Surface-enhanced Raman scattering investigations of 4-nitro(pyridine N-oxide) and 4,4′-azobis(pyridine N-oxide) adsorbed on silver colloidal nanoparticles

A spectroscopic investigation on the adsorption process of 4-nitro(pyridine N-oxide) (NPO) onto silver colloidal nanoparticles has been performed by Raman and UV-visible absorption measurements. Discrepancies between the Raman spectra of NPO in aqueous solution and in Ag hydrosol prompted an extension of the SERS study to include an investigation on the reactivity of this compound in silver colloidal suspension. The data are diagnostic of a reduction of the nitrogroup of NPO with subsequent formation of 4,4'-azobis( pyridine N-oxide) (APO). Hence, the adsorption mechanism and the molecular arrangement of APO adsorbed on the silver surface have been investigated by means of the SERS technique with the help of Density Functional Theoretical calculations of models of APO bound to a silver surface adatom.