Facile Synthesis\nof Soft Core–Shell Nanospheres\nfor Constructing Multiresponsive Photonic Crystal Security Patterns

Responsive photonic crystals (RPCs) presenting tunable\nstructural\ncolors under external stimuli are widely applied in the fields of\ndynamic displays, sensors, and anticounterfeiting. However, the development\nof multiresponsive photonic crystal (MRPC) films possessing versatile\nvariable optical properties remains a significant challenge due to\nthe tedious synthetic procedure of multifunctional building blocks\nand complex assembly processes, thereby constraining their extensive\napplications. In the present work, a series of soft nanospheres with\na hydrophobic cores and responsive hydrophilic shells have been synthesized\nby a facile one-step surfactant-free emulsion polymerization method.\nThe MRPC patterns were then prepared by depositing soft nanosphere\nemulsions onto the 3D-printed substrates with a topological structure\nfollowed by drying at room temperature. The shells of soft nanospheres\ndeformed and fused with each other, resulting in the formation of\ntransparent MRPC film patterns. The MRPC patterns exhibited brilliant\nstructural color in a wet state but lost the color again after complete\ndrying. Such a reversible structural color was ascribed to the change\nof the refractive index (RI) of the hydrophilic shell layers of nanospheres\naccording to wetting/drying state shifting. Moreover, the on-demand\ndesigned MRPC patterns could rapidly respond to external stimuli of\ntemperature, pH, and organic solvents in a reversible manner, and\nmultichannel encrypted security labels were also demonstrated. We\npostulate that our facile and feasible approach can be applied to\nthe systematic design and large-scale production of MRPC patterns\nfor a variety of applications.