Researchers from the Standards Institute and Rice University have developed a cheap process for obtaining self-organizing nanotubes - that is, their self-assembly, in perfect rows, just like in cotton fabric
Nanotubes, tiny honeycomb-like cylinders of carbon atoms a few nanometers in diameter, may be the key ingredient in modern engineering research, but the practical attempt to organize those atomic cylinders is like the complex challenge of leading a pack of cats. However, new research from the National Institute of Standards and Technology (NIST) and Rice University (Rice) suggests an inexpensive process for obtaining self-organizing nanotubes - that is, their self-assembly, in perfect rows, just like cotton fabric.
A wide range of new materials and electronics technologies exploits the unique physical, optical and electronic properties of carbon nanotubes, but most of them - nanoscale conductors or "nanowires" - depend on their ability to organize themselves in some orderly fashion. Unfortunately, simply mixing them in a solution will only lead to a black sticky, tar-like substance. Admittedly, they can be coated with other compounds to prevent them from sticking to each other - sometimes DNA compounds are used. To that end - but from the moment you separate the mixture and dry all you get will be a tangled and random cocoon of nanotubes. There are various mechanical approaches to organize carbon nanotubes on a surface, but a more elegant and tempting solution would be the ability to force them to do it themselves - self-assembly.
Institute researchers are looking at more efficient ways to sort and clean carbon nanotubes to prepare standard samples of these materials using bile acids that coat the nanotubes to prevent them from sticking to each other. "Bile acids," says Chemical Institute researcher Erik Hobbie, "is a biological surfactant and most of them contain a part that attracts water and another that repels it. Bile acid is a more complex surfactant because instead of having a "head" and a "tail" - the usual geometry, it has two different ends (sides) - one that is attracted to water and the other that repels water." When mixed with water, these hydrophilic/hydrophobic compounds tend to self-assemble into hollow spheres with their hydrophobic tails protected from the inside, the researcher explains, but the unique geometry of bile acid leads them to organize into hollow cylinders instead. As a result, these hollow cylinders can host cylindrical carbon nanotubes within them.
As it turned out, the above process has an additional bonus. After one day, the bile acid coating causes the nanotubes to start aligning in an orderly manner, end to end, into an array of long threads, and then these threads begin to fit together into a coiled fiber, similar to a coiled copper wire. The discovery is still far from being a perfect solution for the organization of nanotubes, warns the lead researcher, and there is still much room for its development. As a first step, there is a need to remove the bile acids after the nanotubes are properly positioned, but this process has proven to be difficult and complex. Since bile acids are toxic to living cells, they will prevent use in most biomedical applications if not removed. On the other hand, he points out, it is already a very simple and cheap method for researchers interested, for example, in examining the optical properties of carbon nanotubes. "The method provides a recipe for preparing organized and ordered arrays of individual carbon nanotubes. There is no need to use any external magnetic or electric field and you do not need to dry the test tubes or heat the mixture. It is possible to have significant complexes with highly organized arrays only as a result of this self-organization."
The original news of the research institute
