New Applications Of Uranyl-Organic Frameworks As Materials For Radiation Detection And Metal Ion Probes

Depleted uranium is an important byproduct of the process of nuclear fuel production,whose radioactivity is lower than that of natural uranium.The safe disposal and recycling of depleted uranium is an important factor to guarantee the development of nuclear energy and environmental safety.Therefore,scientists have been trying to explore new applications of depleted uranium,and found that uranium has excellent properties in many fields,such as being used as fluorescent material,single molecular magnets,superconductor,photocatalytic,electrocatalysis and environmental decontamination materials.Our group has been engaged in the design and synthesis of uranium materials and taking advantages of the special fluorescence properties of uranyl to explore its application in related fields,including radiation detection,metal ion detection,and decontamination.In this study,a series of fluorescent uranyl-organic compounds have been synthesized by utilizing different carboxylate ligands and adjusting the reaction conditions via hydrothermal and solvothermal method.X-ray single crystal diffraction technique was used to obtain precise crystal structure of those compounds.In addition,X-ray powder diffraction data,solid state UV-vis absorption spectra,fluorescence spectra,Raman spectra,and thermogravimetric data were collected.Based on the structural elucidation and spectral characterization,potential application of these compounds was explored.Concrete research content is as follows:1)Using oxalic acid and succinic acid as ligands,tetrabutylammonium bromide as template,a new uranyl-organic framework?denoted as compound 1?has been synthesized via hydrothermal method.Single-crystal-X-ray-diffraction studies revealed that 1 crystallizes in the monoclinic and centrosymmetric space group P2₁/n and exhibits a 3D network structure.The color of the compound is light green,indicating that it has a strong fluorescence emission?the quantum yield is about 0.49?.Interestingly,the fluorescence of the compound was gradually quenched under continuous UV radiation?365 nm?.Inspired by this phenomenon,it is proposed that this uranyl-organic framework material can be used to detect the intensity of ionizing radiation.Based on the above conjecture,compound 1 was used in X-ray andg-ray detection experiments.The results show that the material is highly sensitive to low dose X-ray andg-ray,and the detection limit was as low as 10^-4Gy,representing an improvement of approximately two orders of magnitude,compared to the reported solvent-assisted photoluminescence quenching dosimeter.The mechanism of uranyl fluorescence quenching was further studied by collecting EPR data and single crystal structure data of the compound before and after irradiation.In the EPR spectra,the g value is about 2,indicating that free radicals were generated during the radiation exposure.By comparing the bonds length before and after irradiation,significant shrinks were found in the U-O and U-U distance.The above results stongly evidenced that it was the radiation-induced free radicals which affected the coordination environment of uranyl and consequently quenched the fluorescence of it.In addition,after 200?heating for 12 h,the fluorescence of irradiated sample is recovery,indicating that the radiation detection materials can be recycled.2)A new 1D compound?denoted as compound 2?was constructed by uranyl and5-Nitroisophthalic acid via solvothermal method.Fluorescence characterization showed that compound 2 also show strong fluorescent emission.Compound 2 can maintain its crystallinity in aqueous media within a wide pH range from 2 to 12.In addition,the stability of 2 was also checked in various solutions containing 200 ppm of cations such as M=Na⁺,Mg²⁺,Co²⁺,Cu²⁺,Cr³⁺,Mn²⁺,Ni²⁺,Sr²⁺,Fe²⁺,and Fe³⁺.Based on the aqueous stability tests,compound 2 was expected to be a fluorescent probe to detect metal ions in aqueous solution.Bulk crystalline samples of 2 were dispersed in various solutions containing 200 ppm cations including Na⁺,Mg²⁺,Co²⁺,Cu²⁺,Cr³⁺,Mn²⁺,Ni²⁺,Sr²⁺,and Fe³⁺,and the fluorescence intensities of each sample were recorded.It was found that only Fe³⁺could quench the fluorescence of uranyl.Subsequently,we conducted a detection experiment on Fe³⁺using compound 2 and the detection limit was calculated to be 6.3×10^-3ppm.The UV-vis absorption spectra show that the absorption peaks of uranyl and Fe³⁺overlap between 225 and 450 nm,indicating that it is the competeing light absorption between Fe³⁺on the surface of the crystal and the uranyl ions in the structure that quenches the fluorescence of the uranyl unit.