If not stabilised by use of special substances, nanoobjects, and also carbon nanotubes, tend to agglutinate very rapidly. The agglomerates can be larger than one cell. Under such conditions, the cells would rather “choke” on (frustrated phagocytosis) than take up carbon nanotubes. Smaller agglomerates are taken up by the cells via pinocytosis (cell drinking) and can be detected by means of TEM (Transmission Electron Microscopy) in cell inclusions (so-called vesicles). The vesicle membrane protects the remaining cell components from the carbon nanoparticles, i.e. although being in the cell, the particles are still encapsulated. However, a topical study shows that CNT may well be able to escape from such encapsulations (compartments) to get into the cytoplasm, which is the liquid interior of the cell^[3].
Functionalised carbon nanotubes (fCNT), which can penetrate the biological barriers due to modified surface properties, are of great interest to medical and biotechnological applications. fCNT, in contrast to non-modified CNT, do not agglutinate in aqueous environment and can be available as individual tubes^[2]. Due to their shapes and sizes, fCNT can take an alternative, vesicle-independent path into the cells. Present research wants to throw light on this uptake mechanism and its implications as regards nanomaterials.
It has not yet been adequately explained so far on which path the CNT leave the cells again and whether they are accumulated in the cells.
 

Literature

1. Conner SD and Schmid SL 2003, Nature 422(6927): 37-44
   
2. Kostarelos K et al 2007, Nat Nanotechnol 2: 108-113
   
3. Mu Q et al 2009, Nano Lett 9(12): 4370-4375.

A study shows that unmodified water-insoluble carbon nanotubes injected directly into the bloodstream of test animals can be subsequently detected in small quantities in the brain^[1]. This observation indicates that under certain conditions, carbon nanotubes can pass the blood-brain barrier. The actual conditions and the positive (e.g. dose delivery in the brain) and negative effects (e.g. cell death, brain damage) that may result from them remain to be investigated in more detail.

Literature

1. Yang, S et al, 2007, J. Phys. Chem. C 111, 17761-17764

Often, they do not exist as single tubes, but agglomerated as bundles. But the behaviour in the environment depends strongly on the type of tubes and which characteristics they have. For example, they can be soluble in water as well as insoluble, which influence their behaviour in the environment tremendously. Therefore general statements regarding the environmental behaviour are difficult to make because of the large variety of CNT.
A good example for this is provided by two studies on the mobility of CNT in soil, which should clarify, whether CNT may reach e.g. the ground water. Single walled tubes with a surface modification ensuring a good solubility in water were hardly transported in sandy soils and settled ^[1] , whereas multi walled tubes, good soluble in water as well, were found to be very mobile ^[2] .
These investigations are also of relevance for future filter technologies, as efficient sand bed or other filters represent a good way to minimize the release of CNT into waterbodies. Once nanotubes reach surface waters, they may be stabilized by dissolved organic matter and hence settling on the ground of water is prevented ^[3] .
As known from other carbon-based materials (e.g. activated carbon, which is used in filters) CNT can effectively absorb other substances. If this process occurs at CNT in the environment, several environmental pollutants may be absorbed and hence be enriched ^[4,5] . First results suggest a degradation of CNT by certain plant enzymes.  
 

Literature:

1. Jaisi, D. P.Elimelech, M. (2009), Environ Sci Technol 43, 9161-9166.
2. Liu, X. Y. et al. (2009), Environ Sci & Technol 43, 8153-8158.
3. Hyung, H. et al. (2007), Environ Sci Technol 41, 179-184.
4. Yang, K. et al. (2006), Environ Sci & Technol 40, 1855-1861.
5. Yang, K. et al. (2008), Environ Sci & Technol 42, 7931.