Solvent-Programmable Photonic Hydrogels with Tunable Transmittance, Fluorescence, and Diffraction

Photonic hydrogels capable of stimulus-responsive optical modulation are of growing interest for dynamic encryption, anticounterfeiting, and intelligent display technologies. However, achieving multimodal and reversible optical switching in a single hydrogel system remains challenging. Herein, we report a solvent-programmable poly(vinyl alcohol)/polyacrylamide (PVA/PAAm) hydrogel doped with red-emissive carbon dots (CDs), which integrates soft lithographic micropatterning with nonsolvent-induced phase separation (NIPS) to enable dynamic control over transmittance, fluorescence, and diffraction behavior. Upon exposure to ethanol/water mixtures, the hydrogel exhibits reversible microphase separation. This phenomenon triggers three concurrent effects: reduced optical transmittance due to enhanced light scattering, amplified fluorescence resulting from solvent polarity effects and scattering-assisted confinement of excitation light, and tunable diffraction shifts induced by microstructure contraction. These three distinct optical responses, governed by a unified phase separation mechanism, are fully reversible and repeatable across multiple solvent exchange cycles. Furthermore, a solvent-programmable smart window is constructed by integrating patterned hydrogels as encrypted motifs, exhibiting trimodal decryption under white light, UV excitation, and oblique illumination, enabling orthogonal, reversible, and stepwise optical readout. This work demonstrates a simple yet versatile strategy for constructing multimodal photonic hydrogels with reconfigurable surface architectures and tunable light–field interactions, offering opportunities for responsive optical materials in information encryption and programmable photonic devices.