Engineering Thiol–Ene Click Chemistry for the Fabrication of Novel Structurally Well-Defined Multifunctional Cyclodextrin Separation Materials for Enhanced Enantioseparation

Facile synthesis of a new chiral polyimine macrocycle and its application for enantioseparation in high-performance liquid chromatography

Macrocyclic compounds such as crown ethers and cyclodextrins play an important role in the field of chromatography and show excellent separation performance. The design of simple and convenient methods for the efficient synthesis of novel chiral macrocycles for chromatographic separation is of great significance. In this work, a novel chiral polyimine macrocycle (PIMC) was designed and synthesized by the simply one-step reaction of 2,6-diformyl-4-tert-butylphenol with (S)-(-)-1,2-propanediamine. Then, it was bonded onto silica by the thiol-ene click reaction to construct a new chiral stationary phase (CSP) for high-performance liquid chromatography (HPLC). The chiral separation performance of the proposed CSP was examined by separating various racemates in the normal-phase (NP) and reversed-phase (RP) HPLC. In total, twelve and nine racemates, including ethers, esters, amines, alcohols, organic acids, ketones, and epoxides, were separated to varying degrees via NP-HPLC and RP-HPLC, respectively, Moreover, the CSP offered good chiral separation complementarity to Chiralcel OD-H and Chiralpak AD-H columns for resolution of these test racemates, and it can separate several racemic compounds that either cannot be separated or cannot be separated well be separated by the two commercially available columns. After the column was used for hundreds of injections, the relative standard deviations of the retention time and resolution were below 0.56 % and 0.45 %, respectively, showing the good reproducibility and stability of the CSP. This study provides a simple and convenient approach to synthesize a novel chiral macrocycle and CSP and also indicates the broad application prospects of such chiral PIMCs in HPLC chiral separation.

Preparation and evaluation of a 1,1'-bi-2-naphthol-based chiral macrocycle bonded silica chiral stationary phase for high performance liquid chromatography

Macrocycles play vital roles in supramolecular chemistry and chromatography. 1,1'-Bi-2-naphthol (BINOL)-based chiral polyimine macrocycles are an emerging class of chiral macrocycles that can be constructed by one-step aldehyde-amine condensation of BINOL derivatives with other building blocks. These macrocycles exhibit good characteristics, such as facile preparation, rigid cyclic structures, multiple chiral centers, and defined molecular cavities, that make them good candidates as new chiral recognition materials for chromatographic enantioseparations. In this study, a BINOL-based [2+2] chiral polyimine macrocycle was synthesized by one-step condensation of enantiopure (S)-2,2'-dihydroxy-[1,1'-binaphthalene]-3,3'-dicarboxaldehyde with (1R,2R)-1,2-diaminocyclohexane. The product was modified with 5-bromo-1-pentene and then attached to thiolated silica using click chemistry to construct a new chiral stationary phase (CSP). The enantioselectivity of the new CSP was explored by separating various racemates under normal phase (NP) and reversed phase (RP) high performance liquid chromatography (HPLC). Thirteen racemates and eight racemates were enantioseparated under the two separation modes, respectively, including chiral alcohols, phenols, esters, ketones, amines, and organic acids. Among them, nine racemates achieved baseline separation under NP-HPLC and seven racemates achieved baseline separation under RP-HPLC. High resolution separation was observed with benzoin (Rs = 5.10), epinephrine (Rs = 4.98), 3-benzyloxy-1,2-propanediol (Rs = 4.42), and 4,4'-dimethylbenzoin (Rs = 4.52) in NP-HPLC, and with 4-methylbenzhydrol (Rs = 4.72), benzoin ethyl ether (Rs = 3.79), 1-phenyl-1-pentanol (Rs = 3.68), and 1-(3-bromophenyl)ethanol (Rs = 3.60) in RP-HPLC. Interestingly, the CSP complemented Chiralcel OD-H, Chiralpak AD-H, and CYCLOBOND I 2000 RSP columns for resolution of these test racemates, separating several racemic compounds that could not be well separated by the three commercially available columns. The influences of injected sample amount on separation were also evaluated. It was found that the column exhibited excellent stability and reproducibility after hundreds of injections, and the relative standard deviations (n = 5) of the retention time and resolution were less than 0.49% and 0.69%, respectively. This study indicates that the BINOL-based chiral macrocycle has great potential for HPLC enantioseparation.

Fabrication of ionic liquid functionalized silica with different anions and the application in mixed-mode and chiral chromatography

To realize the potential of ionic liquid functionalized silica to prepare mixed-mode and chiral stationary phases, two ionic liquid silane reagents with different anions were synthesized via a high-efficiency click reaction. Then they were decorated onto the surface of silica by a one-step bonding reaction. The functionalized silica was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and elemental analysis (EA). Two stationary phases provided satisfactory performance when compared with a commercial mixed-mode column. Notably, Sil-C10Im-D-BCS with D-3-bromocamphor-8-sulfonate (D-BCS) as anion presented chiral separation capacity towards 1,2,3,4-Tetrahydro-1-naphthol. The separation mechanism was investigated through multiple pathways, and the results revealed that the prepared stationary phases can retain and separate solutes through multiple interactions, like hydrophobic effect, ion exchange, hydrogen-bond interaction, etc. Quantum chemical calculation (QC) was employed to obtain the optimized structures and the binding energy of anions to cations. The results provided some insights into the retention mechanism from a molecular perspective. This work demonstrated the superiority of ionic liquid functionalized silica as mixed-mode stationary phases and the potential of chiral ionic liquid as chiral selectors.

Synergetic Multichiral Covalent Organic Framework for Enantioselective Recognition and Separation

In enantiomer recognition and separation, a highly enantioselective approach with universal applicability is urgently desired but hard to realize, especially in the case of chiral molecules. To resolve the trade-off between enantioselectivity and universality, a glutathione (GSH) and methylated cyclodextrins (MCD)-functionalized covalent organic framework (GSH-MCD COF) with porosity and abundant chiral surfaces is presented that was designed and synthesized for recognition and separation of various enantiomers. As expected, the GSH-MCD COF can be used as chiral stationary phases for the separation of various enantiomers, including aromatic alcohols, aromatic acids, amides, amino acids, and organic acids, with performance and versatility even superior to some widely used commercial chiral chromatographic columns. Furthermore, the synthesized GSH-MCD COF shows high enantioselectivity for the rapid recognition and identification of enantiomers and chiral metabolites when coupling to Raman spectroscopy. Molecular simulations suggest that the COF provides a confined microenvironment for cyclodextrins and peptides that dictates the separation and recognition capability. This work provides a strategy to synthesize synergetic multichiral COF and achieve separations and recognitions of enantiomers in complex samples.

