Direct electrochemistry of redox proteins and enzymes promoted by carbon nanotubes

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, E-0', was almost independent on the scan rates, the average value of E0' for Hb, HRP and GOx was - 0.343 +/- 0.001, - 0.319 +/- 0.002 and - 0.456 +/- 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 (k(s)) 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.