Superparamagnetic iron oxide nanoparticles (SPION) offer many applications in biomedicine such as magnetic resonance imaging (MRI) for contrast enhancement, drug delivery , stem cell tracking, or magnetic separation technologies (e.g. rapid DNA sequencing) and ultrasensitive diagnostic assays. These particles present a variety of advantages compared to other tools such as (a) the controllable sizes ranging from about 10 to several hundred nanometres (in beads) or (b) the custom surface derivatization. Accordingly, they can selectively bind to a defined biologic entity (like cells or degraded extracellular matrix molecules) and deliver molecules and drugs to specific sites. It is also possible to retain the particles e.g. in the joint by external magnetic fields, for applications including extended drug delivery. In addition, they respond to magnetic fields and are therefore used in molecular resonance imaging or Hyperthermia. First introduced as MRI contrast agents in the mid-1980s, there are nowadays numerous SPION compounds already FDA-approved for use in the clinic as well as others undergoing clinical trials.
Magnetic nanoparticles have been synthesized with a number of different compositions and phases. Several popular methods include co-precipitation, thermal decomposition and/or reduction, micelle and hydrothermal synthesis, and laser pyrolysis and each method has its advantages and disadvantages. Co-precipitation is an easy and convenient approach to synthesize iron oxides from aqueous solutions, and size, shape, and composition of the resulting particles strongly depend on the reaction conditions, in particular the used reactants, pH, and ionic strength of the media. The experimental challenge lies in the control of the particle size and since the blocking temperature depends on particle size, a broad size distribution will result in a non-ideal magnetic behaviour for many applications.
Polymeric stabilizing agents play an important role in increasing the stability of the SPIONs suspension by preventing or decreasing particle agglomeration and providing a means for conjugation to carrier molecules or, in some cases, to serve as targeting molecules themselves. However, the coating of nanoparticles with polymers has shown drastic limitations due to poor interaction between the nanoparticles and the polymer molecules. This means that under given circumstances, the coating is separated from the particles. This can also happen e.g. during surface derivatization but far more importantly
Characterization of nanoparticles in biological environments (for e.g. toxicological studies or a potential biomedical application) is complex, with a variety of material attributes to be considered, including measurements of size and size distribution, shape and other morphological features (e.g., crystallinity, porosity, and surface roughness), bulk chemistry of the material, solubility, surface area, state of dispersion, surface chemistry and other physico-chemical properties in addition to the interaction of the particle with environmental molecules such as proteins. There are numerous techniques for measuring particle size distributions and an ensemble method of measurement is normally preferred. In one project we focus on the analytical investigation of particle size (distributions) in complex environments.