Molecular Detection and Force Spectroscopy in Solid-State Nanopores with Integrated Optical Tweezers - Nanopores: Sensing and Fundamental Biological Interactions - page 40

Fig. 2.4 Typical dsDNA captures. (a) Simultaneous measurement of nanopore current ( top ) and
PSD voltage on the position-sensitive detector (which measures the bead position, bottom ) under
varying applied voltage to the trans chamber. See text for descriptions. (b) Close-up of the dsDNA
capture event circled in (a). The dashed lines mark the difference in time between the conductance
drop ( top ) and the change in bead position ( bottom )
agreement with the (unfolded) event depth measured in translocation experiments
for the same molecule [ 15 ] (Fig. 2.5a ) and is thus similarly indicative of the size
sensitivity of nanopore measurements. Second, although the molecule is statically
captured inside the nanopore so the measured conductance does not return to a
baseline value as in translocation, temporal information can also be garnered from
these optical tweezer measurements. When a molecule is captured in the nanopore,
its presence immediately affects the trans-membrane conductance, but there is a
delay in the bead position change, because the entire molecule must be threaded
through the pore before it is pulled taut and the force is transmitted to the trapped
bead. This delay time (Fig. 2.4b , red lines) is a measure of the molecular contour
length in the same way that the dwell time is in free translocation [ 5 ]. In fact,
measurements of the delay consistently match the dwell times for the same length
of molecule quantitatively (Fig. 2.5b ).
Once a single dsDNA is statically held inside the nanopore, the piezo stage can be
used to change the bead-membrane separation and control the position of the molecule
relative to the nanopore. An example of this is shown in Fig. 2.6 , where the bead is
slowly moved away from the membrane surface to a distance greater than the contour
length of the molecule (16.5 mm). Here, the measured pore conductance changes
suddenly back to the original open-pore level, indicating that the opening is no longer
occluded and that the dsDNA has been removed. This occurs at a total distance of
15 mm from the membrane surface, in reasonable agreement with the contour length.
The slight difference is due to settling in the system over the long measurement time