Polarization of Gold in Nanopores Leads to Ion Current Rectification

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Letter

Polarization of Gold in Nanopores Leads to Ion Current Rectification Crystal Yang, Preston Hinkle, Justin Menestrina, Ivan V. Vlassiouk, and Zuzanna S. Siwy J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01971 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 3, 2016

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The Journal of Physical Chemistry Letters

Polarization of Gold in Nanopores Leads to Ion Current Rectification Crystal

Yang,#1

Preston

Hinkle,#2

Justin

Menestrina,2

Ivan

V.

Vlassiouk,3

Zuzanna S. Siwy1,2,4* 1

Department of Chemistry, University of California, Irvine, CA 92697

2

Department of Physics and Astronomy, University of California, Irvine, CA 92697

3

Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, TN, 37831, United States

4

Department of Bioengineering, University of California, Irvine, CA 92697, United States

Abstract Biomimetic nanopores with rectifying properties are relevant components of ionic switches, ionic circuits, and biological sensors. Rectification indicates that currents for voltages of one polarity are higher than currents for voltages of the opposite polarity. Ion current rectification requires the presence of surface charges on the pore walls, achieved either by the attachment of charged groups, or in multi-electrode systems by applying voltage to integrated gate electrodes. Here, we present a simpler concept for introducing surface charges via polarization of a thin layer of Au present at one entrance of a silicon nitride nanopore. In an electric field applied by two electrodes placed in bulk solution on both sides of the membrane, the Au layer polarizes such that excess positive charge locally concentrates at one end and negative charge at the other end. Consequently, a junction is formed between zones with enhanced anion and cation concentrations in the solution adjacent to the Au layer. This bipolar double-layer together with enhanced cation concentration in a negatively charged silicon nitride nanopore leads to voltage-controlled surface-charge patterns and ion current rectification. The experimental findings are supported by numerical modeling that confirm modulation of ionic concentrations by the Au layer and ion current rectification even in low-aspect ratio nanopores. Our findings enable a new strategy for creating ionic circuits with diodes and transistors.

#

*

These authors contributed equally; Corresponding Author: [email protected], Tel. 949-824-8290

ACS Paragon Plus Environment


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Membrane bound biological channels and pores are the basis for intercellular signaling and are therefore fundamental to the existence of multicellular organisms. Ion channels regulate transport of ions in and out of the cells, and are often responsive to external stimuli such as voltage, binding of a chemical and mechanical stress.1,2 Rectifying systems create an important sub-class of channels, which facilitate transport of ions in one direction, crucial e.g. in nerve signaling in an axon. Ion current rectification is observed as an asymmetric current-voltage (I-V) curve so that currents of voltages of one polarity are higher than currents for voltages of the opposite polarity. 3-11 Mimicking ion current rectification in synthetic systems has attracted the interest of researchers from various fields because rectifying nanopores can be used as ionic switches, components of ionic circuits,

17,18

12-16

and the basis of chemical sensors.19,20 The highest

degrees of rectification are achieved by nanopores that contain a junction between two zones with different surface charge properties.12-14 These systems, called ionic diodes, are typically characterized by static surface charges determined by attached chemical groups and their ionization level. Introducing dynamic control over the surface potential of nanofluidic diodes was reported as well, but required building multi-terminal systems with gate electrodes.21-24 In this manuscript we report rectifying nanopores that operate in a simple two-electrode set-up, but feature surface charge patterns that are induced by an external electric field via polarization of gold (Au).25-27 The magnitude of the induced surface charges is proportional to the transmembrane potential, and when combined with a charged nanopore leads to ion current rectification. The mechanism of ion current rectification proposed here is especially applicable to low-aspect ratio nanopores, which were predicted not to rectify according to electrostatic mechanisms developed for long nanopores.28 Short pores are important for creating biomimetic systems and preparation of structures with both diameter and length at the nanoscale. When an external potential difference is applied across the pore, free charges in the metallic Au layer redistribute accordingly to negate the local external electric field. The resulting charge distribution has a bipolar charge pattern, with one end containing excess positive charge and the other end containing excess negative charge.25-27 When



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placed in an electrolyte solution, the polarized surface modifies local ionic concentrations,

forming

separate

regions

with

enhanced

anion

and

cation

concentrations. The capacity for metals to induce bipolar electrical double layers was confirmed experimentally by observations of vortical electroosmotic fluid flow at the surface of a floating electrode in an external electric field.25,26 Polarization of Au is also used in electrochemistry as a bipolar electrode applied to concentrate charged molecules and perform redox reactions.27,29 In this manuscript we demonstrate that polarization of an Au film placed at the entrance of a nanopore allows for voltage control of local ionic concentrations and the formation of an ionic diode. Our experimental findings are supported by numerical modeling of ionic current performed with coupled Poisson-Nernst-Planck and Navier-Stokes equations.

