We show that diffusion currents for a membrane containing a single conical nanopore with a fixed surface charge and small enough opening diameter depend on the concentration gradient direction. We interpret the results based on the effect of salt concentration on the thickness of the electrical double layer within the nanopore associated with the nanopore's surface charge and the distribution of electric fields inside the pore. The experimental observations are described by a diffusional model based on the Smoluchowski-Nernst-Planck equation.

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. Instr. Meth. B 184, 337 (2001); Z. Siwy, et al., Europhys. Lett. 60, 349 (2002)]. Here, the ion current rectification is studied within a reduced 1D Poisson-Nernst-Planck (PNP) model of synthetic nanopores. A conical channel of a few nm to a few hundred of nm in diameter, and of few µm 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 onedimensional reduction of the three-dimensional problem in terms of corresponding entropic effects is put forward. The ion transport is described by the non-equilibrium 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.

Ion transport through nanopores is modelled by the Smoluchowski equation. The model, based on properties of a bulk electrolyte solution and containing no adjustable parameters, is compared with ion current data recorded for single cylindrical nanopores in polymeric films. It predicts qualitatively a possibility of constructing an ion current rectifier based on the symmetric nanopore with asymmetric charge distribution. Our experimental data, however, indicate that the bulk-type diffusion approach does not describe the surface current, which gives a significant contribution to the total ion current through nanopores.

No abstract

The asymmetric diffusion through conical nanopores is described by the diffusional model. Diffusion is several times faster; when the concentration gradient points from the wide towards the narrow opening of the cone than in the opposite direction. The asymmetric diffusion appears either when the diffusion coefficient depends on the concentration or when the diffusing substances interact with the channel (i.e., ions moving through channels with charged walls). These results suggest that asymmetric nanopores can act as molecular (ionic) filters which could be used for retrieving the molecules of a given component from solutions in which its concentration fluctuates strongly, and only occasionally attains high values.

Ion transport in biological and synthetic nanochannels is characterized by such phenomena as ion current fluctuations, rectification, and pumping. Recently, it has been shown that the nanofabricated synthetic pores could be considered as analogous to biological channels with respect to their transport characteristics [P. Yu. Apel et al., Nucl. Instrum. Methods Phys. Res. B 184, 337 (2001); Z. Siwy et al., Europhys. Lett. 60, 349 (2002)]. The ion current rectification is analyzed. Ion transport through cylindrical nanopores is described by the Smoluchowski equation. The model is considering the symmetric nanopore with asymmetric charge distribution. In this model, the current rectification in asymmetrically charged nanochannels shows a diodelike shape of I-V characteristic. It is shown that this feature may be induced by the coupling between the degree of asymmetry and the depth of internal electric potential well. The role of concentration gradient is discussed.

We simulated the single-file motion of K+ ions through a model channel with the gate which opens and closes under influence of white noise and of interactions with ions present inside the channel. There is a range of the model parameters, in which the power spectrum of the ion net current through the channel has the characteristics of the flicker noise. The flicker noise is accompanied by the long-tail dwell-time distributions. The stochastic analysis of the calculated currents reveals their self-similarity. The open-state currents scale with the scaling exponent β=-1.0±0.15. To our best knowledge, our results are the first derivation of 1/f noise directly from Langevin equations with simple electrostatic interactions and white noise.

No abstract