Classes of Electrochemical Biosensors

Potentiometric enzyme electrodes were the first biosensors. In such sensors, an IBS was coated by an enzyme layer acting as a biocatalyst for reaction of a specific substance. The reaction product subsequently was detected by the ISE. This feature had been transferred soon to ISFETs. A special term, ENFET, has even been informally proposed for enzyme-modified ISFETs. Attempts were also made to utilize further biological interactions for recognition of analytes and construction of potentiometric biosensors. Immunologic sensors on the basis of antigen-antibody reaction may be called IMFETs if they are built on top of a MOSFET. Immunologic reactions, however, maybe used much more efficiently in combination with other transducers besides potentiometric 

Among the oldest biosensors is a urea sensor (Guilbault and Montalvo 1969). As depicted in Fig. 7.29, the enzyme urease is fixed in a hydrogel layer of polyacrylamide which is coated on the surface of a glass electrode with nylon gauze as support. Urea diffuses into the gel, where a catalysed hydrolysis takes place  

The reaction products carbonate and ammonia change the pH, which is detected by the glass electrode, thus allowing one to estimate the urea content. A better function can be achieved by utilization of a special ammoniiun-sensitive glass electrode, as proposed by the inventors. 

Actual potentiometric biosensors are commonly designed on the basis of ISFETs, i.e. they are ENFETs. Inclusion in hydrogels is no longer done. Covalent bonding, preferably at carbon surfaces, is preferred. Alternatively, embedding in polymer layers is performed, today preferably in polymers generated elec-trochemically. Also widespread are P VC coatings with specific softeners (as solvents for active substances) as well as layers of silicon rubber and polyurethane. 

The formation of a stationary concentration of reaction product at the electrode surface is a prerequisite for proper fimctioning of potentiometric enzyme sensors. Stable conditions are estabhshed after a certain settling time where reaction speed is equal to the diffusional transport of the analyte to the electrode (Fig. 7.30). This time is represented by the response time of the sensor, hi stationary state, the maximum concentration of reaction product is located at the electrode surface (Vadgama 1990). The enzyme layer should be designed to ensure that this optimum state is reached again following a change in sample composition. Response times of potentiometric biosensors are different. Commonly they assume values of some minutes. 

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