Synthesis of a novel chiral DA-TD covalent organic framework for open-tubular capillary electrochromatography enantioseparation

Tuning interlayer stacking of 2D covalent organic frameworks for high-resolution separation of C8 aromatic isomers

Covalent organic frameworks (COFs) has shown great potential as stationary phase in gas chromatography separation. However, designing COF stationary phases with high separation performance remains challenging. Here, we report a novel strategy to enhance the separation ability of COF stationary phases through tuning the interlayer stacking of COF. A rare interlayer modulation of 2D COFs from eclipsed-AA to slipped-AA was achieved through a two-step synthesis method. Simply changing the solvent used in step 1 allowed an interlayer modulation from slipped-AA to eclipsed-AA. As the proof-of-concept, 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H-porphyrin (Tph) and 2,5-dihydroxyterephthalaldehyde (DHTA) were condensed to prepare 2D COF Tph-DHTA. The interlayer stacking of the 2D COF Tph-DHTA was tuned from eclipsed-AA model to slipped-AA by changing the solvent from o-dichlorobenzene + n-butanol (3:1, v/v) to tetrahydrofuran + n-butanol (1:7, v/v) in step 1. The as-prepared Tph-DHTA with slipped-AA stacking (s-Tph-DHTA) showed higher resolution and faster separation of C8 aromatic isomers than that with eclipsed-AA stacking (e-Tph-DHTA). The formation of slipping stacking of s-Tph-DHTA facilitated the thermodynamics, but did not affect the mass transfer resistance for the separation of C8 aromatic isomers. This work not only provides a promising way to modulate the stacking structure of COFs, but also opens a new strategy to design COF stationary phases for the separation of intractable isomers.

Chiral-induced covalent organic framework as novel chiral stationary phase for chiral separation using open-tubular capillary electrochromatography

As a novel class of chiral stationary phase (CPS) material, chiral covalent organic frameworks (CCOFs) have already shown great promise in open-tubular capillary electrochromatography (OT-CEC) for chiral separation. The synthesis methods of CCOFs used in OT-CEC mainly include bottom-up, post modification and chiral induction. The CCOFs synthesized by bottom-up and post modification strategies already have lots of applications in capillary electrochromatography, however, the chiral-induced synthesized via an asymmetric catalytic strategy has not yet been reported for using as the chiral stationary phase (CPS) in OT-CEC or even in chromatographic separation. Herein, the chiral-induced COF (Λ)-TpPa-1 was synthesized by asymmetric catalytic synthesis and coated on the inner surface of a capillary by an in-situ growth strategy as the CPS for chiral drug separation. The baseline separation of six enantiomers was achieved within 14 min, with a high-resolution (Rs) range from 1.85 to 6.75. Moreover, the resolution and migration time of the capillary keep stable within 160 runs, showing its superior stability and repeatability. This research provides a new idea for the development and application of novel CPS materials in the field of capillary electrochromatography separation, also shows the new application of chiral induced COFs. Furthermore, the chiral-induced CCOFs can be easily applied to other chromatographic separation fields, exhibiting its extensive application value in chiral analysis separation.

Three Birds with One Sulfur: Construction of Sulfur-Bridged Porous Organic Polymers for Efficient Gold Adsorption

Sulfur is a massive byproduct of the petrochemicals industry and hardly employed as a building block for porous organic polymers (POPs). Here, a new family of sulfur-bridged POPs has been prepared via a C-H insertion reaction between sulfur and polycyclic aromatic hydrocarbons. Sulfur works as a solvent, external cross-linker, and porogen simultaneously during the polymerization process. The products demonstrate high porosity and maximum surface area of 1050 m2 g-1 with abundant accessible active sites, contributing to the nanometerization of sulfur and significantly enhancing the inherent affinity between heteroatoms toward soft metal ions. Therefore, they exhibit a high absorption capacity for Au(III) of 3287 mg g-1 and excellent absorption selectivity and removal efficiency via a performance evaluation even in real electronic wastewater. This synthetic strategy to prepare high added-value functional POPs with sulfur not only sheds light on designing high-performance gold adsorption materials and emerging POPs, but also promotes a sustainable development protocol.

