An Environmental Perception Self‐Adaptive Discolorable Hydrogel Film toward Sensing and Display

Abstract Vision perception, especially color change, is the most direct and convenient way of feedback among numerous information recognition modes. Because of unique discoloration property upon stimulation, photonic crystals have attracted wide attention. Herein, with a facile UV polymerization process under the magnetic field, a sensitive photonic hydrogel film with vivid structural color is made. The structural color comes from the 1D chain‐like structures assembled by the carbon‐encapsulated Fe 3 O 4 colloidal nanocrystal clusters (Fe 3 O 4 @CNCs), which are embedded in the flexible hydrogel matrix. The hydrogel can adapt its size to the environment, resulting in the regulation of intergranular distance and apparent color variation. Thus, a hydrogel film with humidity, ion concentration, and stress responsivity can be obtained, showing high sensitivity, stability, and full reversibility in a wide range of visible spectrum. It can be designed as a reusable display board or applied to colorimetric measurements similar to pH test paper. Innovatively, the hydrogel film is integrated as back reflectors to improve the efficiency of semitransparent polymer solar cells while protecting devices. This structural color hydrogel film makes it easier to recognize environmental dynamic changes and provide a wider application prospect in optics devices, sensing, display, and diagnostics.