Fabrication and Characterization of a Flexible Non-Enzymatic Electrochemical Glucose Sensor Using a Cu Nanoparticle/Laser-Induced Graphene Fiber/Porous Laser-Induced Graphene Network Electrode

Abstract

We demonstrate a flexible electrochemical biosensor for non-enzymatic glucose detection under different bending conditions. The novel flexible glucose sensor consists of a Cu nanoparticle (NP)/laser-induced graphene fiber (LIGF)/porous laser-induced graphene (LIG) network structure on a polyimide film. The bare LIGF/LIG electrode fabricated using an 8.9 W laser power shows a measured sheet resistance and thickness of 6.8 Ω/□ and ~420 μm, respectively. In addition, a conventional Cu NP electroplating method is used to fabricate a Cu/LIGF/LIG electrode-based glucose sensor that shows excellent glucose detection characteristics, including a sensitivity of 1438.8 µA/mM∙cm², a limit of detection (LOD) of 124 nM, and a broad linear range at an applied potential of +600 mV. Significantly, the Cu/LIGF/LIG electrode-based glucose sensor exhibits a relatively high sensitivity, low LOD, good linear detection range, and long-term stability at bending angles of 0°, 45°, 90°, 135°, and 180°.

Keywords: Cu nanoparticle; flexible sensor; laser-induced graphene; non-enzymatic electrochemical detection.

Figure 1

Schematic of the fabrication process: (A) CO₂ irradiation of PI film to form the LIGF/LIG electrode pattern, (B) fabricated LIGF/LIG electrode-based sensor, (C) removal of LIGF from the electrical path and pad with N₂ gas stream, (D) formation of Ag/AgCl electrode in RE area and preparation of the passivation layer with PI tape and Ag epoxy, (E) completed Cu NP/LIGF/LIG electrode-based glucose sensor obtained by electroplating Cu NPs on the WE area, (F) photograph of the fabricated Cu NP/LIGF/LIG electrode-based glucose sensor.

Figure 2

(A) Measured sheet resistance and (B) XPS results showing atomic percentages of C1s, O1s, and N1s of the fabricated LIGF/LIG electrodes at laser powers of 8.5, 8.6, 8.7, 8.8, 8.9, and 9 W. (C) XRD pattern and (D) Raman spectrum of the fabricated LIGF/LIG electrode at 8.9 W.

Figure 3

FE-SEM images of the top view of (A) the fabricated Cu NP/LIGF/LIG electrode (×100), (B) Cu NP-coated LIGF (×1k), (C) Cu NP-coated LIGF (×30k) at higher magnification, (D) EDS results for the Cu-NP-coated LIGF/LIG electrode, and (E,F) a cross-sectional view of the LIGF/LIG electrode fabricated under an 8.9 W laser power.

Figure 4

Electrochemical characterization of the bare LIG, Cu NP/LIG, the bare LIGF/LIG, and Cu NP/LIGF/LIG electrodes in [10 mM K₃[Fe(CN)₆] + 1 M KNO₃] (1:1) ferricyanide solution. Scan rate: 10, 20, 30, 50, 100, and 200 mV/s. Randles–Sevcik plots with linear fitting curves of (A) bare LIG electrode, (B) Cu NP/LIG electrode, (C) bare LIGF/LIG electrode, and (D) Cu NP/LIGF/LIG electrode.

Figure 5

(A) CV curves of the Cu NP/LIGF/LIG electrode at glucose concentrations of 0, 1, 2, 3, and 4 mM in 10 mL of 0.1 M NaOH solution. (B) Amperometric current response at +600 mV; glucose concentration was increased every 10 s by pipetting 250 μL of glucose at different concentrations into 10 mL of 0.1 M NaOH solution [50 μM (twice), 100 μM (nine times), and 1 mM (three times) in sequence], and then the amperometric current response was measured for 1, 2, 3, and 4 mM of glucose in 10 mL of 0.1 M NaOH solution. (C) Linear fitting curve from amperometric current response results. (D) Selectivity test using chronoamperometry, where 250 μL of 0.3 mM solutions of glucose, AA, UA, lactose, fructose, sucrose, and glucose were pipetted sequentially into 10 mL of 0.1 M NaOH solution.

Figure 6

Cu NP/LIGF/LIG electrode-based glucose sensor in bent states with different bending angles (θ) for sheet resistance and CV measurements: (A) 0°, (B) 45°, (C) 90°, (D) 135°, and (E) 180°. In (B–E), the curvature radius is 1 cm. (F) Sheet resistance and (G) CV results of Cu NP/LIGF/LIG-electrode-based glucose sensor at glucose concentrations of 1, 2, 3, and 4 mM for each bending angle.

Figure 7

(A) Amperometric current response at +600 mV with time of applying different glucose concentrations for five different bending angles and (B) amperometric current response with respect to glucose concentration to investigate its linearity at bending angles of 0°, 45°, 90°, 135°, and 180°.

Figure 8

Stability of the fabricated Cu NP/LIGF/LIG electrode stored at ambient conditions over 15 days with the addition of 1 mM glucose under bending state at (A) 0°, (B) 45°, (C) 90°, (D) 135°, and (E) 180°.