Large-size porous spherical 3D covalent organic framework for preconcentration of bisphenol F in water samples and orange juice

Large-size spherical sorbents with particle size of 10-50 μm are widely applied in separation fields, however it is still a great challenge to synthesize such large-size spherical covalent organic framework (COF). In this work, a type of large-size porous 3D COF was size-controablly synthesized via a two-step strategy, in which a large-size porous 3D spherical polymer was prepared first through a Pickering emulsion polymerization using nano silica as the stabilizer, and subsequently it was converted into porous spherical 3D COF by a solvothermal method. The as-prepared porous spherical COF (COF-320 as a model) showed size-controllable uniform spherical morphology within 15-45 μm, large specific surface area, fine crystalline structure, and good chemical stability. When used as the sorbent for dispersive solid-phase extraction (d-SPE) of bisphenol F (BPF), the porous spherical COF-320 (15 μm) displayed high adsorption capacity (Qmax = 335.6 mg/g), high enrichment factor (80 folds), and good reusability (at least five cycles). By coupling the d-SPE method to HPLC, a new analytical approach was developed and successfully applied to the determination of trace BPF in two water samples, an orange juice and a standard sample with recoveries of 96.0-102.2 % (RSD = 1.1-1.5 %), 95.7-97.4 % (RSD = 1.4-4.4 %) and 98.7 % (RSD = 2.3 %), respectively. The limit of detection (S/N = 3) and limit of quantification (S/N = 10) were 0.1 and 0.3 ng/mL, respectively. The new synthesis strategy opens a viable way to prepare large-size porous spherical COFs, and the developed analytical method can be potentially applied to sensitively detect the trace BPF in water samples and beverages.

Rapid and in-situ preparation COFs coated capillary by adsorption method for the separation and determination of phthalate ester using CEC

As a novel class of stationary phase materials, covalent organic frameworks (COFs) have shown great promise in open-tubular capillary electrochromatography. However, the current preparation of COFs coating capillaries heavily relies on tedious and time-consuming covalent bond methods. In this work, a novel, simple and rapid adsorption method was developed for fabrication of TPB-DMTP COF (fabricated from 1,3,5-tris(4-aminophenyl)benzene (TPB) and 2,5-dimethoxyterephthalaldehyde (DMTP)) coated capillary. Due to the crystallization process of the COF is greatly shortened because pre-modification capillary does not require silane coupling agent, this method enables the rapid preparation of COFs-coated capillaries. The organic molecular building units only need 25 min to form a stable COFs coating on the inner wall of a capillary by this method. To our knowledge, this is the shortest method for preparing COFs coated capillary up to now. The performance of the TPB-DMTP COF coated capillary was evaluated by using phthalate esters as model analytes. The results demonstrated that the TPB-DMTP COF coated capillary has excellent repeatability and stability. The relative standard deviations (RSDs) of the analyte's retention time of intra-day, inter-day and column-to-column were in the range of 0.05 %-0.27 %, 0.31 %-0.63 % and 0.31 %-0.88 %, respectively. And, no significant changes were observed in separation efficiency and retention time after over 200 runs. Finally, the TPB-DMTP COF coated capillary was applied for the determination of phthalates in marketed plastic bag and the recovery ranged from 88.0 % to 114.0 %.