Engineering thiol-ene click chemistry for the preparation of a chiral stationary phase based on a [4+6]-type homochiral porous organic cage for enantiomeric separation in normal-phase and reversed-phase high performance liquid chromatography

In this study, a new chiral stationary phase (CSP) was fabricated by covalent bonding of a [4+6]-type homochiral porous organic cage (POC) CC19-R onto thiolated silica via a thiol-ene click reaction. The CC19-R was synthesized via Schiff-base reaction between 2-hydroxybenzene-1,3,5-tricarbaldehyde and (1R, 2R)-(-)-1,2-diaminocyclohexane. The enantioseparation capability of the resulting CC19-R-based CSP was systematically evaluated upon separating various chiral compounds or chiral pharmaceuticals in normal phase HPLC (NP-HPLC) and reversed phase HPLC (RP-HPLC), including alcohols, organic acids, ketones, diols, esters, and amines. Fifteen racemates were enantioseparated in NP-HPLC and 11 racemates in RP-HPLC. Some racemates have been well separated, such as 4-chlorobenzhydrol, cetirizine (in the form of dihydrochloride), 1,2-diphenyl-1,2-ethanediol, and 3-(benzyloxy)propane-1,2-diol whose resolution values reached 3.66, 4.23, 6.50, and 3.50, respectively. When compared with a previously reported chiral POC-based column (NC1-R column), eight racemates were not separated on the NC1-R column in NP-HPLC and five racemates were not separated in RP-HPLC, but were well resolved on this column, revealing that the enantioselectivity and separable range of chiral POCs-type columns could be significantly widened using this fabricated CC19-R column. Moreover, the resolution performance of the CC19-R column was also compared with commercial Chiralpak AD-H [CSP: Amylose tris(3,5-dimethylphenylcarbamate)] and Chiralcel OD-H [CSP: Cellulose tris(3,5-dimethylphenylcarbamate)] columns. The column also can separate some racemates that could not be separated or not well be separated by the two commercial columns, showing its good complementarity to the two commercial columns on chiral separation. In addition, the column also had good stability and reproducibility with the relative standard deviation (n = 5) of the retention time and resolution lower than 1.0% and 1.8%, respectively, after it had undergone multiple injections (100, 200, 300, and 400 times). This work indicated that the features of good resolution ability and simple synthesis methods using with this POC-based CSP provided chiral POCs with potential application prospects in HPLC racemic separation.

Investigation of the chiral recognition role of cyclodextrin hydroxyl moieties via high performance liquid chromatography

Cyclodextrin (CD) is known to afford excellent enantioselectivities due to its hydrophobic cavity and external H-bonding sites from hydroxyl moieties. However, there is still a lack of direct and comprehensive evidence clearly illustrating the origin of the important H-bonding effect. Regarding this issue, herein, four allylimidazole CD derivatives by selective substitution of the primary (6-position) and/or secondary (2,3-position) CD were synthesized and clicked onto silica surfaces to afford the corresponding chiral stationary phases (CSPs). The chiral chromatographic performances were systematically evaluated by separating 35 racemic analytes including isoxazolines, dansyl-amino acids, flavonoids and other racemates under reversed-phase HPLC. The chiral selection factors (α) and retention times (k) of the analytes on the as-prepared CSPs were comprehensively compared and it reveals that the enantioseparation ability was significantly altered due to the selective substituents of CD hydroxyl groups. The natural allylimidazole CD CSP (AICDCSP) was superior to the 6-O-tert-butyldimethylsilyl AICDCSP (6-TBDMAICDCSP) for most analytes. Dansyl amino acids and Ar-Pys were well separated on AICDCSP and 6-TBDMAICDCSP, where dansyl amino leucine gained the highest resolution up to 4.72 on AICDCSP, and flavonoids and Ar-Oprs were only separated on AICDCSP. These interesting separation results demonstrate that the secondary hydroxyl groups play a pivotal role in the separation of chiral compounds. In addition, the size of the CD cavity and the choice of solute also have an effect on the separation of substances. The mechanism involved in enantioselective discrimination of the selectively substituted CDs was further investigated by the molecular docking simulation.

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.

A novel pillar[3]trianglimine macrocycle with a deep cavity used as a chiral selector to prepare a chiral stationary phase by thiol-ene click reaction for enantioseparation in high-performance liquid chromatography

A chiral pillar[3]trianglimine (C60 H72 N6 O6 ) with a deep cavity has been developed as a chiral selector and bonded to thiolated silica by thiol-ene click reaction to fabricate a novel chiral stationary phase for enantioseparation in high-performance liquid chromatography. The enantioseparation performance of the fabricated chiral stationary phase has been evaluated by separating various racemic compounds, including alcohols, esters, amines, ketones, amino acids, and epoxides, in both normal-phase and reversed-phase elution modes. In total, 14 and 17 racemates have been effectively separated in these two separation modes, respectively. In comparison with two widely used chiral columns (Chiralcel OD-H and Chiralpak AD-H), our novel chiral stationary phase offered good chiral separation complementarity, separating some of the tested racemates that could not be separated or were only partially separated on these two commercial columns. The influences of analyte mass, mobile phase composition, and column temperature on chiral separation have been investigated. Good repeatability, stability, and column-to-column reproducibility of the chiral stationary phase for enantioseparation have been observed. After the fabricated column had been eluted up to 400 times, the relative standard deviations (n = 5) of resolution (Rs) and retention time of the separated analytes were < 0.39% and < 0.20%, respectively. The relative standard deviations (n = 3) of Rs and retention time for column-to-column reproducibility were < 4.6% and < 5.2%, respectively. This study demonstrated that the new chiral stationary phase has great prospects for chiral separation in high-performance liquid chromatography.

