  Nanopores spanning synthetic membranes have been used as key components in
proof-of-principle nanofluidic applications, particularly those involving
manipulation of biomolecules or sequencing of DNA. The only practical way of
manipulating charged macromolecules near nanopores is through a voltage
difference applied across the nanopore-spanning membrane. However, recent
experiments have shown that salt concentration gradients applied across
nanopores can also dramatically enhance charged particle capture from a low
concentration reservoir of charged molecules at one end of the nanopore. This
puzzling effect has hitherto eluded a physically consistent theoretical
explanation. Here, we propose an electrokinetic mechanism of this enhanced
capture that relies on the electrostatic potential near the pore mouth. For
long pores with diameter much greater than the local screening length, we
obtain accurate analytic expressions showing how salt gradients control the
local conductivity which can lead to increased local electrostatic potentials
and charged analyte capture rates. We also find that the attractive
electrostatic potential may be balanced by an outward, repulsive electroosmotic
flow (EOF) that can in certain cases conspire with the salt gradient to further
enhance the analyte capture rate.
