Surface-enhanced raman scattering of 2,2′-bipyridine adsorbed on colloidal silver and stabilized AgBr sols

SERS investigations of 2,2′-bipyridine molecules adsorbed on Ag colloids and AgBr sols are reported. Absorption spectra and electron micrographs of colloidal systems were studied and evolution of silver particles formed at photolysis of silver bromide was described. The character of SERS spectra and their dependence on 2,2′-bipyridine concentration evidence the formation of two types of complexes bound to the surface in edge-on orientation through the lone-pair electrons of the N atoms. At low concentrations the strongly chemisorbed complex with a Lewis acid coordination structure prevails and at high concentrations the complex with usual coordination σ bonds analogous to that in complex ion Ag (bpy)⁺₂ forms. The bromide ion participation in creating the active sites for the formation of strongly chemisorbed surface complexes is proposed. The electromagnetic mechanism of Raman scattering enhancement is assumed to prevail in the systems studied, although a certain contribution of the chemical enhancement is not excluded.