Understanding the nature of the DNA-assisted separation of single-walled carbon nanotubes using fluorescence and Raman spectroscopy

Certain DNA sequences form a highly stable monolayer at the surface of single-walled carbon nanotubes, enabling their dispersion in aqueous solution. Ion-exchange chromatography can be used to separate these systems on the basis of their diameter in distinct, eluted fractions. Raman and fluorescence spectroscopy are used to characterize the nature of the separation by a comparison of radial breathing mode intensities at 457-, 532-, 633-, and 785-nm excitation wavelengths. Trends reveal a strong diameter dependence of the fractions, with larger diameters eluting later than smaller diameters. Average diameters of each fraction varied from 0.816 and 1.084 nm for the first and last eluted fractions, respectively. This conclusion is supported by a systematic shifting of the Raman tangential mode with increasing elution as well as changes in the relative intensities of radial breathing modes, Fluorescence emission from each fraction reveals the absence of relative shifting or broadening and hence uniformity in the chemical environment and the absence of doping-related complications for each fraction. The separation also reveals the fine structure of the G' two-phonon mode, which is described using a composite model with contributions from transition-dependent phonons.