Carbon Nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1 significantly larger than for any other material. These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology.
SWCNT (Single Wall Carbon Nanotube)
Single-wall nanotubes (SWNT) are tubes of graphite that are normally capped at the ends. They have a single cylindrical wall. The structure of a SWNT can be visualized as a layer of graphite, called graphene, which is rolled into a seamless cylinder.
Most SWNT typically have a diameter of close to 1 mm. The tube length, however, can be many thousands of times longer.
SWNT are more pliable yet harder to make than MWNT. They can be twisted, flattened, and bent into small circles or around sharp bends without breaking.
SWNT have unique electronic and mechanical properties which can be used in numerous applications, such as field-emission displays, nanocomposite materials, nanosensors, and logic elements. These materials are on the leading-edge of electronic fabrication, and are expected to play a major role in the next generation of miniaturized electronics.
MWCNT (Multi Wall Carbon Nanotube)
Multi-wall nanotubes can appear either in the form of a coaxial assembly of SWNT similar to a coaxial cable, or as a single sheet of graphite rolled into the shape of a scroll.
The diameters of MWNT are typically in the range of 5 nm to 50 nm. The interlayer distance in MWNT is close to the distance between graphene layers in graphite.
MWNT are easier to produce in high volume quantities than SWNT. However, the structure of MWNT is less well understood because of its greater complexity and variety. Regions of structural imperfection may diminish its desirable material properties.
The challenge in producing SWNT on a large scale as compared to MWNT is reflected in the prices of SWNT, which currently remain higher than MWNT.
Laser method for the manufacturing of carbon nanotubes
Laser ablation was first used for the synthesis of fullerenes. In this method, a graphite target is vaporized by laser irradiation under flowing inert atmosphere at temperatures near 1200 oC , as shown in the figure. The laser vaporization produces carbon species which are swept by the owing gas and accumulate on a water-cooled collector. SWCNTs are very uniform in diameter and can be produced when the graphite target is doped with a small amount of transition metals such Ni and Co.
Applications of CNTs:
- Carrier for Drug delivery: Carbon nanohorns (CNHs) are the spherical aggregates of CNTs with irregular horn like shape. Research studies have proved CNTs and CNHs as a potential carrier for drug delivery system.
- Functionalised carbon nanotubes are reported for targeting of Amphotericin B to cells.
- Cisplatin incorporated oxidized SWNHs have showed slow release of Cisplatin in aqueous environment. The released Cisplatin had been effective in terminating the growth of human lung cancer cells, while the SWNHs alone did not show anticancer activity.
- Anticancer drug Polyphosphazene platinum given with nanotubes had enhanced permeability, distribution and retention in the brain due to controlled lipophilicity of nanotubes.
- Antibiotic, Doxorubicin given with nanotubes is reported for enhanced intracellular penetration.
- The gelatin CNT mixture (hydro-gel) has been used as potential carrier system for biomedicals.
- CNT-based carrier system can offer a successful oral alternative administration of Erythropoietin (EPO), which has not been possible so far because of the denaturation of EPO by the gastric environment conditions and enzymes.
- They can be used as lubricants or glidants in tablet manufacturing due to nanosize and sliding nature of graphite layers bound with van derwaals forces.