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

Abstract

A novel metal–organic framework based on octa-nuclear zinc clusters, namely [Zn₄(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 μ₃-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 × 10³ M⁻¹ and 1.49 × 10⁴ 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.