Selective Electrochemical Sensing of NADH and NAD+ Using Graphene/Tungstate Nanocomposite Modified Electrode

Reduced form of beta-nicotinamide adenine dinucleotide (NADH) and its oxidized form (NAD(+)) are the main cofactors involved in more than 300 dehydrogenase reactions. NAD(+) is one of the important oxidizing agent for the oxidation of alcohol, aldehyde and ketones. Similarly, NADH has been used for the treatment of Alzheimer and Parkinson's diseases. Herein, we have reported synthesis of graphene flakes by electrochemical exfoliation of graphite in sodium tungstate solution. It was found that tungstate (WO42-) and hydroxyl (OH-) ions were intercalated into graphite layers and enabled the production of graphene flakes. As-obtained graphene flakes were characterized by UV-Visible (UV-Vis), Fourier transform infrared (FT-IR), Raman, Field emission scanning electron microscopy (FE-SEM), Energy dispersive X-ray (EDX) and High-resolution transmission electron microscopies (HR-TEM). FT-IR, Raman and EDX analysis were confirmed that tungstate (WO42-) ions were present on the surface of graphene flakes. Moreover, graphene-WO42- (Gr-W) dispersion was prepared by probe-sonication to form a thin-film on the surface of glassy carbon electrode (Gr-W/GCE). Interestingly, Gr-W modified GCE reduced the overpotentials of NADH oxidation and NAD(+) reduction. This new senor was also showed linear responses for NADH and NAD(+) from 10-270 mu M and 100-500 mu M, respectively. Furthermore, the selectivity of the Gr-W/GCE was tested in the presence of L-tyrosine, L-isoleucine, L-alanine, glutathione, dopamine (DA), ascorbic acid (AA), uric acid (UA), oxalic acid (OA), glucose, hydrogen peroxide (H2O2)(,) acetaminophen (PA), potassium chloride (KCl) and sodium chloride (NaCl). It was found that selective detection of both NADH and NAD(+) could be achieved by using Gr-W/GCE. Finally, the real application of the sensor was demonstrated by accurately detecting spiked NADH concentrations in human blood serum. The recovery analysis was also confirmed that Gr-W/GCE could be useful to detect NADH in biological samples.