A fast-response and water-resistant flexible humidity sensor based on a composite of ionic liquid and hydrogen-bonded organic frameworks encapsulated with a hydrophobic coating

Facile preparation of hydrogen-bonded organic framework (HOF-1) @ ionic liquid (IL-1). • Flexible humidity sensor (FHS) of HOF-1@IL-1 encapsulated with hydrophobic coating (POTs). • HOF-1@IL-1@POTs FHS exhibits short response/recovery time (2 s/3 s), minimal hysteresis (∼1% RH). • HOF-1@IL-1@POTs FHS shows good water-resistance. Flexible humidity sensors (FHS) with fast-response and good water-resistance are highly favored for applications in sophisticated scenarios. Herein, a composite of 2D hydrogen-bonded organic framework (HOF) with ionic liquid (IL) was encapsulated with a hydrophobic coating (POTs) to construct FHS. Specifically, a 2D HOF was obtained by self-assembly of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine, and then the IL of 1-ethyl-3-methylimidazolium tetrafluoroborate was anchored onto the HOF as driven by intermolecular hydrogen-bonding interactions. Moreover, POTs was obtained from hydrolysis and condensation of triethoxy(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane. The IL@HOF@POTs FHS showed fast response (response time: 2 s, recovery time: 3 s), minimal hysteresis (relative humidity of ∼1%) and excellent long-term stability. Furthermore, it demonstrated exceptional water-resistance as revealed by negligible change in humidity response after immersion in water for 30 min. Density functional theory calculations and electrochemical impedance spectroscopy analysis were carried out to explore the sensing mechanism, and it was proposed that the modulation of the interaction energy of the composite with adsorbed water molecules accelerated the response. Meanwhile, the FHS was successfully employed for real-time respiratory monitoring, speech recognition, contactless human-machine interfaces and generation of Morse code. The study reveals that the composite of HOF and IL are promising in the construction of advanced sensors. Download: Download high-res image (145KB) Download: Download full-size image