic rings or a combination of both mechanisms, leading to the formation of humic-like substances. The influence of pH and light irradiation on these catalytic processes was investigated in relation to the importance of these mechanisms. © 2001 John Wiley & Sons, Ltd.