(567g) Stabilization of Superparamagnetic Iron Oxide Nanoparticles with Low Molecular Weight Dispersants

The biocompatibility and favourable magnetic properties of superparamagnetic iron oxide nanoparticles (SPIONs) render them attractive for many different applications especially, but not exclusively, in the biomedical field. Prominent examples of biomedical applications of SPIONs are cell separation and magnetic resonance imaging (MRI) where they are used as contrast agents.

For a successful application, these SPIONs need to retain high stability even in diluted suspensions under physiologic conditions. Furthermore, many applications require a close control over the hydrodynamic diameter and interfacial chemistry of such nanoparticles. These stringent requirements can only be met if such SPIONs are sterically stabilized.

High molecular weight dispersants typically encapsulate multiple iron oxide cores within one cluster by physisorption, leading to poorly defined hydrodynamic diameters which are many times larger than the iron oxide core. Furthermore, constant rearrangements of the dispersants on the nanoparticle surface impede a close control over the interfacial chemistry, which is necessary to further functionalize the nanoparticles in a controlled way.

Low molecular weight dispersants which consist of one high affinity binding group covalently linked to a spacer molecule such as poly(ethylene) glycol (PEG) are an attractive alternative to high molecular weight dispersants. Such dispersants adsorb on the nanoparticle surface in a well defined way and thus allow close control over the interfacial chemistry. Moreover, the hydrodynamic diameter of such stabilized nanoparticles is determined by the core diameter and the molecular weight of the dispersant, both of which can be individually controlled. A well known and often used binding group for stabilizing iron oxide nanoparticles is dopamine. However, we recently observed that the binding affinity of dopamine towards iron oxide is too low to stabilize SPIONs at elevated temperatures and under dilute physiologic conditions.

We have found derivatives of dopamine which as anchor groups bind irreversibly to iron oxide nanoparticles. Therefore, stability of iron oxide nanoparticles individually stabilized with dispersants consisting of these high affinity binding groups covalently linked to PEG (Figure 1) vastly exceeds that of nanoparticles stabilized with low molecular weight dispersants used today. Furthermore, we found that these nanoparticles can easily be diluted under physiologic conditions and repeatedly be heated up to 90°C without agglomeration. Additionally, these dispersants allow for simple co-adsorption of differently functionalized dispersants. This allows us to create multifunctional and targeted SPIONs through an easy route, thus paving the way for numerous new future applications.

Figure: Individually stabilized iron oxide particles