Advances in covalent organic frameworks in separation science

One-pot fabrication and evaluation of β-ketoenamine covalent organic frameworks@silica composite microspheres as reversed-phase/hydrophilic interaction mixed-mode stationary phase for high performance liquid chromatography

Covalent organic frameworks (COFs) show promise as a stationary phase in high performance liquid chromatography (HPLC). However, there are only a few COFs-based stationary phases developed for HPLC separation so far. Therefore, it is crucial to not only develop more varieties of COFs-type stationary phases for HPLC separation, but also to explore the retention mechanism of solutes on these stationary phases. In this paper, a new in-situ growth method was developed to prepare β-ketoenamine COF-TpPa-1@SiO2 composite microspheres, using spherical silica as the core material and COF-TpPa-1 fabricated by covalent conjugation of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa-1) as the COF shells. The resulting microspheres exhibit uniform morphology, good monodispersity, large specific surface area, narrow size distribution, and high stability. Due to diverse functional groups in the structure of COF-TpPa-1, the microspheres can offer multiple interactions, such as hydrophobic, π-π stacking and electron-donor-acceptor (EDA) between COFs and analytes. As a result, the COF-TpPa-1@SiO2 composite microspheres can be used as a mixed-mode stationary phase for HPLC separation. The chromatographic performance and retention mechanism of the COF-TpPa-1@SiO2 packed column were investigated by separating polar and non-polar solutes, as well as isomers, in various HPLC modes, including reversed-phase liquid chromatography (RPLC), hydrophilic interaction chromatography (HILIC), and RPLC/HILIC mixed-mode chromatography. The results showed successful separation of non-polar alkylbenzene homologues, polycyclic aromatic hydrocarbons (PAHs), and polar amines and phenols in RPLC mode. The "U-shaped" curves of retention factor with the ACN concentration in mobile phase for four nucleobases indicated that the solute retention on the column followed a mixed mode mechanism of RPLC/HILIC. Compared to a traditional C18 column, the COF-TpPa-1@SiO2 column exhibited superior separation efficiency, stability, repeatability and reproducibility in the separation of analytes with different polarities. The column enhanced the aromatic, shape and planar selectivity for PAHs and isomers through π-π interaction and improved the separation efficiency for electron-deficient compounds due to EDA effect. At last, the column was successfully used to separate and detect the residues of 5 phenylurea herbicides (PUHs) in soil. All these results indicate the potential of COFs for chromatography applications.

Constrution of Quinazoline-Linked Covalent Organic Frameworks via a Multicomponent Reaction for Photocatalysis

Quinazoline (Qz)-linked covalent organic frameworks (COFs) have been constructed via a three-component reaction of ortho-acylanilines, benzaldehydes and NH4OAc. The structure of Qz-COFs has been confirmed by solid-state nuclear magnetic resonance spectroscopy, Fourier transform infrared and powder X-ray diffraction patterns. The Qz-COFs possess high chemical stability, showing good endurance to strong acid, strong base, oxidant, reductant and other conditions. Particularly, Qz-COF-3 can catalyze the aerobic photooxidation of toluene and other compounds containing C(sp3)-H bonds.

Facile preparation of covalent-organic framework composites for magnetic solid-phase extraction of naphthaleneacetic acid in food prior to HPLC-UV analysis

Novel magnetic covalent organic frameworks (COFs) were prepared by one-pot synthetic strategy and employed as an efficient adsorbent for magnetic solid-phase extraction (MSPE) of naphthaleneacetic acid (NAA) in food samples. Depending on the predesigned the hydrogen bonding, π-π and hydrophobic interactions of magnetic COFs, the efficient and selective extraction process for NAA was achieved within 15 min. The magnetic COFs adsorbent combined with HPLC-UV was devoted to develop a novel quantitative method for NAA in complex food. The method afforded good coefficient in range of 0.002-10.0 µg mL-1 and low limit of detection was 0.0006 µg mL-1. And the newly established method afforded less adsorbent consumption, wider linearity and lower LODs than the reported analytical methods. Ultimately, the method was successfully applied to determine NAA in fresh pear, tomato and peach juice. The magnetic COFs based MSPE coupled with HPLC-UV method provided a simple, efficient and dependable alternative to monitor trace NAA in food samples.

Carboxyl-functionalized covalent organic frameworks for the extraction of malachite green and crystal violet in environmental water samples prior to quantification by high-performance liquid chromatography

In this study, monodisperse, uniform, and spherical covalent organic frameworks (COFs) were synthesized using 1,3,5-tris (4-aminophenyl) benzene and 1,3,5-tricarboxaldehyde benzene at room temperature. Post-modification of 6-aminocaproic acid on the COFs yielded carboxyl-modified COFs (COFs-COOH). The modification enhanced the hydrophilicity and adsorption efficiencies of COFs-COOH for malachite green (MG) and crystal violet (CV). A COFs-COOH-based dispersive solid-phase extraction coupled with high-performance liquid chromatography was developed for the analysis of MG and CV. The method showed a linear range from 10 to 1000 ng/mL with detection limits of 1.82 and 0.70 ng/mL for MG and CV detection, respectively. The recoveries of MG and CV from water samples collected from fish farms and markets ranged from 91.63% to 107.10% with relative standard deviations below 5%. Reproducibility tests demonstrated that the adsorption efficiencies of COFs-COOH were maintained at above 85.86% over 15 cycles. The study verified the potential of COFs-COOH as sorbents for the enrichment and separation of triphenylmethane dyes from complex samples.

Sustainable remediation of pesticide pollutants using covalent organic framework - A review on material properties, synthesis methods and application

Covalent organic frameworks (COF) have emerged as a potential class of materials for a variety of applications in a wide number of sectors including power storage, environmental services, and biological applications due to their ordered and controllable porosity, large surface area, customizable structure, remarkable stability, and diverse electrical characteristics. COF have received a lot of attention in recent years in the field of environmental remediation, It also find its way to eliminate the emerging pollutant from the environment notably pesticide from polluted water. This review more concentrated on the application of COF in pesticide removal by modifying COF structure, COF synthesis and material properties. To increase the adsorption ability and selectivity of the material towards certain pesticides removal, the synthesis of COF involves organic linkers with various functional groups such as amine, carboxylic acid groups etc. The COF have a high degree of stability and endurance make them suitable for intermittent usage in water treatment applications. This review manifests the novel progress where modified COFs employed in a prominent manner to remove pesticides from polluted water. Some examples of COF application in the eradication of pesticides are triformyl phenylene framework functionalized with amine groups has capacity to remove up to 50 mg/l of Organophosphorus - chlorpyrifos. COF modified to improve their photocatalytic capacity to breakdown the pesticide under visible light irradiation. COF tetraphenyl ethylene linked with carboxylic acid group shows efficient photocatalytic degradation of 90% of organochlorine insecticide endosulfan when subjected to visible light. Atrazine and imidacloprid are reduced from 100 ppm to 1 ppm in aqueous solutions by COF based on high adsorption capacity. In addition, the strategies, technique, synthesis and functional group modification design of COF are discussed.

