Abstract
We introduce herein the induced-charge electrokinetic phenomenon to nanometer fluidic systems; the design of the nanofluidic ion diode for field-effect ionic current control of the nanometer dimension is developed by enhancing internal ion concentration polarization through electrochemical transport of inhomogeneous inducing-counterions resulting from double gate terminals mounted on top of a thin dielectric layer, which covers the nanochannel connected to microfluidic reservoirs on both sides. A mathematical model based on the fully-coupled Poisson-Nernst-Plank-Navier-Stokes equations is developed to study the feasibility of this structural configuration causing effective ionic current rectification. The effect of various physiochemical and geometrical parameters, such as the native surface charge density on the nanochannel sidewalls, the number of gate electrodes (GE), the gate voltage magnitude, and the solution conductivity, permittivity, and thickness of the dielectric coating, as well as the size and position of the GE pair of opposite gate polarity, on the resulted rectification performance of the presented nanoscale ionic device is numerically analyzed by using a commercial software package, COMSOL Multiphysics (version 5.2). Three types of electrohydrodynamic flow, including electroosmosis of 1st kind, induced-charge electroosmosis, and electroosmosis of 2nd kind that were originated by the Coulomb force within three distinct charge layers coexist in the micro/nanofluidic hybrid network and are shown to simultaneously influence the output current flux in a complex manner. The rectification factor of a contrast between the ‘on’ and ‘off’ working states can even exceed one thousand-fold in the case of choosing a suitable combination of several key parameters. Our demonstration of field-effect-tunable nanofluidic ion diodes of double external gate electrodes proves invaluable for the construction of a flexible electrokinetic platform for ionic current control and may help transform the field of smart, on-chip, integrated circuits.
Highlights
With the incessant development of nanoscience, interfacial force effects have received increasing attention from the micro/nanofluidic community, due to their benign scaling with larger surface-to-volume ratios in miniaturization systems
Nanofluidic ducts with characteristic dimensions on the level of double-layer thickness are capable of possessing unique ion motion characteristics; with an evident electrical double layer (EDL) extension, the surface charge density naturally developed on the channel sidewalls, resulting in the ionic contents being in a high degree of asymmetry, with a global difference in ion concentrations between cations and anions [17,18,19]
To make the device smaller for portable applications, we present a simple but useful way to set up the field-effect-tunable nanofluidic ion diode by adjusting the internal rather than the external ion concentration distribution of a central nanochannel connected to two reservoirs on both sides, under the help of bipolar induced-charge electrokinetics [37,38,39,40,41,42,43] from the double external gate terminals of the opposite gate polarities (Figure 1)
Summary
With the incessant development of nanoscience, interfacial force effects have received increasing attention from the micro/nanofluidic community, due to their benign scaling with larger surface-to-volume ratios in miniaturization systems. Diffuse-charge dynamics next to a Debye screening cloud constitute a significant polarization phenomenon at the sharp material interface between a solid surface and the adjacent saline solution: the native free surface charge chemically adsorbed on the channel walls is always compensated for by a thin electrical double layer (EDL) of mobile counterionic charges in the liquid phase from which coions are forced to leave [1,2,3,4]. Nanofluidic ducts with characteristic dimensions on the level of double-layer thickness are capable of possessing unique ion motion characteristics; with an evident EDL extension, the surface charge density naturally developed on the channel sidewalls, resulting in the ionic contents being in a high degree of asymmetry, with a global difference in ion concentrations between cations and anions [17,18,19]. In the presence of ion-selective motion with respect to the wall charge of opposite polarity, ionic current flux can be controlled within surface-conduction-dominated nanofluidic channels, while it is not possible to realize this in microchannels governed by bulk conductance [29,30,31]
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