In bulk powders, the oxidation of magnetite to maghemite is seen by a change in color from black to red, but in nanoparticles it is not nearly so easy to distinguish the two phases. Image: Lara Bogart.Iron oxide nanoparticles are used in sentinel node detection, iron replacement therapy and other biomedical applications. In a paper in Applied Physics Letters, researchers now describe a new technique for investigating how these materials age, and how aging may change their functional or safety profiles.
For the first time, by combining lab-based Mössbauer spectroscopy with ‘center of gravity’ analysis, Quentin Pankhurst and his colleagues at University College London in the UK have been able to quantify the diffusive oxidation of magnetite into maghemite, and track the process. Their work is poised to help understand the aging mechanisms in nanomaterials, and how this aging changes the way nanomaterials interact with the human body.
"It's almost an unasked question about how this material oxidizes over time," said Pankhurst. "We need more information about it. This technique helps us know what's happening as products are sitting on the shelf." Distinguishing the two forms of iron oxide in nanoparticles is so difficult that it has led to an unofficial convention of naming samples ‘magnetite/maghemite’ when their composition isn't known.
One way to distinguish the two forms is via their oxidation state. Using nuclear gamma rays, Mössbauer spectroscopy is able to measure how much of a sample has iron atoms with the +2 charge of magnetite, compared to the +3 charge that predominates in maghemite. Pankhurst and his colleagues then process these subtle measurements with center of gravity calculations, which combines the data to create a bigger picture for the sample. Because the test doesn't destroy the samples, the researchers can track the oxidation of iron oxide nanoparticles over long periods of time.
Next, the group is looking to extend its technique to a broader range of magnetite and maghemite samples, and to help other researchers better understand how a nanomaterial's age correlates with its functional properties.
"We've raised a question about whether the oxidative aging affects the particles, but we haven't seen if that's the case or not," he said. "Now there's this idea that aging is going on, and that's a whole other parameter we haven't been measuring. I'd be delighted if other people explored this correlation between function and aging in their own materials."
This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.