Octadecylimidazolium ionic liquids-functionalized carbon dots and their precursor co-immobilized silica as hydrophobic chromatographic stationary phase with enhanced shape selectivity

Construction of MOFs@COFs composite material as stationary phase for efficient separation of diverse organic compounds

BACKGROUND

The development of efficent chromatographic stationary phases (SP) with mixed-mode or multiple interactions in high-performance liquid chromatography (HPLC) for the separation of complex samples is a challenging task. Metal organic frameworks (MOFs)-based SP can provide desired multiple interactions and enable the separation of a diverse range of solutes, but have limitations of low column efficiency and poor stability.

RESULTS

Herein, the hybrid MOFs@Covalent organic frameworks (COFs) materials were used as SP in HPLC due to their synergistic structural features. The SiO2@NH2-UiO-66@CTF SP was synthesized by integration of NH2-UiO-66 and covalent triazine framework (CTF) onto silica surface. Due to the unique structure of SiO2@NH2-UiO-66@CTF with hierarchical-pores, this column showed higher column efficiency (up to 49,369 plates m-1 for alkylbenzenes) than the reported columns packed with MOFs-based SP. Owing to the Zr4+-N coordination bonding between CTF and NH2-UiO-66, the structural stability of SiO2@NH2-UiO-66@CTF can be improved. Furthermore, this new column exhibited remarkable column stability with relative standard deviation of retention time of <0.40% after 400 injections. With the combined advantages of multifunctional properties, high column efficiency, and good stability, SiO2@NH2-UiO-66@CTF SP showed excellent selectivity for the separation of a variety of hydrophobic, aromatic, heteroatomic, and hydrophilic analytes.

SIGNIFICANCE AND NOVELTY

This work not only offers a promising SP with multiple retention mechanisms for HPLC, but also provides an efficient strategy for development of high column efficiency MOFs-based SP with good stability. Moreover, the MOFs@COFs hybrid materials were expanded in application area through this study, and the research results can also afford the foundation for further explore its structural characteristics.

Tetraethylenepentamine-derived carbon dots and tetraethylenepentamine co-immobilized silica stationary phase for hydrophilic interaction chromatography

In this work, tetraethylenepentamine (TEPA) was used as precursor and reaction medium to prepare tetraethylenepentamine-functionalized carbon dots (TEPACDs), the resultant mixture was subsequently silanized and then grafted on the surface of bare silica. The obtained tetraethylenepentamine-functionalized carbon dots and tetraethylenepentamine co-modified silica stationary phase (Sil-TEPA/CDs) was characterized by multiple ways, such as Fourier transformed infrared spectroscopy (FTIR), elemental analysis and transmission electron microscope, which revealed the successful preparation of the mixed stationary phase and higher density of functional groups on co-modified stationary phase than precursor single-modified stationary phase. The synergistic effect of TEPACDs and TEPA was proved by comparing the separation performance of Sil-TEPA/CDs and Sil-TEPA toward amino acids, nucleosides, and nucleobases, which distinctly enhanced the selectivity of Sil-TEPA/CDs. Thus, 12 nucleosides and nucleobases and 11 amino acids was nicely separated on Sil-TEPA/CDs. By study the influences of the changes of mobile phase composition, mobile phase buffer concentration and buffer pH on the retention behaviors of Sil-TEPA and Sil-TEPA/CDs, it was found that both hydrophilic partitioning and adsorption of analytes on Sil-TEPA/CDs were enhanced benefit from the co-existence of TEPA and TEPACDs, which provided the analytes better separation performance. By comparing the column quality of Sil-TEPA/CDs with four commercially available columns, Sil-TEPA/CDs exhibited the best peak asymmetry of 0.98, and second best column efficiency of 43895 m-1 using guanosine as analyte. The RSD (n = 9) of the retention times of five selected analytes on Sil-TEPA/CDs were within 0.30-0.61% during 40 h of continuously elution, which implied excellent stability of prepared packing material.

Hydrophobic/hydrophilic separation performance evaluation of a mixed-mode ionic liquid embedded stearyl thioglycolate functionalized silica stationary phase

In this work, a novel imidazolium ionic liquid embedded multifunctional chromatographic stationary phase (Sil-AVI-ST) was synthesized by the radical-mediated thiol-ene click reaction. A wide range of samples including hydrophilic sulfonamides, vitamins and nucleosides/bases as well as hydrophobic phthalates, bisphenols, alkylphenols and steroid hormones were selected to evaluate the separation ability of the newly obtained Sil-AVI-ST. As expected, an efficient separation of the above tested analytes was successfully achieved in different chromatographic modes. It was proved that multiple stationary phase-analyte interaction forces promoted the selective separation. The Sil-AVI-ST column provided multiple retention mechanisms, enabling the efficient separation of diverse analytes with different polarity. More importantly, embedding a polar ligand (1-allyl-3-vinyl-imidazolium) could improve the separation efficiency of long-chain alkyl bonded stationary phases for hydrophilic analytes, and the developed Sil-AVI-ST column could also realize the detection of hydrophobic analytes under water-rich conditions, which is impossible for the conventional hydrophobic columns. Therefore, the newly prepared Sil-AVI-ST stationary phase has a good practical application potential.

