Non-conjugated flexible network for the functional design of silica-based stationary phase for mixed-mode liquid chromatography

Glutathione-functionalized covalent organic frameworks@silica as a hydrophilic-hydrophobic balanced mixed-mode stationary phase for highly efficient separation of compounds with a wide range of polarity

BACKGROUND

Covalent organic frameworks (COFs) are a highly promising stationary phase for high-performance liquid chromatography (HPLC), but the separation of polar compounds is limited by their low hydrophilicity. Therefore, it is crucial to develop novel COFs-based stationary phases with balanced hydrophilicity-hydrophobicity for the efficient separation of different polar compounds.

RESULTS

In this paper, glutathione (GSH)-functionalized COFs@silica microspheres (GSH-COFs@SiO2) were synthesized via a two-step, post-synthesis modification strategy. The COFs particles was constructed onto silica surface by the covalent conjugation of 1,3,5-tris(4-aminophenyl)benzene and 2,5-divinylterephthalaldehyde. GSH containing abundant -NH2 and -COOH groups was bonded onto the surface of COFs@SiO2 to further enhance hydrophilicity. The resulting GSH-COFs@SiO2 exhibited balanced hydrophilicity-hydrophobicity and can be used in hydrophilic/reversed-phase liquid chromatography modes through multiple retention mechanisms. Consequently, a variety of compounds with different polarity, including nucleosides/bases, benzoic acids, anilines, phenols, alkylbenzenes and polycyclic aromatic hydrocarbons, were well separated with ideal resolution, satisfactory column efficiency and good peak shapes. Furthermore, this novel column exhibited remarkable column stability, as evidenced by intra-day relative standard deviations of 0.08 %-0.18 % for retention time and 0.45 %-1.47 % for peak area.

SIGNIFICANCE AND NOVELTY

This work demonstrates the superior hydrophilic-hydrophobic selectivity of GSH-COFs@SiO2 stationary phases towards compounds with a wide range of polarity and provides a very facile and easily popularized post-synthetic modification route for hydrophilic-hydrophobic balanced COFs-based HPLC stationary phases.

An adhesive hydrogel functionalized silica sphere for polar analytes separation and analysis

In response to the challenges associated with the chromatographic separation of polar compounds, this study aims to devise a solution by introducing a novel stationary phase. Hydrogels, characterized by a three-dimensional network structure, have aroused wide attention owing to its functional designability, multiple interaction sites and good adhesion, etc. In this work, an adhesive hydrogel functionalized silica stationary phase (Sil@PVA/TA) was synthesized using physical coating technique. Due to the co-existence of hydroxyl and benzene ring in the hydrogel structure, the obtained composites materials exhibited excellent separation performance for various of compounds and excellent column efficiency up to 71385.6 plates/m for thymidine. Furthermore, the hydrogel functionalized silica demonstrated superior selectivity to bare silica, diol-column and NH2-column for the separation of various of polar molecules, including, nucleosides/bases, alkaloids, organic acids, antibiotics and amino acids. Notably, for alkaloids, which frequently encounter peak tailing issues, Sil@PVA/TA demonstrated superior peak shape compared with C18 column. In short, this study successfully synthesized a hydrogel functionalized silica stationary phase, offering a novel method for the separation and analysis of polar compounds.

Deep eutectic supramolecular polymers based HPLC stationary phase: Green synthesis strategy and promising application prospects

BACKGROUND

Deep eutectic solvents (DESs) have been widely and significantly applied in various fields due to their outstanding features such as low cost, easy preparation and good biodegradability. As novel derivatives of DESs, deep eutectic supramolecular polymers (DESPs) combine the macroscopic state of DESs with the covalent interactions of supramolecular polymers, which also possess the properties of DESs as multifunctional materials. Therefore, DESPs are believed to be promising candidates for separation science. However, there are no studies on the application of DESPs as stationary phases for HPLC analysis.

