ION PUMPING IN NANOCHANNELS USING AN
ASYMMETRIC ELECTRODE ARRAY
Wouter Sparreboom Cristian F. Cucu, Jan C.T. Eijkel and Albert van den Berg
MESA+ Institute for Nanotechnology, THE NETHERLANDS
ABSTRACT
We demonstrate an ion pump, consisting of a nanochannel with an AC driven asymmetric electrode array. Our system enables us to actively pump ions using a low driving voltage. In all experiments the electrical double layers are overlapping. Via viscous coupling ion pumping is accompanied by liquid pumping. Actuation below 500 mV at 10 Hz results in a liquid velocity of ~10 μm/s, corresponding to an electrical ion current of ~400 fA. Finite element simulations support the experimen-tal data.
KEYWORDS: Nanochannel, Nanofluidics, Asymmetric, AC Electrokinetics INTRODUCTION
Liquid pumping with asymmetric electrode arrays in microchannels is first de-scribed theoretically by ref. [1]. The first experiments were presented in ref. [2]. Ref. [3] mentions the possibility to scale down and shows it theoretically. However, literature up to now only describes the effects in microfluidic channels without dou-ble layer overlap. Whereas in microchannels transport is largely a viscous effect, (i.e. only a small part of the ions in the system are directly manipulated and the rest is dragged along) in systems with double layer overlap direct manipulation of ions becomes feasible [4]. This device can thus be regarded as an ion pump.
THEORY
Figure 1. Equations and boundary conditions used to solve the problem. The equa-tions defined in the channel and its boundary condiequa-tions represent the Poisson,
Nernst-Planck and Navier-Stokes equations, respectively. (+− −) = ∇ F c c ε ψ 2 u c c D c RT D F z J u c c D c RT D F z J − − − − − − − + + + + + + + − ∇ − ∇ − = − ∇ − ∇ − = ψ ψ t c J ∂ ∂ = ∇ ± (c c) p F u 2 + − ∇ =∇ ∇ + − ψ η Nanochannel Boundary conditions Walls V=-0.1 V Blocking current No slip Electrodes V=A*sin(2*pi*f*t) V Blocking current No slip Entrance/Exit V=0 V c±=100 μM P=1e5 Pa
Theoretically we describe the device behavior as a multiphysics time variant problem in COMSOL®. Here we used the Poisson and the Nernst-Planck equations
to solve the potential and ion distributions and coupled this to the Navier-Stokes equation for incompressible flow to describe liquid movement. Figure 1 gives the equations and boundary conditions used.
EXPERIMENTAL
We fabricated 50 nm high nanochannels with integrated asymmetric gold trodes by sacrificial layer etching of chromium [5]. The dimensions of the elec-trodes are derived from the optimal case given in [3]. Figure 2 shows a micrograph of the fabricated device. Fluidic connections to the nanochannels are made via a mi-crochannel network in PDMS. The chip is interfaced electrically and fluidically via a home-made chip holder. The driving signals are applied with a potentiostat, which also measures the actuation current. In our experiments we start with a 20 μM KNO3 solution and replace it by a 100 μM KNO3 solution by engaging the pump.
The change in measured current as a function of time is used to derive the flow ve-locity (figure 3). The ion current involved then follows from a theoretical approxi-mation.
RESULTS AND DISCUSSION
In figure 4 preliminary experimental results are given and compared to the finite element model. As shown experimental results are comparable to the theoretical predictions.
In the near future we plan to characterize the device more thoroughly by per-forming measurements and simulations over a wide range of electrode dimensions, actuation voltages and frequencies and different ionic solutions. The low driving voltage renders our system interesting for application in nanofluidic channels as a
Figure 2: Micrograph of the fab-ricated 50 nm high nanochannel and asymmetric electrode array.
Figure 3. Measured current during electrolyte replacement by
ion pumping. t1 and t2 corre-spond to the moments in time where the higher concentration electrolyte enters and completely
substitution for DC-EOF which in general requires high driving voltages. Further-more, the pumping velocity of ions in this system at a given driving frequency de-pends on their mobility, opening possibilities for ionic separation.
Figure 4. Experimental and simulated induced current and velocity. The error bar on the measured data represents the measured experimental variation. Channel
height is 50 nm, 20 to 100 μM KNO3, 400 mV @ 10 Hz.
CONCLUSIONS
The application of AC voltages to an asymmetric electrode array in a 50 nm high nanochannel results in both ion and liquid pumping. At an applied voltage with an amplitude of 400 mV and a 10 Hz frequency a 10 μm/s pumping velocity corre-sponding to a ~400 fA electrical ion current is found. Experimental results are sup-ported by finite element simulations.
ACKNOWLEDGEMENTS
Johan Bomer is gratefully acknowledged for technical assistance. For financial support we thank Nanoned.
REFERENCES
[1] Asymmetric Pumping of Particles, J. Prost, J. Chauwin, L. Peliti, A. Ajdari, Physical Review Letters, 72, 16, pp. 2652-2655 (1994).
[2] AC-Electric Field Induced Fluid Flow in Microelectrodes. A. Ramos, H. Mor-gan, N.G. Green, A. Castellanos, Journal of Colloid and Interface Science, 217, 2, pp. 420-422 (1999).
[3] AC electrokinetic micropumps: The effect of geometrical confinement, Faradaic current injection, and nonlinear surface capacitance, L.H. Olesen, H. Bruus, A. Ajdari, Physical Review E 73, 056313, (2006).
[4] Nanochannel fabrication for chemical sensors, M.B. Stern, M.W. Geis, J.E. Curtin, Journal of Vacuum Science and Technology B, 15, 6, pp. 2887-2891 (1997).
[5] Rapid sacrificial layer etching for the fabrication of nanochannels with inte-grated metal electrodes, W. Sparreboom, J.C.T. Eijkel, J. Bomer, A. van den Berg, Lab Chip, 8, pp. 402-407, (2008).
