Anisotropic cellulose nanocrystal composite hydrogel for multiple responses and information encryption

Anisotropic composites based on well-ordered cellulose nanocrystals (CNCs) have received increasing attention due to their fascinating optical properties. In this study, we fabricated a multi-functional hydrogel with nematic organization of CNC by soaking a pre-stretched polyvinyl alcohol (PVA)/CNC cyrogel in Na₂SO₄, CaCl₂ solution and DI water in sequence. The crystalline domains, water content and transmittance of the hydrogel are regulated via the Hofmeister effect and hydrophobic interactions, which makes the birefringence of the hydrogel observable by interference colors. The aligned CNC not only enhance the mechanical strength of hydrogel, but also endow it with stimuli-responsive ability to the varying environment such as ion strength (0 to 5 M NaCl solution) and external forces (pressure of 0 to 248 kPa). Moreover, the patterned hydrogels are successfully fabricated by the method of local solvent displacement. Due to the low light transmission, the pattern can be encrypted and only be observed between crossed polarizers. These optical properties made the hydrogel a promising candidate for environmental monitoring and anti-counterfeit material.