Metal ion and ultrasonication assisted assembling chiral nematic coatings towards humidity-responsive and anti-counterfeiting

Humidity-responsive sensitive materials have garnered considerable interest in a wide range of humidity sensors and information encryption applications. However, developing an eco-friendly sensitive material with mechanical robustness and response efficiency remains a significant challenge. Herein, we report a facile and smart strategy for assembling humidity-responsive cellulose nanocrystal (CNC)/polyacrylic acid (PAA)-Na iridescent coatings, assisted by metal ion regulation and ultrasonication. Owing to the enhanced electrostatic attraction between the positive charge (Na+) and negative charge (OSO3− and COO−), as well as the abundant hydrogen bonding networks, PAA addition can be as high as 50 wt%, which enables the design of CNC/PAA-Na iridescent coatings with excellent toughness and strength. Moreover, CNC/PAA-Na iridescent coatings possess a precise chiral nematic arrangement with a tunable helical pitch and a high g-factor above 0.5. The chiral nematic characteristics and absorption capacities are related to the humidity-responsive performance. Conditioned under variable humidity stimuli, CNC/PAA-Na iridescent coatings exhibit sensitive humidity-responsive behavior. In particular, the CNC/PAA50-Na/S iridescent coating achieves a wider humidity-responsive wavelength of 144 nm with a relative humidity (RH) above 43%. Interestingly, the CNC/PAA-Na iridescent coatings could regulate the transmitted colors under cross-polarization. Based on chiral nematic triggered by humidity, an “RH-rotation angle-color” ternary colorimetric system is established, which highlights the potential of CNC/PAA-Na iridescent coatings regarding sensitive humidity-response and information encryption. [Display omitted] •A chiral nematic iridescent film is assembled with Na+ and ultrasonication.•The iridescent films possess tunable helical pitch and g-factor above 0.5.•Interfacial bonding and a highly twist structure endow mechanical robustness.•Sensitive humidity-responsivity is achieved with a larger redshift of 144 nm.•A colorimetric humidity-response system is proposed for anti-counterfeiting.