The construction of a multifunctional luminescent Eu-MOF for the sensing of Fe3+, Cr2O72− and amines in aqueous solution

Abstract

A water stable Eu³⁺-organic framework (1-Eu), [Eu(HL)(H₂L)]·7.5H₂O (H₃L = 3-(3,5-dicarboxylphenyl)-4-carboxylpyridine) was constructed through solvothermal synthesis and studied by structural analyses and luminescence sensing. Crystallographic studies revealed that 1-Eu was a 3D framework in which two adjacent Eu³⁺ ions with a single isophthalate bridge were interlinked by organic ligands HL or H₂L and contained uncoordinated carboxylate groups. 1-Eu represents the (3,3,6,6)-connected (4²·10)(4²·10)(4³·8⁵·10⁵·12²)(4⁴·8²·10⁴·12⁵) topology. As expected, 1-Eu exhibits a strong red-light emission at room temperature. And 1-Eu also shows good thermal stability and high chemical stability in common solvents and even in acidic or basic solutions (pH = 3 to 12). Therefore, the strong luminescence, good stability and presence of free carboxylate oxygen atoms in the framework allow 1-Eu to serve as an excellent luminescence sensor for probing Fe³⁺ and Cr₂O₇²⁻ ions in water with quenching constant (K_SV) values of 3.38 × 10⁴ M⁻¹ and 2.23 × 10⁴ M⁻¹, respectively. In addition, 1-Eu can be further used as a sensor for amines with high (K_SV) values and relatively low limit of detection (LOD), e.g. the K_SV and LOD values of ethylenediamine (EN) are 6.25 × 10⁶ M⁻¹ and 0.23 μM, respectively. Meanwhile, the luminescence intensity of the quenched 1-Eu samples will be recovered after washing with water, indicating that 1-Eu can act as a multiresponsive luminescence sensor for Fe³⁺, Cr₂O₇²⁻ and amines with high selectivity, sensitivity and recyclability. Moreover, the luminescence sensing mechanisms for the above different analytes were proposed.