Experimental comparison of chiral metal-organic framework used as stationary phase in chromatography

Preparation of UiO-66 MOF-Bonded Porous-Layer Open-Tubular Columns Using an In Situ Growth Approach for Gas Chromatography

The thermally stable zirconium-based MOF, UiO-66, was employed for the preparation of bonded porous-layer open-tubular (PLOT) GC columns. The synthesis included the in situ growth of the UiO-66 film on the inner wall of the capillary through a one-step solvothermal procedure. SEM-EDX analysis revealed the formation of a thin, continuous, uniform, and compact layer of UiO-66 polycrystals on the functionalized inner wall of the column. The average polarity (ΔIav = 700) and the McReynolds constants reflected the polar nature of the UiO-66 stationary phase. Several mixtures of small organic compounds and real samples were used to evaluate the separation performance of the fabricated columns. Linear alkanes from n-pentane to n-decane were baseline separated within 1.35 min. Also, a series of six n-alkylbenzenes (C3-C8) were separated within 3 min with a minimum resolution of 3.09, whereas monohalobenzene mixtures were separated at 220 °C within 14s. UiO-66 PLOT columns are ideally suited for the isothermal separation of chlorobenzene structural isomers at 210 °C within 45 s with Rs ≥ 1.37. The prepared column featured outstanding thermal stability (up to 450 °C) without any observed bleeding or significant impact on its performance. This feature enabled the analysis of various petroleum-based samples.

as a Versatile and Predictable Chiral Stationary Phase for the Resolution of Racemic Mixtures

Metal-organic frameworks (MOFs) have become promising materials for multiple applications due to their controlled dimensionality and tunable properties. The incorporation of chirality into their frameworks opens new strategies for chiral separation, a key technology in the pharmaceutical industry as each enantiomer of a racemic drug must be isolated. Here, we describe the use of a combination of computational modeling and experiments to demonstrate that high-performance liquid chromatography (HPLC) columns packed with TAMOF-1 as the chiral stationary phase are efficient, versatile, robust, and reusable with a wide array of mobile phases (polar and non-polar). As proof of concept, in this article, we report the resolution with TAMOF-1 HPLC columns of nine racemic mixtures with different molecular sizes, geometries, and functional groups. Initial in silico studies allowed us to predict plausible separations in chiral compounds from different families, including terpenes, calcium channel blockers, or P-stereogenic compounds. The experimental data confirmed the validity of the models and the robust performance of TAMOF-1 columns. The added value of in silico screening is an unprecedented achievement in chiral chromatography.

The development of chiral metal-organic frameworks for enantioseparation of racemates

MIL-101(Cr), an achiral metal-organic framework, made up of a terephthalic acid ligand and a metal chromium ion was selected as a template. Its structural features are unsaturated Lewis acid sites that can be easily activated and it has an extremely high specific surface area, big pore size, and good thermal/chemical/water stability. This achiral framework was modified to introduce chirality within the structure to develop chiral metal-organic frameworks (CMOFs). Here, natural chiral ligands, amino acids (l-proline, l-thioproline and l-tyrosine), were selected for post synthetic modification (PSM) of MIL-101(Cr). This is a very simple, clean and facile methodology with respect to the reactants and reaction conditions. CMOFs 1-3 abbreviated as MIL-101-l-proline (CMOF-1), MIL-101-l-thioproline (CMOF-2) and MIL-101-l-tyrosine (CMOF-3) were prepared by introducing l-proline, l-thioproline and l-tyrosine as chiral moieties within the framework of (Cr). These CMOFs were characterized by FTIR, PXRD, SEM, and thermo gravimetric analysis. Chirality within these CMOFs 1-3 was established by circular dichroism (CD) and polarimetric methods. These three CMOFs 1-3 showed enantioselectivity towards RS-ibuprofen, RS-mandelic acid and RS-1-phenylethanol to varying extents. Their enantioselectivity towards racemates was studied by chiral HPLC and polarimetry.

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).

Recent progress in the design, synthesis and applications of chiral metal-organic frameworks

Chiral Metal-Organic Frameworks (CMOFs) are unique crystalline and porous class of materials which is composed of organic linkers and metal ions. CMOFs surpass traditional organic and inorganic porous materials because of their tunable shape, size, functional diversity, and selectivity. Specific applications of CMOFs may be exploited by introducing desired functional groups. CMOFs have chiral recognition abilities, making them unique for chiral compound synthesis and separation. The CMOFs can be synthesized through different approaches. Two main approaches have been discussed, i.e., direct and indirect synthesis. Synthetic strategies play an essential role in getting desired properties in MOFs. CMOFs find potential applications in adsorption, asymmetric catalysis, luminescence, degradation, and enantioselective separation. The MOFs' porosity, stability, and reusability make them an attractive material for these applications. The plethora of applications of CMOFs have motivated chemists to synthesize novel MOFs and number of MOFs have been ever-escalating. Herein, the synthetic methods of CMOFs and their various applications have been discussed.

