One-pot synthesis of SiO2@SiO2 core-shell microspheres with controllable mesopore size as a new stationary phase for fast HPLC separation of alkyl benzenes and β-agonists

Simultaneous determination of fourteen β2-agonist enantiomers in food animal muscles by liquid chromatography coupled with tandem mass spectrometry

Illegal drug residues in animal derived foods are closely related to human's life and health. Studies on illegal drug residues and the metabolism, such as β2-agonists in animals have attracted more and more attention. In most cases, β2-agonists are suppliedand used astheracemate. The metabolic process and distribution of the two enantiomers in animal tissues are different. Therefore, it is very necessary to develop a simple and fast method for chiral resolution of these drugs in animal tissues. In this paper, a reliable resolution and determination method was presented using liquid chromatography-tandem mass spectrometry (LC-MS/MS) for fourteen enantiomers of seven β2-agonist racemates, clenbuterol (CLE), salbutamol (SAL), cimaterol (CIM), terbutaline (TER), clorprenaline (CLO), tulobuterol (TUL), penbuterol (PEN) in pork, beef, and lamb muscle samples. The samples were added the internal standard solution (IS) and extracted in the alkaline medium with acetonitrile. The further sample purification was accomplished through MCX solid phase extraction cartridge. Chromatographic chiral separation was carried out on a VancoShell chiral column (100 mm × 4.6 mm, 2.7 μm) with an isocratic mobile phase consisting of methanol and 10 mmol mL-1 ammonium formate aqueous solution (85:15, v/v). Under the optimized conditions, the resolution (R) of CIM was 2.0, CLE and PEN were 1.5, the others were all greater than 1.0. Enantiomeric determination was performed in the positive electrospray ionization mode using multiple reaction monitoring (MRM). The correlation coefficient (r) in the range of 0.2-25.0 μg L-1 was above 0.993. The average recoveries at the three spiking levels ranged from 95.3% to 117.7% with the relative standard deviation (RSD) lower than 15%. The limit of detection (LOD) and the limit of quantification (LOQ) of β2-agonist enantiomers was 0.2 μg kg-1 and 0.5 μg kg-1 respectively. The method was successfully applied in the analysis and evaluation of β2-agonist enantiomers in positive food animal muscle samples, CLE, SAL, TEB and CIM enantiomers were detected. The concentrations of the corresponding enantiomers were in the range of 1.06-17.3 μg kg-1, the lowest enantiomer fraction (EF) value was 0.42, and the highest value was 0.69. The work is expected to provide a method for chiral separation and enantiomeric determination of the further study of pharmacology, toxicity and residue elimination of β2-agonist enantiomers.

Fabrication of two-dimensional metal-organic framework nanosheets/PDA composites as mixed-mode stationary phase for chromatographic separation

The synthesis of two-dimensional metal-organic frameworks (2D MOFs)/polymer core-shell composites is reported which were composed of polydopamine modified 2D Zr-1,3,5-(4-carboxylphenyl)-benzene (2D Zr-BTB) nanosheets and silica microspheres via a double-solvent approach. In this way, the composites were obtained under the condition of two solvents with different polarities to avoid agglomeration and uneven modification of most MOFs particles on the surface of the silica, existing inevitably in the one-pot method. Compared with the reported MOFs@silica composites adopting one-pot solvent method, the prepared composites exhibited significantly enhanced separation performance for sulfonamides, antibiotics, nucleosides, and polycyclic aromatic hydrocarbons compounds. Furthermore, the retention mechanisms were demonstrated by studying the relationships of chromatographic retention factors of tested analytes versus a variety of parameters under RPLC and HILIC modes, respectively. The superior chromatographic repeatability and stability were validated through the relative standard deviations of the retention time and/or column efficiency, which were found to be less than 0.8% and 0.9%, respectively. The material showed efficient separation ability for several types of compounds and provided another selectivity for preparing composites based on 2D MOFs nanosheets and other functional molecules.

Synthesis of fully porous silica microspheres with high specific surface area for fast HPLC separation of intact proteins and digests of ovalbumin

Fully porous silica microspheres (FPSM) with high specific surface area and hierarchical pore as matrix for HPLC were prepared. First, the porous silica nanospheres with controllable particle size and pore diameter were successfully synthesized using a dual-templating approach, the pore size of nanospheres can be increased to 18.4 nm by changing the molar ratios of octyltrimethylammonium bromide (TOMAB) and cetyltrimethyl ammonium bromide (CTAB), which is suitable for separation and analysis of biomolecules without pore enlargement. Then, the micron FPSM with hierarchical pore were synthesized by polymerization-induced colloid aggregation (PICA) using the porous nanospheres as a silicon source, which has a large mesoporous structure (35.2 nm) and high specific surface area (560 m² g^-1). Subsequently, the FPSM modified with octadecyltrichlorosilane were studied as stationary phase for separation of cytochrome C, lysozyme, ribonuclease A, and ovalbumin, bovine serum albumin, and the baseline separation of five proteins was achieved within 1 min. The prepared column was also applied to the fast separation of digests of ovalbumin, and more chromatographic peaks were obtained compared to a commercial column under the same gradient elution conditions. In addition, the static-binding capacity of the functionalized FPSM for bovine serum albumin (BSA) was measured to be 276 mg g^-1, which was nearly twice the static adsorption given in literature. Therefore, these FPSM with high specific surface area and hierarchical pore structure are expected to have great potential for the separation of complex biological samples using HPLC. Graphical abstract A synthetic strategy was provided towards FPSM with hierarchical pores and high specific surface area using porous nanospheres as silicon source. The outstanding performance of the FPSM is that it has a high specific surface area while maintaining a large mesoporous size, which overcomes the disadvantage of sacrificing the specific surface area when increasing the pore size of porous silica microspheres prepared by using the traditional PICA method.

