Silica gel microspheres decorated with covalent triazine-based frameworks as an improved stationary phase for high performance liquid chromatography

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.

Chiral-induced synthesis of chiral covalent organic frameworks core-shell microspheres for HPLC enantioseparation

Two chiral covalent organic frameworks (CCOFs) core-shell microspheres based on achiral organic precursors by chiral-induced synthesis strategy for HPLC enantioseparation are reported for the first time. Using n-hexane/isopropanol as mobile phase, various kinds of racemates were selected as analytes and separated on the CCOF-TpPa-1@SiO2 and CCOF-TpBD@SiO2-packed columns with a low column backpressure (3 ~ 9 bar). The fabricated two CCOFs@SiO2 chiral columns exhibited good separation performance towards various racemates with high column efficiency (e.g., 19,500 plates m-1 for (4-fluorophenyl)ethanol and 18,900 plates m-1 for 1-(4-chlorophenyl)ethanol) and good reproducibility. Some effects have been investigated such as the analyte mass and column temperature on the HPLC enantioseparation. Moreover, the chiral separation results of the CCOF-TpPa-1@SiO2 chiral column and the commercialized Chiralpak AD-H column show a good complementarity. This study demonstrates that the usage of chiral-induced synthesis strategy for preparing CCOFs core-shell microspheres as a novel stationary phase has a good application potential in HPLC.

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.

Core-shell silica@pyridyl conjugated microporous polymer as a stationary phase for high performance liquid chromatography

BACKGROUND

Because of the advantages of good selectivity, high sensitivity, and fast analysis, high performance liquid chromatography (HPLC) has become one of the modern analytical techniques in wide application range, such as biological analysis, environmental detection, pharmaceutical and food inspection, agriculture and other fields. The stationary phase greatly decides the chromatographic separation performance, so the development of novel stationary phase is most important for HPLC.

RESULTS

Pyridyl conjugated microporous polymers (P-CMP) with one to four layers were modified on the surface of amino silica to obtain a novel core-shell material (SiO2@P-CMP) by the layer-by-layer assembly strategy and Chichibabin reaction. The relationship between the structure of SiO2@P-CMP and chromatographic performance was carefully investigated, and the retention mechanism was revealed. The interactions including π-π stacking, hydrophobic effect and hydrogen bond gradually enhanced with the increase of P-CMP layers on the silica surface. Compared with C18 column, SiO2@P-CMP columns displayed better separation selectivity for polycyclic aromatic hydrocarbons (PAHs). According to the relative retention values (α), the separation performance of SiO2@P-CMP columns (α = 1.144-1.884) for PAH isomers and other analytes was obviously better than that of C18 column (α = 0.998-1.487). Furthermore, the SiO2@P-CMP column with four layers was selected to separate different types of analytes (eight PAHs, four bisphenols, four estrogens and nine phthalates), and the peak order of analytes was different from that on the C18 column due to the influence of hydrogen-bonding and π-π interactions. The relative standard deviations (n = 10) of retention time and peak area on SiO2@P-CMP column were between 0.28 % and 1.98 %.

SIGNIFICANCE AND NOVELTY

Pyridyl conjugated microporous polymer was introduced as the stationary phase for the first time in HPLC. The proposed column displayed better separation characteristics compared to Zorbax SB-C18 column. It provided a new idea for the separation of small molecules and the development of chromatographic packing or extraction material.

