Zn(ii)-based metal–organic frameworks with tailored architectures: optimizing fluorescence performance for cell imaging and cancer therapy

Abstract

Metal–organic frameworks (MOFs) show great potential in bioimaging and cancer therapy owing to their tunable structures and unique photophysical properties. Here, based on the modulation of reactant loads and autoclave loading level, two Zn(II)-containing MOFs with the formula C₂₂H₁₈N₄O₆Zn (1) and C₂₂H₁₈N₄O₆Zn (2) are synthesized with 2,5-dihydroxyterephthalic acid (DHTA) and 1,4-bis((1H-imidazol-1-yl)methyl)benzene (bix) ligands. Structurally, 1 forms a 3D supramolecular network constructed by π–π stacking between 2D layers, while 2 is indicative of an interpenetrated 3D framework, where the Zn(II) center exhibits a more ideal tetrahedral coordination geometry. In ultrapure water, both compounds exhibit intense green fluorescence, originating from ligand-centered charge-transfer transitions, with compound 2 demonstrating superior photochemical properties (E_m1 = 530 nm and E_m2 = 522 nm). Compound 2 possesses a markedly higher fluorescence quantum yield (Φ = 13.78%) compared to compound 1 (Φ = 6.02%), and the average lifetimes for 1 and 2 are 0.83 and 0.98 µs, respectively. Moreover, both MOFs maintain robust fluorescence stability against diverse biological cations and anions, with 2 showing relatively high emission intensity across all ionic conditions. Biologically, 1 exhibits potent cytotoxicity against HeLa cells via co-activating apoptosis, ferroptosis, endoplasmic reticulum stress, and autophagy, while 2 is highly biocompatible with HeLa cells and exhibits cell-type-adaptive subcellular localization, enabling the clear imaging of the HeLa, SH-SY5Y, and A549 cells. Notably, both compounds show concentration-dependent dual functionality in the SH-SY5Y cells: low concentrations enable fluorescence imaging, while high concentrations inhibit cell proliferation. The structure–function relationship is also explored.