Preparation, characterization, and separation mechanism of a dehydroabietic-acid-based shape-selective chromatographic stationary phase1

Chromatographic stationary phases with molecular-shape selectivity are advantageous for the separation and analysis of geometric isomers. Herein, dehydroabietic acid is bonded on the surface of silica microspheres via 3-glycidoxypropyltrimethoxysilane to form a monolayer dehydroabietic-acid stationary phase (Si-DOMM) with a racket-shaped structure. Various characterization techniques indicate that Si-DOMM is successfully prepared, and the separation performance of a Si-DOMM column is evaluated. The stationary phase has a low silanol activity and metal contamination and a high hydrophobicity and shape selectivity. The resolutions of lycopene, lutein, and capsaicin on the Si-DOMM column confirm that the stationary phase exhibits high shape selectivity. The elution order of n-alkyl benzene on the Si-DOMM column indicates its high hydrophobic selectivity and suggests that the separation is an enthalpy-driven process. Repeatability experiments reveal highly stable preparation processes of the stationary phase and column and indicate that the relative standard deviations of retention time, peak height, and peak area are less than 0.26%, 3.54%, and 3.48%, respectively. Density functional theory calculations using n-alkylbenzenes, polycyclic aromatic hydrocarbons, amines, and phenols as model solutes provide an intuitive and quantitative description of the multiple retention mechanisms. The Si-DOMM stationary phase exhibits superior retention and high selectivity for these compounds via multiple interactions. The bonding phase of the monolayer dehydroabietic acid stationary phase with a racket-shaped structure has a unique affinity for benzene, strong shape selectivity, and good separation performance for geometrical isomers with different molecular shapes.

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.

Preparation and characterization of glucose-based covalent organic polymer coated silica as stationary phase for high-performance liquid chromatography

Carbohydrate is a renewable, sustainable, hydrophilic, and biodegradable natural product, which is widely used in the field of adsorption. In this study, a glucose-based covalent organic polymer (COP) coated silica was fabricated by facile solvent knitting reaction between tetrabenzylglucose and silica-phenyl with anhydrous aluminum trichloride as catalyst, forming a core-shell stationary phase (donated as SiO₂@COP_Bn-glu) for high performance liquid chromatography. The prepared SiO₂@COP_Bn-glu was characterized by scanning electron microscopy, transmission electron microscopy, Fourier-transform infrared spectrometry, solid-state ¹³C nuclear magnetic resonance spectrometry, X-ray photoelectron spectroscopy, and N₂ adsorption-desorption experiments. Owing to the coexistence of benzene units and alkyl, hydroxyl and ether groups in the skeleton of COP_Bn-glu shell, the developed chromatographic packing exhibited reversed-phase/hydrophilic interaction mixed-mode with multiple retention mechanisms, such as hydrophobic, π-π, hydrogen bonding, and electron donor-acceptor interactions. The results revealed that the SiO₂@COP_Bn-glu column demonstrated excellent selectivity and retention behavior for both hydrophilic and hydrophobic compounds with good repeatability and stability. Meanwhile, the chromatographic performance of the prepared SiO₂@COP_Bn-glu column was compared with a C₁₈ column to assess the role of the coating COP_Bn-glu shell. Therefore, the development of the SiO₂@COP_Bn-glu stationary phase expands the potential application of COPs in separation field.

Fabrication and evaluation of dodecyl imide maleopimaric acid glycidyl methacrylate ester modified silica with multiple retention mechanisms for reversed phase liquid chromatography

