Biomedical Engineering Reference
In-Depth Information
In another experiment, the same authors demonstrated how to separate
CNTs on the basis of the tubes' characteristics and diameter, by simply adsorbing
DNA onto their external sidewall. 14 Anion exchange chromatography showed
that the best separation of the tubes was obtained with sequences rich in two
alternating nucleotides, deoxyguanylate (dG) and deoxythymidylate (dT),
expressed as d(GT) n (with n = 10 to 45). Such sequences were responsible for
the formation of self-assembled helical wires around individual nanotubes
involving hydrogen-bonding interactions among different strands (Fig. 5.2a).
More precisely, two complementary (antiparallel) d(GT) n strands tended to
form a double-stranded strip, which subsequently surrounded CNTs in a
densely packed wrapping around their sidewalls. The mechanism that enabled
the separation of the tubes was attributed to electrostatic interactions among
the CNT-DNA hybrids. In fact CNT-DNA complexes carried a net negative
charge on the basis of the deprotonated backbone phosphate groups on the
DNA (Fig. 5.2b); the negative charges interacted with the positive charges
present in the ion exchange resin, while the eluting salt solution depended on
the effective linear charge density.
R
R
a)
R
3 '
R
5'
H
N
H
N
N
O
N
O
N
N
N
N
H
N
H
N
N
N
H
H
HN
HN
O
O
N
N
O
O
O
O
O
O
H
H
H
H
N
N
N
N
N
N
N
N
H
H
O
N
N
N
O
N
5 '
R
R
R
R
3'
b)
d(GT)n:d(GT)n charge strip
Salt elution
H 2 O
Figure 5.2 (a) Proposed hydrogen-bonding interactions between two d(GT) n strands
that lead to the formation of a “d(GT) n :d(GT) n charge strip”. (b) Schematic for anion
exchange separation process. At lower salt concentration, the surface-bound state is
favoured in which positive ions on the resin attract the negative surface charge on the
DNA-CNT. With increasing salt concentration, the surface and DNA-CNT interactions
are screened, favouring elution. Image modiied from Zheng et al . 14
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