Electron-transfer pathways between redox enzymes and electrode surfaces: reagentless biosensors based on thiol-monolayer-bound and polypyrrole-entrapped enzymes.

Based on previous results which showed that quinohemo-protein alcohol dehydrogenase (QH-ADH) entrapped within polypyrrole is able to directly transfer electrons via the conducting polymer to the electrode surface, the electron-transfer properties of this multi-cofactor enzyme adsorbed and covalently-bound to self-assembled thiol monolayers and bare electrode surfaces has been investigated more closely. While the dissolved enzyme is able to transfer electrons to the electrode via heme c as well as via the more deeply buried PQQ (fast adsorption-chemical reaction-desorption mechanism), an orientation of adsorbed QH-ADH on hydrophobic electrode surfaces, as well as of adsorbed and covalently bound QH-ADH on negatively-charged thiol monolayers could be observed. In these cases the heme c units are pointing towards the electrode surfaces resulting in an optimised direct ET rate.

Electron-transfer mechanisms in amperometric biosensors.

The function of amperometric biosensors is related to electron-transfer processes between the active site of an (immobilized) enzyme and an electrode surface which is poised to an appropriate working potential. Problems and specific features of architectures for amperometric biosensors using different electron-transfer pathways such as mediated electron transfer, electron-hopping in redox polymers, electron transfer using mediator-modified enzymes and carbon-paste electrodes, direct electron transfer by means of self-assembled monolayers or via conducting-polymer chains are discussed.

Polypyrrole-entrapped quinohemoprotein alcohol dehydrogenase. Evidence for direct electron transfer via conducting-polymer chains.

It is reported for the first time that direct electron-transfer processes between a polypyrrole (PPY) entrapped quinohemoprotein alcohol dehydrogenase from Gluconobacter sp. 33 (QH-ADH) and a platinum electrode take place via the conducting-polymer network. The cooperative action of the enzyme-integrated prosthetic groups--pyrroloquinoline-quinone and hemes--is assumed to allow this electron-transfer pathway from the enzyme's active site to the conducting-polymer backbone. A hypothetical model of the electron transfer is proposed which is supported by the influence of various parameters, such as, e.g., ionic strength and nature of the buffer salts. This unusual electron-transfer pathway leads to an accentuated increase of the K M app value (102 mM) and hence to a significantly increased linear detection range of an ethanol sensor based on this enzyme.