OVERVIEW ON THE MAJOR RESEARCH ACTIVITIES ON CARBON NANOTUBES BEING DONE IN AMERICA, EUROPE AND ASIA - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 266

of SEC, 95 which, in combination with IEC, has enabled selective sorting of
SWCNTs with narrow diameter, length and chiral angle distributions. At the
moment, however, the success of chromatographic sorting is limited to small-
diameter (<1.2 nm) tubes.
Moreover, Hersam et al . have proposed another post-synthetic sorting
approach through ultracentrifugation. The eficacy of this technique suffers
from convolution among multiple structural parameters (e.g., diameter and
length), because it sorts SWCNTs according to their sedimentation coeficient,
which is inluenced by several parameters. Therefore, in order to be successful,
it has to be associated, for example, with surfactant encapsulation or chemical
derivatisation of SWCNTs. This has allowed the separation between metallic
and semiconducting SWCNTs, with 99% purity. 96
In particular, density gradient ultracentrifugation (DGU), combined with
surfactant-encapsulated SWCNTs, has demonstrated several advantages for
large-scale production due to its compatibility with several raw materials
and non-covalent and reversible functionalisations. In fact, co-surfactant
mixtures of sodium cholate (SC) and sodium dodecyl sulphate (SDS) have led
to diameter and electronic-type sorting by DGU. The mechanism at the basis
of this successful separation could be attributed to different binding of the
two surfactants as a function of the SWCNTs' polarisability, which is directly
correlated with the tubes' electronic properties.
In addition to their separating properties, sodium cholate encapsulated
SWCNTs have been investigated to determine both the linear density of
surfactant molecules along the length of the tubes and the molar volume of
sodium cholate on SWCNTs' surfaces. 97 The obtained results, of 3.6 ± 0.8 nm −1
and 270 ± 20 cm 3 ·mol −1 , respectively, represent a valuable contribution in
understanding the hydrodynamic properties of SWCNTs and the interactions
between SWCNTs and surfactants in aqueous solution.
In a very recent publication, 98 DGU showed the ability to separate double-
walled CNTs (DWCNTs), which was very useful to study the inter-wall
interactions in CNTs. Usually, it is very dificult to synthesise or separate this
type of tubes through conventional techniques, since mixtures with single-
and multi-walled tubes are generally produced. Typically, a DWCNT with
the same outer diameter of a SWCNT possesses similar SC encapsulation
properties to the SWCNT. However, the DWCNT is expected to present a
much higher density, given by the contribution of the additional DWCNT's
inner wall. Taking advantage of this difference, DGU was able to selectively
separate DWCNTs from a mixture of tubes. Four sets of bands located at
different densities were clearly observed in the centrifuge tube. The upper
two bands corresponded to small- and large-diameter SWCNTs, the third
band represented the DWCNTs and the inal thick black region was populated
by CNT bundles, multi-walled tubes (MWCNTs) and carbonaceous impurities
(as conirmed by optical absorbance measurements). DWCNTs were found to