Carbon Nanotubes: Uses, Properties and Limitations
Carbon nanotubes (CNTs) have been deemed a wonder material due to their remarkable and highly unique physical and chemical properties. They have received much attention over the past decade as a promising material, particularly in the trending field of nanotechnology.
CNTs are super strong and yet highly versatile. They have applications across a broad range of fields including materials science, engineering, medicine, agriculture, and plant biotechnology.
What are Carbon Nanotubes?
CNTs are a graphitic carbon-based nanomaterial forming a tubular structure. They are made up of rolled graphene sheets of cylindrical shape and containing either one or multiple graphene layers. They are normally classified according to their wall structure as follows:
CNTs are renowned for their excellent optical, electrical, and mechanical properties. They demonstrate good flexibility, high mechanical and thermal stability, and low density. They can conduct electricity, which makes them a prime candidate for nanotechnological applications. CNTs have relatively large specific surface areas, enhancing their potential absorbency (in applications where this characteristic is desired). CNTs can effectively absorb substances like lead, heavy metal ions, dyes, and many other compounds.
The possibility of surface modifications to CNTs offers the potential to enhance certain characteristics, making them more water resistant. CNTs are chemically stable and highly resistant to acid and alkaline conditions, thus reducing the risk of corrosion. When taken together, all of these amazing and relatively rare properties mean CNTs have attracted a lot of attention. CNTs are the most widely utilized nanomaterial - other examples being nanodiamonds, fullerenes (hollow carbon molecules) and graphene. Nanotechnology is a growing field concerned with structures between 1 and 100 nm and involves the production of minute materials or microelectronics.
CNTs were a much anticipated “material of dreams” just half a century ago (Endo et al. 2006). They were first synthesised by Endo in 1976. Then in 1991, Iijima reported the preparation of carbon structures consisting of needle-like tubes, in the prestigious journal “Nature”. Electron microscopical analysis gave a detailed picture of their molecular structure, revealing coaxial (where several 3D linear or planar forms share a common axis) tubes of graphite sheets ranging from 2 to around 50 nm. The synthesis of these nanometre size tubes drew a great deal of interest amongst researchers, not least for their unlimited potential and prospects for engineering these microstructures on a much larger scale.
What Are CNTs Used For?
Nowadays CNTs are in high demand and are utilized across a diverse range of applications. Research into CNTs is a highly interdisciplinary effort involving diverse fields such as physics, chemistry, biology, medicine, materials science, and engineering. CNTs can be used in nanotechnology, automotive parts, electrical circuitry, supercapacitors, photovoltaic technology - including solar panels, LEDs, sensors, transistors, field emitting devices, fuel cells, actuators (devices that power physical movement), ceramics, batteries, absorbents, catalysts, storage devices, polymer- and metal oxide-based nanocomposites. Graphene and fullerenes can be added to a material or substance to enhance strength. This is useful, for example, in the manufacture of sportswear, or materials used in the deflection of projectiles including bullet-proof vests.
CNTs also have important applications in pharmaceuticals, medicine, and agriculture. Their tubular structure enables them to be filled with substances like pharmaceutical drugs, thus making them an ideal candidate for drug delivery and other therapeutic applications. This means CNTs can play a significant role in the treatment of cancer. As well as drug therapy, CNTs also have potential applications in biosensing, bioimaging, nanorobotics, gene therapy and tissue regeneration, where, for example, CNTs can play a direct role in the formation of artificial cellular scaffolding resembling the extracellular matrix (ECM) - the structural architecture of the cell. In agriculture and the environmental sciences, CNTs have an important part to play in bioremediation, water purification and wastewater treatment. Here CNTs harbour much potential to filter pollutants and microorganisms through the mechanisms of absorption, catalysis, and disinfection.
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Limitations and Further Research of CNTs
Though deemed the “king of materials”, this wonder structure is not without its limitations. What these are depends upon their potential use. The following properties are seen as problematic to the utilization of CNTs in nanomedicine:
- Impurities
- Non-uniformity in morphology and structure
- Hydrophobicity
- Tendency to bundle
Meanwhile this capacity to bundle is a favourable feature in other cases. CNT bundles resembling the hairs of Gecko feet have been used in the manufacture of a novel kind of sticky tape. In this guise they have the capacity to stick to a variety of materials like glass and Teflon.
The main drawbacks for CNTs are related to cost-effectiveness, the ability to produce them without defects, and questions about their potential toxicity. The fibrous nature of CNT structure means they resemble asbestos fibres. In addition, their small size and their strength means they are able to penetrate cellular membranes within the body. Their hydrophobicity (water repellence) means they can easily accumulate and persist within the body.
We also need to learn more about the potential toxic effects of CNTs including how they are distributed within the body and how they might potentially be broken down. Current research into the latter area involves investigation into the biodegradation of CNTs by white blood cells (macrophages - cells that play a key role in immunity). In the meantime, limitations remain for scaling up the production of CNTs from the bench (laboratory) onto a much grander industrial scale and for doing so in a cost-effective way. This means that at present biomedical applications remain somewhat limited. Further research is yet needed to fully realise the potential of CNTs.
References
- Bullis, K. Climbing Walls with Carbon Nanotubes.(2007) MIT Technology Review. Online: www.technologyreview.com/2007/06/25/224884/climbing-walls-with-carbon-nanotubes
- Endo, M. et al. (2006) Development and Application of Carbon Nanotubes. Japanese Journal of Applied Physics, 45. DOI: 10.1143/JJAP.45.4883.
- Iijima, S. Helical microtubules of graphitic carbon. (1991) Nature 354, 56–58. DOI: 10.1038/354056a0 National Institute for Occupational Safety and Health (2013) Current Intelligence Bulletin 65: Occupational Exposure to Carbon Nanotubes and Nanofibers. Centers for Disease Control and Prevention (CDC). Online: www.cdc.gov/niosh/docs/2013-145/
- Rastogi, V. (2014) Carbon Nanotubes: An Emerging Drug Carrier for Targeting Cancer Cells. J. Drug Delivery. DOI: 10.1155/2014/670815.
- Shariatinia, Z. (2021) Applications of Carbon Nanotubes. Handbook of Carbon–Based Nanomaterials, pp. 321-364.
- Simon, J, et al. (2019). Overview of Carbon Nanotubes for Biomedical Applications. Materials 12 (4), 624. DOI: 10.3390/ma12040624.
- Sridharan, R. et al. (2022) Carbon nanomaterials and its applications in pharmaceuticals: A brief review. Chemosphere, 294. DOI: 10.1016/j.chemosphere.2022.133731
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- Yang, M and M. Zhang. (2019) Biodegradation of Carbon Nanotubes by Macrophages. Frontiers in Materials, 6. DOI: 10.3389/fmats.2019.00225.
- Zhang, et al. (2021) Carbon Nanotubes: A Summary of Beneficial and Dangerous Aspects of an Increasingly Popular Group of Nanomaterials. Frontiers in Oncology, DOI: 10.3389/fonc.2021.693814.