Browsing by Author "Weisman, Bruce R."

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    Coating Displacement Studies on Single-Wall Carbon Nanotubes
    (2023-08-09) Lei, Kunhua; Weisman, Bruce R.
    We have investigated the process by which coatings of sodium dodecyl sulfate (SDS) adsorbed on the surface of single-wall carbon nanotubes (SWCNTs) are replaced by short oligomers of single-stranded DNA (ssDNA). The kinetic and equilibrium measurements focused on the influence of ssDNA length, concentration, SWCNT structure, and temperature. Our experiments involved samples of SWCNTs dispersed in a low concentration aqueous solution of SDS, to which we injected specific amounts of ssDNA solution, using oligomers composed of from 3 to 20 GT nucleobase units. We used absorption and emission spectroscopy to monitor the displacement process and measured time-dependent fluorescence spectra for kinetic studies. Our results show that the SDS coating displacement kinetics have a strong inverse dependence on the length of the ssDNA oligomer. Moreover, for displacement by ssDNA oligomers such as (GT)5, the initial rate constant depends not only on nanotube diameter but also on nanotube chiral angle. We further found that higher temperatures and higher ssDNA concentrations lead to faster displacement kinetics. Based on the experimental results, we propose a two-step displacement mechanism in which disruption of the initial SDS coating is followed by conformational relaxation of the newly adsorbed ssDNA. The first step is rate-determining for short oligomers whereas the second step becomes rate-determining for longer oligomers. In addition, we conducted equilibrium coating displacement experiments that measured stable spectral shifts of the emission peaks from multiple nanotube species resulting from the calibrated addition of ssDNA to aqueous suspensions of SWCNTs in SDS. Data were compiled to construct “titration” curves showing diameter-dependent inflection points at which the nanotubes were equally coated by SDS and the ssDNA. Smaller diameter nanotubes showed coating displacement at significantly lower ssDNA concentrations as compared to larger diameter nanotubes. Finally, this differential coating affinity result was applied in the development of a new structural sorting method that successfully separated smaller from larger diameter SWCNTs through simple steps including filtration. Last but not the least, we studied the steady-state emission spectra of stirred SWCNTs samples and built a model that interpreted the relationship between emission intensity and stirring rate. We found that stirring increases the emission intensity of SWCNT samples. This is attributed to a relatively slow photo-induced oxidation process on SWCNTs that partially quenched their emission. However, rapid stirring replaces photo-oxidized nanotubes in the probed volume with fresh nanotubes from the surrounding solution and restores the unquenched emission intensity.