Adham Ahmed

Core-shell particles: preparation, fundamentals and applications in high performance liquid chromatography.

The challenges in HPLC are fast and efficient separation for a wide range of samples. Fast separation often results in very high operating pressure, which places a huge burden on HPLC instrumentation. In recent years, core-shell silica microspheres (with a solid core and a porous shell, also known as fused-core or superficially porous microspheres) have been widely investigated and used for highly efficient and fast separation with reasonably low pressure for separation of small molecules, large molecules and complex samples. In this review, we firstly show the types of core-shell particles and how they are generally prepared, focusing on the methods used to produce core-shell silica particles for chromatographic applications. The fundamentals are discussed on why core-shell particles can perform better with low back pressure, in terms of van Deemter equation and kinetic plots. The core-shell particles are compared with totally porous silica particles and also monolithic columns. The use of columns packed with core-shell particles in different types of liquid chromatography is then discussed, followed by illustrating example applications of such columns for separation of various types of samples. The review is completed with conclusion and a brief perspective on future development of core-shell particles in chromatography.

Silica SOS@HKUST-1 composite microspheres as easily packed stationary phases for fast separation

Metal–organic frameworks (MOFs) have been investigated for separations including chromatography. Typically, MOF particles are directly packed into columns for the separations. The irregular shapes and wide size distributions of MOF particles have led to difficulty in column packing and low column efficiency or high back pressure. We describe here the preparation of MOF–silica microspheres as packing materials for fast and efficient liquid chromatography. Spheres-on-sphere (SOS) silica particles are prepared, modified with –COOH and –NH2 groups, and then used as support to grow HKUST-1. HKUST-1 nanocrystals and films are formed and attached firmly onto the SOS particles with adjustable porosity. The composite microspheres, showing core–shell properties, are directly packed into columns to offer separation capability of MOFs and efficient packing and support of silica microspheres. These columns show separation of toluene/ethylbenzene/styrene and toluene/o-xylene/thiophene within 1.5 minutes. Although HKUST-1 is not good for separating xylene isomers, the separation can be achieved in 5 minutes using the composite microspheres after conditioning the column with dichloromethane or toluene. Remarkably, it is observed that conditioning with DCM can change retention time and selectivity (elution order) of xylene isomers. It is also possible to produce other types of MOFs (e.g., ZIF-8) on the SOS particles, indicating the potential of this method for wider applications.

Systematic tuning of pore morphologies and pore volumes in macroporous materials by freezing

Freezing and its combination with emulsion-templating are investigated to systematically tune pore morphologies and volumes in macroporous materials. Macroporous structures with controllable pore morphologies are formed under defined freezing conditions. Oil-in-water emulsions are processed to produce porous polymeric materials with a controlled proportion of ice-templated pores and emulsion-templated pores by systematically changing the volume ratio of the internal oil droplet phase to aqueous continuous phase in the emulsions. Pore morphology, bulk density, and pore volumes of these macroporous materials can thus be systematically tuned. Chemical crosslinking and sol–gel processing are further employed to produce porous polymeric and inorganic materials (silica, silica–alumina, and zirconia) with enhanced mechanical stability and hierarchical porosity.

Freeze‐Align and Heat‐Fuse: Microwires and Networks from Nanoparticle Suspensions

The use of a simple freezing technique to produce microwires from a wide range of materials is demonstrated. Freezing has been used previously to prepare porous materials with new structures and useful properties. For example, Deville et al. have shown how to replicate the nacre structure of shells by using a simple freezing path. Also, highly porous fibers were synthesized by electrospinning into a cryogenic liquid. Hierarchical biohybrid materials were prepared by an icesegregation-induced self-assembly process. We have demonstrated the synthesis of three-dimensional aligned porous materials by directional freezing of aqueous/organic solutions and emulsions, and liquid CO2 solutions. [8] Microwires can be produced by electrospinning but this requires the use of soluble or fusible polymers in solution. Silicon and silica microwires were synthesized using a vapor– liquid–solid process. In addition, capillaries, fungi, and porous membranes/plates have all been used as templates to produce microwires. A self-seeding technique was adopted to prepare single-crystal polythiophene microwires on a silicon substrate, and a photolithographic route was applied to fabricate micro/nanowires of semiconductors. An external perpendicular magnetic field was applied to a CoPt3-nanocrystal sol to drive the formation of microwire structures. Electrically functional microwires were also assembled from metallic nanoparticles by dielectrophoresis. Herein, we show that directional freezing is a simple and flexible method that can be applied to a range of particulate materials (metal nanoparticles, metal-oxide nanoparticles, polymer colloids) with diameters ranging from 10 to 500 nm. The scope of the approach encompasses materials such as metals, metal oxides, and polymers. The method is also easy to scale up for bulk-quantity microwire synthesis, and the aggregated nanoparticles may in some cases be fused together by heat treatment. To produce gold microwires, a concentrated suspension of gold nanoparticles (GNPs; 15 nm in diameter) was injected dropwise into liquid nitrogen. The GNP sol droplets were observed to float for a few seconds at the liquid surface before freezing and sinking into the liquid nitrogen. Porous bead structures were produced after freeze-drying. Figure 1a shows the porous structure of a sectioned surface of a single gold-microwire bead. The bead consists of a randomly oriented porous shell surrounding radially aligned microwires, focusing at the center of the bead.