A review on chiral metal-organic frameworks: synthesis and asymmetric applications

Chiral metal-organic frameworks (CMOFs) have the characteristics of framework structure diversity and functional tunability, and have important applications in the fields of chiral identification, separation of enantiomers and asymmetric catalysis. In recent years, the application of CMOFs has also been extended to other research fields, such as circularly polarized fluorescence and chiral ferroelectrics. Compared with achiral MOFs, the design of CMOFs only considers the modes of introduction of chirality, and also takes into account the crystallization and purification. Therefore, the synthesis and characterization of CMOFs face many difficult challenges. This review discusses three effective strategies for constructing CMOFs, including direct synthesis of chiral ligands, spontaneous resolution of achiral ligands or chiral template-induced synthesis, and post-synthetic chiralization of achiral MOFs. In addition, this review also discusses the recent application progress of CMOFs in chiral molecular recognition, enantiomer separation, asymmetric catalysis, circularly polarized fluorescence, and chiral ferroelectrics.

l-Cysteine modified metal-organic framework as a chiral stationary phase for enantioseparation by capillary electrochromatography

A new kind of chiral zirconium based metal-organic framework, l-Cys-PCN-222, was synthesized using l-cysteine (l-Cys) as a chiral modifier by a solvent-assisted ligand incorporation approach and utilized as the chiral stationary phase in the capillary electrochromatography system. l-Cys-PCN-222 was characterized by X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, Fourier-transform infrared spectra, nitrogen adsorption/desorption, circular dichroism spectrum, zeta-potential and so on. The results revealed that l-Cys-PCN-222 had the advantages of good crystallinity, high specific surface area (1818 m2 g-1), thermal stability and chiral recognition performance. Meanwhile, the l-Cys-PCN-222-bonded open-tubular column was prepared using l-Cys-PCN-222 particles as the solid phase by 'thiol-ene' click chemistry reaction and characterized by scanning electron microscopy, which proved the successful bonding of l-Cys-PCN-222 to the column inner wall. Finally, the stability, reproducibility and chiral separation performance of the l-Cys-PCN-222-bonded OT column were measured. Relative standard deviations (RSD) of the column efficiencies for run-to-run, day-to-day, column-to-column and runs were 1.39-6.62%, and did not obviously change after 200 runs. The enantiomeric separation of 17 kinds of chiral compounds including acidic, neutral and basic amino acids, imidazolinone and aryloxyphenoxypropionic pesticides, and fluoroquinolones were achieved in the l-Cys-PCN-222-bonded OT column. These results demonstrated that the chiral separation system of the chiral metal-organic frameworks (CMOFs) coupled with capillary electrochromatography has good application prospects.

Differentiation of Epoxide Enantiomers in the Confined Spaces of an Homochiral Cu(II) Metal-Organic Framework by Kinetic Resolution

TAMOF-1, a homochiral metal-organic framework (MOF) constructed from an amino acid derivative and Cu(II), was investigated as a heterogeneous catalyst in kinetic resolutions involving the ring opening of styrene oxide with a set of anilines. The branched products generated from the ring opening of styrene oxide with anilines and the unreacted epoxide were obtained with moderately high enantiomeric excesses. The linear product arising from the attack on the non-benzylic position of styrene oxide underwent a second kinetic resolution by reacting with the epoxide, resulting in an amplification of its final enantiomeric excess and a concomitant formation of an array of isomeric aminodiols. Computational studies confirmed the experimental results, providing a deep understanding of the whole process involving the two successive kinetic resolutions. Furthermore, TAMOF-1 activity was conserved after several catalytic cycles. The ring opening of a meso-epoxide with aniline catalyzed by TAMOF-1 was also studied and moderate enantioselectivities were obtained.

