ISFET-Based Differential pCO₂ Sensors Employing a Low-Resistance Gas-Permeable Membrane

Abstract

A new type of ISFET-based differential pCO2 sensor system is devised by employing a low-resistance gas-permeable membrane. Both the pCO2-FET and the reference FET (REFET) are made by coating their gate surfaces with an RTV silicone rubber film doped with valinomycin. The pCO2 FET is covered with a hydrogel-based recipient layer inside the gas-permeable RTV membrane, but the REFET does not include the hydrogel layer. Because of low resistance of the gas-permeable membrane, the reference electrode can be placed outside as a common reference for both FETs. Output signals from the pCO2FET and the REFET are measured against the common reference, and the difference of these two signals is measured through a differential amplifier; therefore, calibration drifts of the two identical base ISFETs, caused by their thermal sensitivity, for example, cancel out. It is also shown that the ion response or the potential drifts caused by both parts of the RTV membranes cancel out exactly the same way as in the case of the base ISFETs.