Electrocatalytic activity of carbon spheres towards NADH oxidation at low overpotential and its applications in biosensors and biofuel cells

The excellent electrocatalytic activity of a micro-structured carbon material, carbon hollow spheres (CS), to the oxidation of dihydronicotinamide adenine dinucleotide (NADH) is demonstrated here. Compared to conventional bare glassy carbon electrodes, a substantial decrease by 450 mV in the overpotential of NADH electrooxidation was observed using CS coatings, with oxidation starting at ca. -0.10 V (vs. Ag/AgCl, pH 7.0). The CS-coated glassy carbon electrode (CS/GC) thus allows highly sensitive and direct amperometric detection of NADH at lower potential, ranging from 0.20 to 100 mu M with a high sensitivity of 7.3 +/- 0.2 nA mu M-1 (i.e., 103.3 +/- 2.8 nA mu M-1 cm(-2)), low detection limit of 0.08 +/- 0.03 mu M, and minimization of surface fouling. With lactate dehydrogenase (LDH) as a model, a lactate biosensor with the LDH-CS/GC electrode was constructed and the biosensor shows rapid and highly sensitive amperometric response to lactate ranging from 0.5 to 12 mM with a detection limit of 3.7 +/- 0.2 mu M, a sensitivity of 4.1 +/- 0.2 nA mu M-1 (i.e., 57.9 +/- 2.8 nA mu M-1 cm(-2)), good reproducibility and excellent stability. Furthermore, the promoted direct electron transfer (DET) of bilirubin oxidase (BOD) on CS/GC electrode was investigated, and thus a membrane-less lactate/oxygen biofuel cell was assembled by using LDH-CS/GC as bioanode for lactate oxidation and BOD-CS/GC electrode for oxygen reduction, with a high open-circuit potential of 0.60 V. Such ability of CS to decrease the NADH oxidation overpotential and promote DET of blue-copper oxidases suggests great promise for dehydrogenase-based amperometric biosensors and biofuel cells.