A β-cyclodextrin covalent organic framework used as a chiral stationary phase for chiral separation in gas chromatography

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.

Modulating the cavity micro-environments of Fe-MOFs for high-performance gas chromatographic separations

Precise modulation of cavity micro-environment in metal-organic frameworks (MOFs) is important for achieving significant separation performance. Herein, the Fe-MOF (MIL-142-BTB-BDC) with different tridentate and bidentate ligands to form cavities, was chosen as the platform to precisely modulate the cavity micro-environment and investigate the influence of cavity windows and cavity walls on gas chromatographic separations. Changing tridentate ligands on the cavity walls led to MIL-142-TATB-BDC while changing bidentate ligands on the cavity windows produced MIL-142-BTB-BDC-NH2. Mechanism investigation revealed that for MIL-142-BTB-BDC and MIL-142-TATB-BDC, altering the ligands of cavity walls had little influence on the thermodynamic interactions between MOFs and analytes while slightly reducing the kinetic diffusion rate of analytes. On the contrary, introducing amino groups on cavity windows in MIL-142-BTB-BDC-NH2 not only increased the thermodynamic interactions with analytes but also slowed down the kinetic diffusion of analytes, which resulted in poor separation performance of the MIL-142-BTB-BDC-NH2 coated column. This work provides a guide to precisely modulating the cavity micro-environment and analyzing the relationship between the cavity micro-environment and application properties of MOFs.

Chiral ionic organic single-crystal and its exfoliated two-dimensional nanosheets with enhanced enantioseparation

A chiral ionic organic single-crystal (CIOC) was prepared for the first time through ionic self-assembly using bipyridium chiral ionic liquid (CIL) and 4,4'-biphenyldisulfonic acid (BDA). The CIOC can be ultrasonically exfoliated to produce two-dimensional nanosheets (2D-NSs). The 2D-NSs presented enhanced enantioseparation compared to the CIOC and CIL when used as gas chromatography stationary phase, which may be due to the exfoliated 2D-NSs exhibiting greater exposure of functional groups. Additionally, better resolution of other organic compounds such as positional isomers, n-alkanes and n-alkanols, Grob mixture, phenols and anilines was obtained in 2D-NSs than CIOC and CIL. This work not only provides a reference for the preparation of chiral ionic organic single-crystals and two-dimensional nanosheets for chiral separation, but also stimulates the preparation of such new ionic organic single-crystals via self-assembly for other potential applications.

Preparation of alkyl microporous organic network-based capillary column for an efficient gas chromatographic separation of position isomers

The large surface area, excellent thermal stability and easy modification make microporous organic networks (MONs) good candidates in the field of gas chromatography (GC). Due to the limited species and highly conjugated networks of MONs, their applications are still in infancy and restricted. To accelerate their developments and to enrich their types in GC, here we report the first example of synthesizing alkyl MON and its capillary column for GC separation of position isomers. Linear 1,8-dibromooctane is used as the alkyl monomer instead of traditional aromatic ones to construct novel alkyl MON to decrease the inherent conjugated characteristic of MONs. The alkyl MON exhibits good thermal stability (up to 350°C), large surface area (1173 m2 g-1), and non-polar character, allowing good resolution for alkanes, alkyl benzenes, alcohols, ketones, and diverse position isomers, including dichlorobenzene, trichlorobenzene, bromotoluene, nitrotoluene, methylbenzaldehyde, and ionone with the limits of detection (0.003 mg mL-1) and limits of quantitation of (0.10 mg mL-1). The in situ growth-prepared alkyl MON column demonstrates remarkable duration time and precisions for the retention relative standard deviations, (RSDs%, intra-day, n = 7), 0.06%-0.53% (intra-day, n = 7), and 2.87%-10.59% (column-to-column, n = 3). In addition, the fabricated alkyl MON-coated capillary column offers better resolution than three commercial GC columns for the resolution of methylbenzaldehyde, bromotoluene, and chlorotoluene isomers. This work reveals the practicability for synthesizing alkyl MONs and demonstrates their prospects for position isomers separation.

