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

ABSTRACT 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.