Single-Walled Carbon Nanotubes: Separation Using Capillary Electrophoresis
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Fulfilling the promise that carbon nanotubes hold for new lightweight, high-performance materials and nanoscale electronics and sensors, among other areas, will require production of bulk quantities of pure material. Much progress has been made in production, including electric-arc, laser ablation, and catalytic gas-phase growth methods, which are beginning to prove themselves for large-scale production of raw nanotube material. Initial purification work has focused on breaking up the carbon mat resulting from production and elimination of amorphous carbon and catalyst material from the end product.
With the increasing availability of relatively pure product, it is becoming feasible to pursue geometry selective separations of carbon nanotubes. Structure-based purification for isolation of specific properties will be an important tool for advancing nanotube science. For example, differences in the geometrical parameters (n, m) will determine whether the nanotubes are conducting or semiconducting. Changes in (n, m) also result in differences in diameter. A (10, 6) tube will be semiconducting and have diameter of 1.11 nm, while a (10, 10) tube is conducting, with diameter of 1.32 nm. Thus the ability to separate tubes based on geometry will also ultimately provide a degree of selectivity over electronic properties. Field-flow fractionation (FFF) and size-exclusion chromatography have demonstrated some success at length-based separations, but result in fractions with broad length distributions. However, recent work using flow field-flow fractionation demonstrates progress in producing narrower distributions. Electrophoretic separations may also be promising. Bulk electrophoresis has been used for alignment of nanotubes and is capable of separating nanotubes from particulate impurities. Gel electrophoresis has produced fractions with similar length distributions as found for FFF separations.
Capillary electrophoresis (CE) has only been recently investigated for purification of carbon nanotubes. The high-resolution separations available with this method are intrinsically dependent on solute geometry and may prove valuable in the separation of nanotubes by geometric specification. The first reported use on electric arc produced nanotubes demonstrated its ability to separate nanotubes according to length. More recent work has shown that this approach may be capable of producing diameter-specific separations as well. These recent studies are described in this article.