Chiral hydroxyl-controlled covalent organic framework-modified stationary phase for chromatographic enantioseparation

Chiral covalent organic frameworks (CCOFs) possess a superior chiral recognition environment, abundant pore configuration, and favorable physicochemical stability. In the post-synthetic chiral modification of COFs, research usually focused on increasing the density of chiral sites as much as possible, and little attention has been paid to the influence of the density of chiral sites on the spatial structure and chiral separation performance of CCOFs. In this article, 1,3,5-tris(4-aminophenyl) benzene (TPB), 2,5-dihydroxyterephthalaldehyde (DHTP), and 2,5-dimethoxyterephthalaldehyde (DMTP) served as the platform molecules to directly establish hydroxyl-controlled COFs through Schiff base condensation reactions. Then the novel chiral selectors 6-deoxy-6-[1-(2-aminoethyl)-3-(4-(4-isocyanatobenzyl)phenyl)urea]-β-cyclodextrin (UB-β-CD) were pended into the micropore structures of COFs via covalent bond for further construction the [UB-β-CD]x-TPB-DMTP COFs (x represents the density of chiral sites). The chiral sites density on [UB-β-CD]x-TPB-DMTP COFs was regulated by changing the construction proportion of DHTP to obtain a satisfactory CCOFs and significantly improve the ability of chiral separation. [UB-β-CD]x-TPB-DMTP COFs were coated on the inner wall of a capillary via a covalently bonding strategy. The prepared open tubular capillary exhibited strong and broad enantioselectivity toward a variety of chiral analytes, including sixteen racemic amino acids and six model chiral drugs. By comparing the outcomes of chromatographic separation, we observed that the density of chiral sites in CCOFs was not positively correlated with their enantiomeric separation performance. The mechanism of chiral recognition [UB-β-CD]x-TPB-DMTP COFs were further demonstrated by molecular docking simulation. This study not only introduces a new high-efficiency member of the COFs-based CSPs family but also demonstrates the enantioseparation potential of CCOFs constructed with traditional post-synthetic modification (PSM) strategy by utilizing the inherent characteristics of porous organic frameworks.

Chiral zeolite beta used as stationary phase for enantioseparation in miniaturized open tubular capillary electrochromatography with amperometric detection

BACKGROUND

With the rapid growth of the demand for optically pure compounds in the fields of biology, medicine and stereospecific synthesis, it is of great importance to develop efficient, economical, simple enantioseparation and analysis methods. Open tubular capillary electrochromatography (OT-CEC) has attracted much attention in the field of chiral separation, but its column capacity and the sensitivity of common-used optical detection are relatively low. Zeolite beta nanomaterial is both enantioselective and size-selective, providing suitable chiral microenvironment for chiral recognition, and amperometric detection (AD) avoids the low sensitivity caused by the short optical path in optical detection to some extent.

RESULTS

Zeolite beta nanomaterials with different particle sizes (25, 50 and 200 nm) were synthesized, and the morphology and structure were characterized by scanning electron microscopy and X-ray diffraction. Then, a novel chiral OT column was prepared by one-step method using zeolite beta nanomaterial as chiral stationary phase, and its separation performance was characterized by miniaturized CEC with AD (mini-CEC-AD) device. Under the optimum conditions, six groups of chiral drugs achieved baseline separation. Norepinephrine enantiomers were used for evaluating the inter-day, intra-day and inter-column reproducibility of the prepared open-tubular column. The relative standard deviations of migration time, peak area, resolution and selectivity factor were within 8.7 %. The limits of detection for norepinephrine enantiomers were 0.18 μg mL-1 (S/N = 3), and the average recoveries were in range of 96.7-105.0 %. This developed method has been successfully applied to the analysis of impurity enantiomer in potassium dichromate (+)-norepinephrine injection sample.

SIGNIFICANCE

Zeolite beta nanomaterial was used as the stationary phase to prepare chiral OT columns for the first time, and this one-step preparation method is simple and easy. The introduction of zeolite beta enriches the types of chiral stationary phase materials in electrochromatographic columns, and mini-OT-CEC-AD system provides an alternative for fast enantioseparation of chiral compounds.

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.

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.

