Electrode Potential-Based Coupled Coil Sensor for Remote pH Monitoring

We present a coupled coil pH sensor for high-resolution remote pH monitoring. The sensor is based on a passive LC coil resonator whose resonant frequency is monitored remotely by measuring the change in impedance of an interrogator coil coupled to the sensor coil. The sensor resonator consists of an inductive coil connected in parallel with a voltage dependent capacitor and a pH combination electrode. When the pH of the contact solution changes, the electrode potential changes the capacitance, and therefore the resonant frequency of the sensor. A linear response with a 0.1 pH resolution is achieved over a 2-12 pH dynamic range at room temperature. The response time of the sensor is demonstrated to be less than 30 s and is limited by the response time of the pH combination electrode. Effects of varying separation distance and temperature change on the sensor's performance are shown. The described sensor technology has potential application for remote pH monitoring in numerous fields such as biomedical sensing, environmental monitoring, industrial and chemical processing, and structural health monitoring.