Magnetic Self-Assembled Fe₃O₄ Colloidal Nanocrystals in Structural Color Hydrogels for Environmental Humidity Monitoring

The structural color hydrogel has attracted considerable attention due to its unique color-changing ability in response to external stimuli. It is committed to exploring possibilities in the fields of sensing, display, and anticounterfeiting and enhancing its functionality to achieve practical applications. In this study, structural color hydrogels are synthesized through the concurrent polymerization of Fe3O4 colloidal nanocrystals and gel monomers under an external magnetic field. In the presence of an external magnetic field, Fe3O4 colloidal nanocrystals rapidly assemble into 1D chain structures, creating structural colors. The hydrogel matrix stabilizes the one-dimensional chain structures, preserving the structural colors even after the external magnetic field is removed. As environmental humidity changes, the hydrogel absorbs water, expanding in volume and increasing the spacing between Fe3O4 colloidal nanocrystals, which results in a red shift. By adjusting the relative humidity (from 11% to 97%), the structural color hydrogel undergoes expansion, altering the distance between Fe3O4 colloidal nanocrystals, resulting in a color change from green to red, and the diffraction peak shifts from 494 to 720 nm, spanning a significant variation range of 226 nm, achieving humidity responsiveness. These findings will pave the way for the designing of desirable environmental humidity monitoring applicable in optical equipment, sensors, displays, etc.