NiO NF@Ex-G Electrodes for Glucose Sensing

• The paper demonstrates a novel electrode design combining phase-pure NiO nanoflowers with exfoliated graphite to achieve high sensitivity (~304 µA·mM⁻¹·cm⁻²) and selectivity in glucose detection.
• It details a multistep synthesis involving oxidative intercalation for Ex-G and hydrothermal fabrication of NiO nanoflowers, ensuring uniform catalyst dispersion and durable electrode assembly.
• The sensor achieves a low detection limit (100 µM) and broad linear range (1–10 mM), highlighting its promise for practical, disposable biomedical applications.

Nickel oxide nanoflower-decorated exfoliated graphite (NiO NF@Ex-G) electrodes constitute a class of non-enzymatic electrochemical sensors that leverage the high surface area and catalytic properties of transition metal oxide nanostructures integrated with highly conductive graphite substrates. These systems have demonstrated enhanced glucose sensing performance metrics, outperforming analogous assemblies on conventional glassy carbon electrodes. NiO NF@Ex-G electrodes employ phase-pure NiO nanoflowers as catalytically active sites supported on flexible, large-area exfoliated graphite sheets, offering significant improvements in both sensitivity and selectivity within physiologically relevant glucose detection ranges (Choudhary et al., 24 Jan 2026).

1. Fabrication Protocols and Material Characterization

The synthesis of NiO NF@Ex-G electrodes involves distinct multistep chemical routes for both the exfoliated graphite support and the NiO nanoflower catalyst, followed by their integration as functional electrochemical substrates.

1.1 Exfoliated Graphite Synthesis

Exfoliated graphite (Ex-G) is prepared via oxidative intercalation of natural graphite flakes with a concentrated acid mixture (H₂SO₄/HNO₃), potassium permanganate (KMnO₄) as oxidant, and subsequent microwave-induced expansion at 800 W for 2 min. Post-expansion, the product is pressed at 20 tonnes for 6 h into 0.5 mm thick flexible sheets.

1.2 NiO Nanoflower Synthesis

Nickel oxide nanoflowers are synthesized hydrothermally. A NiCl₂·2H₂O precursor in ethanol is mixed with polyvinylpyrrolidone (PVP) and an aqueous urea solution, followed by autoclave treatment at 160 °C for 10 h, producing a Ni(OH)₂ precipitate. This is washed, dried, then annealed at 450 °C for 4 h to yield phase-pure NiO with a uniform nanoflower morphology.

1.3 Electrode Assembly

5 mm disks are punched from compressed Ex-G sheets and mounted with copper wire via silver paste; exposed basal planes serve as working faces. NiO nanoflowers are drop-cast at several loadings (0.5–3 mg·mL⁻¹), followed by Nafion coating for mechanical stability, then dried to produce the final NiO NF@Ex-G electrodes (NF-0.5 → NF-3).

1.4 Structural and Morphological Characterization

Table 1 summarizes principal findings.

Technique	Key Findings
XRD	NiO fcc phase (peaks at 37.2°, 43.2°, 62.3°, 75.2°, 79.8°); 16.3 nm crystallite size; Ex-G peaks at 26.5°, 54.9°
Raman	LO₁ 505 cm⁻¹, LO₂ 1075 cm⁻¹ (phase-pure NiO); TO mode not resolved
FTIR	Ni–O at 426, 813 cm⁻¹; adsorbate bands at 1320, 1628, 3450 cm⁻¹
FESEM (NiO NF)	0.8–1.2 µm “marigold” flowers; petals of ~20 nm subunits; porous
FESEM (Ex-G)	Layered, smooth planar sheets at micron scale
FESEM (NF@Ex-G)	Dense, uniform coverage; petal-like clusters 1–2 µm
TEM/BET	Not reported

These analyses reveal that the NiO retains its nanoflower morphology after deposition, and the Ex-G offers a high-surface-area platform for catalyst dispersion (Choudhary et al., 24 Jan 2026).

2. Electrochemical Configuration and Analytical Methodologies

Electrochemical characterization is performed in a three-electrode cell (0.1 M NaOH electrolyte, volume 20 mL) with NiO NF@Ex-G as the working electrode, Ag/AgCl (3 M KCl) as reference, and platinum wire counter. Cyclic voltammetry (CV) is executed from 0.00 to +0.80 V with scan rates between 10–200 mV·s⁻¹, following preconditioning with 25 cycles at 200 mV·s⁻¹. Amperometric (chronoamperometric) detection is performed at +0.60 V (NiOOH/Ni(OH)₂ redox region), recording current transients after stepwise addition of glucose (1 mM increments, 0–10 mM) with baseline stabilization prior to analysis.

