Surface-enhanced Raman scattering spectroscopy and density functional theory calculation studies on adsorption of o-, m-, and p-nitroaniline on silver and gold colloid

Abstract

Raman and surface-enhanced Raman scattering (SERS) spectra of o-, m-, and p-nitroaniline in silver and gold colloidal solutions were measured under off-resonance conditions. The SERS effect was observed for o- and p-nitroaniline but not for m-nitroaniline. The energies, structures, electrostatic properties, vibrational frequencies, and Raman scattering intensities of the three nitroanilines were calculated by use of the density functional theory (DFT) method. The DFT calculation suggested that not only an inherent dipole moment of a molecule but also its induced dipole moment associated with the polarizability on the electric field produced by the surface plasmon contribute to the adsorption of the molecule to metal colloid. In addition, transition dipole moments and transition polarizabilities associated with the bands, which give rise to the great SERS effect for o- and p-nitroaniline, were calculated by DFT method. It was suggested from the DFT calculation as well as the SERS spectra that o-nitroaniline has different orientation from p-nitroaniline on the metal colloids, though the two nitroanilines have the same structural formula. Consequently, the substitution effect in the nitroanilines gives the different adsorption orientations on the metal colloids. The combination of SERS spectroscopy and DFT calculation is quite useful for studies of the adsorption orientation of a molecule on a metal colloid.