A chiral metal-organic framework core-shell microspheres composite for high-performance liquid chromatography enantioseparation

The unique features of uniform and adjustable cavities, abundant chiral active sites, and high enantioselectivity make chiral metal-organic frameworks popular as an emerging candidate for enantioselective separation. However, the wide particle size distribution and irregular shape of as-synthesized metal-organic frameworks result in low column efficiency, undesired chromatographic peak shape, and high column backpressure of such metal-organic frameworks packed columns. Herein, we report the fabrication of chiral core-shell microspheres [Cu₂ (d-Cam)₂ (4,4'-bpy)]_n @SiO₂ composite for high-performance liquid chromatography enantioseparation to overcome the above-mentioned problems. The [Cu₂ (d-Cam)₂ (4,4'-bpy)]_n @SiO₂ packed column gave high-resolution separation of racemates under low column backpressure (10-22 bar), indicating its synergistic effect of the good column packing property of the SiO₂ microspheres and the chiral recognition ability of [Cu₂ (d-Cam)₂ (4,4'-bpy)]_n crystals. Thirteen kinds of chiral compounds including alcohols, amines, ketones, epoxides, and organic bases were well separated with good peak shapes and high column efficiency (18200 plates/m for 1-(9-anthryl)-2,2,2-trifluoroethanol) on the [Cu₂ (d-Cam)₂ (4,4'-bpy)]_n @SiO₂ packed column. Among them, seven pairs of enantiomers achieved baseline separation and the resolution value for 1-(9-anthryl)-2,2,2-trifluoroethanol reached 11.22. Some effects such as column temperature, and analytes mass on the enantioseparations have been investigated. In addition, the [Cu₂ (d-Cam)₂ (4,4'-bpy)]_n @SiO₂ packed column exhibited good stability and repeatability for the separation of chiral compounds. The relative standard deviations for five replicate separations of 1-phenylethanol were less than 1.0, 1.5, 3.0, and 2.0% for the retention time, peak area, number of theoretical plates, and resolution, respectively. The research results demonstrated the development of chiral metal-organic frameworks core-shell microspheres composite provide a promising platform for their practical application in chiral separation fields.

Homochiral Metal-Organic Framework [Co(L)(bpe)2(H2O)2]·H2O Used for Separation of Racemates in High-Performance Liquid Chromatography

A homochiral metal-organic framework (MOF) comprising [Co(L)(bpe)2(H2O)2]·H2O was prepared using (1R,2R)-(-)-1,2-cyclohexanedicarboxylic acid (H2L) and 1,2-bis(4-pyridyl)-ethylene as organic ligands. The homochiral MOF [Co(L)(bpe)2(H2O)2]·H2O was explored as chiral stationary phase (CSP) for high-performance liquid chromatography (HPLC) separation of racemates. Nine racemates including naphthol, alcohol, diol, amine, ketone, ether and organic acid were well separated on the homochiral MOF [Co(L)(bpe)2(H2O)2]·H2O column (250 mm long × 4.6 mm i.d.). The relative standard deviation for five replicate separations of 1,1'-bi-2-naphthol is 0.69% for the retention time, indicating that the good reproducibility and stability of the homochiral MOF column for HPLC enantioseparation. The results indicated that the homochiral MOF as CSP is practical, which promotes the application of homochiral MOFs in HPLC.

Past, present, and future developments in enantioselective analysis using capillary electromigration techniques

Enantioseparation of chiral products has become increasingly important in a large diversity of academic and industrial applications. The separation of chiral compounds is inherently challenging and thus requires a suitable analytical technique that can achieve high resolution and sensitivity. In this context, CE has shown remarkable results so far. Chiral CE offers an orthogonal enantioselectivity and is typically considered less costly than chromatographic techniques, since only minute amounts of chiral selectors are needed. Several CE approaches have been developed for chiral analysis, including chiral EKC and chiral CEC. Enantioseparations by EKC benefit from the wide variety of possible pseudostationary phases that can be employed. Chiral CEC, on the other hand, combines chromatographic separation principles with the bulk fluid movement of CE, benefitting from reduced band broadening as compared to pressure-driven systems. Although UV detection is conventionally used for these approaches, MS can also be considered. CE-MS represents a promising alternative due to the increased sensitivity and selectivity, enabling the chiral analysis of complex samples. The potential contamination of the MS ion source in EKC-MS can be overcome using partial-filling and counter-migration techniques. However, chiral analysis using monolithic and open-tubular CEC-MS awaits additional method validation and a dedicated commercial interface. Further efforts in chiral CE are expected toward the improvement of existing techniques, the development of novel pseudostationary phases, and establishing the use of chiral ionic liquids, molecular imprinted polymers, and metal-organic frameworks. These developments will certainly foster the adoption of CE(-MS) as a well-established technique in routine chiral analysis.

