Preparation of Aminopyridine Grafted Carbon Nanotube and Its Interaction with Horseradish Peroxidase

Carboxylic-functionalized multiwalled carbon nanotube (MWCNT-COOH) is obtained by oxidation with potassium bichromate and further modification by amide condensation afforded aminopyridine-grafted MWCNT (MWCNT-AP). The MWCNT-AP was characterized by Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance spectroscopy (H-1-NMR) and X-ray photoelectron spectroscopy (XPS). Transmission electron microscopy (TEM) results suggest that MWCNT-COOH aggregates in ethanol and that MWCNT-AP is stable and well dispersed in solution. Horseradish peroxidase (HRP) physically adsorbed onto the surfaces of MWCNT-AP and MWCNT-COOH and the adsorption amounts were 187.5 and 153.0 mu g.mg(-1), respectively. UV-Vis spectra showed that the Soret band of HRP red-shifted markedly after adsorption onto MWCNT-AP or MWCNT-COOH indicating that the binding site of MWCNT-AP or MWCNT-COOH is near the heme pocket of HRP. Circular dichroism spectral results demonstrate that the secondary structure of HRP is influenced by MWCNT-AP. Enzyme-kinetic studies show that MWCNT-AP may adsorb HRP and its substrate 3,3', 5,5'-tetramethylbenzidine (TMB) effectively, and the maximum reaction rate (v(max)) of HRP increases significantly after interaction with MWCNT-AP.