Integrated, electrically contacted NAD(P)⁺-dependent enzyme-carbon nanotube electrodes for biosensors and biofuel cell applications

Integrated, electrically contacted β-nicotinamide adenine dinucleotide- (NAD⁺) or β-nicotinamide adenine dinuclcotide phosphate- (NADP⁺) dependent enzyme electrodes were prepared on single-walled carbon nanotube (SWCNT) supports. The SWCNTs were functionalized with Nile Blue (1), and the cofactors NADP⁺ and NAD⁺ were linked to 1 through a phenyl boronic acid ligand. The affinity complexes of glucose dehydrogenase (GDH) with the NADP⁺ cofactor or alcohol dehydrogenase (AlcDH) with the NAD⁺ cofactor were crosslinked with glutaric dialdehyde and the biomolecule-functionalized SWCNT materials were deposited on glassy carbon electrodes. The integrated enzyme electrodes revealed bioelectrocatalytic activities, and they acted as amperometric electrades for the analysis of glucose or ethanol. The bioelectrocatalytic response of the systems originated from the biocatalyzed oxidation of the respective substrates by the enzyme with the concomitant generation of NAD(P)H cofactors. The electrocatalytically mediated oxidation of NAD(P)H by 1 led to amperometric responses in the system. Similarly, an electrically contacted bilirubin oxidase (BOD)-SWCNT electrode was prepared by the deposition of BOD onto the SWCNTs and the subsequent cross-linking of the BOD units using glutaric dialdehyde. The BOD-SWCNT electrade revealed bioelectrocatalytic functions for the reduction of O₂ to H²O. The different electrically contacted SWCNT-based enzyme electrodes were used to construct biofuel cell elements. The electrically contacted GDH-SWCNT electrode was used as the anode for the oxidation of the glucose fuel in conjunction with the BOD-SWCNT electrode in the presence of O₂, which acted as an oxidizer in the system. The power output of the cell was 23 μWcm^-2. Similarly, the AlcDH-SWCNT electrode was used as the anode for the oxidation of ethanol, which was acting as the fuel, with the BOD-SWCNT electrode as the cathode for the reduction of O₂. The power output of the system was 48 μWcm^-2.