A dual fluorescent chemosensor for Al ³⁺ and Zn ²⁺ ions based on inhibition of ESIPT can be applied in bioimaging.

A new Dy(III)–Zn(II) heterometallic-organic framework [H(H₂O)₈][DyZn₄(imdc)₄(im)₄] (Dy–Zn MOF) (H₃imdc = 4,5-imidazole dicarboxylic acid, im = imidazole) was synthesized under the hydrothermal conditions and characterized by single-crystal X-ray diffraction. The Dy–Zn MOF possesses the characteristic Dy(III) emission with a lifetime and a quantum yield of 54.65 μs and 5.5%, respectively, when excited at 290 nm. The luminescence of the Dy–Zn MOF can be tuned by the excitation wavelength to give white-light and yellow emissions based on emissions of Dy³⁺ and the ligands imdc³⁻ and im. The energy transfer pathways at different excitation wavelengths were discussed. Furthermore, the Dy–Zn MOF can act as a turn-off chemosensor for Fe³⁺ with high selectivity and sensitivity. The resultant species at different molar ratios of Dy–Zn MOF and Fe³⁺ in the response system were obtained and have the general formula [H(H₂O)₈][DyZn_4−xFe_x(imdc)₄(im)_4−x(OH)_x] with the same framework as the Dy–Zn MOF. The sensing mechanism could be described as the adsorption of Fe³⁺ and displacement of Zn²⁺ by paramagnetic Fe³⁺ in the Dy–Zn MOF to form the Dy–Zn–Fe MOF, resulting in fluorescence quenching. The quenching constant was determined to be 9.29 × 10⁵ M⁻¹.