Optimized α‑NiS Nanosphere Films for Long‑Term, Non‑Enzymatic Glucose Sensing

Abstract

We report the synthesis of α‑nickel sulfide (α‑NiS) nanosphere films that retain functional stability for over five years at ambient conditions. The films were produced by electrodepositing a nickel nanosheet layer onto indium tin oxide (ITO) glass, followed by sulfurization in vacuum‑sealed ampoules at 300–500 °C. Comprehensive characterization—including XRD, SEM, EDS, HR‑TEM, CV, EIS, UV‑Vis‑NIR, and PL—confirmed the α‑phase and nanosphere morphology, particularly at 400 °C for 4 h. Electrochemical tests in 0.1 M NaOH and Krebs buffer revealed a linear glucose response from 1–35 µM and 0–40 µM, respectively, with a sensitivity of 8.4 µA µM⁻¹ cm⁻². The films maintained their electrochemical performance after five and a half years stored at room temperature, demonstrating their suitability for durable, non‑enzymatic glucose sensors.

Background

Nickel sulfide (NiS) is a well‑known conductive material with applications ranging from lithium‑ion batteries to photocatalysis and non‑enzymatic glucose sensing. While many sensing strategies exist, non‑enzymatic electrodes offer superior shelf‑life and resistance to biological fouling. We focused on α‑NiS because of its high conductivity and stability, aiming to create a sensor that can be stored for years without loss of sensitivity. To our knowledge, no prior work has documented a non‑enzymatic glucose sensor based on α‑NiS that remains functional after five years of ambient storage.

Methods

Preparation of the α‑NiS Films

Film synthesis involved a two‑step process. First, nickel nanosheets were electrodeposited onto 0.5 × 1 cm² ITO glass using a Pt anode and a 3.0 V DC potential in a 0.1 M NiSO₄·6H₂O / 0.05 M NaOH bath at pH 7.7 and 40 °C for 10 min. The resulting films had a thickness of ~500 nm and grain sizes of 0.01–0.3 µm. Second, the nickel layers were sulfurized by placing the samples in vacuum‑sealed glass ampoules with sulfur sheets and heating at 300, 400, or 500 °C for 4 h. For optimization, 400 °C samples were also annealed for 3 and 6 h. The final α‑NiS films were then rinsed with deionized water and dried under nitrogen.

Characterization of the α‑NiS Film

Phase identification was performed with a SHIMADZU XRD‑6000 (Cu Kα). Morphology and composition were examined by VVSEM (HITACHI S‑3000N) and FE‑SEM/EDS (HITACHI S‑4800, 3 kV). Electrochemical behavior was measured in a three‑electrode cell using a JIEHAN ECW‑5000 potentiostat with an Ag/AgCl reference. CV scans (0–0.8 V, 20 mV s⁻¹) were conducted in 0.1 M NaOH and Krebs buffer (pH 7.4). Amperometric responses were recorded at 0.6 V for glucose concentrations ranging from 1 to 35 µM (NaOH) and 0–40 µM (Krebs). EIS was carried out in 0.1 M KCl + 1.5 mM Fe(CN)₆³⁻/⁴⁻ (Zennium IM6). UV‑Vis‑NIR spectra (HITACHI U‑3501) and PL (RF‑5301PC) were obtained for optical analysis. HR‑TEM (JEOL TEM‑2010) provided lattice spacing and crystallite size data.

Results and Discussion

Electrodeposition produced smooth nickel nanosheets with uniform grain sizes; SEM images confirmed thicknesses of ~500 nm. XRD patterns matched JCPDS 870–712, confirming pure Ni metal. After sulfurization at 400 °C for 4 h, α‑NiS nanospheres (~0.1–0.2 µm) appeared densely on the surface, with EDS showing S/Ni ≈ 1.02 (wt% S ≈ 35.8 %, Ni ≈ 64.2 %). The 400 °C condition yielded the best morphology; 300 °C produced irregular particles (~0.5–2 µm), while 500 °C produced chain‑like structures (~1–5 µm).

CV studies revealed clear oxidation peaks at ~0.6 V, indicating Ni²⁺/Ni⁺ redox activity. In 0.1 M NaOH, the peak current increased linearly with glucose concentration (1–35 µM), yielding a sensitivity of 8.4 µA µM⁻¹ cm⁻² (R² = 0.99). Amperometry at 0.6 V showed excellent anti‑interference against dopamine, uric acid, and lactic acid. EIS data showed lower charge‑transfer resistance (R_ct = 42.1 Ω) for the 400 °C films compared to 300 °C (R_ct = 71 Ω), correlating with higher surface area.

Long‑term stability tests (ambient 16–26 °C, 50–65 % RH) over five and a half years demonstrated no loss of peak current or sensitivity. The films remained fully functional in both NaOH and Krebs buffer, with linear glucose detection ranges of 1–35 µM (NaOH) and 0–40 µM (Krebs). This durability surpasses typical enzymatic sensors, which often degrade within weeks.

Optical measurements revealed bandgaps of 1.08 eV (300 °C), 1.80 eV (400 °C), and 0.66 eV (500 °C), consistent with quantum‑confinement effects in the nanospheres. PL spectra displayed peaks at 448 nm and 369 nm (excited at 277 nm), confirming high crystallinity.

HR‑TEM images of the 400 °C/4 h sample showed nanospheres 150–250 nm in diameter with clear (101) lattice fringes (0.7786 nm). Selected‑area electron diffraction confirmed the α‑NiS phase.

Conclusion

α‑NiS nanosphere films produced at 400 °C for 4 h exhibit robust structural, optical, and electrochemical properties suitable for non‑enzymatic glucose sensing. Their long‑term stability (≥ 5 years at room temperature) and high sensitivity make them promising candidates for portable, low‑maintenance glucose monitoring devices.

Abbreviations

CV

Cyclic voltammogram

EDS

Energy‑dispersive spectrometer

FE‑SEM

Field emission scanning electron microscopy

HR‑TEM

High‑resolution transmission electron microscopy

NiS

Nickel sulfide

PL

Photoluminescence

PVT

Physical vapor transport

SD

Standard deviation

UV/Visible/NIR

Ultraviolet/visible/near‑infrared

VVSEM

Variable vacuum scanning electron microscopy

wt%

Percentage by weight

XRD

X‑ray diffraction