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High current ionic diode using homogeneously charged asymmetric nanochannel network membrane Eunpyo Choi, Cong Wang, Gyu Tae Chang, and Jungyul Park Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04246 • Publication Date (Web): 18 Mar 2016 Downloaded from http://pubs.acs.org on March 18, 2016

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High current ionic diode using homogeneously charged asymmetric nanochannel network membrane Eunpyo Choi, Cong Wang, Gyu Tae Chang, and Jungyul Park* Department of Mechanical Engineering, Sogang University, 35 Baekbeom-ro (Sinsu-dong), Mapo-gu, Seoul 121-742, Korea

ABSTRACT. A high current ionic diode is achieved using an asymmetric nanochannel network membrane (NCNM) constructed by soft lithography and in situ self-assembly of nanoparticles with uniform surface charge. The asymmetric NCNM exhibits high rectified currents without losing a rectification ratio because of its ionic selectivity gradient and differentiated electrical conductance. Asymmetric ionic transport is analyzed with diode-like I-V curves, and visualized via fluorescent dyes, which is closely correlated with ionic selectivity and ion distribution according to variation of NCNM geometries. KEYWORDS: Ionic diode, ion selectivity, high current, self-assembly, nanoparticle

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Ionic transport through nanoscale geometries such as nanopore clusters and nanochannels has been intensively studied in recent decades. Ionic current rectification is a representative example of transport characteristic induced by nanoscale effects. In ion current rectification, the current for voltages of one polarity are higher compared to that for voltages of the same amplitude but opposite polarity.1 This property can be exploited for use as switches for ions and charged molecules in solution, and in diodes and transistors for micro/nanofluidic circuits.2, 3 To date, there have been several studies of rectification systems with homogenous surface charges, including tapered cone-shaped nanopores, nanotubes, and nanocapillaries. After Wei et al.4 introduced glass nanopipettes for conical nanopores, Umehara et al.5 improved their rectification properties by chemically modifying the inner wall, and Liu et al. utilized the system as a biosensor.6 Siwy and co-workers1,

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developed a nanofluidic diode using track-etched

conically shaped nanopores with homogeneously charged inner walls. To control the ion transport through these track-etched nanopores with the homogeneously charged inner surfaces, Ali et al.8 used the layer-by-layer deposition of a polyelectrolyte onto the inner wall, and Zhang et al.9 suggested a light- and pH-cooperative nanofluidic diode with a spiropyran-functionalized inner wall. Although many types of ionic rectification systems have been reported, they rely on asymmetric nanoscale openings comparable with the thickness of the electrical double layer (EDL, λD). That is, if the pore or capillary on one side is sufficiently narrow and comparable with the thickness of the EDL, but the other side is wider, then different counter ion distributions occur and ionic rectification is induced. However, these reported ion rectification systems have several disadvantages, especially for applications in micro total analysis systems (µ-TAS), due to their low ionic currents and the high fluidic resistance of the nanopores/channels (for example, the reported values of rectified


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ionic currents of homogeneously charged systems are only a few tens of nano-amperes (nA)). Some studies have attempted to increase the ionic current or reduce the fluidic resistance by using multiple arrays of nanochannels or nanopores,10-15 or the aforementioned chemical modification of the inner wall.2, 5, 6, 8, 9 These methods, however, still have issues, including the following. An array of nanochannels must be realized in two dimensions because of the intrinsic limitations from a top-down fabrication process, and these ion-track-etched nanopores and ebeam lithography-based nanochannels require extensive fabrication processes and/or expensive and specialized equipment. In addition, chemical modification inside nanopores/nanochannels is time consuming and cannot be easily controlled. Other disadvantages are the technical difficulties associated with visualizing ion transport inside the nanopores (for example, using fluorescent probes for the study of electrokinetics phenomena), and also with the integration with other components in the µ-TAS for further applications. The heterogeneously charged membrane has been suggested to enhance the rectification system.2 The mechanism of this bipolar membrane is similar with “p-n diode”, which can suppress ionic flow in one direction almost entirely, so that the rectification ratio could be higher than the rectification relying on asymmetric geometry with homogenous surface charge.3 For example, in case of asymmetric nanoscale geometry with homogenous surface charge, the rectification factor is approximately 4 to 30,3, 5, 8, 9 but the rectification factor in bipolar case can reach over 300.16 However, we only focused on the rectification using homogenously charged membrane because these two approaches (hetero and homogeneously charged membrane) depend on different rectification mechanism. In addition, the study based on homogeneously charged asymmetric membrane is able to contribute to revealing the still-unclear fundamental mechanism for ionic transport depending on nanoscale geometry, because the counter-ion


