Molecular Detection and Force Spectroscopy in Solid-State Nanopores with Integrated Optical Tweezers - Nanopores: Sensing and Fundamental Biological Interactions - page 42

(largely a result of laser heating of the membrane and optics). The presented retraction
is performed at a speed of 20 nm/s; more than five orders of magnitude slower than a
common free translocation event (~8 nm/ms).
2.4 Force Spectroscopy
The optically trapped bead can be used as more than merely a handle, however. The
net forces acting on the captured molecule are transferred to the bead itself,
displacing it from its initial position in the potential well of the optical tweezer.
Therefore, changes in force result in relative bead displacements that are measur-
able through the PSD signal, allowing the system to perform one-dimensional force
curves on a single-molecule basis. A typical force curve for dsDNA is shown in
Fig. 2.7a (taken in 1 M KCl and at a starting bead-membrane distance of 4 mm),
demonstrating the linear relation between applied voltage and measured force.
Fig. 2.7 Nanopore force measurements on dsDNA. (a) A typical dsDNA force curve (nanopore
radius 12 nm, 1 M salt conditions) showing the linear dependence of measured force on applied
voltage ( gray line is a linear fit to the data). The observed relationship between the slope of the
force curve and both nanopore radius (b) and salt concentration (c). The solid gray line in each
represents the theoretical relation ( 2 ) and the dashed line represents the same relation calculated
with a reduction of dsDNA charge of 50%