Figure 1a shows a scheme of the prepared nanoporous device based on a 50 nm thick film of silicon nitride (SiN) with a layer of metal consisting of 3 nm of evaporated chromium (Cr) together with 12 nm or 27 nm thick layer of Au. Single nanopores were drilled through the SiN-Cr-Au film using the electron beam in a transmission electron microscope (TEM).30 TEM drilled nanopores are known to be hour-glass shaped31 with a cone half-opening angle of ~5 degrees. All pores examined in this study had opening diameter, as imaged on the membrane surface, between 9 and 16 nm (Figure S1). Transport properties of the nanopores were first characterized by measuring ion current in 500 mM and 100 mM potassium fluoride (KF). The fluoride salt was chosen as the supporting electrolyte, rather than KCl, because unlike Cl- , F- does not adsorb to Au, which allows us to consider the effects of the induced charge only.32 Thus, in KF the Au layer can be considered effectively uncharged if no voltage is applied across the membrane. In all recordings, the working electrode was placed on the same side of the membrane as the Au layer. In order to increase wettability of the membranes, measurements of ion current were performed from solutions prepared in 50% water and 50% ethanol. Ion current signals were recorded as time-series using Axopatch 200B, Digidata 1322A (Molecular Devices, Inc) to ensure that the signals were stable in time


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and not significantly fluctuating. Current-voltage curves were measured by applying a voltage for 20 seconds and averaging the last 0.5 seconds of each recording. Example recordings for 10 - 15 nm in diameter single nanopores are shown in Figures 1b,c, S2, S3. The I-V curves produced from the current time-series indicate the system rectifies the current. Ionic currents were larger for negative voltages, which correspond to potassium ion migration from the free side of the SiN to the Au-covered side. The ion current rectification is more pronounced in lower salt concentrations (Figure S2). We concluded the rectification observed was induced by the presence of the Au layer, because SiN nanopores without Au exhibited a nearly symmetric behavior (Figure S4).

(a)

(b)

(c)



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Figure 1. (a) Scheme of a single nanopore in a 50 nm thick SiN film containing an Au layer on one side; the polarity of the transmembrane potential used in all measurements is indicated. (b) Current-voltage curve created by time-averaging the ion current signals shown in (c) for a single 12 nm in diameter SiN pore with 30 nm Cr,Au layer. The current signals in (c) are 20 s long and were recorded for a voltage range between -2V and +2V with 100 mV steps.

The direction of rectification observed here is reversed compared to what was reported before for Au coated nanopores in polymer and solid-state films.33-35 The previous nanopore systems were studied in KCl and rectified such that higher currents were observed when potassium ions moved from the side with Au towards the membrane surface without metal. In KCl however, as reported before, the Au surface is negatively charged due to chloride adsorption,32,36 so that a nanopore/Au structure can be considered equivalent to conically shaped ionic rectifiers and unipolar diodes reported before.3-11

In solution containing only F- as anions, the Au layer is expected not to have any adsorbed ions and the effective charge is solely due to the metal polarization in the transmembrane potential. It is known that the density of induced charges is directly proportional to the magnitude of the axial component of the electric field.25,26 We will first analyze the case when a negatively biased electrode is placed on the side of the membrane with deposited Au. For this polarity of the externally applied transmembrane potential, Au polarization makes the pore entrance effectively positively charged leading to a negatively charged screening layer in solution with enhanced concentration of fluoride ions (Figure 2). 25-27,29 Inside the pore, it is the concentration of potassium ions that is enhanced due to the negative surface charge of the SiN nanopore37 as well as the induced negative charge on the Au layer. Since both fluoride and potassium ions are readily sourced to pass through the pore, this direction of electric field creates a


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diode-like pattern of surface charges with an ‘on’ state of the device.12 For the opposite voltage polarity i.e. when a positively biased electrode is placed on the Au side, the polarized Au and negative charge on the SiN pore create a surface charge pattern that is reminiscent of the bipolar junction transistor reported before (Figure 2, panel on the right).38,39 The junction created between the deposited Au and negative surface charges on the SiN pore wall is reversed biased, and becomes responsible for the ‘off’ state of the device. It is important to note that the presence of the charged nanopore is necessary for the rectification to occur. The voltage-independent surface charges of a SiN nanopore integrated with the Au layer allow symmetry breaking of the electrochemical potential of the system. As a result, ionic concentrations are enhanced for one voltage polarity, and a depletion zone is formed for the opposite polarity.

