Synthesis, self-assembly and potential applications of cobalt-based nanoparticles with tailored magnetic properties

This dissertation includes a comprehensive study of the wide range of issues involving the synthesis, self-assembly, magnetic behavior and bio-sensing applications of cobalt and related binary-element nanoparticles.; We began with the development of a robust chemical method for the reproducible synthesis of monodispersed, metallic cobalt nanoparticles with good size control, narrow size distribution and well-defined shapes, including spheres and anisotropic nanodiscs with specific aspect ratios. We then investigated the controlled self-assembly behavior of the single component, cobalt nanoparticle system by a systematic variation of their size, shape and inter-particle interactions. It was discovered that the self-assembly of the arrays could be tuned to selectively achieve square packing, hexagonal close packing, linear chains, spatially segregated arrays as a function of particle size and lyotropic liquid-crystal-like arrays with orientation order. This richness in self-assembly behavior was obtained when one of the competing forces (steric, van der Waals, depletion, or magnetostatic) was chosen to dominate and determine their subsequent self-organization.; Further, the strategies developed from the cobalt synthesis were successfully extended to fabricate binary-element nanoparticles. Using immiscible Co-Au and miscible Co-Pt systems, combined with a heterogeneous nucleation theory that we developed based on experimentally accessible parameters, Co-Au core-shell nanoparticles with dual magnetic/optical functionality and Co-Pt nanoparticles with controlled morphologies were synthesized. This methodology makes it possible to specifically engineer the chemical composition of the binary nanoparticles. As a result, binary nanoparticles with tailored magnetic properties (from soft to hard) were synthesized to meet the various needs for biological applications. For example, soft FeCo nanoparticles and hard FePt nanoparticles were fabricated for targeting, bio-sensing or delivery applications.; Finally, we demonstrated the potential of magnetically blocked nanoparticles in bio-sensing application. This was performed by studying the AC relaxation behavior of 20nm diameter cobalt nanoparticles with various coatings in different solution environments. It was discovered that the interactions between the surface coatings could be probed with a relaxation peak at very low frequency. This opens up a number of possibilities in biological sensing, diagnosis and detection, such as study of protein configuration, activity of biomolecular binding sites etc.