Surface Enhanced Raman Spectroscopic Study of Isomeric Formyl Pyridines in Silver Colloid

Abstract

Surface enhanced Raman (SER) spectra of ortho, meta,and paraformyl pyridines in silver sol are compared with their normal Raman spectra in bulk and aqueous solution. Significant enhancement of Raman bands has been observed. The SER spectra suggest an almost normal orientation of the adsorbate molecular planes to the metal surface and also reveal that orthoand paraisomers exist in a hydrate form in the surface adsorbed state. The metaisomer, however, retains its original nicotinaldehyde form. On addition of the formyl pyridines (FP), the ν maxof the bands that appear on the longer wavelength side of the electronic absorption band of stable, neat silver sol show a parallel relationship with the Hammett σ (σ H) values of the isomers. Raman excitation wavelength dependence suggests that resonance to the aggregation band contributes significantly to SER spectra. The concentration dependence of the enhancement ratio shows a maximum, suggesting monolayer formation at a specific concentration for each isomer. This “first-layer effect” and also the excitation wavelength dependence of SER spectra imply a charge transfer contribution to enhancement.