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

A chiral porous organic cage-modified restricted-access material achieves online analysis of serum samples containing enantiomers and positional isomers

Restricted-access materials (RAMs) allow biological samples to directly enter the chromatographic column for analysis owing to the steric exclusion function ability for biomolecules and extraction function for small-molecule analytes, which promoting the development of rapid, efficient, and automated in vivo drug analysis. Few reports on chiral RAMs that have been used to analyze enantiomers and positional isomers in serum by direct injection in currently. In this study, a chiral porous organic cage material RCC3 was innovatively introduced into the inner surface of silica gel and modified the outer surface with polyethylene glycol to prepare a novel type of chiral RAM-RCC3, and reported the use of chiral RAM-RCC3 as a stationary phase for the separation of chiral compounds and positional isomers in blank serum using high-performance liquid chromatography. The novel RAM-RCC3 column exhibited good performance in the online analysis of nine enantiomers and five positional isomers in serum samples. The effects of analyte mass, temperature, and composition of the mobile phase on the separation of o-, m-, and p-nitrophenol in serum samples using the RAM-RCC3 column were also investigated. Even after 300 injections, the RAM-RCC3 column exhibited good reproducibility and stability. These results indicate the potential of the chiral RAM-RCC3 column as a stationary phase for direct injection analysis of both chiral separation and positional isomers in biological samples, which also rendering it suitable to be further developed as a new type of RAM for online analysis of various small molecules in biological samples.

Cyano-modified molecular cage silica gel stationary phase: Multi-functional chromatographic performance by high-performance liquid chromatography

This study successfully prepared different loading levels of cyano-functionalized RCC3 molecular cage silica gel stationary phase (RCC3-CN@SiO2) through aldehyde-amine condensation reaction and subsequent modification strategies. Fourier transform infrared spectroscopy, thermogravimetric analysis, nitrogen adsorption-desorption, and scanning electron microscopy confirmed the successful synthesis of RCC3-CN@SiO2 chromatographic stationary phase. The research demonstrates that due to hydrophobic/hydrophilic interactions, π-π interactions, hydrogen bonding, and size-selective porous structure, the stationary phase effectively separates moderately polar and weakly polar compounds in reversed-phase liquid chromatography (RPLC) mode, exhibiting hydrophobic selectivity comparable to the commercial DaisoC18-RP columns. Additionally, the tertiary amine and cyanogen groups on the molecular cage surface enhance the interaction with polar compounds, successfully separating nucleosides, sulfonamides, amino acids, and sugars in hydrophilic interaction chromatography (HILIC) mode. Further applications in the separation analysis of acidic drugs, alkaline drugs, cinnamic acid natural products, and chiral compounds demonstrate the multifunctional chromatographic capabilities for diverse compound types. Compared to Unitary Diol commercial columns, the prepared stationary phase showed significant advantages in wide polarity range separation performance. Moreover, through nucleoside compound separation mode switching analysis, RCC3-CN@SiO2 stationary phase further validates its favorable performance in both RPLC and HILIC modes, demonstrating extensive potential applications in the field of analytical chemistry. Importantly, the stationary phase exhibits efficient separation of nucleoside compounds in pure water systems, aligning with the principles of green analysis.

Preparation of D-NCCDs and its application in fluorescent/colorimetric dual-mode discrimination of glutamine enantiomers

A new type of carbon dots (D-NCCDs) was synthesized by 3, 5-diaminobenzoic acid, N,N-dimethyl-o-phenylenediamine, and D-cysteine. The morphology and structure of D-NCCDs were investigated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and FT-IR spectra, and the chirality was characterized by circular dichroism. In the presence of hydrogen peroxide, the fluorescence of D-NCCDs at 487 nm (λex = 410 nm) showed great discrimination ability towards glutamine enantiomers. The ratio of fluorescence intensity (F/F0) to the concentration of D-Gln showed good linearity in the range 0.5-10 mM, with a detection limits of 0.11 mM. Meanwhile, the color of the solution gradually changed from light yellow to yellow-brown. The UV-Vis absorption ratio (A/A0) at 410 nm showed good linearity with the concentration of D-Gln in the range 0.5 to 20 mM; the detection limit is 7.7 μM. But the fluorescence and absorbance of D-NCCDs showed no significant change after the addition of L-glutamine. Thus, fluorescence and colorimetry dual-mode discrimination of glutamine enantiomers was achieved. The fluorescence enantioselectivity of Gln (FL-Gln/FD-Gln) is 1.62, and the colorimetric enantioselectivity of Gln (AD-Gln/AL-Gln) is 2.14. The chiral discrimination mechanism of D-NCCDs to Gln enantiomers was also investigated systematically. This work not only can discriminate glutamine enantiomers with high sensitivity and convenience, but also offers a new strategy for preparing new dual mode chiral nanoprobes.

