A flexible and wearable NO₂ gas detection and early warning device based on a spraying process and an interdigital electrode at room temperature

Nitrogen dioxide (NO₂) poses serious health risks, causing damage to lung tissues; hence, real-time monitoring is critical, especially in leak-prone industrial environments such as chemical plants. The study addresses the need for a flexible, wearable NO₂ detection and early-warning device that operates at room temperature and across a wide concentration range. To overcome limitations of existing sensors (rigid form factors, high operating temperatures, narrow detection ranges), the authors propose a flexible polyimide-based sensor using Cu/Au interdigital electrodes and an rGO/SnO₂ composite sensing film deposited via a low-cost, consistent spraying process. They investigate how rGO/SnO₂ composition and annealing temperature affect sensing performance and integrate the sensor with a flexible warning module for wearable applications.

Metal-oxide semiconductor sensors (e.g., WO₃, SnO₂, In₂O₃, ZnO) offer high sensitivity and fast response but typically require elevated temperatures and rigid substrates. Prior works: SnO₂ hollow spheres on ceramic tubes showed higher NO₂ response at 160 °C; Pa-coated SnO₂ nanofibers detected H₂ with 0.25 ppm limit but required ≥160 °C; ZnO/SnO₂ core–shell nanorods for ethanol had response 0.866 at 225 °C; Cu/SnO₂ xerogel formaldehyde sensors achieved high response (S = 50–96%) at 275–325 °C. Most existing devices are rigid, limiting wearability. Flexible NO₂ sensors exist but generally operate near 100 °C and target low concentrations: e.g., nanostructured composite on a flexible heater for 1–20 ppm NO₂ at 100 °C; Al-doped ZnO flexible sensors for 0.2–2 ppm NO₂ at 100 °C. Therefore, developing a flexible, wide-range NO₂ sensor that functions at room temperature is a significant unmet need. rGO offers good electron transport and surface functional groups, while SnO₂ is a stable n-type oxide; rGO modification of SnO₂ can enhance low-temperature gas sensing.

Materials and synthesis: Graphene oxide (GO) was prepared by Hummer’s method and thermally reduced to rGO. Commercial SnO₂ nanoparticles (50 nm, 99.9%) were used. A 0.05 wt% rGO aqueous dispersion was prepared (12.5 mg rGO in 25 mL ultrapure water) via magnetic stirring and ultrasonication. The 25 mL was divided into five parts; SnO₂ nanoparticles (10, 20, 30, 40 mg) were added to four parts, yielding rGO/SnO₂ suspensions with rGO solid contents of 20, 11.1, 7.7, and 5.9 wt% after ultrasonication. Spraying platform: A custom low-cost spraying setup comprised an N₂ gas supply with a pressure-reducing valve, a U-STAR-S120 airbrush (0.2 mm needle), and an adjustable-stage platform. N₂ flow aided solvent evaporation and minimized deposition on the electrode surface. Spraying conditions (air pressure, nozzle height, needle opening) were tuned for uniform films. Sensor fabrication: Flexible sensors were fabricated on a PI substrate with Cu/Au interdigital electrodes (line width/spacing 50 µm each; electrode area 5 mm × 5 mm; overall sensor size 7 mm × 11 mm). rGO or rGO/SnO₂ suspensions were spray-deposited onto the electrodes (typical spray volume 0.25 mL), followed by vacuum drying at 60 °C for 1 h. Films were annealed in N₂ at 100, 250, or 400 °C for 1 h to adjust defect density and recovery behavior. Characterization and testing: SEM and AFM examined morphology and roughness; I–V characteristics were measured with a semiconductor device analyzer; Raman spectroscopy analyzed rGO and SnO₂ features; XRD identified crystalline phases; XPS characterized elemental states. Gas sensing was conducted at 25 °C in dry air as carrier, with NO₂ concentrations controlled by a mass flow controller (Sevenstar CS200). Resistance was recorded with a Keysight 34461A multimeter. Sensor response S was defined as (R₀ − Rₐ)/R₀, with response time t_res as time to 90% of total response upon exposure and recovery time t_rec as time to 10% above baseline in air. Film consistency assessment: rGO-only films produced by three identical sprays (0.25 mL each) were cross-sectioned to measure thickness and correlate with resistance; surface roughness on electrode vs PI groove regions was profiled by AFM.

