Size Does Matter—Mediation of Electron Transfer by Gold Clusters in Bioelectrocatalysis

Metal nanostructures are often used in bioelectrocatalytic systems to increase the electrode surface area or to improve the conductivity of biofilms. We demonstrate, that decreasing the size of gold nanoparticles below 2 nm may result in a change of the mechanism of electron transfer (ET) between the enzyme active site and the electrode from direct to mediated ET. Clusters with diameters smaller than 2 nm exhibited molecule-like behavior reflected in the appearance of oxidation and reduction peaks separated by a clearly developed HOMO—LUMO gap. The redox activity of the nanoparticles was found to contribute to the ET mechanism of fructose dehydrogenase switching it to gold cluster mediated electron transfer instead of direct ET. In the presence of gold clusters at the electrode, the overpotential of the catalyzed fructose oxidation reaction was 100 mV lower and the catalytic reaction rate constant was 2.5 times larger confirming the unique mediating role of the Au clusters.