Redox coenzyme functionalization of electrochemically grown Prussian blue films

Thin solid films of the molecular semiconductor rubidium Prussian blue, RbFe2(CN)(6), are electrochemically deposited on quartz-crystal microbalance electrodes. The films are found to surface-coordinate tightly both the oxidized and reduced forms of the redox coenzyme nicotinamide adanine dinucleotide (NAD(+)), presumably via the diphosphate linkage. Gravimetric and electrode potential measurements provide evidence that the surface-bound NAD(+) can act as an electron-transfer mediator from solution to the Prussian blue. The electron transfer appears to result from an inner-sphere mechanism between NADH and Fe-2(CN)(6)(-); unbound NADH in solution does not reduce the Prussian blue films. Conversion of surface-bound NAD(+) to NADH can be enacted enzymatically, with subsequent electron transfer regenerating NAD(+). The semiconductive properties of the film transport surface electrons to the bulk, enabling near-quantitative reduction, with the bound coenzyme acting catalytically as an electron-transfer mediator. The potential application of these coenzyme-functionalized molecular semiconductor films to the area of biosensing is explored.