Redox mediation and electron transfer through supramolecular arrays of ferrocene-labeled streptavidin on biotinylated gold electrodes

In this work we have studied electron transfer between ferrocyanide species In solution and Au electrodes redox-mediated by ferrocene labels linked to supramolecular bioconjugates. This configuration leads to the so-called electrochemical rectification where the ferrocyanide oxidation is mediated by electrogenerated ferricenium, as evidenced by a unidirectional current flux. Our aims were focused at gaining insight into the molecular aspects of the rectified current and exploring the limits of the electrocatalytic amplification when bioconjugates are used as a platform. We used the Alleman-Weber-Creager model to rationalize and describe in a more quantitative manner the influence of the donor species in solution on the measured electrocatalytic current. Moreover, this model enabled us to estimate homogeneous and heterogeneous rate constants associated with the electron transfer process. We observed that the characteristics of the redox-mediated amplification are highly dependent on the concentration of donor species. This can be described in two regimes depending on whether the bimolecular reaction (k(cross)) or the electrogeneration of the mediator (k(o)) is the limiting step in the overall electron transfer process. The estimation of k(cross) is in good agreement with that calculated from Marcus theory. On the other hand, the k(o) values suggest that electron transfer between the redox labels in the protein conjugate and the Au electrode is mainly controlled by electron tunneling through the blotinylated platform.