Multifunctional Magnetic Nanocomposites: Tailored Surface Engineering for Biomedical Applications, Photonic Harvesting, and Energy Conversion

Abstract

Magnetic iron oxide nanoparticles (Fe₃O₄) have gained significant attention in nanotechnology due to their high surface-to-volume ratio, strong magnetic responsiveness, and superparamagnetic behavior. These properties make them ideal building blocks for multifunctional nanosystems that integrate biochemical, optical, and energy-related functionalities within a single platform. This dissertation focuses on the design, synthesis, and surface engineering of Fe₃O₄-based nanocomposites for applications spanning enzyme immobilization, controlled coating processes, photothermal energy conversion, and the construction of hierarchical hybrid structures. The first part of this work examines the functionalization of Fe₃O₄ nanoparticles using 3-aminopropyl)triethoxysilane (APTES) to introduce amine groups for enzyme attachment. Trypsin was selected as a model enzyme and immobilized via glutaraldehyde coupling, resulting in a magnetically recoverable biocatalyst that retained significant catalytic activity while exhibiting improved stability and reusability. The second part extends this approach by coating Fe₃O₄ nanoparticles with a dextran coating to enhance aqueous dispersion and biocompatibility. Trypsin was successfully immobilized onto these dextran-coated nanoparticles, and detailed kinetic analyses confirmed effective enzyme attachment and activity. These results underscore the versatility of Fe₃O₄-based nanocarriers and demonstrate their potential for immobilizing other proteins, enzymes, and antibodies. The third part of the dissertation addresses the growth kinetics of polydopamine (PDA) coatings on Fe₃O₄ nanoparticles. Although PDA is widely used as a universal coating material, its thickness evolution and deposition kinetics remain poorly understood. In this study, PDA shell formation was systematically monitored by scanning electron microscopy (SEM), enabling the development of a kinetic model that describes its exponential growth. These insights provide a reproducible framework for designing PDA-based nanomaterials with controlled coating thickness and improved functional reliability. The final part presents the synthesis of a hollow Fe₃O₄@Au@mTiO₂ nanocomposite that integrates magnetic, plasmonic, and photocatalytic properties. The hollow Fe₃O₄ core, plasmonic gold layer, and mesoporous TiO₂ shell collectively enable strong magnetic responsiveness, efficient NIR-induced photothermal heating, and enhanced structural stability. This multifunctional architecture shows strong potential for applications such as photothermal therapy, environmental remediation, and solar-driven energy conversion. Overall, this dissertation highlights how controlled surface chemistry and hierarchical nanostructure design can be used to engineer advanced multifunctional magnetic nanocomposites tailored for biomedical, optical, and energy-related technologies.