Three-dimensional fluorinated covalent organic frameworks coated capillary for the separation of fluoroquinolones by capillary electrochromatography

In this work, a three-dimensional fluorinated covalent organic frameworks (3D FCOFs) JUC-515 was synthesized from tetra(4-aminophenyl)methane (TAM) and 2,3,5,6-tetrafluoroterephthalol (TFA) by an ionic liquid method. JUC-515 was introduced into the capillary column and bonded to the inner wall of the capillary column by chemical bonding. Through a variety of characterization results, JUC-515 was successfully synthesized and introduced into the capillary column. The effects of buffer solution concentration, organic additive content and pH of the buffer solution on the separation of fluoroquinolones (FQs) were investigated in detail. The JUC-515-coated capillary column showed good resolution (>1.5) and reproducibility. The relative standard deviations (RSDs) of the retention time for intraday, interday, column-to-column and interbatch precision were less than 0.88%, 2.45%, 2.74% and 3.32%, respectively. The RSDs of the peak area for intraday, interday, column-to-column and interbatch precision were less than 3.79%, 4.31%, 3.33% and 5.62%, respectively. The JUC-515-coated capillary column could be used no less than 150 times. The results showed that the JUC-515-coated capillary column had good separation performance. In addition, by separating fluorinated β-phenylalanine analogs, β-phenylalanine and trifluoromethyl β-phenylalanine analogs, the separation mechanism based on fluorine interactions was discussed. In conclusion, JUC-515 had good potential as a stationary phase for capillary electrochromatography.

In-situ immobilization of covalent organic frameworks as stationary phase for capillary electrochromatography

A new kind of covalent organic framework (COF) was first utilized as an stationary phase for open-tubular electrochromatography (OT-CEC) by in situ synthesis immobilized method at room temperature. On the basis of our previous work, 4,4',4″-(1,3,5-Triazine-2,4,6-triyl)trianiline (TZ) and 2,5-bis(2-propyn-1-yloxy)-1,4-benzenedicarboxaldehyde (BPTA) were employed as building blocks for the synthesis of COF TZ-BPTA. The coated capillary and COF TZ-BPTA were characterized by scanning electron microscopy (SEM). Then, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) were also applied to characterize COF TZ-BPTA and the modified column. In SEM, it can be seen that COF TZ-BPTA was the spherical shape and the modified capillary was covered with globular particles equably. The COF TZ-BPTA coated column exhibited good separation resolution and efficiency towards two antiepileptic drugs and other kinds of small organic molecules involving alkylbenzene, sulfonamides, polycyclic aromatic hydrocarbon (PAH), parabens, amino acids and herbicides. The maximum column efficiency was over 2.8 × 105 plates·m-1. In addition, the precisions (RSDs) of the retention times for the alkylbenzenes of intra-day runs (n = 3), inter-day runs (n = 3) and column-to-column runs (n = 3) were all less than 1.70% and separation performance was without obvious change within 100 times run. In addition, the real sample was tested on COF TZ-BPTA coated column. Hence, COF TZ-BPTA showed great potential in the separation domain.

Preparation of β-cyclodextrin covalent organic framework-immobilized poly(glycidyl methacrylate) nanoparticle-coated open tubular capillary electrochromatography column for chiral separation

A new enantioselective open-tubular capillary electrochromatography was developed employing poly(glycidyl methacrylate) nanoparticles/β-cyclodextrin covalent organic frameworks chemically immobilized on the inner wall of the capillary as a stationary phase. A pretreated silica-fused capillary reacted with 3-aminopropyl-trimethoxysilane followed by poly(glycidyl methacrylate) nanoparticles and β-cyclodextrin covalent organic frameworks through a ring-opening reaction. The resulting coating layer on the capillary was characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The electroosmotic flow was studied to evaluate the variation of the immobilized columns. The chiral separation performance of the fabricated capillary columns was validated by the analysis of the four racemic proton pump inhibitors including lansoprazole, pantoprazole, tenatoprazole, and omeprazole. The influences of bonding concentration, bonding time, bonding temperature, buffer type and concentration, buffer pH, and applied voltage on the enantioseparation of four proton pump inhibitors were investigated. Good enantioseparation efficiencies were achieved for all enantiomers. In the optimum conditions, the enantiomers of four proton pump inhibitors were fully resolved within 10 min with high resolutions of 9.5-13.9. The column-to-column and inter- to intra-day repeatability of the fabricated capillary columns through relative standard deviation were found better than 9.54%, exhibiting satisfactory stability and repeatability of the fabricated capillary columns.