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concentration in nanoscale is not solely determined by the surface charge density but also by the geometry and the ionic concentration of solution. This paper reports a high current ionic diode, which was achieved by designing an asymmetric nanochannel network membrane (NCNM) constructed by the in situ self-assembly of nanoparticles with homogeneous surface charges (figure 1(a)). Recently, the micro/nano fluidic systems that were integrated with assembled nanostructures, which enabled the occurrence of electrokinetic phenomena in microfluidics, have been suggested.17-24 For examples, the assembled nanoparticles in the micropore from KOH-etched silicon wafer were utilized: A microplatform based on reverse electrodialysis (RED) was presented by Wei et al., which could generate a high ionic current through a nano-interstices between the assembled nanoparticles for high-power energy harvesting.18 Lei et al. reported an approach that utilized a suspended nanoparticle crystal as an electrical read-out biosensor based on a nanofluidic electrokinetics principle.20 Moreover, the straight microchannel with assembled nanoparticles was suggested: Zeng et al. suggested a biomolecular sieving system, based on ordered colloidal arrays that defined the sieve structure within the microfluidic device.19 Under moderate electric fields, they achieved the fast separation of DNA and proteins over a wide size range. Also, ion concentration polarization (ICP) phenomena around the assembled nanoparticles were characterized by Chen et al.

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and an electrokinetic preconcentration system based on ICP near

the nanoparticle assembly which allowed the rapid concentration of DNA and the protein samples was suggested by Syed et al.23

However, these methods could not realize the

nanoparticle assembly in the desired area and shape, so that it makes difficult to visualize ion transport using fluorescent dye at the interface between microchannel and nanochannels for studying nanoscale electrokinetics by changing the geometry of microchannels. We suggested


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the novel method to form the nanochannel networks between two microfluidic channels using geometrically controlled in situ self-assembled nanoparticles.17,

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The advantage of our

system is that the electrical performance can be tuned quantitatively or even optimized by changing the geometry of the microchannel and the ion transport according to the variations of surface properties can be visualized with fluorescent dyes, so that it can contribute to revealing the still-unclear fundamental mechanism of ion transport. In this study, we present the first report of ionic current rectification using a homogeneously charged nanoparticle assembly; to date, asymmetric nanoscale openings have been considered as the only way to achieve differentiated ion selectivity. The proposed system realizes the ion selectivity gradient (higher cation/anion ratio toward tip region), i.e. spatially inhomogeneous fixed charge distribution, stemming from the asymmetric NCNM geometry that can be easily defined by standard microfabrication. There have been many researches about the spatially inhomogeneous fixed charge distribution at the macroscopic ion-exchange membrane with asymmetric geometries and there effects on the ionic transport and selectivity properties of the membrane.26-33 Once the ion selectivity gradient is built up in NCNM, the ions do not redistribute immediately right after the external electric-field is applied and the ion current can be proportional to the ion concentration at equilibrium. If forward bias is applied, then the inward current is greater than outward current and each ion species will accumulate in the NCNM. On the contrary, if there is more outward current than inward current (reverse bias), ion depletion takes place as shown in figure 1(b). Therefore, a high current through three dimensional NCNM can be achieved without losing the rectification ratio (frec, the ratio of the absolute values of currents recorded at voltages of the same amplitude but opposite polarities) compared to conventional systems using nanopores or nanochannels with homogeneously


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charged inner walls.1-9 Moreover, the current can be tuned quantitatively and flexibly by changing the size of nanoparticle and geometry of the NCNM via standard photolithography, and ion transport inside the NCNM can be visualized by using two differently charged fluorescent dyes, and characterized together with electrical measurements. In the Supporting Information, the formation of the NCNM using nanoparticles within a shallow channel is described, as reported previously.17,

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The spatially controlled self-

assembly of nanoparticles within the microchannels was realized by a multi-layered design of the microchannels and the control of the microdroplets containing the nanoparticles. Figure 1(c-d) shows the top view of the fabricated ionic diode and the SEM images of the asymmetric NCNM fully filled with the self-assembled nanoparticles in a face-centered-cubic (FCC) structure. The nano-interstices formed between these close-packed homogeneously sized nanoparticles, which consist of equivalent nanopores (nanopore size, dn, is ~15% of the sphere size, Dn),19 serve as the pores in the ion-selective membrane because the EDL (λD) is comparable to the size of the nanointerstices.34 The ion-selective gradient (inhomogeneous fixed charge distribution) in the asymmetric NCNM is able to be established by following steps. The whole channel is filled with a sufficiently low concentrated KCl to create the cation selectivity through the entire NCNM. Subsequently, a higher concentrated KCl is introduced at the wide-opening side of the deep channel (deep channel-B in figure 1). This high enough concentration is able to make the overlapped EDLs collapse, which results in low ion selectivity, and then it allows both anions and cations from the bulk to be transported through the wide-opening side. However, owing to the steep ion selectivity gradients induced by the asymmetric geometry of NCNM and the high electrical resistance of NCNM by the physical confinement at the small tip17, 21, 35 ( K n ∝ ws ,


