A dual ligand Eu-MOF with high stability and quantum yield for the detection of trace Fe³⁺, CrO₄²⁻ and Cr₂O₇²⁻ in aqueous environments

The design and preparation of fluorescent sensors for detecting toxic ions in aqueous environments are of paramount importance. In this study, a europium metal-organic framework (Eu-MOF), constructed from Eu³⁺, the rigid 2,2'-biphenyldicarboxylic acid (2,2'-H₂bpdc) ligand, and the auxiliary 1,10-phenanthroline (Phen) ligand, was synthesized through hydrothermal synthesis. The synthesized Eu-MOF demonstrated remarkable water stability, pH stability, thermal stability, and photostability. It showed a high quantum yield of 42.6%, along with an average fluorescence lifetime of 0.9002 ms, which could enhance its sensitivity when employed as a fluorescent probe. The Eu-MOF could function as a fluorescent probe to quantitatively detect trace amounts of Fe³⁺, CrO₄^2- and Cr₂O₇^2-<i>via</i> fluorescence quenching. Their detection sensitivities are 1.527 × 10⁴ M^-1, 1.378 × 10⁴ M^-1 and 1.463 × 10⁴ M^-1 respectively, with detection limits as low as 0.28 μM, 0.19 μM and 0.09 μM. Additionally, the Eu-MOF sensor exhibited excellent selectivity, anti-interference ability, repeatability (after undergoing five cycles) and practical feasibility. Furthermore, a detailed investigation of its fluorescence properties and sensing mechanisms was carried out. Based on density functional theory calculations, two energy transfer pathways in the process of Eu³⁺ ion luminescence sensitized by the double ligands 2,2'-H₂bpdc and Phen were identified. The fluorescence sensing mechanism of the Eu-MOF for detecting Fe³⁺, CrO₄^2- and Cr₂O₇^2- involved photoinduced electron transfer from the antenna ligand to the organic molecules and competitive absorption of excitation energy. Owing to its high stability and sensitivity, the Eu-MOF holds great potential for practical applications as a sensor for Fe³⁺, CrO₄^2- and Cr₂O₇^2- in aqueous environments.