Monodisperse core-shell silica particles as a high-performance liquid chromatography packing material: Facile in situ silica sol-gel synthesis

In recent years, core-shell silica particles (CSSPs) have been increasingly used for highly efficient separation at fast flow rates and relatively low back pressures in high-performance liquid chromatography (HPLC). However, material synthesis techniques for producing CSSPs economically in batch processes remain elusive. In this report, a practical and straightforward method for the preparation of CSSPs is presented. By refluxing freshly prepared nonporous silica particles in ammonia-water solution in the presence of poly(diallyldimethylammonium chloride) at 70-100 °C, CSSPs with shell thicknesses of up to 300 nm and pore sizes from 8 to 25 nm were easily prepared. The effects of the synthetic conditions on the shell thickness, surface area, and pore size were investigated in detail, and the method reproducibility was evaluated in scale-up experiments. A mechanism of CSSP formation is also proposed. The CSSPs were characterized via scanning electron microscopy, transmission electron microscopy, laser particle size (dynamic light scattering) analysis, and nitrogen adsorption and desorption experiments. The synthesized 3.4-μm CSSPs were functionalized with dimethyloctadecylchlorosilane and used as an HPLC packing material, exhibiting excellent separation performance for both small molecules and large biomolecules. In summary, we report the simplest method developed thus far for the preparation of monodisperse core-shell silica particles suitable for HPLC column packing.

Controlled manipulation of TiO2 nanoclusters inside mesochannels of core-shell silica particles as stationary phase for HPLC separation

Based on a detailed study of the hydrolysis process of tetrabutyl orthotitanate (TBOT), TiO₂ nanoclusters were modified inside the pores of SiO₂ core-shell particles instead of the outside. The pore size distribution of SiO₂ core-shell spheres modified with TiO₂ (SiO₂@dSiO₂@TiO₂) was analyzed by Barrett-Joyner-Halenda (BJH) method and density functional theory (DFT) method, respectively. The results of the DFT calculations demonstrate that the TiO₂ nanoclusters are always first formed in bulk solution and then enter the pores. By regulating the rate of hydrolysis of TBOT, almost all of the TiO₂ nanoclusters are modified into the pores and the structure of the original SiO₂ core-shell sphere is hardly affected. The morphology of the particles was characterized by scanning electron microscopy and transmission electron microscopy. The crystal phase of TiO₂ was measured by XRD. SiO₂@dSiO₂@TiO₂ spheres functionalized with C18 were packed into a stainless steel column. The chemical stability of SiO₂@dSiO₂@TiO₂ spheres under alkaline was tested by flushing of a mobile phase at pH 13 for 7 days. The efficiency of the column after the alkali solution treatment still reaches 98,430 plates m^-1, which is only about 1.6% lower than that before the alkali solution treatment. A series of basic and acidic analytes were also separated on the column. Graphical abstract TiO₂ nanocrystals were coated into the pore of core-shell silica spheres. The prepared particles were packed into the column and separation performance up to 98,430 plates per meter was achieved.

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.

Metal Organic Framework@Polysilsesequioxane Core/Shell-Structured Nanoplatform for Drug Delivery

Modern pharmaceutics requires novel drug loading platforms with high drug loading capacity, controlled release, high stability, and good biocompacity. Metal-organic frameworks (MOFs) show promising applications in biomedicine owing to their extraordinarily high surface area, tunable pore size, and adjustable internal surface properties. However, MOFs have low stability due to weak coordinate bonding and limited biocompatibility, limiting their bioapplication. In this study, we fabricated MOFs/polysilsesquioxane (PSQ) nanocomposites and utilized them as drug carriers. Amine-functionalized MOF (UiO-66-NH2) nanoparticles were synthesized and encapsulated with epoxy-functionalized polysilsesquioxane layer on the surface via a facile process. MOFs possessed high surface area and regular micropores, and PSQs offered stability, inertness, and functionality. The obtained UiO-66-NH2@EPSQ nanocomposites were utilized as carriers for ibuprofen, a drug with carboxylic groups on the surface, and demonstrated high drug loading capacity and well-controlled release property. The UiO-66-NH2@EPSQ nanocomposite exhibited low cytotoxicity to HeLa cells within a wide concentration range of 10-100 µg/mL, as estimated by the MTT method. The UiO-66-NH2@EPSQ drug release system could be a potential platform in the field of controlled drug delivery.

Functional Metal Organic Framework/SiO2 Nanocomposites: From Versatile Synthesis to Advanced Applications

Metal organic frameworks (MOFs), also called porous coordination polymers, have attracted extensive attention as molecular-level organic-inorganic hybrid supramolecular solid materials bridged by metal ions/clusters and organic ligands. Given their advantages, such as their high specific surface area, high porosity, and open active metal sites, MOFs offer great potential for gas storage, adsorption, catalysis, pollute removal, and biomedicine. However, the relatively weak stability and poor mechanical property of most MOFs have limited the practical application of such materials. Recently, the combination of MOFs with inorganic materials has been found to provide a possible strategy to solve such limitations. Silica, which has excellent chemical stability and mechanical properties, shows great advantages in compounding with MOFs to improve their properties and performance. It not only provides structured support for MOF materials but also improves the stability of materials through hydrophobic interaction or covalent bonding. This review summarizes the fabrication strategy, structural characteristics, and applications of MOF/silica composites, focusing on their application in chromatographic column separation, catalysis, biomedicine, and adsorption. The challenges of the application of MOF/SiO2 composites are addressed, and future developments are prospected.