Direct electron transfer in a mediator-free glucose oxidase-based carbon nanotube-coated biosensor

A novel biosensor was prepared by immobilizing glucose oxidase on multi-walled carbon nanotube (MWCNT)-coated electrospun gold fibers. Homogeneous coating of the electrospun gold fibers by MWCNTs was achieved by electrophoretic deposition at 20 V (40 V cm−1), a deposition time of 30 s and a solution concentration of 0.25 mg mL−1. Scanning electron microscopy confirmed the complete coverage of MWCNTs on the fiber surface. The carboxylated MWCNTs on the gold fibers provided an anchor for covalent immobilization of glucose oxidase (GOX). GOX covalently coupled to conductive carbon nanotubes demonstrated direct electron transfer between the enzyme and the electrode surface without the need for a redox active mediator. Electrochemical characterization of the fabricated sensor by cyclic voltammetry revealed that the immobilized GOX exhibited a surface-confined reversible two-electron and two-proton reaction, with an electron transfer rate constant, ks, of 1.12 s−1 and a surface coverage of 1.1 × 10−12 mol cm−2. The sensor produced a linear response to glucose concentration up to 30.0 mM with a sensitivity of 0.47 μA mM−1 cm−2 and a detection limit of 4 μM.