RESULTS

In this work, a novel DESP based HPLC stationary phase (Poly(DES)@SiO2) was developed for the first time through a green synthesis method by using DES as the polymerization monomer as well as the reaction medium. The results manifest that this novel Poly(DES)@SiO2 column can well interact with analytes through various mechanisms, and realize selective separation of a wide range of structurally similar hydrophilic/hydrophobic substances. More importantly, the separation of hydrophobic analytes on the Poly(DES)@SiO2 column is less time-consuming with fewer organic eluent, although the column efficiency is slightly lower than that of commercial C18 column. Furthermore, the Poly(DES)@SiO2 column exhibits excellent mechanical stability and satisfactory separation repeatability for steroid hormones. Therefore, a reliable method was established for detecting steroid hormones in actual samples with the recoveries ranging from 94.56 % to 103.84 %, which can meet the detection needs of commonly seen steroid hormones in food and the environment.

SIGNIFICANCE

In summary, this work provides some valuable theoretical references for the synthesis of new DESP based stationary phases through a green and facile strategy, and meanwhile, verifies the feasibility of DESP for effective HPLC separations. In addition, the promising application prospect of DESP based stationary phases in the analysis of complex samples has also been demonstrated, expanding the potential application of DES in separation science.

The series of L-lysine-derived gelators-modified multifunctional chromatography stationary phase for separation of chiral and achiral compounds

In this study, using chiral L-lysine as the molecular skeleton, three kinds of L-lysine-derived gelators (GBLB, GBLF and GFLF) were synthesized and then bonded to the surface of silica matrix (5 μm) by amide condensation to prepare a series of multifunctional chromatography stationary phases (GBLB-SiO2, GBLF-SiO2, and GFLF-SiO2) were prepared. The L-lysine-derived gelators not only possess chiral recognition ability, but also can spontaneously form oriented and ordered network structures in liquid medium through the interaction of non-covalent bonding forces such as hydrogen bonding, π-π stacking, and van der Waals forces. The comprehensive effect of multiple weak interaction sites enhances the molecular recognition ability and further improves the separation diversity of different types of compounds on stationary phases. The separation and evaluation of chiral compounds showed that benzoin, 1-phenyl-ethanol, 1-phenyl-propanol and 6-hydroxyflavanone could be separated in normal phase mode (NPLC). The separation of different types of non-chiral compounds, such as sulfonamides, nucleosides, nucleobases, polycyclic aromatic hydrocarbons (PAHs), anilines, and aromatic acids, were achieved in hydrophilic interaction/reversed-phase/ion-exchange mode (HILIC/RPLC/IEC), and the separation of polarized compounds could be performed under the condition of ultrapure water as the mobile phase, which has the typical retention characteristics of per aqueous liquid chromatography (PALC). The effects of organic solvent content, temperature, pH value, and buffer salt concentration on the retention and separation performance of the column were investigated. Comparison of the three prepared columns showed that the separation performance (such as aromatic selectivity) could be improved by increasing the types of functional groups on the surface of the stationary phase and the number of aromatic groups. In a word, the prepared stationary phase have multiple retention properties, can simultaneously separate chiral compounds and various types of achiral compounds. This work provides an idea for developing multifunctional liquid chromatography stationary phase materials, and further expands the application of gelators in separation science.

Facile synthesis of a novel polymer/covalent organic framework@silica composite material in deep eutectic solvent for mixed-mode liquid chromatographic separation

