Covalent Organic Frameworks Analyzed Using New Solid-Phase Extraction

Scientists from Wuhan Textile University in Wuhan, China and the University of Oklahoma in Norman, Oklahoma, tested a new solid-phase extraction system for analyzing covalent organic frameworks (COFs). Their findings were published in Analytica Chimica Acta (1).

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Covalent organic frameworks (COFs) are a type of crystalline porous polymeric material that are constructed by organic monomers via different covalent linkages. Recently, COFs have drawn increasing attention in catalysis, sensing, gas storage, photoelectric behavior, energy storage, and especially separation applications. This stems from their various unique properties, such as readily pre–designable structure, predictable pore sizes, tailored functionalities, excellent chemical stability and large specific surface area. Of the various linkages that can occur, imine COFs, which are obtained via Schiff–base condensation of amino– and aldehyde–monomers are attractive.

COFs are viewed as excellent separation materials, being directly used as sorbents for pollutant remediation, disperse solid–phase extraction (SPE), magnetic SPE, micro-SPE, solid–phase microextraction, off–line SPE [23,24], and flow-injection on-line SPE combined to inductively coupled plasma mass spectrometry (ICP–MS), in addition to acting as stationary phases for gas chromatography and capillary electrochromatography. However, COFs are rarely directly used as the packing materials of on-line SPE coupled to high-performance liquid chromatography and HPLC. These COFs are usually synthesized via solvothermal methods, where the size and morphology of COF particles are uncontrolled under harsh reaction conditions, such as high temperature, high pressure, or a sealed tube. This causes the COFs’ nano/submicron size and irregular shapes, lead to ultrahigh back pressure or low column efficiency in on-line SPE and HPLC.

In this study, two large spherical 3D COFs (COF–320 AND COF–300) were size-controllably synthesized within 10–90 µm using a two-step strategy. First, large-size spherical imine-linked polymers were synthesized using a new emulsion polymerization method. Afterwards, they were transformed into corresponding large-size spherical COFs via a solvothermal approach. COF–320 was chosen first due its being well-studied previously, with the obtained spherical COF–320 showing high specific surface area, fine crystallinity, good chemical/thermal/mechanical stability, and good reproducibility. To verify this two-step strategy, spherical COF–300, which is another 3D imine COF, was also synthesized with a highly specific surface area and good crystallinity.

As an application case, the large-size spherical COF–320 was used as the on-line SPE sorbent for bisphenol F (BPF), an endocrine disrupting compound, while coupled to HPLC. Overall, the spherical COF–320 showed high binding capacity (Qmax of 452.49 mg/g) low column back pressure (6–8 psi at a flow rate of 1 mL/min), and good reusability (lasting at least 30 cycles). The corresponding SPE cartridges showed low back pressure and the developed analytical method presented high recovery and sensitivity. The developed on-line–SPE–HPLC–UV method also showed good analytical performance, having an enrichment factor of 667 folds, a linear range of 1.0–400 ng/mL, and recovery ranges of 100.3–103.2% (with RSDs of 2.0–3.5%) and 95.2–97.0% (with RSDs of 4.3–5.6%) for tap water and lake water samples, respectively.

According to the scientists, “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” (1). This method can be potentially applied to rapidly and sensitively detect trace bisphenol F in environmental water samples.

Reference

(1) Chen, Y.; He, Q.; Liu, Y.; Wang, Q.; et al. Size–Controllable Synthesis of Large–Size Spherical 3D Covalent Organic Frameworks as Efficient On–Line Solid-Phase Extraction Sorbents Coupled to HPLC. Anal. Chim. Acta 2024, 1287, 342061. DOI: 10.1016/j.aca.2023.342061