Optically switchable chiral photonic crystal composite films for multimodal anti-counterfeiting labels and plant...

The combination of chiral photonic crystals and fluorescent nanomaterials has garnered significant attention due to their exceptional light modulation abilities and versatile stimulus-responsive characteristics. In this study, we present a flexible chiral photonic crystal fluorescent film composed of rod-shaped cellulose nanocrystals (CNCs) and orthogonal upconversion nanoparticles (OUCNPs). This composite film maintains the photonic order of the CNCs chiral matrix while preserving the orthogonal excitation-emission properties of OUCNPs, with fluorescence shifting from blue to red as the excitation wavelength changes from 980 nm to 808 nm. By adjusting ultrasonic energy, the photonic bandgap (PBG) of the composite film can be tuned, and the bandgap effect is found to reduce OUCNPs fluorescence emission. Furthermore, by altering factors such as the observation background, viewing angle, polarizer, humidity, and excitation wavelength, the composite film can transition between distinct optical states, effectively integrating stimulus-responsive chiral structural color with fluorescence and exhibiting a sophisticated quintuple stimulus-response functionality. Due to the film's remarkable flexibility, its practical applications in multimodal anti-counterfeiting and humidity sensing are successfully demonstrated using a plant model. This work shows great potential for the development of innovative sensors with advanced environmental response capabilities.