Surface-Enhanced Vibrational Spectroscopy: SERS and SEIRA

With respect to the origin of single-molecule sensitivity in surface-enhanced Raman scattering, elastic scattering and emission spectra were investigated for Ag particles adsorbed with dye. The scattering peak observed at 600–650 nm was extinguished during the inactivation process of an enormous SERS signal, whereas localized surface plasmon (LSP) peaks located at 520 nm and 730 nm did not change significantly. The scattering peak at 600–650 nm arises from increased electromagnetic coupling between the LSP of adjacent Ag particles through dye molecules. In addition, distinct emission peaks were observed at 550–600 nm and 600–750 nm for hot Ag particles with adsorbates. These bands were attributed to emissive relaxation of metal electrons and fluorescence of molecules, respectively. Furthermore, the shorter wavelength peak showed invariant Stokes shift irrespective of excitation wavelengths, most probably arising from inelastic scattering of excited electrons by adsorbed molecules. The adsorbed state of CO and related species on the Pt film electrode was investigated using attenuated total reflection—surface-enhanced infrared absorption spectroscopy. Intermediate species were found on the bare Pt surface in 1 mM CH₃OH + HClO₄ solutions at +0.2 V ≤ E ≤ +0.6 V that give absorption peaks at 1405 cm^?1 and 1300 cm^?1. These bands can be attributed to carbonate species or -COH. Water molecules located at the hydrophobic interfaces between CO and electrolyte solutions were evidenced by a quite high OH stretch absorption at 3664–3646 cm^?1, as well as a lower broad peak at ca. 3480 cm^?1.