ITO/Glass as Extended-Gate of FET: A Low-Cost Method for Differential pH-Sensing in Alkaline Solutions

Abstract

An electrochemical pH-sensor for aqueous solutions is designed based on the working principle of extended-gate field-effect transistor (EGFET). A commercially available metal-oxide-semiconductor field-effect transistor (MOSFET) was employed as the transducer in combination with an indium tin oxide (ITO) coated glass slide as the sensing membrane. The ITO/glass slide was patterned and encapsulated independently of any cleanroom process. The unique packaging technique provides a 6 μL cell, allowing for a low volume sample measurement. A custom electrical circuit, driving two MOSFETs in differential mode, eliminates the sensor drift, temperature, and common mode disturbances. The differential measurement enables the sensor to operate in a reference-electrode free mode, making sensor miniaturization feasible. The sensor showed a pH-sensitivity of 52.31 mV/pH with linearity (square regression) of 0.995 in the pH-range of 8.00 to 10.80 on the first working day. Electrochemical impedance spectroscopy (EIS) technique was applied to study the corrosion of the ITO in contact with multiple buffer solutions to determine a reliable working pH-region for the sensor. A fluidic module was employed to carry out the differential measurement and calibrate the sensor to compensate for the pH-sensitivity decrease. Therefore, a low-cost and disposable EGFET-based sensor is introduced for trustworthy pH-measurements over time.