Solid-State Nanopores: Methods of Fabrication and Integration, and Feasibility Issues in DNA Sequencing - Nanopores: Sensing and Fundamental Biological Interactions - page 193

Fig. 8.11 The concept of
reverse translocation [ 27 ].
Used with permission.
Copyright Nanotechnology
2009
By ramping the magnetic field slowly, the DNAs in all the nanopores can be pulled
out slowly during one ramping step.
In the experiment of Peng and Ling [ 27 ], as shown in Fig. 8.12 , the ionic current
signals were used to detect DNA being captured into or being pulled out of the
nanopore. To make sure that the ionic current data is not due to bead blocking the
pore or leaving the pore, 0.1 M KCl is used instead of the more standard 1.0 M KCl
buffer. It is known (Chang et al. [ 29 ] and Smeets et al. [ 30 ]) that the presence of a
DNA in a nanopore has two competing effects for the nanopore conductance: the
physical volume of the DNA leads to a reduction in total ion population in the
nanopore, thereby reducing nanopore conductance; the negatively charged DNA
brings in extra counterions, leading to a conductance enhancement. The net effect
of a translocating DNA on the nanopore conductance depends on the ionic strength
of the buffer solution. At 1.0 M KCl, the DNA entry is signaled by a drop in ionic
current, while at 0.1 M KCl the ionic current signal due to a DNA entering the pore
is an increase [ 29 , 30 ]. At 0.1 M KCl, DNA (attached to a bead) captured into the
nanopore and then pulled out of the nanopore by the bead will be indicated by
a current increase and then decrease. This “Chang-Bashir effect” [ 29 ] provides a
convenient way to distinguish a true DNA capture signal from the bead blocking the
nanopore.