ABSTRACT
Chemically modified SWCNTs with distinct density, surface charge or dimension can be readily separated using a variety of techniques, namely centrifugation, electrophoresis, chromatography, extraction, and adsorption. Some of these techniques have been introduced in the earlier section. Here, we only highlight three most successful separation techniques. Although a recent review has discussed their latest developments [10], we emphasize their fundamental principles, advantages, and current technological limitations. Centrifugation, especially density gradient ultracentrifugation (DGU), separates SWCNTs via buoyant density. As exemplified in Fig. 4.3A, surfactants (SDS and sodium cholate) can disperse m/s-SWCNTs differently by non-covalent functionalizations
[93-95], leading to their density variation. A density gradient can be created in a centrifuge tube [96]. Surfactant-dispersed SWCNTs are inserted in the middle of the density gradient. Under a centrifugal field, SWCNTs move toward locations, where their densities match the local density of the gradient. The key advantages of DGU come from its tunability and scalability. DGU has a good sorting tunability because the density of SWCNTs can be simply changed using different covalent or non-covalent functionalization methods [43, 93, 94]. Moreover, the density gradient, centrifugal force and time can also be easily modified to yield different separation selectivities [97, 98]. The scalability and economic viability of DGU have already been reported in the literature. To date, DGU is the only separation technique that can deliver high-purity m/s-SWCNT commercial products (NanoIntegris). However, its technological limitations are also bounded by its scalability and economic viability. As there are only tiny density differences among dispersed SWCNT species, high centrifugal fields (about 200,000 g) are required to obtain good separation selectivities. It remains a challenge to realize large scale production at such a high centrifugal field [99]. Consequently, the cost of purified m/s-SWCNTs is still too high for widespread SWCNT applications.