Phenanthroline-Decorated Covalent Organic Framework for Catalytic Synthesis of 2-Aminobenzothiazoles in Water

2-Aminobenzothiazoles are widely used in the fields of pharmaceuticals and pesticides. Herein, we report a metal-free protocol for the preparation of 2-aminobenzothiazoles by a covalent organic framework (COF) catalyzed tandem reaction. In the presence of catalytic amount of phenanthroline-decorated COF (Phen-COF), a variety of 2-aminobenzothiazoles are obtained in excellent yields by the cross-coupling of 2-iodoanilines with isothiocyanates at room temperature in water. In addition, the COF-catalyst is very stable and can be reused at least seven times without loss of its catalytic activity.

Covalent organic frameworks: spotlight on applications in the pharmaceutical arena

Covalent organic frameworks (COFs) have much potential in the field of analytical separation research due to their distinctive characteristics, including easy modification, low densities, large specific surface areas and permanent porosity. This article provides a historical overview of the synthesis and broad perspectives on the applications of COFs. The use of COF-based membranes in gas separation, water treatment (desalination, heavy metals and dye removal), membrane filtration, photoconduction, sensing and fuel cells is also covered. However, these COFs also demonstrate great promise as solid-phase extraction sorbents and solid-phase microextraction coatings. In addition to various separation applications, this work aims to highlight important advancements in the synthesis of COFs for chiral and isomeric compounds.

High Selectivity of A Novel Pillar[5]arene with Ester Units as a Gas Chromatographic Stationary Phase toward Aromatic Isomers

This work reports the first example of employing ester-functionalized pillar[5]arene (P5A-C10-OAc) stationary phase for gas chromatography (GC) separations. The as-fabricated P5A-C10-OAc column achieved improved column efficiency of 4270 plates/m and separation performance in contrast to the P5-C10-Br column. The P5A-C10-OAc column showed good separation performance for a wide range of analytes such as alkanes, bromoalkanes, ketones, fatty acid methyl esters, aldehydes, alcohols, halobenzenes, anilines, phenols, naphthalenes, and showed sharp and symmetrical peak shapes for analytes that are liable to peak-tailing in GC analysis. As testified by the challenging isomer mixtures (bromonitrobenzene, chloronitrobenzene, bromobenzaldehyde, chlorobenzaldehyde, nitrobenzaldehyde), the P5A-C10-OAc column exhibited comprehensively higher separation capability than the P5A-C10-Br, P5A-C10 and commercial HP-35 columns. This work demonstrates the great potential of pillararene-based stationary phases as a new type of stationary phases for GC separations.

Immobilization of two dendritic organic phases onto silica and their molecular shape recognition for polycyclic aromatic hydrocarbons, tocopherols and carotenoid isomers

BACKGROUND

Molecular shape selectivity, based on the size and shape parameters of the molecule, such as length and planarity, is a separation process that can be used for compounds with restricted shapes, such as isomers. The separation of geometric isomers is challenging because these compounds have similar physicochemical properties but differ slightly in molecular shape. The ability to separate and quantify these isomers is important in high performance liquid chromatography (HPLC), which is one of the most widely used techniques in separation science today, because the shape of the molecule has a strong influence on biological processes.

RESULTS

We prepared symmetrical discoidal dendrimeric organomolecule gelators (GSDM) and o-phenylenediamine-derived low-molecular-weight dendrimeric organomolecule gelators (G1) and bonded them to silica surfaces. The dendritic organic compound-grafted silica (SiO2@GSDM and SiO2@G1) was used as HPLC stationary phases for the separation of shape-restricted isomers of polycyclic aromatic hydrocarbons (PAHs), carotenoids and tocopherols. The two phases exhibit a very high molecular shape selectivity compared to the commercially available alkyl phases. There are differences in molecular shape selectivity between the two stationary phases. Changes in the chemical structure of dendritic organic compounds can alter the orientation of the molecules, as well as changes in the molecular recognition ability. It was found that SiO2@GSDM has high molecular linear selectivity for PAHs at different temperatures, even at 50 °C. The planar selectivity of SiO2@GSDM was better for triphenylene and o-terphenyl benzenes compared to SiO2@G1.

SIGNIFICANCE

This separation behavior may be attributed to the combined effect of weak interaction centers, which allowed the effective separation of bioactive and shape-restricted isomers through multiple interactions. Furthermore, SiO2@GSDM showed better separation of tocopherols and carotenoids, suggesting that the backbone and ordered structure of organic molecular gelators is an effective way to improve the shape selectivity of the molecules, whereas the molecular orientation of the functional groups influences the separation mechanism of the shape-restricted isomers.

Regenerable and Highly Stable Two-Dimensional Imine-Based Covalent Organic Framework for Simultaneous Rapid Detection and Adsorption of Cu2+ Ions

The development of efficient fluorescent probes and adsorbents for detecting and removing Cu2+, which pose potential environmental and health risks, is a highly active area of research. However, achieving simultaneously improved fluorescence detection efficiency and enhanced adsorption capacity in a single porous probe remains a significant challenge. In this study, we successfully synthesized a two-dimensional imine-based TAP-COF using 2,4,6-triformylphloroglucinol and tri(4-aminophenyl)amine as raw materials. TAP-COF exhibited excellent properties, including a large specific surface area of 685.65 m2·g-1, exceptional thermal stability (>440 °C), chemical stability, temporal stability, and recyclability. Fluorescence testing revealed that TAP-COF exhibited remarkable specificity and high sensitivity for detecting Cu2+. The fluorescence mechanism, in which the excited state intramolecular proton transfer was impeded by the interaction of Cu2+ with C═O and C-N bonds on TAP-COF upon the addition of Cu2+, was further elucidated through experimental and theoretical methods. Furthermore, the adsorption capacity of TAP-COF toward Cu2+ was investigated, confirming the excellence of TAP-COF as a fluorescent probe and adsorbent for the specific detection and removal of Cu2+. This work holds significant implications for improving environmental and human health concerns associated with Cu2+ contamination.