The solvothermal synthesis of covalent organic framework (COF) modified silica gel usually requires the use of harmful organic solvents, tedious steps, and harsh reaction conditions. In pursuit of green chemistry, a new strategy for the facile preparation of COF@SiO2 composite material was realized in this work by using a low-toxicity and low-cost deep eutectic solvent as the reaction medium. Additionally, a flexible polyacrylic acid (PAA) was introduced for the purpose of improving the hydrophilic selectivity and separation efficiency of COF@SiO2. Based on the above ideas, a novel PAA/COF@SiO2 composite was successfully developed as a liquid chromatographic packing material. Performance evaluation of the slurry-packed PAA/COF@SiO2 column showed that diverse types of analytes were effectively separated, and the retention behavior of polar nucleosides showed a U-shaped trend, indicating mixed-mode of hydrophobic/hydrophilic retention mechanisms. Thermodynamic studies revealed that the separation mechanism was largely independent of temperature. This work verifies the feasibility of synthesizing polymer/COF@SiO2 composite material in the deep eutectic solvent. This strategy provides a theoretical reference for the green and facile preparation of COF@SiO2 as an efficient liquid chromatographic stationary phase.

A composite hydrogel modified silica stationary phase for mixed‑mode liquid chromatography

A novel composite hydrogel functionalized silica core-shell stationary phase was prepared by the surface modification of silica sphere. The successful synthesis of the new stationary phase (T-Sil@PAM/SA/UiO-66-NH2) was proven by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), etc. Due to the coexistence of amide, hydroxyl, long carbon chain and UiO-66-NH2 in composite hydrogel shell, the obtained stationary phase can be used in hydrophilic/reversed-phase liquid chromatography with multiple retention mechanisms, such as hydrophilic, hydrophobic and π - π interactions. The chromatographic retention behavior of T-Sil@PAM/SA/UiO-66-NH2 demonstrated that the new stationary phase showed excellent separation performance for both polar analytes (such as alkaloids, saccharides, etc.) and nonpolar analytes (such as substituted benzene and polycyclic aromatic hydrocarbon (PAHs), etc.). Furthermore, compared with NH2 column and commercial C18 column, the T-Sil@PAM/SA/UiO-66-NH2 exhibited a certain superiority. Moreover, the relative standard deviation (RSD) of PAHs' retention time with eight replicates consecutive elution was found to range from 0.03% to 0.17%. Therefore, the successful use of T-Sil@PAM/SA/UiO-66-NH2 in mixed‑mode liquid chromatography expanded the potential applications of hydrogels in the field of separation.

System to screen and purify active ingredients from herbal medicines using hydrogel-modified human umbilical vein endothelial cell membrane chromatography coupled with semi-preparative high-performance liquid chromatography-offline-high-performance liquid chromatography-mass spectrometry

Analytical screening and validation systems based on a combination of cell membrane chromatography and two-dimensional chromatography-tandem mass spectrometry are incapable of providing prepared samples containing the active ingredients found in traditional Chinese medicine; therefore, these samples cannot be directly used in subsequent studies. In this study, a semi-preparative cell membrane chromatography column was developed using a hydrogel-modified carrier and human umbilical vein endothelial cells to optimize prepared conditions, such as hydrogel polymerization, cell fragmentation, and cell membrane volume. This increased the binding ratio of membrane protein and carrier to 15.79 mg/g. The column was systematically evaluated using multitarget tyrosine kinase inhibitors that displayed good specificity and reproducibility. Subsequently, using the column coupled with a semi-preparative high-performance liquid chromatography-offline-high-performance liquid chromatography-mass spectrometry system, 15 active ingredients were screened and purified from Indigo naturalis, and five main components were identified: l-lysine, oxyresveratrol, tryptanthrin, isorhamnetin, and indirubin. Furthermore, the pharmacological effects of the ingredients were confirmed using cell proliferation and apoptosis assays. Results revealed potent proliferation-inhibiting and apoptosis-promoting abilities on human chronic myelogenous leukemic cells and human promyelocytic leukemic cells (p < 0.001). Overall, the system presented screening and purification functions that could be used to prepare I. naturalis samples acting on the epidermal growth factor receptor and vascular endothelial cell growth factor.

