Magnetic nanoparticles are of interest in a variety of applications that take advantage of their manipulation using externally applied magnetic fields. In this talk I will summarize our recent work on characterizing the diffusion and stability of magnetic nanoparticles in complex and biological fluids through their response to time-varying magnetic fields. I will discuss the physical principles of this measurement technique and then demonstrate how quantitative measurements can be made accurately in simple fluids. Then I will show results for nanoparticles suspended in complex fluids, such as gelling solutions, polymer solutions, and polymer melts. Finally, I will present results for measurements of magnetic nanoparticle rotational dynamics in biological fluids, as a means of studying protein adsorption, concentrated protein solutions, and nanoparticle behavior in tissues and blood
