Novel Sensor Using ISFET and Pt Electrodes for Water pH and Flow Speed Measurement

Multiple water quality sensors are required to construct a network for continuous and precise monitoring of water quality in natural environments. Therefore, low-cost sensors are necessary for guaranteeing scalability and widespread adoption. In this letter, we developed a sensor that can measure natural water pH and flow speed using only one ion-sensitive field-effect transistor (ISFET) and two Pt electrodes. The proposed sensor does not require an external reference electrode (RE) or electrolyte, because one of the Pt electrodes temporally acts as a reference electrode by electrolysis. Therefore, the sensor system for a reference electrode is omitted, and the electrode does not undergo electrolyte deterioration. Furthermore, the fabrication process is much simpler than that of pH sensors integrated with reference electrodes. We experimentally verified the proposed sensor. We confirmed that its pH measurements showed roughly similar tendency to those obtained using an Ag/AgCl reference electrode and achieved the sensitivity at least 20 mV/pH. In addition, we could measure flow speed from 5 to 25 cm/s by measuring the time variation of the Pt electrode potential.