Slow effects in the pH-step response of silicon nitride ISFET sensors

Investigations has been carried out with S3N4-gateinsulator ISFETs to get a more complete database and a deeper understanding of the underlying physical and electrochemical effects. In this study the ISFET sensors were operated in phosphate buffer under stirred conditions at constant temperature. By adding the appropriate amount of buffer, pH-steps were performed in a range from pH 5 to pH 8. To extract the different amplitudes and time constants the sensor signal was fitted to a multi-exponential step-response curve. As a result, the following effects have been found, which in part are not described up to now in literature: A rapidly decaying exponential effect in the short-time domain with a mean time constant of about 400 sec. and a pH-independent amplitude; a slowly decaying exponential effect in the long-term region with a mean time constant of about 2700 sec., whose amplitude is strongly pH-dependent and increases with increasing pH; a significant influence of the step direction on the time constants (a pH-step from a low to a high value produces higher time constants than the same pH-step in opposite direction), however; no influence of the pH-step direction on the amplitudes and, which were found to be almost completely reversible in a pH-cycle.