From individuals to families: design and application of self-similar chiral nanomaterials

Establishing an intimate relationship between similar individuals is the beginning of self-extension. Various self-similar chiral nanomaterials can be designed using an individual-to-family approach, accomplishing self-extension. This self-similarity facilitates chiral communication, transmission, and amplification of synthons. We focus on describing the marriage of discrete cages to develop self-similar extended frameworks. The advantages of utilizing cage-based frameworks for chiral recognition, enantioseparation, chiral catalysis and sensing are highlighted. To further promote self-extension, fractal chiral nanomaterials with self-similar and iterated architectures have attracted tremendous attention. The beauty of a fractal family tree lies in its ability to capture the complexity and interconnectedness of a family's lineage. As a type of fractal material, nanoflowers possess an overarching importance in chiral amplification due to their large surface-to-volume ratio. This review summarizes the design and application of state-of-the-art self-similar chiral nanomaterials including cage-based extended frameworks, fractal nanomaterials, and nanoflowers. We hope this formation process from individuals to families will inherit and broaden this great chirality.

A singular chromatographic column breakthrough: Achieving full polarity range separations with the epoxy propanol molecular cage bonded silica stationary phase

The epoxy propanol molecular cage bonded silica stationary phase, RCC3-GLD@silica, synthesized through the ring-opening reaction of secondary amine with epoxy propanol using RCC3-R as the scaffold unit, was successfully prepared as confirmed by infrared spectroscopy, thermogravimetric analysis, and nitrogen adsorption-desorption characterization. This stationary phase demonstrated excellent separation performance in both reversed-phase and hydrophilic chromatography modes, effectively separating a wide variety of compounds including alkylbenzenes, polycyclic aromatic hydrocarbons, phenols, anilines, sulfonamides, nucleosides, amino acids, sugars, and acids. The development of RCC3-GLD@silica benefits from the synergistic effects of its hydrophobic and hydrophilic actions, as evidenced by the U-shaped characteristic of the retention factor for nucleoside compounds with changes in the aqueous content of the mobile phase, further confirming the simultaneous presence of reversed-phase and hydrophilic chromatography mechanisms. Not only did this stationary phase successfully separate 33 compounds in reversed-phase chromatography mode, but it also separated 54 compounds in hydrophilic interaction chromatography mode, showcasing its broad separation capability from weakly polar to strongly polar compounds on a single chromatographic column. This indicates a wide application prospect in the field of chromatographic analysis.

Recent progress of porous cage materials in sample preparation, chromatographic separation, and detection

Porous cage materials with certain dimensions, sizes, shapes, and functions have been regarded as promising materials for sample preparation, chromatographic separation, and detection process. In contrast to infinite frameworks such as metal-organic frameworks or covalent organic frameworks, porous cage materials are constructed from discrete molecules containing at least one internal cavity. The well-defined cavities in porous cage materials provide opportunities for non-covalent interactions. These interactions can be programmed into the ligand design or supramolecular cage constructing using the cages as building blocks, offering various host-guest recognition with great selectivity. In this review, we desire to elucidate the fundamental principles governing the design and fabrication of porous cage materials with well-defined cavities, good solvent processability, and modifiable groups, the applications of these porous cage materials in sample preparation, chromatographic separation, and detection were discussed. The recent advantages of porous cage materials for the analysis process were summarized. We state the potential of these materials and provide an outlook for further application strategies. We expect that this review can inspire interest in the porous cage materials research area for analysis.