Subcomponent self-assembly construction of tetrahedral cage FeII4L4 for high-resolution gas chromatographic separation

Metal organic cages (MOCs), as an emerging discrete supramolecular compounds, have received widespread attention in separation, biomedicine, gas capture, catalysis, and molecular recognition due to their porosity, adjustability and stability. Herein, we present a new chiral MOC FeII4L4 coated capillary column prepared for gas chromatographic (GC) separation of different types of organic compounds, including n-alkanes, n-alcohols, alkylbenzenes, isomers, especially for racemic compounds. There are 20 different kinds of racemates (e.g., alcohols, ethers, epoxides, esters, alkenes, and aldehydes) were well resolved on the FeII4L4 chiral column and a maximum resolution value for 1-phenyl-1-propanol reaches 6.17. The FeII4L4 coated column exhibited high column efficiency (3100 plates m-1 for n-dodecane) and good enantiomeric resolution complementary to that of a commercial β-DEX 120 column and the previously reported chiral MOC [Fe4L6] (ClO4)8 coated column. The relative standard deviation (RSDs) of the peak area and retention time of glycidol and nitrotoluene were below 1.2 %. This study reveals that chiral MOCs have good application prospects in chromatographic separation.

Surface molecularly imprinted polymer/covalent organic framework/silica composite material with specific recognition ability and excellent chromatographic performance

Facing with the difficulty of specific chromatographic separation of nucleoside drugs, this study prepared a surface molecularly imprinted polymer (SMIP) modified covalent organic framework (COF) coated silica stationary phase based on the specificity of molecular imprinting technology and the powerful chromatographic separation performance of COF. This novel SMIP-COF@SiO2 stationary phase can not only specifically identify template molecule and structural analogs, but can also be used to separate multiple types of analytes, such as B vitamins, sulfonamides, alkylbenzenes, phenyl ketones, polycyclic aromatic hydrocarbons and environmental endocrine disruptors, which satisfies the need for complex sample separation. Various retention mechanisms have been investigated and multiple interactions between the SMIP-COF@SiO2 stationary phase and the analytes are discovered. The chromatographic performance of SMIP-COF@SiO2 is far superior to that of the SMIP@SiO2 and COF@SiO2. Furthermore, the SMIP-COF@SiO2 stationary phase can be successfully used to analyze polycyclic aromatic hydrocarbons in the environmental water sample and detect whitening ingredient in skincare product, indicating its great potential for application in various fields.

Solvent and monomer regulation synthesis of core-shelled magnetic β-cyclodextrin microporous organic network for efficient extraction of estrogens in biological samples prior to HPLC analysis

The abnormal estrogens levels in human body can cause many side effects and diseases, but the quantitative detection of the trace estrogens in complex biological samples still remains great challenge. Here we reported the fabrication of a novel core-shell structured magnetic cyclodextrin microporous organic network (Fe3O4@CD-MON) for rapid magnetic solid phase extraction (MSPE) of four estrogens in human serum and urine samples prior to HPLC-UV determination. The uniform spherical core-shell Fe3O4@CD-MONs was successfully regulated by altering the reactive monomers and solvents. The Fe3O4@CD-MONs owned high specific surface area, good hydrophobicity, large superparamagnetism, and abundant extraction sites for estrogens. Under optimal conditions, the proposed MSPE-HPLC-UV method provided wide linearity range (2.0-400 μg L-1), low limits of detection (0.5-1.0 μg L-1), large enrichment factors (183-198), less adsorbent consumption (3 mg), short extraction time (3 min), and good stability and reusability (at least 8 cycles). The established method had also been successfully applied to the enrichment and detection of four estrogens in serum and urine samples with a recovery of 88.4-105.1 % and a relative standard deviation of 1.0-5.9 %. This work confirmed the feasibility of solvent and monomer regulation synthesis of Fe3O4@CD-MON composites, and revealed the great prospects of magnetic CD-MONs for efficient enrichment of trace estrogens in complex biological samples.

