Controllable Preparation And Character Of Fe₃O₄/SiO₂ Magnetic Composite Particles

Superparamagnetic nanoparticles have attracted much research interesting in the field of the bioscience such as separation of the biomolecules, biolabeling, site-specific drug delivery and magnetic fluid hyperthermia et al. Usually, the size of surperparamagnetic particles is very small, and the magnetic strength is rather limited, especially in actually usages, a protect layer is often coated on these superparamagnetic particles, decreasing the content of the magnetic cores, which further limited the magnetic strength and confined their various applications. Coating many superparamagnetic nanoparticles in one sphere and controlling the core number can solve the problem well. Recently there are many reports on preparing magnetic polymer spheres with many superparamagnetic nanoparticles based on emulsion, miniemulsion and microemulsion systems. These methods require tedious steps to separate the magnetic composite particles from the surfactants in the emulsion system and the particles size is usually large which not suitable in the in vivo applications. Silica is often considered as one of the most ideal materials for protection of supermagnetic nanoparticles due to its good chemical stability, dispersing ability and biocompatible property. However, the content of magnetic cores of silica magnetic composite particles synthesized now is so low that the overall magnetic strength is often rather limited. This limited strength presents a major hurdle to their various applications, especially in magnetic carrier technology. Among a variety of the reported nanomagnetic materials, the magnetite (Fe₃O₄) nanoparticles show many outstanding advantages in immunoassay and targeting drug-delivery system not only due to their simple preparation techniques, lower cost and higher saturation magnetization, but also because they are nontoxic, nonimmunogenic, biodegradable and are easily transported though serials of physiological barrier to the site of choice. At present, the magnetite nanomagnetic materials have become the preferred materials for the biological applications. In this thesis, the preparation of silica magnetic composite particles is studied in detail. By controlling the interaction of the superparamagnetic nanoparticles and silica matrix, we have studied the effect of the surface properties of the particles on the preparing magnetic composite particles and different coating mechanisms.The main content of the thesis as follow:1. Coating the Fe₃O₄ nanoparticles with silica. The colloidal stability of the Fe₃O₄ nanoparticles is ensured by a balance among magnetic, Van der waals and static electric repulsion interaction, which may produce a secondary minimum in the interaction potential and allows reversible clusters of particles without loss of colloidal stability. We changed the static electric repulsion interaction of the Fe₃O₄ nanoparticles, which influenced the clusters size, after coating them with silica, the silica magnetic composite particles with different cores are obtained. We also found that the shape of the clusters of the cores determine the shape of the final silica magnetic composite particles. When the silica coating process is carried out under applied magnetic field, the final composite particles present chain-like structure due to the dipole-dipole attraction induced between the magnetic nanoparticles. And then we modified the composite particles surface by APS under different ammonia concentration. It is found that there are more amido on the particles when the reaction carried out under low ammonia concentration. We further coated the composite particles with polymer after modified their surface with MPTMS with emulsion method. When added some AA into the reaction mixture, we can get carboxyl-modified particles.2. We study the mechanism of the silica formation by st?ber method in detail.The reaction was followed by by TEM, DLS, conductivity and monomer measurement under different catalyst concentration. It is found that the tendency of silica particles diameter variation at the beginning of the reaction is different by changing the catalyst concentration. Under high ammonia concentration, the particles formation is more likely the Zukoski model that the primary particle aggregated to form the particles. And under low ammonia concentration, the mechanism obeys the monomer additional model. Based on the aggregation nuclei mechanism, we coat silica on the magnetite directly and the magnetite is not used any surface modification. Moreover this method can also be used to coat other nanoparticle with silica such as QD or metal nanocrystal.3. Monodisperse magnetizable silica composite particles were prepared from heteroaggregate's of carboxylic polystyrene latex and Fe₃O₄ nanoparticles. we demonstrate that PS latex and Fe₃O₄ nanoparticles can achieve effective aggregation when there are opposite charges on the surfaces. The heteroaggregate formed under acidic conditions is stable under both acidic and alkaline environment due to the existence of coordination of carboxyl groups and Fe3+ ions at the interface of PS latex and the Fe₃O₄ nanoparticles. After coating with silica and a complete removal of PS latex by calcination, hollow monodisperse magnetizable silica composite particles are successfully prepared. We further synthesized magnetic-fluorescent silica composite spheres from these monodisperse magnetizable silica composite particles. The composite spheres present enhanced emission of fluorescein after the further deposition of Ag nanoparticles on their surface. It is found that the surface coverage of the Ag nanoparticles on the sphere surface is dependent on the pH value used for the deposition. At low pH value, more Ag nanoparticles can be absorbed on the sphere surface as a result of intensified electrostatic interactions between the spheres and the Ag nanoparticles. Therefore, the Ag nanoparticles coated composite spheres prepared at low pH value show stronger fluorescent intensity than those prepared at high pH value.