Chirality-Controlled Mercapto-β-cyclodextrin Covalent Organic Frameworks for Selective Adsorption and Chromatographic Enantioseparation

Chiral covalent organic frameworks (CCOFs) benefit from superior stability, abundant chiral environment, and homogeneous pore configuration. In its constructive tactics, only the post-modification method allows for the integration of supramolecular chiral selectors into achiral COFs. Here, the finding utilizes 6-deoxy-6-mercapto-β-cyclodextrin (SH-β-CD) as chiral subunits and 2,5-dihydroxy-1,4-benzenedicarboxaldehyde (DVA) as the platform molecule to synthesize chiral functional monomers through thiol-ene click reactions and directly establish ternary "pendant-type" SH-β-CD COFs. The chiral site density on SH-β-CD COFs was regulated by changing the proportion of chiral monomers to obtain an optimal construction strategy and remarkably improve the ability of chiral separation. SH-β-CD COFs were coated on the inner wall of the capillary in a covalently bound manner. The prepared open tubular capillary was achieved for the separation of six chiral drugs. By combining the outcomes of selective adsorption and chromatographic separation, we observed the higher density of chiral sites in the CCOFs, and poorer results were achieved. From the perspective of spatial conformational distribution, we interpret the variation in the performance of these chirality-controlled CCOFs for selective adsorption and chiral separation.

In situ growth preparation of a new chiral covalent triazine framework core-shell microspheres used for HPLC enantioseparation

The manufacturing of chiral covalent triazine framework core-shell microspheres CC-MP CCTF@SiO₂ composite is reported as stationary phase for HPLC enantioseparation. The CC-MP CCTF@SiO₂ core-shell microspheres were prepared by immobilizing chiral COF CC-MP CCTF constructed using cyanuric chloride and (S)-2-methylpiperazine on the surface of activated SiO₂ through an in-situ growth approach. Various racemates as analytes were separated on the CC-MP CCTF@SiO₂-packed column. The experimental results indicate that 19 pairs of enantiomers were well separated on the CC-MP CCTF@SiO₂-packed column, including alcohols, phenols, amines, ketones, and organic acids. Among them, there are 17 pairs of enantiomers that can achieve baseline separation with good peak shapes. Their resolution values on this chiral column are between 0.4 and 5.61. The influences of analyte mass, column temperature, and composition of the mobile phase on the resolution of enantiomers were studied. In addition, the chiral resolution ability of CC-MP CCTF@SiO₂-packed column was compared with the commercial chiral chromatographic columns (Chiralpak AD-H and Chiralcel OD-H columns) and some CCOF@SiO₂ chiral columns (β-CD-COF@SiO₂, CTpBD@SiO₂, and MDI-β-CD-modified COF@SiO₂). The CC-MP CCTF@SiO₂-packed column exhibited some unique advantages and can complement these chiral columns in chiral separations. The research results show that the CC-MP CCTF@SiO₂ chiral column offered high column efficiency (e.g., 17680 plates m^-1 for ethyl mandelate), low column backpressure (5-9 bar), high enantioselectivity, and excellent chiral resolution ability for HPLC enantioseparation with good stability and reproducibility. The relative standard deviations (RSD) (n = 5) of the retention time, and peak areas by repeated separation of ethyl mandelate are 0.23% and 0.67%, respectively. It demonstrates that the CC-MP CCTF@SiO₂ core-shell microsphere composite has great potential in enantiomeric separation by HPLC.

