Response of Photonic Hydrogels of Homogeneous Particles to Uranyl Ions in Aqueous Solutions

Abstract

Abstract We study here the response of photonic hydrogels (PHs), made of photonic crystals of homogeneous silica particles in polyacrylamide hydrogels (SPHs), to the uranyl ions UO 2 2+ in aqueous solutions. It is found that the reflection spectra of the SPH show a peak due to the Bragg diffraction, which exhibits a blue shift in the presence of UO 2 2+ . Upon exposure to the SPH, UO 2 2+ gets adsorbed on the SPH and forms complex coordinate bonds with multiple ligands on the SPH, which causes shrinking of hydrogel and leads to the blue shift in the diffraction peak. The amount of the blue shift in the diffraction peak increases monotonically up to UO 2 2+ concentrations as high as 2300µM. The equilibration time for the shift in the Bragg peak upon exposure to UO 2 2+ is found to be ~30 min. These results are in contrast to the earlier reports on photonic hydrogels of inhomogeneous microgel particles hydrogel (MPH), which shows the threshold UO 2 2+ concentration of ~600 µM, below which the diffraction peak exhibits a blue shift and a change to a red shift above it. The equilibration time for MPH is ~300min. The observed monotonic blue shift and the faster time response of the SPH to UO 2 2+ as compared to the MPH are explained in terms of homogeneous nature of silica particles in the SPH, against the porous and polymeric nature of microgels in the MPH. We also study the extraction of UO 2 2+ from aqueous solutions using the SPH. The extraction capacity estimated by the arsenazo-III analysis is found to be 112 mM/kg.