A self-assembled metal-organic framework for enhanced UO22+ fluorescence sensing: integration of an octa-nuclear zinc cluster with hexakis(4-carboxyphenoxy)cyclotriphosphazene

A novel metal-organic framework based on octa-nuclear zinc clusters, namely [Zn-4(L)(OH)(2)(H2O)(4)] (Zn-MOF) (H6L = hexakis(4-carboxylatophenoxy)cyclotriphosphazene), has been synthesized and structurally characterized by a solvothermal method. Single-crystal X-ray diffraction shows that eight zinc ions form a linear octa-nuclear cluster via a mu(3)-OH group, and each hexakis(4-carboxylatophenoxy)-cyclotriphosphazene connects four octa-nuclear clusters to construct 3D frameworks with one-dimensional pore structures. Zn-MOF exhibits suitable fluorescence properties, water stability as well as thermal stability, and can be applied as a convenient fluorescent probe for the detection of radioactive uranyl cations (UO22+) and ferric ions (Fe3+) through fluorescence quenching behavior. The corresponding K-sv values can reach 7.44 x 10(3) M-1 and 1.49 x 10(4) M-1. Meanwhile, the theoretical calculations fully indicated that the introduction of UO22+ effectively inhibited the metal-to-ligand charge transfer (MLCT) of Zn-MOF, which in turn led to the fluorescence quenching phenomenon. Therefore, Zn-MOF prepared using crystal engineering can be used to construct an efficient and reliable fluorescent probe used in the fields of environmental monitoring and nuclear waste treatment.