Double-walled carbon nanotubes (DWNTs) can be separated from mixtures of single- (SWNTs) and multiwalled nanotubes (MWNTs) by using a centrifugation method recently developed at Northwestern University (Nat. Nanotechnol., DOI: 10.1038/nnano.2008.364). Today's nanotube synthesis methods yield mixtures containing many types of tubes. SWNTs can be eliminated from the mixtures via high-temperature oxidation, but the process degrades the electrical and optical properties of DWNTs and does not remove MWNTs. Now, a Northwestern team has shown that by treating the tubes with a surfactant and suspending them in a fluid with a density gradient, the mixtures can be separated into their components by ultracentrifugation. The researchers used microscopy and spectroscopy to confirm the technique's ability to consistently isolate DWNTs. DWNTs are ideal systems for studying interwall interactions that influence the properties of nanotubes with two or more walls, says professor Mark C. Hersam, who conducted the study with graduate student Alexander A. Green. In addition, DWNTs have properties that lie between those of SWNTs and MWNTs. DWNTs provide enhanced performance in field-effect transistors and improved resolution in scanning-probe microscopy.