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where Kn and ws represent the electrical conductance and width of the membrane, respectively) can cause a drastic decrease of ion transportation the bulk toward the tip-opening side. Moreover, the initial cation selective state of macroscopic NCNM might serve as a barrier to anion transport from wide to tip opening. In these reasons, anions could not be reached to the tip side of NCNM, but most cations could. However, the membrane became narrow in the tip region and high resistance induced from the narrow tip makes high traffic for counter-ion transport, which results in trapping them at the tip region.27 It has previously been discussed that these trapped counterion clusters (fixed charge concentration) give rise to a potential within the membrane which impedes co-ion relative to counter-ion transport, increasing ion selectivity.27, 31, 36 Once this ion selectivity gradient is established, it is preserved steadily after the both deep channels are filled with the same concentration solution. It seems because the spatially fixed charge distribution by trapped cation clusters at the tip region has a greater selectivity than the permselectivity predicted by a homogeneous model,27 so that the mobile anions were hard to enter NCNM from the deep channel-A (depicts in figure 1) to the tip opening. To verify the ion selectivity distribution in asymmetric NCNM, we visualized the ion transportation using mixed fluorescent dyes: the negatively charged Alexa-488 (green, representing Cl−) and positively charged Atto-633 (red, representing K+). Figure 2(a) shows a sequence of fluorescence images taken after the mixed fluorescent dyes in 1 mM KCl solution were introduced into the deep channel-B for 10 min. In this case, the ratio of the widths of the tip-opening (wt) to the wide-opening (ww) was 10 : 200 µm, and the NCNM was assembled from nanoparticles of Dn = 700 nm (nanopores size of ~105 nm and a λD value of ~10 nm at 1 mM KCl). At the wide-opening side, the red intensity was lower than the green intensity due to the low ion selectivity, and it became bulk region. At the tip-opening, however, the red intensity was


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higher than the green intensity, which indicates high cation selectivity, and it became to tip region. Finally, we introduced the same concentrated KCl into deep channel-A (tip-opening side of the deep channel) and confirmed that the ion selectivity was maintained continuously. The sequential fluorescence images in figure 2(b) show a long term preservation of high ion selectivity (red intensity) in the tip region after then the same mixed fluorescent solution was injected into deep channel-A. These results support that the higher fixed charge concentration occurred at the tip region and it was steeply decreased from tip to wide opening in hyperbolic function. The important parameters for the gradients of ionic selectivity in the asymmetric NCNM are the relationship between the ratio of Debye length to nanopore size (R = dn/λD) and the width of the wide opening. If too low concentrated bulk solution is used, the NCNM becomes entirely high cation selective, and consequently bulk region is diminished. On the other hand, high concentrated bulk solution increases the concentration of the solution near the tip-opening side, resulting in the collapse of the ion selectivity of the tip region. Figure 3(a) shows the fluorescence images at different bulk KCl concentrations under the fixed conditions of wt : ww = 10 : 200 µm and Dn = 700 nm (dn = 105 nm). At the concentration of 0.01 mM KCl with the mixed fluorescent dyes, the red intensity (high cation selectivity) was dominant throughout the NCNM; the EDLs were fully overlapped throughout the NCNM (λD was about 97 nm in 0.01 mM, R ~ 1). At high concentration (10 mM KCl, λD = 3 nm, R ~ 34), a dramatic decrease in the red colored tip region and wider green colored bulk region were observed because of the collapse of the overlapped EDLs. In moderate concentration (1 mM KCl, R ~ 11), it was shown that the apparently distinguishable tip and bulk regions. The similar tendency was observed when the


