One of the most frequently considered enzymes for use as the cathodic biocatalyst in biofuel cells is laccase. It belongs to the multi-copper group of oxidases, and drives the four-electron reduction of the co-substrate O₂ to H₂O effectively, while it is quite unspecific with respect to the electron donating substrate. The dianion ABTS^2- (2,2’ – Azinobis (3 – ethylbenzothiazoline – 6 – sulfonate)) undergoes a reversible oxidation - reduction reaction and  is commonly used with oxygen reducing enzymes as a mediator. ABTS was mostly considered as a diffusional mediator but recently attempts have been made to immobilize ABTS together with laccase on the electrode surface. Immobilization of the enzyme and mediator eliminates the need to separate the anode and cathode compartments with a membrane.

In order to improve the effectiveness of the bio-electrocatalytic system for the electro-reduction of oxygen, we have employed carbon nanotubes (CNTs). Carbon nanotubes exhibit very good mechanical and electronic properties and they facilitate electron transfer in bioelectrocatalytic systems. We have prepared colloidal suspensions of carbon nanotubes modified with ABTS utilizing the procedure proposed by us for the fabrication of phosphomolybdate-stabilized carbon nanotubes [1]. The bioelectrocatalytic film based on CNTs_ABTS and laccase exhibits very good electrocatalytic activity towards oxygen reduction. The combination of CNTs and ABTS gives a highly effective mediation system able to propagate charge between the active center of the enzyme and the electrode surface.

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