Preparation of Chiral Porous Organic Cage Clicked Chiral Stationary Phase for HPLC Enantioseparation

Porous organic cages (POCs) are a new subclass of porous materials, which are constructed from discrete cage molecules with permanent cavities via weak intermolecular forces. In this study, a novel chiral stationary phase (CSP) has been prepared by chemically binding a [4 + 6]-type chiral POC (C120H96N12O4) with thiol-functionalized silica gel using a thiol-ene click reaction and applied to HPLC separations. The column packed with this CSP presented good separation capability for chiral compounds and positional isomers. Thirteen racemates have been enantioseparated on this column, including alcohols, diols, ketones, amines, epoxides, and organic acids. Upon comparison with a previously reported chiral POC NC1-R-based column, commercial Chiralpak AD-H, and Chiralcel OD-H columns, this column is complementary to these three columns in terms of its enantiomeric separation; and can also separate some racemic compounds that cannot be separated by the three columns. In addition, eight positional isomers (iodoaniline, bromoaniline, chloroaniline, dibromobenzene, dichlorobenzene, toluidine, nitrobromobenzene, and nitroaniline) have also been separated. The influences of the injection weight and column temperature on separation have been explored. After the column has undergone multiple injections, the relative standard deviations (RSDs) for the retention time and selectivity were below 1.0 and 1.5%, respectively, indicating the good reproducibility and stability of the column for separation. This work demonstrates that POCs are promising materials for HPLC separation.

Chiral Covalent-Organic Framework MDI-β-CD-Modified COF@SiO2 Core-Shell Composite for HPLC Enantioseparation

The chiral covalent-organic framework (CCOF) is a new kind of chiral porous material, which has been broadly applied in many fields owing to its high porosity, regular pores, and structural adjustability. However, conventional CCOF particles have the characteristics of irregular morphology and inhomogeneous particle size distribution, which lead to difficulties in fabricating chromatographic columns and high column backpressure when the pure CCOFs particles are directly used as the HPLC stationary phases. Herein, we used an in situ growth strategy to prepare core-shell composite by immobilizing MDI-β-CD-modified COF on the surface of SiO₂-NH₂. The synthesized MDI-β-CD-modified COF@SiO₂ was utilized as a novel chiral stationary phase (CSP) to explore its enantiomeric-separation performance in HPLC. The separation of racemates and positional isomers on MDI-β-CD-modified COF@SiO₂-packed column (column A) utilizing n-hexane/isopropanol as the mobile phase was investigated. The results demonstrated that column A displayed remarkable separation ability for racemic compounds and positional isomers with good reproducibility and stability. By comparing the MDI-β-CD-modified COF@SiO₂-packed column (column A) with commercial Chiralpak AD-H column and the previously reported β-CD-COF@SiO₂-packed column (column B), the chiral recognition ability of column A can be complementary to that of Chiralpak AD-H column and column B. The relative standard deviations (RSDs) of the retention time and peak area for the separation of 1,2-bis(4-fluorophenyl)-2-hydroxyethanone were 0.28% and 0.79%, respectively. Hence, the synthesis of CCOFs@SiO₂ core-shell composites as the CSPs for chromatographic separation has significant research potential and application prospects.

Preparation and evaluation of a chiral porous organic cage based chiral stationary phase for enantioseparation in high performance liquid chromatography

Porous organic cages (POCs) are a new kind of porous molecular materials, which have gained widespread interest in many fields due to their intriguing properties, including excellent molecular solubility, inherent molecular cavity and rich host-guest chemistry. To date, many chiral POCs have been explored as chiral stationary phases (CSPs) for gas chromatographic (GC) separation of enantiomers. However, the applications of chiral POCs for high performance liquid chromatography (HPLC) enantiomeric separation is extremely rare. In this study, we report the construction of thiol-ene click reaction for the preparation of CSP for HPLC by using a [4+8]-type chiral POC NC4-R as chiral selector. The fabricated CSP showed good chiral resolution performance not only in normal-phase HPLC (NP-HPLC) but also in reversed-phase HPLC (RP-HPLC). Seventeen and ten racemates were well resolved in the two separation modes, respectively, including ketones, esters, alcohols, phenols, amines, ethers, organic acids, and amino acids. Moreover, the fabricated column also shows good chiral recognition complementarity to two popular chiral HPLC columns (Chiralpak AD-H and Chiralcel OD-H columns) and previously reported chiral POC NC1-R-based HPLC column, which can resolve some racemates that unable to be resolved by the two commercially available chiral HPLC columns and NC1-R-based column. The relative standard deviation (RSD) values (n = 4) of retention time and resolution (Rs) of analytes separated on the column were less than 0.3 % and 0.5 % after it was subjected to different injections, showing the good reproducibility and stability of the NC4-R-based column. This work demonstrated high potentials of chiral POCs for HPLC enantioseparation and the applicability of chiral POC-based HPLC columns can be broadened by developing more chiral POCs with diverse structures as chiral selector for HPLC.

Preparation of chiral stationary phase based on a [3+3] chiral polyimine macrocycle by thiol-ene click chemistry for enantioseparation in normal-phase and reversed-phase high performance liquid chromatography

Polyimine macrocycles are a new class of organic macrocycles with cyclic structures, well-defined molecular cavities, and multiple cooperative binding sites, which have recently aroused considerable research interest in molecular recognition and separation. Herein, we report the bonding of a [3+3] chiral polyimine macrocycle (H₃L, C₇₈H₇₈N₆O₃) on thiol-functionalized silica gel using thiol-ene click chemistry to prepare a chiral stationary phase (CSP) for high performance liquid chromatography (HPLC). The fabricated column exhibited excellent chiral separation capability under both normal-phase and reversed-phase conditions. Fourteen and 10 racemates were well resolved on the column in normal-phase mode (using n-hexane/isopropanol as the mobile phase) and reversed-phase mode (using methanol/water as the mobile phase), respectively, including alcohols, esters, ethers, ketones, aldehydes, epoxides and organic acids. Moreover, the column also shows good selectivity toward positional isomers. Six positional isomers (dinitrobenzene, chloroaniline, bromoaniline, iodoaniline, nitrobrobenzene and nitrochlorobenzene) were well separated on the column. In addition, the effects of the injection mass and mobile phase composition on the separation were investigated. The column shows good reproducibility and stability after multiple injections with the relative standard deviation (RSD) (n = 5) of the retention time and resolution being < 0.96 % and 0.65 %, respectively. This study indicates that this type of chiral polyimine macrocycles is a promising chiral selector for HPLC enantioseparation and will push forward the applications of more novel chiral macrocycles for chiral chromatographic separation.

