Anal. Chem. 2008, 80, 2514-2523
Size Separation of Single-Wall Carbon Nanotubes by Flow-Field Flow Fractionation Jaehun Chun, Jeffrey A. Fagan, Erik K. Hobbie, and Barry J. Bauer*
Polymers Division, National Institute of Standards and Technology Gaithersburg, Maryland 20899
Flow-field flow fractionation (flow-FFF) is used to separate single wall carbon nanotubes (SWNTs) dispersed in aqueous medium by the use of DNA. Online measurements are made of SWNT concentration, molar mass, and size by using UV-vis absorption and multiangle light scattering (MALS). Separations are made of both unfractionated SWNTs and SWNT fractions made by use of size exclusion chromatography (SEC). The SEC fractions are well resolved by flow-FFF. SWNT hydrodynamic volume from calibrations with polymer latex particles in flow-FFF are compared to calibrations of hydrodynamic volume from the SEC fractions derived from dissolved polymers. Rod lengths of the SWNTs are calculated from online measurements of MALS and those are compared to rod lengths from hydrodynamic models based on latex sphere calibrations. Samples with varied sizes were prepared by fracturing SWNTs through extended sonication. Flow-FFF of these fractured samples shows very broad size distributions compared to the original SEC and flow-FFF fractions. Single wall carbon nanotubes (SWNTs) are rodlike molecular objects whose length and diameter are about 100-1000 nm and 0.5-2 nm, respectively. They have the structure of a rolled graphene sheet, and direction and magnitude of a roll-up vector determine the chirality and diameter. Because of unique mechanical and electrical properties, SWNTs have been proposed for many potential applications such as high strength and conductive composites, energy storage, sensors, field emission displays, radiation sources, hydrogen storage media, and nanometer-sized semiconductor devices.1 SWNTs usually exhibit distributions of length, chirality, and diameter in the as-produced material. However, separation of SWNTs by those characteristics (i.e., length, chirality, and diameter) still remains a challenge and thus defers many practical applications. Separation of SWNTs by length is especially important not only because the length is associated with optical properties2 and quantum yields3 but also because it may be necessary as a preliminary step for separation by chirality.4,5 Separation of SWNTs by length can be achieved through several approaches, * To whom correspondence should be addressed. Phone: 301-975-6849. Fax: 301-975-4924. E-mail:
[email protected]. (1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (2) Fagan, J. A.; Simpson, J. R.; Bauer, B. J.; Lacerda, S. H.; Becker, M. L.; Chun, J.; Migler, K. B.; Hight Walker, A. R.; Hobbie, E. K. J. Am. Chem. Soc. 2007, 129, 10607. (3) Crochet, J.; Clemens, M.; Hertel, T. J. Am. Chem. Soc. 2007, 129, 8058.
2514 Analytical Chemistry, Vol. 80, No. 7, April 1, 2008
size exclusion chromatography (SEC), gel electrophoresis (GE), capillary electrophoresis (CE), or field flow fractionation (FFF). Recent studies have shown that SEC can produce reasonable resolution of SWNTs by length.5-7 However, the exclusion limit of the SEC column, controlled by the pore size, restricts the maximum length SWNTs (
