PSM-Assisted Luminescent Functionalized Eu-MOFs: Sorption Studies with U(VI), Sensing Behavior of Distinct Metal Ions, and Density Functional Theory Calculations

Four distinct functionalized, isostructural, and highly luminescent europium-based metal–organic frameworks (MOFs), namely, Eu-BDC-PC (BDC: benzene dicarboxylic acid; PC: 2-pyridine carboxaldehyde), Eu-BDC-MA (maleic anhydride), Eu-BDC-SA (sulfamic acid), and Eu-BDC-DPC (diphenyl phosphoryl chloride), were synthesized through postsynthetic modification methodology using Eu-BDC-NH2. The ability of these functionalized Eu-MOFs for the sorption of U(VI) from aqueous solutions was investigated in batch mode. U(VI) sorption studies confirmed that these Eu-MOFs are favorable adsorbents for the removal of U(VI), and interestingly, the functionalized Eu-BDC-SA exhibited very rapid (∼5 min) and high sorption efficiency, ca. ∼99% at pH 6. The utility of these functionalized Eu-MOFs was also investigated for the sensing of distinct fission product elements. Interestingly, fluorescence sensing studies of fission products along with U(VI) confirmed that these Eu-MOFs exhibited a high and selective sensing ability to detect Pd(II) from a pH 2 solution over other metal ions. However, Eu-BDC-NH2 showed excellent sensing ability toward Pd(II) with a detection limit of 0.4 ppm. Additionally, we have investigated the nature of Pd(II) interactions on MOFs by applying density functional theory (DFT). The high binding energies obtained for the complexes between the ligand motifs and Pd2+ may be correlated with the high fluorescence quenching of the MOFs by palladium. The charge transfer interactions occurring within the complexes were also probed by natural bond orbital analysis. The work established here will pave the way for the extraction of U(VI) in the nuclear industry across various fields and will also greatly enhance the utility of these MOFs for the highly selective sensing of precious metal ions, such as Pd(II).