STM imaging of FAD-modified gold electrodes with electrocatalytical properties

MARÍA ANTONIETA DAZA MILLONE; MARIANO H. FONTICELLI; GUILLERMO BENÍTEZ; MARÍA ELENA VELA; ROBERTO C. SALVAREZZA

Mar del Plata (Pcia. Bs. As.)

Simposio; 4th Latin American Symposium on Scanning Probe Microscopy; 2007

Flavin adenine dinucleotide (FAD) is an enzyme cofactor that plays a central role in aerobic metabolism through its ability to catalyze two-electron dehydrogenations of numerous substrates and to participate in one-electron transfers to various metal centers [1]. Free FAD in solution shows very poor catalytic activity. The enzyme environment stabilizes FAD structure in a planar configuration through hydrogen bridges, Van der Waals forces,  stacking, electrostatic forces, etc. enabling the reaction with the substrate, i. e., nicotinamide adenine dinucleotide (NAD). FAD based biosensors try to mimic the enzyme properties in a simple and inexpensive way [2]. In this work, we used two kinds of substrates to immobilized FAD: Au(111) modified with a n-dodecanethiol (C12) monolayer and bare Au(111). Both substrates were dipped in aqueous 10 -4 M FAD solution for time periods between 1min and 48hs. STM images of FAD/Au electrodes show tridimensional aggregates of molecules all over the surface. In cleaner zones, spots of 1 - 5 nm could be visualized. For the same incubation times, FAD/C12/Au electrodes show less amount of material: few zones of aggregates but more isolated spots covering the C12 monolayer. For 1h of FAD incubation, XPS measurements give a Nitrogen amount of 12.5% with respect to FAD/Au. To evaluate its catalytical properties, cyclic voltammetries with NAD+ in solution were performed. FAD/Au electrodes could reduce NAD+ only in first cycle and then a fouling process begins. Meanwhile FAD/C12/Au electrodes were able to catalyze NAD+/NADH conversion in a reversible way for over 50 cycles. References: [1] Lehninger, A., Nelson, D and Cox, M. Principles of Biochemistry, W. H. Freeman 4th edition, New York, 2004 [2] Cunningham, A. J. Introduction to Bioanalytical sensors, John Wiley & Sons, Inc., New York, 1998