Short Communication THE pH-STATIC ENZYME SENSOR An ISFET-based Enzyme Sensor, Insensitive to the Buffer Capacity of the Sample

Enzyme sensors based on the measurement of pH consist basically of a pH-sensitive electrode to which an enzyme-loaded membrane is attached. The conversion of substrate in the immobilized enzyme membrane results in a local change in pH which can then be measured. The pH-sensitive electrode used is usually a glass membrane electrode or an ISFET. In 1980, Caras and Janata [l] described the first ISFET-based enzyme sensor sensitive to penicillin. The name ENFET was introduced for this sensor. The response of enzyme sensors based on the measurement of pH is sensitive to the buffering capacity of the sample. Furthermore, these electrodes have a non-linear response for two reasons. First, the buffering capacity of the sample solution is pH-dependent; this implies that pH changes in the immobilized enzyme membrane depend on the initial pH of the sample solution. A second, and perhaps even more important reason for the nonlinear response is the pHdependent enzyme kinetics. It is well known that each enzyme has its own optimal pH value at which its activity is greatest. Thus, at high substrate concentrations, the pH changes in the enzyme membrane may become so large that the enzyme is actually inhibiting its own performance. The complex response characteristics of these enzyme electrodes have been the subject of a number of papers (see, e.g. [2,3] ) and the general conclusion is that the practical applicability is limited. Recently, ISFET-based sensors have been developed for the performance of coulometric acid/base titrations on a very small scale. This is achieved through the application of a noble-metal electrode that is arranged closely around the pH-sensitive gate of the ISFET. Through the electrolysis of water, H’ or OHions are formed, depending on the direction of the current from this electrode to a distantly located counter electrode. In this way, a