Asymmetric catalytic synthesis of chiral covalent organic framework composite (S)-DTP-COF@SiO2 for HPLC enantioseparations by normal-phase and reversed-phase chromatographic modes

A novel CCOF core-shell composite material (S)-DTP-COF@SiO2 was prepared via asymmetric catalytic and in situ growth strategy. The prepared (S)-DTP-COF@SiO2 was utilized as separation medium for HPLC enantioseparation using normal-phase and reversed-phase chromatographic modes, which displays excellent chiral separation performance for alcohols, esters, ketones, and epoxides, etc. Compared with chiral commercial chromatographic columns (Chiralpak AD-H and Chiralcel OD-H columns) and some previously reported chiral CCOF@SiO2 (CC-MP CCTF@SiO2 and MDI-β-CD-modified COF@SiO2)-packed columns, there are 4, 3, 13, and 15 tested racemic compounds that could not be resolved on the Chiralpak AD-H column, Chiralcel OD-H column, CC-MP CCTF@SiO2 column, and MDI-β-CD-modified COF@SiO2 column, respectively, which indicates that the resolution effect of (S)-DTP-COF@SiO2-packed column can be complementary to the other ones. The effects of the analyte mass, column temperature, and mobile phase composition on the enantiomeric separation were investigated. The chiral column exhibits good reproducibility after multiple consecutive injections. The RSDs (n = 5) of the peak area and retention time were less than 1.5% for repetitive separation of 2-methoxy-2-phenylethanol and 1-phenyl-1-pentanol. The chiral core-shell composite (S)-DTP-COF@SiO2 exhibited good enantiomeric separation performance, which not only demonstrates its potential as a novel CSP material in HPLC but also expands the range of applications for chiral COFs.

Recent progress for chiral stationary phases based on chiral porous materials in high-performance liquid chromatography and gas chromatography separation

Chirality is a fundamental property of nature. Separation and analysis of racemates are of great importance in the fields of medicine and the production of chiral biopharmaceutical intermediates. Chiral chromatography has the characteristics of a wide separation range, fast separation speed, and high efficiency. The development and preparation of novel chiral stationary phases with good chiral recognition and separation capacity is the core and key of chiral chromatographic separation and analysis. In this work, the representative research progress of novel chiral porous crystal materials including chiral covalent organic frameworks, chiral porous organic cages, chiral metal-organic frameworks, and chiral metal-organic cages used as chiral stationary phases of capillary gas chromatography and high-performance liquid chromatography over the last 4 years is reviewed in detail. The chiral recognition and separation properties of the representative studies in this review are also introduced and discussed.

Click preparation of triazole-bridged teicoplanin-bound chiral stationary phases for efficient separating amino acid enantiomers

Enantioseparation of amino acids is considered as a challenging task due to the extreme structural similarity of their enantiomers. Herein, teicoplanin was modified with different chemical equivalents of azide groups and attached to silica particles by employing Click Chemistry for resolution of chiral amino acids for the first time. Interestingly, teicoplanin modified with 5-fold the chemical equivalent of azide groups (TK-2 CSP) exhibited superior amino acid separation ability compared to two other columns: one modified with only 1-fold the chemical equivalent of azide groups (TK-1 CSP), and the other modified with excess azide groups (TK-3 CSP). Additionally, the TK-2 CSP exhibited superior enantioselectivity when separating amino acids containing hydrophobic alkyl side chains in comparison to other teicoplanin-based CSPs. The TK-2 CSP column allows the baseline separation of 7 native amino acids. Molecular docking demonstrates that effective enantioseparation arises from distinct patterns of interaction between the host and guest molecules. Moreover, (p-methyl) phenylcarbaminoylated-teicoplanin CSP (TK-4, TK-5 CSP) were prepared by post-modification from TK-1 CSP and TK-2 CSP to isolate Fmoc-modified amino acids. This work explores the impact of various modification methods on the enantioseparation effects of host molecules and paves the way for expanding the potential applications of teicoplanin and macrocyclic glycopeptide molecules.

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.

