Toxicity of nanotubes

Carbon nanotubes consist of molecular cylinders of pure, hexagonally-arranged carbon atoms with a diameter measuring a few nanometres and a length of many microns. They occur in two main types, the single-wall carbon nanotube composed of a single cylinder of carbon and the multi-wall version consisting of concentric tubes or cylinders of carbon. But very little fact is known about how these nanomaterials affect the environment, since some researchers indicate that one class of SWNTs produces cell-damaging reactive oxygen species (ROS) when dispersed in water and exposed to sunlight.

ROS are chemically-reactive molecules containing oxygen such as oxygen ions and peroxides. They are highly reactive due to the presence of unpaired valence shell electrons. ROS can result in significant damage to cell structures and cumulates into a situation known as oxidative stress. Reactive oxygen species are implicated in cellular activity to a variety of inflammatory responses including cardiovascular disease. They may also be involved in hearing impairment via cochlear damage induced by elevated sound levels, ototoxicity of drugs such as cisplatin, and in congenital deafness in both animals and humans. Redox signaling is also implicated in mediation of apoptosis or programmed cell death, ischaemic injury and can oxidize and damage an organism's DNA, lipids and proteins. Specific examples include stroke and heart attack.

Large quantities of CNTs are produced, used and disposed globally in industries and research labs and hence it should be known how they are transformed in the environment and whether their products are more or less harmful than the parent materials.

Simulated environmental exposure studies revealed that by shining natural sunlight on glass tubes filled with carboxylated SWNTs suspended in water, after 80 hours with carboxylated swnts produced ROS. Similarly, SWNTs when suspended in acidic water and irradiated, clumped together and came out of suspension as aggregate, thus getting chemically altered in composition. This could be toxic to aquatic organisms when they come in contact with water body of the environment.

However it has been known that cells are normally able to defend themselves against ROS damage with a host of enzymes. Small molecule antioxidants such as ascorbic acid (vitamin C), tocopherol (vitamin E), uric acid, and glutathione also play important roles as cellular antioxidants. Similarly, polyphenol antioxidants assist in preventing ROS damage by scavenging free radicals. Intensive research is going on to understand how toxicity of these engineered materials will affect the environment when they are released.