“Now that we have managed to purify double-walled nanotube materials, we will be able to work on integrating them into working devices without having to be concerned about impurities,” says team leader Benjamin Flavel from the Karlsruhe Institute of Nanotechnology in Germany. “DWCNTs could be ideal as biosensors, for example. Here, we chemically modify the nanotubes’ outer walls while leaving their inner walls intact for signal transduction.”

Flavel and colleagues’ technique involves placing an as-prepared solution of nanotubes and the surfactant sodium dodecyl sulphate in an ultrasonic bath for eight hours at 15 °C. The researchers then pass the solution through a column containing the chemical gel Sephacryl S-200. Finally, they pass sodium cholate through the column as an eluent and collect the separated material at the other end.

The team, which includes researchers from Flinders University in Australia, found that DWCNTs travel faster through the S-200 gel compared with the SWCNTs and therefore come out first from the column.

Nanotubes cut to different lengths

“We believe that the DWCNTs and SWCNTs present in the raw starting material are cut to different lengths in the ultrasonic bath,” Flavel told nanotechweb.org. “We hypothesize that this may be partly thanks to the fact that DWCNTs contain an inner wall that makes them more stable to sonication than their SWCNT counterparts. This means that the DWCNTs are not cut as small and have larger average lengths overall. However, it is important to remember that the interplay between the two different surfactants used and the well known diameter-dependent wrapping of carbon nanotubes also play a role."

The technique produces two very distinct populations of CNTs, ideal for size exclusion chromatography where smaller species takes longer to pass through a gel medium than the larger ones – simply because they have a greater interaction area with the gel, he added.

Relatively simple alternative

“It certainly would not be fair to say that our method is better than other techniques (such as density gradient ultracentrifugation) for separating out DWCNTs, but it is different,” he explained. “It is also a relatively simple alternative and does not require any expensive centrifugation equipment.”

At this point, the researchers have only shown that they can separate out DWCNTs from other types of CNT in the raw material starting mix. “DWCNT separation is much more complex than this and the individual walls of a DWCNT can be either semiconducting or metallic too,” added Flavel. “We now thus plan on focusing our efforts on separating DWCNTs by their electronic type.”

The current work is detailed in ACS Nano DOI: 10.1021/nn500756a.