Effect of stereoconfiguration of aromatic ligands on retention and selectivity of terphenyl isomer-bonded stationary phases

The structure of ligands has a significant influence on the separation properties of alkyl and aromatic phases in reversed-phase liquid chromatography. Compared with alkyl phases, the effect of stereoconfiguration of aromatic ligands on the retention and selectivity of stationary phases has rarely been addressed. To illustrate the issue, three terphenyl isomer-bonded stationary phases were prepared via the coupling chemistry of isocyanate with terphenyl amine isomers, 3,4-diphenylaniline, 2,4-diphenylaniline and 4-amino-p-terphenyl, respectively. The retention behaviors of stationary phases were assessed in terms of retention strength, selectivity, hydrophobic and π-π interaction by five kinds of solutes. It is found that the selectivity towards the solutes is slightly larger on the branched m-terphenyl-bonded phase (m-π³) than o-terphenyl-bonded phase (o-π³) but is significantly improved on the chain p-terphenyl-bonded phase (p-π³). The results can be interpreted by the ease self-adjustment of the conformation of the chain p-terphenyl ligand and the smaller steric effect of p-π³ towards the insertion of solutes into the ligand brushes. In addition, the p-π³ yields excellent selective separation towards aromatic solutes. These findings are of significance in the design of aromatic stationary phases.

Hydrogel Coating with Temperature Response Retention Behavior and Its Application in Selective Separation of Liquid Chromatography

We reporte the double-layer hydrogel-coated mesoporous silica material as a new stationary phase for liquid chromatography. The method of combining physical coating and chemical coating was to apply hydrogel coating on the surface of silica, and finally, a new type of liquid chromatography stationary phase with in situ coating of the functional hydrogel on silica was obtained. This hydrogel-functionalized liquid chromatography stationary phase also exhibits a certain temperature responsiveness. Experimental results show that this temperature response is mainly due to changes in the hydrogen bonding between the stationary phase and the analyte at different temperatures in the column oven, which leads to changes in retention behavior. The hydrogel-coated mesoporous silica microspheres showed excellent selectivity for many polar analytes. An excellent column efficiency was obtained (139 000 plates/m for terephthalic acid) after optimization of chromatographic conditions. In addition to rapid separation of some analytes, this new hydrogel stationary phase also has certain superiority in chromatographic performance compared with other new excellent liquid chromatography stationary phases functioned by three-dimensional cross-linking systems. The important thing is that this strategy is relatively easy to prepare a new stationary phase with different properties.

Mesoporous nanomaterial-assisted hydrogel double network composite for mixed-mode liquid chromatography

By introducing functional groups such as quaternary amine groups, sulfonic acid groups, triazine groups, and other mespore nanomaterials into the hydrogel, better separation effect of some organic framework materials has been obtained. Due to a reasonable design and preparation strategy, the hydrogel composite-modified silica can be used in the selective separation of various analytes such as pesticides, alkylbenzenes, polycyclic aromatic hydrocarbons, nucleosides/bases, benzoic acids, antibiotics, and carbohydrates. Through the exploration of chromatographic retention behavior, it is proved that the column can be used in mixed-mode liquid chromatography. The intra-day relative standard deviation for retention time of this new stationary phase is 0.12-0.16% (n = 10), and the inter-day relative standard deviation is less than 0.39% (n = 5). This new stationary phase can also be used for separation in complex samples. The limit of detection (LOD) for chlorotoluron in farm irrigation water is 0.21 µg/L and the linear range is 2-250 µg/L. After optimizing the chromatographic conditions, the highest efficiency of the hydrogel column in RPLC and HILIC modes has reached 32,400 plates/m (chlorobenzuron) and 41,300 plates/m (galactose). This new type of hydrogel composite is a porous network material with flexible functional design and simple preparation method and its application has been expanded in liquid chromatography separation successfully. The hydrogel composed of triallyl cyanate cross-linking agent and 3-(2-(methacryloyloxy) ethyl) dimethylamine) propane-1-sulfonate (SBMA) monomer which were co-modified on the surface of mesoporous silica with MOF-919 for separation in mixed-mode liquid chromatography.