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different size of nanoparticle under the fixed conditions of wt : ww = 10 : 200 µm was used for the I-V curve analysis (see below for detailed discussion). Even though R is fixed, the gradient of ionic selectivity in NCNM can be changed as varying the width of wide-opening (ww). Here, wt fixed at 10 µm in 1 mM KCl (figure 3(b)). As ww increased from 10 to 200 µm, the chance of ion diffusion from bulk solution through the wide-opening side increased, and therefore, the bulk regions could be established and became wider. However, at ww = 400 µm, the excessive increase of ion diffusion from the bulk to the tip region reduced the cation selectivity at the tip also. In conclusion, the careful selection of the ratio of Debye length to nanopore size, as well as the optimal design of the asymmetric microchannel geometry for the NCNM is necessary for high performance. Figure 4(a) shows a schematic of the differentiated ionic transport regarding to the type of ions in the asymmetric NCNM when voltage biases with different polarities are applied. For positive voltage application (forward bias), high inward ion currents occurs and both ion species are transferred into the transition region and accumulated inside the NCNM, which generates a high ionic current. The cations from the tip region can move toward the bulk region freely owing to the low electrical resistance. With the application of a negative voltage at the tip (reverse bias), cations in the transition region are transferred towards the tip region, and anions in the transition region move toward the bulk region. Due to the low electrical resistance near the bulk region, the anions in the bulk region can move out to the deep main channel rapidly. Therefore, both ions are depleted in the transition region, and consequently, a lower ionic-current value is achieved. These phenomena can be supported by observing the movement of positively and negatively charged fluorescent dyes inside the NCNM, as shown in figure 4(b). The fixed width ratio condition was wt : ww = 10 : 200 µm. The voltage from a source meter was applied through


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platinum electrodes. To analyze the changes in the fluorescent intensity from the initial equilibrium status more clearly, a normalization process for the subtracted images was introduced: first, we captured the fluorescence image before applying the voltage and converted it into grey scale to normalize the intensity (I0). Then, we subtracted this intensity from the greyscaled fluorescence intensity (I1) captured after applying the voltage. Finally, we color mapped the normalized subtracted intensity profile. Green to red (I1 − I0 > 0) represents an increase in the intensity (accumulated ions), whereas green to blue (I1 − I0 < 0) indicates a decrease in the intensity (depleted ions). When zero potential was applied, since it is the initial equilibrium status, there was no change in the intensity: the color is green (I1 − I0 = 0). However, with negative (−10 V) and positive (+10 V) potentials at the tip opening, the shifting of colors toward the blue with I1 − I0 < 0 (depletion) and toward the red with I1 − I0 > 0 (accumulation) was observed inside the transition region, respectively. The experimentally acquired intensity profiles for cation and anion concentrations at each status were depicted in figure 4(b) also. These profiles show good agreement with our description for ion transportation in asymmetric NCNM and the conceptual profiles in figure 1(b). They are also similar with the change of concentration distribution in a single tapered nanopores.1 Moreover, these results were supported by the numerical simulation (more details are in Supporting Information). In this simulation, we focused on the nanopore size of 36 nm because of following reasons: in the visualization experiment, the best conditions for the concentration distribution for diode-like performance were obtained when wt : ww = 10 : 200 µm with R ~ 11 (dn = 105 nm, λD = ~10 nm, and 1 mM KCl). However, to analyze the I-V characteristics which will be shown later, only K+ and Cl− ions should be considered, not the fluorescent dyes, so that the size of the nanopore has to be adjusted to match the value of R. The size of both K+ and Cl-


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ions (~0.3 nm)37 have approximately three times smaller than the sizes of the fluorescent dyes (the Alexa-488 and Atto-633 dyes have similar diameters of roughly ~1 nm).38, 39 Therefore, approximately three times smaller nanopores (~36 nm, Dn = 240 nm) and λD = ~3 nm (KCl 10 mM) should be considered to match the R close to 11. To verify this assumption, the simulation was conducted under aforementioned conditions (wherein R is approximately 12 and wt : ww = 10 : 200 µm). As a result, the depletion and accumulation of ions were observed in the transition region when the negative and positive potential was applied, respectively (figure 4(c)). However, when the same condition with dn = 36 nm case but only the size of nanopore was equal to the visualization study case, i.e. dn = 105 nm, such phenomena became significantly reduced under the same DC bias (figure S5 (b) in Supporting Information). The current-voltage (I–V) curves were then measured to analyze the ionic-current rectifying performance of the proposed asymmetric NCNM. First, the effect of R = dn/λD to rectification ratio at the fixed geometry conditions was studied (figure 5(a) and (b)) and second, the effect of geometry to rectification ratio by changing the width of wide opening under the fixed condition of nanoparticles diameter and electrolyte concentration was investigated (figure 5(c)). As for the former studies, there are two ways to adjust the value of R: (1) changing the Debye length by using different concentration or (2) nanopore size by varying size of nanoparticles. In case (1), I-V characteristics were examined under the same conditions with simulation studies in figure 4(c): wt : ww = 10 : 200 µm, dn = 36 nm (Dn = 240 nm), and 10 mM KCl for R ~ 12. As revealed in figure 5(a), the ionic current increased proportionally with positive voltage, but for negative voltages, much lower ionic currents with a nearly constant zero value were observed. The rectification factor (frec) reached ~55 at ±10 V. However, when the


