Graphene-Based Ion-Selective Field-Effect Transistor for Sodium Sensing

Abstract

Field-effect transistors have attracted significant attention in chemical sensing and clinical diagnosis, due to their high sensitivity and label-free operation. Through a scalable photolithographic process in this study, we fabricated graphene-based ion-sensitive field-effect transistor (ISFET) arrays that can continuously monitor sodium ions in real-time. As the sodium ion concentration increased, the current-gate voltage characteristic curves shifted towards the negative direction, showing that sodium ions were captured and could be detected over a wide concentration range, from 10^-8 to 10^-1 M, with a sensitivity of 152.4 mV/dec. Time-dependent measurements and interfering experiments were conducted to validate the real-time measurements and the highly specific detection capability of our sensor. Our graphene ISFETs (G-ISFET) not only showed a fast response, but also exhibited remarkable selectivity against interference ions, including Ca²⁺, K⁺, Mg²⁺ and NH₄⁺. The scalability, high sensitivity and selectivity synergistically make our G-ISFET a promising platform for sodium sensing in health monitoring.

Keywords: graphene; ion-selective field-effect transistor; real-time monitoring; sodium ions.

Figure 1

(a) Optical image of as-fabricated GFETs, (b) Optical image of the graphene channel and source/drain electrode. (c) Raman spectrum of a graphene channel after the fabrication process. Two characteristic peaks were found: G peak at ~1580 cm⁻¹ and 2D peak at ~2700 cm⁻¹ (d) The line scan profile of the as-annealed GFET, Inset: AFM image with scan line indicated. The thickness of the graphene channel is ~0.5 nm.

Figure 2

(a) Current-gate voltage characteristic of an array of 100 graphene field-effect transistors. Histograms and Gaussian fits (black lines) of (b) Dirac voltage and (c) hole mobility extracted from the curves in panel a. (d) Current-gate voltage characteristic curves before and after ionophore deposition.

Figure 3

(a) Schematic of a back-gated G-ISFET. (b) Schematic of ionophore-functionalized GFET. The sodium ions captured in ionophores lead to a doping effect of the GFET.

Figure 4

(a) Transport characteristic curves of G-ISFET against different Na⁺ concentrations from 10⁻⁸ to 10⁻¹ M with a bias voltage of 100 mV. (b) G-ISFET response as a function for different target sodium concentrations at the logarithmic scale. A response of 152.4 mV/dec was observed.

Figure 5

(a) Real-time response of the source–drain current against a series of Na⁺ molar concentrations (V_g = 0 V). (b) The linear response in I_DS with different sodium concentrations from the real–time measurements in panel a.

Figure 6

(a) The selectivity test against several interfering ions. Error bars are the standard deviation of the mean. (b) Real-time responses of G-ISFET against human sweat samples collected from a sporting volunteer.