Bioinspired Cholesteric Photonic Crystals with 3D Cross-Linked Network Structure for Humidity Sensors

Inspired by photonic crystals found in nature, smart humidity sensors with structural colors were designed. These sensors differed from conventional ones by excluding memory metals in the construction process and eliminating the need for energy input during the response phase. They were notable for low cost and ease of fabrication into label-like layers, offering strong competition to advanced electronic sensors and carrying significant research implications. In this study, with hydrophilic monomer 1-vinyl-2-pyrrolidone (NVP) as a precursor of stimulus-response component, cellulose nanocrystals ((CNC)-NVP) cholesteric photonic crystal materials were formed through coevaporation-introduced self-assembly of cellulose nanocrystals (CNCs) and NVP, followed by ultraviolet (UV) curing. The effect of varying poly-NVP contents on the optical properties, chemical properties, microstructure, and humidity response behavior of photonic crystal materials was systematically investigated, and a humidity response mechanism was proposed. Compared to composite materials using small molecular substances as stimulus-response components, the materials developed in this study formed a three-dimensional (3D) cross-linked network, which mitigated the risks of component migration, environmental pollution, and performance deterioration. This research contributed to the development of environmentally friendly, low-cost, and sustainable intelligent sensors.