Size–controllable synthesis of large–size spherical 3D covalent organic frameworks as efficient on–line solid-phase extraction sorbents coupled to HPLC

Covalent organic frameworks (COFs) have found promising applications in separation fields due to their large surface area and high adsorption capacity, but the exiting COFs can not be directly used as the packing materials of on–line solid-phase extraction (SPE) coupled to HPLC and HPLC because their nano/submicron size or irregular shapes might cause ultrahigh column back pressure and low column efficiency. To synthesize the large-size spherical COFs larger than 3 μm as sorbents might be able to address these problems, however it is still a great challenge till now. In this work, two large-size spherical 3D COFs (COF–320 and COF–300) were size–controllably synthesized within 10–90 μm via a two–step strategy. These two spherical COFs showed large surface area, fine crystallinity, good chemical/mechanical stability, and good reproducibility. As an application case, when used as the on–line SPE sorbents coupled to HPLC, the large–size spherical COF–320 displayed high binding capacity for bisphenol F (Qmax of 452.49 mg/g), low column back pressure (6–8 psi at flow rate of 1 mL/min), and good reusability (at least 30 cycles). The developed on–line–SPE–HPLC–UV method presented good analytical performance with enrichment factor of 667 folds, linear range of 1.0–400 ng/mL, limit of detection (LOD, S/N = 3) of 0.3 ng/mL, limit of quantification (LOQ, S/N = 10) of 1.0 ng/mL, and recoveries of 100.3–103.2 % (RSDs of 2.0–3.5 %) and 95.2–97.0 % (RSDs of 4.3–5.6 %) for tap water and lake water samples, respectively. This is the first case to synthesize the large–size spherical COFs within 10–90 μm, and this work made it possible to directly use COFs as the filling materials of on–line SPE coupled to HPLC and HPLC. The developed analytical method can be potentially applied to the rapid and sensitive detection of trace bisphenol F in environmental water samples.