Lanthanoid hydrogen-bonded organic frameworks: Enhancement of luminescence by the coordination-promoted antenna effect and applications in heavy-metal ion sensing and sterilization

Abstract

Enhancing the antenna effect to increase energy transfer to lanthanoid ions for generating bright luminescence is an effective strategy to construct lanthanoid luminophores with excellent performance. However, the antenna effect often depends on the structure, connection, and configuration of immobilized ligands. We found for the first time that the introduction of the Zn(II) ion coordination can greatly enhance the solid-state fluorescence and phosphorescence emission of lanthanoid hydrogen-bonded organic frameworks (Ln-HOFs). Notably, relative to that of Tb-HOF, the solid-state fluorescence intensity, quantum yield, and lifetime of TbZn-HOFs had increased by 17.09, 38.81, and 12.96 times, respectively. The chelating coordination of the Zn(II) ions effectively hindered the vibration of organic ligands to weaken nonradiative transitions and stabilized and reduced the triplet excited state of the ligands through the spin–orbit coupling effect, which led to the increased efficiency of energy transfer with Tb(III) ions. In aqueous solution, LnZn-HOFs released Ln(III) ions effectively and captured low concentrations of the heavy metal ion Cu(II), and the efficient ligand-to-Cu(II) ion energy transfer quenched ligand luminescence. Notably, the limit of detection (LOD) was calculated to be 1.91 nM, which was considerably lower than the maximum level of Cu(II) ions in drinking water (20 μM) specified by the United States Environmental Protection Agency and was better than the LOD of other previously reported probes. Furthermore, LnZn-HOFs can effectively inhibit the proliferation of classical Gram-positive bacteria, such as Bacillus subtilis and Staphylococcus aureus, via ion release and thus showed a high sterilization effect. This work is the first to find that HOFs can exert efficient bacteriostatic and sterilization effects through ion release.