A Water‐Driven Switchable Material for Optical and Electronic Information Security: Electromagnetic Shielding and Optical Encryption

A multifunctional hydrogel capable of both wet-state microwave absorption and dry-state phosphorescence is designed. Incorporating Cu-doped carbon dots (Cu_x-CDs) into a polyacrylamide/carboxymethyl cellulose (PAM/CMC) network enables synergistic modulation of electromagnetic and optical behaviors. Cu doping effectively regulates the hydrogen-bond network and water state within the hydrogel, promoting charge migration and dipolar polarization, thereby significantly enhancing dielectric loss and absorption efficiency. The optimized hydrogel exhibits a minimum reflection loss of -62.67 dB and an effective absorption bandwidth of 5.94 GHz within the 2-18 GHz range. Density functional theory (DFT) calculations reveal that Cu-N coordination reduces the energy gap and enhances electronic delocalization, facilitating charge transfer. Upon dehydration, the polymer network reconstructs into a dense and rigid framework that suppresses nonradiative transitions, yielding stable blue and green room-temperature phosphorescence. Such a transition from wet-state absorption to dry-state emission, driven by Cu doping and hydrogen-bond engineering, provides a new paradigm for constructing stimuli-responsive hydrogels. This work offers a versatile strategy for designing multifunctional materials with potential applications in electromagnetic protection, information encryption, and optical anti-counterfeiting.