Adsorption and chemical modification of phenols on a silver surface

The adsorption of phenols of different natures on silver colloidal particle s is studied here by surface-enhanced Raman spectroscopy (SERS). The studie d compounds can be classified in three groups: (a) cinnamic acic derivative s: caffeic and isoferulic acids; (b) catechol; and (c) the phenols derived from benzoic acid: m- and p-hydroxybenzoic acids and salicylic, vanillic, a nd gallic acids. The interest of these compounds lies in the fact that they are naturally occurring molecules with significant importance in relation to plant metabolism, soil chemistry, and vegetal food stability. In additio n, many of these compounds have antioxidant properties derived from their h igh affinity toward atmospheric oxygen. They exhibit high reactivity that m ay be enhanced in the presence of a metal surface such as those employed fo r SERS spectroscopy. From the SERS results it can be deduced that a clear c hemical change of caffeic and gallic acid and catechol occurred. The chemic al modification consists mainly of polymerization connected to existence in the molecule of o-diphenol moieties. In the case of m-hydroxybenzoic acid the chemical change may occur at low pH at which a reorientation of the mol ecule on the surface takes place, while in the o-hydroxybenzoic acid the on ly chemical change seems to be the internal H bond breakdown induced by the complexation with the metal. Finally, isoferulic and p-hydroxybenzoic acid s do not show any chemical modification upon adsorption on the metal, which takes place through the carboxylate group adopting the molecule a standing up orientation. The case of vanillic acid is not so clear, although possib le chemical modification is also possible for this adsorbate. From the resu lts found in this work it can be inferred that the factors influencing poss ible chemical modification are the chemical structure of the adsorbate and its orientation and interaction with the surface. (C) 2000 Academic Press.