Joined: 16 Mar 2004
|Posted: Mon Feb 04, 2008 11:27 am Post subject: Telling left from right - carbon nanotubes
Researchers in Japan have separated left and right-handed carbon nanotubes. Naoki Komatsu of Shiga University of Medical Science and colleagues achieved their breakthrough result using special, chemical "nano-tweezers" that can selectively pick between the two different types of nanotube from a mixture. This technique is the first to produce optically active nanotubes and could be a valuable step in preparing these materials for applications in nano- and optoelectronics.
As-prepared samples of single-walled carbon nanotubes contain equal amounts of left and right-handed helical structures. Researchers have previously sorted nanotubes by diameter or length but they have paid little attention to separating the chiral forms – known as optical isomers – that have opposite helical twists. The word chiral comes from the Greek for "hand" and describes an object that cannot be superimposed on its mirror image – like a human hand.
In 1849, Louis Pasteur was the first to separate chiral molecules by meticulously sorting asymmetric crystals of tartaric acid by hand using tweezers and a microscope. Using this concept, Komatsu and colleagues have now designed pairs of nano-tweezers that can discriminate between the two carbon nanotube forms. The chiral tweezers consist of two porphyrin units and bridged spacers .
The technique works by molecular recognition: diporphyrin molecules bind with different affinities to the left and right-handed nanotube isomers, to form complexes with differing stabilities that can be easily separated. Once separation is complete, the diporphyrins can be removed to leave behind samples enriched in either the left or right-handed nanotubes.
The samples are optically active because they differ in the way they absorb circularly polarized light – one strongly absorbs right-handed light while the other absorbs the left-handed form. Before separation, the mixture absorbed equal quantities of both right and left-handed light.
The ability to separate the nanotubes in this way will ultimately lead to a better understanding and control of their optical properties, offering unique opportunities for applications in photonics and quantum optics.
This work was reported in Nature Nanotechnology.
Story posted: 23rd May 2007