| Magnetic particles have attracted much research interest in the past decade and are considered as one of the most ideal materials for data storage, biolabeling and separation of biomolecules etc. The aim of this thesis is to prepare magnetic particles which can be applied to biotechnologies such as biolabeling, bioanalysis, immunoassay, enzyme immobilization, and especially bioseparation. Application in biotechnology impose strict requirements on the magnetic particles'physical and chemical properties, including: (1) paramagnetism or superparamagnetism, which means that after eliminating the magnetic field the particles no longer show magnetic interaction; (2) good magnetic response; (3) well dispersed in a liquid, normally in water; (4) good biocompatibility; (5) surface characteristics provided functional groups for further coupling with biomolecules; (6) chemical stability; (7) controllable sizes; (8) low cost. To achieve these requirements, magnetic particles should be stable large entities composed of a high concentration of superparamagnetic nanoparticles. Silica is often considered as one of the most ideal materials for protection of magnetic nanoparticles to form silica magnetic composite particles due to its good chemical stability and dispersing ability. Another advantage of silica is that its surface can be modified with various silane coupling agents to covalently attach specific bioligands such as antibody or an enzyme to the surfaces of the magnetic particles. Two different approaches have been commonly used to prepare silica magnetic composite particles. The first method is based on microemulsion synthesis, in which micelles or inverse micelles are used as mini-reactor to control the silica coating on the magnetic nanoparticles. This method requires tedious steps to separate the silica magnetic composite particles from the surfactants in the microemulsion system. The other method relies on the well-known St?ber process, which comprises the hydrolysis and the polycondensation of tetraethoxysilane under alkaline conditions in ethanol. This method can be directly used to synthesize silica magnetic composite particles of controlled sizes under mild condition. But the content of magnetic cores of silica magnetic composite particles synthesized using St?ber method 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. In this thesis, the preparation of silica magnetic composite particles is extensively studied, and silica magnetic composite particles with different sizes and magnetic response are synthesized. The structures and surface properties of silica are manipulated to satisfy the requirement of bioapplication. In addition, technique for purifying DNA using silica magnetic silica particles is studied. The Fe3O4 nanoparticles with superparamagnetism and high magnetic susceptibility prepared using coprecipitaton method are used as the magnetic cores. The utilisation of Fe3O4 nanoparticles has the added advantage that the body is designed to process excess iron. In the human body iron is stored primarily in the core of the iron storage protein ferritin. Iron that is contained in endosomes and lysosomes (post-cell uptake) is known to be metabolized into elemental iron and oxygen by hydrolytic enzymes, where the iron joins normal body stores. Iron homeostasis is well controlled by adsorption, excretion and storage, and thus it is postulated that, following the administration of iron nanoparticles,iron in the body can be processed. However it should also be noted that although iron plays an important role in virtually all living tissues, it has a rather limited bioavailability, and in some situations it can also be toxic to cells. There are mainly three parts in this thesis: 1. Control over the size, shape and the magnetic property of the silica magnetic composite particles is one of the criteria for their practical applications. For Fe3O4 nonoparticles sol, the colloidal stability is ensured by a balance among magnetic, Van der waals, and static electric repulsion interactions of particles, which may produce a secondary minimum in the interaction potential and allows reversible clusters of particles without loss of colloidal stability. These clusters determine the size, shape, and content of magnetic cores of the final silica magnetic composite particles. The silica magnetic composite particles with different sizes and magnetic response are synthesized using Fe3O4 nonoparticles with different surface treatment: (1) We used different amount of TMA (tetramethylammonium hydroxide) to peptize the Fe3O4 nanoparticles. Electrophoresis measurements show that the mobility of the Fe3O4 nanoparticles is dependent on the amount of TMA used. With certain amount of TMA used, monodisperse Fe3O4/SiO2 composite nanoparticles with several magnetic cores is synthesized; (2) We employed ultrasonic treatment instead of TMA to peptize the magnetic Fe3O4 nanoparticles. It is found that the stability of the magnetic nanoparticles is dependent on time of the ultrasonic treatment. After growth of the silica protection layer, Fe3O4/SiO2 composite nanoparticles with different amount of the magnetic cores were obtained from the Fe3O4 nanoparticles with different time of ultrasonic treatment. And Fe3O4/SiO2 composite nanoparticles with multi cores are parepared from the Fe3O4 nanoparticles with 10 min of ultrasonic treatment. (3) The Fe3O4/SiO2 micrometer-sized composite particles with good magnetic response are synthesized from the Fe3O4 nanoparticles without sureface treatment. 2. Analyses of pH, conductivity, and dissolution of silica have been carried out in order...
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