Artificial Chiral Probes and Bioapplications

The development of artificial chiral architectures, especially chiral inorganic nanostructures, has greatly promoted research into chirality in nanoscience. The nanoscale chirality of artificial chiral nanostructures offers many new application opportunities, including chiral catalysis, asymmetric synthesis, chiral biosensing, and others that may not be allowed by natural chiral molecules. Herein, the progress achieved during the past decade in chirality-associated biological applications (biosensing, biolabeling, and bioimaging) combined with individual chiral nanostructures (such as chiral semiconductor nanoparticles and chiral metal nanoparticles) or chiral assemblies is discussed.

Chiral Macroporous MOF Surfaces for Electroassisted Enantioselective Adsorption and Separation

The development of surfaces with chiral features is a fascinating challenge for modern materials science, especially when they are used for chiral separation technologies. In this contribution, the design of hierarchically structured chiral macroporous zeolitic imidazolate framework-8 (ZIF-8) electrodes is presented. They are elaborated by an electrochemical deposition-dissolution technique based on the electrodeposition of metal through a colloidal crystal template, followed by controlled electrooxidation. This generates locally metal cations, which can interact with a chiral ligand present in the solution to form metal-organic frameworks (MOFs). The macroporous structure facilitates the access of the chiral recognition sites, located in the mesoporous MOF, and thus helps to overcome mass transport limitations. The efficiency of the designed functional materials for chiral adsorption and separation can be fine-tuned by applying an adjustable electric potential to the electrode surfaces. This hierarchical chiral ZIF-8 structure was deposited at the walls of a microfluidic device and used as a stationary phase for enantioselective separation. The potential-controlled interaction between the stationary phase and the chiral analytes allows baseline separation of two enantiomers. This opens up interesting perspectives for using hierarchically structured chiral MOFs as an efficient material for the selective adsorption and separation of chiral compounds.

Chiral Metal-Organic Framework d-His-ZIF-8@SiO2 Core-Shell Microspheres Used for HPLC Enantioseparations

Chiral metal-organic frameworks (MOFs) have aroused great attention in the chiral separation field based on their excellent characteristics, including abundant topological structures, large surface area, adjustable pore/channel sizes, multiple active sites, and good chemical stability. However, the irregular morphology and nonuniformity of the synthesized MOF particles cause low column efficiency and high column backpressure for MOF-packed columns, which significantly affects their separation performance. Herein, we prepared a homochiral d-his-ZIF-8@SiO₂ composite by growing of d-his-ZIF-8 on the carboxylic-functionalized SiO₂ microspheres via a simple one-pot synthesis approach. The d-his-ZIF-8@SiO₂ core-shell microspheres with uniform particles and narrow size distribution were applied as the chiral stationary phase (CSP) for enantioseparations in HPLC. Various racemates were separated on the d-his-ZIF-8@SiO₂-packed columns with n-hexane/isopropanol as the mobile phase. Eighteen racemates including alcohol, phenol, amine, ketone, and organic acid were well resolved on the homochiral d-his-ZIF-8@SiO₂ CSP. The d-his-ZIF-8@SiO₂ core-shell microspheres' CSP possesses an excellent chiral resolution ability toward various racemic compounds with good reproducibility and stability. Hence, the fabrication of chiral MOF@SiO₂ core-shell microspheres is an effective strategy to improve the application of homochiral MOFs as CSPs in the field of chromatography.

An easy and low-cost method of embedding chiral molecules in metal–organic frameworks for enantioseparation

A facile post-synthetic modification method has been demonstrated here to prepare chiral metal–organic frameworks for enantioseparation.

Amphiphilic Block Copolymer PCL-PEG-PCL as Stationary Phase for Capillary Gas Chromatographic Separations

This work presents the first example of utilization of amphiphilic block copolymer PCL-PEG-PCL as a stationary phase for capillary gas chromatographic (GC) separations. The PCL-PEG-PCL capillary column fabricated by static coating provides a high column efficiency of 3951 plates/m for n-dodecane at 120 °C. McReynolds constants and Abraham system constants were also determined in order to evaluate the polarity and possible molecular interactions of the PCL-PEG-PCL stationary phase. Its selectivity and resolving capability were investigated by using a complex mixture covering analytes of diverse types and positional, structural, and cis-/trans-isomers. Impressively, it exhibits high resolution performance for aliphatic and aromatic isomers with diverse polarity, including those critical isomers such as butanol, dichlorobenzene, dimethylnaphthalene, xylenol, dichlorobenzaldehyde, and toluidine. Moreover, it was applied for the determination of isomer impurities in real samples, suggesting its potential for practical use. The superior separation performance demonstrates the potential of PCL-PEG-PCL and related block copolymers as stationary phases in GC and other separation technologies.

