Microgel-hydrogel composite photonic crystals to monitor and extract uranyl ions in aqueous solutions

We fabricate microgel-hydrogel composite photonic crystals and investigate their spectroscopic response to uranyl ion (UO₂²⁺) in aqueous solution. Upon adsorption of UO₂²⁺, the UV–visible reflection spectra of the composite photonic crystal (CPC) exhibit a peak due to Bragg diffraction, which is intense and blue shifted as compared to that of unadsorbed CPC. At low concentrations, the blue shift and intensity of the diffraction peak increases with UO₂²⁺. However, at higher concentrations of UO₂²⁺ (>600 μM), the peak shows a red-shift. Reasons for this altered behaviour of the Bragg diffraction at different UO₂²⁺ concentrations are discussed in terms of chelation of UO₂²⁺ with ligand groups in the CPC. The uranium extraction capacity of the CPC (at 600 μM, pH = 5.5) estimated using Arsenazo-III method is found to be 487 mM/kg of composite photonic crystals. Our observations indicate that microgel-hydrogel composite photonic crystals can be good candidates for extraction as well as detection of uranium, along with in situ monitoring of adsorption of uranium during extraction by simply monitoring their Bragg diffraction.