Picking armchairs from zigzags

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  • Published: Jan 14, 2008
  • Author: Jon Evans
  • Channels: Ion Chromatography
thumbnail image: Picking armchairs from zigzags

Nanotechnology researchers have spent the past few years developing a whole range of methods for separating different types of single-walled carbon nanotube (SWNT), from centrifuging them in a density gradient to attaching diazonium salts and then separating them by electrophoresis. But it now looks as though ion exchange chromatography (IEC) might provide the best solution, not just being able to separate metallic SWNTs from semi-conducting SWNTs but able to separate every type of SWNT according to its specific electrical properties.

SWNTs are tiny tubes of carbon with walls only one-atom thick, as though a single sheet of graphite has been rolled into a tube. They have a number of useful properties, including various electrical and optical properties. The precise nature of these properties are dictated by the alignment of the carbon bonds in the tube (or, in other words, by the direction in which the graphite sheet is rolled up) and by the tube's diameter.

These two features of SWNTs can be represented by a pair of integers (n,m) that essentially describe the number of carbon molecules along two directions of the graphite sheet forming the tube. When n=m, the SWNTs are known as armchair; when n or m equal zero, the SWNTs are known as zigzag; and when n and m take any other non-identical values, the SWNTs are known as chiral. All armchair SWNTs have metallic properties and are able to conduct electricity very efficiently, whereas both zigzag and chiral SWNTs are semi-conductors, with the precise level of conductivity depending on the specific values taken by n and m.

Although there is great excitement over the use of SWNTs in the next generation of electrical devices, this will require being able to supply SWNTs with specific electrical properties. Unfortunately, current production techniques, which generally involve growing SWNTs by passing a carbon-based gas such as methane over metal catalysts, can only produce a broad selection of SWNTs with various n and m values. Hence the interest in developing methods that can separate and isolate different types of SWNT.

So far, most of these methods have concentrated on separating metallic SWNTs from semi-conducting SWNTs. But the ultimate goal is to be able to separate SWNTs according to their specific n and m values. In 2003, a group of researchers from DuPont Central Research and Development at Wilmington, the University of Illinois at Urbana-Champaign and the Massachusetts Institute of Technology showed how this might be achieved with IEC.

They found that a specific DNA strand composed entirely of the bases guanine and thymine would naturally bind with SWNTs and that the resultant DNA-SWNT hybrids could then be separated by anion exchange chromatography (AEC) according to the SWNTs' electrical properties. The researchers proposed that this separation mechanism depended on variations in the electrical densities of the DNA-SWNT hybrids, which influenced how strongly they bonded with the AEC stationary phase. These variations are most likely a function of the way in which the negative charge of the phosphate groups on the DNA backbone are modulated by the specific electrical properties and diameter of the SWNTs.

Since then, a number of groups have built on this finding to perform ever more sensitive SWNT separations. In 2007, two DuPont researchers combined this IEC separation technique with size-exclusion chromatography to separate (9,1) SWNTs from (6,5) SWNTs, which have the same diameter but different molecular alignments.

Most recently, researchers from Stanford University, California, and the University of Arkansas, Little Rock combined this IEC separation technique with a novel iron-ruthenium catalyst that is able to produce a very narrow range of SWNT types. Growing SWNTs at 850°C, the researchers discovered their novel catalyst produced just four different types of semi-conducting SWNT (with different n and m values), which they were then able to separate from each other using the IEC separation technique.

So thanks to IEC, the day of being able to pick and choose SWNTs with defined electrical properties may soon be upon us.


Single-walled carbon nanotube
The diameter of single-walled carbon nanotubes and the alignment of their carbon bonds dictate their electrical properties.
Credit: NASA Ames Research Center 

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