Kinetics of Single-Wall Carbon Nanotube Coating Displacement by Single-Stranded DNA Depends on Nanotube Structure

Time-resolved fluorescence spectroscopy has been used to study the displacement of adsorbed sodium dodecyl sulfate (SDS) from the surface of single-wall carbon nanotubes (SWCNTs) by short strands of single-stranded DNA. Intensity changes in near-infrared emission peaks of various SWCNT structures were analyzed following the addition of six different (GT) oligomers (n from 3 to 20) to SDS-coated nanotube samples. There is a strong kinetic dependence on the oligomer length, with (GT)₃ giving an initial rate more than 300 times greater than that of (GT)₂₀. For shorter oligos in the (GT) series, we observe an inverse dependence of the displacement rate on the SWCNT diameter, with SDS displaced from (6,5) more than twice as fast as from (8,7). However, this diameter dependence is reversed for oligos with more than six (GT) units. There is also a systematic dependence of the displacement rate on the nanotube chiral angle that is strongest for (GT)₅, leading to a factor of ∼3 initial rate difference between (9,1) and (6,5) despite their identical diameters. To account for these findings, we propose a simple two-step kinetic model in which disruption of the original SDS coating is followed by conformational relaxation of ssDNA on the nanotube surface. The relaxation is relatively fast for ssDNA oligos shorter than 12 bp, making the first step rate-determining. Conversely, relaxation of the longer oligomers is slow enough that the second step becomes rate-determining.