Magnetic Self-Assembled\nFe<sub>3</sub>O<sub>4</sub> Colloidal Nanocrystals in Structural Color\nHydrogels for Environmental\nHumidity Monitoring

The structural color hydrogel has attracted considerable\nattention\ndue to its unique color-changing ability in response to external stimuli.\nIt is committed to exploring possibilities in the fields of sensing,\ndisplay, and anticounterfeiting and enhancing its functionality to\nachieve practical applications. In this study, structural color hydrogels\nare synthesized through the concurrent polymerization of Fe<sub>3</sub>O<sub>4</sub> colloidal nanocrystals and gel monomers under an external\nmagnetic field. In the presence of an external magnetic field, Fe<sub>3</sub>O<sub>4</sub> colloidal nanocrystals rapidly assemble into\n1D chain structures, creating structural colors. The hydrogel matrix\nstabilizes the one-dimensional chain structures, preserving the structural\ncolors even after the external magnetic field is removed. As environmental\nhumidity changes, the hydrogel absorbs water, expanding in volume\nand increasing the spacing between Fe<sub>3</sub>O<sub>4</sub> colloidal\nnanocrystals, which results in a red shift. By adjusting the relative\nhumidity (from 11% to 97%), the structural color hydrogel undergoes\nexpansion, altering the distance between Fe<sub>3</sub>O<sub>4</sub> colloidal nanocrystals, resulting in a color change from green to\nred, and the diffraction peak shifts from 494 to 720 nm, spanning\na significant variation range of 226 nm, achieving humidity responsiveness.\nThese findings will pave the way for the designing of desirable environmental\nhumidity monitoring applicable in optical equipment, sensors, displays,\netc.