A study of pyridinethiolate derivative complexes adsorbed on gold by surface-enhanced Raman scattering

Gold surfaces were modified with [M(CN)5(pyS)]4− (pyS = 4-mercaptopyridine, M = Fe or Ru) type complexes by two procedures related to the electrode activation. These procedures were carried out by oxidation–reduction cycles in situ (with complexes in solution) and ex situ (in the absence of complexes in solution). The SERS (surface-enhanced Raman scattering) spectra for the surface modified by the in situ procedure indicate that the adsorption occurs through the CN group. The heterogeneous electron transfer of the cytochrome c (cyt c) was not successfully assessed by using this modified electrode. On the other hand, by using the ex situ procedure, the SERS spectra indicate that the adsorption occurs through the sulfur atom of the pyS moiety. In this case, the redox process of the cyt c is clearly visualized. For this modified electrode, the pyridinethiolate vibrational modes showed to be strongly related to the half-wave formal potential of the inorganic complexes, which reflects the π-back-bonding interaction from the metal atoms.