Credit: © 2006 ACS

The magnetic properties of iron nanoparticles make them promising candidates for biomedical applications such as sensing, separation and imaging. However, making stable dispersions of iron nanoparticles is not straightforward as they are easily oxidized in air to form iron oxide. This reaction reduces the magnetic moment of the nanoparticles and makes them clump together and precipitate.

Now, Shouheng Sun and co-workers1 from Brown University in the USA have shown that by controlling the oxidation process, stable core–shell structures can be made in which an Fe core is surrounded by a layer of iron oxide. In the first step of the process, Fe nanoparticles are formed by the thermal decomposition of a precursor material (iron pentacarbonyl) in the presence of oleylamine — a stabilizing organic compound. Rather than allowing the nanoparticles to oxidize in air, Sun and co-workers use precise quantities of an oxygen transfer agent to convert the outer surface of the nanoparticles into iron oxide. Unlike the reaction in air, which produces an amorphous structure, this process forms an ordered crystalline shell of iron oxide, the thickness of which can be tuned by varying the amount of transfer agent.

Dispersions of these core–shell nanoparticles are found to be stable when exposed to air, indicating that the crystalline iron oxide crust protects the inner Fe core from further oxidation. Furthermore, by exchanging the organic coating for a hydrophilic one based on dopamine-like molecules, the nanoparticles could be dispersed in a saline solution — a more biologically relevant medium.