Adsorption of 3-Thiophene Carboxylic Acid on Silver Nanocolloids: FTIR, Raman, and SERS Study Aided by Density Functional Theory

The concentration and pH dependent surface-enhanced Raman scattering (SERS) study of the industrially and biologically significant 3-thiophene carboxylic acid (3-TCA) molecule has been investigated. The SERS spectra of the molecule at different C-Ad values are compared with its FTIR spectrum and normal Raman spectra (NRS) in varied environments. The optimized structural parameters and computed vibrational wavenumbers of the neutral and the anionic (3-TCA(-)) forms of the molecule have been estimated from density functional theory (DFT) calculations. The vibrational signatures of the molecule have been assigned from the potential energy distributions (PEDs). The concomitance of the Raman bands representing vibrational signatures emanating from the neutral and the anionic forms of the molecule signify the presence of both the forms of the molecule in the solid state and in acetonitrile (ACN) solution. However, detailed vibrational analysis reveals that 54% of 3-TCA is prevalent in the solid state while 63% of 3-TCA(-) is predominate in ACN solution at neutral pH. Concentration dependent SERS spectra reveal that the anionic form of the molecule is adsorbed on the nanocolloidal silver surface with the molecular plane tilted or nearly flat with respect to the surface. The genesis of selective enhancements of the Raman bands in the SERS spectra of the molecule has been unveiled from the view of Albretcht's A contribution and Herzberg-Teller's charge transfer contribution.