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Biodegradation of carbon nanotubes

Carbon nanotubes which are composed of thick rolls of graphite and stronger than steel have found use in a variety of applications, including reinforcing plastics, conducting electricity in electronics and as sensitive chemical sensors.
The development and application of such nanomaterials have led to serious concern about their potential toxicity. Consumer related commercial products developed with nanomaterials have attracted concern about their safe use, particularly when placed in contact with human beings and the proper disposal of nanomaterials.
Research reports reveal that the toxicity of carbon nanotubes is controversial. There are some reports that with the growing use of carbon nanotubes, there is an increasing chance of environmental pollution and exposure. For example, carbon nanotubes, if inhaled, can cause respiratory inflammation in a similar manner to asbestos. But identification of a potential safety problem is more complicated because carbon nanotubes have been produced in many different forms. Hence, development of a safe method for decomposing carbon nanotubes is desired, but no such technique is available until now.
Safe decomposition of carbon nanotubes is quite challenging due to the nature of the material such as strongest carbon carbon bonds in carbon nanotubes. Researchers have identified a natural enzyme called horseradish peroxidase (HRP) as a potential candidate to biodegrade carbon nanotubes when used with hydrogen peroxide.
HRP is known to biodegrade different organic compounds, including poly aromatic hydrocarbons that are close in composition to carbon nanotubes. This enzyme is also very robust and works well over a broad range of temperatures according to the researchers. HRP when added to the carboxylated carbon nanotubes in the presence of hydrogen peroxide and incubated for few weeks at low temperature and neutral pH in the dark degrades carbon nanotubes. Electron spin-resonance analysis indicates that the HRP remained viable and was not deactivated by the carbon nanotubes.
The degradation can be attributed to the formation of a highly reactive intermediate. The active site of the HRP is an iron (III) porphyrin ring complex. Oxidation with hydrogen peroxide generates an iron (IV) porphyrin radical cation that decomposes carbon nanotubes by reducing their length.

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