Nanopore-Based DNA Sequencing and DNA Motion Control - Nanopores: Sensing and Fundamental Biological Interactions - page 274

Fig. 11.23 Translocation
time is increased when the
temperature is decreased.
At low voltage (40 mV) and
low temperature (4 C) in
1.6 M KCl-TE buffer
containing 20% glycerol,
the translocation time is
about 800 m s. Image adapted
from [ 31 ]
11.2.2 Effective Electric Driving Force on DNA
in the Nanopore and Optical Tweezers
Approach for Motion Control
Optical tweezers use a focused laser beam to trap, as well as control the motion of a
micrometer-sized particle. After characterizing the stiffness k of the interaction
between a laser beam and the particle, the optical tweezers can be used to measure
forces from a few pico-Newtons to about 100 pico-Newtons. As a single molecule
technique, optical tweezers are widely used to manipulate biological molecules and
interrogate biological processes, such as stretching DNA [ 32 ] and probing the
stepping of a kinesin molecule on micortubules [ 33 ]. Recently, optical tweezers
have been used to measure forces on DNA in a solid-state nanopore [ 34 , 35 ].
To facilitate a DNA sequencing process, ideally, optical tweezers can drive DNA
through a solid-state nanopore at an arbitrarily slow speed [ 36 ].
In the experiment reported in [ 34 ], one end of a DNA molecule was attached
to a bead via a standard streptavidin-biotin bond while the other end of DNA
was electrically driven through a solid state nanopore. The effective electric driving
force F el that is balanced by the force from the optical tweezers can be computed
from the measured displacement z of a bead, i.e. F el ¼kz (see Fig. 11.24a ).
When increasing the biasing electric field across the solid membrane, the bead
was dragged towards the membrane until the effective electric driving force was
balanced by the restorative force from the optical tweezers. Figure 11.24b shows
that the effective electric driving force increases linearly with the voltage drop
across the membrane. Interestingly, the slope that characterizes the effective
charge q eff of DNA does not vary with the ion concentration in an electrolyte.
This experiment elegantly demonstrated the successful combination of two
nanotechnologies to obtain mechanical and electrical interactions between DNA
and a nanopore. The nature of q eff , however, was not determined, as it could result
from either counterion condensation or hydrodynamic friction or both.