Ever since their discovery in 1991, carbon nanotubes have attracted interest from a variety of sciences. Research into these microscopic tubes of connected carbon atoms has grown exponentially, and the tubes have shown their versatility across countless disciplines, being utilized in electronics, optics, structural reinforcements and medicine.
Carbon nanotubes are tiny cylinders of interlocked atoms wrapped around an empty middle. Their shape, size and structure possess properties that set them apart from other materials and make them applicable in a wide range of scenarios.
There are two types of nanotubes: the "single-walled nanotubes" (SWNTs), which are single cylinders, and the tube-within-a-tube "multi-walled nanotubes" (MWNTs). The carbon-to-carbon bond, the building block of these nanotubes, is the strongest bond in the known universe. This makes the tubes unbelievably stable, strong, light and elastic.
A versatile molecule
Much of the nanotube’s versatility comes from its structure. It can be more than 10,000 times longer than it is thick, and with all its atoms on the surface. It therefore has a huge surface area for binding to other molecules, and it is extremely invasive. Even when a small percentage of nanotubes is mixed into a material, the tubes form a web-like network that helps bear the material’s stress, increase conductivity and reorganize the material to be more efficient.
The electronic properties of the nanotube are determined by the geometric pattern the carbon atoms make when bonded to each other. One pattern can make the tube a semiconductor, while another pattern turns it into a conductor. The tubes easily accept electrons and have a special quality called ballistic transport, in which electrons fly at super-fast speeds along the length of the tube. Nanotubes can endure much higher currents than copper and do not deteriorate with overheating or constant use. The tube's size constraints allow both heat and electricity to flow down it efficiently. Because of their electronic diversity, carbon nanotubes can be utilized in almost any part of the circuit: wire, transistor, memory, heat sink, emitter for a display, battery, or sensor.
During the manufacturing process of carbon nanotubes, engineers can tweak their length, thickness and purity, and the tubes can be tailored easily to a specific need. They also can be accessorized with other molecules, or "functionalized," in countless ways. Through a set of reactions and catalysts, the tubes' structure can incorporate DNA, proteins, polymers, metals, or practically any molecule one would study in organic chemistry.
Functionalized nanotubes bring a whole new approach to medicine.
Nanotubes can enter "directly into the cells by a passive mechanism that we described as nanoneedle penetration," said Alberto Bianco, a researcher at Institut de Biologie National Research Council in Strasbourg, France, who has been on the forefront of finding bio-applications for nanotubes.
Unlike other drug-delivery mechanisms, which have to be surrounded by a bubble of cell membrane to gain access, nanotubes just spear their way through, with the payload ready to go.
Nanotubes also seem perfect for bio-sensing, and fluorescent properties in the semiconducting nanotubes could image inside the body with great clarity. (These bio-applications have been worked on only in labs at this point, with no clinical trials.)
For all of the versatility carbon nanotubes offer, there have been some obstacles to their use. Scaling up, or producing enough cost-effective nanotubes for the human-size application market, has been a major issue. Currently, MWNTs are being produced en masse, but SWNTs have a way to go until they are produced in a commercially viable way.