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KCl concentration was less than 10 mM, which means lower than R ~ 12, the frec was reduced, with values of ~13 and ~7 at ±10 V for 1 mM (R ~ 4) and 0.1 mM KCl (R ~ 1), respectively. This is because cation selectivity becomes dominant throughout the NCNM, and consequently, the transition and bulk regions are diminished. According to the Donnan theory, at high ion selectivity, the total ion concentration and the conductance of the nanopores are a universal invariance, making it difficult for rectification to occur.40, 41 Moreover, Yan et al.42 suggested that the external field focusing and the concentration depletion at the interface of NCNM and microchannel are able to reduce the rectification when the dimensionless parameter X is much higher than 1. Here, X represents the density of the surface charge, extrapolated over the entire pore volume, relative to the bulk electrolyte concentration, and is expressed as X =

2σ , zFC0 d n

where, σ, z, F, C0, and dn is the negative surface charge density, valency, Faraday constant, symmetric electrolyte of concentrations, and the nanopore of diameter, respectively. If the negative surface charge density is fixed to be 0.01 C/m2,21and nanoparticle diameter of 240 nm (dn ~ 36 nm) is used, the value of X will be about 57, 5.7, 0.57, and 0.057 for KCl concentration

of 0.1, 1, 10, and 100 mM, respectively. It means that the external polarization phenomena are able to produce a rectification inversion for the lower concentration than 10 mM (X > 1). For 10 mM of KCl used in our electrokinetic study , since X < 1, the intra-pore ion transport controls the current and internal ion accumulation/depletion at positive/reverse biased voltage, which is responsible for current rectification42 and the obtained value of frec was ~55 at ±10 V. That is, internal concentration polarization is dominant under our experimental condition. However, a lower frec was obtained at much higher concentration (i.e. higher R) despite of lower value of X than 10 mM KCl case (frec reached ~23 at ±10 V for 100 mM KCl, R ~ 35), because the cation selectivity in the tip region might be collapsed as shown in figure 3(a).


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As for the second methods, we used the dn of 200 nm, 240 nm, 300 nm, and 700 nm which has the nanopore size with 30 nm, 36 nm, 45 nm, and 105 nm, respectively. The fixed conditions were wt : ww = 10 : 200 µm and 10 mM KCl. Figure 5(b) shows the related results as depicting the frec and R depends on the different size of nanoparticle. The results showed the similar tendency with the former case: the highest frec was observed when R was ~12 (dn = 36 nm) and it became reduced at lower R (~10 for dn = 30 nm) and at higher R (~15 and ~34 for dn = 45 nm and 105 nm, respectively). It seems because when the nanopore was larger than 36 nm, the ion selectivity might be more collapsed so that both ions would transport into the tip region as a similar result in right fluorescent image in figure 3(a). Otherwise, when the nanopore was lower than 36 nm, the cation selectivity would be dominant from tip to bulk region which makes the bulk region diminish like the left fluorescent image in figure 3(a). At last, we investigated the I-V characteristics by changing ww under fixed conditions of wt = 10 µm, 10 mM KCl, and Dn = 240 nm. As revealed in figure 5(c), frec rises as ww is increased from 10 to 200 µm: frec reaches ~1, ~8, and ~55 at ww = 10, 100, and 200 µm, respectively. At ww = 400 µm, however, the further influx from the bulk to the tip region makes the cation selectivity in the tip region collapse, and consequently, leads to a lower frec value (~7 at ±10 V). The visualization studies using the mixed fluorescent dyes shown in figure 3(a) also support these results, in terms of the ionic concentration distributions represented by fluorescent intensities and the ratio of the Debye length to nanopore size. In this study, we achieved the highest rectification ratio of ~55 at ±10 V for wt : ww = 10 : 200 µm and Dn = 240 nm in 10 mM KCl. The measured current was ~0.8 µA at 10 V. Under the conditions of wt : ww = 10 : 200 µm, Dn = 240 nm, and 100 mM KCl, we obtained the maximum current, ~3.6 µA at 10 V, and a frec of ~23 at ±10 V. Table 1 presents the comparison of the


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maximum currents and rectification ratios with the previously reported nanofluidic diodes with homogeneously charged inner walls. The reported values of the rectified ionic currents do not exceed a few tens of nano-amperes, but the proposed system shows much higher currents (submicro-ampere (µA)) under the same conditions. The reason is the increased number of effective nanochannels, which are stacked in three dimensions and are interconnected. In addition, the rectification ratio was similar or higher to the systems prepared by conventional methods that had homogeneously charged inner walls, as depicted in Table 1. In principle, further current increase can be achieved by increasing the height of the shallow channel, decreasing the size of the nanoparticles, or using nanoparticles that are highly surface-charged (e.g., polystyrene, chemically modified nanoparticles). We recently showed that higher shallow channels promoted ion current in NCNM, smaller nanoparticles enhanced the conductance of the NCNMs, and the hydrophobicity of polystyrene (as a surface property) could amplify the hydrodynamic slippage, resulting in an increase in the conductance of the NCNM.17, 21, 24

