Sensitive luminescent chemosensing of fluoride based on Eu-doped Zn-LMOF in aqueous media: structural and spectroscopic studies

Abstract

Selective anion sensing/recognition by luminescent compounds able to operate in aqueous media is a central topic of supramolecular analytical chemistry that impacts biological and environmental chemistry. In this work, a novel blue-emitting (λ_em = 370 nm) 3D metal–organic framework {[Zn₃(BDC)₃(EtOH)₂](EtOH)_0.6}_∞, Zn-LMOF (BDC = 1,4-benzenedicarboxylate), was synthesized and structurally analysed by single-crystal X-ray diffraction. Subsequently, an Eu(III)-doped Zn-MOF was obtained using Zn-LMOF by a post-synthetic ball milling reaction, and this compound was studied in detail as a luminescent chemosensor for anions (e.g., halides, pseudohalides, oxyanions, and carboxylates) in 20% aqueous ethanol. The new Eu@Zn-LMOF is a hydrostable material with a long-lived pink emission originated by an efficient energy transfer from the excited energy levels of Zn-LMOF toward the ⁵D state of Eu(III) centers, as evidenced by its strong emission signals at 591, 616, 650, and 698 nm, lifetime (τ = 0.68 ms), and quantum yield (Φ_PL = 0.32). The addition of anions to aqueous ethanolic dispersions of Eu@Zn-LMOF modified their emission intensities corresponding to Eu(III) ions, with a pronounced selectivity and quenching response (K_SV = 7.27 × 10³ M⁻¹) toward F⁻ over common interfering anions such as acetate, phosphate, and heavy halides. The detection limit in the presence of potentially interfering anions is 13.70 μmol L⁻¹. On the basis of multiple spectroscopic tools, such as SEM-EDS analysis, the optical change is attributed to the efficient release of Eu(III) ions from the Zn-LMOF matrix with the simultaneous formation of EuF₃. These results demonstrate the usefulness of lanthanide-doped Zn-LMOFs as analytical tools for the selective quantification of a neurotoxic and environmental anion in aqueous media.