Preparation and application of a novel imine-linked covalent organic framework@silica composite for reversed-phase and hydrophilic interaction chromatographic separations

The composites of covalent organic frameworks (COFs) and silica gel have been considered to be promising chromatographic separation materials due to the distinct advantages such as large specific surface area, good mechanical strength and high porosity. In the present study, a novel imine-linked COF@silica composite was prepared by in-situ growth of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and 2,5-dihydroxyterephthalaldehyde (DHTA) monomers on the surface of aminated silica gel (SiO2-NH2). The successful surface-modification of TAPT-DHTA-COF distinctly enhanced the separation selectivity and efficiency of SiO2-NH2. Multiple types of analyte-stationary phase interactions contributed to the selective retention of structurally similar analytes. The designed TAPT-DHTA-COF@SiO2 was observed to effectively separate hydrophobic phenyl ketones, phthalate esters and steroid hormones. Moreover, the polar amino and hydroxyl groups of TAPT-DHTA-COF facilitated the selective determination of hydrophilic nucleosides/bases. The kinetic performance and thermodynamic behavior of TAPT-DHTA-COF@SiO2 column were particularly explored. It was found that column efficiency was mainly affected by the mass transfer resistance, and the retention of nucleosides/bases on the TAPT-DHTA-COF@SiO2 column was temperature dependent. The developed versatile TAPT-DHTA-COF@SiO2 column was finally applied for detecting environmental hormones as well as water-soluble nicotinamide in real samples. In summary, the potential application of TAPT-DHTA-COF@SiO2 composite material for liquid chromatographic separations was first explored and verified. The TAPT-DHTA-COF@SiO2 was proved to be a promising chromatographic separation material.

Macrocyclic Antibiotics as Effective Chiral Selectors in Liquid Chromatography for Enantiomeric Separation of Pharmaceutical Compounds: A Review

Chiral separation techniques play a crucial role in the pharmaceutical industry, where the enantiomeric purity of drugs can have a significant impact on their efficacy and safety. Macrocyclic antibiotics are highly effective chiral selectors used in various chiral separation techniques, including LC, HPLC, SMB, and TLC, offering reproducible results and a wide range of applications. However, developing robust and efficient immobilization mechanisms for these chiral selectors remains a challenge. This review article focuses on various immobilization approaches, such as immobilization, coating, encapsulation, and photosynthesis, that have been applied to immobilize macrocyclic antibiotics on their support. Commercially available macrocyclic antibiotics for conventional liquid chromatography include Vancomycin, Norvancomycin, Eremomycin, Teicoplanin, Ristocetin A, Rifamycin, Avoparcin, Bacitracin, and others. In addition, capillary (nano) liquid chromatography has also been used in chiral separation utilizing Vancomycin, Polymyxin B, Daptomycin, and Colistin Sulfate. Macrocyclic antibiotic-based CSPs have been extensively applied due to their reproducible results, ease of use, and broad range of applications, capable of separating a large number of racemates.

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.

Pyridine-linked covalent triazine frameworks with bidirectional electron donor-acceptor for efficient organic pollution removal

Simultaneous regulation of adsorption and photocatalytic performance of covalent triazine frameworks (CTFs) to achieve efficient control of organic pollution in water is a promising strategy, but remains a formidable challenge. Herein, pyridine linkers were innovatively introduced into pristine CTF (p-CTF) and the bidirectional electron donor-acceptor (EDA) system of contaminant-to-pyridine and pyridine-to-triazine was constructed inside. Experimental results combined with theoretical calculations revealed that pyridine units with π-deficient properties performed as electron acceptors and electron donors in the adsorption and photocatalytic processes, respectively. This special structure provided a directional pathway for electron transfer, which endowed CTFs with excellent adsorption and photocatalytic properties. Compared to p-CTF, pyridine-linked CTF (M-CTF) showed a 16-fold increase in adsorption capacity for naphthalene (973.4 μmol·g^-1). Benefiting from the optimized light absorption and electron transfer form (n → π*transition), M-CTF exhibited high regeneration efficiency after adsorption of both bisphenol A (94 % after 4 cycles) and naphthalene (95 % after 4 cycles). Besides, the removal performance of organic micropollutants from natural water showed a great advantage thanks to the bidirectional EDA system. Overall, the present study provides new insights into the optimization of electronic structures for carbon-based environmental functional materials applied to organic pollution control in water.

