  The translocation of double-stranded DNA through a solid-state nanopore may
either decrease or increase the ionic current depending on the ionic
concentration of the surrounding solution. Below a certain crossover ionic
concentration, the current change inverts from a current blockade to current
enhancement. In this paper, we show that the crossover concentration for
bundled DNA nanostructures composed of multiple connected DNA double-helices is
lower than that of double-stranded DNA. Our measurements suggest that
counterion mobility in the vicinity of DNA is reduced depending on the
three-dimensional structure of the molecule. We further demonstrate that
introducing neutral polymers such as polyethylene glycol into the measurement
solution reduces electroosmotic outflow from the nanopore, allowing
translocation of large DNA structures at low salt concentrations. Our
experiments contribute to an improved understanding of ion transport in
confined DNA environments, which is critical for the development of nanopore
sensing techniques as well as synthetic membrane channels. Our salt-dependent
measurements of model DNA nanostructures will guide the development of
computational models of DNA translocation through nanopores.
