Flying Droplet Sensor Based on H-GQDs/CNF Network Structure and Its Applications.

Respiratory infectious diseases continue to pose a serious threat to public health. Their transmission relies on respiratory droplets that are small in size and evaporate rapidly, making real-time monitoring difficult. In this study, we propose a flying droplet sensor based on hydroxylated graphene quantum dots (H-GQDs) deposited on a cellulose nanofiber (CNF) film. Owing to the network water-locking structure of the film, the sensor is capable of detecting coughing droplets transmitting over long distances (≥2.4 m). By placing multiple such sensors at equal intervals along the cough airflow direction, the spatiotemporal transmission features of coughing droplets were obtained, including arrival time and signal intensity at different distances. On this basis, we investigated the transmission behavior of cough droplets from multiple individuals at various distances. The results show that regions close to the volunteer's mouth exhibit a broader droplet size distribution, whereas at longer distances, the remaining droplets are mainly composed of small-size and low-intensity components. We further found that coughing droplets typically take 4-5 s to travel more than 2 m, indicating that a time window for timely protection still exists at this distance. In addition, the relationships between cough sound level and droplet signal intensity as well as transmission time were evaluated. No clear correlation was observed, suggesting that acoustic indicators alone cannot directly reflect the infection risk associated with cough droplet transmission. This study provides important insights for the real-time monitoring and protection against respiratory droplet transmission.