Surface-enhanced Raman Spectroscopic Studies of Ellagic Acid in Silver Colloids

Ellagic acid is believed to perpendicularly coordinate to an Ag+ adatom on the basis of the strong in‐plane vibrational bands observed in its surface‐enhanced Raman scattering spectrum and agreement with the computed vibrational characteristics for the configuration. Evidently this natural dye adsorbs on the Ag surface most effectively in near neutral solution, and the effects of excitation wavelength are also discussed. Surface‐enhanced Raman scattering (SERS) spectroscopy has been applied for the vibrational characterization of ellagic acid (EA), a natural organic dye, using citrate‐reduced silver colloids. The infrared and FT‐Raman spectra of EA in the solid state were examined for comparison. Spectral assignments of the observed bands were aided by density functional theory (DFT) calculations employing the B3LYP functional. The variations in the SERS spectrum with pH and excitation wavelength were analyzed to gain information on the adsorption‐ and pH‐dependent behaviors of the natural dye on the metal surface. On the basis of the observed spectral features and DFT calculations, the adsorption configuration of EA on the silver metal surface is proposed.