Iron oxide nanoparticles
Iron oxide nanoparticles are generally of three types: simple iron oxide <math>FeO</math>, magnetite <math>Fe_3O_4</math> and hermatite <math> Fe_2O_3</math>. These particles are generally used for their magnetic properties. Magnetite in particular is used in super-paramagnetic beads. It occurs naturally in certain magnetic bacteria such as Magnetospirillum magnetotacticum. Together these can form a colloidal suspension who's stability depends on charges, magnetic forces, and steric stabilization from anything used to functionalize the surface of the particles.
In the bacteria that they exist in, iron oxide nanoparticles act as a compass to allow bacteria to navigate using the earth's magnetic field. This is explored in more detail in: Magnetic_Colloids_from_Magnetotactic_Bacteria:_Chain_Formation_and_Colloidal_Stability.
For humans, there has been much research in using these particles for new diagnostic and therapeutic techniques. For instance, in Development of functionalized superparamagnetic iron oxide nanoparticles for interaction with human cancer cells, the authors explore the creation of an iron oxide nanoparticle to target cancer.
In all these applications, the iron oxide nanoparticles alone are of little use besides their magnetic properties. This is where soft matter comes in. The particles are often sterically stabilized by functionalizing their surface. This also allows them to target specific proteins, cells, etc.
Unfortunately, some recent studies have shown that these nanoparticles can be toxic to nerve cells.
 Wikipadia "Iron Oxide"
 Science Daily article on potential risks