Higher-Order Assemblies of Flavin-Wrapped, Single-Walled Carbon Nanotubes

Individualization of carbon nanotubes is of paramount importance in being able to access their unique physical and optoelectronic properties[1]. While stable dispersions of individualized carbon nanotubes can be prepared through a number of different means[2], these dispersions lack the uniformity of nanotube diameters and chiralities required of many device-driven applications. Accordingly, one of the most pressing challenges facing the field of carbon nanotube research is the ongoing effort to isolate single chirality nanotubes. In 2008, Ju and Papadimitrakopoulos reported a facile method for the 85% enrichment of the (8,6) chirality nanotube based on the helical self-assembly of flavin mononucleotide (FMN) and its significantly higher affinity for the (8,6) nanotube[3]. In process of understanding the mechanism and favorability of (8,6) selection, it was discovered that FMN-nanotubes form novel, higher-order aggregates. Atomic force microscopy reveals that these higher-order structures are composed of individual, FMN-wrapped nanotubes twisted about one another in a periodic fashion. Spectroscopic binding studies suggest that these structures are in a dynamic equilibrium with the individual nanotubes they are comprised of. The direction of this equilibrium shifts depending on the how much FMN is present and the amount of time the system is allowed to incubate. We propose that the formation of these previously undiscovered structures may have had a favorable role in the reported enrichment of (8,6) nanotubes. Further exploration of these structures can help clarify the underlying mechanisms of chiral separation, while the structures themselves have the potential to be integrated into nano/biological devices as structural and sensor elements.