Recent progress of covalent organic frameworks as attractive materials for solid-phase microextraction: A review

Solid-phase microextraction (SPME) is a green, environmentally friendly, and efficient technique for sample pre-treatment. Covalent organic frameworks (COFs), a class of porous materials formed by covalent bonds, have gained prominence owing to their remarkable attributes, including large specific surface area, tunable pore size, and robust thermal/chemical stability. These characteristics have made COFs highly appealing as potential coatings for SPME fiber over the past decades. In this review, various methods used to prepare SPME coatings based on COFs are presented. These methods encompass physical adhesion, sol-gel processes, in situ growth, and chemical cross-linking strategies. In addition, the applications of COF-based SPME coating fibers for the preconcentration of various targets in environmental, food, and biological samples are summarized. Moreover, not only their advantages but also the challenges they pose in practical applications are highlighted. By shedding light on these aspects, this review aims to contribute to the continued development and utilization of COF materials in the field of sample pretreatment.

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

BACKGROUND

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.

RESULTS

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.

SIGNIFICANCE

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.

Magnetic covalent organic frameworks for extraction and determination of endocrine-disrupting chemicals in beverage and water samples

BACKGROUND

Phenolic endocrine-disrupting chemicals (EDCs) are widespread and easily ingested through the food chain. They pose a serious threat to human health. Magnetic solid-phase extraction (MSPE) is an effective sample pre-treatment technology to determine traces of phenolic EDCs.

RESULTS

Magnetic covalent organic framework (COF) (Fe3 O4 @COF) nanospheres were prepared and characterized. The efficient and selective extraction of phenolic EDCs relies on a large specific surface and the inherent porosity of COFs and hydrogen bonding, π-π, and hydrophobic interactions between COF shells and phenolic EDCs. Under optimal conditions, the proposed magnetic solid-phase extraction-high-performance liquid chromatography-ultra violet (MSPE-HPLC-UV) based on the metallic covalent organic framework method for phenolic EDCs shows good linearities (0.002-6 μg mL-1 ), with R2 of 0.995 or higher, and low limits of detection (6-1.200 ng mL-1 ).

CONCLUSION

Magnetic covalent organic frameworks (Fe3 O4 @COFs) with good MSPE performance for phenolic EDCs were synthesized by the solvothermal method. The magnetic covalent organic framework-based MSPE-HPLC-UV method was applied successfully to determine phenolic EDCs in beverage and water samples with satisfactory recoveries (90.200%-123%) and relative standard deviations (2.100%-12.100%). © 2023 Society of Chemical Industry.

Synthesis of composite materials combining magnetic metal-organic frameworks and conjugated organic frameworks for selective extraction of carbendazim and thiabendazole residues from Chinese herbal medicine samples

A magnetic metal-organic framework MIL-68(Al) and a covalent organic framework were used as magnetic solid-phase extraction (MSPE) adsorbents in combination with high-performance liquid chromatography ultraviolet detection (HPLC-UV) to detect carbendazim (CBZ) and thiabendazole (TBZ). The main parameters affecting the extraction in the MSPE process were studied and optimized. Fe3O4@MIL-68(Al) coated with 1,3,5-tris(4-aminophenyl)benzene and terephthaldehyde (Fe3O4@MIL-68(Al)@TAPB-PDA-COF) was analyzed and verified. The material was proven to be suitable for adsorbing CBZ and TBZ. Various adsorption models were used to study its adsorption mechanism. The adsorption results were in good agreement with the pseudo-second-order kinetic model and Langmuir isotherm model. The maximum adsorption capacities of Fe3O4@MIL-68(Al)@TAPB-PDA-COF over CBZ and TBZ were 54.24 and 67.87 mg g-1, respectively, and the equilibrium adsorption time was 200 min. Fe3O4@MIL-68(Al)@TAPB-PDA-COF with excellent recyclability showed higher adsorption capacity and selectivity. A method based on Fe3O4@MIL-68(Al)@TAPB-PDA-COF combined with HPLC-UV was established under the optimal extraction conditions and used to separate and detect trace imidazole drugs in Chinese herbal samples, achieving a low limit of detection (0.65-1.30 μg L-1) with excellent linear correlation (r > 0.999). The recovery rate and relative standard deviation were 86.05-99.78 % and 0.15-4.90 %, respectively. Therefore, the Fe3O4@MIL-68@TAPB-PDA-COF can be regarded as an effective adsorbent for the pretreatment of CBZ and TBZ drugs in Chinese herbal samples.

General Two-Step Method for the Fabrication of Covalent-Organic Framework-Bound Open-Tubular Capillary Columns for High-Resolution Gas Chromatography Separation of Isomers

Covalent organic frameworks (COFs) are promising as stationary phases for gas chromatography (GC). The successful anchoring of COFs to the inner walls of the capillary with good uniformity is an important prerequisite to ensure the excellent separation performance of columns. However, current methods for the fabrication of COF-based capillary columns cannot always meet this requirement when faced with different COFs, which hampers the further development of COF-based GC stationary phases. Here, we show a general two-step method for the fabrication of COF-bound capillary column. The first step enables the formation of uniform amorphous polymer layer on the inner walls of capillary, while the second step allows the facile transformation of the amorphous polymer layer into a highly crystalline COF layer. COF-bound capillary columns with different framework structures were fabricated successfully by the developed two-step method. Impressively, the COF layers bound on the inner walls of these capillary columns showed good uniformity and high crystallinity. More importantly, as an example, the fabricated Tab-DHTA-bound capillary column showed good resolution (R > 1.5) and high column efficiency (700-39,663 plates m-1) for the tested isomers of ethylbenzene, xylene, dichlorobenzene, chlorotoluene, pinene, 1,3-dichloropropene, and propylbenzene with good precision (RSD, run-to-run, n = 5) (retention time, 0.2-0.6%; peak area, 0.5-1.1%; and peak height, 0.5-1.4%). In general, the fabricated Tab-DHTA-bound capillary column exhibited better performance for the separation isomers than commercial columns DB-5 and HP-FFAP. These results indicate that the two-step method is an efficient way to fabricate the COF-bound capillary column with excellent separation performance.

