Preparation of cationic hierarchical porous covalent organic frameworks for rapid and effective enrichment of perfluorinated substances in dairy products

Perfluorinated substances (PFASs) are harmful pollutants that have environmental persistence and high bioaccumulation. Effective sample pretreatment must be performed to detect trace or even ultra-trace PFASs in actual samples because of their extremely low contents in complex samples. In this study, a cationic hierarchical porous covalent organic frameworks (C-H-COF) were customized via a template-assisted strategy using polystyrene spheres (PS) as sacrificial materials and a post-synthetic modification method. C-H-COF showed good adsorption selectivity for PFASs owing to the dual effects of the full utilization of the internal adsorption sites and electrostatic interaction. The key role of electrostatic attraction in the extraction of PFASs using C-H-COF was further proven by density functional theory (DFT) calculations. The maximum adsorption capacity of the C-H-COF for perfluorooctanoic acid (PFOA) was 400 mg·g⁻1, which was superior to that of microporous COFs (M-COF) and hierarchical porous COFs without cationic functionalization (H-COF). Accordingly, an analytical method for sensitively detecting five PFASs was established by employing C-H-COF as a dispersive solid phase extraction (DSPE) adsorbent combined with ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS), and the limits of detection were 0.011‒0.29 ng·L⁻1. Moreover, the hierarchical porous structure of the C-H-COF accelerated the mass transfer of analytes so that the extraction process could be completed within 10 min. This method was employed to analyze PFASs in dairy products, in which the ultra-trace levels of analytes were quickly determined with spiked recoveries of 80.1‒112.6%. This work not only provides a rational synthetic strategy for novel ionic hierarchical porous COFs but also helps to expand the application of COFs in sample pretreatment.