Biomedical Engineering Reference
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Moreover, this analysis suggested that the helix angle is the most inluential
property in IEC separation. SWCNTs can shield DNA backbone charges, which
may contract the helical pitch. The tubes' polarisability may further modulate
the helical pitch because of induced dipolar interactions between the DNA
backbone and the SWCNTs.
Alternatively to the partitiong approach on the basis of their diameters
and chirality, CNTs can also be separated through the size-exclusion
chromatography (SEC) process,
17
which helps purify DNA-wrapped carbon
nanotubes (DNA-CNTs) from graphitic impurities and sort them into fractions
of uniform length (Fig. 5.4). The advantage of this technique is the narrow
length variation of 10% in each of the measured fractions, in which length
decreases monotonically from 500 nm in the early fractions to 100 nm in the
late fractions.
Figure 5.4
AFM images of three representative SEC fractions deposited onto alkyl
silane-coated SiO
2
substrates. Reproduced from Huang
et al.
17
with permisssion.
In another study, photoluminescence (PL) spectra from HiPco nanotube
dispersed with salmon DNA (SaDNA) showed that the genomic sequence
mediates selective stabilisation of (6,5) SWCNTs, providing >86% of (6,5)
chirality enrichment without requiring additional separation through ion
exchange or density gradient columns.
18
More precisely, the supernatant
collected after ultracentrifugation of the SWCNT-SaDNA samples showed
that the relative emission intensity of semiconducting (6,5) SWCNTs was
significantly enhanced
(
Fig. 5.5a),
while the redispersed precipitate showed
peaks corresponding to the remaining ((7,6), (8,4) and (7,5)) chiral tubes
(Fig. 5.5b). Therefore, selective solubilisation of (6,5) tubes in aqueous media
was observed using SaDNA. Such evidence was further supported by radial
breathing mode (RBM) peaks from resonance Raman spectra (RRS) [19],
which were obtained from the 633 nm excitation laser line. It was observed
that the relative intensity of semiconducting SWCNT-SaDNA peaks was
increased, suggesting that SaDNA preferentially stabilised semiconducting
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