Nanopores spanning synthetic membranes have been used as key components in proof-of-principlenanofluidic applications, particularly those involving manipulation of biomolecules or sequencing ofDNA. The only practical way of manipulating charged macromolecules near nanopores is througha voltage difference applied across the nanopore-spanning membrane. However, recent experimentshave shown that salt concentration gradients applied across nanopores can also dramaticallyenhance charged particle capture from a low concentration reservoir of charged molecules at oneend of the nanopore. This puzzling effect has hitherto eluded a physically consistent theoreticalexplanation. Here, we propose an electrokinetic mechanism of this enhanced capture that relies onthe electrostatic potential near the pore mouth. For long pores with diameter much greater than thelocal screening length, we obtain accurate analytic expressions showing how salt gradients controlthe local conductivity which can lead to increased local electrostatic potentials and charged analytecapture rates. We also find that the attractive electrostatic potential may be balanced by an outward,repulsive electro-osmotic flow that can in certain cases conspire with the salt gradient to furtherenhance the analyte capture rate.
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