Biocatalysis of carbon nanotube-attached enzymes for biofuel cells

Carbon nanotubes(CNTs)-attached glucose oxidase was examined for biofuel cell applications. The reaction kinetics of glucose oxidation reaction in the presence of a mediator, hydroquinone, was probed by using a Ping-Pong Bi-Bi model. It was observed that the value of K_m' of the enzyme varied from 29 mM at free state to 38 mM upon immobilization onto suspended CNTs, while the values of V_max only showed a very mild change from 1.1×10^-3 to 1.3×10^-3 mM/s, indicating the immobilization only slightly changed the kinetics of the mediated reactions. The use of CNTs showed a 6-fold enhancement of current density as compared to biofuel cells applied the same amount of enzyme but without the use of CNTs. That was believed to be a result of the increased surface area and improved mass transfer property of the CNTs-containing electrodes. Studies also revealed that the maximum electrical current density of the fuel cells only matched less than 1% of the reactivity the enzyme mounted on the electrodes. This indicated that electron transfer and other electrical resistances within the fuel cells, instead of enzyme activity and availability, are the limiting factors of the power density of the biofuel cell systems examined in this work.