Magnetic Solid-Phase Extraction of Benzoic Acids in Fruit Juices Using Hydrophilic Metal-Organic Framework MIL-101(Cr) Coated with Magnetic Nanoparticles

In this study, magnetic Fe₃O₄-coated metal-organic framework composites, CONH₂-MIL-101(Cr)/Fe₃O₄, were synthesized by depositing Fe₃O₄ particles on the surface of amidated MIL-101(Cr). The scanning electron microscope images show that the MIL-101(Cr) crystals were uniformly coated by Fe₃O₄ particles. The magnetic hysteresis curve of CONH₂-MIL-101(Cr)/Fe₃O₄ reveals a saturated magnetization value of 23.47 emu/g. The water contact angle decreases from 61.00° to 12.88° after amidation and Fe₃O₄ coating. The magnetic solid-phase extraction (MSPE) efficiency of CONH₂-MIL-101(Cr)/Fe₃O₄ for benzoic acid derivatives was studied. Under optimal conditions, the adsorption and desorption processes can be completed within 5 min, and the maximum adsorption capacity to protocatechuic acid (PCA) and vanillic acid (VA) is 10.31 and 17.30 mg/g. Coupling the MSPE with high-performance liquid chromatography-ultraviolet detection, the detection limit for PCA and VA was 0.29 and 0.58 ng/mL, respectively. The mean recoveries of PCA and VA in spiked samples ranged from 82.85%–91.27% with relative standard deviations lower than 0.33%–5.33%. This study provides inspiration for the fabrication of novel magnetic sorbents, and a reliable method for the adsorption and analysis of benzoic acid derivatives.