Dynamic Pressure/Temperature Behaviour of GaN-Based Chemical Sensors

Long-term operation of chemical sensors under dynamic pressure/temperature conditions for industrial or environmental monitoring is desirable and yet rarely achieved. We have investigated long-term (>20 hours) operation of a novel ion sensor technology under varying temperature and pressure conditions whilst controlling for chemical effects by use of chemically inert hydrocarbon oil as the sensor analyte. The gallium nitride-based transistor-like sensor technology under study has previously been demonstrated for sensing a wide-range of heavy metal and nutrient ions and offers potential improvements regarding chemical stability but until now only short-term testing (1 to 3 hours) in water has been reported, and not under multi-variate environmental conditions. We investigated the response under varying. We show that for measurements under continuously cycling pressure (between 10.3 and 172.3 bar) and temperature (between 30 and 60 °C) conditions, within any two-hour observation window, the sensor response is linear and repeatable with a high coefficient of determination. Furthermore the temperature dependence strongly dominates the pressure dependence. However, when the observation window during pressure and/or temperature variation is extended to approximately 20 hours, anomalous variations in response that are uncorrelated to any environmental conditions (temperature and pressure which were measured, and light and ions, which were not present) become evident. Yet this anomalous behaviour is not present when the pressure and temperature are invariant. This finding has significant implications for implementing these sensors in dynamic environments.