"The skin of mice injected with high doses of gold nanoparticles turned blue," explains Warren Chan from the University of Toronto and team leader in the research project. "We've been working on nanoparticles for 15 years and never seen skin colour changing, but then before we never injected at such high doses." He tells nanotechweb.org that these higher dosage studies were prompted by the suggestion that nanoparticle toxicity is probably linked to dosage.

"Now we've proved that the concentration of nanoparticles in the skin is linked to concentrations in other organs and that there is a direct correlation," says Chan. "This means you can see nanoparticle accumulation through colour change in the skin or fluorescence under a UV lamp and then you can have a small skin biopsy to assess how much you've been exposed."

Nanoparticle 'spill-over'

Spectroscopic studies of skin samples confirmed that the nanoparticle accumulation in the skin increased linearly with dosage. The researchers then microscopically analysed skin samples taken from mice at times ranging from 4 to 504 hours after injection. They found that at low dosages, nanoparticles accumulated in macrophage cells, but at higher dosages the particles were found in the space between cells.

"Macrophages gobble up foreign materials and break them down for removal from the body," explains Chan. "But when the dosage is too high, the macrophages in the skin cannot handle them all so there’s what we call a 'spill-over effect', where the nanoparticles reside in the space between cells."

Comparing samples from the skin, liver and spleen demonstrated a linear relation between the accumulation levels in these organs. This finding implies that the concentration accumulated in the liver or spleen could be quantitatively determined simply by multiplying the measured skin content by a constant.

In depth with UV

The research team also investigated whether skin accumulation occurred for other types of nanoparticle. Although the skin did not seem to change colour when quantum dots were used instead of gold nanoparticles, under a UV lamp the mice began to glow. Different alloys of quantum dots fluoresced green, yellow and red when exposed to UV light, which is in line with the known spectral properties of the quantum dots, confirming that the quantum dots and not the associated ions were residing in the skin. As for the gold nanoparticles, the quantity of quantum dots in the skin was linearly correlated with the injection dose.

At low dosages, the fluorescence itself was patchy, but the studies of skin samples indicated that quantum dots were in fact still present. A probable explanation for this finding is that the quantum dots had accumulated at different depths in the skin.

"UV light doesn’t penetrate deep into the body," explains Chan, "So now, because we know a large concentration of nanoparticles accumulate in the skin, we may be able to use UV light to manipulate the nanoparticles' function." The researchers are now investigating the potential of using UV light to trigger drug release.

Outlook

The results imply that fewer animals may be needed for future toxicology studies. Perhaps more significantly, this method of visualizing and quantifying nanoparticles may enable human exposure to be monitored. "At the moment there’s no way of measuring this," adds Chan.

Full details are available in Nature Communications.