Relation between surface enhanced Raman intensity and surface concentration for 2-amino 5-nitro pyridine adsorbed on roughened polycrystalline silver electrodes

Abstract

A quantitative correlation between surface enhanced Raman (SER) intensity and surface coverage was obtained for 2-amino 5-nitro-pyridine (ANP) adsorption on roughened polycrystalline silver from aqueous 0.09 M KClO 4 + 0.02 M NaOH by recording in the same cell and on the same electrode surface the SER spectrum of ANP at −0.4 V vs. the molar calomel electrode and, immediately after, its surface concentration Γ ANP. The latter concentration was determined by stepping the applied potential from −0.4 V, where ANP is electroinactive, to −1.2 V where the adsorbed ANP molecules are electroreduced instantaneously, and by measuring the corresponding charge using the double-potential-step chronocoulometric method. In this way the effect of any irreproducibilities in the roughening procedure on the SER intensity vs. Γ ANP relationship was minimized. The surface roughening was made quite reproducible and stable by carrying out the oxidation-reduction cycle (ORC) in neutral 0.09 M KClO 4 in the absence of the adsorbate; this ORC ensures the reversibility of ANP adsorption, which satisfies the Frumkin isotherm with a standard Gibbs energy of adsorption of −43 kJ mol −1. The stability of the surface roughness was checked by measuring the photoemission current at −1.4 V on the freshly roughened electrode in the absence of scavengers. The SER intensity of ANP increases proportionally to Γ ANP up to maximum coverage. The SER spectra of ANP on colloidal silver and on silver electrodes in 10 −3 M HClO 4 were also recorded and an assignment of the bands of all these spectra was provided.