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Are Nanotubes Safe?

Researchers at Rice University find a way to make the nanoparticles less toxic.
November 7, 2005

Nanoparticles are prized for their unique electronic and biological properties. Their size allows them to slip through biological barriers, to deliver drugs to cancer cells, for example.

But researchers and advocacy groups are concerned that these same features may make nanoparticles dangerous in some applications, allowing them to infiltrate and damage healthy cells.

Now a growing number of research groups are trying to get a handle on how these novel structures interact with biology, so that scientists can take advantage of their great promise without unwittingly causing harm.

As findings from studies start to come in, an intriguing – and more complex – possibility is emerging: researchers may be able to regulate the toxicity of nanoparticles, allowing them to be more toxic, for instance, if the goal is to kill cancer cells, or less toxic for drug delivery or imaging applications.

The most recent study concerning this ability, which is due to appear in an upcoming issue of Toxicology Letters, suggests that the more researcher modify the surfaces of carbon nanotubes, the less toxic they become.

“Nanotechnology is a tool. It can be beneficial but also disastrous. We need to be careful, and we need to understand nanoparticles before we use them,” says Chang-Yu Wu of the University of Florida, Gainesville, co-author of a recent review article on the environmental impact of nanoparticles.

Some scientists worry that because nanoparticles can have significantly different properties than bulk materials, they could exhibit unanticipated dangers. For example, Wu notes that, although ordinary gold is normally safe for use in biological applications, some gold nanoparticles kill bacteria. Also, nanoparticles can slip past the blood-brain barrier into the brain – a characteristic that could be bad or good, depending on the particle, Wu says.

In the current study, researchers at Rice University were able to change the toxicity of carbon nanotubes by using some of the precise chemistry that has made nanotechnology possible. They attached chemical groups to the outside of the nanotubes, which are cylindrical cages of carbon atoms, resembling rolled-up chicken wire. These chemical groups make the tubes more soluble, which is an important feature if they are to be used inside the watery environment of the body. The researchers found that the nanotubes most modified in this way also killed the fewest cells in a culture. They were so safe, in fact, that no more of the cells exposed to these tubes died than those that died when exposed to a control solution without nanotubes.

“The study sounds encouraging,” says Pat Roy Mooney, Executive Director of the ETC Group, an organization that opposes the current use of some nanoparticles in consumer products, such as sunscreen, saying the particles should be fully tested first. Although the Rice results are promising, Mooney emphasizes that “we need a lot more studies” to understand the potential dangers of nanoparticles.

These latest findings are part of ongoing work at Rice on nanoparticles and the environment. Describing this work at a recent conference, Vicki Colvin, Director of the Center for Biological and Environmental Nanotechnology (CBEN) at Rice and one of the authors of the study, says that finding ways to “modify and control biological effects by changing the particles” will help chemists to design safer nanotechnologies.

An earlier study at the center dealt with large carbon molecules, fullerenes (also called “buckyballs”) that are known to damage cell membranes. By “stealing” electrons, unmodified fullerenes create free radicals that can cause a chain reaction, destroying molecules. The study found that modifying the surface of the buckyballs made them much less toxic.

Modifying buckyballs and nanotubes will not work for all applications, though. Altering the surface of a buckyball to make it safe also makes it less attractive for use in, for example, photovoltaics, where its ability to grab electrons is essential. On the other hand, the modified structures maintain other useful traits, perhaps including the ability to encapsulate a drug.

Likewise, modified carbon nanotubes may also lose functions, such as the ability to fluoresce, while retaining other possible uses, such as delivering imaging agents.

CBEN’s goal, says chemist Kevin Ausman, Executive Director of the center and a co-author of the study, is to develop a “predictive feeling for which nanomaterials in which contexts are going to be dangerous and which ones are not.”

To this end, researchers from several institutions organized by the D.C.-based Risk Science Institute last month published guidelines for screening nanomaterials, rules that should help to standardize experiments and allow researchers to incorporate data from different sites.

“This is a very exciting area,” Ausman says, “But it’s too big a problem for any one group to handle, so we welcome everybody to get into it.”

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