Structure and electrochemical behavior of a flavin sulfide monolayer adsorbed on gold

The electrochemical behavior and structure of monolayers of the synthetic flavin 10-(3′-methylthiopropyl)-isoalloxazinyl-7-carboxylic acid adsorbed on gold is reported. The redox behavior of the compound is quasi-reversible. The surface concentration is estimated to be 1.5×10^-10 mol cm^-2. An electron transfer rate constant of 340 s^-1 is estimated for the cathodic process and a value of 540 s^-1 is estimated for the anodic process. The reduction potential of the monolayer is found to shift with pH as expected for a 2e^-, 2H⁺ process. The monolayers have also been characterized by X-ray photoelectron spectroscopy (XPS) and low voltage field emission secondary electron microscopy (LVFESEM). The most likely orientation would have the long axis parallel to the surface with the carboxyl group exposed to the solution. A comparison of the C1s XPS spectra at glancing and normal emission indicates that the carboxyl group is at the film surface. The LVFESEM images indicate that the molecules pack in domains that do not follow the topology of the gold grains. Semi-quantitative examination of the micrographs shows that 10-15% of the gold surface is uncovered. The bare gold substrate catalyzes the oxidation of NADH; the presence of the flavin film reduces the observed catalysis by the gold substrate.