Capture and Translocation of Nucleic Acids into Sub-5 nm Solid-State Nanopores - Nanopores: Sensing and Fundamental Biological Interactions - page 249

Luan and coworkers also found that although the counterion distribution around
DNA is independent of solution viscosity, the stall force itself is viscosity-dependent,
in apparent contradiction to classical Manning theory. It was noted that the redistri-
bution of ions around the negatively charged DNA surface results in an unbalanced
total electrolyte charge, which is driven by the electric field, generating an electro-
osmotic flow of water near the DNA. The direction of the additional drag on the DNA
due to this flow is opposite to that of the electrical field, thus reducing the effective
force sensed by the spring when the DNA is at rest [ 28 ].

10.3.4 Effect of Salt Gradients on the Transport Dynamics

After identifying the importance of counterion electroosmosis in DNA/small pore
systems, we expect that increasing the hydrodynamic profile of the counterions
around the DNA will reduce the effective force acting on it in the pore. In Fig. 10.18 ,
we show the normalized translocation times under asymmetric salt conditions for
two DNA lengths [ 50 ]. In these experiments, we have systematically reduced the
salt concentrations in the cis chamber and recorded DNA translocations at each
indicated C cis value. We then compare the mean transport time of the DNA under a
salt gradient to its mean transport time under symmetric 1 M/1 M salt conditions.
We have observed a systematic retardation for both DNA lengths (400 and
2,000 bp) when C cis is reduced: for the shorter DNA, we find that translocation is
slowed by a factor of ~50% when the salt concentration is reduced by a factor of 5,
whereas translocation is slowed by nearly fourfold for the longer DNA in the same
salt conditions. It is not surprising that the shorter DNA molecules slow down less
than the longer molecules; given the 1.40 power law that we find for DNA

< 3 kbp,

Fig. 10.18 Dependence of
applied salt gradient on DNA
transport for 400 and 2,000 bp
DNA molecules. The quantity
t T / t T (1 M) is the ratio of the
mean transport time at the
indicated salt gradient
( C trans 1 M) to the transport

time under symmetric 1 M/
1 M KCl conditions. DNA
slows down upon reduction of
salt concentrations,
suggesting counterion
electroosmosis further damps
the electrical force on the
DNA. Reproduced with
permission from Wanunu
et al. [ 50 ], Copyright Nature
Publishing Group

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