FT surface-enhanced Raman evidence of the oxidative condensation reactionsof caffeic acid in solution and on silver surface

Fourier transform (FT)-Raman spectroscopy and FT surface-enhanced Raman spe ctroscopy (FT-SERS) are applied for the first time to study the chemical ox idative changes undergone by caffeic acid (CA) in solution and when adsorbe d on silver colloids. UV-visible spectroscopy was also applied to follow th e polymerization of CA, Normal Raman techniques have only a limited applica tion in the characterization of CA oxidation products owing to the high flu orescence background emitted by them. Nevertheless, the use of silver collo ids may both quench the fluorescence signal and catalyze the CA oxidation, with the additional advantage of analyzing very small concentrations of the CA derivatives formed ill situ on the surface. The influence of the solven t (water or ethanol) in the oxidation of the stock solution, the pH, and th e storage time on the SERS spectra was investigated and analyzed on the bas is of the oxidation and polymerization mechanisms proposed by several autho rs in the literature, The results found for CA were compared with those for other chemically related molecules as catechol and isoferulic acid in orde r to assign the vibrational spectra obtained at the different conditions em ployed to carry out the study of CA oxidation, The results found in this st udy demonstrate that the polymerization mechanism followed by CA depends on the conditions of CA storage and those at which the SERS spectra are recor ded, The conclusions derived from this work can be very useful in the under standing of the chemical processes undergone by CA, which lead to possible changes in the nutritional value of the foods where this molecule appears. The advantage of this technique in relation to those employed so far in sim ilar investigations is the ability to carry out ill situ time-resolved meas urements, thus allowing assignments of the different mechanisms of CA oxida tion under the different experimental conditions employed.