NO₂ is an air pollutant that poses documented threats to human health and the ecological environment. Consequently, developing portable and low-power gas sensors for real-time NO₂ monitoring is an ongoing research focus in environmental science. Traditional resistive gas sensors often require continuous energy consumption due to their structural reliance on external power sources and elevated operating temperatures. To address these limitations, this study aims to develop a self-powered, room-temperature NO₂ gas sensor. By combining the room-temperature gas-sensing properties of composite nanomaterials with the mechanical energy-harvesting capabilities of triboelectric nanogenerators (TENG), an autonomous detection system was constructed. Specifically, an SnO₂/MXene composite was utilized as the core gas-sensing material to detect NO₂ concentrations, and a TENG was employed to harvest ambient mechanical energy to drive the sensing system without the need for external batteries or power grids.

In this study, an SnO₂/MXene composite material was integrated into a sensing platform, leveraging the physical and chemical interactions between the catalytic properties of SnO₂ and the electrical conductivity and specific surface area of MXene. For the energy-harvesting module, a contact-separation mode TENG was fabricated based on the principles of triboelectrification and electrostatic induction. The friction layers of this TENG were constructed using a commercial nylon film and an Ecoflex silicone film as positive and negative triboelectric materials, respectively, based on their differing electron affinities. A triboelectric energy-powered NO₂ sensor (TENS) was constructed by electrically coupling the TENG with the resistive SnO₂/MXene gas sensor through a rectifying and voltage-stabilizing circuit. This circuit uses a bridge rectifier to convert the alternating current output generated by the TENG into a direct current output to supply power. The gas-sensing characteristics of the TENS were evaluated in a custom-built gas testing chamber at room temperature, using standard NO₂ gas to calibrate and control internal concentrations.

The testing procedures demonstrated the electrical output metrics of the fabricated TENG. Under mechanical excitation, the TENG achieved a maximum peak-to-peak voltage of 540 V and a maximum output power of 12 mW. Power generation capacity was visually verified by lighting 50 commercial light-emitting diodes simultaneously, which indicates sufficient power supply for driving specific low-power electronic components. Following electrical characterization, the self-powered sensing performance of the TENS for NO₂ quantitative detection was evaluated. Room-temperature gas-sensing tests showed the response behavior of the SnO₂/MXene-based sensor. When exposed to 100 μL/L of NO₂, the self-powered sensor exhibited a response value of 39. Furthermore, the dynamic sensing curves indicated that the response and recovery times of the sensor were 33 and 80 s, respectively. This performance correlates with the heterojunction between SnO₂ and MXene, which affects localized charge transfer and active adsorption site distribution for target gas molecules.

In summary, this study constructed a self-powered NO₂ gas sensor driven by a nylon/Ecoflex TENG using an SnO₂/MXene nanocomposite as the active sensing layer. The integration of mechanical energy harvesting with room-temperature gas sensing eliminates reliance on external power sources for standard sensor operations. The proposed TENS aligns with technical demands for the miniaturization, low power consumption, and simplified maintenance protocols of environmental monitoring equipment. By enabling self-sustained operation, this work provides empirical data and a technical reference for the research and development of portable atmospheric pollutant detection devices in Internet of Things applications.

Respiratory monitoring is essential for determining the vital characteristics of the human body and diagnosis of sleep-disordered breathing, critical diseases, and neonates. Moreover, micro/nanosensors play an important role in healthcare. In particular, they can detect changes in humidity around the nose and mouth for analysis of respiratory behavior. Herein, a high-performance flexible wearable humidity sensor was developed using a chitosan/conductive carbon black (CS/CB) hybrid film as the sensitive layer for respiratory detection.

The humidity sensor was fabricated by screen-printing a CS/CB film on a flexible poly (ethylene terephthalate) substrate with interdigitated electrodes. Scanning electron microscopy (SEM) was conducted to demonstrate the successful synthesis and combination of the CS/CB hybrid film. The SEM results showed that the CB particles are uniformly embedded in the CS/CB film with porous and rough nanostructures, which can help water molecules diffuse into the sensitive film. The humidity performance of the humidity sensor was tested by switching the device between different RH levels established by several saturated salt solutions (11%–97% RH) at room temperature. The resistance of the CS/CB sensor was measured with a high-precision data logger. The CS/CB humidity sensor had good electrical conductivity, and its resistance was increased with the increase in RH with good linearity. The sensor was experimentally evaluated for its performance parameters, such as sensitivity, response recovery characteristics, and repeatability. The humidity switching test demonstrated that the sensor has a fast response rate and good discrimination to different humidity environments. The sensor under the humidity rise reduction test showed good symmetry and coincidence, indicating that the hysteresis effect can be ignored. The CS/CB humidity sensor demonstrated good repeatability, rapid response/recovery time, and negligible hysteresis. A flexible wearable film-type breath detection platform was developed using a single-chip microcomputer and LabVIEW technologies, which achieved real-time data reading and processing of humidity, communication between the microcontroller and computer terminal, and display of breath waveforms. The detection platform has low production costs and is easy to operate, providing new insights into the practical application of high-performance humidity sensor detection technology. The sensor is placed in the mask for respiratory behavior monitoring, including normal breathing, slow breathing, rapid breathing, stop breathing, Biot’s breathing, wave breathing, tidal breathing, febrile patient breathing, etc. The breathing frequency and intensity can be observed clearly.

The flexible wearable humidity sensor could monitor and analyze human respiratory conditions with high accuracy and reliability. It demonstrated good application in real- time human breath monitoring. The humidity-sensitive mechanism of the sensor was ascribed to the good hydrophilicity and many active sites (hydrophilic groups and vacancies) on the CS/CB nanomaterials to absorb water molecules. In addition, the CS/CB nanomaterial with high surface area had numerous surface-active sites to adsorb water molecules.

The developed CS/CB humidity sensor has great practical potential in wearable applications, especially in humidity sensing detection. It also promotes students’ in-depth understanding of the experiments and applications of flexible wearable sensors, effectively enhancing their motivation and effectiveness of teaching.