The term "carbon nanotubes" (CNTs) comprises a group of carbon-containing nanomaterials with diverse properties, which for example differ in their surface structure or in the number of walls. These features significantly affect the strength of the toxic effects of the CNTs. Many of the observed effects are due to the fibre-like form of the carbon nanotubes, which may cause indirect effects upon contact with the surface of the environmental organisms. Universally valid statements about the environmental toxicity of carbon nanotubes are not yet possible.

 

Due to their fibre-like form carbon nanotubes cause indirect toxic effects in bacteria by acting as needles which pierce the cell envelope leading to mechanical damage. Via those defects in the sheath the growth of bacteria is severely restricted or totally inhibited. This effect on bacteria gets stronger the better the CNTs are distributed and isolated in the nutrient solution [1,2].

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Single-celled organisms such as ciliates and amoebae, which are present in all water bodies, are able to take up CNTs but equally able to excrete those as well. Their viability is not affected by the carbon nanotubes, however, food intake (bacteria) and digestion is inhibited [3].

 

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Various types of algae are also indirectly affected by carbon nanotubes. Pure and coated CNTs inhibit the growth of algae by shading the sunlight or by forming agglomerates between algae and CNTs [4,5,6]. If vital light is missing, the algae cells can produce less energy and consequently, growth is inhibited. Other factors such as the nature of the surface of the CNTs or the agglomeration state in the aqueous environment have little effect on algal growth [4]. In addition, the carbon nanotubes can also induce oxidative stress in algae [5,6].

 

Icon BlumeWhen adding carbon nanotubes to the soil of various crops such as wheat or rape, the plant roots take up only very small amounts (0.005 %) of the offered CNTs [7]. Hence an accumulation of CNTs in the food chain via crop is considered unlikely. The growth of the plants was not affected. In tomato plants, incorporated carbon nanotubes could be detected in roots, shoots and fruits. Also carbon nanotubes were found to activate stress genes in the plant cells, indicating an increased stress to the plant [8]. Perhaps the plant is actively trying to excrete the CNTs. But it is not yet clear how the carbon nanotubes induce stress in the plant cells.

 

Icon WasserflohWater fleas take up carbon nanotubes regardless of their surface charge and excrete them likewise [9]. Generally, the surface charge contributes significantly to the behaviour of the CNTs in aqueous media (stability, agglomeration) and can influence various parameters such as uptake and distribution. In this study, however, the differences observed in the strength of the toxic effects could not be attributed to the uptake, but rather to the coating of the carbon nanotubes [9,10].

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The effects of carbon nanotubes with various surface charges were also studied in earthworms [11]. As well as in water, the behaviour of the CNTs in soil varies depending on the particular surface properties. Similar to water fleas the differences in carbon nanotube behaviour in soil had no influence on absorption and excretion in earthworms. There are currently no results regarding toxic effects in worms available.

 

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Larvae of the African clawed frog show an acute toxic reaction towards carbon nanotubes. The CNTs "clog" the gills and intestine thereby severely restricting the growth and thus the survival of the larvae [12]. No genotoxic effects were observed.

 

Icon FischSingle-walled carbon nanotubes added to the food of the rainbow trout are incorporated by the fish, but they do not exert harmful effects [13]. If the carbon nanotubes however are added directly to the water, then the trout shows an irritation of the gills accompanied by an accelerated respiration [14]. Similar to situation with the frog larvae, the CNTs clog the gills of the fish due to their fibrous form and cause the mentioned observed effects.

Functionalized multi-walled carbon nanotubes that were directly injected into zebrafish embryos are no longer detectable in the animals after 4 days, and were, therefore, effectively eliminated [15]. The further development of fish embryos was not affected. However the next generation showed significantly reduced survival rates.

 

When looking at the entire life cycle of carbon nanotubes - from production and use to disposal - then the manufacturing of the CNTs has a much greater impact on environmental organisms than the direct release of the CNTs in the environment [16]. For example, indirect effects to the environmental organisms can be caused by environmental damage that may occur due to mining of raw materials or due to exhaust gases created.

 

Finally, the toxicity of carbon nanotubes is currently difficult to assess due to their diversity. Mechanical and indirect effects due to the fibre-like form are observed in many organisms, particularly in studies in which very high doses were tested. For very low concentrations, as they are currently expected in the environment, no risk to environmental organisms is currently estimated.

 

 

Literatur arrow down

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  2. Chung, H et al. (2011), Ecotoxicol Environ Saf, 74(4): 569-575.
  3. Chan, TS et al. (2013), Nanotoxicology, 7(3): 251-258.
  4. Schwab, F et al. (2011), Environ Sci Technol, 45(14): 6136-6144.
  5. Wei, L et al. (2010), Aquat Toxicol, 100(2): 194-201.
  6. Long, Z et al. (2012), Environ Sci Technol, 46(15): 8458-8466.
  7. Larue, C et al. (2012), J Hazard Mater, 227-228 155-163.
  8. Khodakovskaya, MV et al. (2011), PNAS, 108(3): 1028-1033.
  9. Petersen, EJ et al. (2011), Environ Sci Technol, 45(3): 1133-1138.
  10. Kennedy, AJ et al. (2009), Environ Toxicol Chem, 28(9): 1930-1938.
  11. Petersen, EJ et al. (2011), Environ Sci Technol, 45(8): 3718-3724.
  12. Mouchet, F et al. (2008), Aquat Toxicol, 87(2): 127-137.
  13. Fraser, TW et al. (2011), Nanotoxicology, 5(1): 98-108.
  14. Smith, CJ et al. (2007), Aquat Toxicol, 82(2): 94-109.
  15. Cheng, J et al. (2009), Toxicol Appl Pharmacol, 235(2): 216-225.
  16. Eckelman, M.J. et al. (2012), Environ Sci Technol, 46(5): 2902-2910.
  17. Kennedy, AJ et al. (2008), Environ Toxicol Chem, 27(9): 1932-1941.

 

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