Low-cost EGFET-based pH-sensor using encapsulated ITO/PET-electrodes

This paper presents a truly low-cost modular sensor using laser-micromachined Indium Tin Oxide (ITO) on Polyethylene Terephthalate (PET) as pH-sensitive material for a differential Extended Gate Field Effect Transistor (EGFET). In this method, the gate terminal of a commercially available MOSFET is connected to ITO/PET-electrodes providing an electrochemical measurement cell in combination with a reference electrode. Hence, a pH-variation of the solution in contact with the ITO/PET-electrode can be sensed by the MOSFET due to a surface potential change at its “liquid gate” terminal. A serial flow cell configuration has been designed to enable differential sensing, in order to overcome long-term drift and common mode disturbances and eliminate the need for a glass reference electrode. A special encapsulation method of the ITO/PET-foil conserves its pH-sensitivity. The whole sensor and its package are realized without any need for expensive cleanroom processes. The combination yields to a true low-cost modular flow-through-sensor with high precision and robustness to sense pH.