Recognition Mechanisms of Chiral Selectors: An Overview

Stereospecific recognition of chiral molecules plays an important role in nature as the basis of the interaction of chiral bioactive compounds with the chiral target structures. In separation sciences such as chromatographic and capillary electromigration techniques, interactions between chiral analytes and chiral selectors, i.e., the formation of transient diastereomeric complexes in thermodynamic equilibria, are the basis for chiral separations. Due to the large structural variety of chiral selectors, different structural features contribute to the overall chiral recognition process. This introductory chapter briefly summarizes the present understanding of the structural enantioselective recognition processes for various types of chiral selectors.

Recent development trends for chiral stationary phases based on chitosan derivatives, cyclofructan derivatives and chiral porous materials in high performance liquid chromatography

The separation of enantiomers by chromatographic methods, such as gas chromatography, high-performance liquid chromatography and capillary electrochromatography, has become an increasingly significant challenge over the past few decades due to the demand of pharmaceutical, agrochemical, and food analysis. Among these chromatographic resolution methods, high-performance liquid chromatography based on chiral stationary phases has become the most popular and effective method used for the analytical and preparative separation of optically active compounds. This review mainly focuses on the recent development trends for novel chiral stationary phases based on chitosan derivatives, cyclofructan derivatives, and chiral porous materials that include metal-organic frameworks and covalent organic frameworks in high-performance liquid chromatography. The enantioseparation performance and chiral recognition mechanisms of these newly developed chiral selectors toward enantiomers are discussed in detail.

Open-tubular capillary electrochromatographic determination of ten sulfonamides in tap water and milk by a metal-organic framework-coated capillary column

In this study, a metal-organic framework (MOF), [Mn(cam)(bpy)], was synthesized and characterized by thermogravimetric analysis, scanning electron microscopy, and Fourier transform infrared spectrometry. An open-tubular capillary column was fabricated from [Mn(cam)(bpy)] via the amide coupling method. Ten types of sulfonamides were separated through the fabricated capillary column, which showed a good limits of detection (<0.07 μg/mL) and linear ranges (1-100 or 5-100 μg/mL) with a high correlation coefficients (R² > 0.9987). The intra-day, inter-day and column-to-column relative standard deviations (RSDs) in the migration times ranged from 0.44 to 4.87%, and the peak area RSDs ranged from 0.80 to 7.28%. The developed capillary electrochromatography method can be successfully utilized for the determination of sulfonamides in tap water and milk samples.

Recent advances in metal-organic frameworks and covalent organic frameworks for sample preparation and chromatographic analysis

In the field of analytical chemistry, sample preparation and chromatographic separation are two core procedures. The means by which to improve the sensitivity, selectivity and detection limit of a method have become a topic of great interest. Recently, porous organic frameworks, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely used in this research area because of their special features, and different methods have been developed. This review summarizes the applications of MOFs and COFs in sample preparation and chromatographic stationary phases. The MOF- or COF-based solid-phase extraction (SPE), solid-phase microextraction (SPME), gas chromatography (GC), high-performance liquid chromatography (HPLC) and capillary electrochromatography (CEC) methods are described. The excellent properties of MOFs and COFs have resulted in intense interest in exploring their performance and mechanisms for sample preparation and chromatographic separation.

A Rapid and Facile Detection for Specific Small-Sized Amino Acids Based on Target-Triggered Destruction of Metal Organic Frameworks

Most of the reported metal organic frameworks (MOFs)-based DNA sensors were developed by utilizing the different adsorption capacities of MOFs to different structural DNAs (for example, single-stranded DNAs (ssDNAs) and double-stranded DNAs (dsDNAs)) or ssDNAs with different lengths. Herein, we introduced another strategy for the design of MOFs-based biosensing platforms. We found that specific small-sized amino acids (for example, glycine and serine) could lead to the destruction of the MOFs formed by [Cu(mal)(bpy)]·2H₂O], thus recovering the fluorescence of a fluorophore-labeled ssDNA that had been quenched by MOFs. The corresponding working mechanism was discussed. On the basis of this finding, a mix-and-detect fluorescence method was designed for the turn-on detection of specific small-sized amino acids. The feasibility of its use in real serum samples was also demonstrated. Besides biosensing applications, the discovery of amino acids-triggered destruction of MOFs can also enrich the building blocks of molecular logic gate. As an example, a biomolecular logic gate that performs OR logic operation was constructed using glycine and a DNA strand as inputs.