Advanced porous organic materials for sample preparation in pharmaceutical analysis

The development of novel sample preparation media plays a crucial role in pharmaceutical analysis. To facilitate the extraction and enrichment of pharmaceutical molecules in complex samples, various functionalized materials have been developed and prepared as adsorbents. Recently, some functionalized porous organic materials have become adsorbents for pharmaceutical analysis due to their unique properties of adsorption and recognition. These advanced porous organic materials, combined with consequent analytical techniques, have been successfully used for pharmaceutical analysis in complex samples such as environmental and biological samples. This review encapsulates the progress of advanced porous materials for pharmaceutical analysis including pesticides, antibiotics, chiral drugs, and other compounds in the past decade. In addition, we also address the limitations and future trends of these porous organic materials in pharmaceutical analysis.

Covalent organic framework in situ grown on the metal-organic framework as fiber coating for solid-phase microextraction of polycyclic aromatic hydrocarbons in tea

A novel MIL-88-NH2@COF composite was produced by in situ growth of covalent organic framework (COF) on the metal-organic framework (MOF) surface. To obtain a coating fiber for solid-phase microextraction (SPME), the MIL-88-NH2@COF composite physically adhered to the stainless steel wire. Combined with gas chromatography-flame ionization detection (GC-FID), various analytes such as chlorophenols (CPs), phthalates (PAEs), and polycyclic aromatic hydrocarbons (PAHs) were extracted and determined to evaluate the extraction performance of MIL-88-NH2@COF coated fibers and explore their extraction mechanism. This composite exhibit excellent extraction performance and adsorption capacity for various analytes, especially for PAHs with enrichment factor up to 9858. The SPME-GC-FID method based on MIL-88-NH2@COF fiber was established for the determination of five PAHs after the main extraction conditions were optimized. Under optimal conditions, the proposed technique showed a wide linear range (1-150 ng mL-1) with a low limit of detection (0.019 ng mL-1) and a high coefficient of determination (R2 > 0.99). The developed SPME-GC-FID method was used to determine PAHs in green tea and black tea samples, with good recoveries of 51.70-103.64% and 68.56-103.64%, respectively. It is worth mentioning that this is the first time MIL-88-NH2@COF composites have been prepared and applied to SPME. The preparation method of the composite provides a new idea in adsorbent preparation, which will contribute to the field of SPME.

In Vivo Fluorescence Molecular Imaging Using Covalent Organic Nanosheets Without Labeling

Organic nanomaterials, as nanocarrier platforms, have tremendous potential for biomedical applications. The authors successfully prepared novel two-dimensional covalent organic nanosheets (CONs) that can be used as efficient in vivo bioimaging probes by condensing 1,3,5-triformylglucinol (Tp) and 2,7-diaminopyrene (Py) to produce TpPy covalent organic frameworks (COFs). TpPy COFs are then subjected to a liquid exfoliation process to obtain TpPy CONs (< 200 nm in size and < 1.7 nm in thickness). TpPy CONs disperse well in water to provide a stable, homogeneous colloidal suspension, which shows favorable photoluminescence properties. Cell viability tests using MDA-MB-231 and RAW 264.7 cells reveal that TpPy CONs are low in cytotoxicity. Confocal microscopy reveals clear fluorescent cell images after incubation with TpPy CONs for 24 h, without reduction in cell activity or cytosolic aggregation. To investigate the biological behavior of the TpPy CONs, the authors perform an in vivo fluorescence imaging study using MDA-MB-231 tumor-bearing mice. After intravenous injection of TpPy CONs disperse in phosphate-buffered saline (PBS), persistent and strong fluorescence signals are observed in the tumor region, with low background signals from normal tissues at 1, 3, 12, and 24 h after injection. Furthermore, these in vivo imaging results concurred with ex vivo biodistribution and histological results.

In situ growth of porous organic framework on iron wire for microextraction of polycyclic aromatic hydrocarbons

In this work, a novel spherical metal organic framework (MOF) was first in situ grown on the surface of iron wire (IW), in which IW served as the substrate and metal source for MOF (type NH₂-MIL88) growth without adding additional metal salts in the process, while spherical NH₂-MIL88 provided more active sites for further construction of multifunctional composites. Subsequently, a covalent organic framework (COF) was covalently bonded to the surface of the NH₂-MIL88 to obtain the IW@NH₂-MIL88@COF fibers, which were used for headspace solid-phase microextraction (HS-SPME) of polycyclic aromatic hydrocarbons (PAHs) in milk samples prior to determination by gas chromatography-flame ionization detection (GC-FID). Compared with the fiber prepared by physical coating, the IW@NH₂-MIL88@COF fiber prepared by in situ growth and covalent bonding exhibits better stability and possesses more uniform layer. The extraction mechanism of the IW@NH₂-MIL88@COF fiber for PAHs was discussed, which mainly owed to π-π interactions and hydrophobic interactions. After optimization of the primary extraction conditions, the SPME-GC-FID method was established for five PAHs with a wide linear range (1-200 ng mL^-1), good linearity coefficient (0.9935-0.9987) and low detection limits (0.017-0.028 ng mL^-1). The relative recoveries for PAHs detection in milk samples ranged from 64.69 to 113.97%. This work not only provides new ideas for the in situ growth of other types of MOF, but also provides new methods for the construction of multifunctional composites.

Efficient selective removal of uremic toxin precursor by olefin-linked covalent organic frameworks for nephropathy treatment

Indoxyl sulfate is a protein-bound uremic toxin synthesized from indole that cannot be efficiently removed by the hemodialysis method and thus becomes a key risk factor for the progression of chronic kidney disease. Here, we develop a non-dialysis treatment strategy to fabricate an ultramicroporous olefin-linked covalent organic framework with high crystallinity in a green and scalable fashion for selectively removing the indoxyl sulfate precursor (i.e., indole) from the intestine. Various analyses show that the resulting material exhibits excellent gastrointestinal fluid stability, high adsorption efficiency, and good biocompatibility. Notably, it realizes the efficient and selective removal of indole from the intestine and significantly attenuates serum indoxyl sulfate level in vivo. More importantly, the selective removal efficacy of indole is substantially higher than that of the commercial adsorbent AST-120 used in the clinic. The present study opens up a new avenue to eliminate indoxyl sulfate by a non-dialysis strategy and further expands the in vivo applications of covalent organic frameworks.