Covalent Organic Frameworks-Based Membranes as Promising Modalities from Preparation to Separation Applications: An Overview

Covalent organic frameworks (COFs) are a promising class of porous crystalline materials made up of covalently connected and periodically protracted network topologies through organic linkers. The tailorability of organic linker and intrinsic structures endow COFs with a tunable porosity and structure, low density, facilely-tailored functionality, and large surface area, attracting increasing amount of interests in variety of research areas of membrane separations. COF-based membranes have spawned a slew of new research projects, ranging from fabrication methodologies to separation applications. Herein, we tried to emphasis the major developments in the synthetic approaches of COFs based membranes for a variety of separation applications such as, separation of gaseous mixtures, water treatment as well as separation of isomeric and chiral organic compounds. The proposed methods for fabricating COF-based continuous membranes and columns for real world applications are also thoroughly explored. Finally, a viewpoint on the future directions and remaining challenges for COF research in the area of separation is provided.

Hydrogel Coating with Temperature Response Retention Behavior and Its Application in Selective Separation of Liquid Chromatography

We reporte the double-layer hydrogel-coated mesoporous silica material as a new stationary phase for liquid chromatography. The method of combining physical coating and chemical coating was to apply hydrogel coating on the surface of silica, and finally, a new type of liquid chromatography stationary phase with in situ coating of the functional hydrogel on silica was obtained. This hydrogel-functionalized liquid chromatography stationary phase also exhibits a certain temperature responsiveness. Experimental results show that this temperature response is mainly due to changes in the hydrogen bonding between the stationary phase and the analyte at different temperatures in the column oven, which leads to changes in retention behavior. The hydrogel-coated mesoporous silica microspheres showed excellent selectivity for many polar analytes. An excellent column efficiency was obtained (139 000 plates/m for terephthalic acid) after optimization of chromatographic conditions. In addition to rapid separation of some analytes, this new hydrogel stationary phase also has certain superiority in chromatographic performance compared with other new excellent liquid chromatography stationary phases functioned by three-dimensional cross-linking systems. The important thing is that this strategy is relatively easy to prepare a new stationary phase with different properties.

Chiral core-shell microspheres β-CD-COF@SiO2 used for HPLC enantioseparation

Chiral covalent organic frameworks (CCOFs) have potential application in enantioseparation due to their advantages, such as large surface area, abundant chiral recognition sites and good chemical stability in organic solvents. However, the application of CCOFs in high performance liquid chromatography (HPLC) for enantioseparation has been rarely reported because of the shortcomings of CCOFs, such as light weight, irregular shape, and wide particle size distribution. In order to overcome the above shortcomings, a one-pot synthetic method was adopted to prepare a core-shell composite (β-CD-COF@SiO₂) via the growth of chiral β-CD COF on the surface of amino-functionalized SiO₂ microspheres. The as-prepared β-CD-COF@SiO₂ microspheres were used as a stationary phase for HPLC enantioseparation. The resolution ability of the β-CD-COF@SiO₂-packed column toward various chiral compounds was investigated using n-hexane/isopropanol as the mobile phase. The results show that the chiral β-CD-COF@SiO₂-packed column exhibited excellent chiral recognition ability for 24 pairs of chiral compounds with good reproducibility. These successful applications indicate that the preparation of the chiral COFs@SiO₂ core-shell microspheres as a novel stationary phase for enantioseparation has good application prospects in HPLC.

