Tuning the fluorescence response of Zn-MOFs through ligand replacement for ultrasensitive metronidazole detection

The presence of metronidazole residues in food and environmental samples necessitates the use of sensitive, selective, and rapid monitoring methods. In this study, two zinc-based metal-organic frameworks (MOFs), Zn-MOF-PSA and Zn-MOF-TPA, were synthesised using a solvothermal method. X-ray diffraction and Rietveld refinement confirmed the formation of orthorhombic and monoclinic single crystal phases. Of the synthesised materials, Zn-MOF-PSA exhibited intense red-orange fluorescence and was used as a fluorescent probe to detect MTZ. It showed a linear response over the range of 6 to 90 nM and a detection limit of 1.32 nM. This sensor demonstrated significantly higher sensitivity than Zn-MOF-TPA and other fluorescent probes. Tests on real samples showed satisfactory recoveries ranging from 91.3% to 105.0%. Mechanistic investigations combining UV-Vis spectroscopy, Stern-Volmer analysis, and TD-DFT calculations revealed that fluorescence quenching is primarily governed by a photoinduced electron transfer (PET) process. These results demonstrate that ligand engineering is an effective strategy for enhancing the detection performance of MOFs, establishing Zn-MOF-PSA as a promising platform for ultrasensitive MTZ detection.