Biodegradation of carbon nanotubes could mitigate potential toxic effects
(Nanowerk Spotlight) The toxicity issues surrounding carbon nanotubes (CNTs) are highly relevant for two reasons: Firstly, as more and more products containing CNTs come to market, there is a chance that free CNTs get released during their life cycles, most likely during production or disposal, and find their way through the environment into the body. Secondly, and much more pertinent with regard to potential health risks, is the use of CNTs in biological and medical settings. CNTs interesting structural, chemical, electrical, and optical properties are explored by numerous nanomedicine research groups around the world with the goal of drastically improving performance and efficacy of biological detection, imaging, and therapy applications. In many of these envisaged applications, CNTs would be deliberately injected or implanted in the body. For instance, CNT-based intercellular molecular delivery vehicles have been developed for intracellular gene and drug delivery in vitro (see: Nanotechnology based stem cell therapies for damaged heart muscles).
"While it has been shown that carbon nanotubes can indeed act as a means for drug delivery, negative effects such as unusual and robust inflammatory response, oxidative stress and formation of free radicals, and the accumulation of peroxidative products have also been found as a result of carbon nanotubes and their accumulated aggregates" Dr. Alexander Star tells Nanowerk. "As a possible solution, we have provided compelling evidence of the biodegradation of carbon nanotubes by horseradish peroxidase and hydrogen peroxide over the period of several weeks. This marks a promising possibility for nanotubes to be degraded by horseradish peroxidase in environmentally relevant settings."
Star, an Assistant Professor in Advanced Functional Materials, Nanosensors, Physical Organic Chemistry at the University of Pittsburgh, together with Dr. Valerian Kagan and collaborators from Departments of Chemistry and Environmental & Occupational Health at the University of Pittsburgh, have demonstrated the natural biodegradation of single-walled carbon nanotubes through enzymatic catalysis. They have published their findings in the October 28, 2008 online edition of Nano Letters ("Biodegradation of Single-Walled Carbon Nanotubes through Enzymatic Catalysis").
Researchers have been experimenting with ways to alleviate the potential negative side effects of carbon nanotubes either by functionalizing them to make them more biocompatible or by degrading them after their use. So far, methods for degrading nanotubes, or 'cutting' nanotubes, involved the use of a harsh solvent consisting of sulfuric acid and high concentrations of hydrogen peroxide.
"When dealing with environmental issues it is important not to introduce any contaminants harsher than what is being cleaned" says Star. "Our method provides a mild, natural approach for the safe removal of carbon nanotube material."
In their work, the University of Pittsburgh scientists show the natural biodegradation of single-walled carbon nanotubes through enzymatic catalysis.
"By incubating carbon nanotubes with a common enzyme, horseradish peroxidase and low levels of hydrogen peroxide under static conditions, these nanomaterials are oxidized" says Brett Allen, a chemistry Ph.D. candidate in Star’s lab and first author of the report. "The formation of a highly oxidizing intermediate from this enzyme, known as Compound I, facilitates this biodegradation process. These results mark promising possibilities for nanotubes to be degraded in environmentally relevant settings."
TEM tracks the biodegradation of carbon nanotubes as incubation time increases. Length decrease is seen at week 8 with some globular material, while at week 12 mostly globular material is present. This change is attributed to nanotube biodegradation. (Reprinted with permission from American Chemical Society)
In their 16-week experiment, the researchers started to observe a substantial decrease in the average nanotube length and the appearance of globular material after eight weeks. By the end of the incubation period after 16 weeks, they found that it had become difficult to account for any nanotube structure at all.
Allen says that examination of the samples at 12 weeks already revealed that the bulk of nanotubes were no longer present, and globular material had amassed.
These findings could lead to the development of immobilized horseradish peroxidase/hydrogen peroxide mixtures into a chemical spill kit to clean up carbon nanotubes in environment, thus mitigating carbon nanotube toxicity.
"It is tempting to speculate that other peroxidases in plants and animals may be effective in oxidative degradation of carbon nanotubes" says Star. "If so, enhancement of these catalytic biodegradation pathways may be instrumental in avoiding their cytotoxicity in drug delivery, gene silencing, and tumor imaging."
He and Allen point out that, with further insight into this type of biodegradation process, it will be possible to engineer better, more efficient drug delivery platforms, where the patient need not worry about the injection of materials that could possibly accumulate causing cytotoxic effects.
"We are currently in collaborative efforts with Dr. Kagan to investigate the effects and degradation of carbon nanotubes using other relevant peroxidases" Star explains the team's current efforts. "Furthermore, we are at a need to understand the products of formation. While it appears that nanotubes are degraded, we have still not identified those products of degradation."
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