Two-Dimensional Disposable Graphene Sensor to Detect Na+ Ions

Abstract

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.

Keywords: ISFET; Na+ ion; disposable sensor; fluorinated graphene; fluorobenzene; reference electrode.

Figure 1

The characteristics of graphene before and after fluorobenzene treatment for 30 s: (a) Raman spectra; (b) water contact angles; and (c) I_DS–V_GS of G–ISFET and FG–ISFET.

Figure 2

Evaluation of sensitivity of G–ISFET and FG–ISFET with Ag/AgCl–RE. Linear fits were used to extract sensitivity: (a) experimental setup using G–ISFET with Ag/AgCl–RE; (b) I_DS–V_GS of FG–ISFET depending on Na⁺ ion concentration; (c) pH sensitivity of G–ISFET and FG–ISFET; (d) Na⁺, K⁺, and Ca²⁺ sensitivity of G–ISFET; (e) Na⁺, K⁺, and Ca²⁺ sensitivity of FG–ISFET; and (f) long–term stability of G–ISFET and FG–ISFET in Tris–HCl buffer, in which 100 mM NaCl was dissolved for 6 h.

Figure 3

Schematic diagram and image of the 2D structural sensing device: (a) fabrication process of the fluorinated graphene reference electrode (FG–RE); (b) G–ISFET–ISM and FG–RE were integrated on an SD card–type printed circuit board (PCB).

Figure 4

(a) Experimental setup using G–ISFET–ISM with FG–RE; (b) I_DS–V_GS of G–ISFET–ISM with FG–RE depending on Na⁺ ion concentration; (c) the sensitivity of G–ISFET–ISM with Ag/AgCl–RE or FG–RE to Na⁺ ions. Linear fits were used to extract sensitivity; (d) real–time detection of Na⁺ ions using G–ISFET–ISM with FG–RE.

Figure 5

(a) Sensitivity of G–ISFET–ISM with FG–RE to K⁺ and Ca²⁺ ions in Tris–HCl buffer, in which 100 mM NaCl was dissolved. Linear fits were used to extract sensitivity; (b) sensitivity to pH in Carmody buffer; and (c) sensitivity to Na⁺ ions in Tris–HCl buffer, in which 100 mM KCl was dissolved.

Figure 6

Detection of Na⁺ ions in real human patient urine samples using G–ISFET–ISM with FG–RE. Linear fits were used to extract sensitivity: (a) I_DS–V_GS plots at different Na⁺ concentrations in the same urine sample (added by titration); (b) I_DS–V_GS plots of three different urine samples; and (c) I_DS–V_GS of G–ISFET–ISM with FG–RE in Tris–HCl buffer between measurements of three different patient urine samples.