Mesoporous Ag‑Doped α‑Fe₂O₃ Electrochemical Sensor Achieves Ultra‑Sensitive Liquid Ethanol Detection

Abstract

We report a highly sensitive, low‑cost electrochemical sensor for liquid ethanol based on mesoporous α‑Fe₂O₃ decorated with sub‑20 nm Ag nanoparticles. The hybrid nanostructure was prepared via a modified sol‑gel route using Pluronic F‑127 as a template, followed by photochemical reduction of Ag⁺. Characterization by XRD, FTIR, Raman, UV‑Vis, PL, and N₂ sorption confirms a highly crystalline, mesoporous matrix with Ag surface decoration. Electrochemical testing on an Ag/α‑Fe₂O₃‑modified glassy carbon electrode (GCE) demonstrates a sensitivity of 41.27 µA mM⁻¹ cm⁻² in the 0.05–0.8 mM range and 2.93 µA mM⁻¹ cm⁻² in the 0.8–15 mM range, with a limit of detection (LOD) of 15.4 µM. The sensor exhibits rapid response, excellent reproducibility, and superior selectivity over methanol and isopropanol. These results establish Ag‑doped α‑Fe₂O₃ as a promising platform for high‑performance ethanol detection in aqueous media.

Background

Chemical sensors are pivotal in diagnostics, food safety, environmental monitoring, and agriculture. Electrochemical sensors, in particular, offer high sensitivity, fast response, and low cost. The performance of such sensors is governed by the active material’s physicochemical properties. Metal‑oxide semiconductors, especially α‑Fe₂O₃ (hematite), are attractive due to their stability, nontoxicity, and catalytic activity. Recent advances demonstrate that mesoporosity and noble‑metal doping can significantly enhance electron‑transfer kinetics and adsorption sites. However, efficient, low‑concentration ethanol detection remains challenging.

Materials and Methods

Materials

Fe(NO₃)₃·9H₂O, AgNO₃, ethanol, Pluronic F‑127 (EO106–PO70–EO106), and standard laboratory reagents were used without further purification.

Synthesis of Mesoporous α‑Fe₂O₃

A sol‑gel mixture of Fe(NO₃)₃·9H₂O, F‑127, ethanol, HCl, and acetic acid (molar ratio 1:0.02:50:2.25:3.75) was stirred, gelled, aged at 40 °C/40 % RH for 12 h, then at 65 °C for 24 h. Calcination at 450 °C (1 °C min⁻¹) yielded mesoporous α‑Fe₂O₃ nanocrystals.

Photochemical Reduction of Ag

1 g of mesoporous α‑Fe₂O₃ was suspended in 100 mL aqueous methanol (1 % v/v). AgNO₃ (9.4 × 10⁻⁵ mol) was added, and the mixture was irradiated with a 2.0 mW cm⁻² Hg lamp for 12 h. The resulting Ag/α‑Fe₂O₃ was centrifuged, washed, and dried at 110 °C.

Characterization

Phase purity: XRD (Cu Kα). Functional groups: FTIR (400–4000 cm⁻¹). Vibrational modes: Raman. Optical bandgap: UV‑Vis (200–800 nm). Photoluminescence: 315 nm excitation. Morphology: FE‑SEM, TEM (HR‑TEM, SAED). Surface area & porosity: N₂ sorption at 77 K (BET, BJH).

Electrochemical Measurements

GCEs (0.071 cm²) were polished, coated with Ag/α‑Fe₂O₃ in a binder mixture, and dried at 65 °C. Measurements were performed in 0.1 M PBS (pH 7) with ethanol concentrations 0.05–15 mM. Current–potential (I‑V) sweeps at 50 mV s⁻¹ and cyclic voltammetry (CV) were recorded. Electrochemical impedance spectroscopy (EIS) assessed interfacial properties.

Results and Discussion

Structural and Optical Properties

XRD patterns match the hematite phase (JCPDS 01-086-0550) with no secondary phases; Ag peaks are undetectable due to low loading. FTIR shows O‑H stretching (~3350 cm⁻¹), Fe–O deformation (~566 cm⁻¹), and residual C–H (~2900 cm⁻¹). Raman spectra display α‑Fe₂O₃ modes (221, 290, 405, 495, 609, 1315 cm⁻¹) and Ag‑related bands (1370, 1683 cm⁻¹), indicating successful decoration and local field enhancement. UV‑Vis absorption reveals a red‑shifted peak (~450 nm) due to Ag surface plasmon resonance, while PL intensity is reduced, confirming suppressed electron–hole recombination.

Morphology and Porosity

SEM shows semi‑spherical α‑Fe₂O₃ particles (25–70 nm). TEM confirms Ag nanoparticles (<20 nm) uniformly distributed on the α‑Fe₂O₃ matrix. N₂ sorption isotherms (type IV, H1 loop) yield BET surface areas of 3.55 m² g⁻¹ (α‑Fe₂O₃) and 3.74 m² g⁻¹ (Ag/α‑Fe₂O₃). Pore size distribution centers at 8 nm for α‑Fe₂O₃ and 4 nm after Ag deposition, reflecting mesoporosity and nanocluster formation.

Electrochemical Behavior

CV in 5 mM ethanol shows negligible response on bare GCE, moderate on α‑Fe₂O₃‑modified GCE (Iₐₙₒₙ = 4.5 µA), and doubled response on Ag/α‑Fe₂O₃‑modified GCE (Iₐₙₒₙ = 8.4 µA). EIS reveals lower charge‑transfer resistance for Ag/α‑Fe₂O₃, confirming enhanced electron transfer.

Ethanol Sensing Performance

Current–potential sweeps (50 mV s⁻¹) across 0.05–15 mM ethanol yield two linear regimes: (1) 0.05–0.8 mM (sensitivity = 41.27 µA mM⁻¹ cm⁻², R² = 0.9623) and (2) 0.8–15 mM (sensitivity = 2.93 µA mM⁻¹ cm⁻², R² = 0.9876). LOD calculated as 15.4 µM (S/N = 3). Selectivity tests show ethanol > methanol > isopropanol responses.

Reaction Kinetics

CV at varying scan rates (25–500 mV s⁻¹) for 0.2 mM ethanol shows peak current ∝ scan rate (R² = 0.995) and ∝ √scan rate (R² = 0.995), indicating mixed surface‑controlled and diffusion‑controlled processes.

Stability and Reproducibility

Relative standard deviation < 5 % across five electrodes; repeatability < 5 % over five cycles. Continuous operation for 45 min shows minor drift; storage for 5 weeks maintains performance.

Conclusions

The mesoporous Ag/α‑Fe₂O₃ nanocomposite delivers exceptional ethanol detection at room temperature, combining high sensitivity, low LOD, and robust stability. Its superior performance stems from the synergy of mesoporosity, small Ag nanoparticles, and enhanced electron‑transfer kinetics, making it a compelling candidate for future electrochemical alcohol sensors.