A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range (nanoscale). They are one of the allotropes of carbon.
Carbon nanotubes are members of the fullerene family. Although the first fullerene molecules were discovered in 1985, it was not until Sumio Iijima reported his findings in 1991 about needlelike carbon tubes in Nature that carbon nanotubes came to public awareness.
Mainly three types of nanotubes i.e., single-walled, double-walled and multiwalled carbon nanotubes are manufactured.
Single-walled carbon nanotubes (SWCNTs)
Single-walled carbon nanotubes (SWCNTs) are seamless cylinders, made of a single layer of graphene. SWCNTs have diameters around 0.5–2.0 nanometres, about 100,000 times smaller than the width of a human hair. They can be idealised as cutouts from a two-dimensional graphene sheet rolled up to form a hollow cylinder.
Multi-walled carbon nanotubes (MWCNTs)
Multi-walled carbon nanotubes (MWCNTs) consist of nested single-wall carbon nanotubes in a nested, tube-in-tube structure. MWCNTs consist of multiple rolled layers of graphene.
Structure of SWCNTs
The structure of an ideal (infinitely long) single-walled carbon nanotube is that of a regular hexagonal lattice drawn on an infinite cylindrical surface, whose vertices are the positions of the carbon atoms.
Synthesis of Carbon nanotubes
Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, chemical vapor deposition (CVD) and high-pressure carbon monoxide disproportionation (HiPCO).
Chemical vapor deposition (CVD) is the most promising way to produce carbon nanotubes on an industrial scale. This process utilizes high energy (600–900 °C ) to atomize gaseous carbon sources, such as methane, carbon monoxide, and acetylene. The resulting reactive carbon atoms diffuse toward a catalyst-coated substrate and condense to form carbon nanotubes.
Properties of Carbon nanotubes
Carbon nanotubes can exhibit remarkable properties, such as exceptional tensile strength and thermal conductivity because of their nanostructure and strength of the bonds between carbon atoms. Some SWCNT structures exhibit high electrical conductivity while others are semiconductors.
In addition, carbon nanotubes can be chemically modified. These properties are expected to be valuable in many areas of technology, such as electronics, optics, composite materials (replacing or complementing carbon fibres), nanotechnology (including nanomedicine), and other applications of materials science.
Some important properties of Carbon nanotubes are:
֍ Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus.
֍ Unlike graphene, which is a two-dimensional semimetal, carbon nanotubes are either metallic or semiconducting along the tubular axis.
֍ Carbon nanotubes have useful absorption, photoluminescence (fluorescence), and Raman spectroscopy properties.
֍ All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as “ballistic conduction”, but good insulators lateral to the tube axis.
֍ The cylindrical carbon molecules feature high aspect ratios (length-to-diameter values), with diameters from about 1 nanometer up to tens of nanometers and lengths up to millimeters. This unique one-dimensional structure and concomitant properties endow carbon nanotubes with special features, rendering them with unlimited potential in nanotechnology associated applications.
Applications of Carbon nanotubes
Carbon nanotubes are currently used in multiple industrial and consumer applications. These include battery components, polymer composites, to improve the mechanical, thermal and electrical properties of the bulk product, and as a highly absorptive black paint.
Many other applications are under development, including field effect transistors for electronics, high-strength fabrics, biosensors for biomedical and agricultural applications, and many others.
For biomedical applications, carbon nanotubes show promise as vehicles for targeted drug-delivery and nerve cell regeneration.
Carbon nanotubes as nanosensors
Due to continuous progress in the development of detection strategies, there are numerous examples of the use of SWCNTs as highly sensitive nanosensors .
Examples include the detection of reactive oxygen and nitrogen species , neurotransmitters other small molecules, lipids, proteins, sugars, DNA/RNA, enzymes as well as bacteria.
Potential future applications include biomedical and environmental applications such as monitoring plant health in agriculture, standoff process control in bioreactors, research/diagnostics of neuronal communication and numerous diseases such as coagulation disorders, diabetes, cancer, microbial and viral infections, testing the efficacy of pharmaceuticals or infection monitoring using smart implants.
In industry, SWCNTs are already used as sensors in the detection of gases and odors in the form of an electronic nose or in enzyme screening.
Are Carbon nanotubes biodegradable?
Carbon nanotubes were once considered to be resistant to chemical damage due to their rigid and perfect chemical structure, which rendered them immune to biodegradation. However, enzymes like peroxidase were found to play an important role in the process of biodegradation of carbon nanotubes.
Environmental issues associated with Carbon nanotubes
Nanoscale particles may pose a greater health risk than bulk materials due to a relative increase in surface area per unit mass. Increase in length and diameter of CNT is correlated to increased toxicity and pathological alterations in lung.
In particular, exposure to carbon nanotubes has been associated with mesothelioma, a cancer of the lung lining. If inhaled, it is believed that nanotubes can scar lung tissues in a manner similar to asbestos fibers, a cause for concern because nanotubes are already used in many common products, such as bicycle frames, automobile bodies, and tennis rackets.
Must read: Carbon fibers – Production , Advantages and Applications
External link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8353320/
PRACTICE QUESTIONS
QUES . With reference to carbon nanotubes, consider the following statements: UPSC 2020
1 . They can be used as carriers of drugs and antigens in the human body.
2 . They can be made into artificial blood capillaries for an injured part of human body.
3 . They can be used in biochemical sensors.
4 . Carbon nanotubes are biodegradable.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2, 3 and 4 only
(c) 1, 3 and 4 only
(d) 1, 2, 3 and 4
Ans (c)
