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.