Rectification in synthetic conical nanopores: A one-dimensional Poisson-Nernst-Planck model

Ion transport in biological and synthetic nanochannels is characterized by phenomena such as ion current fluctuations and rectification. Recently, it has been demonstrated that nanofabricated synthetic pores can mimic transport properties of biological ion channels [ P. Yu. Apel et al., Nucl. Instrum Methods Phys. Res. B 184, 337 (2001 ); Z. Siwy et al., Europhys. Lett. 60, 349 (2002 ) ]. Here, the ion current rectification is studied within a reduced one- dimensional (1D ) Poisson- Nernst- Planck (PNP ) model of synthetic nanopores. A conical channel of a few nm to a few hundred nm in diameter, and of a few mu m long is considered in the limit where the channel length considerably exceeds the Debye screening length. The rigid channel wall is assumed to be weakly charged. A one- dimensional reduction of the three- dimensional problem in terms of corresponding entropic effects is put forward. The ion transport is described by the nonequilibrium steady- state solution of the 1D Poisson- Nernst- Planck system within a singular perturbation treatment. An analytic formula for the approximate rectification current in the lowest order perturbation theory is derived. A detailed comparison between numerical results and the singular perturbation theory is presented. The crucial importance of the asymmetry in the potential jumps at the pore ends on the rectification effect is demonstrated. This so constructed 1D theory is shown to describe well the experimental data in the regime of small- to- moderate electric currents.