Biomedical Engineering Reference
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due to its unique electronic and optical properties. In addition, SWNT provides some
unique properties such as structural chirality that determines it to be an electron
conductor or a semiconductor. We could therefore expect DNA-functionalized
SWNTs to be a new family of material building blocks for DNA-programmable
self-assembly, which are distinctively different from previously investigated metal
and quantum dot nanoparticles.
Researchers have used covalent bonding to make DNA-functionalized SWNTs
[
50
,
51
]. In a typical process, chemical oxidation assisted by ultra-sonication was
employed to break SWNTs and introduce carboxylic groups, based on which amide
bond could be formed between amino-terminated DNA oligonucleotides and the
carboxylated SWNTs via a chemical condensation reaction. As oxidation tends to
happen at the two termini of a chemically cut open-ended SWNT, where more
structural defects exist (in another word, the chemical oxidant finds an initial
defect on an SWNT and then cut the SWNT from that point by forming various
oxygen-containing groups including carboxyls) [
52
,
53
], it provides a chance to
achieve end functionalization of SWNTs. However, this method is not suitable
to realize a high-density sidewall decoration of DNA on the SWNTs, which
is necessary for DNA-directed surface alignment of the nanotubes in specified
orientations. On the other hand, chemical treatments require very harsh oxidative
and acidic conditions that would cause unwanted alterations to the nanotube's
electronic band structure. Also, the formation of a covalent bond between the
nanotube and a DNA strand is relatively complicated and thus less efficient.
More importantly, DNA hybridization may be hindered due to strong
staking
interactions between DNA's nucleobases and SWNT. These challenges thus call
for a new strategy to prepare DNA-functionalized SWNT, which should be able to
produce highly dispersible SWNTs with a high-density DNA decoration via a much
simpler process, with minimized DNA adsorption on the SWNT.
Zheng et al. achieved highly water-dispersible DNA-wrapped SWNTs by non-
covalent
-
stacking interactions, based on which length and chirality separations
of SWNTs were realized by size-exclusion or anion-exchange chromatography as
well as gel electrophoresis [
54
-
56
]. The findings by Zheng et al. provided a novel
way to interface DNA with SWNTs and gave us a freedom to develop a new DNA-
functionalization technique for SWNTs [
57
]. One challenging task was to reduce the
surface adhesion of a grafted DNA sequence on the nanotubes in order to maintain
its hybridization activity. A tail strategy was developed by us to address this problem
(Fig.
8.9
). In a typical process, an excess amount of a DNA-hybridization strand was
added to a solution containing the DNA-wrapped SWNTs, such that one segment of
the added DNA could base pair with a corresponding domain in the disperser strand
wrapping on the SWNTs. A prolonged incubation between the hybridization and
the disperser strands ensured an efficient hybridization, resulting in a double helix
insert flanked by a relatively free single-stranded tail as one part of the hybridization
strand and the SWNT binding domain of the disperser DNA (see Fig.
8.9
).
We hypothesized that the double helix insert sitting between the hybridization
and the binding domains of the DNA complex would play a crucial role in reducing
the adsorption of the DNA tail on the SWNT, benefiting from its relatively high
-
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