Pressure inverse solubility and polymorphism of an edible γ-cyclodextrin-based metal–organic framework

An edible metal–organic framework, γ-CD-MOF-1, has been obtained in a new trigonal form. The γ-CD-MOF-1 high-pressure dissolution as well as the cubic and trigonal polymorphs are closely related and regulated by the guest framework interactions and adsorption in the pores.

γ-Cyclodextrin metal–organic framework for efficient separation of chiral aromatic alcohols

A γ-cyclodextrin metal–organic framework was applied as an efficient chiral stationary phase for HPLC separation of chiral aromatic alcohols.

Application of Homochiral Alkylated Organic Cages as Chiral Stationary Phases for Molecular Separations by Capillary Gas Chromatography

Molecular organic cage compounds have attracted considerable attention due to their potential applications in gas storage, catalysis, chemical sensing, molecular separations, etc. In this study, a homochiral pentyl cage compound was synthesized from a condensation reaction of (S,S)-1,2-pentyl-1,2-diaminoethane and 1,3,5-triformylbenzene. The imine-linked pentyl cage diluted with a polysiloxane (OV-1701) was explored as a novel stationary phase for high-resolution gas chromatographic separation of organic compounds. Some positional isomers were baseline separated on the pentyl cage-coated capillary column. In particular, various types of enantiomers including chiral alcohols, esters, ethers and epoxides can be resolved without derivatization on the pentyl cage-coated capillary column. The reproducibility of the pentyl cage-coated capillary column for separation was investigated using nitrochlorobenzene and styrene oxide as analytes. The results indicate that the column has good stability and separation reproducibility after being repeatedly used. This work demonstrates that molecular organic cage compounds could become a novel class of chiral separation media in the near future.

Approaches for enantioselective resolution of pharmaceuticals by miniaturised separation techniques with new chiral phases based on nanoparticles and monolithis

This article discusses new developments in the preparation of nanoparticles and monoliths with emphasis upon their application as the stationary and pseudo-stationary phases for miniaturised liquid phase separation techniques, which have occurred in the last 10 years (from 2006 to the actuality). References included in this review represent current trends and state of the art in the application of these materials to the analysis, by EKC, CEC and miniaturised chromatography, of chiral compounds with environmental interest such as pharmaceuticals. Due to their extraordinary properties, columns prepared with these new chiral stationary or pseudo-stationary phases, based on materials such as gold nanoparticles, metal-organic frameworks, ordered mesoporous silicas, carbonaceous materials, polymeric-based and silica-based monoliths or molecularly imprinted materials, can usually show some improvements in the separation selectivity, column efficiency and chemical stability in comparison with conventional chiral columns available commercially.

Chiral recognition in separation science - an update

Stereospecific recognition of chiral molecules is an important issue in various aspects of life sciences and chemistry including analytical separation sciences. The basis of analytical enantioseparations is the formation of transient diastereomeric complexes driven by hydrogen bonds or ionic, ion-dipole, dipole-dipole, van der Waals as well as π-π interactions. Recently, halogen bonding was also described to contribute to selector-selectand complexation. Besides structure-separation relationships, spectroscopic techniques, especially NMR spectroscopy, as well as X-ray crystallography have contributed to the understanding of the structure of the diastereomeric complexes. Molecular modeling has provided the tool for the visualization of the structures. The present review highlights recent contributions to the understanding of the binding mechanism between chiral selectors and selectands in analytical enantioseparations dating between 2012 and early 2016 including polysaccharide derivatives, cyclodextrins, cyclofructans, macrocyclic glycopeptides, proteins, brush-type selectors, ion-exchangers, polymers, crown ethers, ligand-exchangers, molecular micelles, ionic liquids, metal-organic frameworks and nucleotide-derived selectors. A systematic compilation of all published literature on the various chiral selectors has not been attempted.

Synthesis and characterization of a multimode stationary phase: Congo red derivatized silica in nano-flow HPLC

Congo red derivatized silica has been developed as a mixed mode stationary phase and used for nano-flow HPLC.

Separation of amino acid enantiomers by a capillary modified with a metal–organic framework

Covalent bonding of homochiral metal–organic framework in capillaries for amino acid enantiomer separation by capillary electrochromatography.