"Next, we hope to add environmental variables to the assays, for example, to mimic ultraviolet exposure or river water conditions in the solution to see how they affect toxicity," says Weiwei Zhong. "We also want to study the biological mechanism by which some particles are toxic to worms." (Credit: iStockphoto)

materials science

Tiny worms test nanoparticles for safety

Scientists tested 20 types of nanoparticles on roundworms and found that 5, including carbon-60 molecules called “buckyballs,” showed little to no toxicity.

Others were moderately or highly toxic to Caenorhabditis elegans, which the researchers observed over several generations to see the particles’ health effects.

“Nanoparticles are basically new materials, and we don’t know much about what they will do to human health and the health of the ecosystem,” says Qilin Li, an associate professor of civil and environmental engineering and of materials science and nanoengineering at Rice University.

“There have been a lot of publications showing certain nanomaterials are more toxic than others. So before we make more products that incorporate these nanomaterials, it’s important that we understand we’re not putting anything toxic into the environment or into consumer products.

“The question is, ‘How much cost can we bear?'” she says. “It’s a long and expensive process to do a thorough toxicological study of any chemical, not just nanomaterials.”

She says that due to the large variety of nanomaterials being produced at high speed and at such a large scale, there is “an urgent need for high-throughput screening techniques to prioritize which to study more extensively.”

The new pilot study proves it’s possible to gather a lot of toxicity data at low cost, says coauthor Weiwei Zhong, an assistant professor of biosciences, who has performed extensive studies on C. elegans, particularly on their gene networks.

Materials alone for each assay, including the worms and the bacteria they consumed and the culture media, cost about 50 cents, she says.

4 ways to test a nematode

The researchers used four assays to see how worms react to nanoparticles: fitness, movement, growth, and lifespan. The most sensitive assay of toxicity was fitness. In this test, the researchers mixed the nanoparticles in solutions with the bacteria that worms consume. Measuring how much bacteria they ate over time served as a measure of the worms’ “fitness.”

“If the worms’ health is affected by the nanoparticles, they reproduce less and eat less,” Zhong says. “In the fitness assay, we monitor the worms for a week. That is long enough for us to monitor toxicity effects accumulated through three generations of worms.”

C. elegans has a life cycle of about three days, and since each can produce many offspring, a population that started at 50 would number more than 10,000 after a week. Such a large number of tested animals also enabled the fitness assay to be highly sensitive.

The researchers’ “QuantWorm” system allowed fast monitoring of worm fitness, movement, growth, and lifespan.

3 types of nanoparticles

The researchers studied a representative sampling of three classes of nanoparticles: metal, metal oxides and carbon-based. “We did not do polymeric nanoparticles because the type of polymers you can possibly have is endless,” Li explains.

They examined the toxicity of each nanoparticle at four concentrations. Their results showed C-60 fullerenes, fullerol (a fullerene derivative), titanium dioxide, titanium dioxide-decorated nanotubes, and cerium dioxide were the least damaging to worm populations.

Their “fitness” assay confirmed dose-dependent toxicity for carbon black, single- and multiwalled carbon nanotubes, graphene, graphene oxide, gold nanoparticles, and fumed silicon dioxide.

They also determined the degree to which surface chemistry affected the toxicity of some particles. While amine-functionalized multiwalled nanotubes proved highly toxic, hydroxylated nanotubes had the least toxicity, with significant differences in fitness, body length and lifespan.

Adding variables

Zhong says the method could prove its worth as a rapid way for drug or other companies to narrow the range of nanoparticles they wish to put through more expensive, dedicated toxicology testing.

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“Next, we hope to add environmental variables to the assays, for example, to mimic ultraviolet exposure or river water conditions in the solution to see how they affect toxicity,” she says. “We also want to study the biological mechanism by which some particles are toxic to worms.”

Rice postdoctoral researcher Sang-Kyu Jung and alumna Xiaolei Qu, now an associate professor at Nanjing University, are lead authors of the paper. Coauthors are Rice research scientist Boanerges Aleman-Meza, graduate students Tianxiao Wang and Zheng Liu and alumna Celeste Riepe, now a graduate student at the University of California, Berkeley.

The National Institutes of Health and a Searle Scholar grant funded the research, which appears in the journal Environmental Sciences and Technology. The findings are also available on the researchers’ open-source website.

Source: Rice University

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