Preparation of Novel Chiral Stationary Phases Based on the Chiral Porous Organic Cage by Thiol-ene Click Chemistry for Enantioseparation in HPLC

Porous organic cages (POCs) are an emerging class of porous materials that have aroused considerable research interest because of their unique characteristics, including good solubility and a well-defined intrinsic cavity. However, there have so far been no reports of chiral POCs as chiral stationary phases (CSPs) for enantioseparation by high-performance liquid chromatography (HPLC). Herein, we report the first immobilization of a chiral POC, NC1-R, on thiol-functionalized silica using a mild thiol-ene click reaction to prepare novel CSPs for HPLC. Two CSPs (CSP-1 and CSP-2) with different spacers have been prepared. CSP-1, with a cationic imidazolium spacer, exhibited excellent enantioselectivity for the resolution of various racemates. Twenty-three and 12 racemic compounds or chiral drugs were well enantioseparated on the CSP-1-packed column under normal-phase and reversed-phase conditions, respectively, including alcohols, diols, esters, ethers, ketones, epoxides, organic acids, and amines. In contrast, chiral resolution using CSP-2 (without a cationic imidazolium spacer)-packed column B was inferior to that of column A, demonstrating the important role of the cationic imidazolium spacer for chiral separation. The chiral separation capability of column A was also compared with that of two most popular commercial chiral columns, Chiralpak AD-H and Chiralcel OD-H, which exhibits good chiral recognition complementarity with the two commercial chiral columns. In addition, five positional isomers dinitrobenzene, nitroaniline, chloroaniline, bromoaniline, and iodoaniline were also well separated on column A. The effects of temperature, mobile phase composition, and injected analyte mass for separation on column A were investigated. Column A also showed good stability and reproducibility after repeated injections. This work demonstrates that chiral POCs are promising chiral materials for HPLC enantioseparation.

Design, synthesis, and characterization of a series of novel β-cyclodextrin functional monomers

In order to increase its solubility in water and improve β-cyclodextrin combination with guest molecules, chemical modification of β-cyclodextrin is a feasible and effective method. A variety of β-cyclodextrin derivatives are designed and introduced for pharmaceutical complexation and analytical chemistry application. In this study, a series of β-cyclodextrin derivatives containing unsaturated bonds is designed and synthesized. The products are characterized by MS, FTIR, 1H NMR, and 13C NMR. Some of the functional monomers may be used in the preparation of molecularly imprinted polymers, and preliminary studies have shown excellent molecular recognition ability. The prepared β-cyclodextrin functional monomers have potential application value in molecular recognition materials based on polymers.

Surface-up construction of quinine bridged functional cyclodextrin for single-column versatile enantioseparation

Seeking for single-column versatile chiral separation methodology is the ultimate goal for analysts engrossed in enantioseparation. However, the versatility and selectivity are always contradictory due to negative influence among the recognition domains and the relatively low surface concentrations in a limited support surface area. Herein, we reported a novel series of quinine (QN) bridged cyclodextrin (CD) chiral stationary phases (CSPs) with satisfied surface concentration of both selectors, prepared via a facile surface-up 'thiol-ene click' approach, where QN and CD can not only nicely exhibit their individual resolution capability but also afford possible synergism in resolving difficult-to-separate analytes. QN bridged phenylcarbamoylated CD CSP exhibits powerful resolution ability by positive combining the resolving ability of QN and functional CD and achieves the resolution of almost a double number of racemates over QN or CD CSPs. Meanwhile, it exhibits comparable and even better chiral selectivity over the widely used chemical-bonded chiral column (CHIRALPAK® IA, CHIRALPAK® IB and CHIRALPAK® IC of Daicel) for the studied analytes. This work thus advances the duplex QN-CD structure as a relatively versatile platform for chiral resolution and commendably promotes the design of functional CSPs with chiral molecular bridge.

Preparation and chiral resolution properties of bridged bis(cyclodextrin)s hybrid spheres for high performance liquid chromatography

Selenium-bridged bis(β-cyclodextrin)s organic-inorganic hybrid silica material with regular spherical shape as new type of chiral stationary phase was directly synthesized under the one-pot hydrothermal synthesis method, and the chiral stationary phase was further characterized by infrared spectroscopy, scanning electron microscopy, thermogravimetry, and elemental analysis. The results of chiral separation showed that eight chiral compounds including various types of chiral alcohols and flavanone were successfully separated in the reversed-phase separation mode by high performance liquid chromatography, which showed the better chiral resolution effect than that on the C2 position of single β-cyclodextrin. The mechanism of chiral separation was likely due to multiple interactions such as inclusion, hydrogen bonding, electrostatic interaction, dipole-dipole interaction, and the synergistic effect of two cyclodextrins during the chiral resolution process. The synergy of the two cyclodextrins has great potential for development in chiral resolution.

Recent progress in the development of chiral stationary phases for high-performance liquid chromatography

Separations and analyses of chiral compounds are important in many fields, including pharmaceutical production, preparation of chemical intermediates, and biochemistry. High-performance liquid chromatography using a chiral stationary phase is regarded as one of the most valuable methods for enantiomeric separation and analysis because it is highly efficient, is broadly applicable, and has powerful separation capability. The focus for development of this method is the identification of novel chiral stationary phases with superior recognition performance and good stability. The present article reviews recent progress in the development of new chiral stationary phases for high-performance liquid chromatography between January 2018 and June 2021. These newly reported chiral stationary phases are divided into three categories: small organic molecule-based (cyclodextrin and its derivatives, macrocyclic antibiotics, cinchona alkaloids, and other low molecular weight chiral molecules), macromolecule-based (cellulose and amylose derivatives, chitin and chitosan derivatives, and synthetic helical polymers) and chiral porous material-based (chiral metal-organic frameworks, chiral covalent organic frameworks, and chiral inorganic mesoporous silicas). Each type of chiral stationary phase is discussed in detail.

