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Sensors 2005, 5, 220-234 sensors ISSN 1424-8220 ?

2004 by MDPI http://www.

mdpi.net/sensors Direct Electrochemistry of Redox Proteins and Enzymes Promoted by Carbon Nanotubes Yajing Yin, Yafen Lü, Ping Wu and Chenxin Cai* Department of Chemistry, Nanjing Normal University, Nanjing 210097, China * Corresponding author. E-mail: [email protected], [email protected]. Received:

5 June

2004 / Accepted:

11 September

2004 / Published:

27 April

2005 Abstract: The redox protein and enzyme, such as hemoglobin (Hb), horseradish peroxidase (HRP) and glucose oxidase (GOx), was immobilized on the surface of the carbon nanotube modified glassy carbon (CNT/GC) electrode, respectively. The cyclic voltammetric results indicated that the redox protein and enzyme underwent effective and stable direct electron transfer reaction with a pair of nearly symmetrical redox peaks. The formal redox potential, E0 '

, was almost independent on the scan rates, the average value of E0 '

for Hb, HRP and GOx was C0.343 ± 0.001, C0.319 ± 0.002 and C0.456 ± 0.0008 V (vs. SCE,pH 6.9), respectively. The dependence of E0 '

on the pH solution indicated that the direct electron transfer of Hb and HRP was a one-electron-transfer reaction process coupled with one- proton-transfer, while the GOx was a two-electron-transfer coupled with two-proton- transfer. The apparent heterogeneous electron transfer rate constant (ks) was 1.25 ± 0.25, 2.07 ± 0.69 and 1.74 ± 0.42 s-1 for Hb, HRP and GOx, respectively. The method presented here can be easily extended to immobilize other redox enzymes or proteins and obtain their direct electrochemistry. Keywords: carbon nanotube, direct electrochemistry, hemoglobin, horseradish peroxidase, glucose oxidase 1. Introduction Electron transfer in biological systems is one of the leading areas of biochemical and biophysical sciences, and has been received more and more attention [1-9]. The direct electron transfer of enzymes (proteins) with electrodes can be applied to study enzymes-catalyzed reactions in biological systems Sensors 2005,

5 221 and this has developed into an electrochemical basis for the investigation of the structure of enzymes (proteins), mechanisms of redox transformations of enzyme (protein) molecules, and metabolic processes involving redox transformations. Enzyme-modified electrodes provide a basis for constructing biosensors, biomedical devices and enzymatic bioreactors. From these studies, one can find potential applications in biotechnology. For example, if an enzyme immobilized on an electrode surface is capable of the direct electron transfer and keeping its bioactivity, it can be used in biosensors and biofuel cells without the addition of mediators or promoters onto the electrode surface or into the solution. Unfortunately, it is difficult for an enzyme (a protein) to carry out the direct electrochemical reaction due to several factors. For example, enzymes (proteins) would be adsorbed on the electrode surface, resulting in the denaturation and loss of their electrochemical activities and bioactivities. In addition, usually, the larger three-dimensional structure of enzymes (proteins) and the resulting inaccessibility of the redox centers have made it generally difficult to obtain direct electron transfer between enzymes (proteins) and electrode surfaces, so that the promoters and mediators are needed to obtain their electrochemical responses. For the applications in biosensors, the enzymes (proteins) should be immobilized on the electrode surface to avoid many complications linked to the solution systems. Therefore, the suitable electrode materials and immobilization methods of enzymes (proteins) onto the electrode surface are important for obtaining their direct electrochemical reaction and keeping their bioactivities. Since their initial discovery by Iijima [10] in