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 265

These goals can be achieved, provided SWCNTs undergo strategic
functionalisations, and they are processed through several techniques,
including ultracentrifugation, electrophoretic separation and/or
chromatographic methods. In particular, covalent sidewall chemistries have
been shown to afford the best selectivity: for example, the functionalisation
with p -hydroxybenzene diazonium salt induces a negative charge on metallic
SWCNTs through deprotonation in alkaline solutions, thus enabling tube
separation by electrophoresis. 84 Alternatively, diazonium compounds
with a long alkyl tail render the metallic SWCNTs selectively soluble in
tetrahydrofuran (THF). 85 Otherwise, dichlorocarbene converts metallic
tubes into semiconducting SWCNTs, while nitronium ions selectively attack
small-diameter metallic SWCNTs, reducing them to amorphous carbon. 86
In addition, separation of semiconducting from metallic SWCNTs has been
achieved by cycloaddition of azomethine ylides. 87 However, since covalent
functionalisation affects nanotube integrity remarkably, non-covalent
strategies (e.g., using surfactants, 88 wrapped polymers, 89 solvents 90 and
porphyrins 91 ) have been pursued, though the vast majority of work on
selective non-covalent functionalisation has been focused on relatively small-
diameter SWCNTs (diameter < 1.2 nm).
Among the techniques of CNT separation, conventional electrophoresis
sorts SWCNTs in response to a direct current (DC) electric ield. The tubes with
the smallest molecular weight travel most quickly, thus leading to sorting on
the basis of SWCNTs' length and diameter. Moreover, capillary electrophoresis
can also provide separation of individual SWCNTs from bundles. 92 It has
been demonstrated that alternating current (AC) dielectrophoresis can sort
SWCNTs on the basis of their dielectric constants, which are considerably
different between metallic and semiconducting SWCNTs. In fact, the more
polarisable species (i.e., metallic tubes) are selectively deposited onto the
substrate between the microelectrodes, while the electric ield also induces
SWCNT alignment. A big disadvantage of this technique is the limited
throughput, for which as low as 100 pg of metallic SWCNTs could be isolated. 92
Therefore, recent work has attempted to scale up dielectrophoresis using, for
example, dielectrophoretic ield-low fractionation (FFF). 93
Chromatographic techniques (e.g., IEC) have also provided successful
separations of SWCNTs, especially when the tubes are pre-encapsulated
with single-stranded DNA in aqueous solution. Separation by electronic type
and diameter can be followed through optical absorbance measurements,
which demonstrated that the sorting quality depends on DNA sequence,
with a sequence of (GT) n (where n is between 10 and 45) leading to the best
results. 94 Even better separations have been achieved with the association