Probing the dynamics of dithiooxamide coordinated to gold nanoparticles using SERS

The substitution kinetics and specific binding modes of the dithiooxamide (dto) ligand at the metal surface have been investigated and interpreted on the light of theoretical and experimental Raman studies. By monitoring the vibrational properties of this ligand, it was shown that gold nanoparticles employed as surface-enhanced effects (SERS) probes are strongly dependent on the coordination chemistry of the ligands at the metal interface. The binding of dto to the gold nanoparticles led to linkage isomers, and at low concentrations, the bidentate form, corresponding to bis-thione-Au bond in cis configuration {Au-S(NH2)CC(NH2)S-Au), was the predominating species. Even at high concentrations, the cis-configuration was observed first; but as the ligand competition dynamics proceeded, the binding mode gradually changed into a monodentate thione-Au mode (AuS(NH2)CCS(NH2)). In this mode, the dto ligand adopts a thermodynamically favorable trans-conformation geometry, producing dramatic changes in the SERS spectra. Such dynamic behavior of the ligand at the metal plasmonic surface is of crucial importance, as one of the possible factors associated with lack of reproducibility of the SERS spectra frequently reported in the literature.