β-Cyclodextrin-Calcium Complex Intercalated Hydrotalcites as Efficient Catalyst for Transesterification of Glycerol

β-cyclodextrin derivative intercalated MgAl-hydrotalcites (β-CD-Ca/LDH) was synthesized to convert glycerol into high value-added glycerol carbonate(GC) by transesterification of dimethyl carbonate (DMC) and glycerol in this paper. β-cyclodextrin-metal complexes and β-CD-Ca/LDH was characterized by XRD, FT-IR, SEM, XPS and nitrogen adsorption-desorption. The enrichment of organic reactants in the hydrophobic cavity of β-cyclodextrin improved the collision probability of reactants. The intercalation of β-cyclodextrin-calcium complex (β-CD-Ca) increased the pore size and basic strength of catalyst. The experiment results showed that the glycerol conversion was 93.7% and the GC yield was 91.8% catalyzed by β-CD-Ca/LDH when the molar ratio of DMC and glycerol was 3:1, the catalyst dosage was 4 wt.%, the reaction temperature was 75 °C and the reaction time was 100 min while the glycerol conversion was 49.4% and the GC yield was 48.6% catalyzed by MgAl-LDH under the same conditions.

Chiral covalent organic framework-monolith as stationary phase for high-performance liquid chromatographic enantioseparation of selected amino acids

The separation of amino acid (AA) enantiomers shows significance for chemistry, food, and biology, but remains challenging due to their similar properties. A promising nanoporous chiral covalent organic framework (COF) as a stationary phase for high-performance liquid chromatography (HPLC) suffers from the irregularity and widely distributed particle size of the chiral COF. Herein, we show the facile preparation of a chiral COF-monolith as a stationary phase for HPLC enantiomeric separation of AAs via orthogonal experiments. The CTzDa-monolith is prepared by the incorporation of the model chiral COF named CTzDa into the porous poly(ethylene dimethacrylate-co-methacrylate) monolith and reveals great permeability and mechanical stability. The corresponding CTzDa-monolithic column gives better chiral HPLC separation of AAs than the commercial Poroshell 120 chiral-T column. Thermal dynamic analysis and molecular docking calculations imply the involvement of stereoscopic hydrogen, π-π, and van der Waals interactions between the CTzDa and AAs during HPLC enantioseparation. The facile incorporation of the chiral COF into the porous monolith will promote the potential of a chiral COF as a stationary phase for HPLC.

Effects of amino acid-derived chiral ionic liquids on cyclodextrin-mediated capillary electrophoresis enantioseparations of dipeptides

The synergistic effect of chiral ionic liquids composed of tetraalkylammonium ions and the amino acids Asn, Asp or Pro on the enantioseparations of dipeptides mediated by β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin in capillary electrophoresis was studied. Addition of a chiral ionic liquid resulted in a concentration-dependent increase in the enantioresolutions compared to the sole presence of a cyclodextrin in the background electrolyte. The extent varied with the tetraalkylammonium cation (tetramethylammonium versus tetrabutylammonium) as well as the amino acid component of the ionic liquid. The presence of a chiral ionic liquid did not counteract the pH-dependent reversal of the enantiomer migration order of the dipeptides Ala-Phe, Ala-Tyr and Phe-Phe when increasing the pH of the background electrolyte from 2.5 to 3.5. Comparing the effect of a chiral ionic liquid based on Asp with the addition of equimolar concentrations of the individual components of the ionic liquid, a diverse picture was observed. In some cases, higher resolution values were obtained with the chiral ionic liquid, while for other cases superior enantioseparations were obtained upon separate addition of the amino acid component and a tetraalkylammonium chloride. With regard to the stereochemistry of the amino acid, a superior effect was typically observed using the l-configured amino acid, but in some cases higher resolution values were found in the presence of d-Asp. The rationale for the diverse observations is not obvious and may be due to the zwitterionic nature of analytes as well as the amino acid component of the chiral ionic liquid.

Anhydride-linked β-cyclodextrin-bonded silica stationary phases with enhanced chiral separation ability in liquid chromatography

β-Cyclodextrin can be functionalized by derivation of reactive hydroxyl on the ring due to its special chiral environment and structural characteristics, which can be used to identify or separate a variety of chiral substance. In this manuscript, a series of excellent chiral stationary phases for high-performance liquid chromatography were developed for enantioseparation by using anhydride modified β-cyclodextrin bearing chiral (R/S)-α-phenethylamine or (S)-(+)-2-amino-1-propanol. They were characterized by elemental analysis, Fourier transform infrared spectra (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and BET. These chiral stationary phases presented good resolution and repeatability, about 17 kinds of enantiomers were effectively separated. And most of enantiomers were separated better than those reported in the literature in the same both normal and reversed phase modes. The RSD values of R_s for repeatability and column-to-column were below 0.44% and 2.83%, respectively. All results revealed that these new CSPs show great prospect for chiral separation in actual applications.

Click preparation of multiple-thioether bridged cyclodextrin chiral materials for efficient enantioseparation in high-performance liquid chromatography

A highly efficient covalent immobilization procedure is considered as an essential tool for obtaining stable and reliable cyclodextrin (CD) chiral stationary phases (CSPs). This work reports the "thiolene" click immobilization of heptakis(6-mercapto-6-deoxy)-β-CD-CSP onto alkene functional silica to afford novel multiple-thioether bridged CD CSPs by controlling the surface CD concentration. Solid-state NMR, FTIR, TGA and X-ray photoelectron diffraction spectroscopy (XPS) results proved the successful preparation of the desired CSPs with different surface CD loadings. The surface CD concentrations were calculated to be 0.49 and 0.68 μmol m-2 according to the elemental analysis results. More than 60 chiral enantiomers including isoxazolines, chiral lactides, chiral ketones, dansyl amino acids, small molecule acids and alkalis as well as some flavonoids were resolved or partially separated in the reversed-phase HPLC mode. Compared with the previously prepared single thiolene bridged CD-CSP, the current multiple-thioether CD-CSP afforded much better enantioseparation ability due to the existence of the thiol moiety and a confined structure.