Figure 2. Scheme of electric-field induced surface charges and distribution of ionic concentrations in a nanopore with an Au layer. Red (blue) color in the Au layer indicates induced negative (positive) charge. The transmembrane potential causes formation of zones with effective positive and negative charges on Au; a bipolar double-layer is formed in the solution. The arrangement of the zones is dependent on the polarity of transmembrane potential.



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When examining rectifying behavior of Au coated pores, we noticed it was of crucial importance to probe stability of the ion current signals in time. It is known from previous studies that ion current through single SiN nanopores can fluctuate very heavily due to difficulties in wetting or formation of gas bubbles.40 We have prepared 10 Au coated nanopores with opening diameter between 9 and 16 nm, which produced stable ion currents (e.g. Figures 1c, S2, S3). We found that the current rectification in KF as shown in Figure 1b was dominant: 8 out of the 10 pores rectified the current so that negative currents were higher than positive currents, one pore exhibited a nearly linear behavior, and one pore rectified in the opposite direction. If signals of ion current in time were however unstable with sudden jumps or decreases of ion current upon change of voltage and/or in time (especially evident in higher voltages), the pores could effectively rectify in either direction (Figure S5). Rapid random fluctuations of the current suggest that the system behavior might be dominated by wetting instabilities and/or bubble formation, and not electrokinetic phenomena induced by Au polarization. Ion current instabilities could also result from electrooxidation of ethanol, as discussed below.

In order to support the hypothesis that the current rectification observed is due to Au polarization, experiments were also performed in KCl as the background electrolyte. In KCl, the effective charge on the Au layer is expected to be a superposition of the adsorbed chloride and the induced charge,25 thus reduction or even inversion of the dominant rectification mode was expected. Three different pores were first characterized in 100 mM (or 500 mM) KF to confirm they rectify as shown in Figure 1, and subsequently ion current through the same pores was measured in KCl. In two pores, changing salt from KF to KCl indeed reduced the ion current rectification (Figures S6, S7), and one pore showed a linear current-voltage curve. We did not observe inverted rectification as reported in previous work,33-35 possibly due to the presence of ethanol in the solution, depositing Au before drilling the nanopores, or the higher transmembrane electric fields used in this study. Adsorption of ethanol to Au41 is expected to lower the density of adsorbed chloride ions to the Au layer, enhancing the


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effect of metal polarization on local ionic concentrations. Drilling nanopores through a metal-SiN sandwich structure (versus Au evaporation after nanopore drilling) can lead to structures with homogenous thickness of Au layer around the pore opening, which will also make the effect of Au polarization more dominant. Previous studies of polymer pores with an embedded Au layer could not rectify according to the Au polarization effect due to their larger, microscopic length.34,35 Electric fields in these channels are orders of magnitude smaller than in our system, resulting in negligible induced charge densities. Observing the same character of ion current rectification in KCl and KF provided evidence that indeed, it was the Au polarization effect, which determined the system transport properties, and not the static charge on the Au layer.

Polarized Au can also be used as a bipolar electrode to perform electrochemical reactions.42 As an example, electrolysis of water was observed in a microfluidic channel with an embedded bipolar electrode, and protons and hydroxide ions generated in the process significantly contributed to the measured ion signal signal.27,29,42 We performed a simple analysis to verify that electrolysis of water most probably did not take place in our system and did not influence the measured current. Electrochemical reactions occur at the poles of a bipolar electrode, when a voltage drop across its length, , reaches a threshold value of a given redox reaction: ∆ =

where is the applied voltage. Due to Au polarization, the transmembrane potential drops primarily over the SiN pore,26,29 whose length, , needs to include not only the geometrical length of 50 nm but also the zone at pore entrances, which constitutes the so-called access resistance. 43 Access resistance takes into account the distortion of electric field lines in the vicinity of the pore opening, and is often included in calculations as an extension of pore length by a factor of 0.8 times the pore diameter. Thus, in the case of the SiN nanopores considered here, if the pore opening diameter is 10 nm, the effective length of the pore will be = 58 . Electrolysis of water requires



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∆ = 1.23, thus for =2V, the electrode length, , would have to be ~36 nm to enable electrolysis of water. It is also important to consider possible oxidation of ethanol in our solution, which requires ∆ ~0.4 and of only ~13 nm.44 Even though electrooxidation of ethanol is known to be kinetically slow,45,

