Correction of instability in ion-selective field effect transistors (ISFETs) for accurate continuous monitoring of pH

A method for correction of instability in pH-sensitive ISFETs is presented. This method is based on monitoring the differential of the drain current for operation with a constant applied reference potential, or the differential of the feedback voltage applied to the reference electrode for operation in the feedback mode. Device instability in pH-sensitive ISFETs, commonly known as drift, is associated with a relatively slow, monotonic change in the measuring signal (i.e. the drain current or the compensating feedback voltage) in the absence of variations in the pH. Typically, over a sufficiently small interval of time, the variation in the measuring signal due to drift is essentially negligible in applications, such as cardiopulmonary bypass, where changes in pH are abrupt. Therefore, the differential of the measuring signal reflects only the variations in the pH for a device with a relatively high sensitivity. The validity of this method is examined using a Si₃N₄-gate pH-sensitive ISFET to monitor pH values in the 3.5-10.1 range. This approach to correction of drift may be generally applicable to all biosensors in which the instantaneous drift rate is small compared to the typical rate of change of the analyte concentration