Preparation and evaluation of two chiral stationary phases based on imidazolyl-functionalized bromoethoxy pillar[5]arene-bonded silica

Chiral pillar[5]arene-based mesoporous silica, an emerging class of chiral structure, possesses excellent characteristics such as abundant chiral active sites, encapsulated cavity and excellent chiral modification, which make them a promising candidate as new chiral stationary phases (CSPs) in enantioseparation. In this study, two imidazole-containing (S)-1-(4-phenyl-1H-imidazol-2-yl)ethanamine and (S)-Histidinol were respectively modified to bromoethoxy pillar[5]arene-bonded silica to construct new chiral stationary phases (sPIE-BP5-Sil and sHol-BP5-Sil) for the separation and analysis of enantiomers. The separation conditions such as mobile phase composition, flow rate and temperature were optimized. Under optimal conditions, both sPIE-BP5-Sil and sHol-BP5-Sil showed good separation performance for different types of enantiomers. Interestingly, sPIE-BP5-Sil and sHol-BP5-Sil showed better enantioselectivity for chiral aromatic compounds and chiral aliphatic compounds, respectively. This enantioseparation result was closely related to the presence of additional aromatic rings and abundant hydroxyl groups in the side chains of the two chiral groups. In addition, the enantioseparation process was further studied by molecular docking simulation. Therefore, this work provided a new strategy for the preparation and application of imidazolyl-derived pillar[5]arene-based chiral stationary phases, which can be efficiently used for screening and separating enantiomers.

Polar alcohol guest molecules regulate the stacking modes of 2-D MOF nanosheets

The modulation of two-dimensional metal-organic framework (2-D MOF) nanosheet stacking is an effective means to improve the properties and promote the application of nanosheets in various fields. Here, we employed a series of alcohol guest molecules (MeOH, EtOH and PrOH) to modulate Zr-BTB (BTB = benzene-1,3,5-tribenzoate) nanosheets and to generate untwisted stacking. The distribution of stacking angles was statistically analyzed from high-angle annular dark-field (HAADF) and fast Fourier transform (FFT) images. The ratios of untwisted stacking were calculated, such as 77.01% untwisted stacking for MeOH, 83.45% for EtOH, and 85.61% for PrOH. The obtained untwisted Zr-BTB showed good separation abilities for different substituted benzene isomers, superior para selectivity and excellent column stability and reusability. Control experiments of 2-D Zr-TCA (TCA = 4,4',4''-tricarboxytriphenylamine) and Zr-TATB (TATB = 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzoic acid) nanosheets with similar pore sizes and stronger polarity regulated by the alcohol guests exhibited moderate separation performance. The electron microscopy images revealed that polar alcohol regulation dominantly generated the twisted stacking of Zr-TCA and Zr-TATB with various Moiré patterns. Polar guest molecules, such as alcohols, provide strong host-guest interactions during the regulation of MOF nanosheet stacking, providing an opportunity to design new porous Moiré materials with application prospects.

Increasing Mass Transfer Resistance of MOFs as a Reverse Tuning Strategy to Achieve High-Resolution Gas Chromatographic Separation

In separation science, precise control and regulation of the MOF stationary phase are crucial for achieving a high separation performance. We supposed that increasing the mass transfer resistance of MOFs with excessive porosity to achieve a moderate mass transfer resistance of the analytes is the key to conducting the MOF stationary phase with a high resolution. Three-dimensional UiO-67 (UiO-67-3D) and two-dimensional UiO-67 (UiO-67-2D) were chosen to validate this strategy. Compared with UiO-67-3D with overfast mass transfer and low retention, the reduced porosity of UiO-67-2D increased the mass transfer resistance of analytes in reverse, resulting in improved separation performance. Kinetic diffusion experiments were conducted to verify the difference in mass transfer resistance of the analytes between UiO-67-3D and UiO-67-2D. In addition, the optimization of the UiO-67-2D thickness for separation revealed that a moderate diffusion length of the analytes is more advantageous in achieving the equilibrium of absorption and desorption.

