Photonic Crystal Hydrogels Fabricated from Nanoparticles of Fe₃O₄/SiO₂ with Programmable Colors through Photopolymerization for Applications as Anticounterfeiting Applications

Photonic crystal (PC) hydrogels with unique micro/nanostructures have excellent prospects in many fields, such as sensing, displays, anticounterfeiting, information encryption, and adaptive camouflage. Herein, magnetic core–shell Fe 3 O 4 /SiO 2 nanoparticles with a controlled size were synthesized, followed by PC hydrogels of arbitrary structures that were fabricated via the magnetically inducing self-assembly of magnetic nanoparticles in the poly(ethylene glycol) diacrylate (PEGDA) pregel solution and a subsequent digital micromirror device (DMD)-based UV polymerization process. By controlling the size of nanoparticles and magnetic field intensity, various PC hydrogel patterns with tunable structural colors were fabricated. Combining magnetic field control with DMD-based light modulation, high-throughput preparation of structural color PC hydrogels was achieved. Furthermore, the Fe 3 O 4 /SiO 2 nanoparticles were assembled into poly(2-hydroxyethyl methacrylate) (PHEMA) with solvent responsiveness. Interestingly, the obtained PC hydrogels could swell in ethanol, resulting in a change in the structural color. Meaningfully, a dual-component PC hydrogel consisting of a PEGDA pattern and a PHEMA background was fabricated, and the invisible pattern in the initial state could be displayed and hidden through cyclic exposure to ethanol and ethylene glycol. The presented strategy for constructing the PC hydrogels from Fe 3 O 4 /SiO 2 nanoparticles could provide valuable insights for the development of intelligent structural color materials, which exhibit significant potential in applications such as information storage, visual sensing, and anticounterfeiting.