FT Surface-Enhanced Raman Evidence of the Oxidative Condensation Reactions of Caffeic Acid in Solution and on Silver Surface

Fourier transform (FT)-Raman spectroscopy and FT surface-enhanced Raman spectroscopy (FT-SERS) are applied for the first time to study the chemical oxidative changes undergone by caffeic acid (CA) in solution and when adsorbed on silver colloids. UV-visible spectroscopy was also applied to follow the polymerization of CA. Normal Raman techniques have only a limited application in the characterization of CA oxidation products owing to the high fluorescence background emitted by them. Nevertheless, the use of silver colloids 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 in situ on the surface. The influence of the solvent (water or ethanol) in the oxidation of the stock solution, the pH, and the storage time on the SERS spectra was investigated and analyzed on the basis of the oxidation and polymerization mechanisms proposed by several authors in the literature. The results found for CA were compared with those for other chemically related molecules as catechol and isoferulic acid in order to assign the vibrational spectra obtained at the different conditions employed to carry out the study of CA oxidation. The results found in this study demonstrate that the polymerization mechanism followed by CA depends on the conditions of CA storage and those at which the SERS spectra are recorded. The conclusions derived from this work can be very useful in the understanding 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 similar investigations is the ability to carry out in situ time-resolved measurements, thus allowing assignments of the different mechanisms of CA oxidation under the different experimental conditions employed.