Low-angle-dependent CdS@SiO₂ photonic crystal hydrogel material for visual detection and removal of uranyl ions

A low-angle-dependent photonic crystal hydrogel (LAD-PCH) material was developed to simultaneously detect and remove uranyl ions (UO₂²⁺). Different from traditional SiO₂ photonic crystal hydrogel with the problem of angle dependency, the LAD-PCH material overcomes the restriction of observation direction. The LAD-PCH is a composite material with the photonic crystal array of 180-nm monodisperse CdS@SiO₂ particles embedded into the functional hydrogel. As one UO₂²⁺ can bind to multiple carboxyl groups and amide groups, the functional hydrogel fabricated by acrylic acid and acrylamide will shrink after chelating. These changes in the hydrogel volume alter the array spacing and trigger a blue shift of diffraction wavelength and naked-eye visual color changes of LAD-PCH. The color can vary from orange-red to orange, yellow, green, and cyan, corresponding to the determination range of 100 pM-100 μM. The LAD-PCH material detects UO₂²⁺ sensitively as the lowest detectable concentration is about 100 pM, and removes UO₂²⁺ high-efficiently as the maximum adsorption capacity of U(VI) is about 1010 mg g^-1 at 298 K. This LAD-PCH material is convenient and has potential to simultaneously monitor and remove UO₂²⁺ from uranium-polluted water. Schematic representation of the low-angle-dependent photonic crystal hydrogel (LAD-PCH) material for UO₂²⁺ detection and removal: The structural colors of LAD-PCH material overcome the restriction of observation angles. After the ligands complex with UO₂²⁺, the networks of LAD-PCH show different degrees of shrinkage; these volume changes of hydrogel trigger obvious naked-eye visual color changes of LAD-PCH.