Synthesis of Nanospheres-on-Microsphere Silica with Tunable Shell Morphology and Mesoporosity for Improved HPLC

Core–shell particles have a wide range of applications. Most of the core–shell particles are prepared in two or multiple steps. Core–shell silica microspheres, with solid core and porous shell, have been used as novel packing materials in recent years for highly efficient liquid chromatography separation with relatively low back-pressure. These core–shell silica microspheres are usually prepared by the time-consuming layer-by-layer technique. Built on our previous report of one-pot synthesis of core–shell nanospheres-on-microspheres (termed as SOS particles for “spheres-on-spheres”), we describe here a two-stage synthesis for the introduction of shell mesoporosity into SOS particles with tunable shell morphology by co-condensation of tetraethyl orthosilicate (TEOS) with 3-mercaptopropyltrimethoxysilane (MPTMS) in the presence of surfactant in the second stage. With MPTMS as the primary precursor at the first stage, some other silica precursors (apart from TEOS) are also employed at the second stage. Expansion of the surfactant-templated mesopores with swelling agents during the reaction and by hydrothermal postsynthesis treatment is then performed to allow the pore sizes (> 6 nm) suitable for separation of small molecules in liquid chromatography. Compared to the standard SOS silica (both the nanospheres and microspheres contain nearly no mesopores), the introduction of mesoporosity into the nanosphere shell increases the separation efficiency of small molecule mixtures by 4 times as judged by the height equivalent plate number, while the separation of protein mixtures is not negatively affected

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