A Simulation Analysis of Nanofluidic Ion Current Rectification Using a Metal-Dielectric Janus Nanopore Driven by Induced-Charge Electrokinetic Phenomena.

Qisheng Wu,Chunlei Song,Hui Yan,Yukun Ren,Weiyu Liu,Yongjun Sun

doi:10.3390/mi11060542

Qisheng Wu, Chunlei Song + Show 4 more

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https://doi.org/10.3390/mi11060542

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Abstract

We propose herein a unique mechanism of generating tunable surface charges in a metal-dielectric Janus nanopore for the development of nanofluidic ion diode, wherein an uncharged metallic nanochannel is in serial connection with a dielectric nanopore of fixed surface charge. In response to an external electric field supplied by two probes located on both sides of the asymmetric Janus nanopore, the metallic portion of the nanochannel is electrochemically polarized, so that a critical junction is formed between regions with an enriched concentration of positive and negative ions in the bulk electrolyte adjacent to the conducting wall. The combined action of the field-induced bipolar induced double layer and the native unipolar double layer full of cations within the negatively-charged dielectric nanopore leads to a voltage-controllable heterogenous volumetric charge distribution. The electrochemical transport of field-induced counterions along the nanopore length direction creates an internal zone of ion enrichment/depletion, and thereby enhancement/suppression of the resulting electric current inside the Janus nanopore for reverse working status of the nanofluidic ion diode. A mathematical model based upon continuum mechanics is established to study the feasibility of the Janus nanochannel in causing sufficient ion current rectification, and we find that only a good matching between pore diameter and Debye length is able to result in a reliable rectifying functionality for practical applications. This rectification effect is reminiscent of the typical bipolar membrane, but much more flexible on account of the nature of a voltage-based control due to induced-charge electrokinetic polarization of the conducting end, which may hold promise for osmotic energy conversion wherein an electric current appears due to a difference in salt concentration. Our theoretical demonstration of a composite metal-dielectric ion-selective medium provides useful guidelines for construction of flexible on-chip platforms utilizing induced-charge electrokinetic phenomena for a high degree of freedom ion current control.

Highlights

With the rapid development of nanoscience and nanotechnology, interfacial mechanical phenomena have received unprecedentedly increasing attention from the microfluidic society, due to their favorable scaling with greater surface-to-volume ratios in small-scale systems [1,2]
We demonstrate by direct numerical simulation that electrochemical polarization of a conducting nanochannel located at the entrance of a metal-dielectric Janus nanopore permits voltage-biased control of local charge concentrations for the development of a nanofluidic ion diode
First and foremost, since the nanofluidic ion diode is operated on the basis of induced-charge electrokinetics (ICEK) behavior of a metal-dielectric Janus nanopore, we should pay attention to the importance of the length proportion of the metallic wall compared to the overall axial extension of the asymmetric ion-selective medium

Summary

Introduction

With the rapid development of nanoscience and nanotechnology, interfacial mechanical phenomena have received unprecedentedly increasing attention from the microfluidic society, due to their favorable scaling with greater surface-to-volume ratios in small-scale systems [1,2]. In the presence of an appreciable extension of the EDL, the surface charge density natively adsorbed on channel walls results in a high degree of symmetry breaking in the ion contents, with a global concentration difference between cations and anions. In this way, only counterions are allowed to pass through nanoscale channels, while the axial motion of coions is forbidden. Because of the ion-selective delivery relative to the channel wall charge of opposite sign, electric current flux along the length direction is highly controllable by surface conduction within nanofluidic channels, while it is almost impossible to achieve this in microscale ducts where the electrophoretic transport of ionic species is dictated by bulk Ohmic conductance [14,15,16]

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