GOx/CNT/Silica Composites for Bioelectrodes

June 1, 2008

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

Composites of glucose oxidase (GOx), carbon nanotubes (CNTs), and biologically synthesized silica have been synthesized and tested. These composites are prototypes of biological/electrical interfacial materials and could enable the development of the next generation of devices for a variety of medical, scientific, industrial, and military applications. In particular, it is envisioned that materials based on these prototypes will be integrated into bioelectrodes for biosensors and biofuel cells.

Cyclic Voltammograms of a GOx/CNT/silica-coated electrode were obtained in scans at a rate of 20 mV/s in the presence and absence of glucose at a concentration of 40 mM. The anodic and cathodic peaks shifted toward oxidative current values in the presence of glucose. This shift is attributed to a decrease, induced by enzyme activity, in the concentration of oxygen at the surface of the electrode.

The basic idea is to immobilize an enzyme (GOx) in a silica matrix intertwined with a CNT matrix that provides electrical conductivity between the enzyme and a carbon electrode substrate. In addition to holding the enzyme at the desired location, the immobilization of the enzyme in the matrix helps to electrochemically stabilize the enzyme. The electrical conductivity of the CNTs makes it possible to closely approach the ideal of direct electron transfer (DET) between the enzyme and the electrode substrate. For a biofuel cell, DET is advantageous because it obviates complex electron mediators that would otherwise be needed, thereby contributing to miniaturization of electrodes and maximization of output power. For a biosensor, DET is advantageous in that, at least theoretically, it enables an electrode to function in a potential range close to the redox potential of the enzyme.

GOx was chosen as the enzyme to be incorporated into the prototype composites because (1) it has been widely studied, especially with respect to DET and (2) as such, it serves as a suitable model material for the development of biosensors and fuel cells. In future development, other enzymes could be chosen to satisfy requirements in specific applications.

The GOx/CNT/silica composites were synthesized and deposited on, variously, screen-printed or carbon-paper electrode substrates by use of a modified combination of previously developed immobilization methods. Omitting numerous details for the sake of brevity, the synthesis/deposition process is summarized as follows:

The carbon substrates were coated with lysozyme by soaking them in a lysozyme solution and then washing off the excess lysozyme using a phosphate buffer solution.
A homogeneous GOx/CNT suspension was prepared by mixing CNTs and then GOx into a phosphate buffer solution, all the while sonicating the mixture.
A silica-precipitation reaction mixture was made by combining a phosphate buffer solution and a solution of tetramethyl orthosilicate in dilute hydrochloric acid.
The GOx/CNT suspension was incorporated into the silica-precipitation reaction mixture.
The lysozyme-coated carbon substrates were exposed to the reaction mixture at room temperature, allowing enough time (30 min.) for the formation of silica via precipitation catalyzed by the lysozyme.
The resulting composite-coated carbon substrates were washed with water, then dried.

The chemical compositions of the coatings were analyzed by use of x-ray photo-electron spectroscopy. Geometric aspects of the composite nanostructures were analyzed by use of scanning electron microscopy. Electrochemical properties were analyzed by use of cyclic voltammetry. The figure presents an example of plots of cyclic voltammetric scans made in the presence and absence of glucose, showing stable oxidation and reduction peaks at an optimal potential close to that of the FAD/FADH2 cofactor of immobilized glucose oxidase. [FAD/FADH2" refers to flavin adenine dinucleotide (FAD), a coenzyme that is derived from riboflavin and that becomes FADH2 as it accepts a pair of high energy electrons]. The immobilized GOx was found to be stable for a period of one month and to retain catalytic activity toward the oxidation of glucose.

This work was done by Heather R. Luckarift and Glenn R. Johnson of the Air Force Research Laboratory and Dmitri Invitski, Kateryna Artyuskova, Rosalba A. Rincón, and Plamen Atanassov of the University of New Mexico.