Photoresponsive Nanometer-Sized CdS Colloids Assembled into Photonic Crystals within Polydimethylsiloxane as Multicolored Anticounterfeiting Labels

In this study, we combined CdS photonic crystals with polydimethylsiloxane (PDMS) to develop a photoresponsive CdS/PDMS photonic crystal (CdS/PDMS PC) composite, which formed the photonic crystals of two different structural colors by controlling the size of the nanometer-sized CdS colloidal microsphere and realized the photoresponsive characteristics of the photonic crystals by the addition of photochromic substances. In addition, we studied the effects of the photosensitive material in the photonic crystals on their optical properties and surface features. We used a self-assembly method to periodically arrange CdS nanospheres into photonic crystal templates. The materials with high refractive index such as CdS made photonic crystals less angle-dependent and brightly colored. PDMS combined with spirooxazine is then filled in the photonic crystal matrix, and CdS/PDMS photonic crystal composites of structural red and green colors were obtained by thermal curing. Because of the photochromic effect of spirooxazine, under ultraviolet light irradiation, the photonic crystals had the original structural color and the reversible photochromic chemical color overlaid, exhibiting a mixed color. According to this principle, when irradiated with ultraviolet light, the PDMS substrate changed from colorless to blue and the photonic crystals changed from structural color mode to photochromism mode, and this change was reversible and can be repeated. The structural color of the red photonic crystals fused with the blue PDMS to become purple, and the structural color of the green photonic crystals fused with the blue PDMS to become cyan. When irradiated with visible light, photonic crystals can return to their original structure color. In this approach, we have designed a series of ultraviolet anticounterfeiting labels to expand the application of information anticounterfeiting.