First popularised in 1991 by the work of physicist sumio iijima, and a timeline of its history going back much further back, carbon nanotubes and CNTs have solidified themselves as one of the most fascinating developments of the recent century. It plays a huge role in the fast growing world in nanotechnology and a multitude of industries including optics, drug delivery in the body, electronics and more can be benefited with the introduction of CNTs.
What are CNTs?
Carbon nanotubes or CNTs are cylinder structures containing one or more layers of graphene. Their diameter ranges from 0.8 to 2nm, which makes them extremely small, and therefore an extremely low density material, being a sixth of that of steel. There are currently two types: single walled structures and multi walled structures. In most materials, these mechanical properties such as strength, conductivity etc. can be degraded very substantially due to defects in its structure. For example, steel breaks at only 1% of its theoretical breaking strength. CNTs, however, do not have this problem due to their molecular perfect structure which allows them to retain these incredible properties.
Graphene as a structure is a single layer of graphite, and is a hexagonal lattice of carbon atoms. There is much more scientific detail to delve into but in essence, this lattice of strong covalently bonded atoms gives it unnatural strength, more than 400 times the strength of steel, and has made it the strongest material currently known to man. A carbon atom has 4 valence electrons and can form four bonds with neighbouring carbons. However, it forms three covalent bonds with its neighbouring carbon atoms, leaving a fourth delocalised electron. This electron is free to move through the structure and carry charge. This property makes these structures highly conductive, showcasing a thermal conductivity of more than 3500 K - more than that of a diamond.
They are also generally chemically stable which makes it biocompatible or non-toxic body organs inside an organism. This prospect can be heavily utilised within drug discovery, which is going to be described in higher detail below.
How are they made?
There are currently three main methods to produce carbon nanotubes, however there is one that shows great promise: chemical labour deposition.
While scientists have been able to grow individual nanotubes up to 50cm, CNT forests, or organised arrays of tubes are the key to scaling up production and cut down the production costs massively. However, this has been a struggle to achieve and is now where most of its research is centred upon.
The obstacle lying in its development is figuring out how to produce nanotubes at a larger scale, both to meet industrial demand and to decrease its extremely high price of production. This is because the catalysts responsible for this production deactivate pretty quickly.
One recent significant research was done in Waseda University Japan, where research used chemical labour disposition with an alteration to the catalysts used to grow a forest of carbon nanotubes. They were able to grow a whopping 14cm of forests, the previous maximum being only 2cm. The catalysts were able to stay active for 26 hours, which is an indication that we can alter various catalysts in the process to make them work for much longer. This is initial, but indicates that further experimental research could hopefully lead to a reaction that can carry on for longer periods of time.
The making of a new catalyst was through adding a gadolinium layer to an iron-aluminum oxide catalyst, which is coated onto a silicon substrate. This helped curb the deterioration to a certain extent allowing the forest to grow up to 5cm. From there, they placed the catalyst in a chemical vapor deposition chamber, heating it up to 750C which kept the catalysts active for much longer periods of time. It was also shown that the strength and purity of the tube was maintained,meaning this research opens new doors in nanoscience and catalyst engineering.
Drug delivery: A major potential application for nanomaterials is its use in developing nanocarrier drug delivery systems. Their many beneficial properties can be used in their application in medicine, and mainly in carrying anti cancer drugs to tumour cells. This makes for a better cancer treatment option than chemotherapy and radiation as it drastically reduces the number of healthy cells dying due to its specificity. They have a hollow interior which can be filled with various nanomaterials to shield them from their environment, and also provides them with a high drug loading capacity. It’s incredibly tiny size, as well as its needle-like shape makes it very easy to penetrate cells without causing cell damage. However this is not yet fully researched and confirmed. Nanomaterials are also well suited to medical application due to their significant biocompatibility, controllable size and large surface area to volume ratio.
While all of these developments sound extremely promising, there are certainly a few issues and uncertainties regarding the use of CNTs. To fully ensure that CNTs are indeed biosafe and compatible, accurate complete toxicology tests need to be carried out from a medical and manufacturing perspective. Currently the tests have shown to be quite incomplete and majorly based on animals that may not mimic its activity in an actual human body. Moreover, there are many studies showing that the uptake of nanoparticles by the cells can happen in many different ways depending on the CNT properties- some including direct cell membrane penetration, passive uptake and active uptake. which will have to be further researched on.
Another biomedical research has shown that cells can grow on nanotubes, making them very biocompatible, as previously mentioned. This means that they have a potential use in neuron growth and regeneration. Due to the complexity of the nervous system, recovering function of the injured nerves or repairing damages associated with neurodegenerative conditions is still a major challenge in the biomedical field. If we are able to successfully figure out a way to reconnect neurons, it has the ability to make a paralysed person walk again. CNTs have shown to interact with the nervous system to promote its neural development and boost neural electrical performance
Their hollow structure means they have a reacting surface both outside and inside the nanotube. This greatly increases their surface area. This can open new doors for new catalyst engineering and catalytic behaviour
The applications mentioned here are merely a fraction of the list. Further research can only tell just how much these materials can truly change our society.
Waseda University. (2021). Scientists Grow Carbon Nanotube Forest Much Longer Than Any Other. [online] Available at: https://www.waseda.jp/top/en/news/73953 [Accessed 17 Aug. 2021].#
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