Nitrogen dioxide (NO₂) is a highly toxic gas causing severe lung damage, posing significant threats to human health, particularly in industrial settings like chemical plants. Real-time NO₂ detection and wearable early warning systems are crucial for worker safety. Existing metal-oxide semiconductor gas sensors, while sensitive and responsive, often suffer from limitations such as high operating temperatures (e.g., 160–325 °C for SnO₂, ZnO, and WO₃-based sensors), narrow detection ranges, and rigid structures unsuitable for wearable electronics. Flexible sensors offer advantages for wearable applications, but most flexible NO₂ gas sensors are designed for low concentrations, lacking wide-range capabilities. The high operating temperature of many sensors also leads to increased complexity, power consumption, and reduced suitability for wearable devices. This research aims to address these limitations by developing a flexible, room-temperature NO₂ gas sensor with a wide detection range, based on a rGO/SnO₂ composite material.

The paper reviews existing gas sensors, noting the limitations of rigid structures and high operating temperatures in previous NO₂ sensors. It highlights the use of materials like SnO₂, ZnO, and WO₃ in gas sensing, but points out the challenges associated with their agglomeration and high operating temperatures. The unique properties of reduced graphene oxide (rGO), including its semiconductor and metallic properties, excellent electron transfer capability, and abundance of oxygen-containing functional groups, are discussed as a potential solution for improving gas sensing performance at lower temperatures. The researchers highlight the lack of flexible, wide-range NO₂ gas sensors operating at room temperature as a major gap in the existing literature, motivating their research.

The researchers synthesized rGO/SnO₂ composite films using a novel spraying platform. This platform offers a low-cost and efficient method for creating consistent films. The platform consists of an air-pressure control device, an airbrush, and an operating platform, utilizing nitrogen gas to control evaporation during the spraying process. The rGO content was controlled by varying the spraying time and the SnO₂ nanoparticle content. The synthesized rGO/SnO₂ films were characterized using SEM, AFM, Raman spectroscopy, XRD, and XPS to assess their morphology, roughness, composition, and structure. The effects of rGO content, SnO₂ nanoparticle concentration, and annealing temperature on sensor performance were studied. The sensor’s response (defined as (R₀ - Rₐ)/R₀, where R₀ is resistance in air and Rₐ is resistance in NO₂ gas) was measured using a digital multimeter, and response and recovery times were determined. The sensor consists of a polyimide (PI) substrate with a Cu/Au interdigital electrode, onto which the rGO/SnO₂ film was sprayed and annealed. The sensor's electrical characteristics were evaluated using I–V tests. Finally, a flexible gas early-warning module was integrated with the sensor, embedding it into a woven fabric for wearable device fabrication.

The study found that the optimal rGO/SnO₂ composition for NO₂ gas sensing was 11.1 wt% rGO/SnO₂ with an annealing temperature of 250 °C. This combination yielded the best overall performance. A lower rGO content resulted in incomplete coverage of the electrode, leading to unstable film performance, while a higher content showed a decline in the response despite increased conductivity. The optimal sensor exhibited a response value of 0.2640 at 25°C (room temperature) for 100 ppm NO₂ gas, with response and recovery times of 412.4 s and 587.3 s, respectively. In the concentration range of 20–100 ppm NO₂, the sensor showed a linear relationship between response and concentration (R² = 0.9851). The spraying process demonstrated good film-forming consistency, with thickness remaining relatively constant despite minor variations in resistance. Annealing significantly affected the sensor's performance, with 250°C showing the best balance between response and recovery time. Higher annealing temperatures reduced defects in the rGO, lowering the sensor's response. Raman Spectroscopy confirmed the presence of both rGO and SnO₂ in the composite film, and XRD confirmed the tetragonal rutile structure of SnO₂. XPS analysis showed the film composition to be C, O, and SnO₂ without other impurities. Finally, the successful integration of the sensor into a wearable device demonstrated the feasibility of the design for practical applications.

The results demonstrate the successful development of a highly sensitive, flexible, room-temperature NO₂ gas sensor with a wider detection range than many previously reported sensors. The use of rGO/SnO₂ composite material, optimized through precise control of material ratios and annealing temperature, significantly improved sensing performance. The use of a simple and low-cost spraying technique for film deposition contributes to the overall practicality and scalability of the sensor design. The integration of the sensor into a wearable device significantly expands its potential for real-world applications in environmental monitoring and occupational safety. This design addresses the limitations of previous NO₂ gas sensors, providing a viable solution for detecting wide-range NO₂ gas concentration at room temperature in wearable applications.

This work successfully demonstrated the fabrication of a flexible and wearable NO₂ gas detection and early warning device using a novel spraying process and rGO/SnO₂ composite material. The sensor exhibited high sensitivity, wide detection range, and fast response time at room temperature. The integration with a flexible warning module and embedding in a wearable device highlight its potential for practical application in occupational safety and environmental monitoring. Future research could focus on further enhancing sensitivity, reducing recovery time, improving selectivity, and exploring other wearable device designs.

While the sensor showed excellent performance, some limitations exist. The recovery time, though improved compared to pure rGO sensors, is relatively long. Further optimization of the rGO/SnO₂ composition and annealing conditions could potentially shorten this time. The long-term stability and durability of the sensor under real-world conditions require further investigation. The sensor's selectivity against other interfering gases might require improvement for specific applications.