It is possible to induce the formation of sol-gel layers on electrode surfaces by means of electrochemistry. The concept is based on local pH change near the electrode surface which catalyses the sol condensation and significantly increases the rate of silica deposition only onto the solid surface without disturbing the rest of the sol [1].
Recently it has been shown that this procedure could be applied to the encapsulation of biomolecules in thin silica films while retaining their biocatalytic properties [2]. An interesting perspective of this work concerns the deposition of such composite material on electrodes exhibiting large electroactive surface areas, which would contribute to significantly improve the bioelectrochemical reaction [3].
Carbon nanotubes are important materials for the electrochemical detection of NADH [4]. This molecule is used as a co-factor by a large number of dehydrogenase that can find application in biosensors or enzymatic electrosyntheses.
In this context we have evaluated the interest of carbon nanotubes networks obtained by electrophoretic deposition for the oxidation of NADH and the immobilization of dehydrogenases. D-sorbitol dehydrogenase was used as model enzyme. Immobilization of the biomolecule was achieved by physical entrapment into sol-gel films prepared by the electrochemically assisted deposition.
The carbon nanotubes network catalyses effectively the detection of NADH, this reaction occurs at lower potential than on the glassy carbon substrate. Moreover, the intensity of the oxidation peak depends on the quantity and texture of deposited nanotubes. D-sorbitol dehydrogenase can be immobilized in this network of carbon nanotubes by electrochemically assisted generation of sol-gel layers and the response of the electrode to D-sorbitol is strongly dependent on the electrogeneration time.

[1] R. Shacham, D. Avnir, D. Mandler, Adv. Mater. 1999, 11,  384.
[2] O. Nadzhafova, M. Etienne, A. Walcarius, Electrochem. Commun. 2007, 9, 1189.
[3] Z. Wang, M. Etienne, G.-W. Kohring, Y. Bon-Saint-Côme, A. Kuhn, A. Walcarius, Electrochim. Acta, in press.
[4] M. Wooten, W. Gorski, Anal. Chem. 2010, 82, 1299.

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