In fact, the smaller size of nanoparticle showed the higher current in the positive bias rage as

shown in figure 5(b). Moreover, it is obviously expected that if we use the assembly of two layers with oppositely charged nanoparticles, the large rectification with high current would be also achieved. We believe, therefore, that the electrical properties of NCNM can be easily tailored and that appropriate material selection will lead to enhanced diode-like ionic rectification properties in the near future. In this paper, we proposed a high current ionic diode based on an asymmetric 3D NCNM which constructed by spatially controlled nanoparticle assembly. Although tapered nanopores and nanochannel diodes with homogeneously charged inner walls have been studied previously, there were intrinsic challenges to their expensive and specific fabrication, as well as low ionic


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current, high fluidic resistance, and retarded ion transport. These problems could be overcome successfully by the proposed asymmetric NCNM; with the ionic gradient in its internal structure and differentiated electrical conductance, a higher rectified current was realized without reducing the rectification ratio. Moreover, the ionic current and rectification ratio could be tuned easily by changing the geometry of the NCNM via standard photolithography techniques and the concentration of the electrolyte. The ion transport inside the NCNM could be visualized using fluorescent dyes, which effectively supported the I-V-curve-based analysis. We believe that much higher ionic currents and rectification ratios can be achieved easily by selecting the proper size of the nanoparticles, geometrically controlling the shallow channels, and using different kinds of nanoparticle materials. These results have the potential to be extended to exciting applications such as highly sensitive biosensors, microenergy harvesting, and integrated ionic circuits.

Methods. We used negatively charged silica nanoparticles with diameters (Dn) of 240 nm (Microspheres-Nanospheres Co., USA) and 700 nm (Polysciences Inc., USA). Voltage from a picometer/voltage source (6487, Keithley Instruments) was applied through platinum electrodes and controlled with a general-purpose interface bus (GPIB) and Labview 9.0 (National Instruments). The voltage and current signals were recorded by a computer connected to the picometer/voltage source through a GPIB card (PCI-GPIB, National Instruments). In all experiment, the voltage from a source meter was applied from −10 to +10 V in steps of 0.2 V every 3 s. We repeated the experiment and plotted the I-V curve with error bars (standard deviation, more details in Supporting Information). The potassium chloride electrolyte solution was prepared by dissolving the target solute in deionized (DI) water. A precise scale (AB 264-


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S/31, Mettler-Toledo) with a resolution of 0.01 mg was utilized to accurately measure the concentration of the electrolyte. We measured the pH of the electrolyte solution using a pH meter (SevenMultiTM S47, Mettler-Toledo); the pH was typically 5.6–6.0, similar to that of DI water. The electrolyte solution was allowed to cool after mixing and, after sealing, was stored at room temperature until use. The fluorescent dyes used for the visualization studies were the negatively charged Alexa-488 (Life Technologies Inc., USA, 4 µM) and positively charged Atto-633 (Attotec, Siegen, Germany, 6 µM). The activities of both fluorescent dyes are independent of pH variations.43 The visualization studies were performed with an inverted microscope (IX7, Olympus Co., Tokyo, Japan), and the images were captured on a PC using a CCD camera (CoolSNAP, Photometrics, USA). The differently colored fluorescent dyes were monitored and captured simultaneously using a full multiband laser filter installed in the microscope (BrightLine®, optimized for 405, 488, 561, and 635 nm laser sources, Semrock Inc., USA). The fluorescence intensities could be quantitatively assessed from the captured images using ImagePro Plus (Media Cybernetics, USA) software.

Supporting Information. Formation of the nanochannel networks using nanoparticles within the shallow channel. This material is available free of charge via the Internet at http://pubs.acs.org Corresponding Author *Prof. Jungyul Park. Tel.: +82 2 705 8642; Fax: +82 2 701 7075; Email: [email protected]

Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT The authors would like to thank Prof. Daejoong Kim, Prof. Jungchul Lee, Prof. Sung Jae Kim, and Prof. Ho-Young Kim for their time and valuable advice. This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF-2013R1A1A2073271, NRF-2015R1A2A2A04006181) and by the Industrial Materials Fundamental Technology Development Program (10052981, Development of smart contact lens materials for glaucoma therapy and IOP measurements) funded by the Ministry of Trade, Industry and Energy(MOTIE) of Korea.


