Enhancement of charged macromolecule capture by nanopores in a salt gradient

TL;DR

This paper explains how salt gradients across nanopores can significantly enhance the capture of charged macromolecules by analyzing electrokinetic effects and electrostatic potentials near the pore mouth.

Contribution

It introduces a physically consistent electrokinetic model that accounts for salt gradient effects on charged molecule capture in nanopores, supported by analytic expressions.

Findings

Salt gradients increase local electrostatic potentials near nanopores.

Electroosmotic flow can counteract electrostatic attraction, affecting capture rates.

Analytic expressions quantify how salt gradients control capture enhancement.

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.