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Abstract
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.