Pb²⁺ Transfer‐Enabled Recoverable Hydrogel‐Based H₂S Colorimetric Sensing with Assistance of Multimodal Deep Learning for Multifunctional Applications

Abstract Recoverable and humidity‐tolerant colorimetric sensors are extremely difficult to overcome. In this work, a composite hydrogel sensor based on chelation reaction is proposed for the first time to achieve ultra‐fast recovery and ultra‐low detection limit (LOD) of H₂S colorimetric sensing. The composite hydrogel sensor is constructed by polyvinyl alcohol/boric acid/Pb²⁺ (PVA/Bn/Pb) hydrogel sensing layer and polyacrylamide/sodium alginate (PAM/SA) hydrogel adsorption layer. The change from white to brown is achieved in H₂S and can be restored when placed in the air. The response/recovery mechanism is explained by the transfer of Pb²⁺ between hydrogels and the formation of an “Alginate‐Pb²⁺” with egg‐box structure during the process. The multimodal fusion deep neural network (MF‐DNN) can overcome the error caused by hydrogel aging. Due to the excellent loading and diffusion function provided by the 3D structure of hydrogel, the sensor has a broad detection range of 0.2–100 ppm, a response/recovery time of 10s/32s, and a theoretical LOD of 0.026 ppm. A smartphone‐based H₂S sensing system is developed to enable non‐invasive halitosis diagnosis, egg freshness detection, and remote H₂S monitoring and alarming. The system is recoverable, humidity‐tolerant, capable of ultrafast detection and widely applicable. This work provides inspiration for humidity‐tolerant and recoverable H₂S gas sensing.