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The edge surface of pyrolytic graphite readily adsorbs a wide range of redox-active enzymes and allows facile electron transfer through the graphite layers. Pyrolytic graphite can also be ground into microparticles and coated with complementary pair of redox enzymes to create a selective, reusable, heterogeneous catalyst.
The adsorbed pair effectively function as a short-circuited fuel cell: the reaction of one enzyme is “fuelled” by the continuous electro-oxidation of substrates by the other enzyme. For example, when hydrogenase is used as the electron donor enzyme, molecular hydrogen is oxidised to form two protons and two electrons. The electrons are transported rapidly through the graphite particle to drive the reduction reaction.
The technique has been successfully demonstrated with hydrogenases from Allochromatium vinosum and Ralstonia sp. and, as electron acceptors, Escherichia coli nitrate reductase and E. coli fumarate reductase, however the technology is applicable to a wide range of enzymes and transformations so long as there is sufficient electromotive driving force. The modified particles are dispersed in an aqueous medium in which they are catalytically functional. Simple centrifugation allows the particles to be separated from the liquid medium for reuse.
Enzyme pairs as selective, heterogeneous catalysts: hydrogen oxidation catalysed by hydrogenase (green) supplies electrons through a graphite particle for catalytic reduction at a second enzyme (purple).
Scanning electron micrograph (secondary electron image) of graphite particles ground on to an adhesive support.
- “Enzymatic catalysis on conducting graphite particles.” K.A. Vincent, X. Li, C.F. Blanford, N.A. Belsey, J.H. Weiner and F.A. Armstrong. Nat. Chem. Biol., in press (2007). [PDF]
- “The pyrolytic graphite surface as an enzyme substrate: microscopic and spectroscopic studies.” C.F. Blanford and F.A. Armstrong. J. Solid State Electr. 10(10), 826-832 (2006). [PDF]