Sol-gel technique for the preparation of β-cyclodextrin gold nanoparticles as chiral stationary phase in open-tubular capillary electrochromatography

A cooperation tale of biomolecules and nanomaterials in nanoscale chiral sensing and separation

The cooperative relationship between biomolecules and nanomaterials makes up a beautiful tale about nanoscale chiral sensing and separation. Biomolecules are considered a fabulous chirality 'donor' to develop chiral sensors and separation systems. Nature has endowed biomolecules with mysterious chirality. Various nanomaterials with specific physicochemical attributes can realize the transmission and amplification of this chirality. We focus on highlighting the advantages of combining biomolecules and nanomaterials in nanoscale chirality. To enhance the sensors' detection sensitivity, novel cooperation approaches between nanomaterials and biomolecules have attracted tremendous attention. Moreover, innovative biomolecule-based nanocomposites possess great importance in developing chiral separation systems with improved assay performance. This review describes the formation of a network based on nanomaterials and biomolecules mainly including DNA, proteins, peptides, amino acids, and polysaccharides. We hope this tale will record the perpetual relation between biomolecules and nanomaterials in nanoscale chirality.

Dynamic in situ growth of bonded-phase silica nanospheres on silica capillary inner walls for open-tubular liquid chromatography

Silica nanospheres (SNS) were grown on the inner walls of silica capillaries through a dynamic in situ nucleation process to prepare a highly porous and large accessible surface area substrate. The SNS were then functionalized with octadecyl (C18), 3-aminopropyltriethoxysilane (APTES), beta-cyclodextrin (β-CD), and amino groups to develop robust and efficient chromatographic stationary phases. The modified silica capillaries were exploited for open-tubular liquid chromatography (OT-LC) and open-tubular capillary electrochromatography (OT-CEC) applications. The prepared stationary phases were compared to conventional capillaries in terms of separation performance. The synthesis process was optimized, and the bonded-phase stationary phases were characterized by the electron microscopy technique. The effects of different solvents, additives, and functional groups on the geometry and chromatographic resolving power of the SNS were envisaged. The capillaries modified with octadecyl groups were evaluated for the separation of non-steroidal anti-inflammatory drugs, phenones, alkenylbenzenes, and enantiomers of chlorophenoxy herbicides. As an application instance, an SNS-C18-coated capillary was utilized for the separation of alkenylbenzenes from clove extract and protein digest medium, through OT-LC and OT-CEC techniques, respectively. The β-CD functionalized capillary was applied for the OT-CEC separation of a dichlorprop racemic mixture.

Preparation of β-cyclodextrin covalent organic framework-immobilized poly(glycidyl methacrylate) nanoparticle-coated open tubular capillary electrochromatography column for chiral separation

A new enantioselective open-tubular capillary electrochromatography was developed employing poly(glycidyl methacrylate) nanoparticles/β-cyclodextrin covalent organic frameworks chemically immobilized on the inner wall of the capillary as a stationary phase. A pretreated silica-fused capillary reacted with 3-aminopropyl-trimethoxysilane followed by poly(glycidyl methacrylate) nanoparticles and β-cyclodextrin covalent organic frameworks through a ring-opening reaction. The resulting coating layer on the capillary was characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The electroosmotic flow was studied to evaluate the variation of the immobilized columns. The chiral separation performance of the fabricated capillary columns was validated by the analysis of the four racemic proton pump inhibitors including lansoprazole, pantoprazole, tenatoprazole, and omeprazole. The influences of bonding concentration, bonding time, bonding temperature, buffer type and concentration, buffer pH, and applied voltage on the enantioseparation of four proton pump inhibitors were investigated. Good enantioseparation efficiencies were achieved for all enantiomers. In the optimum conditions, the enantiomers of four proton pump inhibitors were fully resolved within 10 min with high resolutions of 9.5-13.9. The column-to-column and inter- to intra-day repeatability of the fabricated capillary columns through relative standard deviation were found better than 9.54%, exhibiting satisfactory stability and repeatability of the fabricated capillary columns.