Chiral covalent organic framework-based open tubular capillary electrochromatography column for enantioseparation of selected amino acids and pesticides

A novel chiral covalent organic framework (CCOF) was synthesized with an imine covalent organic framework TpBD (synthesized via Schiff-base reaction between phloroglucinol (Tp) and benzidine (BD)) modified using (1S)-(+)-10-camphorsulfonyl chloride as chiral ligand by chemical bonding method for the first time, and was characterized by X-ray diffraction, Fourier-transform infrared spectra, X-ray photoelectron spectroscopy, nitrogen adsorption/desorption, thermogravimetry analysis, and zeta-potential. The results revealed that the CCOF had good crystallinity, high specific surface area and good thermal stability. Then, the CCOF was employed as stationary phase in open-tubular capillary electrochromatography (OT-CEC) column (the CCOF-bonded OT-CEC column) for enantioseparation of 21 single chiral compounds (12 natural amino acids including acidic, neutral and basic, 9 pesticides including herbicides, insecticides and fungicides) and simultaneous enantioseparation of mixture amino acids and pesticides with similar structures or properties. Under the optimized CEC conditions, all the analytes reached the baseline separation with high resolutions of 1.67-25.93 and selectivity factors of 1.06-3.49 in 8 min. Finally, the reproducibility and stability of the CCOF-bonded OT-CEC column were measured. Relative standard deviations (RSDs) of retention time and separation efficiency were 0.58-4.57% and 1.85-4.98%, and not obviously changed after 150 runs. These results demonstrate that COFs-modified OT-CEC explore a promising method to separate chiral compounds.

Separation and identification of cetirizine enantiomers in human urine by capillary electrophoresis and circular dichroism independent of their standards

Enantioseparation and determination of chiral drugs are of vital importance in biochemical and pharmaceutical research due to the different biological activity, mechanism, and toxicity of individual enantiomers. As a second-generation H(1)-antagonist, cetirizine's pharmaceutical activity is mainly derived from the levocetirizine while the dextro-enantiomer is ineffective and even associated with side effects. Herein, the enantiomers of cetirizine were separated by capillary electrophoresis and identified by electronic circular dichroism. Satisfactory linear relationship was found between the circular dichroism signal at λ_max and the electrophoretic peak area difference in the nonracemic mixture of enantiomers. It made possible identification and quantification of cetirizine enantiomers independent of single enantiomer standards. The method's feasibility was demonstrated on the enantiomeric excess experiments of oral drugs measured in human blank urine. Additionally, the separation and determination of cetirizine in human urine after administration were also realized by capillary electrophoresis, indicating the method was sensitive enough for pharmacokinetic study.

Chiral covalent organic framework incorporated organic polymer monolithic capillary column for enantioseparations

A chiral covalent organic framework was synthesized, characterized, and incorporated into organic polymer monolithic capillary columns to provide chiral stationary phases for enantioseparations. The prepared monolithic capillary columns were characterized by scanning electron microscopy and elemental analysis. To obtain better enantioseparations, the columns' preparation conditions, and enantioseparation conditions were optimized. Baseline resolutions of several chiral compounds were obtained with good reproducibility and stability. Furthermore, the mechanism of chiral recognition was investigated using molecular docking with AutoDock. Docking results showed that the enantioselectivity factor rather than resolution is correlated with the binding free energy difference between enantiomers with the chiral covalent organic framework. And abundant acetoxy and nitrile groups as well as benzene rings in the chiral covalent organic framework are responsible for the enantioseparation ability of the chiral monolithic capillary columns.