Functionalized Chiral Materials for Use in Chiral Sensors

Chirality represents a fundamental attribute within living systems and is a pervasive phenomenon in the natural world. The identification and analysis of chiral materials within natural environments and biological systems hold paramount importance in clinical, chemical, and biological sciences. Within chiral analysis, there is a burgeoning focus on developing chiral sensors exhibiting exceptional selectivity, sensitivity, and stability, marking it as a forefront area of research. In the past decade (2013-2023), approximately 1990 papers concerning the application of various chiral materials in chiral sensors have been published. Biological materials and nanomaterials have important applications in the development of chiral sensors, which accounting for 26.67% and 45.24% of the material-related applications in these sensors, respectively; moreover, the development of chiral nanomaterials is closely related to the development of portable and stable chiral sensors. Natural chiral materials, utilized as selective recognition units, are combined with carriers characterized by good physical and chemical properties through functionalization to form various functional chiral materials, which improve the recognition efficiency of chiral sensors. In this article, from the perspective of biological materials, polymer materials, nanomaterials, and other functional chiral materials, the applications of chiral sensors are summarized and the research prospects of chiral sensors are discussed.

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

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

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

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

Strategies for chiral separation: from racemate to enantiomer

Chiral separation has become a crucial topic for effectively utilizing superfluous racemates synthesized by chemical means and satisfying the growing requirements for producing enantiopure chiral compounds. However, the remarkably close physical and chemical properties of enantiomers present significant obstacles, making it necessary to develop novel enantioseparation methods. This review comprehensively summaries the latest developments in the main enantioseparation methods, including preparative-scale chromatography, enantioselective liquid-liquid extraction, crystallization-based methods for chiral separation, deracemization process coupling racemization and crystallization, porous material method and membrane resolution method, focusing on significant cases involving crystallization, deracemization and membranes. Notably, potential trends and future directions are suggested based on the state-of-art "coupling" strategy, which may greatly reinvigorate the existing individual methods and facilitate the emergence of cross-cutting ideas among researchers from different enantioseparation domains.

Recent advances in chiral liquid chromatography stationary phases for pharmaceutical analysis

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

Recent advances in chiral selectors immobilization and chiral mobile phase additives in liquid chromatographic enantio-separations: A review

For decades now, the separation of chiral enantiomers of drugs has been gaining the interest and attention of researchers. In 1991, the first guidelines for development of chiral drugs were firstly released by the US-FDA. Since then, the development in chromatographic enantioseparation tools has been fast and variable, aiming at creating a suitable environment where the physically and chemically identical enantiomers can be separated. Among those tools, the immobilization of chiral selectors (CS) on different stationary phases and the chiral mobile phase additives (CMPA) which have been progressed and studied extensively. This review article highlights the major advances in immobilization of CS together with their different recognition mechanisms as well as CMPA as a cheaper and successful alternative for chiral stationary phases. Moreover, the role of molecular modeling tool as a pre-step in the choice of CS for evaluating possible interactions with different ligands has been pointed up. Illustrations of reported methods and updates for immobilized CS and CMPA have been included.

Homochiral organic molecular cage RCC3-R-modified silica as a new multimodal and multifunctional stationary phase for high-performance liquid chromatography

In this work, homochiral reduced imine cage was covalently bonded to the surface of the silica to prepare a novel high-performance liquid chromatography stationary phase, which was applied for the multiple separation modes such as normal phase, reversed-phase, ion exchange, and hydrophilic interaction chromatography. The successful preparation of the homochiral reduced imine cage bonded silica stationary phase was confirmed by performing a series of methods including X-ray photoelectron spectroscopy, thermogravimetric analysis, and infrared spectroscopy. From the extracted results of the chiral resolution in normal phase and reversed-phase modes, it was demonstrated that seven chiral compounds were successfully separated, among which the resolution of 1-phenylethanol reached the value of 3.97. Moreover, the multifunctional chromatographic performance of the new molecular cage stationary phase was systematically investigated in the modes of reversed-phase, ion exchange, and hydrophilic interaction chromatography for the separation and analysis of a total of 59 compounds in eight classes. This work demonstrated that the homochiral reduced imine cage not only achieved multiseparation modes and multiseparation functions performance with high stability, but also expanded the application of the organic molecular cage in the field of liquid chromatography.

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

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