Magnetic covalent organic frameworks with core-shell structure as sorbents for solid phase extraction of fluoroquinolones, and their quantitation by HPLC

A core-shell structured magnetic covalent organic frameworks of the type Fe3O4@COFs was prepared by using the Fe3O4 nanoparticles as magnetic core, and 4,4”-diamino-p-terphenyl and 1,3,5-tris(p-formylphenyl)benzene as two building blocks. The Fe3O4@COFs were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive spectrum, Fourier transform infrared spectroscopy, zeta potentiometric analysis, X-ray diffraction, vibrating sample magnetometry, thermogravimetric analysis and the nitrogen adsorption-desorption isotherms. The Fe3O4@COFs have core-shell structure with average diameter of 200 ± 2.4 nm, a high specific surface area (124 m2·g-1), uniform pore size distribution (3.1 nm), good magnetic responsivity (36.8 emu·g-1), good thermal and chemical stability. They were applied as the sorbents for magnetic solid phase extraction (MSPE) for fluoroquinolones (FQs) ciprofloxacin, enrofloxacin, lomefloxacin, gatifloxacin, levofloxacin and pefloxacin. The effects of sorbent dosage, extraction time, p H value, ionic strength, desorption solvent and desorption time were investigated. By combining MSPE with HPLC-DAD analysis, a rapid and sensitive method was developed for the enrichment and determination of these FQs. The method had good linearity in the range of 2.5-1500 ng·g-1 FQ concentration range and low limits of detection (0.25-0.5 ng·g-1). The method was successfully applied to the extraction and determination of FQs in (spiked) pork, milk and human plasma samples. Recoveries ranged from 78.7-103.5% (with RSD<6.2%).