COF-SiO2@Fe3O4 Composite for Magnetic Solid-Phase Extraction of Pyrethroid Pesticides in Vegetables

Abstract

Pyrethroid pesticides (PYRs) have found widespread application in agriculture for the protection of fruit and vegetable crops. Nonetheless, excessive usage or improper application may allow the residues to exceed the safe limits and pose a threat to consumer safety. Thus, there is an urgent need to develop efficient technologies for the elimination or trace detection of PYRs from vegetables. Here, a simple and efficient magnetic solid-phase extraction (MSPE) strategy was developed for the simultaneous purification and enrichment of five PYRs in vegetables, employing the magnetic covalent organic framework nanomaterial COF-SiO₂@Fe₃O₄ as an adsorbent. COF-SiO₂@Fe₃O₄ was prepared by a straightforward solvothermal method, using Fe₃O₄ as a magnetic core and benzidine and 3,3,5,5-tetraaldehyde biphenyl as the two building units. COF-SiO₂@Fe₃O₄ could effectively capture the targeted PYRs by virtue of its abundant π-electron system and hydroxyl groups. The impact of various experimental parameters on the extraction efficiency was investigated to optimize the MSPE conditions, including the adsorbent amount, extraction time, elution solvent type and elution time. Subsequently, method validation was conducted under the optimal conditions in conjunction with gas chromatography-mass spectrometry (GC-MS). Within the range of 5.00-100 μg·kg^-1 (1.00-100 μg·kg^-1 for bifenthrin and 2.5-100 μg·kg^-1 for fenpropathrin), the five PYRs exhibited a strong linear relationship, with determination coefficients ranging from 0.9990 to 0.9997. The limits of detection (LODs) were 0.3-1.5 μg·kg^-1, and the limits of quantification (LOQs) were 0.9-4.5 μg·kg^-1. The recoveries were 80.2-116.7% with relative standard deviations (RSDs) below 7.0%. Finally, COF-SiO₂@Fe₃O₄, NH₂-SiO₂@Fe₃O₄ and Fe₃O₄ were compared as MSPE adsorbents for PYRs. The results indicated that COF-SiO₂@Fe₃O₄ was an efficient and rapid selective adsorbent for PYRs. This method holds promise for the determination of PYRs in real samples.

Keywords: covalent organic framework; gas chromatography–mass spectrometry; magnetic solid-phase extraction; pyrethroid pesticides.

Scheme 1

Synthesis of COF-SiO₂@Fe₃O₄ and determination of PYRs by MSPE.

Figure 1

SEM images of Fe₃O₄ (A) and COF-SiO₂@Fe₃O₄ (B), TEM images of COF-SiO₂@Fe₃O₄ (C,D) and EDS spectra of COF-SiO₂@Fe₃O₄ (E–J).

Figure 2

XRD patterns of Fe₃O₄, SiO₂@Fe₃O_4, NH₂-SiO₂@Fe₃O₄ and COF-SiO₂@Fe₃O₄ (A); FTIR spectra of Fe₃O₄ (B−a), SiO₂@Fe₃O₄ (B−b), NH₂-SiO₂@Fe₃O₄ (B−c) and COF-SiO₂@Fe₃O₄ (B−d); N₂ adsorption–desorption isotherms of Fe₃O₄ and COF-SiO₂@Fe₃O₄ (C,D); TGA spectra of NH₂-SiO₂@Fe₃O₄ and COF-SiO₂@Fe₃O₄ (E); and magnetic hysteresis curves of Fe₃O₄, SiO₂@Fe₃O₄, NH₂-SiO₂@Fe₃O₄ and COF-SiO₂@Fe₃O₄ (F).

Figure 3

Optimization of adsorbent amount (A), extraction time (B), elution solvent (C) and elution time (D).

Figure 4

Total ion flow chromatography of blank cabbage sample (A) and spiked cabbage sample (50 μg⋅kg⁻¹) after purifcation by Fe₃O₄ (B), NH₂-SiO₂@Fe₃O₄ (C), COF-SiO₂@Fe₃O₄ (D), respectively. 1—Allethrin, 2—Tetramethrin, 3—Bifenthrin, 4—Fenpropathrin, 5—Cyhalothrin.