- Optimal composition and processing: An rGO/SnO₂ film obtained by spraying 0.25 mL of a 0.4 wt% rGO/SnO₂ mixture and annealing at 250 °C delivered the best overall NO₂ sensing at room temperature. - Performance at room temperature (25 °C): For 100 ppm NO₂, response S = 0.2640; response time t_res = 412.4 s; recovery time t_rec = 587.3 s. The response scaled linearly with concentration from 20–100 ppm with correlation coefficient 0.9851. - Composition dependence: Among tested loadings (rGO solid content 20, 11.1, 7.7, 5.9 wt%), 11.1 wt% rGO/SnO₂ showed the highest response at 50 ppm NO₂. SnO₂ nanoparticles increased surface roughness and active sites, improving response versus rGO-only films. - Annealing effects: Annealing shortened response and recovery times but reduced response magnitude as temperature increased. At 50 ppm NO₂ and 100 °C anneal, S = 0.1591, t_res = 316.1 s, t_rec = 537.2 s with full recovery. At 400 °C anneal, response dropped sharply to S = 0.0197, though dynamics improved. 250 °C anneal provided the best balance of response and recovery. - rGO defect evolution: Raman I_D/I_G decreased with higher annealing temperatures, indicating fewer rGO defects (fewer active sites), consistent with reduced response and film resistance decrease. - Structural confirmation: Raman showed rGO D (1341 cm⁻¹), G (1596 cm⁻¹), 2D (2680 cm⁻¹), and a weak SnO₂ peak (~626 cm⁻¹). XRD peaks at 2θ ≈ 26.53°, 33.81°, 37.91°, 51.72°, 54.80° matched rutile SnO₂ (JCPDS 41-1445). XPS survey detected C, O, and Sn signals with no other impurities. - rGO film behavior and consistency: Increasing rGO content via longer spray times decreased resistance (exponential trend). Pure rGO films showed slow/incomplete recovery. Three identically sprayed rGO films had thicknesses ~18.46, 23.97, 19.23 µm with corresponding resistances 21.23, 16.87, 19.45 kΩ, showing inverse correlation and good film-forming consistency. Electrode surfaces (R_a ~26.6 nm) yielded smoother films than PI grooves (R_a ~142–180 nm). - Wearable integration: A flexible soft-monitoring node with overlimit NO₂ warning was developed and, together with the flexible sensor, embedded into woven fabric (mask or watch form factor), demonstrating practical wearable NO₂ detection.

The study demonstrates a flexible NO₂ sensor capable of room-temperature operation with a linear response over 20–100 ppm, addressing the limitations of high-temperature, rigid sensors and the narrow operating ranges of many flexible counterparts. The rGO/SnO₂ composite leverages rGO’s conductivity and functional groups for adsorption, while SnO₂ nanoparticles increase roughness and provide additional active sites, enhancing sensitivity. Annealing tunes defect density: reduced defects (lower I_D/I_G) diminish active adsorption sites, lowering response but improving recovery and decreasing resistance. An intermediate annealing temperature (250 °C) provides a practical trade-off between response magnitude and reversibility. Film morphology and substrate roughness strongly influence uniformity and electrical stability; smoother electrode surfaces yield more consistent films. The successful integration of the sensor with a flexible warning module and embedding into textiles validates the device’s applicability for wearable early-warning systems in industrial environments.

A low-cost spraying platform was developed to deposit uniform rGO/SnO₂ films on flexible interdigital electrodes, enabling a wearable NO₂ sensor that operates at room temperature with linear response over 20–100 ppm. Optimal performance was achieved using a 0.25 mL spray of a 0.4 wt% rGO/SnO₂ mixture and annealing at 250 °C, yielding S = 0.2640 at 100 ppm with t_res = 412.4 s and t_rec = 587.3 s. Material characterization confirmed the presence and structure of rGO and SnO₂, while annealing was shown to modulate defect density and sensing behavior. A flexible overlimit warning module was designed and integrated with the sensor into wearable textiles (mask/watch), demonstrating practical applicability for real-time NO₂ monitoring in industrial settings.

- Recovery behavior: Pure rGO films exhibited very long and incomplete recovery; rGO/SnO₂ films improved recovery but still struggled without annealing. Even with annealing at 100 °C, recovery times remained >600 s in some cases. - Response–recovery trade-off: Higher annealing temperatures shortened response/recovery times but significantly reduced sensitivity (e.g., S ≈ 0.0197 at 400 °C for 50 ppm NO₂), indicating a trade-off between speed and response magnitude. - Tested range: Reported linearity and performance were demonstrated over 20–100 ppm; lower detection limits and ultra-low concentration performance were not detailed in the provided text. - Selectivity and long-term stability: The provided text does not present selectivity against interfering gases or long-term stability/aging data.