Ionic liquid/covalent organic framework/silica composite material: Green synthesis and chromatographic evaluation

BACKGROUND

Due to their large surface area and distinctive adsorption affinity, covalent organic frameworks (COFs) appear to be good candidates as liquid chromatographic separation materials with good application prospect. The development of COF materials in chromatographic science is currently in an exploratory stage. Especially, the practicability of COF@silica composite materials as liquid chromatographic stationary phases needs further exploration. Reasonably integrating a functional component such as ionic liquid (IL) into the COF@silica composite materials may provide customized functionality to achieve the purpose of synthesizing multi-functional COF based stationary phases.

RESULTS

In this study, an IL modified COF bonded silica composite material (IL-COF@SiO2) was successfully synthesized by using an environmentally friendly deep eutectic solvent as the reaction medium instead of the frequently-used organic solvent. The synthesized IL-COF@SiO2 composite material combines the excellent separation ability of COF and the excellent mass transfer function of spherical porous silica microsphere, and meanwhile, the introduction of IL endows COF@SiO2 with preferable separation performance. The slurry-packed IL-COF@SiO2 liquid chromatographic column could be applied to effectively separate hydrophobic and hydrophilic compounds with preferable separation selectivity and high column efficiency. By investigating the retention behavior and influencing factors, a mixed-mode retention mechanism was found. Multiple interaction forces endow the IL-COF@SiO2 with a hydrophilic-hydrophobic balance performance, demonstrating a good application prospect as a versatile liquid chromatographic separation material.

SIGNIFICANCE

In this study, a new strategy is proposed for greenly synthesizing a novel IL-COF@SiO2 composite material under mild conditions, which expands the potential application of COF materials in chromatographic science. One particular point to note is that the reaction medium in each step of the preparation process is low toxic and degradable deep eutectic solvent, which conforms to the concept of green chemistry.

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.

Recent advances in chiral liquid chromatography stationary phases for pharmaceutical analysis

Chirality is a common phenomenon in nature. Different enantiomers of chiral drug compounds have obvious differences in their effects on the human body. Therefore, the separation of chiral drugs plays an extremely important role in the safe utilization of drugs. High-performance liquid chromatography (HPLC) is an effective tool for the separation and analysis of compounds, in which the chromatographic packing plays a key role in the separation. Chiral pharmaceutical separation and analysis in HPLC rely on chiral stationary phases (CSPs). Thus, various CSPs are being developed to meet the needs of chiral drug separation and analysis. In this review, recent developments in CSPs, including saccharides (cyclodextrin, cellulose, amylose and chitosan), macrocycles (macrocyclic glycopeptides, pillar[n]arene and polyamide) and porous organic materials (metal-organic frameworks, covalent organic frameworks, and porous organic cages), for pharmaceutical analysis in HPLC were summarized, the advantages and disadvantages of various stationary phases were introduced, and their development prospects were discussed.

Tetraethylenepentamine-derived carbon dots and tetraethylenepentamine co-immobilized silica stationary phase for hydrophilic interaction chromatography

In this work, tetraethylenepentamine (TEPA) was used as precursor and reaction medium to prepare tetraethylenepentamine-functionalized carbon dots (TEPACDs), the resultant mixture was subsequently silanized and then grafted on the surface of bare silica. The obtained tetraethylenepentamine-functionalized carbon dots and tetraethylenepentamine co-modified silica stationary phase (Sil-TEPA/CDs) was characterized by multiple ways, such as Fourier transformed infrared spectroscopy (FTIR), elemental analysis and transmission electron microscope, which revealed the successful preparation of the mixed stationary phase and higher density of functional groups on co-modified stationary phase than precursor single-modified stationary phase. The synergistic effect of TEPACDs and TEPA was proved by comparing the separation performance of Sil-TEPA/CDs and Sil-TEPA toward amino acids, nucleosides, and nucleobases, which distinctly enhanced the selectivity of Sil-TEPA/CDs. Thus, 12 nucleosides and nucleobases and 11 amino acids was nicely separated on Sil-TEPA/CDs. By study the influences of the changes of mobile phase composition, mobile phase buffer concentration and buffer pH on the retention behaviors of Sil-TEPA and Sil-TEPA/CDs, it was found that both hydrophilic partitioning and adsorption of analytes on Sil-TEPA/CDs were enhanced benefit from the co-existence of TEPA and TEPACDs, which provided the analytes better separation performance. By comparing the column quality of Sil-TEPA/CDs with four commercially available columns, Sil-TEPA/CDs exhibited the best peak asymmetry of 0.98, and second best column efficiency of 43895 m-1 using guanosine as analyte. The RSD (n = 9) of the retention times of five selected analytes on Sil-TEPA/CDs were within 0.30-0.61% during 40 h of continuously elution, which implied excellent stability of prepared packing material.

