An Eu(III)-functionalized Sr-based metal-organic framework for fluorometric determination of Cr(III) and Cr(VI) ions

An Eu(III)-functionalized strontium-organic framework (Eu³⁺@Sr-MOF) was synthesized by encapsulating Eu³⁺ ions into the pores of Sr-MOF via postsynthetic modification (PSM) and fully characterized by XRD, SEM, EDS, FTIR, and TGA. The luminescent properties of Eu³⁺@Sr-MOF were measured and investigated in detail, which demonstrated that Eu³⁺@Sr-MOF not only possessed excellent water and chemical stabilities in a wide pH range (5-10) but also displayed red luminescence with excitation/emission maxima at 332/617 nm. The Eu³⁺@Sr-MOF can be considered as a highly selective and sensitive recyclable luminescence probe for sensing Cr(III) and Cr(VI) based on strong luminescence quenching effects; it can be recycled at least five times. The luminescence probe for Cr³⁺, CrO₄^2-, and Cr₂O₇^2- shows low detection limits (0.15 μM, 0.17 μM, and 0.13 μM, respectively), relatively broad linear ranges (0.6-150 μM, 0.1-200 μM and 0.4-180 μM, respectively), and high quenching constants K_SV. The signals can be observed by the naked eye under UV light of 365 nm. The method was applied to the determination of total Cr in environmental water samples. Graphical abstract.