Frozen polymerization for aligned porous structures with enhanced mechanical stability, conductivity, and as stationary phase for HPLC

A directional freezing and frozen polymerization method is developed to prepare crosslinked aligned porous polymers with improved mechanical stability. Monomer solutions are directionally frozen in liquid nitrogen to orientate the growth of solvent crystals. The frozen samples are polymerized by UV irradiation. The solvent is removed under vacuum at room temperature to produce aligned porous structure. The mechanical stability is improved by two orders of magnitude compared to the usually freeze-dried porous materials. The materials are modified with graphene and a conducting polymer to achieve conductivity at 1.9 × 10−4 S cm−1 and 5.2 × 10−6 S cm−1, with the stable aligned pore structures maintained. The aligned porous monolith is also assessed by high performance liquid chromatography (HPLC), showing fast separation of hydrocarbon compounds with low back pressure at 59 bar.

One-Pot Synthesis of Spheres-on-Sphere Silica Particles from a Single Precursor for Fast HPLC with Low Back Pressure

Spheres-on-sphere (SOS) silica particles are prepared in a one-pot scalable synthesis from mercaptopropyltrimethoxysilane with hydrophilic polymer and cationic surfactant under alkaline conditions. The SOS particles exhibit solid-core porous-shell properties. The fast separation of small molecules and proteins with low back pressure are demonstrated by high-performance liquid chromatography (HPLC) for the columns packed with SOS-particles.

Tuning Morphology of Nanostructured ZIF-8 on Silica Microspheres and Applications in Liquid Chromatography and Dye Degradation

Zeolitic imidazolate framework-8 (ZIF-8) is one type of metal-organic framework (MOF) with excellent thermal and solvent stability and has been used extensively in separation, catalysis, and gas storage. Supported ZIF-8 structures can offer additional advantages beyond the MOF-only materials. Here, spheres-on-spheres (SOS) silica microspheres are used as support for the nucleation and growth of ZIF-8 nanocrystals. The surface functionalities (-SH, -COOH, and -NH2) of silica and reaction conditions are investigated for their effects on the ZIF-8 morphology. The use of SOS microspheres results in the formation of highly crystalline ZIF-8 nanostructured shell with varied sizes and shapes, ranging from spherical to cubic and to needle crystals. The SOS@ZIF-8 microspheres are packed into a column and utilized for separation of aromatic molecules on the basis of π-π interaction in high-performance liquid chromatography (HPLC). Furthermore, by thermal treatment in air, ZIF-8 nanocrystals can be transformed into ZnO coating on SOS silica microspheres. The SOS@ZnO microspheres show excellent photocatalytic activity, as measured by degradation of methyl orange in water, when compared to ZnO nanoparticles. This study has demonstrated the facile way of using SOS microspheres to prepare core-shell microspheres and their applications.

Macroporous metal–organic framework microparticles with improved liquid phase separation

Typically, metal–organic frameworks (MOFs) exhibit ordered micropores (<2 nm). The control of pore shape, surface functionality and high surface area, which comes with the variety of metal ions and enormously available organic ligands, has rendered a wide range of applications for MOF materials. Due to the limited mass transport of micropores, various approaches have been developed to produce mesoporous MOFs. However, the preparation of macroporous MOFs (with macropores in addition to the micropores) has been scarce, despite this type of material being able to facilitate considerably applications such as separation and catalysis. Here, we report the solvothermal modification of HKUST-1 microparticles with hydroquinone. An etching mechanism is suggested for the formation of macroporous HKUST-1 particles, which presents a high surface area and high macropore volume with the HKUST-1 characteristic pattern. Single X-ray diffraction data shows a cubic unit cell eight times the size of the HKUST-1 unit cell, as a result of slight distortions to the framework. The modified macroporous HKUST-1 particles are further packed into a column, showing fast and improved separation of ethylbenzene and styrene by high performance liquid chromatography.

Hierarchically porous silica monoliths with tuneable morphology, porosity, and mechanical stability

Colloids have been used as sacrificial templates to produce porous materials with controllable morphology and pore sizes. Ceramic particles were employed to prepare porous ceramics and strong composite materials by freeze-casting. However, highly porous monodisperse microspheres have been rarely used as building blocks to obtain hierarchically porous structures. In the present study, porous monodisperse silica microspheres were prepared by a modified Stober method and then used as building blocks to produce porous silica with meso-/micro-pores and macropores by a controlled freezing approach. The macropore morphologies could be tuned with the addition of surfactants in the silica colloidal suspensions during the freezing process. The engineering of porosity and improvement on mechanical stability of the silica materials were achieved via a further soaking and sol–gel process. It was also possible to enhance the mechanical stability through the thermal treatment of the materials.

Dual-tuned drug release by nanofibrous scaffolds of chitosan and mesoporous silica microspheres

A sustained drug release with an initial burst release followed by a linear release to optimally maintain the drug concentration in the therapeutic region is highly desirable for medicine administration. This study describes a method for the preparation of chitosan and chitosan–silica scaffolds and assesses their use for sustained release of curcumin. Porous chitosans with nanofibrous or sheet-like morphologies are fabricated from aqueous curcumin–chitosan solutions by a freeze-drying approach. The pore morphology can influence the release rate; curcumin was released much faster from the nanofibers than from the macroporous scaffolds. Amine-modified mesoporous silica microspheres are added into chitosan as another control; the release rate decreases due to the interaction of amine groups on silica and curcumin with chitosan. The silica microspheres with tuneable loading of curcumin are further incorporated into the curcumin–chitosan scaffold. The curcumin release into phosphate buffer solutions at 37 °C shows a two-stage release profile with distinctly different release rates. The high permeability and hydrophilic property of chitosan result in the initial burst release. The slow near-linear release at the 2nd stage results from the loaded silica microspheres. Loading of curcumin, concentration of chitosan, and content of silica microspheres in the composites are investigated and the effects on the release profiles are demonstrated. The present method is further adapted to prepare thin films with reasonable strength and flexibility by air drying and freeze drying. The tuneable sustained release of curcumin is also shown for the films.