Conductimetric Biosensors

Many biological processes involve changes in the concentrations of ionic species. Such changes can be utilised by biosensors that detect changes in electrical conductivity. A typical example of such a biosensor is the urea sensor, utilising immobilised urease, and used as a monitor during
renal surgery and dialysis (Figure 1.). The reaction gives rise to a large change in ionic concentration at pH 7.0, making this type of biosensor particularly attractive for monitoring urea concentrations.

An alternating field between the two electrodes allows the conductivity changes to be determined while minimising undesirable electrochemical processes. The electrodes are interdigitated to give a relatively long track length (~1 cm) within a small sensing area (0.2mm²). A steady-state response can be achieved in a few seconds, allowing urea to be determined within the range 0.1–10mM. The output is corrected for non-specific changes in pH and other factors by comparison with the output of a non-enzymic reference electrode pair on the same chip.

Figure 1. Parts of a conductimetric biosensor electrode arrangement. (a) Top view; (b) cross-sectional view. The tracks are about 5 μm wide and the thicknesses of the various layers are approximately SiO₂ 0.55 μm, Ti 0.1 μm, Pt 0.1μm, Au 2 μm.

 The method can easily be extended to use other enzymes and enzyme combinations that produce ionic species, for example amidases,
decarboxylases, esterases, phosphatases and nucleases. As molecular iodine molecules change the conductivity of iodine-sensitive phthalocyanine films, they may be used in peroxidase-linked immunoassays where the peroxidase converts iodide ions to iodine. Conductimetric biosensors have also be devised to determine DNA hybridisation.