Biomedical Engineering Reference
In-Depth Information
distinct, dark-coloured aggregates were determined by the sandwich
hybridisation of multi-DP conjugated CNT labels (DPs-CNT) and the multi-CP
bound beads (CPs-beads) in the presence of a complementary target (TG).
This provided a simple and low-cost nucleic acids detection technique in
view of early-stage disease and point-of-care diagnostic applications.
Another sandwich structure was proposed by Huang's research group
79
on the basis of the combination between the intrinsic nature of magnetic iron
oxide (Fe
3
O
4
) particles (MPs) and the dispersing properties of MWCNT-DNA,
with the inal purpose of monitoring the presence of complementary target
DNA. In this study, MPs were initially coupled with a 5
′
-NH
2
-modiied DNA
probe (P1), while MWCNTs were conjugated with a 3
′
-NH
2
-modiied DNA
probe (P2), thus generating a suspension of MPP1 and MWNTP2 probes.
In the absence of target DNA, the light-scattering (LS) signal was intense.
80
Conversely, if complementary DNA (T1) was added, sandwich hybridisation
was formed, with the hybrid of P1-P2 (in the form of a double strand)-T1
as illing material (similar to
Fig. 5.21b)
.
As a consequence, the LS signal
decreased remarkably. Several advantages are associated with this approach:
First of all, MP-P1 hybrids could be reused at least 17 times, indicating the
high stability and reproducibility of the system. Moreover, since CNT-DNA
complexes could disperse in an aqueous medium and have strong LS signals
in the UV-vis region, the proposed method did not involve a visual recognition
element such as luorescent/chemiluminescent labels. Therefore, it represents
an interesting option for the detection of target DNA sequences.
Besides the identiication of target molecules, Kaxiras
et al.
recently
proposed an ultrafast and effective DNA sequencing method. This was
applied to either a periodic dsDNA (with a nanotube itting the major groove
of the DNA)
81
or an ssDNA, with a resolution of 2 Å.
82
In the irst case, the
DNA sequence was held ixed, while the CNT, which was rather stiff and did
not deform signiicantly, was allowed to dock to the DNA at the major groove.
Conversely, in their second investigation, the authors applied a force (
F
) on
a bead attached to one end of ssDNA in close proximity to CNT (Fig. 5.22a).
Therefore, by pulling the ssDNA fragment, the bases along it successively
interacted with the CNT, providing a signal that could be measured by a probe
sensitive to local electronic states, such as a scanning tunneling spectroscopy
(STS) tip. The interactions between the nucleosides and the nanotube
are mainly van der Waals forces and mutual polarisation when each base
approaches the tube. To maximise the sensitivity of the measurements, a
semiconducting (10,0) CNT was used as the substrate, while only the most
stable conigurations of the bases (shown in
Fig. 5.22b
and corresponding
to 65% of the total) were taken into consideration. The nucleoside bound on
the CNT through its base unit was located 3.3 Å away from the CNT's wall,
while the sugar residue was more lexible. The results depicted with the
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