Novel covalent organic frameworks based electrospun composite nanofiber membranes as pipette-tip strong anion exchange sorbent for determination of inorganic arsenic in rice

Novel covalent organic frameworks (COFs) based PAN@TpBD(NH₂)₂ electrospun composite nanofiber membranes (ECNMs) were fabricated as strong anion exchange sorbent by implementing electrospinning technology. The finished sorbent was characterized, and key parameters of pipette-tip solid phase extraction (PTSPE) procedures were investigated. Inorganic arsenic (iAs) was successfully separated from rice under the optimal precondition conditions, and quantified by hydride generation-atomic fluorescence spectrometry (HG-AFS). This PTSPE-HG-AFS methodology achieved 0.015 μg L^-1 detection limit, 4.67 % relative standard deviation, and 86.48～99.11 % recoveries. In this work, preparation and characterization of this novel COFs-based anion exchange sorbent, PAN@TpBD(NH₂)₂ ECNMs, is described and its suitability for PTSPE applications is demonstrated.

Chromatographic separation performance of silica microspheres surface-modified with triazine-containing imine-linked covalent organic frameworks

In this work, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and 1,3,5-tris(4-formylphenyl)benzene (TFPB) were used as monomers to construct a triazine-containing imine-linked covalent organic framework (COF), which was then bonded onto the surface of aldehydized silica (SiO₂-CHO), and finally a COF@silica composite material (TAPT-TFPB COF@SiO₂) was successfully prepared. The chromatographic separation performance of SiO₂-CHO, TAPT-TFPB COF@SiO₂ and TAPT-TFPB COF@SiO₂/SiO₂-CHO (80/20, mass ratio) was evaluated and compared. It was found that separation efficiency was obviously enhanced by adding an appropriate amount of SiO₂-CHO into TAPT-TFPB COF@SiO₂. The obtained TAPT-TFPB COF@SiO₂/SiO₂-CHO showed more favorable separation ability than SiO₂-CHO and TAPT-TFPB COF@SiO₂. Various aromatic compounds including alkylbenzenes, polycyclic aromatic hydrocarbons, environmental endocrine disruptors, foodborne stimulants and phenyl ketones were effectively separated on the TAPT-TFPB COF@SiO₂/SiO₂-CHO column in reversed phase chromatography mode. The silica microspheres surface-modified with triazine-containing imine-linked COFs proved to be a new type of promising chromatographic packing materials.

Design of hydroxyl-functionalized nanoporous organic polymer with tunable hydrophilic-hydrophobic surface for solid phase extraction of neonicotinoid insecticides

As being widely used insecticides, neonicotinoid residues are toxic and harmful to human health and aquatic ecosystems. Thus, the sensitive monitoring of neonicotinoids in water and food samples is highly desirable to reduce their risks to humans. Herein, four novel hydroxyl-functionalized nanoporous organic frameworks (OH-NOP1, OH-NOP2, OH-NOP3 and OH-NOP4) with tunable hydrophilic-hydrophobic surface have been designed and fabricated for the first time by employing luteolin as monomer and 4,4'-bis(chloromethyl)-1,1'-biphenyl as crosslinker at the molar ratio of 3:1, 1:1, 1:3 and 1:6, respectively. When the molar ratio of luteolin to crosslinker was 1:3, OH-NOP3 was obtained and it presented the highest affinity with excellent adsorption performance towards the studied neonicotinoids. The adsorption mechanism was proposed to be the strong hydrogen bond, polar interaction, Lewis acid-base interaction and pore adsorption between OH-NOP3 and neonicotinoids. Then, utilizing OH-NOP3 as sorbent for solid phase extraction cartridges, an effective method for extraction and preconcentration of neonicotinoids followed by high performance liquid chromatography analysis has been developed for quantitative detection of neonicotinoids from water and edible fungi. The method provided good linearity over the range of 0.06-100.0 ng mL^-1 for lake water, 1.5-100.0 ng g^-1 for pleurotus eryngii and sea-shroom. Low detection limit (at the signal to noise ratio of 3) was achieved in the range of 0.02-0.08 ng mL^-1 for water, 0.50-0.60 ng g^-1 for pleurotus eryngii and 0.50-0.80 ng g^-1 for sea-shroom, while the limit of quantification was 0.06-0.25 ng mL^-1, 1.50-1.80 ng g^-1 and 1.50-2.50 ng g^-1, respectively. Satisfactory method recoveries (85.1-112%) were obtained, with relative standard deviations below 8.2%. This study offered a new strategy for designing efficient sorbents to adsorb or remove organic pollutants based on the structure and properties of substrates.

In-situ growth of covalent organic framework on stainless steel needles as solid-phase microextraction probe coupled with electrospray ionization mass spectrometry for rapid and sensitive determination of tricyclic antidepressants in biosamples

Tricyclic antidepressants (TCAs) including amitriptyline (AT), doxepin (DOX) and nortriptyline (NT) are the first-line drugs for the clinical treatment of depression; however, monitoring TCA concentrations in biological fluids and tissues is necessary to improve therapeutic effect and determine the cause of death in patients. It is of great significance to develop a rapid and sensitive method for real-time monitoring of TCAs in various biosamples. In this work, we fabricated a novel covalent organic framework (COF) based solid-phase microextraction (SPME) probe by an in-situ step-by-step strategy, which was obtained by sequentially modifying 1,3,5-tri (4-aminophenyl) benzene (TPB) and 2, 5-divinylbenzaldehyde (DVA) on the surface of polydopamine layer. The TPB-DVA-COF-SPME probe possessed high specific surface area (1244 m²·g ^- 1), regular pores (3.23 nm), good hydrophobicity and stability, resulting in efficient enrichment for TCAs. Furthermore, the combination of TPB-DVA-COF-SPME probe and ambient electrospray ionization mass spectrometry system (ESI/MS) was firstly proposed for rapid and sensitive determination of TCAs in biosamples. As a result, the developed method exhibited low limits of detection (LODs) (0.1-0.5 μg∙L ^- 1), high enrichment factors (39-218), and low relative standard deviations (RSDs) for one probe (1.2-3.8%) and probe-to-probe (2.0-3.7%). Benefiting from these outstanding performance, TPB-DVA-COF-SPME probe was further successfully applied to biosamples (i.e., serum, liver, kidney, and brain) with excellent reusability, indicating the promising applicability of the TPB-DVA-COF-SPME-ESI/MS as a powerful tool for drug monitoring.

