Construction of ultra-stable magnetic imine-cyclization covalent organic frameworks for efficient extraction of phenylurea herbicides in plant beverages.

Imine-linked covalent organic frameworks (COFs) are considered as promising adsorbents to remove the water contaminants. However, the reversibility and hydrolyzation of imine bond may generate limited stability in long-term aqueous environment-based applications, and the improved stability of imine-linked COFs is highly desired. In this work, a carboxyl-quinoline linked COFs (CQ-COF) was synthesized by the cyclization of imine linkage via one-step modification. CQ-COF demonstrated high crystallinity, excellent stability and large surface area. Further integrating with magnetic nanoparticles enables the construction of inorganic-organic heterostructure (Fe3O4@CQ-COF) to serve as a durable and maneuverable adsorbent for the extraction of phenylurea herbicides (PUHs). Compared to imine-linked counterpart, Fe3O4@CQ-COF not only exhibited superior adsorption efficiency, but also enhanced resistance to hydrolysis and good reusability as well. Adsorption mechanism revealed the involvement of hydrogen bond, π-π stacking and hydrophobic interaction between CQ-COF and targets. The extraction method based on Fe3O4@CQ-COF has a wide linearity range of 0.06-200 ng mL-1 and low limit of detection of 0.01-0.06 ng mL-1 for PUHs. This work lays a significant foundation for robust imine-derived COFs and their composites as a promising platform for the extraction of environmental pollutants.