46

products of ethanol

oxidation could contribute to the current instabilities observed in some samples. Our future efforts will focus on optimizing surface chemistry of SiN nanopores to enhance wetting and eliminate the presence of ethanol. Another possible electrochemical reaction to address is oxidation of Au in the presence of KF and KCl electrolyte.47 The choice of the electrolytes was dictated by the necessity to identify two model systems containing anions that do not adsorb to Au (F-),32,33 and anions which were found to adsorb to Au at high densities (e.g. Cl-).32,36 Thus, experiments in KF allowed us to create a system in which the effect of Au polarization on ionic transport was dominant. Measurements in KCl were important, because this salt is used in the majority of nanopore experiments due to its biological relevance and the convenience of having ion species of nearly the same mobility. The effect of Au oxidation can also be minimized by working at lower transmembrane potentials.

Our experimental findings on metal induced ion current rectification, were supported by finite-element analysis of ion currents and local ionic concentrations. The system was modeled by numerically solving coupled Poisson-Nernst-Planck and Navier-Stokes equations, 48 using the COMSOL Multiphysics software package. The geometry of the system is shown in Figure S8. In order to make the simulation results more generally interpretable, we did not transfer the double conical geometry that TEM drilled nanopores exhibit; as we will show, the rectification properties of the pore are sufficiently explained by the diode junction in the pore's interior without having to consider a more complex geometry. The conditions of the numerical modeling are as follows, with a more detailed explanation available in the accompanying Supporting Information. The electrodes were inserted into the simulations as voltage boundary conditions in both reservoirs, with the working electrode placed on the same side of the pore as the


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Au layer. A macroscopic reservoir size was simulated by applying appropriate boundary conditions for ion flux and pressure on the reservoir walls. A 15 nm thick floating potential layer was inserted on the surface of the silicon nitride to represent the polarizable Au material, while a continuous -0.016 C/m2 surface charge distribution was placed on the SiN walls to simulate the silanol charges native to silicon nitride. Bulk electrolyte consisted of 100 mM-concentrated cations and anions with mobilities set equal to the mobility of potassium (and therefore approximately equal to the mobility of chloride). Solutions of the coupled PNP-NS equations were found to be stable with respect to changes in the mesh density for which the solutions converged (Table S1). A moderate mesh density was used to keep convergence time to a minimum while still being able to capture the physics occurring in the ~0.1 – 1 nm-thick electrical double layer. The primary results of the simulations are shown in Figure 3. The simulated I-V curves show a positive rectification ratio for all salt concentrations, i.e. the current is higher at negative voltages than positive voltages (Figure 3a). The rectification ratio |I(-V)/I(+V)| increases with the decrease of ion concentration, which agrees with earlier findings for bipolar electrodes12,48 and our experimental results. We also verified that the predicted I-V curves were only weakly dependent on the dielectric constant of the solution. Simulations shown in Figure 3 were performed with dielectric constant of water (ε=80); lowering dielectric constant to ε=53.4, as reported for 50% water/50% ethanol,49 changed the predicted currents only for negative voltages by as little as few percent (Figure S9). To look for further evidence of our hypothesis that a diode junction forms in the pore and dominates the device behavior, we examine the ion concentration profile along the pore's axis. Figure 3 shows plots of the cross-sectional average of concentrations of potassium, fluoride, and their sums and differences. The concentration of potassium (fluoride) is enhanced in the pore’s interior at the SiN-Au junction for negative (positive) voltages, in agreement with the previously described model of electrical double layer formation due to induced charges in the metal.25,26 Crucially, the plots reveal that a bipolar junction - a division between regions with opposite net charge polarities - forms at the SiN-Au junction. This is most obvious for positive voltages in the data for average



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difference in concentrations of cations and anions , which is a measure of the average charge concentration in the solution (up to dimensional factors). In the vicinity of the SiN-Au junction there is a reversal in the sign of the charge. As the voltage is increased from 0 V to positive values, a region of low total ion concentrations emerges at the SiN-Au junction. This region is the depletion zone and is responsible for the low currents in the system's 'off state'. In order to measure the depletion zone width, we defined it as the interval within the pore where the average total concentration of ions falls below half its bulk value i.e. below 200 mol/m3. According to the above criteria, the depletion zone forms for voltages above 0.2 V (Figure 3e). Oppositely, when the voltage is negative, regions are formed where concentration of cations and anions exceeds bulk values, and the current is correspondingly enhanced. It is also important to note that, as shown previously,