Facile synthesis of Cu2+-immobilized magnetic covalent organic frameworks for highly efficient enrichment and sensitive determination of five phthalate monoesters from mouse plasma with HPLC-MS/MS

Development of a simple, highly selective, and sensitive analytical method for phthalate monoesters (mPAEs) remains a challenge due to the complexity of biological samples. To address this issue, Cu²⁺ immobilized magnetic covalent organic frameworks (Fe₃O₄@TtDt@Cu²⁺ composites) with core-shell structures were prepared to enhance the enrichment efficiency of mPAEs by a facile approach synthesis of COFs shells with inherent bifunctional groups on Fe₃O₄ NPs and further Cu²⁺ immobilization. The composites exhibit high specific surface area (348.1 m² g^-1), outstanding saturation magnetization (34.94 emu g^-1), ordered mesoporous structure, Cu²⁺ immobilization, and excellent thermal stability. Accordingly, a magnetic solid-phase extraction (MSPE) pretreatment technique based on Cu²⁺ immobilized COF composites combined with high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was established, and key parameters including the adsorbent amount, adsorption time, elution solvent, etc. were examined in detail. The developed analytical method showed wide linear ranges (10-8000 ng L^-1), low limit of detections (LODs, 2-10 ng L^-1), and good correlation coefficients (R² ≥ 0.9904) for the five mPAEs. Furthermore, the analytical method was also successfully applied to the highly sensitive detection of metabolite mPAEs in mouse plasma samples, indicating the promising application of the Fe₃O₄@TtDt@Cu²⁺ composites as a quick and efficient adsorbent in the sample pretreatment.

The Preparation of Covalent Bonding COF-TpBD Coating in Arrayed Nanopores of Stainless Steel Fiber for Solid-Phase Microextraction of Polycyclic Aromatic Hydrocarbons in Water

Covalent organic framework (COF)-TpBD was grafted on the arrayed nanopores of stainless steel fiber (SSF) with (3-aminopropyl) triethoxysilane as the cross-linking agent. The prepared SSF bonded with COF-TpBD showed high thermal and chemical stability and excellent repeatability. The prepared SSF bonded with COF-TpBD was also used for the solid-phase microextraction (SPME) of seven kinds of polycyclic aromatic hydrocarbons (PAHs) in actual water samples, followed by gas chromatography with flame ionization detection (GC-FID) determination, which exhibited low limits of detection (LODs), good relative standard deviation (RSD) and high recoveries.

Preparation of spherical silica hydroxyl-functionalized covalent organic polymer composites for mixed-mode liquid chromatography

Covalent organic polymers are an emerging class of amorphous microporous materials that have raised increasing concerns in analytical chemistry due to their unique structural and surface chemical properties. However, the application of covalent organic polymers as mixed-mode stationary phases in chromatographic separations has rarely been reported. Herein, novel spherical silica hydroxyl-functionalized covalent organic polymer composites were successfully prepared via a layer-by-layer approach. The structure and morphology of the materials were carefully characterized by elemental analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, Brunauer-Emmett-Teller, and contact angle measurements. Baseline separations of various alkylbenzenes, polycyclic aromatic hydrocarbons, and nucleosides and bases were achieved on the prepared stationary phase under reversed-phase/hydrophilic interaction mode. The column efficiencies of 23 853 and 36 580 plates/m were obtained for butylbenzene and uracil, respectively, and the relative standard deviation of the retention time for continuous injections was less than 1.38% (n = 10), suggesting satisfactory column efficiency and repeatability. Additionally, this novel stationary phase realized the complete separation of the endocrine-disrupting chemicals in river water. This work affords a new route for synthesizing covalent organic polymers-based mixed-mode stationary phase and further reveals their great potential in chromatographic separation.

Facile room temperature synthesis of a NiFe2O4-based magnetic covalent organic framework for the extraction of tetracycline residues in environmental water samples prior to HPLC

In this study, a functionalized magnetic covalent organic framework (NiFe₂O₄@TAPB-TPA) was fabricated with NiFe₂O₄ nanoparticles as the magnetic core, and 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehyde (TPA) as building blocks by a facile room temperature strategy. Benefitting from the π-π stacking and hydrogen bond interaction, NiFe₂O₄@TAPB-TPA showed great potential as a magnetic adsorbent for the extraction of tetracyclines (TCs). Under optimal conditions, good linearities (R² > 0.9990) were obtained between the peak area and TC concentration in the range of 1-500 μg L^-1 with limits of detection ranging from 0.09 to 0.26 μg L^-1. The intra-day and inter-day relative standard deviations were less than 2.2% and 4.7%, respectively. The established method was successfully applied for the determination of TCs in diverse environmental water samples with satisfactory recoveries in the range of 91.6-102.7%. In addition, NiFe₂O₄@TAPB-TPA showed good reusability with the recoveries for TCs higher than 73.1% after nine recycles, indicating potential application of NiFe₂O₄@TAPB-TPA as an ideal adsorbent for the enrichment of TCs.

Frontier Materials for Adsorption of Antimony and Arsenic in Aqueous Environments: A Review

As highly toxic and carcinogenic substances, antimony and arsenic often coexist and cause compound pollution. Heavy metal pollution in water significantly threatens human health and the ecological environment. This article elaborates on the sources and hazards of compound antimony and arsenic contamination and systematically discusses the research progress of treatment technology to remove antimony and arsenic in water. Due to the advantages of simple operation, high removal efficiency, low economic cost, and renewable solid and sustainable utilization, adsorption technology for removing antimony and arsenic from sewage stand out among many treatment technologies. The adsorption performance of adsorbent materials is the key to removing antimony and arsenic in water. Therefore, this article focused on summarizing frontier adsorption materials' characteristics, adsorption mechanism, and performance, including MOFs, COFs, graphene, and biomass materials. Then, the research and application progress of antimony and arsenic removal by frontier materials were described. The adsorption effects of various frontier adsorption materials were objectively analyzed and comparatively evaluated. Finally, the characteristics, advantages, and disadvantages of various frontier adsorption materials in removing antimony and arsenic from water were summarized to provide ideas for improving and innovating adsorption materials for water pollution treatment.

Core-shell-structured magnetic covalent organic frameworks for effective extraction of parabens prior to their determination by HPLC

Covalent organic framework (COF)-decorated magnetic nanoparticles (Fe₃O₄@DhaTab) with core-shell structure have been synthesized by one-pot method. The prepared Fe₃O₄@DhaTab was well characterized, and parameters of magnetic solid-phase extraction (MSPE) for parabens were also investigated in detail. Under optimized conditions, the adsorbent dosage was only 3 mg and extraction time was 10 min. The developed Fe₃O₄@DhaTab-based MSPE-HPLC analysis method offered good linearity (0.01-20 μg mL^-1) with R² (0.999) and low limits of detection (3.3-6.5 μg L^-1) using UV detector at 254 nm. The proposed method was applied to determine four parabens in environmental water samples with recoveries in the range 64.0-105% and relative standard deviations of 0.16-7.8%. The adsorption mechanism was explored and indicated that porous DhaTab shell provided π-π, hydrophobic, and hydrogen bonding interactions in the MSPE process. The results revealed the potential of magnetic-functionalized COFs in determination of environmental contaminants.

