Photoinduced coupling and adsorption of caffeic acid on silver surface studied by surface-enhanced Raman spectroscopy

The effect of light on the caffeic acid (CA) oxidative coupling is studied in aqueous solution and on silver by surface-enhanced Raman spectroscopy (S ERS). CA can polymerize in aqueous solution or on a metal surface through a n oxidative mechanism involving the formation of the corresponding quinone giving rise to characteristic Raman features in each case. We show here tha t the effect of light in relation to this oxidative coupling is crucial tak ing place mainly in the solution bulk. The products derived from such polym erization can then adsorb on the silver surface employed for SERS measureme nts, thus allowing its detection by Raman spectroscopy. The influence of ir radiation time and the wavelength of the light employed for the photoinduce d coupling was investigated. (C) 1999 Elsevier Science B.V. All rights rese rved.