As green, less toxic, and abundant ligands with rich functional groups, natural products are widely used in synthesis of chromatographic stationary phases. In this work, dodecyl imide maleopimaric acid glycidyl methacrylate ester (C12-MPAGN) was prepared from maleopimaric acid through the imidization and ring-opening based esterification reaction. By using "thiol-ene" click chemistry, it was chemically bonded to the silica and (3-mercaptopropyl) trimethoxysilane (γ-MPS) was used as the coupling agent to obtain dodecyl imide maleopimaric acid glycidyl methacrylate ester bonded silica stationary phase (Sil-C12-MPAGN). Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopies (SEM), and elemental analysis (EA) were utilized to verify that the Sil-C12-MPAGN stationary phase was successfully prepared with C12-MPAGN immobilized on the silica surface. In order to evaluate the chromatographic performance and retention mechanisms of the Sil-C12-MPAGN column and compared with C18 column, a variety of compounds were used, including stander mixture of Tanaka, alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs), phenols and flavonoids. Based on these multiple interactions, including hydrophobic, hydrogen-bonding, and π-π interactions, high selectivity and superior separation performance were demonstrated by the Sil-C12-MPAGN column for probe molecules what had previously been mentioned. In addition, the quantitative determination of paclitaxel content in Yew bark extract was conducted with this column, which was found that the concentration was 83.67 mg/L, respectively. In short, the present study proposes a new strategy for introducing rosin to liquid chromatography with high selectivity and separation performance.

Grafting of Tetraphenylethylene on Silica Surface, Characterizations, and Their Chromatographic Performance as Reversed-Phase Stationary Phases

Surface modification is an effective way to functionalize the materials so as to get some special properties. Tetraphenylethylene (TPE) has been widely investigated as a well-known reagent which has the nature of aggregation-induced emission (AIE), but has never been reported in the liquid chromatography stationary phase. In this work, TPE-grafted silica (Sil-TPE) was obtained successfully using the derivative of 1-(4-hydroxyphenyl)-1,2,2-triphenylethylene as a ligand, and then characterized by elemental analysis, Fourier transform infrared spectra, thermogravimetric analysis, and so forth. Laser scanning confocal microscopy images reflected the AIE phenomenon of grafted TPE because the internal vibration and rotation of TPE molecules were restrained in the confined silica space. The contact angle test showed superhydrophobic properties of Sil-TPE. In order to understand thoroughly the mechanism of chromatographic performance and retention behavior for Sil-TPE, Tanaka test mixture, alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs), and phenols were separated. This reveals that Sil-TPE has strong aromaticity and certain shape selectivity, especially, has excellent separation performance for PAHs and phenols. The thermodynamic properties and repeatability of Sil-TPE were further studied, which showed the stability of Sil-TPE. This work shows that TPE can be successfully grafted on silica surface and it has the potential to be a new kind of promising stationary phases in the future.

Phosphorus-doped deep eutectic solvent-derived carbon dots-modified silica as a mixed-mode stationary phase for reversed-phase and hydrophilic interaction chromatography

In this work, phosphorus-doped carbon dots (P-DESCDs) were successfully prepared using choline chloride/lactic acid type deep eutectic solvent and phosphoric acid as ingredients, and (3-aminopropyl) trimethoxysilane was used as a bridge to graft P-DESCDs onto the silica surface to obtain a new mixed-mode stationary phase (Sil-P-DESCDs) for reversed-phase and hydrophilic interaction liquid chromatography. The successful preparation of the stationary phase was confirmed by laser scanning confocal microscopy, elemental analysis, and Fourier transform infrared spectrometry. Interestingly, the doping of phosphorus greatly improved the separation performance and hydrophilicity of the Sil-P-DESCDs column. The Sil-P-DESCDs column was found to have certain hydrophobicity, hydrogen bonding ability and shape selectivity by Tanaka and Engelhardt standard test mixtures, and a series of hydrophilic and hydrophobic compounds such as alkylbenzenes, polycyclic aromatic hydrocarbons, sulfonamides, aromatic amines, phenols, flavonoids, nucleoside bases, and alkaloids. In addition, the effects of mobile phase ratio, column temperature, flow rate, salt concentration, and pH on the retention of analytes on Sil-P-DESCDs columns were investigated. Finally, the Sil-P-DESCDs column was applied to the qualitative and quantitative analysis of calcein-7-glucoside in the real sample of medicinal Astragalus pellets, and it was found at a concentration of 0.02 mg/mL.

Dendritic organic molecular gel coating with molecular shape selectivity and its application in selective separation by liquid chromatography

Dendritic organic molecular gels are a promising class of three-dimensional network compounds. Here, we have synthesized a new type of dendritic organic molecular gel stationary phase (SiO₂-G3) by using benzyl alcohol as raw material and dimethyl 5-hydroxyisophthalate as growth unit to synthesize a third-generation organic molecular gel G3, which grafted onto the silica surface by cyanogen chloride (CC). The developed stationary phase not only exhibits high molecular shape selectivity but also has a RPLC/HILIC/IEC mixed-mode characteristic for HPLC due to the ordered structure, the multiple strong π-π stacking interactions and the introduction of a hydrophilic triazine fraction during the grafting process. Compared with a commercial C₁₈ column, the developed column exhibited flexible selectivity, enhanced separation performance and excellent separation of monosubstituted benzene, polycyclic aromatic hydrocarbons (PAHs), positional isomers, nucleosides and nucleobases, benzoic acid and aniline compounds. In addition, the new column provided baseline separation of polycyclic aromatic hydrocarbon contaminants in Yellow River water, verifying its potential for application in the analysis of real samples.

