Biomedical Engineering Reference
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Fig. 10.8 The dependence of
the specific capture rate of
dsDNA on voltage. DNA
shorter than ~8,000 bp
displays an exponential
dependence on
V
and longer
DNA displays a linear
dependence on
V
. A linear-
linear plot of the data is
shown in the
inset
to better
illustrate these dependencies.
Reproduced with permission
from Wanunu et al. [
50
],
Copyright Nature Publishing
Group
longer DNA molecules we observe a weaker, linear dependence on voltage (III),
which is predicted for a diffusion-limited process (
10.2
).
The dependence of capture rate on
N
for short molecules in sub-5-nm pores
represents a unique example of capture selectivity towards longer DNA molecules.
Although DNA mobility in free-flow electrophoresis is not length-dependent, here
the rate-limiting step is
R
bar
. Thus, in this regime longer DNA molecules are more
efficiently threaded than short ones, as their higher charge is advantageous for
capture into the pore mouth. Moreover, the threading selectivity has a very steep
dependence on DNA size for molecules in the range 800-8,000 bp [
50
]. For longer
molecules, diffusion to the pore is the rate-limiting step, so the corresponding
translocation rate shows no length dependence. Both regimes are also apparent in
voltage studies, which show that
R
c
is exponentially dependent upon
V
for shorter
molecules, but linearly dependent for very long molecules (Fig.
10.8
).
10.2.3 Capture Enhancement Using Manipulated Fields
The strong impact of
VðrÞ
on the capture rate suggests that manipulation of the
voltage profile outside the pore is potentially useful for increasing throughput. One
effective way to alter the potential profile is the application of an ion gradient across
the pore. This results in selective “pumping” of the positive ions from the
trans
to the
cis
chamber, in the opposite translocation direction of the negatively charged DNA
[
50
]. As it turns out, this phenomenon has two positive outcomes for nanopore-based
platforms: First, the higher density of cations in the vicinity of the pore on the
cis
side
increases the attraction of DNA to the pore (it increases the magnitude of
VðrÞ
),
resulting in a significantly higher capture rate. Second, additional hydrodynamic
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