Post-synthetic click chemistry-mediated surface engineering of fluorinated COFs for ultrahigh PFASs uptake

Fluorinated covalent organic frameworks (F-COFs) have attracted widespread attention, owing to their capacity to enhance the adsorption of per- and polyfluoroalkyl substances (PFASs) through F-F affinity and the intrinsic adsorption properties of COFs. F-COFs were usually synthesized based on "de novo synthesis" with commercial fluorinated monomers. However, challenges still remain due to the limited availability of fluorinated monomers, harsh control of crystal growth, and difficult adjustment of surface fluorination. Post-synthetic surface engineering offers great potential for flexibly fabricating F-COFs with tailored functions by modifying functional moieties into the predefined frameworks. Herein, a post-synthetic strategy was pioneered for the fabrication of novel F-COFs with tunable fluorination degrees (noted as COF-V-x-F, x=0-1.0). The approach integrates two synergistic processes: aniline-modulated room-temperature synthesis of the COF-V precursor, and the flexible surface engineering on COF-V via click chemistry to realize controllable fluorination. Using this method, the optimal COF-V-0.33-F was achieved, which contains sufficient fluorinated groups for F-F affinity, high hydrophobicity, and strong molecular adsorption, thus enabling ultra-high PFAS uptake by solid-phase microextraction (SPME). A highly sensitive detection was achieved with limits of detection at 0.2-1.4 ng·L⁻¹ using COF-V-0.33-F fiber coupled with LC-MS/MS. Compared to the commercial PDMS, COF-V and amorphous COP-V-0.33-F, the designed COF-V-0.33-F fiber exhibited rather high extraction capacity with 2.7-65.8, 2.1-10.8, and 2.8-367.1 fold enhancement, respectively. The proposed method was successfully used for PFASs determination in tap, pond and river water. Seven trace PFASs were detected with satisfactory recoveries of 71.0%-117.9%, 75.7%-109.1% and 82.8%-118.2% at spiked concentrations of 20 ng/L, 200 ng/L and 2000 ng/L, respectively. This work posed a simple but powerful access to efficient synthesis of F-COFs with good crystallinity and flexible adjustment of fluorination.