Synthesis of salinity ultra-sensitive ionic hydrogels for visual salinity detection and usage as an intelligent liquid valve

Functional hydrogel materials with large-scale deformation induced by ultra-low salinity as an external stimulus have promising applications in seawater osmotic energy harvesting, stimulus-responsive separation materials, intelligent sensors, and actuators. A salt-responsive hydrogel undergoing large volumetric changes (∼1/60) with the use of dilute salt solution (0.01–0.6 wt% NaCl) is designed via facile random copolymerization of polyelectrolyte and zwitterionic polymer chains in this study. The salt-responsive swelling/deswelling properties, morphologies, and mechanical properties of the prepared hydrogels are systematically examined. Internal reconfiguration of distinct polymeric networks is characterized by different salinity percentages via Raman spectroscopy. Subsequently, a portable ion-detection device comprising easy-to-access capillary tubes and salt-responsive hydrogel is successfully designed and compared with a commercial water quality monitoring device. Moreover, intelligent liquid valves with autosensing and actuating functions are developed for the regulation of liquid leakage and transport using the same salt-sensitive hydrogel. It is highly expected that these salinity ultra-sensitive ionic hydrogels would have great potential in monitoring and regulating freshwater resources, seawater osmotic energy harvesting, stimulus-responsive separation materials, and intelligent soft robotics and electronics.