Recent development of chiral ionic liquids for enantioseparation in liquid chromatography and capillary electrophoresis: A review

Ionic liquids (ILs), which are salts in a molten state below 100 °C, have become a hot topic of research in various fields because of their negligible vapour pressure, high thermal stability, and tunable viscosity. Chiral ionic liquids (CILs) can be applied in chromatography and capillary electrophoresis fields to improve the performance of enantiomeric separation, such as chiral stationary phases (CSPs) and mobile phase additives in high-performance liquid chromatography (HPLC); CSPs in gas chromatography (GC); and background electrolyte additives (BGE), chiral ligands and chiral selectors (CSs) in capillary electrophoresis (CE). This review focuses on the applications of CILs in HPLC and CE for the separation of enantiomers in the past five years. The mechanism for separating enantiomers was explained, and the prospect of the application of CILs in chiral liquid chromatography (LC) and CE analysis was also discussed.

Homochiral Metallacycle Used as a Stationary Phase for Capillary Gas Chromatographic Separation of Chiral and Achiral Compounds

Metallacycles are a novel class of supramolecular materials with circular structures, internal cavities, and abundant host-guest chemical properties that have exhibited good application prospects in many fields. However, to the best of our knowledge, no research on the use of metallacycles as stationary phases for gas chromatographic (GC) separations has been published yet. In this work, we report for the first time the use of a homochiral metallacycle, [ZnCl2L]2, as a stationary phase for GC separations. [ZnCl2L]2 was synthesized by reaction of (S)-(1-isonicotinoylpyrrolidin-2-yl)methyl-isonicotinate (L) with ZnCl2 via coordination-driven self-assembly. The [ZnCl2L]2-coated column displayed an excellent separation performance not only of organic isomers but also of racemic compounds. Sixteen racemates (including alcohols, esters, amino acid derivatives, ethers, organic acids, and epoxides) and 21 isomeric compounds (including positional, structural, and cis/trans-isomers) were well separated on the [ZnCl2L]2-coated column. Impressively, some racemates were resolved with high resolution values (Rs), including 1,2-butanediol diacetate (Rs = 25.86), ethyl 3-hydroxybutyrate (Rs = 20.97), 1,3-butanediol diacetate (Rs = 18.09), and threonine derivative (Rs = 18.61). Compared with the commercial β-DEX 120 column for separation of the tested racemates, the [ZnCl2L]2-coated column exhibited good enantioseparation complementarity, enabling separation of some racemates that could not be separated, or were not well resolved, by the β-DEX 120 column. In addition, many organic mixtures, such as n-alkanes, alkylbenzenes, n-alcohols, and a Grob test mixture, were also well separated on the [ZnCl2L]2-coated column. The column also has good reproducibility and thermal stability on separation. This work not only reveals the great potential of metallacycles for GC separations but also opens up a new application of metallacycles in separation science.

Trifluoromethyl-Functionalized 2D Covalent Organic Framework for High-Resolution Separation of Isomers

