Noncovalent attachment of NAD+ cofactor onto carbon nanotubes for preparation of integrated dehydrogenase-based electrochemical biosensors.

This study describes a facile approach to the preparation of integrated dehydrogenase-based electrochemical biosensors through noncovalent attachment of an oxidized form of beta-nicotinamide adenine dinucleotide (NAD(+)) onto carbon nanotubes with the interaction between the adenine subunit in NAD(+) molecules and multiwalled carbon nanotubes (MWCNTs). X-ray photoelectron spectroscopic and cyclic voltammetric results suggest that NAD(+) is noncovalently attached onto MWCNTs to form an NAD(+)/MWCNT composite that acts as the electronic transducer for the integrated dehydrogenase-based electrochemical biosensors. With glucose dehydrogenase (GDH) as a model dehydrogenase-based recognition unit, electrochemical studies reveal that glucose is readily oxidized at the GDH/NAD(+)/MWCNT-modified electrode without addition of NAD(+) in the phosphate buffer. The potential for the oxidation of glucose at the GDH/NAD(+)/MWCNT-modified electrode remains very close to that for NADH oxidation at the MWCNT-modified electrode, but it is more negative than those for the oxidation of glucose at the MWCNT-modified electrode and for NADH oxidation at a bare glassy carbon electrode. These results demonstrate that NAD(+) molecules stably attached onto MWCNTs efficiently act as the cofactor for the dehydrogenases. MWCNTs employed here not only serve as the electronic transducer and the support to confine NAD(+) cofactor onto the electrode surface, but also act as the electrocatalyst for NADH oxidation in the dehydrogenase-based electrochemical biosensors. At the GDH/NAD(+)/MWCNT-based glucose biosensor, the current is linear with the concentration of glucose being within a concentration range from 10 to 300 microM with a limit of detection down to 4.81 microM (S/N = 3). This study offers a facile and versatile approach to the development of integrated dehydrogenase-based electrochemical devices, such as electrochemical biosensors and biofuel cells.