Ultrathin g-C ₃ N ₅ Nanosheets with Enhanced Charge Transfer for Room-Temperature NH ₃ Sensing and Breath-Based Diagnosis of Pulmonary Infection

Atomically thin, two-dimensional (2D) semiconductors offer unprecedented opportunities for high-performance gas sensing, while the limited surface accessibility of their bulk counterparts severely restricts their practical applications. Herein, ultrathin graphitic carbon nitride (g-C 3 N 5 ) nanosheets with an average thickness of ∼1.7 nm were successfully synthesized via a facile and scalable sulfuric acid-assisted intercalation–exfoliation strategy using bulk g-C 3 N 5 derived from the thermal polymerization of 3-amino-1,2,4-triazole. The exfoliation process effectively tailors the electronic structure and maximizes the exposure of nitrogen-rich active sites, enabling enhanced surface adsorption and accelerated charge transfer. As a result, the as-prepared g-C 3 N 5 nanosheets exhibit outstanding room-temperature ammonia (NH 3 ) sensing performance, delivering a high sensitivity of 22.27 ppm –1, a low detection limit of 27 ppb, and excellent selectivity under complex gas environments. Notably, the integration of g-C 3 N 5 nanosheets onto a flexible porous substrate further enables robust mechanical stability and efficient gas diffusion, supporting reliable operation under bending and long-term use. More importantly, the sensor demonstrates the capability to discriminate pulmonary infection patients from healthy individuals through exhaled breath analysis, highlighting its potential for non-invasive clinical diagnostics. This work not only provides an effective strategy for constructing high-performance carbon nitride-based gas sensors but also offers a promising platform for wearable healthcare monitoring and early disease detection.