The hydrodynamic diameter and the surface characteristics of the SPIONs are of great importance in terms of clearance, cell response, toxicity, and biocompatibility, especially for medical or biological applications. In general, the spleen and liver as a result of mechanical filtration remove SPIONs approximately of 200 nm in diameter. To characterize the hydrodynamic diameter of the SPIONs we use dynamic light scattering. Dynamic light scattering, more precisely Photon Cross-correlation Spectroscopy (PCCS), is a technique which can be used to determine the size distribution profile of small particles in suspension or polymers in solution. Photon Cross-correlation Spectroscopy is also a technique which allows the simultaneous measurement of precise particle size and stability in the range of about 1 nm to some µm in opaque suspensions and emulsions.

It is important that the particles are stable over a long time without forming agglomerates. Therefore the stability of the SPIONs has to be analysed in different media at different pH values varying from 4.6 to 10.0 at room temperature and e.g. at 37°C. We use different media like PBS and characterized over 6 weeks the hydrodynamic diameter and the stability of the particles.