Deep eutectic solvents-derivated carbon dots-decorated silica stationary phase with enhanced separation selectivity in reversed-phase liquid chromatography

In this work, deep eutectic solvents-based carbon dots (DESCDs) were prepared and bonded to the silica surface for the first time to form a new hydrophobic chromatographic stationary phase (Sil-DESCDs). The successful preparation of DESCDs and Sil-DESCDs were demonstrated by a series of characterizations including transmission electron microscopies, laser scanning confocal microscope, Fourier transform infrared spectrometry, elemental analysis, etc. Retention behavior of Sil-DESCDs was evaluated using Tanaka and Engelhardt standard test mixtures. The results showed that this new stationary phase had excellent separation performance for polycyclic aromatic hydrocarbons, flavonoids, aromatic amines and phenolic compounds. Excellent separation selectivity for the 3-phenylene ring isomers including phenanthrene and anthracene, the 4-phenylene ring isomers including pyrene, triphenylene, chrysene and 1,2-benzanthracene was also obtained. Especially, prednisolone and hydrocortisone, which have very similar structures, can be separated using pure water as the mobile phase. In addition, the flavonoids in Astragalus extracts including calycosin-7-glucoside, ononin, calycosin and formononetin were determined using this new column, their concentrations were 0.050, 0.031, 0.023 and 0.034 mg/mL, respectively.

Effect of phenyl numbers in polyphenyl ligand on retention properties of aromatic stationary phases

Aromatic phase, as one type of reversed-phase stationary phases, shows complementary selectivity to the n-alkyl counterparts especially for certain challenging separation tasks. However, effect of phenyl numbers in aromatic ligands on retention behaviors has rarely been addressed compared with the alkyl stationary phases. To illustrate the issue, a series of polyphenyl stationary phases were facially prepared via the coupling chemistry of isocyanate with amine, including aniline (π¹), 4-aminobiphenyl (π²), 4-amino-p-terphenyl (π³) and [1,1':4',1'':4'',1'''-quaterphenyl]-4-amine (π⁴), respectively. The chromatographic behaviors of the new stationary phases as well as the traditional C18 were systematically compared in terms of retention mode, hydrophobic and aromatic selectivity, shape selectivity and π-π interaction by various analytes, including alkylbenzenes, polycyclic aromatic hydrocarbons congeners and substituted benzenes with electron-withdrawing groups. Due to the homologous structure of four polyphenyl ligands, the hydrophobic selectivity, aromatic selectivity and shape selectivity of stationary phases increase with phenyl numbers in the bonded polyphenyl ligands, whereas the increment becomes insignificant from U-π³ to U-π⁴. This phenomenon is explained by the insertion degree of analytes in the polyphenyl ligand brushes. Compared with the homemade C18, the polyphenyl phases indicate insignificant changes of shape selectivity with temperature. Notably, the new polyphenyl phases demonstrate the great selective separation towards the electron-deficient compounds through the π-π interaction. These findings make up for the understanding of the retention behavior of aromatic stationary phases.

Polymerized phosphonium ionic liquid functionalized silica microspheres as mixed-mode stationary phase for liquid chromatographic separation of phospholipids

There is a large and growing demand for the vigorous development of new high performance liquid chromatography stationary phases in order to solve complex phospholipids separation. Herein, phosphonium-based ionic liquid trioctyl(allyl)phosphonium bromide ([P_888Allyl]Br) was first synthesized with trioctylphosphine and allyl bromide. With [P_888Allyl]Br as the polymerizable monomer, polymerized phosphonium ionic liquid functionalized silica microsphere (PIL@SiO₂) was further synthetized via click chemistry reaction. Significantly, based on the inherent amphiphilic nature of the introduced [P_888Allyl]Br, the packed PIL@SiO₂ column displayed hydrophilic/hydrophobic mixed-mode retention mechanisms. The PIL@SiO₂ column can achieve separation of nucleic acid bases and nucleosides, sulfonamides, amides and anilines with excellent selectivity in a shorter separation time. The column efficiency reached 109,700 N/m for 2-iodoacetamide. One of the important characteristics of the PIL@SiO₂ column is that both phospholipid classes and species can be efficiently separated via the same column, outperforming that of the commercial amino column. Furthermore, the application potential of the PIL@SiO₂ column was further verified via separation of phospholipids extracted from soy lecithin. The proposed PIL@SiO₂ column provides a promising candidate for separation of complex phospholipid samples.