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 approximately 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.

We measured the force required to peel single-stranded DNA molecules from single-crystal graphite using chemical force microscopy. Force traces during retraction of a tip chemically modified with oligonucleotides displayed characteristic plateaus with abrupt force jumps, which we interpreted as a steady state peeling process punctuated by complete detachment of one or more molecules. We were able to differentiate between bases in pyrimidine homopolymers -peeling forces were 85.3 ±4.7 pN for polythymine and 60.8±5.5 pN for polycytosine, substantially independent of salt concentration and the rate of detachment. We developed a model for peeling a freely jointed chain from the graphite surface and estimated the average binding energy per monomer to be 11.5±0.6 k B T and 8.3±0.7 k B T in the cases of thymine and cytosine nucleotides. The equilibrium free-energy profile simulated using molecular dynamics had a potential well of 18.9 k B T for thymidine, showing that non-electrostatic interactions dominate the binding. The discrepancy between the experiment and theory indicates that not all bases are adsorbed on the surface or that there is a population of conformations in which they adsorb. Force spectroscopy using oligonucleotides covalently linked to AFM tips provides a flexible and unambiguous means to quantify the strength of interactions between DNA and a number of substrates, potentially including nanomaterials such as carbon nanotubes.Interactions of polyelectrolytes with solid substrates have several important applications in materials science and engineering. 1, 2 On complexation with neutral particles, polyelectrolytes convert these particles into charged species, enabling dispersions in aqueous media, and find use in detergents, cosmetics, gels, food additives, and oil recovery. In a biological context, our interest is in the interaction between single and double-stranded DNA (ssDNA and dsDNA) and substrates such as graphite and carbon-nanotubes (CNTs) that can potentially play a vital role in biomedicine, 3, 4 nanotechnology, 5,6 and relevant for the understanding of the origin of life. 7 Importantly, ssDNA has been used successfully for dispersion and solution-based manipulation of CNTs -ssDNA forms a stable hybrid with a nanotube by wrapping around the CNT in helical fashion. 8,9 The hybrid is useful for dispersion, sorting, 8,9 and patterned placement of nanotubes, 6 for transportation of DNA into cells, and for killing cancer cells by thermal ablation. 3 Strength of dispersion, ability to sort, and stability in the cellular environment all depend on the interaction between DNA and a CNT. 10 Individual DNA bases also bind to a graphitic surface through non-covalent π-π interactions. [11][12][13][14] Very little is known quantitatively about the strength of binding between ssDNA and CNT's in spite of the fundamental importance of understanding such interactions. 8,[15][16][17] As a first

Single-stranded DNA-carbon nanotube (CNT) hybrids have been used successfully for dispersion and structurebased sorting of CNTs. The structure of the hybrid determines its behavior in solution. Using scaling arguments and molecular dynamics simulations, we have studied various factors that contribute to the free energy of hybrid formation, including adhesion between DNA bases and the CNT, entropy of the DNA backbone, and electrostatic interactions between backbone charges. MD simulations show that a significant fraction of bases unstack from the CNT at room temperature, which reduces effective adhesion between the two per base. For homopolymer wrappings, we show that at low ionic strength, the dominant influences on the structure are adhesion between DNA and the CNT and electrostatic repulsion between backbone charges on the DNA. With a simple analytical model, we show that competition between these two can result in an optimal helical wrapping geometry.