Constructing Surface Roughness of Silver for Surface-Enhanced Raman Scattering by Self-Assembled Monolayers and Selective Etching Process

Abstract

A new approach has been developed to roughen silver plates or foils for surface-enhanced Raman scattering (SERS). An aqueous solution consisting of potassium ferricyanide(III) and potassium thiocyanate was used as an etchant, which could selectively remove the unoccupied sites of the underlying silver. The resulting substrate was found to have a signal-enhancing effect for Raman scattering comparable to that produced by the oxidation-reduction cycle (ORC) technique. The etching time required to achieve optimal enhancement was very sensitive to the structure of adsorbed molecules in static etching. Scanning electron microscopy (SEM) allowed the characterization of surface roughness of the substrate. A step-by-step etching mechanism was revealed by the SEM images obtained from the substrate plates after different lengths of etching time, and a homogeneous distribution of the spheric silver features (diameters on the order of 100 nm) was observed throughout the surface after an optimal etching time. Electrochemical measurements of heterogeneous electron-transfer rates for different electrodes indicated an increase of surface area during etching, and an increase in the monolayer's barrier to electron transfer and the ion penetration upon re-immersion of the self-assembled monolayer (SAM) confined silver substrate in thiol solutions. The SER spectrum indicated that cyanide ion was adsorbed on the freshly formed silver surface. But the cyanide could be easily removed by re-immersing the electrode into the corresponding thiolate or sulfinate solution. Substrates subjected to UV exposure and water rinse were found to still be SERS-active and reusable for other analytes.