a, A 2D DNA sheet structure formed by three anti-parallel ATTTATTT strands. The dotted lines between bases indicate hydrogen bonds. The open arrow in each strand denotes 5′ to 3′ direction. The dashed grey arrow (top right to bottom left) represents the roll-up vector along which the DNA barrel in b is formed. b, A DNA barrel on a (8,4) nanotube formed by rolling up a 2D DNA sheet composed of two hydrogen-bonded anti-parallel ATTTATTTATTT strands. c, The structure in b viewed along the tube axis. Colour coding: orange, thymine; green, adenine; yellow ribbons, backbones
Single-walled carbon nanotubes (SWNTs) are a family of molecules that have the same cylindrical shape but different chiralities. Many fundamental studies and technological applications of SWNTs require a population of tubes with identical chirality that current syntheses cannot provide. The SWNT sorting problem—that is, separation of a synthetic mixture of tubes into individual single-chirality components—has attracted considerable attention in recent years. Intense efforts so far have focused largely on, and resulted in solutions for, a weaker version of the sorting problem: metal/semiconductor separation. A systematic and general method to purify each and every single-chirality species of the same electronic type from the synthetic mixture of SWNTs is highly desirable, but the task has proven to be insurmountable to date. Here we report such a method, which allows purification of all 12 major single-chirality semiconducting species from a synthetic mixture, with sufficient yield for both fundamental studies and application development. We have designed an effective search of a DNA library of 10^60 in size, and have identified more than 20 short DNA sequences, each of which recognizes and enables chromatographic purification of a particular nanotube species from the synthetic mixture. Recognition sequences exhibit a periodic purine–pyrimidines pattern, which can undergo hydrogen-bonding to form a two-dimensional sheet, and fold selectively on nanotubes into a well-ordered three-dimensional barrel. We propose that the ordered two-dimensional sheet and three-dimensional barrel provide the structural basis for the observed DNA recognition of SWNTs.
Eric Drexler’s Comments on the Potential
SNCWs 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 CWNTs [Carbon MultiWall Nanotubes], 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 CWNTs 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.