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.

Here, robust and low-cost sodium sensors based on the ion-sensitive field-effect transistor (ISFET) are fabricated using indium tin oxide (ITO) thin film. The effect of the presence of oxygen during the sputtering of the ITO thin film is characterized on the sensitivity of sodium ion (Na + ) detection. A sodium ionophore III membrane is applied on the ITO substrate to fabricate ISFET (ITO-ISFET). The results reveal that the increase in the oxygen gas flow rate (0-1 sccm) with a fixed argon flow rate at 20 sccm during sputtering increased the sensitivity of ITO-ISFET to Na + from 15.8 to 100.9 mV/decade. In addition, the proper condition with a nearly Nernstian slope is at 0.4 sccm oxygen flow rate. Furthermore, the experimental results reveal that the sensors exhibit a high sensitivity of 58.5 ± 2.1 mV/decade in a wide range (10 −10 to 10 −1 M). The limit of detection (LOD) of ITO-ISFET is 1.8 nM, which is lower than the minimum allowable Na + level in the human body and urine. Furthermore, the proposed ITO-ISFET has the capability to detect Na + in real human patient urine without any dilution process.

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.