Recent applications and chiral separation developments based on stationary phases in open tubular capillary electrochromatography (2019–2022)

Capillary electrochromatography (CEC) plays a significant role in chiral separation via the double separation principle, partition coefficient difference between the two phases, and electroosmotic flow-driven separation. Given the distinct properties of the inner wall stationary phase (SP), the separation ability of each SP differs from one another. Particularly, it provides large room for promising applications of open tubular capillary electrochromatography (OT-CEC). We divided the OT-CEC SPs developed over the past four years into six types: ionic liquids, nanoparticle materials, microporous materials, biomaterials, non-nanopolymers, and others, to mainly introduce their characteristics in chiral drug separation. There also added a few classic SPs that occurred within ten years as supplements to enrich the features of each SP. Additionally, we discuss their applications in metabolomics, food, cosmetics, environment, and biology as analytes in addition to chiral drugs. OT-CEC plays an increasingly significant role in chiral separation and may promote the development of capillary electrophoresis (CE) combined with other instruments in recent years, such as CE with mass spectrometry (CE/MS) and CE with ultraviolet light detector (CE/UV).

Non-porous silica support covalent organic frameworks as stationary phases for liquid chromatography

A new strategy using non-porous silica (NPS) spheres as the support and covalent organic frameworks (COFs) as the porous functional shell for liquid chromatography was developed to ensure the independent effect of the COFs on the separation. As a proof of concept, NPS@TPB-DMTP was prepared for liquid chromatographic analysis using 1,3,5-tris(4-aminophenyl)benzene (TPB) and 2,5-dimethoxy-1,4-benzenedicarboxaldehyde (DMTP) as monomers by in situ polymerisation on the surface of NPS. It is a new way of developing COF-based stationary phases, which will be helpful in understanding what effect the COFs will have on separation.

Challenges and opportunities for chiral covalent organic frameworks

As highly versatile crystalline porous materials, covalent organic frameworks (COFs) have emerged as an ideal platform for developing novel functional materials, attributed to their precise tunability of structure and functionality. Introducing chiral functional units into frameworks produces chiral COFs (CCOFs) with chiral superiorities through chirality conservation and conversion processes. This review summarises recent research progress in CCOFs, including synthetic methods, chiroptical characterisations, and their applications in asymmetric catalysis, chiral separation, and enantioselective recognition and sensing. Challenges and limitations are discussed to uncover future opportunities in CCOF research.

The progress on porous organic materials for chiral separation

Chiral compounds have similar structures and properties, but their pharmacological action is very different or even opposite. Therefore, the separation of chiral compounds has great significance in pharmaceutical and agriculture. Porous organic materials are novel crystalline porous materials, which possess high surface area, controllable pore size, and favorable functionalization. Therefore, porous organic materials are considered to be an ideal material for chiral separation. In this review, we summarized the progress of chiral porous organic materials for chiral separation in recent years. Furthermore, the applications of chiral porous organic materials as chiral separation medias (chromatography stationary phases and membrane materials) in enantioseparation were highlighted. Finally, the remaining challenges and future directions for porous organic materials in chiral separation were also briefly outlined further to promote the development of porous organic materials in chiral separation.

Enhanced Chiral Recognition Abilities of Cyclodextrin Covalent Organic Frameworks via Chiral/Achiral Functional Modification

β-Cyclodextrin covalent organic frameworks (β-CD COFs) show great potential in enantioseparation due to their uniformly distributed chiral recognition sites and good chemical stability. The hydroxyl and amino groups of β-CD COFs enable facile post-modification to introduce the desired functionality into the frameworks. In this study, we perform post-modification of β-CD COF_BPDA with 1,4-butane sultone and [(3R,4R)-4-acetyloxy-2,5-dioxooxolan-3-yl] acetate to construct two kinds of novel functional β-CD COFs. The capillary columns prepared with these two functional β-CD COFs separated chiral dihydropyridines and fluoroquinolones with excellent selectivity and repeatability in capillary electrochromatography, while β-CD COF_BPDA-modified capillary columns did not present the chiral recognition ability for these drugs. The mechanism of chiral recognition and the enhanced enantioselectivity of functional β-CD COFs were further demonstrated by molecular docking simulation. The divergent chiral separation performances of β-CD COFs suggest that the introduction of functional groups enables the modification of β-CD COF properties and tuning of its chiral recognition abilities for the diversity of enantioseparation.