Single-walled carbon nanotubes (SWCNs) are well known for their outstanding strength, stiffness, and electronic properties, but their utility has been limited by the diversity of their structures and the difficulty of separating different kinds. In a new paper in Nature, a DuPont group reports the development of a new method for separation, one that also has implications for how SWCNs could be used in atomically precise nanosystems.
SWCNs can be regarded as tubes of graphene, but these tubes can differ both in radius and in the angle between the tube’s axis and the lattice axes of the graphene sheet*. Different structures are metallic, insulating, or semiconducting, and not functionally interchangeable. Building on previous work that showed how single-strand DNA could wrap and solubilize SWCNs, the DuPont group searched the vast, combinatorial space of DNA sequences for those that would wrap tubes in an orderly and selective way that enables different kinds to be separated.
The results are surprisingly effective, enabling the separation of a dozen kinds of tubes of similar diameter, each to a purity of 60–90% or better. Each kind is preferentially wrapped by a different DNA sequence.
There are two basic strategies for getting atomically precise structures: either make them precisely, or make a mixture of kinds, and separate them. Precise SWCNs are increasingly available by means of the second strategy. Further, the ability to wrap them in well-organized sheaths of engineered biomolecules provides a natural way to interface them to complex biomolecular nanosystems.
*The lattice vectors that determine this angle specify a tube’s “chirality”, but do not distinguish between left- and right-handed twists. “Chirality” therefore specifies everything about a structure except its chirality. Chemists take note.