Biomedical Engineering Reference
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SWCNTs in the aqueous supernatant, while leaving the metallic counterpart
in the precipitate.
Figure 5.5
Normalised PLE emission contour plot (a, b) from SaDNA dispersed SWCNTs
in the ultracentrifuged supernatant (a) and the redispersed-precipitate fractions
(b), respectively. Reproduced from Kim
et al
.
18
with permission. See also Colour
Insert.
Together with the separating properties, another important factor
to consider when dealing with CNT-DNA conjugates is the kinetics of
hybridisation once DNA is adsorbed onto the tubes. For that reason, Jeng
et
al.
reported a mechanistic model to monitor DNA hybridisation on nanotubes
using an energy increase in the NIR luorescence of the (6,5) nanotubes
[20]. Such ampliication is determined by the addition and hybridisation
of complementary DNA (cDNA) to a suspension of ssDNA previously
adsorbed to SWCNTs. It was observed that kinetics were slow when DNA was
adsorbed onto the tubes, while they were much faster when DNA was free in
solution, as described by a simple second-order rate expression. However,
hybridisation on SWCNTs could not be described using the same expression,
so an adsorption step (faster) had to be included before the hybridisation
reaction (slower). It was found that the activation energy of SWCNT-DNA
hybridisation was higher than that of free DNA, providing conirmation that
SWCNTs impeded the hybridisation reaction, while dynamic light scattering
(DLS) excluded that such phenomenon was simply due to sample aggregation.
Moreover, DNA adhered strongly to the nanotubes, which remained stable
over time, thus suggesting that the DNA molecules never left the surface
completely. As a consequence, it seems that one base at a time detached from
the SWCNT surface for hybridisation, after which it was re-adsorbed onto
the tubes. Subsequently, there was an energy decrease from adsorption of
two DNA sequences (the ssDNA and its complementary cDNA) to a nanotube,
between 3.6 and 7.1 kcal/mol, which is very close to the 7.5 kcal/mol in the
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