pH dependent SERS and solvation studies of tyrosine adsorbed on silver colloidal nano particles combined with DFT calculations

The surface enhanced Raman spectra of tyrosine in colloidal Ag solution have been recorded over a range of pH. A line shape analysis of the bands at ∼1359, 1505 and 1577 cm−1 was performed between pH 3.5 and 8.5. The variation of spectral linewidth (FWHM) of the band at ∼1359 cm−1 with pH is explained in terms of two mechanisms in solution: (i) the fluctuation of the pH of a microscopic volume in a solution with an overall uniform pH and/or (ii) the role of changing viscosity and solvation at different pH values due to the intermolecular ionic interactions between different charged states of the tyrosine molecule. The blue shift in three bands with increasing pH has been explained in terms of charge transfer between the different charged states of tyrosine and metal ions upon chemisorption. The experimental spectra are compared with ab initio/DFT calculations of vibrational wavenumbers, bond geometries, binding energy and charge distributions obtained by means of Hartree-Fock (HF) analysis, the nonlocal density functional method (BLYP) and the hybrid functional method (B3LYP). Two basis sets, CEP-31G and lanl2DZ, were used for all calculations.