Self-assembled monolayers formed by [M(CN)5(pyS)]4−(M = Fe, Ru) on gold: a comparative study on stability and efficiency to assess the cyt c heterogeneous electron transfer reaction

Abstract

A comparative study involving SAMs formed by [(CN)5M(pyS)]4− inorganic complexes (M = Fe, Ru; pyS = 4-mercaptopyridine) on gold (MpySAu) has been performed. The characterization data for these complexes suggests that the ruthenium complex exhibit a greater π-back-bonding effect that more strongly stabilizes the MpyC–S bond, thus anticipating its application as a SAM that would better enhance the gold adlayer stability than the iron complex. The MpySAu electrodes were characterized by SERS and electrochemical (LSV) techniques. The ex situ SERS spectra data for both SAMs suggest a σ interaction between the gold and sulfur atom of the complexes, inducing a perpendicular arrangement in relation to the surface. The spectra performed for freshly prepared MpySAu adlayers did not show any significant changes that would reflect the degradation of the adlayer. The LSV desorption curves of the SAMs indicate a better enhancement in the C–S bond strength of the pyS ligand when coordinated to the [Ru(CN)5]3− moiety. Comparatively to the data obtained for the desorption process of the pyS monolayer, the reductive desorption potentials, Erd, present shifts of −170 and −110 mV for the Ru and Fe complexes, respectively. The voltammetric curves of cytochrome c (cyt c) performed with the MpySAu electrodes showed electrochemical parameters consistent with that reported for the native protein. These results taken together reinforce that the π back-bonding effect from the [M(CN)5]3− metal center [Ru (4d) > Fe (3d)] strongly affects the MpySAu adlayer stability, reflecting the adlayer performance on the assessment of the cyt c hET reaction.