Imine-linked porous covalent organic framework used for the solid-phase extraction of estrogens from honey prior to liquid chromatography-tandem mass spectrometry

以亚胺连接的多孔共价有机骨架材料(IL-COF-1)作为固相萃取的吸附剂,建立了液相色谱-串联质谱快速检测蜂蜜样品中痕量雌激素的方法。该研究选择雌二醇、己烯雌酚、雌三醇、β-雌二醇和炔雌醇5种雌激素作为目标分析物。在蜂蜜样品中添加雌激素,采用单因素优化法对影响萃取效果的重要因素进行优化,获得最佳条件:IL-COF-1用量为30 mg,样品流速为3 mL/min,样品溶液pH值为7,以5 mL的1%(v/v)氨水-甲醇溶液进行洗脱,流速为0.4 mL/min,萃取过程中不添加NaCl。采用高效液相色谱-三重四极杆质谱联用技术对提取物中的雌激素进行定量分析。以乙腈和5 mmol/L的乙酸铵溶液作为流动相进行梯度洗脱,经C18色谱柱分离,采用电喷雾离子源、质谱多反应监测和负离子扫描模式,实现了蜂蜜样品中5种雌激素的快速定性定量分析。在最佳条件下,方法验证结果中雌三醇、β-雌二醇和炔雌醇的线性范围为1~500 ng/g,雌二醇和己烯雌酚的线性范围为0.1~100 ng/g,相关系数(r)为0.9934~0.9972。检出限(S/N=3)为0.01~0.30 ng/g,定量限(S/N=10)为0.05~0.95 ng/g。添加50 ng/g 5种雌激素进行重复性实验,日内精密度相对标准偏差(RSD)为3.2%~6.6%,日间精密度RSD为4.2%~7.9%。基于IL-COF-1的固相萃取-液相色谱-串联质谱法具有快速准确、灵敏度高等特点,适用于蜂蜜中雌激素的分析和检测。将该方法应用于4个实际蜂蜜样品中雌激素的检测,均未检出目标物;在低中高3个水平下,5种雌激素的加标回收率为80.1%~115.2%,结果令人满意。

Covalent Organic Framework/Polyacrylonitrile Electrospun Nanofiber for Dispersive Solid-Phase Extraction of Trace Quinolones in Food Samples

The extraction of quinolone antibiotics (QAs) is crucial for the environment and human health. In this work, polyacrylonitrile (PAN)/covalent organic framework TpPa–1 nanofiber was prepared by an electrospinning technique and used as an adsorbent for dispersive solid-phase extraction (dSPE) of five QAs in the honey and pork. The morphology and structure of the adsorbent were characterized, and the extraction and desorption conditions for the targeted analytes were optimized. Under the optimal conditions, a sensitive method was developed by using PAN/TpPa–1 nanofiber as an adsorbent coupled with high-performance liquid chromatography (HPLC) for five QAs detection. It offered good linearity in the ranges of 0.5–200 ng·mL⁻¹ for pefloxacin, enrofloxacin, and orbifloxacin, and of 1–200 ng·mL⁻¹ for norfloxacin and sarafloxacin with correlation coefficients above 0.9946. The limits of detection (S/N = 3) of five QAs ranged from 0.03 to 0.133 ng·mL⁻¹. The intra-day and inter-day relative standard deviations of the five QAs with the spiked concentration of 50 ng·mL⁻¹ were 2.8–4.0 and 3.0–8.8, respectively. The recoveries of five QAs in the honey and pork samples were 81.6–119.7%, which proved that the proposed method has great potential for the efficient extraction and determination of QAs in complex samples.

β-Cyclodextrin covalent organic framework supported by polydopamine as stationary phases for electrochromatographic enantioseparation

In this work, a new open-tubular capillary electrochromatography (OT-CEC) column was prepared using β-cyclodextrin covalent organic framework (β-CD COF) as a stationary phase. Polydopamine was used to assist fabrication of β-CD COF on an inner wall of a fused-silica capillary. The coating layer on the capillary was characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Electroosmotic flow (EOF) was also studied to evaluate the variation of the inner wall of immobilized columns. Furthermore, the chiral separation effectiveness of the fabricated capillary column was evaluated by CEC using enantiomers of several related proton pump inhibitors as model analytes, including omeprazole, lansoprazole, pantoprazole and tenatoprazole. The effects of bonding time and concentration of β-CD COF, the type, concentration and pH of buffer, applied voltage were investigated to obtain satisfactory enantioselectivity. In the optimum conditions, the enantiomers of four analytes were resolved within 15 min with resolutions of 1.63-2.62. The relative standard deviation values for migration times and resolutions of the analytes representing intraday and interday were less than 6.75% and 4.24%, respectively. The results reveal that β-CD COF has great potential as chiral-stationary phases for enantioseparation in CEC.

Ultra-trace levels voltammetric determination of Pb2+ in the presence of Bi3+ at food samples by a Fe3O4@Schiff base Network1 modified glassy carbon electrode

In this research, a highly sensitive electrochemical sensor was developed for the square wave anodic stripping voltammetric determination of Pb²⁺ at ultra-trace levels. A Glassy carbon electrode was modified with an in-situ electroplated bismuth film and the nanocomposite of a recently synthesized melamine based covalent organic framework (schiff base network₁ (SNW₁)) and Fe₃O₄ nanoparticles (Fe₃O₄@SNW₁). The obtained results exhibit clearly that combination of Fe₃O₄@SNW₁ and in-situ electroplated bismuth film enhances the sensitivity of the modified electrode towards Pb²⁺ remarkably. A Plackett-Burman design was implemented for screening experimental factors to specify the significant variables influencing the sensitivity of the electroanalytical method. Afterward, the effective factors were optimized using Box-Behnken design (BBD). Under optimized conditions, the proposed electrode showed a linear response towards Pb²⁺ in the concentration range of 0.003-0.3 μmol L^-1 with the detection limit of 0.95 nmol L^-1. The selectivity of the fabricated electrode towards different ionic species were checked out and no serious interference was observed. At the end, the application of the designed sensor in the determination of Pb²⁺ at 10 different edible specimens were investigated and the obtained recovery values were in the range of (95.56-106.64%) indicating the successful performance of the designed sensor.

