Synthesis of Novel Cd-MOFs from Carboxyl Isomeric Ligands with Tuned Morphology and Sensing Performance for Fluorescent Imidacloprid Detection

This study synthesized three novel cadmium-based metal–organic framework materials (Cd-MOF-1, Cd-MOF-2, Cd-MOF-3) via a one-step hydrothermal method and obtained their structure through single-crystal X-ray diffraction analysis. These materials differ based on the use of three isomeric ligands with distinct carboxyl group positions: 3-(2,4-dicarboxyphenyl)-4-carboxylpyridine (L1), 3-(3,5-dicarboxyphenyl)-4-carboxylpyridine (L2), and 3-(2,5-dicarboxyphenyl)-4-carboxylpyridine (L3). Systematic characterization revealed that subtle variations in the carboxyl position of the ligands significantly modulated the crystal structure, morphology, thermal stability, and fluorescence properties of the Cd-MOFs. Among them, Cd-MOF-2 exhibited outstanding fluorescence intensity, excellent chemical stability, and thermal stability and was successfully applied for highly sensitive fluorescence detection of the neonicotinoid pesticide imidacloprid. Results demonstrate a good linear relationship between fluorescence intensity and imidacloprid concentration within the range of 5–50 μM, with a detection limit as low as 24.73 nM. The sensor exhibits high selectivity and strong resistance to interference for imidacloprid, and even more complex real-world sample matrices do not affect detection performance. Mechanistic studies indicate that the fluorescence quenching process is attributed to the internal filtering effect (IFE). This study not only reveals the structure–property relationship between the position of the ligand carboxyl group and the structural properties of MOFs but also provides new design insights for developing high-performance fluorescent sensing materials. It holds significant scientific and practical value for environmental pollution monitoring, particularly in pesticide residue detection.