Nanosilver Tracked In Rats

Toxicologists worry that increasing use of nanosilver in personal care products and food packaging will lead to greater human exposure. Although scientists know silver is toxic to microbes, they don’t fully understand its effects on other organisms, including people. To get a grip on those effects, researchers must first determine which silver species animals absorb: the nanoparticles themselves or ions that slough off the particles. A new study of silver particles and ions in rats suggests that the ions may be the species to watch (ACS Nano, DOI: 10.1021/nn302649p).

Researchers usually study exposure by feeding animals suspensions of silver nanoparticles. But because the particles shed ions, it’s not clear which silver species an animal absorbs, says Meike van der Zande of Wageningen University & Research Centre, in the Netherlands. To help clear up the debate, she and her colleagues decided to compare animals fed silver nanoparticles with ones fed silver ions.

For 28 days, the team fed male rats one of several forms of silver, including polymer-coated nanoparticles, bare nanoparticles, or a silver nitrate salt solution. They then measured total silver levels in the rodents’ tissues using atomic absorption spectroscopy. The scientists also looked for silver nanoparticles using single-particle inductively coupled plasma mass spectrometry.

No matter which form of silver the rats ate, nanoparticles turned up in the animals’ stomach, liver, spleen, and lungs. But compared with levels in rats that ate particles, total silver levels were 10 times greater in the livers and spleens of rats fed silver nitrate. When rats stopped eating the silver-spiked diet, their bodies started to clear the metal from all tissues except for the brain and testis. Silver lingered in those tissues after the rodents’ last meal containing silver for at least 56 days, the longest time period the researchers studied.

Based on the total silver data, the researchers concluded that the rats’ bodies more readily absorbed silver ions than silver particles.

The researchers think the ion-eating rats had nanoparticles in their tissue because certain ions in the rats’ bodies may precipitate the silver as insoluble salts that form nanoparticles. They tested this hypothesis by adding silver nitrate to a solution that resembled gastrointestinal fluid. Silver nanoparticles started to form.

Erik Huusfeldt Larsen, a researcher at the Technical University of Denmark, says the results support the idea that silver changes form between ions and nanoparticles inside animals, no matter which form the animal ingests. He adds that the data on lingering silver in brain and testis, two vulnerable tissues, will help toxicologists choose which tissues to study. But he points out that the particle doses used in the study, 90 mg of silver per kg of body weight, exceed likely real-world exposure levels.

Other questions remain, including whether silver behaves similarly in people’s and rats’ bodies, says Kenneth Dawson of University College Dublin. But, he says, the new work is the kind of “controlled, exhaustive” study necessary to eventually understand nanoparticle toxicity.