Nanocalipers Get A Grip On Nanotubes

The properties of single-walled carbon nanotubes, or SWNTs, vary depending on the tube size and structure. Most SWNTs, however, are produced as a mixture. Seeking a way to select specific tubes from this jumble, researchers led by Naoki Komatsu of Japan’s Shiga University of Medical Science have developed nano­calipers that can discriminate among SWNTs on the basis of their diameter, handedness (the way the tubes twist), and conductivity (J. Am. Chem. Soc., DOI: 10.1021/ja312519s). The nanocalipers, fashioned from a chiral diporphyrin molecule, are an extension, quite literally, of the group’s previous work with chiral diporphyrin nanotweezers. Nano­tweezers work well at separating nanotubes with diameters less than 1 nm, but their small structural clefts can’t pluck out larger SWNTs. For the nanocalipers, the team put a carbazole-anthracene-carbazole spacer between the porphyrins. The spacer makes the nanocalipers wide enough to accommodate SWNTs with diameters greater than 1 nm. The porphyrins and the anthracene interact with SWNTs via π-π stacking, which is how the calipers pluck out tubes of a specific size. Stereogenic centers at the peripheries of the porphyrins differentiate nanotube handedness. Surprisingly, the nanocalipers also picked out nanotubes with metallike conductivity over semiconducting ones, which the nanotweezers can’t do.