Research into zinc oxide nanoparticles allays sunscreen concerns


In a paper published in ACS Nano in November 2013, researchers from MCN, The Australian Synchrotron, CSIRO, RMIT and Monash University showcased ground-breaking characterisation techniques which were used to enable research into the human immune response to zinc oxide nanoparticles.

Zinc oxide nanoparticles are used in many consumer products such as sunscreen since they are largely insoluble in water and invisible to the eye, unlike larger white zinc particles which remain white on the skin. However, the behaviour of zinc oxide nanoparticles in biological systems is not well understood. This led the team of researchers to delve into what happens to these nanoparticles after they are absorbed into our skin.

The research team used x-ray fluorescence to image immune cells which had been treated with zinc oxide nanoparticles. Using the world-class Maia x-ray fluorescence detector at the Australian Synchrotron, the researchers were able to count how many of these tiny particles had been absorbed into the cells.

In order to observe the nanoparticles once in the immune cells, the researchers used the Focused Ion Beam Scanning Electron Microscope installed at MCN to mill away very thin layers of the cell (100x thinner than a human hair), re-imaging the cell as they went. Combining these different techniques produced a unique collection of high-resolution images that allowed the team to observe the immune cells breaking down the nanoparticles as they moved deeper into the cell.

''This is the first time that we have shown that the cells of the immune system can break down the nanoparticles directly,'' said Australian Synchrotron and CSIRO bioinorganic chemist Simon James. ''Previous work was only able to infer that.''

The images showed that on average, 60,000 nanoparticles made it into a single white blood cell. ''60,000 nanoparticles is a tiny, tiny amount of zinc,” Dr James said, “especially seeing as the cells are able to break the nanoparticles down."

What’s more, 50 to 60 per cent of the zinc oxide nanoparticles were entirely broken down by the macrophages within just 24 hours of entering the cells, with the remaining 40 per cent in the process of decomposition. Dr James explained that in the study, the nanoparticles did not reach the cell nucleus, demonstrating that human macrophages function as expected, ingesting and breaking down any zinc oxide nanoparticles before they can enter the blood stream.

“In response to some concerns, this research shows us conclusively that the nanoparticles in sunscreen are unlikely to cause harm to beach-goers," said Prof Ian Olver from Cancer Council Australia, “unlike the sun’s damaging UV rays which we know lead to higher rates of skin cancer and skin damage.”

The unique imaging techniques used in this study have demonstrated that correlative photon and ion beam imaging can provide both high-resolution and statistically powerful information on the biology of metal oxide nanoparticles at the single-cell level. This approach promises ready application to broader studies of phenomena at the interface of nanotechnology and biology.