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REFERENCES AND NOTES 1.

Siwy, Z. S. Advanced Functional Materials 2006, 16, (6), 735-746.

2.

Cheng, L. J.; Guo, L. J. Chemical Society reviews 2010, 39, (3), 923-38.

3.

Gamble, T.; Decker, K.; Plett, T. S.; Pevarnik, M.; Pietschmann, J. F.; Vlassiouk, I.;

Aksimentiev, A.; Siwy, Z. S. The journal of physical chemistry. C, Nanomaterials and interfaces 2014, 118, (18), 9809-9819. 4.

Wei, C.; Bard, A. J.; Feldberg, S. W. Anal Chem 1997, 69, (22), 4627-4633.

5.

Umehara, S.; Pourmand, N.; Webb, C. D.; Davis, R. W.; Yasuda, K.; Karhanek, M. Nano

Lett 2006, 6, (11), 2486-2492. 6.

Liu, Y.; Yobas, L. Biosensors & bioelectronics 2013, 50, 78-83.

7.

Siwy, Z.; Heins, E.; Harrell, C. C.; Kohli, P.; Martin, C. R. J Am Chem Soc 2004, 126,

(35), 10850-10851. 8.

Ali, M.; Yameen, B.; Cervera, J.; Ramirez, P.; Neumann, R.; Ensinger, W.; Knoll, W.;

Azzaroni, O. J Am Chem Soc 2010, 132, (24), 8338-8348. 9.

Zhang, M. H.; Hou, X.; Wang, J. T.; Tian, Y.; Fan, X.; Zhai, J.; Jiang, L. Adv Mater

2012, 24, (18), 2424-2428. 10.

Kim, S. J.; Song, Y. A.; Han, J. Chemical Society reviews 2010, 39, (3), 912-922.

11.

Kim, D.; Raj, A.; Zhu, L.; Masel, R. I.; Shannon, M. A. Lab on a chip 2008, 8, (4), 625-

628. 12.

Lee, S. J.; Kim, D. Microfluid Nanofluid 2012, 12, (6), 897-906.

13.

Guo, W.; Cao, L. X.; Xia, J. C.; Nie, F. Q.; Ma, W.; Xue, J. M.; Song, Y. L.; Zhu, D. B.;

Wang, Y. G.; Jiang, L. Advanced Functional Materials 2010, 20, (8), 1339-1344.


18

Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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14.

Cao, L.; Guo, W.; Ma, W.; Wang, L.; Xia, F.; Wang, S.; Wang, Y.; Jiang, L.; Zhu, D.

Energy & Environmental Science 2011, 4, (6), 2259. 15.

Yossifon, G.; Chang, Y.-C.; Chang, H.-C. Phys Rev Lett 2009, 103, (15), 154502.

16.

Cheng, L. J.; Guo, L. J. ACS nano 2009, 3, (3), 575-84.

17.

Choi, E.; Kwon, K.; Kim, D.; Park, J. Lab on a chip 2015, 15, (1), 168-78.

18.

Wei, O.; Wei, W.; Haixia, Z.; Wengang, W.; Zhihong, L. Nanotechnology 2013, 24, (34),

345401. 19.

Zeng, Y.; Harrison, D. J. Anal Chem 2007, 79, (6), 2289-2295.

20.

Lei, Y.; Xie, F.; Wang, W.; Wu, W.; Li, Z. Lab on a chip 2010, 10, (18), 2338-2340.

21.

Choi, E.; Kwon, K.; Kim, D.; Park, J. Lab on a chip 2015, 15, (2), 512-23.

22.

Chen, Z.; Wang, Y.; Wang, W.; Li, Z. Applied Physics Letters 2009, 95, (10), 102105.

23.

Syed, A.; Mangano, L.; Mao, P.; Han, J.; Song, Y. A. Lab on a chip 2014, 14, (23), 4455-

60. 24.

Choi, E.; Kwon, K.; Lee, S. J.; Kim, D.; Park, J. Lab on a chip 2015, 15, (8), 1794-8.

25.

Choi, E.; Chang, H. K.; Lim, C. Y.; Kim, T.; Park, J. Lab on a chip 2012, 12, (20), 3968-

75. 26.

Ramirez, P.; Gomez, V.; Cervera, J.; Schiedt, B.; Mafe, S. J Chem Phys 2007, 126, (19),

194703. 27.

Selvey, C.; Reiss, H. Journal of Membrane Science 1985, 23, (1), 11-27.

28.

Manzanares, J. A.; Mafe, S.; Pellicer, J. The Journal of Physical Chemistry 1991, 95,

(14), 5620-5624. 29.