[Recent developments of pesticide adsorbents based on cyclodextrins]

The invention and application of pesticides have greatly increased the yield of crops, greatly contributing to ensuring people's basic livelihoods and gradually improving their livelihoods to a well-off level. However, foods, water sources, and soil, containing high levels of pesticide residues, result in increasingly serious pollution. Pesticide residues usually have the characteristics of micro toxicity, difficult biodegradation, and bioaccumulation, and thus pose serious threat to living organisms and ecosystems. In recent years, pesticide pollution has earned worldwide focus. Thus, methods for the efficient detection of trace pesticides and reduction of the harm caused by pesticide pollution are urgently required. Researchers have used catalysis, electrochemistry, membrane separation, adsorption, and other methods to enrich pesticides from complex matrices. Among these, adsorbents have attracted much attention owing to their advantages of simple operation steps, rapid treatment process, and low amounts of organic solvents required. Research on adsorption materials has always been a very active field, and is also the key to the success of separation and enrichment of pesticides from complex matrices. Development of adsorbents with the advantages of simple synthesis, environment-friendliness, high stability, and strong reusability is of great significance. There has been some progress in the field of pesticide adsorption using supramolecular compounds. Cyclodextrin is a macrocyclic compound with a cavity after crown ether, which can form inclusion complexes via host guest interactions as the main body. Cyclodextrin can also be modified by etherification, esterification, oxidation, and other chemical reactions to improve its adsorption performance. Pesticides can be classified into organic and inorganic substances. One of the most widely used inorganic fungicides is the Bordeaux solution, whose main component is Cu2+. Organic fungicides, insecticides, herbicides, and plant growth regulators are basically organic molecules, whose hydroxyl and carboxyl groups can form complexes with Cu2+. As a matrix, cyclodextrin not only increases the surface area of the materials, but also provides the binding sites of hydroxyl and carboxyl groups, which guarantees efficient enrichment of Cu2+. Organic pesticides with high polarity, high electron density, and strong hydrophobicity could be better adsorbed. In this paper, the application of cyclodextrin-based adsorbents in pesticide adsorption was reviewed, and on this basis, reference to future development directions and application prospects were provided. The adsorption capacity of individual pesticide adsorbents based on cyclodextrin, as reviewed in this paper, is not high enough. Therefore, improving the adsorption capacity is currently a major research target. Some of the above-mentioned adsorbents have unclear degradation mechanisms and can easily cause secondary pollution. Therefore, the development of environment-friendly pesticide adsorbents that are easy to regenerate is a promising research direction for the future. After adsorption, some detection methods are used to determine whether the pesticide residues are up to the standard; however, the detection instruments are expensive. Therefore, the development of a combined detection mechanism that can reduce workload and cost is a promising research direction. Finally, the development of smart cyclodextrin-based adsorbents is also an efficient and rapid method to reduce the cost of detecting residual pesticide concentrations and the risk of pesticide pollution. For example, intelligent materials, whose color changes can be observed by the naked eye, not only adsorb pesticides, but also respond according to the concentration of residual pesticides.

Highly sensitive determination of amanita toxins in biological samples using β-cyclodextrin collaborated molecularly imprinted polymers coupled with ultra-high performance liquid chromatography tandem mass spectrometry

In this work, a β-cyclodextrin functional vinyl monomer was synthesized and the common moiety of five amanita toxins was used as the template for preparing molecularly imprinted polymers (MIPs). Chemical calculation was used to evaluate and describe the binding interactions between the template and the functional monomer. The preparation conditions were optimized and the resultant MIPs were characterized and employed as solid-phase extraction (SPE) sorbents. The SPE conditions including the amount of sorbent, extraction solution, and eluting solution were also optimized for the enrichment of the five toxins. Using an ultra-high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS), detection limits ranging from 0.34-0.42 µg/L, 0.16-0.33 µg/L, and 0.035-0.056 µg/kg were achieved for the five toxins in serum, urine and liver samples, respectively. The proposed method was further applied to the determination of the amanita toxins in suspected samples and showed great potential in the diagnosis of mushroom poisoning.

Recent advances in the enantioseparation promoted by ionic liquids and their resolution mechanisms

More and more various chemical media are being applied in enantioseparation; among them, ionic liquids (ILs) have attracted the long-term attention in this decade as green designable solvents. This paper provides comprehensive overview for the applications of ILs in chiral extraction, gas chromatography, liquid chromatography, capillary electrophoresis and other techniques for enantioseparation. Additionally, the important resolution mechanisms based on ILs have also been summarized and discussed. This review focuses on the latest development of enantioseparation methods by using ILs in various modes, leading to meaningful and valuable information to related fields and thus promotes further research and application of reported methods.

Novel chiral stationary phases based on 3,5-dimethyl phenylcarbamoylated β-cyclodextrin combining cinchona alkaloid moiety

Novel chiral selectors based on 3,5-dimethyl phenylcarbamoylated β-cyclodextrin connecting quinine (QN) or quinidine (QD) moiety were synthesized and immobilized on silica gel. Their chromatographic performances were investigated by comparing to the 3,5-dimethyl phenylcarbamoylated β-cyclodextrin (β-CD) chiral stationary phase (CSP) and 9-O-(tert-butylcarbamoyl)-QN-based CSP (QN-AX). Fmoc-protected amino acids, chiral drug cloprostenol (which has been successfully employed in veterinary medicine), and neutral chiral analytes were evaluated on CSPs, and the results showed that the novel CSPs characterized as both enantioseparation capabilities of CD-based CSP and QN/QD-based CSPs have broader application range than β-CD-based CSP or QN/QD-based CSPs. It was found that QN/QD moieties play a dominant role in the overall enantioseparation process of Fmoc-amino acids accompanied by the synergistic effect of β-CD moiety, which lead to the different enantioseparation of β-CD-QN-based CSP and β-CD-QD-based CSP. Furthermore, new CSPs retain extraordinary enantioseparation of cyclodextrin-based CSP for some neutral analytes on normal phase and even exhibit better enantioseparation than the corresponding β-CD-based CSP for certain samples.