Mesoporous nanomaterial-assisted hydrogel double network composite for mixed-mode liquid chromatography

By introducing functional groups such as quaternary amine groups, sulfonic acid groups, triazine groups, and other mespore nanomaterials into the hydrogel, better separation effect of some organic framework materials has been obtained. Due to a reasonable design and preparation strategy, the hydrogel composite-modified silica can be used in the selective separation of various analytes such as pesticides, alkylbenzenes, polycyclic aromatic hydrocarbons, nucleosides/bases, benzoic acids, antibiotics, and carbohydrates. Through the exploration of chromatographic retention behavior, it is proved that the column can be used in mixed-mode liquid chromatography. The intra-day relative standard deviation for retention time of this new stationary phase is 0.12-0.16% (n = 10), and the inter-day relative standard deviation is less than 0.39% (n = 5). This new stationary phase can also be used for separation in complex samples. The limit of detection (LOD) for chlorotoluron in farm irrigation water is 0.21 µg/L and the linear range is 2-250 µg/L. After optimizing the chromatographic conditions, the highest efficiency of the hydrogel column in RPLC and HILIC modes has reached 32,400 plates/m (chlorobenzuron) and 41,300 plates/m (galactose). This new type of hydrogel composite is a porous network material with flexible functional design and simple preparation method and its application has been expanded in liquid chromatography separation successfully. The hydrogel composed of triallyl cyanate cross-linking agent and 3-(2-(methacryloyloxy) ethyl) dimethylamine) propane-1-sulfonate (SBMA) monomer which were co-modified on the surface of mesoporous silica with MOF-919 for separation in mixed-mode liquid chromatography.

Non-conjugated flexible network for the functional design of silica-based stationary phase for mixed-mode liquid chromatography

The functionalized non-conjugated organic network modified silica microspheres are proposed as the stationary phase of liquid chromatography, which can effectively avoid some defects of organic framework materials in liquid chromatography. Due to the co-existing of pyridine ring, carbonyl group, amide group and triazine ring in the non-conjugated flexible organic network on the silica surface, the developed mixed-mode stationary phase exhibits multiple interactions between the stationary phase and the analytes during the separation process. A variety of nucleoside bases, organic acids, antibiotics, pesticides, alkylbenzenes, polycyclic aromatic hydrocarbons and sulfonamides achieved ideal resolution and flexible selectivity in separation. Compared with the commercial chromatographic columns under their optimized chromatographic conditions, it shows better performance for the separation of complex analytes. The influence of chromatographic conditions on retention behavior indicates that the column's multiple retention mechanisms make it suitable for mixed-mode liquid chromatography. The stationary phase prepared by the new design strategy also has excellent chromatographic reproducibility, repeatability and stability with the intraday RSD of 0.09%-0.12% (n = 10) and the interday RSD of 0.37%-1.64% (n = 5) for the retention time. The separation results of actual samples also prove its potential in the analysis of complex samples. In short, we designed and prepared the non-conjugated flexible network modified silica stationary phase material for liquid chromatography that is different from organic framework materials. Its excellent separation ability shows that we have successfully reported a new kind of liquid chromatography packing with functional design and facile preparation method.

In situ fabrication of chiral covalent triazine frameworks membranes for enantiomer separation

Rapid and high-flux enantiomer separation is significant for drug development. Membrane separation technology provides promising approaches for enantiomer separations. Porous membrane with good selectivity and high permeability is an ideal choice for enantiomer separations. Herein, we demonstrate the preparation of a novel two-dimensional chiral covalent triazine frameworks (CCTF) membrane by "in situ growth" method. Inheriting the strong chirality and specific interactions from CCTF, the CCTF membranes exhibited good enantioselectivity for drug intermediates and drug, including (R)/(S)-1-phenylethanol, (R)/(S)-1,1'-binaphthol and (R)/(S)-ibuprofen. Under optimal separation conditions, the enantiomeric excess value (e.e %) was above 21.7 % for (R)/(S)-1-phenylethanol, 12.0% for (R)/(S)-1,1'-binaphthol and 9.7 % for (R)/(S)-ibuprofen. The mechanism of the CCTF recognizing enantiomers were simulated by quantum mechanical calculations. In addition, the mechanism was also proved by the separation of enantiomers using this CCTF-modified silica column in liquid chromatography. The CCTF membrane may bring about the potentially application for large-scale production of chiral compounds. Meanwhile, this work provides a theoretical guidance for the application of CCOFs in enantiomer separation.