Development of novel functional materials for effective isomer separation is of great significance in environmental science, chemical industry, and life science due to the different functions of isomers. However, the similar physicochemical properties of isomers make their separation greatly challenging. Here, we report the fabrication of trifluoromethyl-functionalized 2D covalent organic framework (COF) TpTFMB with 2,2'-bis(trifluoromethyl)benzidine (TFMB) and 1,3,5-triformylphloroglucinol (Tp) for the separation of isomers. TpTFMB was in situ-grown on the inner surface of a capillary for the high-resolution separation of isomers. The introduction of hydroxyl and trifluoromethyl functional groups with uniform distribution in 2D COFs is a powerful tactic to endow TpTFMB with various functions such as hydrogen bonding, dipole interaction, and steric effect. The prepared TpTFMB capillary column enabled the baseline separation of positional isomers such as ethylbenzene and xylene, chlorotoluene, carbon chain isomers such as butylbenzene and ethyl butanoate, and cis-trans isomers 1,3-dichloropropene. The hydrogen-bonding, dipole, and π-π interactions as well as the structure of COF significantly contribute to the isomer separation. This work provides a new strategy for designing functional 2D COFs for the efficient separation of isomers.

Composite materials based on covalent organic frameworks for multiple advanced applications

Covalent organic frameworks (COFs) stand for a class of emerging crystalline porous organic materials, which are ingeniously constructed with organic units through strong covalent bonds. Their excellent design capabilities, and uniform and tunable pore structure make them potential materials for various applications. With the continuous development of synthesis technique and nanoscience, COFs have been successfully combined with a variety of functional materials to form COFs-based composites with superior performance than individual components. This paper offers an overview of the development of different types of COFs-based composites reported so far, with particular focus on the applications of COFs-based composites. Moreover, the challenges and future development prospects of COFs-based composites are presented. We anticipate that the review will provide some inspiration for the further development of COFs-based composites.

A chiral multi-shelled mesoporous carbon nanospheres used for high-resolution gas chromatography separations

Herein, a chiral multishelled mesoporous carbon nanospheres (MCNs) with unique spiral multishelled hollow mesoporous chiral structure is synthesized; the MCNs can be used as stationary phases for high-resolution gas chromatography (GC) and have good separation capacity. The successful preparation of MCNs is verified by a variety of characterizations. In addition, the MCNs-coated capillary column shows excellent separation performance for n-alkanes, n-alcohols, aromatic compounds, and esters, and it has a faster analysis time than the HP-5 commercial capillary column. The chromatography separation performance for various isomers and racemates of the MCNs stationary phase was evaluated, and it showed good separation capability for amino acid derivatives. The MCNs-coated capillary column has been demonstrated to present good reproducibility and stability. In summary, all of the chromatography experiments in this work indicate that this new stationary phase of the MCNs has good application potential for GC capillary separation.

Tracking the Dissolution Surface Kinetics of a Single Fluorescent Cyclodextrin Metal-Organic Framework by Confocal Laser Scanning Microscopy

The understanding of the dissolution processes of solids is important for the design and synthesis of solids in a controlled and precise manner and for predicting their fate in the aquatic environment. We report herein single-particle-based confocal laser scanning microscopy (CLSM) for tracking the dissolution surface kinetics of a single fluorescent cyclodextrin metal-organic framework (CD-MOF). As a proof of concept, CD-MOF containing fluorescein, named as CD-MOF⊃FL, was synthesized by encapsulating fluorescein into the interior of CD-MOF via a vapor diffusion method and used as a single-particle dissolution model because of its high FL efficiency and unique structure. The morphology of CD-MOF⊃FL and the distribution of fluorescein within CD-MOF⊃FL were characterized. The growth and dissolution processes of CD-MOF⊃FL at the single-particle level were visualized and quantified for the first time by recording the change of the fluorescence emission. Three processes, including nucleation, germination growth, and saturation stage, were found in the growth of CD-MOF⊃FL, and the growth kinetics followed Avrami's model. The dissolution rate at the face of a single CD-MOF⊃FL crystal was slower than that of its arris, and the dissolution rate of the CD-MOF⊃FL crystal was increased with the increase of the water amount in methanol solution. The dissolution process of the CD-MOF⊃FL crystal was a competitive process of erosion and diffusion in different methanol aqueous solutions, and the dissolution kinetics followed the Korsmeyer-Peppas model. These results offer new insights into the nature of dissolution kinetics of CD-MOF⊃FL and provide new venues for the quantitative analysis of solid dissolution and growth at the single-particle level.