A new strategy for the preparation of mixed-mode chromatographic stationary phases based on modified dialdehyde cellulose

A new strategy for the preparation of mixed-mode chromatographic stationary phases based on modified dialdehyde cellulose was proposed. Two novel mixed-mode chromatographic stationary phases, dicarboxyl cellulose-modified silica (DCC/SiO₂) and (S)-α-phenylethylamine-bonded DCC/SiO₂ ((S)-α-PEA/DCC/SiO₂), were prepared by utilizing the easy functionalization characteristics of dialdehyde cellulose. The chromatographic evaluation showed that DCC/SiO₂ column could be used in hydrophilic interaction liquid chromatography (HILIC) and ion exchange chromatography (IEC) modes, (S)-α-PEA/DCC/SiO₂ column could be used in HILIC, IEC and chiral separation modes. The DCC/SiO₂ column and (S)-α-PEA/DCC/SiO₂ column exhibited excellent chromatographic performance by separating strongly polar compounds, phenylamines and chiral compounds in the above separation modes. The preparation method of modified dialdehyde cellulose-based mixed-mode chromatographic stationary phases was simple, and also provided a new idea for the development of the subsequent novel mixed-mode chromatographic stationary phases.

Enantioseparation of acetyltropic acid by countercurrent chromatography with sulfobutyl ether-β-cyclodextrin as chiral selector

Acetyltropic acid is an important synthetic intermediate for preparation of tropane alkaloid derivatives, which can be used as anticholinergic drugs, deliriants, and stimulants. In the present work, acetyltropic acid was successfully enantioseparated by countercurrent chromatography using sulfobutyl ether-β-cyclodextrin as chiral selector. A biphasic solvent system composed of n-butyl acetate/n-hexane/0.1 mol/L citrate buffer at pH = 2.2 containing 0.1 mol/L of sulfobutyl ether-β-cyclodextrin (7:3:10, v/v) was selected, which produced a suitable distribution ratio D_S  = 1.14, D_R  = 2.31 and a high enantioseparation factor α = 2.03. Baseline separation was achieved for preparative enantioseparation of 50 mg of racemic acetyltropic acid. A method for chiral analysis of acetyltropic acid by conventional reverse phase liquid chromatography with hydroxylpropyl-β-cyclodextrin as mobile phase additive was established, and formation constants of inclusion complex were determined. It was found that different substituted β-cyclodextrin should be selected for enantioseparation of acetyltropic acid by countercurrent chromatography and reverse phase liquid chromatography.

Chirality Discrimination at the Single Molecule Level by Using a Cationic Supermolecule Quasi-Gated Organic Field Effect Transistor

Achieving rapid and highly sensitive small molecule chiral discrimination is a great challenge in modern-day analytical sciences. Herein, an organic field effect transistors (OFET) is developed by employing an imidazolium 3,5-dimethylphenylcabamoylated-β-cyclodextrin (Im⁺-Ph-β-CD) as both the recognition unit and a quasi gate, which induces a secondary accumulation channel of electrons in the n-type transistor to achieve the signal transduction and amplification via field effect. The charge of the imidazolium group is partially shielded due to its self-inclusion in the CD cavity, and this shielding effect is reduced at varying degrees in the existence of isomers due to the competitive inclusion. Consequently, the different weak intermolecular interactions related to the target-induced CD-enantiomer complexation with different geometry and stabilization energy for each isomer can be transformed to electronic signals based on the variety of Im⁺-Ph-β-CD's effective charge rather than the intrinsic charge of analytes, hence leading to chiral differentiation, and the hydrogen-bonding network of Im⁺-Ph-β-CD membrane further magnifies the signal. This working strategy even allows chiral discrimination of electrically neutral analytes. The as-prepared sensor affords rapid and real-time discrimination to small molecule enantiomers at single molecule level with a limit of detection of 8.1 × 10^-19 M in a 200 μL volume (about 100 small molecules). Moreover, we prove the great potential of the chiral organic field effect transistor in quantitative analysis of commercial medicines.

Preparation and chromatographic performance of a multifunctional immobilized chiral stationary phase based on dialdehyde microcrystalline cellulose derivatives

A novel high-performance liquid chromatography (HPLC) multifunctional immobilized chiral stationary phase was prepared by bonding dialdehyde microcrystalline cellulose to aminosilica via Schiff base reaction and then derivatized with 3,5-dimethylphenyl isocyanate. The HPLC multifunctional immobilized chiral stationary phase could not only achieve chiral separation but also achieve achiral separation. Chiral separation evaluation showed that 1-(1-naphthyl)ethanol and mandelonitrile got separation in normal phase (NP) mode. Ranolazine, benzoin ethyl ether, metalaxyl, and diclofop were successfully separated in reversed phase (RP) mode. Aromatic compounds such as polycyclic aromatic hydrocarbons (PAHs), anilines, and aromatic acids were selected as analytes to investigate the achiral separation performance of the multifunctional immobilized chiral stationary phase in NP and RP modes. The achiral separation evaluation showed that six PAHs could get good separation within 10 minutes in NP mode. Four aromatic acids were well separated in RP mode. The retention mechanism of aromatic compounds on the stationary phase was discussed, founding that π-π interaction, π-π electron-donor-acceptor (EDA) interaction, and hydrogen bonding interaction played important roles during the achiral separation process. This multifunctional immobilized chiral stationary phase had the advantages of simple bonding steps, short reaction time, and no need for space arm.

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.

Preparation of a new benzylureido-β-cyclodextrin-based column and its application for the determination of phenylmercapturic acid and benzylmercapturic acid enantiomers in human urine by LC/MS/MS

A benzylureido-β-cyclodextrin was synthesized by the reaction of 6-amino-β-cyclodextrin with an active benzyl isocyanate. Then, it was bonded to silica gel by a thiol-ene addition reaction, obtaining a new benzylureido-β-cyclodextrin-based chiral stationary phase (BzCDP). Its chemical structure was characterized by infrared spectroscopy, elemental analysis, and solid-state nuclear magnetic resonance spectroscopy. The BzCDP was successfully used to separate phenylmercapturic acid (PMA) and benzylmercapturic acid (BMA) enantiomers, which were confirmed as biomarkers of exposure to benzene and toluene in human urine. The enantiomeric separations were also optimized through the investigation of related factors. The resolutions of PMA and BMA enantiomers could be up to 2.25 and 2.14, respectively, within 30 min under reversed-phase chromatography. Based on the optimal chromatographic and mass spectrometry conditions, a new LC-MS/MS quantitative method for the PMA and BMA enantiomers was established by negative ion multiple reaction monitoring (MRM) and an isotope-labeled PMA (d²-PMA) as an internal standard. The limits of detection (LODs) of enantiomers were less than 0.17 μg/L for PMA and 0.14 μg/L for BMA, and the averaged recoveries of enantiomers were in the range of 86~100% for PMA and 86~113% for BMA. The method had good reproducibility levels with the RSDs (3.5~11.3% for intra-day and 3.9~13.1% for inter-day). The method was successfully applied to urine testing of 60 painting and printing workers. The results showed that only L-PMA was detected in the urine of the Printers, while a high content of L-PMA (27.5~106 μg/L) and D-PMA (19.9~82.8 μg/L) can be detected simultaneously in the urine of the Painters, indicating that benzene pollution was more serious in this group. The positive rate of BMA was rather higher, indicating that toluene pollution was more common than benzene. BMA also existed in the form of two enantiomers (L-BMA and D-BMA), but the difference between the two types of occupational groups was small. It is a meaningful work to deeply study the existence and content of chiral markers in human urine, which will help to better understand and evaluate the harmful effects of benzene series on human beings. Graphical abstract.

