Just How Toxic Are Carbon Nanotubes?

Carbon nanotubes (CNTs) add strength and design flexibility to many materials, from reinforcing adhesives and materials used in electronics to enhancing the design of carbon fiber composites for use in cars and aerospace applications. But there's been increasing evidence that they, along with other nanoscale materials, may pose risks to human health and the environment. In ongoing research efforts to determine just how toxic they actually are, researchers at Texas Tech University have come up with a new method for detecting CNTs in soils.

The university's environmental biologists built an apparatus that can quantify how much CNT is present in a given soil sample. That's not an easy task since they're so small: mean outer diameters of 13nm to 16nm are common in multi-walled tubes. The apparatus was developed as part of the researchers' ongoing work in locating CNTs in biological environments and examining how they to accumulate in soil, plants such as food crops, or other organisms.

The testing apparatus works by exposing soil samples to microwaves, which can reveal even mere trace quantities of CNTs. That's because, in the presence of microwaves, CNTs produce very high quantities of heat, much higher than most materials. The effect is so extreme that, if you put CNTs in your kitchen microwave oven, the carbon will spontaneously ignite. The researchers used this fact to heat samples to different temperatures to determine different concentration levels. The method has also been used to determine the amount of CNT loading in plant samples and earthworms.

The news about CNTs' toxicity is more than a little unwelcome, since graphene, the tubes' constituent substance, is the hardest known substance. When layers of it are rolled up into tubes, the resulting CNTs constitute a fiber that is 100 times stronger than steel and weighs one-sixth as much. That combination, along with qualities such as mechanical strength and electrical and thermal conductivity, makes them highly unusual and ideal building blocks for industrial uses.

Earlier this year, a joint study by the University of Missouri and the US Geological Survey showed CNTs to be toxic to various species of invertebrate aquatic organisms that live in sediment, including mussels, worms, and crustaceans. These researchers note that CNTs, which may contain metals as well as carbon, tend to accumulate in sediment when released into water. Both the metals -- including nickel, chromium, and other metals used in manufacturing that may remain as impurities -- and the carbon in CNTs can reduce growth rates or even kill certain types of marine life. The degree of toxicity varied depending on the type and source of CNT, the species of test organism, whether the materials had been cleaned using acid, and what method was used for dispersing the materials.

Researchers of the joint study say one of the biggest potential contamination risks occurs during the manufacture of carbon composites, but careful waste management and handling procedures can reduce that risk. More information is also needed on what happens when the composites begin to break down.

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