Properties of carbon fibre microelectrodes as a basis for enzyme biosensors

The electrochemical properties of carbon fibre microelectrodes (CFMEs) are investigated in view of their application for the construction of amperometric biosensors based on oxidase enzymes. Scanning electron microscopy and cyclic voltammetry studies show that an appropriate electrochemical pretreatment has a strong effect on both morphology and electrochemical behaviour of the carbon fibre surface for all tested types of fibres. For AGT/F 10000 carbon fibres chosen for further study the surface becomes very smooth and a half-wave potential for H2O2 oxidation assumes a reproducible value of about of +0.8 V vs. SCE. The electrochemistry of the ferro/ferricyanide redox system is irreversible and quasi-reversible on unpretreated and pretreated electrodes, respectively. In the latter case, however, the addition of 20 mM MgCl2 or CaCl2 to the buffer solution makes the cyclic voltammetry curves reversible. Using glucose oxidase as a model enzyme, a glucose microbiosensor is prepared by enzyme cross-linking onto the carbon fibre surface in glutaraldehyde vapour. When operating in a H2O2 detection mode the biosensor dynamic range extends only up to about 2 mM glucose. The deposition of additional membranes (a reticulated bovine serum albumin and Nafion film) on top of the enzyme layer allows an extension of the dynamic range up to 13 mM and substantial elimination of interferences. When ferricyanide is used as a soluble mediator the biosensor dynamic range extends up to 100 mM glucose. The biosensor response was stable at least for one month when it was stored in air or in a phosphate buffer (pH 7.4) at 4 °C and used for about 30 measurements.