Recent advances of ordered mesoporous silica materials for solid-phase extraction

This review mainly focuses on the development and application of ordered mesoporous silica materials for solid-phase extraction in recent years. It overviews not only bare mesoporous silica but also the functionalized mesoporous silica with organic groups, molecularly imprinted polymers, and magnetic materials. These mesoporous silica materials were used as the extraction adsorbents in cartridge solid-phase extraction, dispersive solid-phase extraction, magnetic solid-phase extraction, micro-solid-phase extraction and matrix solid phase dispersion. Coupled with atomic emission spectrometry, chromatography or other detection methods, these techniques efficiently extracted and sensitively determined various targets, such as metal ions, perfluorocarboxylic acids, pesticides, drugs, endocrine disruptors, phenols, flavanones, polycyclic aromatic hydrocarbons, parabens and so on. Based on unique advantages of mesoporous silica materials, the developed analytical method successfully analyzed different matrix samples, like environmental water samples, soil samples, food samples, biological samples and cosmetics. In addition, the prospects of these materials in solid-phase extraction are presented, which can offer an outlook for the further development and applications.

Evaluation of a porous imine-based covalent organic framework for solid-phase extraction of nitroimidazoles

Covalent organic framework materials (COFs), a kind of porous organic material, have excellent potential application in the field of sample pretreatment due to their high surface areas and thermal stability. In this work, a porous imine-based COF, named BP-COF, was fabricated by a solvothermal method using 2,2'-bipyridine-5,5'-dicarboxaldehyde and 1,3,5-tris(4-aminophenyl) benzene as monomers. After a series of structural characterization studies, the BP-COF was used as a solid-phase extraction (SPE) adsorbent for the extraction and enrichment of nitroimidazoles and their metabolites. Several parameters were investigated during the extraction process, including the sorbent dosage, loading pH value, the washing and elution solvent, and the breakthrough volume. By coupling with UPLC-QTOF-MS/MS, a sensitive and facile method was established for the quantitation analysis of NDZs. Under the optimum conditions, a low detection limit (0.015-0.12 ng mL^-1) and wide linearity (0.2-500 ng mL^-1) with higher R² were obtained. In addition, the developed method was applied for the determination of NDZs in four water samples with recoveries in the range of 71.7-115.9% and RSDs less than 8.3%. The results indicated that the BP-COF shows great potential for the enrichment of NDZs in water samples.

Recent trends on the implementation of reticular materials in column‐centered separations

Advances in the development of column‐based analytical separations are strongly linked to the development of novel materials. Stationary phases for chromatographic separation are usually based on silica and polymer materials. Nevertheless, recent advances have been made using porous crystalline reticular materials, such as metal‐organic frameworks and covalent organic frameworks. However, the direct packing of these materials is often limited due to their small crystal size and nonspherical shape. In this review, recent strategies to incorporate porous crystalline materials as stationary phases for liquid‐phase separations are covered. Moreover, we discuss the potential future directions in their development and integration into suitable supports for analytical applications. Finally, we discuss the main challenges to be solved to take full advantage of these materials as stationary phases for analytical separations.

A covalent organic framework for chiral capillary electrochromatography using a cyclodextrin mobile phase additive

Covalent organic frameworks (COFs) have been recognized as promising solid phases in capillary electrochromatography (CEC). Imine-based COF-coated open-tubular CEC column (COF TpBD-coated OT column) was prepared and characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectra, thermogravimetric analysis (TGA), nitrogen adsorption/desorption (Brunauer-Emmett-Teller [BET]), and scanning electron microscopy (SEM). The results showed that the column efficiency was up to 26,776 plate/m, and the thickness of stationary phase was about 6.00 μm for the column prepared under the optimal conditions. Enantioseparation of 15 kinds of the single chiral compounds (histidine, arginine, lysine, leucine, threonine, methionine, valine, aspartic acid and glutamic acid, fipronil, diclofop, imazamox, quizalofop-p, imazethapyr, and acephate) and 3 kinds of mixed amino acids racemaces (valine, methionine, and glutamic acid) were performed with three methods: capillary electrochromatography with COF TpBD-coated OT column (Method 1), CEC with COF TpBD-coated OT column as the separation channels, and capillary electrophoresis (CE) with HP-β-CD as the chiral mobile phase additive (Method 2) and CE with HP-β-CD as the chiral mobile phase additive (Method 3). Separation efficiency and chiral selectivity of Method 2 was best among the three methods. Under the optimal separation conditions of Method 2, all the enantiomers reached the baseline separation regardless of the single chiral compounds or the mixed amino acids. Relative standard deviation (RSDs) of the mean column efficiency for reproducibility and stability was in the range of 0.46-1.49%. This combination of CEC and CE has great potential for use in chiral separation.

Self-assembly of core-shell structured multiwalled nanotubes@covalent organic frameworks composite for solid-phase extraction of four phytohormones from fruit juices

Covalent organic frameworks (COFs) have attracted considerable attention in sample pretreatment because of their unique characteristics. However, the submicron or micron size of COFs has restricted their wider applications in solid-phase extraction (SPE). Herein, multiwalled nanotubes (MWNTs) were used as substrate materials to synthesize core-shell structured MWNTs@COFs composites (MWNTs@SNW-1) using a simple self-assembly method. The as-prepared MWNTs@SNW-1 composite exhibited a high BET surface area, good thermal stability, and good adsorption capacity. The MWNTs@SNW-1 composite was used as an adsorbent in cartridge-based SPE to extract four phytohormones before determining their levels by high-performance liquid chromatography. The experimental parameters affecting extraction efficiency, including the amount of adsorbents, solution pH, ionic strength, eluent type, and eluent volume, were investigated. The developed method showed a wide linear range (0.37-100 ng mL^-1), low detection limits (0.11-0.32 ng mL^-1), low limits of quantification (0.37-1.07 ng mL^-1), high enrichment factors (45.9-49.3), and good reproducibility (<4.8%) for phytohormones. The developed analytical method was used to analyze trace phytohormones in fruit juices with good recoveries, highlighting the potential of the MWNTs@SNW-1 composite as an adsorbent in sample preparation.