  Solid-state nanopores are promising tools for single molecule detection of
both DNA and proteins. In this study, we investigate the patterns of ionic
current blockades as DNA translocates into or out of the geometric confinement
of such conically shaped pores. We studied how the geometry of a nanopore
affects the detected ionic current signal of a translocating DNA molecule over
a wide range of salt concentration. The blockade level in the ionic current
depends on the translocation direction at a high salt concentration, and at
lower salt concentrations we find a non-intuitive ionic current decrease and
increase within each single event for the DNA translocations exiting from
confinement. We use recently developed DNA rulers with markers and finite
element calculations to explain our observations. Our calculations explain the
shapes of the signals observed at all salt concentrations and show that the
unexpected current decrease and increase are due to the competing effects of
ion concentration polarization and geometric exclusion of ions. Our analysis
shows that over a wide range of geometry, voltage and salt concentrations we
are able to understand the ionic current signals of DNA in asymmetric nanopores
enabling signal optimization in molecular sensing applications.
