Paper #2 comes from a group headed by Nanqiang Li of the College of Chemistry and Molecular Engineering of Peking University, Beijing, China, and it reports the electrochemistry of horse-heart cyctochrome c using cyclic voltammetry carried out with a glassy carbon electrode covered with single-wall carbon nanotubes (SWNTs)—the paper even shows a scanning electron microscope image of them in situ. Without the SWNTs the electrode cannot detect the reduction/oxidation properties of cyctochrome c, but with a surface coating of SWNT there is a well-defined response, and this is put down to much improved conductivity. The electrodes were prepared by dispersing SWNTs in the solvent dimethylformamide by means of ultrasound, then placing a drop of the liquid on a glassy carbon electrode that had been smoothed and polished with alumina slurry. The dimethylformamide was evaporated using the heat from an infrared lamp.

Paper #5 comes from a team headed by Joseph Wang of the Department of Chemistry and Biochemistry at New Mexico State University, working in collaboration with Yuehe Lin of the Pacific Northwest National Laboratory in Richland, Washington, and it too reports how both single-wall and multi-wall nanotubes offer greatly improved electrochemical reactivity. The authors demonstrate how effective the electrodes are by studying the response towards beta-nicotinamide adenine dinucleotide (NADH), noting a substantial decrease in the overvoltage of the NADH oxidation reaction compared to that observed using ordinary carbon electrodes, which suffer from low sensitivity and stability. In contrast, the new system is extremely stable with 96% of the initial activity remaining after an hour of stirring with a solution of 2 x 10^-4 M NADH. These new electrodes make possible highly sensitive monitoring of this important biochemical material, and the authors of this paper believe that these new electrodes offer great promise for making biosensors which would incorporate immobilized dehyrogenase enzymes.

The New Mexico group made their electrodes by dispersing the nanotubes in concentrate sulfuric acid, in which it was possible to obtain a suspension of 1mg per ml, and placing a 10 microliter drop of this on the polished glassy carbon surface. The coating was then dried at 200 degrees C for three hours to remove the acid, followed by washing the electrode with doubly-distilled water.

Paper #10 also comes from Wang’s and Lin’s groups and reveals a method of solubilizing carbon nanotubes in a way that promises to transform the fabrication of sensors, and there is now commercial interest in the new developments. The secret is to "wrap" the SWNTs in the perfluorosulfonated polymer Nafion, which has polar side chains, and these make it soluble in a solvent like alcohol. Paper #10 illustrates the solubility with photographs of SWNTs in different solutions showing the dramatic difference that Nafion makes to its solubility. When these solutions are used to coat glassy carbon electrodes they are found to provide the same enhanced sensitivity as the surface films reported in the papers above, and even with the Nafion still present. This was demonstrated by using such an electrode for the electrocatalytic detection of hydrogen peroxide. (This electrode thereby enables glucose to be measured at very low concentrations.)

Research in this area is moving fast and Wang’s group have already developed a CNT-modified electrode for detecting DNA and proteins (see J. Wang,et al., J. Amer. Chem. Soc., 126[10]: 3010-4, 2004) and one that is capable of sensing alcohol (see J. Wang and M. Musameh, Anal. Lett., 36[9]: 2041-8, 2003).