Chiral covalent organic framework core-shell composite CTpBD@SiO2 used as stationary phase for HPLC enantioseparation

The fascinating framework structures and unique properties of chiral covalent organic frameworks (COFs) make them promising candidates as novel separation medium for high-performance liquid chromatography (HPLC). However, the irregular morphology, inhomogeneous particle size, and low density of conventional COF particles will lead to a low column efficiency, undesirable chromatographic peak shape, and high column backpressure of such COF-packed columns. In this work, a chiral COF CTpBD was synthesized by the Schiff base reaction between benzidine (BD) and chiral organic monomer CTp obtained through the reaction of 1,3,5-triformylphoroglucinol (Tp) and (+)-diacetyl-L-tartaric anhydride ((+)-Ac-L-Ta). The chiral COF CTpBD was immobilized on the surface of amino functionalized silica (SiO₂-NH₂) by an in situ growth approach to prepare the chiral COF core-shell microsphere composite CTpBD@SiO₂, which was used as a novel chiral stationary phase (CSP) for HPLC enantioseparation. Various kinds of racemates were separated on the CTpBD@SiO₂-packed column with a low column backpressure (8-11 bar). Some effects such as the analyte mass and column temperature on the HPLC enantioseparation have been studied in detail. The fabricated CTpBD@SiO₂-packed column exhibited high column efficiency (e.g., 16,800 plates m^-1 for atenolol), high enantioselectivity, and good reproducibility toward various racemates. The highest resolution value, retention factor, and separation factor reach to 2.11, 2.85, and 3.73, respectively. The relative standard deviations (RSD) of peak area, peak height, half-peak width, and retention time of atenolol were all below 3.0%.

Striped covalent organic frameworks modified stationary phase for mixed mode chromatography

Covalent organic frameworks (COFs) have showed expected potential in chromatographic separation due to unique structure and excellent performance. Nowadays, COF materials applied as chromatographic stationary phases is still in its infancy. Here, we modified COF materials on silica using benzene-1,4,5-tetracarboxylic dianhydride (PMDA) and 1,3,5-tris-(4-aminophenyl)triazine (TAPT) monomers by one-pot synthetic method for performing mixed-mode function, named as SiO₂@COF. Five characterization methods including thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FT-IR), elemental analysis (EA) and powder X-ray diffraction (XRD) verified the morphology, structure characteristics and physicochemical properties of the materials. SiO₂@COF for performing the separation of polar and nonpolar analytes on high performance liquid chromatography (HPLC) achieved the desired results. Retention mechanisms of the constructed SiO₂@COF were researched via observing the effects of mobile phase with retention times. Results exhibited that the prepared stationary phase can provide various interaction modes, including hydrophobic, hydrophilic, hydrogen bonding and π-π interactions. In conclusion, the prepared SiO₂@COF stationary phase can execute mixed-mode separation abilities and show potential for complex samples analysis.

β-Cyclodextrin covalent organic framework-modified organic polymer monolith as a stationary phase for combined hydrophilic and hydrophobic aqueous capillary electrochromatographic separation of small molecules

