Particle size, uniformity, and mesostructure control of magnetic core/mesoporous silica shell nanocomposite spheres
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Well-structured and monodisperse nanocomposite spheres with a magnetic core/mesoporous silica shell structure (MCMS) were obtained. The effects on the structure and morphology of the MCMS spheres were investigated under various synthesis conditions, including reaction time, quantity of silicate sources of tetraethoxysilane (TEOS) and n-octadecyltrimethoxysilane (C18TMS), ratio of TEOS/C18TMS, and ratio of H2O/EtOH in the starting solution. The particle size of the MCMS spheres and pore diameter are tunable in a certain range with 100% yield. A synthesis mechanism of the mesoporous silica shell was proposed that proceeds via three main stages. The silica shell proved to be effective on protecting the cores from leaching out in acidic conditions. I. INTRODUCTION
In recent years, magnetic mesoporous materials have attracted considerable attention in targeted drug delivery and liquid separation due to their high surface area and magnetic separability. Some attempts to fabricate nanocomposites of mesoporous materials and magnetic particles have been reported.1–6 However, the particle sizes reported in all the research are too large to be used for drug delivery, in which a particle size range between 50 and 300 nm is required. Above 300 nm, a significant proportion of particles will be trapped in the lungs and liver, while too small a particle size will cause the magnetic forces of these tiny particles to be too weak to be suitable for separation or drug delivery.7 Recently, we reported novel monodisperse magnetic nanospheres with a magnetic core/mesoporous silica shell structure (MCMS) whose particle diameter was about 270 nm.8 Generally, the morphology and size of Fe3O4 particles are difficult to control, which is an inherent drawback for fabricating MCMS nanospheres of uniform size distribution directly from Fe3O4. Herein, a kind of uniform hexahedral hematite particle was used as the initial core, which is easy to prepare. A thin and dense silica layer is deposited on the surface of the hematite particles of desired thickness to protect the iron oxide core from leaching into the mother system under acidic conditions. The mesoporous silica shell was formed from simultaneous sol-gel polymerization of tetraethoxysilane (TEOS) and n-octadecyltrimethoxysilane (C18TMS)
followed by removal of the organic group. The magnetic property was endowed via the final reduction of the hematite cores to magnetite in hydrogen. The capability of the MCMS spheres to act as drug carriers for storing ibuprofen and the in vitro release property were also demonstrated.8 Because the capability and applicability in targeted drug delivery are determined by mesoporous properties, particle size, and dispersibility of mesoporous materials, an important question is whether the morphology and mesoporous structure of the MCMS nanospheres can be controlled by monitoring the synthesis processes, as is the case for conventional mesoporous silica materials. In the work presented here, a detailed description of the effects on the structure and morphology
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