Spontaneous Debundling of Single-Walled Carbon Nanotubes in DNA-Based Dispersions

Abstract

Natural salmon testes DNA has been used to disperse single-walled carbon nanotubes (SWNTs) in water. It has been found that the primary factor controlling the nanotube bundle size distribution in the dispersion is the nanotube concentration. As measured by AFM, the mean bundle diameter tends to decrease with decreasing concentration. The number fraction of individual nanotubes increases with decreasing concentration. At low nanotube concentrations, number fractions of up to 83% individual SWNTs, equating to a mass fraction of 6.2%, have been obtained. Both the absolute number density and mass per volume of individual nanotubes initially increased with decreasing concentration, displaying a peak at ∼0.027 mg/mL. This concentration thus yields the largest quantities of individually dispersed SWNTs. The AFM data for populations of individual nanotubes was confirmed by infrared photoluminescence spectroscopy. The photoluminescence intensity increased with decreasing concentration, indicating extensive debundling. The concentration dependence of the luminescence intensity matched well to the AFM data on the number density of individual nanotubes. More importantly, it was found that, once initially dispersed, spontaneous debundling occurs upon dilution without the need for sonication. This implies that DNA−SWNT hybrids exist in water as a solution rather than a dispersion. The effects of dilution have been compared to the results obtained by ultracentrifuging the samples, showing dilution methods to be a viable and cost-effective alternative to ultracentrifugation. It was found that even after 4 h of ultracentrifugation at 122 000g, bundles with diameters of up to 4 nm remained in solution. The bundle diameter distribution after ultracentrifugation was very similar to the equilibrium distribution for the appropriate concentration after dilution, showing ultracentrifugation to be equivalent to dilution.