AN ISFET-FIA SYSTEM FOR HIGH-PRECISION PH RECORDING

It is well known that precise long-term pH recordings with ISFET sensors are difficult due to the inherent drift and hysteresis effects of these devices. To solve this problem a concept is presented comprising a differential ISFET/REFET set-up, which can be used in continuous flow or flow injection analysis (FIA) systems. In contrast to other ISFET/REFET pairs presented in the literature, differential sensing between two ISFET sensors with identical gate insulator composition is employed, which allows effective suppression of common mode disturbances, including drift, without the need of a glass reference electrode. To test the sensor performance in a practical biomedical application, long-term pH measurements were performed in an isolated rat liver perfusion system (IRLPS); in addition, continuous in-line pH measurements of bile from anaesthetized rats were also conducted. In both applications, the ISFET sensing systems showed a good operational stability, high resolution (typically 0.01 pH) and the feasibility of rapid pH change detection. A stable and accurate readout was achieved at the extremely low flow rates present in the cannulated bile duct of anaesthetized rats. These results clearly demonstrate the application specific advantages of a miniaturized solid state sensor over those of a macroscopic device.