Functional Cyclodextrin-Clicked Chiral Stationary Phases for Versatile Enantioseparations by HPLC

The urgent demand for pure biological and pharmaceutical enantiomers has brought together great efforts in developing chiral techniques. High-performance liquid chromatography employing chiral stationary phases (CSPs) has evolved as a powerful tool for both chiral analysis and manufacture of pure enantiomers. Herein, we describe a facile method to prepare a phenylcarbamate cyclodextrin (CD)-based CSPs via azide/alkyne click chemistry. The functionalities of CD rims are altered to mediate the enantioseparation performance in multimode high-performance liquid chromatography.

Graphene quantum dots functionalized β-cyclodextrin and cellulose chiral stationary phases with enhanced enantioseparation performance

Graphene quantum dots (GQD) functionalized β-cyclodextrin (β-CD) and cellulose silica composites were first prepared and applied in HPLC as chiral stationary phases (CSP) to investigate the effect of GQDs on chiral separation. Through comparing the enantioseparation performance of GQDs functionalized β-CD or cellulose CSPs and unmodified β-CD or cellulose CSPs, we found GQDs enhanced the enantioseparation performance of nature β-CD, β-CD-3,5-dimethylphenylcarbamate derivative and cellulose-3,5-dimethylphenylcarbamate derivative. Molecular modeling was applied to understand and theoretically study the enhancement mechanism of GQDs for enantioseparation. According to molecular simulation results, GQDs provide extra interactions such as hydrophobic, hydrogen bond and π-π interaction when chiral selector interacts with enantiomers, which enhances the chiral recognition ability indirectly. The molecular simulation results showed a good agreement with the experimental results. Our work reveals the enhancement performance of GQDs for chiral separation, it can be expected that GQDs-based chiral composites and chiral GQDs have great prospect in chiral separation and other research fields such as asymmetric synthesis, chiral catalysis, chiral recognition and drug delivery.

Wide pH range enantioseparation of cyclodextrin silica-based hybrid spheres for high performance liquid chromatography

pH plays an important role in the enantiomeric separation process by changing the polarity of the mobile phase and the conformation and ionization state of the enantiomers. Herein, β-cyclodextrin-silica hybrid spheres with n-propyl groups as hydrophobic linkers and ethyl-silica as a support were prepared using a one-pot approach, and then the hydroxyl group was further modified with 3,5-dimethylphenyl isocyanate. The new β-cyclodextrin-silica hybrid chiral stationary phase (CD-HCSP) was prepared and characterized using techniques including scanning electron microscopy and transmission electron microscopy. The enantioseparation properties of CD-HCSP were evaluated with different solvents over a wide pH range (1-10) in reversed phase. 14 enantiomers were successfully resolved, and favorable chiral resolution and high stability was demonstrated for multiple types of enantiomer under different pH conditions. Compared with commercial columns, CD-HCSP showed better chiral resolution and, more importantly, could be used for chiral resolution over a wide pH range. This work combines the high pH tolerance of the hybrid material and excellent chiral recognition of cyclodextrin for enantioseparation of chiral drugs, which could lead to the development of a new type of chiral separation material.

Evaluation of the enantioselectivity of capillary electrokinetic chromatography using ethanediamine-bonded poly (glycidyl methacrylate) microspheres as the pseudostationary phases

In this work, a new capillary electrokinetic chromatography (EKC) approach using ethanediamine-bonded poly (glycidyl methacrylate) (Ami-PGMA) microspheres as pseudostationary phases (PSPs) for enantioseparation with a polysaccharide, chondroitin sulfate E (CSE), as the chiral selector. The CSE@Ami-PGMA EKC system was applied to enantioseparate basic drugs, and distinct improved separations of tested enantiomers were obtained while comparing with the single CSE system (the resolution increased from 0.41 to 1.26 for nefopam, from 1.24 to 2.15 for laudanosine, and from 0.92 to 2.36 for amlodipine). The Ami-PGMA microspheres were fully characterized by scanning electron microscopy (SEM) and Fourier Transform Infrared (FT-IR) spectroscopy, and the results showed Ami-PGMA microspheres were uniform and spherical in size (1 μm). Several principal parameters were systematically investigated, and the optimal chiral separations were obtained with Tris/H₃ PO₄ (20 mM, pH 2.4, and 3.4 for NEF) containing 2.5% (w/v) CSE and 20-μg Ami-PGMA microspheres in 20°C. Subsequently, the concentrations of Ami-PGMA microspheres and CSE were proved to be the dominant factors for the separation in the CSE@Ami-PGMA EKC system by Statistical Product and Service Solutions (SPSS).

Click chemistry at the microscale

This manuscript reviews recent developments in click chemistry in microscale systems.

HPLC Enantioseparation on Cyclodextrin-Based Chiral Stationary Phases

As one of the commonly used chiral separation materials, cyclodextrin-based chiral stationary phases (CD-CSP) have been developed rapidly in the past 30 years. A large number of CD-CSPs have been designed and applied for enantioseparation in high-performance liquid chromatography (HPLC). The development of novel CD-CSPs focuses on two aspects: the immobilization chemistry and the functionalization of the CD skeleton. Although such studies are not regarded as the prime research topic in analytical chemistry, there are still many recent works pushing this research forward tardily. In this chapter, the fabrication procedure of a triazole-bridged duplex CD-CSP and its application to HPLC enantioseparations is described.