Recent applications and chiral separation developments based on stationary phases in open tubular capillary electrochromatography (2019–2022)

Capillary electrochromatography (CEC) plays a significant role in chiral separation via the double separation principle, partition coefficient difference between the two phases, and electroosmotic flow-driven separation. Given the distinct properties of the inner wall stationary phase (SP), the separation ability of each SP differs from one another. Particularly, it provides large room for promising applications of open tubular capillary electrochromatography (OT-CEC). We divided the OT-CEC SPs developed over the past four years into six types: ionic liquids, nanoparticle materials, microporous materials, biomaterials, non-nanopolymers, and others, to mainly introduce their characteristics in chiral drug separation. There also added a few classic SPs that occurred within ten years as supplements to enrich the features of each SP. Additionally, we discuss their applications in metabolomics, food, cosmetics, environment, and biology as analytes in addition to chiral drugs. OT-CEC plays an increasingly significant role in chiral separation and may promote the development of capillary electrophoresis (CE) combined with other instruments in recent years, such as CE with mass spectrometry (CE/MS) and CE with ultraviolet light detector (CE/UV).

Open Tubular Capillary Electrochromatography-Mass Spectrometry for Analysis of Underivatized Amino Acid Enantiomers with a Porous Layer-Gold Nanoparticle-Modified Chiral Column

By developing a novel chiral column, we integrate open tubular capillary electrochromatography into sheathless mass spectrometry (MS) for efficient analysis of underivatized amino acid enantiomers. The chiral column is easily fabricated by modifying the inner surface of a capillary with a three-dimensional porous layer (PL, thickness ∼ 90 nm, pore size ∼ 30 nm) and gold nanoparticles and by introducing a chiral selector, thiol β-cyclodextrin (SH-β-CD), onto the modified surface via Au-S bonds. This approach greatly enhances the specific surface area and thus the ratio of the stationary phase to mobile phase and interaction between the stationary phase and analytes. The proposed PLOT@Au@CD column is coupled to the sheathless CE-ESI-MS system for chiral analysis of amino acid enantiomers. No derivatization of amino acids is required for chiral analysis, and baseline separation of a total of 15 pairs of amino acid enantiomers is achieved within 17 min with high column efficiencies of 5.60 × 10⁴ to 1.82 × 10⁶ N/m, high resolutions of 1.51-10.0, and low limits of detection between 0.02 and 0.09 μg/mL. The separation efficiency and MS intensity are only slightly decreased over 60 runs or after usage for 15 days, showing excellent repeatability and stability of the PLOT@Au@CD column. The proposed method is successfully applied to the determination of amino acid enantiomers in vinegar samples with satisfactory accuracy. Our study provides a new approach for developing a chiral stationary phase in the chromatographic separation technique, which can be easily coupled to sensitive MS detection, thus it would be of value for various applications in the fields of chiral analysis.

Polysaccharide- and β-Cyclodextrin-Based Chiral Selectors for Enantiomer Resolution: Recent Developments and Applications

Polysaccharides, oligosaccharides, and their derivatives, particularly of amylose, cellulose, chitosan, and β-cyclodextrin, are well-known chiral selectors (CSs) of chiral stationary phases (CSPs) in chromatography, because they can separate a wide range of enantiomers. Typically, such CSPs are prepared by physically coating, or chemically immobilizing the polysaccharide and β-cyclodextrin derivatives onto inert silica gel carriers as chromatographic support. Over the past few years, new chiral selectors have been introduced, and progressive methods to prepare CSPs have been exploited. Also, chiral recognition mechanisms, which play a crucial role in the investigation of chiral separations, have been better elucidated. Further insights into the broad functional performance of commercially available chiral column materials and/or the respective newly developed chiral phase materials on enantiomeric separation (ES) have been gained. This review summarizes the recent developments in CSs, CSP preparation, chiral recognition mechanisms, and enantiomeric separation methods, based on polysaccharides and β-cyclodextrins as CSs, with a focus on the years 2019-2020 of this rapidly developing field.

Preparation of a polydopamine/β-cyclodextrin coated open tubular capillary electrochromatography column and application for enantioseparation of five proton pump inhibitors

An open tubular capillary electrochromatography column was prepared by immobilizing β-cyclodextrin on the inner wall of pretreated capillary via noncovalent adsorption of polydopamine. The resulting coating layer on the capillary was characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. Electroosmotic flow was studied to evaluate the variation of the immobilized columns. The prepared columns showed good chiral separation performance toward five proton pump inhibitors including lansoprazole, pantoprazole, tenatoprazole, rabeprazole, and omeprazole. The influences of β-cyclodextrin concentration, coating time, buffer pH, buffer concentration, and applied voltage on separation were investigated. In the optimum conditions, the enantiomers of five analytes were fully resolved within 15 min with high resolutions of 4.57 to 8.13. The method was extensively validated in terms of accuracy, precision, and linearity and proved to be robust. The relative standard deviation values for migration times and peak areas of the analytes representing intraday and interday were less than 1.9 and 3.6%, respectively. Further, the polydopamine/β-cyclodextrin coated capillary column could be successively used over 100 runs without showing significant decrease in the separation efficiency.