27,29,42

the axial electric field across the part of the

pore with Au is nearly zero, thus the voltage drop occurs primarily across the uncovered, ‘bare’ nanopore (Figure S10). Finally, we considered the same nanopore-Au system, but with the SiN walls uncharged. The predicted I-V curve is shown in Figure S11 and confirms that the presence of surface charges on silicon nitride is crucial for breaking the electrochemical symmetry of the system. With no surface charge on the SiN nanopore, the currentvoltage curve follows a symmetric sub-linear behavior of the current, because the nanopore presents a resistive element that limits ionic sourcing. For positive (negative) voltages at the membrane side with Au, the flow of anions (cations) is limited by the nanopore resistance. Our results therefore suggest that Au polarization provides a robust mechanism for ion current rectification in nanopores that carry surface charge. The Au-nanopore system rectifies due to its electric potential modulated charge distribution, which in turn regulate ionic concentrations in the nanopore.


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Figure 3. Results of numerical modeling of ionic current and ionic concentrations at various voltages through a single 10 nm in diameter nanopore with an integrated 15 nm thick Au layer (marked with a yellow line on the x-axis between 50 and 65 nm). (a) Current-voltage curve for 0.1 M and (inset) 0.5 M KF. (b) and (c) average concentration of K+ and F- along the pore axis for transmembrane potentials between -1V and +1V; the colors of the lines correspond to the colors of the symbols in (a). (d) width of the depletion zone calculated as the width of the region for which the total concentration of ions falls below half its bulk value; (e) concentration of all ions in the pore, and (f) difference in the concentration between potassium and fluoride ions along the pore axis. The native surface charge density on SiN walls was assumed -0.016 C/m2. Coupled Poisson-Nernst-Planck and Navier-Stokes equations were solved.

In conclusion, we presented a single nanopore rectifier whose surface charge density is controlled in a two-electrode system by the transmembrane potential. An Au layer placed at the nanopore entrance undergoes polarization so that a junction between regions with enhanced cation and anion concentrations is formed inside the pore. The



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induced surface charge density and ionic concentrations in the pore are regulated by the external electric field magnitude and direction leading to ion current rectification. The mechanism of rectification proposed here is applicable to low-aspect ratio nanopores that can be used for preparation of synthetic analogues of voltage-gated biological channels. We expect metal polarization in nanofluidic devices will lead to a variety of systems with applications as ionic switches and components of nanoscale ionic circuits. Inducing surface charge density and surface charge pattern with transmembrane potential only, without directly applying voltage to the metal, provides a new and simplified route to create voltage-responsive systems.

Experimental Section Preparation of nanopores. A 3 nm thick layer of Cr and 27 nm (or 12 nm) thick layer of Au were deposited onto the membrane side of a 50 nm thick silicon nitride film (SPI Supplies, 0.1 mm width window).

The metals were deposited in a cleanroom

environment, using a Temescal CV-8 E-beam evaporator. A single pore was then drilled through the etched side of the silicon nitride using an FEI Titan 80-300 S/TEM at 300 kV in STEM mode.

Recordings of ion current. Silicon nitride chips containing single nanopores were placed between two chambers of a custom made PDMS conductivity cell created from a 3D printed mold. Two Ag/AgCl pellet electrodes (A-M Systems, Sequim, WA) were used in measurements. Signals of ion current in time were recorded at 20 kHz sampling frequency using Axopatch 200B and 1322A Digidata (Molecular Devices, Inc.) and 2 kHz low-pass Bessel filter. Ion transport properties were investigated in symmetric electrolyte conditions using KF and KCl solutions prepared in 50% water and 50% ethanol to improve wettability of the membranes.

Comsol modeling of ion currents. Coupled Poisson-Nernst-Planck (PNP) and NavierStokes (NS) equations were solved using the commercially available Comsol Multiphysics 4.3 package.48 The Au layer was treated as a floating electrode. The mesh was adjusted to assure convergence of the model to the point when no change in the


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observed concentration profiles and currents was observed upon further mesh decrease (Table S1). The diffusion coefficient for potassium and fluoride ions was assumed to be 210-9 m2/s. Further details of modeling are provided in the Supporting Information.

Supporting Information Available: Ion transport through additional nanopores as well as details of numerical modeling are presented. Acknowledgements This research was supported by the National Science Foundation (CHE 1306058). Hospitality of the research group of Prof. Meni Wanunu and training in TEM are greatly acknowledged. We are grateful to Prof. Henry S. White and Prof. Ulrich F. Keyser for discussions.

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Polarization of Gold in Nanopores Leads to Ion Current Rectification

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