Characteristics of Bio-Sensors

Only a few general comments concerning the properties of these sensors can be made here, because the properties of the biologically active phases serving as a basis in each individual case (enzyme, antibody, hormone receptor, etc.) play a central role. 

In general, the bio-sensors of the above sandwich constmction type exhibit a nearly Nernstian relationship between the substrate concentration and the EMF. The response times of constructions of this sort are influenced by many different factors [182] The enzyme reaction itself procedes at a finite rate which depends upon the initial substrate concentration and the Michaelis constant. The product of this reaction must then diffuse to and from the surface of the indicator electrode as its concentration changes. 

The concentration of the indicated material (substrate or product) at the indicator electrode surface under steady state conditions depends on the Michaelis constants, the activity of the biologically active compound in the membrane, the thickness of this membrane, and the diffusion coefficients of the substrate and product. It can be shown that for any given system, the response time of the enzyme electrode is given by the ratio d lD (d = layer thickness D = effective diffusion coefficient), except if the rate-determining step is the diffusion of a gas through a gas-permeable membrane, as in the case of the membrane-covered gas sensors. In practice layer thickness as low as 30 jit can be used. Depending on the substrate concentration, response times between 10 seconds and 15 minutes can be achieved. 

It is more difficult to remove interfering anions (such as the nitrate anion with urease). On the other hand, this allows one to indirectly determine the inhibitor concentration [454]. The lifetime of enzyme electrodes can also be shortened by bacterial degradation. To suppress this, 10 M sodium azide can be added [187,188]. Certain activators (such as phosphate with ureas, sodium cholate with cholesterolesterhydrolase) can effectively prolong the lifetime of these electrodes. 

Since bio-sensors based on enzymes are simple and easy to use, they are already in use in the Technicon Autoanalyzer for clinical work, and show excellent agreement vith analyses obtained with other, more troublesome methods. Bio-sensors based on antibodies (immuno-electrodes) are already in the testing phase [173,189], and those based on hormones are in the stage of theoretical conception [173]. 

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