Open tubular capillary column immobilized with sulfobutylether-β-cyclodextrin for chiral separation in capillary electrochromatography

A novel chiral open tubular capillary column was fabricated with sulfobutylether-β-cyclodextrin and glycidyl methacrylate for enantioseparation in capillary electrochromatography. First, the pretreated silica-fused capillary was treated with 3-trimethoxysilyl propyl methacrylate to attach double bond ligand onto the surface. A copolymer layer was formed on the surface of capillary using glycidyl methacrylate and ethylene dimethacrylate by in situ one-pot polymerization. Sulfobutylether-β-cyclodextrin was encapsulated inside the copolymerized layer. The morphology of the developed column was characterized by field emission scanning electron microscopy. The effect of organic percentage and pH value of the mobile phase on electroosmotic flow and resolution was also investigated. The performance of the fabricated column was validated by separation of amlodipine besilate, 2,3-diphenylpropionic acid, tropic acid, and pantoprazole enantiomers with good resolutions of 3.67, 4.82, 3.34, and 2.61, respectively. The repeatabilities of column-to-column and day-to-day through relative standard deviation were found better than 4%, exhibiting satisfactory repeatability of the developed column. The results reveal that open tubular capillary columns modified with β-cyclodextrin show a great prospect for enantioseparation of chiral drugs in capillary electrochromatography.

Hydroxypropyl-β-cyclodextrin encapsulated stationary phase based on silica monolith particles for enantioseparation in liquid chromatography

Hydroxypropyl-β-cyclodextrin-encapsulated stationary phase incorporated on silica monolith particles was prepared by physical embedding, providing a new method for the development of chiral stationary phase for enantioseparation in liquid chromatography. Ground silica monolith particles of about 2.0 μm were prepared via sol-gel reaction followed by differential sedimentation. Initially, the silica monolith particles were pretreated with 3-trimethoxysilyl propyl methacrylate to attach double-bonded ligands onto the surface, then a network structure was formed onto the surface of the particle using N-isopropyl acrylamide as functional monomer. Hydroxypropyl-β-cyclodextrin was encapsulated inside N-isopropyl acrylamide copolymerized layer on the surface of silica monolith particles. The effect of the amount of chiral selector on the chromatographic efficiency of the chiral stationary phase was examined. The glass lined stainless steel columns (1 mm internal diameter, 300 mm length) were packed with the stationary phase for estimation of the efficiency by separation of phenylsuccinic acid, oxybutynin, equol, and naproxen enantiomers in high-performance liquid chromatography, with the resolutions of 1.54, 1.72, 2.54, and 2.31, respectively. The column to column repeatabilities through relative standard deviation were found better than 3%. The experimental results indicate that the sol-gel ground silica particles modified with β-cyclodextrin provide a promising way for the separation of chiral enantiomers.

Preparation of a thiols β-cyclodextrin/gold nanoparticles-coated open tubular column for capillary electrochromatography enantioseparations

Inspired by the distinct chemical and physical properties of nanoparticles, here a novel open-tubular capillary electrochromatography column was prepared by electrostatic assembly of poly(diallydimethylammonium chloride) onto the inner surface of a fused-silica capillary, followed by self-adsorption of negatively charged SH-β-cyclodextrin/gold nanoparticles. The formation of the SH-β-cyclodextrin/gold nanoparticles coated capillary was confirmed and characterized by scanning electron microscopy and energy dispersive spectrometry. The results of scanning electron microscopy and energy dispersive spectrometry studies indicated that SH-β-cyclodextrin/gold nanoparticles were successfully coated on the inner wall of the capillary column. The performance of the SH-β-cyclodextrin/gold nanoparticles coated capillary was validated by the analysis of six pairs of chiral drugs, namely zopiclone, carvedilol, salbutamol, terbutaline sulfate, phenoxybenzamine hydrochloride, and ibuprofen. Satisfactory enantioseparation results were achieved, confirming the use of gold nanoparticles as the support could enhance the phase ratio of the open-tubular capillary column. Additionally, the stability and reproducibility of the SH-β-cyclodextrin/gold nanoparticles coated capillary column were also investigated. Then, this proposed method was well validated with good linearity (≥0.999), recovery (90.0-93.5%) and repeatability, and was successfully used for enantioseparation of ibuprofen in spiked plasma samples, which indicated the new column's potential usage in biological analysis.