3. Analytical Performance and Sensor Metrics

The NiO NF@Ex-G electrodes exhibit high analytical accuracy and sensitivity across a broad dynamic range. Key quantitative results are summarized in Table 2.

Parameter	Value
Calibration Fit	$$I(\mu\mathrm{A}) = 65.0\,\mu\mathrm{A\,mM^{-1}}\,C + 53.23\,\mu\mathrm{A}$$ ($R^2=0.9925$)
Sensitivity	$304.1\,\mu\mathrm{A\,mM^{-1}\,cm^{-2}}$ (average); best: $339.28\,\mu\mathrm{A\,mM^{-1}\,cm^{-2}}$
LOD	$100\,\mu$M
Linear Range	$1$–$10$ mM ($R^2 = 0.9925$)

Calculation of sensitivity uses the geometric area ($A = 0.1963$ cm² for 5 mm disk), and LOD follows IUPAC convention: $\mathrm{LOD} = 3\,\sigma_\mathrm{blank}/m$ with $\sigma_\mathrm{blank} \approx 2.17\,\mu$A, $m = 65\,\mu$A/mM.

4. Non-Enzymatic Glucose Oxidation Mechanism

The sensor mechanism is governed by surface redox chemistry of the Ni(II)/Ni(III) couple under alkaline conditions. Electrochemical activation occurs via:

$$\begin{aligned} \text{(1)}\quad & \mathrm{NiO} + \mathrm{OH}^- \rightleftharpoons \mathrm{NiOOH} + \mathrm{e}^- \ \text{(2)}\quad & \mathrm{NiOOH} + \text{Glucose} \rightarrow \mathrm{NiO} + \text{Gluconolactone} \end{aligned}$$

The NiOOH species generated in step (1) acts as the oxidant in step (2), chemically converting glucose to gluconolactone. The observed peak current’s dependence on $\nu^{1/2}$ (from CV scan-rate studies) indicates diffusion-controlled kinetics with some degree of surface adsorption. The high density of catalytically active sites on the nanoflower surface, combined with the large electrochemically active surface area afforded by the Ex-G substrate, results in reduced overpotential and enhanced electrode performance (Choudhary et al., 24 Jan 2026).

5. Selectivity, Stability, and Reproducibility

5.1 Selectivity

Selectivity was assessed amperometrically at +0.60 V (0.1 M NaOH) by comparing signal responses to 1 mM glucose with various interferents at 0.1 mM (ascorbate, fructose, sucrose) and 0.1 M NaCl.

Analyte	ΔI (µA)	% Relative to Glucose
Glucose	65.0	100 %
Ascorbate	4.3	6.6 %
Fructose	3.7	5.7 %
Sucrose	2.9	4.5 %
NaCl	3.1	4.8 %

The low cross-responses (<7%) underscore high analyte selectivity under practical conditions.

5.2 Stability and Reproducibility

• CV baseline remains stable after 25 cycles (no drift >5%).
• Inter-electrode relative standard deviation (RSD) is <4% for five independently prepared electrodes.
• Slight fouling and adsorption effects are observed over repeated use; single-use operation is recommended for analytical consistency.

6. Comparative Evaluation of NiO NF@Ex-G versus Other Sensor Platforms

Performance benchmarking against alternative NiO-based systems and substrates is shown in Table 3.

Electrode	Sensitivity (µA·mM⁻¹·cm⁻²)	LOD (µM)	Linear Range (mM)
NiO NF@Ex-G (this work)	304.1	100	1–10
NiO NF@GCE (3 mg·mL⁻¹)	245.1	100	1–8
NiO hollow microspheres	151	5	0.1–3
NiO nanosheets	160	15	0.5–5
NiO@Ni nanowires	280	0.7	0.01–2

The 3 mg·mL⁻¹ NiO NF@Ex-G configuration offers a sensitivity enhancement of ~24% compared to NiO NF on glassy carbon electrode (GCE). Ex-G’s higher ECSA and conductivity are key contributors to this improvement.

7. Conclusions and Implications for Sensing Applications

NiO NF@Ex-G electrodes manifest a confluence of high catalytic NiO nanoflower surfaces and the advantageous ECSA and electrical properties of exfoliated graphite supports, enabling sensitive, selective, and reproducible non-enzymatic glucose detection with a sensitivity of ~304 µA·mM⁻¹·cm⁻², a LOD of 100 µM, and a linear range spanning 1–10 mM. The simple fabrication protocol, low materials cost, and robust selectivity (interference <7%) highlight the utility of these electrodes as promising single-use, disposable sensors in analytical and biomedical contexts (Choudhary et al., 24 Jan 2026).

A plausible implication is that incorporating Ex-G substrates and tailoring NiO architectures hold considerable potential for optimizing next-generation non-enzymatic electrochemical biosensors for clinical and environmental monitoring.