DNA Wrapping Causes Strain in Single-Wall Carbon Nanotubes

Abstract

Short strands of single-stranded DNA (ssDNA) can disperse and individually suspend single-wall carbon nanotubes (SWCNTs) to form a class of hybrids with unusual properties and applications in nanotube structural sorting and fluorescence-based bioanalysis. In these hybrids, the ssDNA strands tend to physisorb onto the nanotube surface, coating it in helical or ring-shaped structures. We have carefully compared fluorescence spectral peak positions in samples of SWCNTs coated by ssDNA and in conventional surfactants such as SDS. In addition to the general and well-known spectral red-shift for the ssDNA-coated samples, we find a systematic dependence of both the E11 and E22 shifts on the mod 1 vs. mod 2 identity of the (n,m) species. This spectral pattern is characteristic of SWCNTs that are distorted by mechanical strain. We infer that the ssDNA wrapping exerts radial forces on the nanotube, causing this strain. The magnitude of the observed effect depends on the ssDNA oligo and on SWCNT diameter, with large values found for (GT)6 and nanotubes with diameters near 0.9 nm. Atomistic molecular dynamics simulations show that (GT)6 forms rings when coating such SWCNTs. Further insights into strain in ssDNA/SWCNT hybrids will be presented based on additional computations and experimental results.