Wearable gas sensors are increasingly important for personal air quality monitoring and breath analysis. Nitrogen oxides (NOx), pollutants causing respiratory illnesses, and nitric oxide (NO), a biomarker for airway inflammation, are key targets. While graphene-based sensors offer advantages, they often suffer from low sensitivity and selectivity. Laser-induced graphene (LIG), a highly porous 3D material, offers a cost-effective and scalable solution with high surface area. However, humidity significantly impacts gas sensor performance. This paper addresses this challenge by designing a moisture-resistant, stretchable LIG-based NOx sensor. The sensor's design incorporates a semi-permeable polydimethylsiloxane (PDMS) membrane to protect the LIG sensing layer from moisture. The study systematically optimizes the LIG fabrication process by controlling laser processing parameters to achieve high sensitivity, fast response, and low detection limits while maintaining stretchability and moisture resistance. The goal is to develop a robust, wearable sensor suitable for continuous monitoring of environmental NOx levels and breath analysis for early disease diagnostics.

Existing literature explores various nanomaterials for NOx gas sensing, including graphene, metal oxides, conducting polymers, and carbon nanotubes. Graphene-based sensors, while promising due to their low noise and high mechanical strength, often suffer from low sensitivity and selectivity without surface modification. LIG offers a significant advantage due to its high specific surface area and low contact resistance, facilitating gas-solid interactions for improved sensing. Previous studies have explored LIG for NO2 detection, but LIG-based NOx sensors remain under-developed. Addressing the humidity effect in gas sensing has involved strategies like hydrophobic coatings, moisture barrier layers, heating elements, and electronic nose algorithms. However, these methods often complicate fabrication and increase costs. This work aims to overcome these limitations by integrating a moisture-resistant encapsulant within the sensor design.

The study fabricated a moisture-resistant, stretchable LIG-based NOx gas sensor with the LIG sensing/electrode region sandwiched between a semi-permeable PDMS membrane and a soft Ecoflex substrate. The LIG was created using a CO2 laser system, varying parameters like laser power, image density, and defocus distance to optimize LIG morphology (sheet, needle, rose petal, collapsed hole-like). Needle-like LIG, showing high specific surface area (296 m²/g) and low defect density (ID/IG = 0.46), was found optimal. A serpentine Ag/LIG electrode design provided stretchability. The sensor's fabrication involved laser scribing LIG patterns onto a PI film, transferring to the Ecoflex substrate, coating with Ag ink, and encapsulating with PDMS. The sensor's performance was characterized by measuring its response (ΔR/Ro) to NO and NO2 at various concentrations and humidity levels. The effects of laser processing parameters on the LIG's morphology, specific surface area, and sensing performance were systematically investigated using SEM, Raman spectroscopy, XPS, and BET measurements. The sensor's stretchability and moisture resistance were also tested. Finally, the sensor was used to monitor outdoor air quality and analyze breath samples from healthy subjects and patients with respiratory diseases.

The optimized needle-like LIG sensor demonstrated high sensitivity to NO (4.18% ppm⁻¹) and NO2 (6.66% ppm⁻¹), with ultralow detection limits of 8.3 ppb for NO and 4.0 ppb for NO2. The sensor exhibited fast response/recovery times (e.g., 113/296 s for NO). The serpentine electrode design enabled a 30% stretchability, and the PDMS encapsulation ensured excellent moisture resistance at 90% RH. The sensor successfully differentiated breath samples from healthy individuals and patients with respiratory diseases, indicating its potential for non-invasive disease diagnostics. The sensor also showed excellent repeatability and long-term stability.

The results demonstrate the successful fabrication of a high-performance, wearable NOx gas sensor with significant improvements in sensitivity, detection limit, response time, and moisture resistance compared to previous graphene-based sensors. The sensor's stretchability and moisture resistance are particularly crucial for wearable applications, enabling continuous and reliable monitoring of both environmental NOx levels and exhaled breath. The ability to differentiate breath samples from healthy individuals and patients with respiratory diseases highlights the sensor's potential for non-invasive disease diagnostics. Future studies could explore the integration of this sensor into fully functional wearable platforms for real-world applications, investigating its performance in diverse environments and with a larger patient population.

This study successfully designed and demonstrated a highly sensitive, selective, stretchable, and moisture-resistant NOx gas sensor based on laser-induced graphene. The sensor exhibits excellent performance characteristics and successfully classifies breath samples for respiratory disease diagnosis. This work paves the way for the development of advanced wearable sensors for environmental monitoring and non-invasive healthcare applications. Future work could focus on integrating this sensor into a complete wearable system for continuous and personalized health monitoring, and investigating its potential for detecting other biomarkers.

While the sensor demonstrates promising results, further research is needed to validate its performance in diverse real-world scenarios and with larger sample sizes. The study focused on NOx detection; investigating its sensitivity to other gases and potential cross-interference should be addressed. Long-term stability testing in more challenging environmental conditions is also recommended. The current operating temperature of 60°C might necessitate a thermal management strategy for comfortable skin contact. Finally, a more comprehensive clinical study with a larger cohort of patients would be necessary to confirm the diagnostic accuracy of this sensor.