On the Use of Ag/AgCl Reference Electrode in Silicon Nanowire Field Effect Transistors for pH and Biosensing Applications

Silicon nanowire field-effect transistors (Si NWFETs), thanks to their highly efficient integration into numerous electronic devices, represent promising tools for pH measurements and molecular biosensing due to their high sensitivity to ionic charges on their surface. To obtain a highly sensitive, stable, and low-noise signal in liquid media, the use of a reference electrode is necessary. Among potential reference electrodes, Ag/AgCl electrodes have proven to be the easiest to miniaturize and, consequently, the most widely used in Si NWFETs. However, their integration in such devices remains complex. The choice of the type of Ag/AgCl reference electrode and its positioning vary considerably depending on the device configuration and the analyte, and whether the measurements are carried out in dynamic or static environments. Here, we report a consolidated overview focused on the reference electrode implementation in Si NWFETs, presenting the advantages and disadvantages of different strategies encountered, and illustrating our points with several examples of miniaturized electrochemical biosensors and ion-sensitive field-effect transistors. Finally, in order to understand the effects of the Ag/AgCl electrode in Si NWFETs and its influence on the polarization of the device or the analyte, a comparative study of different device configurations from the literature is also presented.