Water-stable mixed-ligand Cd(II) metal-organic frameworks as bis-color excited fluorescent sensors for the detection of vitamins and pesticides in aqueous solutions

The synthesis and characterization of three 2D to 3D metal-organic frameworks (MOFs) with mixed ligands, namely {[Cd(DINT)(TPA)(H2O)]ACN}n (BAO24) (H2TPA = terephthalic acid, DINT = 1,4-di(imidazol-1-yl)naphthalene, and ACN = acetonitrile), {[Cd(DINT)(IPA)]DMF}n (BAO25) (H2IPA = isophthalic acid and DMF = N,N-dimethylformamide), and [Cd(DINT)(PA)]n (BAO26) (H2PA = phthalic acid), were reported herein. The structures and purities of the MOFs were revealed and confirmed by single-crystal X-Ray diffraction, Fourier transform infrared spectroscopy, powder X-Ray diffraction, thermogravimetric analysis, and elemental analysis. The MOFs exhibited a fluorescence emission peak around λem = 340 nm with relative excitation wavelengths at both 230 and 290 nm. Since the MOFs showed similar fluorescence properties, BAO24 was selected as a representative MOF in the fluorescent sensing processes. The result showed that BAO24 exhibited a high sensitivity for vitamin B2. However, when sensing pesticides, the sensor performed differently at each excitation wavelength. At λex = 230 nm, the sensor was quenched the most significantly with the nitenpyram analyte, while at λex = 290 nm, the highest fluorescence quenching was achieved by the imidacloprid analyte.