A β-Cyclodextrin covalent organic framework (β-CD COF) was successfully prepared under ambient temperature with a mild chemistry strategy from heptakis(6-amino-6-deoxy)-β-cyclodextrin and terephthalaldehyde. It was embedded into the poly[(glycidyl methacrylate)-co-(ethylene dimethacrylate)] [poly(GMA-co-EDMA)] monolith and served as the β-CD COF material-incorporated monolith. The synthetic materials were characterized by field emission scanning electron microscopy, energy-dispersive X-ray mapping analysis, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, and N₂ adsorption-desorption isotherm. The β-CD COF material-incorporated monolith achieved baseline separation in capillary electrochromatographic separation of three amides, three amino acids, three nucleosides, four aromatic acids, and three positional isomers (with resolution values of three amides, 1.75 and 1.54; three amino acids, 5.24 and 1.75; three nucleosides, 2.56 and 1.77; four aromatic acids, 6.96, 2.74, and 1.64; three positional isomers, 1.61 and 1.50). In comparison with the original monolith, the β-CD COF material-incorporated monolith shows significantly enhanced resolution for mixed molecules. The effect of pH and concentration of buffer and applied voltage were discussed in detail. The fabricated monolith showed good stability and reproducibility (relative standard deviation (RSD) < 6.9%). Molecular modeling illuminated the interactions between the small molecules and stationary phase, and provided a sufficient theoretical basis for experimental data. Graphical abstract Schematic presentation of the preparation of β-cyclodextrin covalent organic framework (β-CD COF) material-incorporated organic polymer monolith for separating the amides, amino acids, nucleosides, aromatic acids, and positional isomers. β-CD COF materials were synthesized and incorporated into the monolith as the stationary phase. Then, the incorporated monolith was applied in the capillary electrochromatography system for separating small molecules.

Covalent organic frameworks for separation applications

Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with highly tuneable structures and functionalities. COFs have been proposed as ideal materials for applications in the energy-intensive field of molecular separation due to their notable intrinsic features such as low density, exceptional stability, high surface area, and readily adjustable pore size and chemical environment. This review attempts to highlight the key advancements made in the synthesis of COFs for diverse separation applications such as water treatment or the separation of gas mixtures and organic molecules, including chiral and isomeric compounds. Methods proposed for the fabrication of COF-based columns and continuous membranes for practical applications are also discussed in detail. Finally, a perspective regarding the remaining challenges and future directions for COF research in the field of separation has also been presented.

Controlled Fabrication of Silica@Covalent Triazine Polymer Core-Shell Spheres as a Reversed-Phase/Hydrophilic Interaction Mixed-Mode Chromatographic Stationary Phase

The unique properties of covalent triazine-based organic framework/polymers, including large surface area, hydrophilic-lipophilic-balanced adsorption, and economical preparation, make it a promising candidate as a stationary phase for high-performance liquid chromatography. However, irregular shapes and wide size distributions of such particles hinder column packing, resulting in a low column efficiency or a high back pressure. Herein, we describe the fabrication of SiO₂@ covalent triazine-based organic polymer (CTP) core-shell microspheres with a distinct sphere-coating-sphere appearance using aminosilica as the supporting substrate to grow the CTP shell. By adjusting the amount of reactants, the thickness of the CTP shell, which consists of triazine and 1,3,5-triphenylbenzene monomers, was easily controlled. The developed core-shell microspheres were characterized via scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, solid-state ¹³C nuclear magnetic resonance analysis, and N₂ adsorption experiments. The synergism of the triazine and aromatic moieties on CTP provides the new stationary phase with multiple retention mechanisms, including hydrophobic, π-π, electron donor-acceptor, hydrogen-bonding interactions, and so forth. On the basis of these interactions, successful separation and higher shape selectivity were achieved among several analytes that vary in polarity under both reversed-phase and hydrophilic interaction liquid chromatography conditions. Therefore, SiO₂@CTP microspheres combine the advantages of good column packing properties of the uniform monodisperse silica microspheres and the recognition performance of CTP, generating flexible selectivity and application prospect for both hydrophilic and hydrophobic analytes.

Well-Defined Materials for High-Performance Chromatographic Separation

Chromatographic separation has been widely applied in various fields, such as chemical engineering, precision medicine, energy, and biology. Because chromatographic separation is based on differential partitioning between the mobile phase and stationary phase and affected by band dispersion and mass transfer resistance from these two phases, the materials used as the stationary phase play a decisive role in separation performance. In this review, we discuss the design of separation materials to achieve the separation with high efficiency and high resolution and highlight the well-defined materials with uniform pore structure and unique properties. The achievements, recent developments, challenges, and future trends of such materials are discussed. Furthermore, the surface functionalization of separation ma-terials for further improvement of separation performance is reviewed. Finally, future research directions and the challenges of chromatographic separation are presented.

