Direct electrochemistry and electrocatalysis of hemoglobin immobilized in poly(ethylene glycol) grafted multi-walled carbon nanotubes

The novel poly(ethylene glycol) (PEG) grafted multi-walled carbon nanotubes (PEG-g-MWCNTs) were synthesized by the covalent functionalization of MWCNTs with hydroxyl-terminated PEG chains, exhibited excellent hydrophilicity and biocompatibility. The PEG-g-MWCNTs were characterized by Fourier transform infrared spectra, transmission electron microscopy, and thermogravimetric analysis, which verified that PEG chains were grafted onto the surface of the MWCNTs. The PEG-g-MWCNTs were then used as substrates for the immobilization of hemoglobin (Hb) and their bioelectrochemical behaviors were studied. Electrochemical impedance spectroscopy was used to confirm the adsorption of Hb onto the surface of PEG-g-MWCNTs. The Hb immobilized in PEG-g-MWCNTs retained its near-native conformations as characterized by the UV–vis spectroscopy. The cyclic voltammetry results of Hb/PEG-g-MWCNT modified electrode showed a pair of well-defined and quasi reversible redox peaks centered at approximate −0.34 V (vs. saturated calomel electrode), which was the characteristic peaks of Hb Fe(III)/Fe(II), in pH 7.0 phosphate buffer solution. Hb immobilized onto the surface of PEG-g-MWCNTs demonstrated good bioelectrocatalytic activities for the reduction of nitrite.