Bioinorganic composites for enzyme electrodes

Sparingly soluble redox salts were combined with a model enzyme, glucose oxidase, in a host matrix of a biopolymer chitosan to form bioinorganic composite films on the surface of glassy carbon electrodes. Four redox salts, each containing the Ru(NH3)6(3+) cation and a selected anion, such as Ru(CN)6(4-), Fe(CN)6(4-), Co(CN)6(3-) or IrCl6(3-), were studied. The composition and catalytic properties of such composite materials toward glucose oxidation were investigated by spectroscopic and electrochemical methods. The composite films provided an oxygen-independent electrical communication between the enzyme's redox centers and a glassy carbon surface at a potential as low as -0.10 V vs Ag/AgCl(3 M Cl-). The nature of the electrical communication is discussed in terms of redox mediation by the Ru(NH3)6(3+)-containing ion pairs formed inside the biocomposites. The kinetic significance of the mediator's charge is considered by postulating that neutral ion pairs are more efficient redox mediators of the enzymatic reaction than those negatively charged. The low operating potential of enzyme electrodes based on the bioinorganic composites allows for an interference-free determination of glucose. The design of the biocomposites is generic and can incorporate oxidoreductase enzymes other than glucose oxidase to provide a host of biosensors for biologically and environmentally important analytes.