title: Surface-enhanced Raman scattering of hydroxybenzoic acids adsorbed on silver nanoparticles.
abstract: Surface-enhanced Raman scattering (SERS) of hydroxybenzoic acids has been studied on silver sols in H(2)O and D(2)O solutions. The adsorption behavior of 4-hydroxybenzoic acid (4HBA) is different from that of salicylic (2HBA) and 3-hydroxybenzoic (3HBA) acids. It was concluded that 4HBA is adsorbed on silver nanoparticles (Ag(n)) as either oxidobenzoate (A(2-)) or hydroxybenzoate (A(-)), depending on the pH of the solution, given rise to a flat orientation. Both 2HBA and 3HBA acids are always adsorbed as hydroxybenzoates anions (A(-)) at pH >or=5 and link to the metal through the carboxylate group (Ag(n)--A(-)), standing more or less perpendicular to the metal surface. In the case of these monoanions, the selective enhancement of the bands is due mainly to a resonant electron or charge transfer process (ET or CT) from the metallic nanoparticles to the adsorbates, yielding the transient formation of the respective radical dianions (Ag(+)(n)--A(2-)). It is found that the enhanced bands, and especially the mode 8a;nu(ring), are related to the difference between the equilibrium structures of the adsorbate in its ground (A(-)) and CT-excited (A(2-)) states. In the SERS spectrum of 4HBA dianion, the contribution of CT mechanism is not observed.
authors.affiliation: Department of Physical Chemistry, Faculty of Sciences, University of Malaga, E-29071 Malaga, Spain.
authors.forename: J L
authors.initials: JL
authors.lastname: Castro
authors.forename: J F
authors.initials: JF
authors.lastname: Arenas
authors.forename: M R
authors.initials: MR
authors.lastname: López-Ramírez
authors.forename: D
authors.initials: D
authors.lastname: Peláez
authors.forename: J C
authors.initials: JC
authors.lastname: Otero