A Composite Structure of In Situ Cross-Linked Poly(Ionic Liquid)s and Paper for Humidity-Monitoring Applications

This paper describes a humidity sensor fabricated by cross-linking poly(ionic liquid)s (PILs) in the cellulose fibers of printing paper via an in situ photoinitiated “thiol-ene” click reaction. The resultant sensors inherit the strong hydrophilicity and high ionic conductivity of the cellulose and PILs, respectively, which are the most important factors for a high-performance impedance-type humidity sensor. The introduction of PILs improved the sensitivity, hysteresis, response speed, and stability of the paper-based devices. The optimal paper-based device sensitivity increased by a factor of 1.4 after adding PILs, and the response time decreased to 25 s, which is comparable to that of the best reported paper-based humidity sensors.