Electroactive films of heme protein-coated multiwalled carbon nanotubes

A novel method for fabricating protein-MWNT films on pyrolytic graphite (PG) electrodes was described. Positively charged hemoglobin (Hb) or myoglobin (Mb) in buffers at pH 5.5 or 5.0 was first adsorbed on the surface of acid-pretreated, negatively charged multiwalled carbon nanotubes (MWNTs) mainly by electrostatic interaction, forming a core-shell structure. The aqueous dispersion of protein-coated MWNTs was then cast on PG electrodes, forming protein-MWNT films after evaporation of solvent. The protein-MWNT films exhibited a pair of well-defined, quasi-reversible cyclic voltammetric peaks, characteristic of heme Fe(III)/Fe(II) redox couples. The protein films were characterized by voltammetry, UV-vis spectroscopy, and scanning electron microscopy (SEM). This approach for assembly of protein-MWNT films showed higher surface concentration of electroactive proteins than the simple cast method, and the amount of proteins in the films could be controlled more precisely compared with the dipping method. Furthermore, the film assembly using this method was more stable than that using simple cast method. The proteins in MWNT films retained their near-native structure, and electrochemically catalyzed reduction of oxygen and hydrogen peroxide, suggesting the potential applicability of the films as the new type of biosensors or bioreactors based on direct electrochemistry of enzymes.