A Flow-through-cell Module containing inexpensive ChemFETs for Differential pH-sensing in Aqueous Solutions

In this publication, an Extended-Gate Field-Effect-Transistor (EGFET) is presented for pH-sensing in conjunction with a differential flow-through measurement set-up. As a pH, sensor a commercially available MOSFET (MOS: Metal Oxide Semiconductor) is used with its gate electrode virtually 'extended' by connecting it to a pH-sensitive indium tin oxide (ITO) layer. This layer is brought into contact with aqueous solution thus creating an electrochemical cell together with a stainless-steel reference electrode. The pH-value of the sample solution inside the cell is sensed through the pHdependent surface potential present at the ITO layer which changes the drain current through the connected MOSFET. The ITO/PET sheet was structured via laser-micromachining to form a robust dual-sensing zone inside a PMMA housing suitable for differential sensing. The use of two flow-through cells in combination with a special, serial flow-through procedure enabled the sensor array to operate without a conventional glass reference electrode, using a simple steel electrode as a pseudo reference electrode instead. This feature contributes significantly to miniaturization and robustness of the sensor.