(Invited) Kinetics of SWCNT Coating Displacement By ssDNA Depends on SWCNT Structure

Abstract

Non-covalent hybrids formed by suspending single-wall carbon nanotubes (SWCNTs) in single-stranded DNA (ssDNA) present novel phenomena and suggest potential applications in biomedicine and nanotube sorting. Although the structures of these hybrids have been simulated using molecular dynamics calculations, much more experimental research is needed to understand their properties. In this study, selective interactions between SWCNTs and ssDNA are probed by measuring the kinetics by which ssDNA displaces SDS (sodium dodecyl sulfate) coatings on the nanotube surfaces. We used samples initially suspended in a low concentration of SDS to allow the coating displacement to occur and to suppress formation of free SDS micelles. The displacement process was monitored by time-dependent measurements of SWCNT fluorescence intensities and wavelengths. Our experiments showed very smooth decreases in fluorescence intensity and red-shifts in fluorescence wavelengths during the displacement process. Through a series of measurements, we found that ssDNA sequences with repeated guanine (G) and thymine (T) bases showed systematic patterns in the coating displacement kinetics. One result is that SDS displacement depends strongly on oligo length, with (GT)3 showing an initial rate constant 500 times greater than that of (GT)20. For shorter oligos in the (GT) n series, we observe an inverse dependence of initial rate constant on SWCNT diameter, with SDS displacement from (6,5) more than twice as fast as from (8,7). However, this diameter dependence is not present for (GT) n oligos with n greater than 6. We also find a distinct and systematic dependence of initial rate constant on nanotube chiral angle for (GT)5 and (GT)6. This effect gives a factor of ~3 difference between (9,1) and (6,5) despite their identical diameters. We empirically find these initial rate constants to vary as (SWCNT diameter)-4(cos 3θ)-y with y = -1/2 for (GT)5 and -1 for (GT)6. In addition, the full displacement kinetic traces are well fitted by double exponential kinetics, with rate constants and amplitudes dependent on SWCNT diameter. A two-step kinetic model for the displacement process will be presented to account for these findings.