Gold nanoparticles-functionalized monolithic column for enantioseparation of eight basic chiral drugs by capillary electrochromatography

Poly(glycidyl methacrylate)-co-(ethylene dimethacrylate) [poly(GMA-co-EDMA)] monoliths were prepared, and used as a support to attach gold nanoparticles (AuNP) via Au-S bond. Pepsin, acting as a chiral selector, was linked to the surface of the carboxyl-modified AuNP through a hydrochloride/N-hydroxysuccinimide coupling reaction. The material was characterized by scanning electron microscopy, energy dispersive X-ray spectrometry, transmission electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy and N₂ adsorption-desorption isotherm. The pepsin@AuNP@poly(GMA-co-EDMA) monolith showed preferable enantioselectivity for hydroxychloroquine (HCQ), chloroquine (CHQ), hydroxyzine (HXY), labetalol (LAB), nefopam (NEF), clenbuterol (CLE), amlodipine (AML) and chlorpheniramine (CHL) in capillary electrochromatography (CEC). These racemic drugs were monitored at the maximum absorption wavelength (220 nm for HXQ, CHQ, HXY, LAB, NEF; 240 nm for AML; 215 nm for CLE, CHL). In comparison with the pepsin@poly(GMA-co-EDMA) monolith loaded with 5 nm AuNP, the pepsin@poly(GMA-co-EDMA) monolith loaded with 13 nm AuNP shows significantly enhanced enantiomeric resolution (HCQ: 0.62 → 3.45; CHQ: 0.60 → 2.11; HXY: 0.49 → 2.30; LAB: 1.03 → 2.45, 1.45 → 3.46, 0 → 0.67; NEF: 0.53 → 1.29; CLE: 0.42 → 0.56; AML: 0 → 0.83; CHL: 0.24 → 0.55). Pepsin concentration, buffer pH value, buffer concentration and applied voltage were investigated in detail with (±) HCQ and (±) HXY as model analytes. The reproducibility of intra-day, inter-day and column-to-column were explored, and found to be satisfactory. Graphical abstractSchematic presentation of the preparation of gold nanoparticles (AuNP) modified.

Preparation and evaluation of biselector bonded-type multifunctional chiral stationary phase based on dialdehyde cellulose and 6-monodeoxy-6-monoamino-β-cyclodextrine derivatives

A novel biselector bonded-type multifunctional chiral stationary phase (MCSP) was prepared by covalently crosslinking dialdehyde cellulose (DAC) with 6-monodeoxy-6-monoamino-β-cyclodextrine (CD) via Schiff base reaction. The biselector bonded-type MCSP had good chiral and achiral chromatographic performance in normal phase (NP) and reversed phase (RP) modes. Seven and eight enantiomers were successfully separated on the prepared biselector bonded-type MCSP in NP and RP modes, respectively. The biselector bonded-type MCSP showed enhanced chiral resolution ability compared with single selector chiral stationary phases due to the simultaneous introduction of DAC and 6-monodeoxy-6-monoamino-β-CD on the surface of silica gel. Aromatic compounds including polycyclic aromatic hydrocarbons, anilines, phenols, phenylates, and aromatic acids were choosed as analytes to investigate the achiral chromatographic performance of the biselector bonded-type MCSP in NP and RP modes. Chromatographic evaluation results showed that the above aromatic compounds were essentially capable of achieving baseline separation by hydrophobic interaction, π-π interaction, and π-π electron-donor-acceptor interaction. Moreover, the host-guest inclusion effect of 6-monodeoxy-6-monoamino-β-CD and the multiple interactions made the biselector bonded-type MCSP have good steric selectivity. The preparation method of the biselector bonded-type MCSP was simple and provided a new idea and strategy for the preparation of the subsequent novel biselector MCSP.

Recent developments of monolithic and open-tubular capillary electrochromatography (2017-2019)

Capillary electrochromatography, which combined the high selectivity of high-performance liquid chromatography and the high separation efficiency of capillary electrophoresis, is an attractive separation tool. In this review, the developments on monolithic and open tubular capillary electrochromatography during 2017 to August 2019 are summarized. Considering the development of novel stationary phases is the most active research field in capillary electrochromatography, monolithic capillary electrochromatography is classified according to the polymer-based and hybrid monolithic columns, while open-tubular capillary electrochromatography is categorized by cyclodextrin, silica, polymer, nanomaterials, microporous materials, and biomaterials-based open tubular columns.