Enhancement of Direct Electrocatalytic Activity of Horseradish Peroxidase on Polyaniline Nanotubes

Abstract

The advantages of nanotubes over compact conducting polymer films include the large surface area available for enzyme immobilization, facile diffusion of analyte molecules and enhanced conductivity. Heat treatment of PANI induces the formation of phenazine cross-links between PANI chains, which influences the electrocatalytic response of the immobilized enzymes. In this study, both polyaniline (PANI) bulk films and nanotubes were studied as a support for horseradish peroxidase (HRP). Two types of nanotubes, with diameters of 100 and 400 nm, were investigated. HRP was immobilized by physical adsorption or covalent bonding via the carbodiimide reaction. The influence of the heat treatment of PANI prior to enzyme immobilization on the electrochemical responses of the PANI/HRP electrodes was studied. The electrocatalytic current due to hydrogen peroxide (H2O2) reduction was observed in all of the studied samples. However, the current values observed for HRP immobilized on PANI nanotubes with diameters of 100 nm (PANI/100 nm) were an order of magnitude higher than that of the bulk film. The electrocatalytic currents for the PANI/400 nm nanotubes had intermediate values between PANI/100 nm and the PANI bulk film. Linear responses to H2O2 were observed over a wide concentration range for all of the HRP/PANI systems. The formation of phenazine cross-links due to the thermal treatment of PANI was confirmed from the Raman spectra. The cross-linking contributes to the enhanced mechanical stability of the PANI nanotubes. However, this cross-linking does not enhance significantly the electrocatalytic activity of the immobilized enzyme.