Superparamagnetic γ-Fe₂O₃ @ SiO₂ Mesoporous Microspheres: Preparation, Characterization And Applications

Magnetic mesoporous silica microspheres have drawn considerable attention in recent years owing to their advantageous characteristics such as large surface area, versatile surface functional groups, excellent biocompatibility, ease of separation from solution and being amenable to automation. These magnetic particles can be used in various areas including catalysis, immobilized enzymes, DNA purification, cell separation, and controlled drug delivery. There are a variety of approaches available for the preparation of magnetic mesoporous silica microspheres, including the methods of impregnation, encapsulation and template synthesis. One common problem with all these methods is that they mostly give rise to ferromagnetic microspheres with relatively high residual magnetism that causes particle aggregation even if the external magnetic field is removed. The particle aggregation prevents the recycled uses of these microspheres, thus lowering the performance of these materials in many applications. The objectives of this thesis are to develop new methods for preparation of superparamagnetic mesoporous silica microspheres and to further demonstrate their applications through surface modification. Firstly, ferromagneticγ- Fe₂O₃@ SiO₂ microspheres were prepared by oxidation of Fe₃O₄@ SiO₂ microspheres, which were obtained through urea-formaldehyde resin templated synthesis route. The materials were used as magnetic matrices for immobilization of penicillin acylase. This work was followed by fabrication of two types of suerparamagneticγ- Fe₂O₃@ SiO₂ microspheres through modifications to the templated method developed previously and exploration of their applications in DNA purification, solid-phase extraction, and chiral separation. The detailed contents of this thesis are as follows:1) Through oxidation of Fe₃O₄@SiO₂ mesoporous microspheres which were prepared with iron sol as magnetic precursor in 300oC to transform Fe₃O₄ intoγ- Fe₂O₃. Theγ-Fe₂O₃ nanoparticles have similar saturation magnetization but higher chemical stability than Fe₃O₄, implying that the particles could retain their magnetization longer when exposed to air during storage. Futher, we applied the modified microspheres on immobilization of penicillin acylase and studied the effects of immobilization conditions on immobilized enzyme activity. 2) A modification to the above method was introduced to prepare superparamagn etic mesoporous silica microspheres. Fe₃O₄ @SiO₂ @UF microspheres were prepared by polymerization of magnetic fluid and silica sol with urea-formaldehyde as template. The composite particles were then subject to calcinations for removal of the template and transformation of Fe₃O₄ intoγ-Fe₂O₃ to yield superparamagneticγ-Fe₂O₃@SiO₂ mesoporous microspheres. The prepared microspheres were used for solid phase extraction of genomic DNA from pepper and pea. The DNA templates isolated were amplifiable by polymerase chain reaction (PCR) and therefore potentially applicable for detection of genomic modification organism in plants.3) The superparamagneticγ-Fe₂O₃@SiO₂ mesoporous microspheres are ideal to serve as matrix for restricted access materials (RAM) because of their average pore diameters around 6nm allowing discrimination of small molecules from large molecules such as proteins. The internal surface of the above magnetic microspheres was bonded with hydrophobic alky chains whereas the external surface was made hydrophilic by encapsulation with diol groups. The alkyl/diol functionalized magnetic RAM was used for extraction ofα,β-naphthols in the presence of bovine serum albumin (BSA). This type of materials could be used in the pretreatment of biologic samples for clinical applications.4) Magnetic mesoporous microspheres with large pore sizes are desirable for enzyme immobilization. A further modification to the templated synthesis method was introduced to prepare the superparamagnetic core-shell mesoporous microspheres with pore diameters in the range around 60 nm. Theγ-Fe₂O₃ mesoporous microspheres were served as the magnetic cores whereas the porous silica shells were formed by sol-gel process involving hydrolysis and condensation of tetraethoxyl silane (TEOS) in the presence of cetyl quaternary amine bromide (CTAB). The removal of the CTAB molecules from the composite particles leaves the material with regular pores of diameter about 60 nm. The prepared core-shell magnetic mesoporous microspheres were superparamagnetic and could be repeatedly used for chiral separation ofβ-amino acids after immobilization with lipase.