Covalent organic framework incorporated chiral polymer monoliths for capillary electrochromatography

A covalent organic framework, Schiff base network-1 (SNW-1), was synthesized and incorporated into cellulase based poly(glycidyl methacrylate-co-ethylene dimethacrylate) (cellulase@poly(GMA-EDMA-SNW-1)) monolith to afford a novel chiral stationary phase for capillary electrochromatography (CEC). SNW-1 is attractive as a stationary phase for CEC because it not only features high surface areas but also provides conjugate structures and abundant amine groups to give π-π electrostatic stacking and hydrogen bonding property. Incorporation of SNW-1 into monolithic column could improve the column efficiency and increase the interactions between the tested racemates and the stationary phase thus significantly improved their CEC separation. The obtained monoliths were characterized by scanning electron microscopy, elemental analysis and nitrogen adsorption. Moreover, effects of SNW-1 concentration, immobilization pH of cellulase and CEC conditions were also investigated. Under the optimized conditions, the cellulase@poly(GMA-EDMA-SNW-1) monolith exhibited excellent enantioseparation performance for eight pairs of different classes of chiral drugs including β-blockers, antihistamines and anticoagulants. Satisfactory repeatability was achieved with relative standard deviations for intra-day, inter-day and column-to-column runs less than 4.5%, and batch-to-batch runs less than 6.8%. The experiment results reveal that the combination of the versatile features of monoliths and unique properties of SNW-1 could be a promising strategy for chiral separation.

Emerging porous materials in confined spaces: from chromatographic applications to flow chemistry

Porous materials confined within capillary columns/microfluidic devices are discussed, and progress in chromatographic and membrane separations and catalysis is reviewed.

Porous Organic Frameworks: Advanced Materials in Analytical Chemistry

Porous organic frameworks (POFs), a general term for covalent-organic frameworks (COFs), covalent triazine frameworks (CTFs), porous aromatic frameworks (PAFs), etc., are constructed from organic building monomers with strong covalent bonds and have generated great interest among researchers. The remarkable features, such as large surface areas, permanent porosity, high thermal and chemical stability, and convenient functionalization, promote the great potential of POFs in diverse applications. A critical overview of the important development in the design and synthesis of COFs, CTFs, and PAFs is provided and their state-of-the-art applications in analytical chemistry are discussed. POFs and their functional composites have been explored as advanced materials in "turn-off" or "turn-on" fluorescence detection and novel stationary phases for chromatographic separation, as well as a promising adsorbent for sample preparation methods. In addition, the prospects for the synthesis and utilization of POFs in analytical chemistry are also presented. These prospects can offer an outlook and reference for further study of the applications of POFs.

Advances in covalent organic frameworks in separation science

Covalent organic frameworks (COFs) are a new class of multifunctional crystalline organic polymer constructed with organic monomers via robust covalent bonds. The unique properties such as convenient modification, low densities, large specific surface areas, good stability and permanent porosity make COFs great potential in separation science. This review shows the state-of-the art for the application of COFs and their composites in analytical separation science. COFs and their composites have been explored as promising sorbents for solid phase extraction, potential coatings for solid phase microextraction, and novel stationary phases for gas chromatography, high-performance liquid chromatography and capillary electrochromatography. The prospects of COFs for separation science are also presented, which can offer an outlook and reference for further study on the applications of COFs.

Layer-by-layer preparation of 3D covalent organic framework/silica composites for chromatographic separation of position isomers

A layer-by-layer approach was developed to fabricate COF-300@SiO₂ for HPLC separation of position isomers.