Ligand-locked post-synthetic metalation of a Eu(III) metal–organic framework achieving the ratiometric fluorescence sensing for pH and neurotransmitter

Lanthanide metal–organic frameworks (Ln-MOFs) show promise for ratiometric biosensors owing to their tunable photoluminescence and rich energy transfer processes among Ln 3+ , organic ligand and photosensitizer, yet face challenges in environmental stability and susceptibility of energy transfer to external stimuli. Herein, a ligand-locked post-synthetic metalation (PSMet) strategy, namely grafting Cu(acac) − units to the free bipyridine sites of 2,2′-bipyridine-5,5′-dicarboxylate (bpydc 2− ) linkers in Eubpydc, was applied to develop a water-stable and dual-emissive Eu-MOF sensor, Cu@Eubpydc. Due to the conformation locking of bipyridine moiety via chelating to Cu(acac) − , the MOF network rigidity is reinforced and water stability is improved. Meanwhile, this modification impedes the energy transfer from the organic linkers to Eu 3+ , boosting ligand-based fluorescent emission, which is more sensitive to external factors than Eu 3+ emission. The post-synthetic Cu@Eubpydc as a ratiometric fluorescence (RF) sensor exhibited a sensitive response to a narrow pH change between 3.1 and 4.0, as well as low detection limits of 80 nM, 107 nM, and 64 nM for common neurotransmitters aspartic acid, glutamic acid, and glutathione, respectively. The pH response mechanism involves the binding of H + to the N-donor of the unmetallized bpydc 2− ligand through hydrogen bonding. In contrast, neurotransmitters sensing relies on hydrogen bonding interaction, absorbance-caused enhancement, as well as acidity enhancement aroused by redox reaction. Additionally, Cu@Eubpydc was finally applied to neurotransmitters determination in serum samples, and its satisfactory recovery rate confirm its practical utility. This work uncovered that the ligand-locked PSMet strategy opens a new avenue for constructing Ln-MOFs with excellent water stability and RF sensing capabilities.