Chiral metal-organic frameworks and their composites as stationary phases for liquid chromatography chiral separation: A minireview

Chiral metal organic frameworks (CMOFs) are a kind of crystal porous framework material that has attracted increasing attention due to the customizable combination of metal nodes and organic ligands. In particular, the highly ordered crystal structure and rich adjustable chiral structure make it a promising material for developing new chiral separation material systems. In this review, the progress of CMOFs and their different types of composites used as chiral stationary phases (CSPs) in liquid chromatography for enantioseparation are discussed. The characteristics of CMOFs and their composites are summarized, aiming to provide new ideas for the development of CMOFs with better performance and further promote the application of CMOFs materials in enantioselective high-performance liquid chromatography (HPLC).

Macrocycle-Based Covalent Organic Frameworks

Macrocycles with well-defined cavities and the ability to undergo supramolecular interactions are classical materials that have played an essential role in materials science. However, one of the most substantial barriers limiting the utilization of macrocycles is their aggregation, which blocks the active regions. Among many attempted strategies to prevent such aggregation, installing macrocycles into covalent organic frameworks (COFs), which are porous and stable reticular networks, has emerged as an ideal solution. The resulting macrocycle-based COFs (M-COFs) preserve the macrocycles' unique activities, enabling applications in various fields such as single-atom catalysis, adsorption/separation, optoelectronics, phototherapy, and structural design of forming single-layered or mechanically interlocked COFs. The resulting properties are unmatchable by any combination of macrocycles with other substrates, opening a new chapter in advanced materials. This review focuses on the latest progress in the concepts, synthesis, properties, and applications of M-COFs, and presents an in-depth outlook on the challenges and opportunities in this emerging field.

Converting an amorphous covalent organic polymer to a crystalline covalent organic framework mediated by a repairing agent

We herein report a new approach to converting an amorphous covalent organic polymer to a crystalline heteropore covalent organic framework (COF), which is promoted by using an additive for structure repair. This provides a new method for the construction of COFs from cross-linked polymers.

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.

Cu Nanoparticle-Decorated Boron-Carbon-Nitrogen Nanosheets for Electrochemical Determination of Chloramphenicol

In the present work, irregular Cu nanoparticle-decorated boron-carbon-nitrogen (Cu-BCN) nanosheets were successfully synthesized. A Cu-BCN dispersion was deposited on a bare glassy carbon electrode (GCE) to prepare an electrochemical sensor (Cu-BCN/GCE) for the detection of chloramphenicol (CAP) in the environment. Cu-BCN was characterized using high-resolution scanning transmission electron microscopy (HRSTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS). The performance of the Cu-BCN/GCE was studied using electrochemical impedance spectroscopy (EIS), and its advantages were proven by electrode comparison. Differential pulse voltammetry (DPV) was used to optimize the experimental conditions, including the amount of Cu-BCN deposited, enrichment potential, deposition time, and pH of the electrolyte. A linear relationship between the CAP concentration and current response was obtained under the optimized experimental conditions, with a wide linear range and a limit of detection (LOD) of 2.41 nmol/L. Cu-BCN/GCE exhibited high stability, reproducibility, and repeatability. In the presence of various organic and inorganic species, the influence of the Cu-BCN-based sensor on the current response of CAP was less than 5%. Notably, the prepared sensor exhibited excellent performance in real-water samples, with satisfactory recovery.

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.

Construction of hydrophilic covalent organic frameworks and their fast and efficient adsorption of cationic dyes from aqueous solution

TFPB-Pa-SO3H COF and TFPB-BDSA COF were synthesized and showed fast adsorption of MLB (1 and 2 min) and high adsorption uptakes of CV (1559 and 1288 mg g−1). TFPB-Pa-SO3H COF as adsorbing material was used for the removal of dye molecules in real water samples.

Multi-sulfonated functionalized hydrophilic covalent organic framework for highly efficient dye removal from real samples

A hydrophilic covalent organic framework (BTA-BDSA-COF) was successfully erected by introducing multi-sulfonated groups into a covalent framework structure and it can be easily applied to capture the cationic dye in real water samples.