Controllable Synthesis of Hydrangea-Shaped Covalent Organic Frameworks by Solvent Tuning at Room Temperature for Solid-Phase Extraction of Herbicides

Flower-shaped covalent organic frameworks (COFs) with hierarchically porous structures are highly desirable for rapid and high-efficiency adsorption applications, yet precisely controlling the morphology of COFs to uniform and monodisperse flower shapes under mild conditions remains a significant challenge. Herein, tetrafluoro-p-benzoaldehyde and 4,4',4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine were used as building blocks for the controllable synthesis of hydrangea-shaped COFs at room temperature by solvent regulation. The use of aprotic solvents facilitated the formation of spherical COFs; however, the addition of a suitable fraction of nonpolar mesitylene in dioxane led to the formation of hydrangea-shaped COFs. The as-synthesized COFs were demonstrated to be good adsorbents for triazine and amide herbicides, benefiting from the hydrangea-shaped structure and rich triazine, imine, and tetrafluorobenzene groups for hydrogen bonding and reinforced π-π interactions. Coupling hydrangea-shaped COFs-based solid-phase extraction with high-performance liquid chromatography, analyses of five herbicides were realized in a wide linear range (1-100 μg/L) and low limits of detection (0.50-0.70 μg/kg). In addition, the relative recoveries of the analytes from spiked soil samples ranged from 85.48% to 103.64% with relative standard deviations less than 3.16%. This study elucidates the effects of solvent properties on the morphologies of COFs and provides inspiration for rational design and controllable synthesis of COFs with well-defined structures and functional groups by facile solvent regulation at room temperature to realize high sorption performances.