Surface-enhanced Raman scattering study of the adsorption of croconate violet on colloidal silver particles

► Silver colloidal suspension with localized surface plasmon resonance in the near infrared. ► Assignment of the SERS spectrum of croconate violet based on theoretical calculations. ► SERS spectrum of the adsorbate allowed to infer the adsorption geometry. Very stable silver particle suspension has been synthesized for use in surface-enhanced Raman scattering (SERS) spectroscopy with near-infrared exciting radiation. Such citrate-stabilized silver particles were obtained through a suitable control of the nucleation and growth process during the synthesis. The SERS spectra of the bis(dicyanomethylene) croconate dianion or croconate violet (CrocV) were obtained, with excitation in the near-infrared and in the visible region. The differences in the spectral patterns were correlated with a pre-resonance Raman effect of the adsorbate. The vibrational frequencies of CrocV isolated and interacting with silver surface were obtained through theoretical calculations using DFT method that together the surface selection rules allowed to perform the vibrational assignment of the SERS spectra and to infer the adsorption geometry.