Characterization of magnetic nanoparticles

Magnetic nanoparticles composed of iron oxide facilitate attractive possibilities in biomedical applications. The controllable size of few nanometers can be tailored with very tight tolerances, enabling them to get close to biological entities of interest. The additional possibility to coat the particles with biological molecules allows for interaction or binding with cells, proteins or even genes. Along with the magnetic properties this allows to manipulate them by an external magnetic field gradient or to make them resonantly respond to a time-varying magnetic field. These unique properties prospect manifold non-invasive in vivo diagnostics or therapies. Magnetic resonance imaging, hyperthermia treatment and drug delivery are only few of the many biomedical applications with promising results. It is almost disconcerting that - in prospect of the apparent advantages - only very few magnetic nanoparticles are licensed for in vivo applications, not least due to lack of standardized characterization methods. This thesis is concerned with the size characterization of different classes of magnetic nanoparticles via frequency and time dependent magnetization and relaxation measurements. A multitude of magnetic nanoparticle samples has been prepared including dilution series, immobilization experiments and exposure to magnetic fields to assess distinct features such as particle interactions, anisotropy energy or size distributions. Several analysis techniques have been utilized to perform measurements on unmodified and altered samples under different conditions. Reviewing the well established models and implementing necessary modifications to derive further information, a comprehensive size characterization of all samples has been carried out. The derived parameters are compared and the origins of deviations are investigated in order to harmonize the size characterization of magnetic nanoparticles. Further challenges of a standardized size characterization of magnetic nanoparticles are investigated and incorporated into the analysis. As a result, the findings are condensed into a self-consistent size characterization and compiled into standard operating procedures for alternating current susceptometry, magnetorelaxometry and the rotating magnetic field method.