In our research we have exploited unique electronic and mechanical characteristics of multi-walled carbon nanotubes (MCNTs) to construct the efficient anodic and cathodic bioelectrocatalytic materials for biofuel cell applications. To stabilize composite films, we utilize MCNTs modified with ultra-thin layers of organic (e.g. 4-(pyrrole-1-yl) benzoic acid [1]. We expect here attractive electrostatic interactions between anionic adsorbates and positively charged domains of the enzymatic sites. Other important issues are stability and mediating capabilities of adsorbates. We have also utilized metalloporphyrin redox centers (at which the reduction of oxygen, mostly to hydrogen peroxide, is initiated) and such an enzyme as horseradish peroxidase (HRP), or cabbage peroxidase (CP), that is capable of catalyzing electroreduction of hydrogen peroxide to water as a final product. Co-existence of the above components leads to synergistic effect that is evident from some positive shift of the oxygen reduction voltammetric potentials (more than 50 mV in citrate buffer) and significant increase of voltammetric currents (relative to those of the enzyme-free system). The film has also exhibited relatively higher activity towards reduction of hydrogen peroxide. It is reasonable to expect that the reduction of oxygen is initiated at cobalt porphyrin redox centers, and the undesirable hydrogen peroxide intermediate is further reduced at the horseradish or cabbage peroxidase enzymatic sites.

The development of bioanode has also been investigated. To facilitate electron transfer between the electrode surface and the redox protein centers, the concept of co-deposition of MCNTs within the bio-electrocatalytic film has also been pursued here. First, MCNTs have been modified with ultra-thin layers of tetrathiafulvalene (TTF) to form stable colloidal suspensions of carbon nanostructures [2]. They have been utilized to produce Nafion-containing inks for sequential deposition of components. The presence of TTF is expected to facilitate an effective flow of electrons from the redox centers of glucose oxidase to the glassy carbon electrode. TTF and its derivatives constitute a group of redox molecules that were successfully used as redox mediators in the enzyme electrochemistry. As before, MCNTs have supported transport of electrons within the bio-electrocatalytic film. Our highly MCNT-based porous films have presumably acted as three-dimensional network of nanowires around the enzyme molecules and have promoted the efficient electron transfers. Thus we have produced a catalytic system capable of effective oxidation of glucose in 0.1 M phosphate buffer (pH = 7).

References:

[1] B. Kowalewska, M. Skunik, K. Karnicka, K. Miecznikowski, M. Chojak, G. Ginalska,
A. Belcarz, P. J. Kulesza, Electrochim. Acta, 53 (2008) 2408.

[2] B. Kowalewska, P.J. Kulesza, Electroanalysis, 21 (2009) 351.

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