||In this thesis, we designed and synthesized multifunctional nanoparticles based on magnetic nanoparticles that have increasing applications in biology and medicine. There are eight chapters in the thesis. In chapter 1, we gave the detailed review on the synthesis and applications of magnetic nanoparticles in the past two decades. In chapter 2, we described a simple, rapid, and ultra-sensitive procedure to detect bacteria in human blood or blood products using biofunctional magnetic nanoparticles in combination with a fluorescent probe. In chapter 3, we reexamined and demonstrated the stability of dopamine or L-dopa modified iron oxide nanoparticles using Fe2O3 hollow nanoparticles and Fe3O4 nanoparticles as two representative substrates. The reinvestigation of this strategy should warrant the potential biomedical applications of iron oxide magnetic nanoparticles that are functionalized by the molecules based on dopamine anchors. In chapter 4, we discussed a magnetic dipolar interaction induced self-assembly to guide the synthesis of wires of hollow nanocrystals of cobalt chalcogenide because of nanoscale Kirkendall effect. In chapter 5, we reported the evaluation of cytotoxicity of a new type of engineered nanomaterials—FePt@CoS2 yolk-shell nanoparticles. The exceptional high toxicity of FePt@CoS2 yolk-shell nanoparticles (about 7 times higher than that of cisplatin in term of Pt) may lead to a new design of anticancer nanomedicine. Following the chapter 5, we described FePt@Fe2O3 yolk-shell nanoparticles can serve as a potential MRI contrast and potent anticancer agent in chapter 6. This type of yolk-shell nanoparticles may lead a new strategy for the combination of disease detection and drug delivery. In chapter 7, we studied the synthesis of fluorescent magnetic nanoparticles with core-shell, sponge-like, or heterodimer nanostructures through the sequential addition of reagents and different solvents in a one-pot procedure. Back to back, we demonstrated the successful intracellular spatial control of fluorescent magnetic nanoparticles by a magnetic force in chapter 8, which may offer a new strategy for studying the polarized cells. The thesis focused on the biological and medical applications of nanomaterials, known as bionanotechnology, which is emerging as an interdisciplinary field for chemists, material scientists, physicist, biologist, and biomedical researchers. We hope our works may contribute more or less to the developments of bionanotechnology.