Facile Synthesis of Soft Core–Shell Nanospheres for Constructing Multiresponsive Photonic Crystal Security Patterns

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