The Integration of Reference Electrode for ISFET Ion Sensors Using Fluorothiophenol-Treated rGO.

Ion-sensitive field-effect transistors (ISFETs) detect specific ions in solutions that enable straightforward, fast, and inexpensive sensors compared to other benchtop equipment. However, a conventional reference electrode (RE) such as Ag/AgCl is limited on the miniaturization of the sensor. We introduce reduced graphene oxide (rGO), which serves as a new RE, when fluorinated (F-rGO) using fluorothiophenol through the π-π interaction. The circular RE is integrated between a fabricated microscale two-channel ISFET, which is capable of detecting two kinds of ions on an indium tin oxide (ITO) thin-film substrate, using the photolithography process. F-rGO bound to this circular region to function as an RE in the ISFETs sensor, which operated stably in solution and showed a relatively high transconductance (gm) value (1.27 mS), low drift characteristic (3.2 mV), and low hysteresis voltage (±0.05 mV). It detected proton (H+) ions in a buffer solution with high sensitivity (67.1 mV/pH). We successfully detected Na+ (62.1 mV/dec) and K+ (57.6 mV/dec) ions in human patient urine using a two-channel ISFET with the F-rGO RE. The F-rGO RE will be a suitable component in the fabrication of low-cost, mass-produced, and disposable ISFETs sensors.

Two-Dimensional Disposable Graphene Sensor to Detect Na+ Ions.

The monitoring of Na+ ions distributed in the body has been indirectly calculated by the detection of Na+ ions in urine. We fabricated a two-dimensional (2D) Na+ ion sensor using a graphene ion-sensitive field-effect transistor (G-ISFET) and used fluorinated graphene as a reference electrode (FG-RE). We integrated G-ISFET and FG on a printed circuit board (PCB) designed in the form of a secure digital (SD) card to fabricate a disposable Na+ ion sensor. The sensitivity of the PCB tip to Na+ ions was determined to be -55.4 mV/dec. The sensor exhibited good linearity despite the presence of interfering ions in the buffer solution. We expanded the evaluation of the PCB tip to real human patient urine samples. The PCB tip exhibited a sensitivity of -0.36 mV/mM and linearly detected Na+ ions in human patient urine without any dilution process. We expect that G-ISFET with FG-RE can be used to realize a disposable Na+ ion sensor by serving as an alternative to Ag/AgCl reference electrodes.