I don’t often plug Nature Nanotechnology papers here on the blog, but I couldn’t resist this one… (mostly because of the title I get to use!)
A couple of days ago we published a paper on our website by Naoki Komatsu and co-workers, entitled, “Optically active single-walled carbon nanotubes”. As we all know, however you go about cooking up a batch of nanotubes, you get a mixture – different diameters, different lengths and different chiralities. Now, what is meant by ‘different chiralities’ in this context, is different degrees of twist when you roll up your graphene sheet – not really ‘chirality’ as the chemist knows it. Methods have been developed to sort carbon nanotubes by length and diameter (and also by degree of twist to some extent – see this free paper here and associated News & Views article here).
What is largely ignored/forgotten, is that chiral nanotubes are, wait for it…, chiral! They come as left- and right-handed forms, depending on which may you roll your graphene sheet, i.e., you can either curl the edges up and over to form a tube, or down and under. (If you want to try this at home, take a couple of overhead transparencies – for those of you under 25 years of age, you may want to look up what one of those is – and draw a hexagonal lattice on each one and roll them up in opposite directions – hey presto, enantiomeric nanotubes!).
Now, Pasteur, all those years ago, separated the enantiomers of tartaric acid by painstakingly sorting through mirror-image crystals, presumably with a microscope and a pair of tweezers. What Komatsu and colleagues have done is to make what they call ‘nano-tweezers’ – chiral gable-type diporphyrins that can discriminate between left- and right-handed nanotubes. One enantiomer of the nano-tweezers forms a stronger complex with either the left- or right-handed nanotubes and these diastereoisomers have different solubility properties, which means that they can be separated by centrifugation. You can then wash away the tweezers and record a CD spectrum of your resolved nanotubes!
Stuart Cantrill (Associate Editor, Nature Nanotechnology)