Ion exchange chromatography (IEC) has already demonstrated its skill at separating the two main types of single-walled carbon nanotube (SWNT) - metallic and semiconducting (see Picking armchairs from zigzags).

Now US scientists have gone a step further and developed an improved IEC process that is able to separate and purify specific kinds of semiconducting SWNT. But in order to do this they first had to sift through more DNA sequences than there are stars in the observable universe.

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 great strength, fluorescence and electrical conductivity. The precise nature of these properties, especially electrical conductivity, are dictated by the alignment of the carbon bonds making up 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). When n=m, the SWNTs possess metallic properties and are able to conduct electricity very effectively. When n and m take non-identical values above zero, the resultant SWNTs are semi-conductors, with the precise level of conductivity depending on the specific values taken by n and m.

Unfortunately, current production techniques, which generally involve growing SWNTs on 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 purify specific types of SWNT.

In 2003, a group of US scientists led by Ming Zheng from the US chemical giant DuPont 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. The resultant DNA-SWNT hybrids could then be separated by IEC according to the SWNTs' electrical conductivity, with metallic SWNTs eluting before semiconducting SWNTs.

But if one DNA strand can distinguish metallic from semiconducting SWNTs, perhaps a whole range of DNA strands can distinguish SWNTs according to their specific n and m values, reasoned Zheng. So he and his team set about trying to find such DNA strands. This was no mean feat given that strands consisting of up to 100 bases can now be synthesised to order, potentially offering a library of 4¹⁰⁰ (or 10⁶⁰) different strands to sift through (in comparison, it has been estimated that there are just 7x10²² stars in the observable universe).

So to make this task slightly more manageable, they limited themselves to strands 28 or 30 bases long made up of simple repeated sequences, which amounted to around 350 different strands. Then they tested the ability of these strands to separate different SWNTs in IEC, shortening those strands that showed promise to see whether length had any effect on their separation ability.

In the end, they came up with 20 strands, most of which consisted of between eight and 16 bases, that between them were able to separate 12 SWNTs with different n and m values from a varied mixture, producing fractions with a purity of 99%.

The scientists are still unsure about exactly how this separation process works. They suspect that the DNA strands may be combining into tubular sheets that are able to house SWNTs within them. As different strands form different size tubes only those SWNTs possessing the correct n and m values are able to fit inside them. Such SWNT-containing DNA structures may then be interacting less with the IEC stationary phase than bare SWNTs, leading these structures to elute first.

There is still a lot to do before this method can form the basis of a large-scale SWNT separation process, not least because DNA strands are fairly expensive and because so far the process works much better for semiconducting SWNTs than metallic ones. But it has already thrown up some interesting questions.

'What does this say about how DNA interacts with nanomaterials,' asks team member Anand Jagota of Lehigh University. 'Will they be harmful inside the body? Can we take advantage of the interaction for therapeutic applications? It's a big open field.'

Related links:

• Nature, 2009, 460, 250 - 253: "DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes" 

Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.