Manzanares, J. A.; Mafe, S.; Pellicer, J. Journal of the Chemical Society, Faraday

Transactions 1992, 88, (16), 2355-2364.


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Page 20 of 29

30.

Koter, S. Journal of Membrane Science 1995, 108, (1–2), 177-183.

31.

Hsu, J.-P.; Ting, K.-C.; Shieh, Y.-H. The Journal of Physical Chemistry B 2000, 104,

(15), 3492-3495. 32.

Yamamoto, R.; Matsumoto, H.; Tanioka, A. The Journal of Physical Chemistry B 2003,

107, (38), 10615-10622. 33.

Choi, E.-Y.; Strathmann, H.; Park, J.-M.; Moon, S.-H. Journal of Membrane Science

2006, 268, (2), 165-174. 34.

Pu, Q. S.; Yun, J. S.; Temkin, H.; Liu, S. R. Nano Lett 2004, 4, (6), 1099-1103.

35.

Chang, C. C.; Yeh, C. P.; Yang, R. J. Electrophoresis 2012, 33, (5), 758-64.

36.

Hsu, J. P.; Ting, K. C. Journal of The Electrochemical Society 1998, 145, (4), 1088-1092.

37.

Volkov, A. G.; Paula, S.; Deamer, D. W. Bioelectrochemistry and Bioenergetics 1997,

42, (2), 153-160. 38.

Masuda, A.; Ushida, K.; Okamoto, T. Biophysical Journal 2005, 88, (5), 3584-3591.

39.

Illaste, A.; Laasmaa, M.; Peterson, P.; Vendelin, M. Biophysical Journal 2012, 102, (4),

739-48. 40.

Cervera, J.; Schiedt, B.; Neumann, R.; Mafe, S.; Ramirez, P. The Journal of chemical

physics 2006, 124, (10), 104706. 41.

Momotenko, D.; Cortes-Salazar, F.; Josserand, J.; Liu, S.; Shao, Y.; Girault, H. H.

Physical chemistry chemical physics : PCCP 2011, 13, (12), 5430-40. 42.

Yan, Y.; Wang, L.; Xue, J.; Chang, H. C. J Chem Phys 2013, 138, (4), 044706.

43.

Harned, H. S.; Nuttall, R. L. J Am Chem Soc 1949, 71, (4), 1460-1463.

44.

Cheng, L. J.; Guo, L. J. Nano Lett 2007, 7, (10), 3165-3171.


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Figure 1. High current ionic diode with homogenous surface charge (a) Schematic diagram for the high current ionic diode based on asymmetric nanochannel networks membrane (NCNM). (b) Asymmetric cation/anion concentration in the proposed device at the equilibrium and its change right after the external electric fields (forward/reverse bias) are applied. The red solid (green dashed) lines represent the cation (anion) concentration profiles in NCNM. If the inward current is greater than outward current (forward bias), each ion species will accumulate in the NCNM when the system reaches the steady state. On the contrary, if there is more outward


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current than inward current (reverse bias), ion depletion takes place in the NCNM. (c) Top view and (d) SEM image (a cross-section view in the direction of A-A’) of the proposed system fabricated by soft lithography and homogeneously charged nanoparticle assembly.


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Figure 2. (a) Sequential fluorescence images when the mixed fluorescent dyes in 1 mM KCl solution were introduced into deep channel-B for 10 min. (b) The high red intensity (indicating high ion selectivity) in the tip region was preserved after the same mixed fluorescent solution was injected into both deep channels.


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Figure 3. (a) Fluorescence images at different concentrations of KCl bulk solution under the fixed conditions (wt : ww = 10 : 200 µm, Dn = 700 nm). The ratio R is approximately 1, 11, and 35 at the KCl concentration of 0.01 mM,. 1mM, and 10 mM, respectively (b) Fluorescence images by varying the width of the wide-opening (ww) under the fixed conditions, wt = 10 µm, 1 mM KCl, and R ~ 11, with mixed fluorescent dyes.


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Figure 4. (a) Schematic for ionic transport in the asymmetric NCNM. The asymmetric NCNM can be divided into three distinguishable regions with respect to the ion selectivity at equilibrium. The tip region shows the highest ion selectivity and in the bulk region, the co-ion concentrations are relatively higher than that of the tip region. The characteristics of the


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transition region depend on the polarity of the applied voltage. If the forward bias is established, cations and anions are accumulated compared to the equilibrium. In the reverse bias, the both ions are depleted. (b) Normalized subtracted intensity images (I1 − I0 : intensity difference between the equilibrium status and the grey-scaled fluorescence image captured 10 s after applying the voltage) and corresponding cation/anion concentration profiles for equilibrium, forward and reverse bias. (c) Numerical simulation supporting the experimental results. Under these conditions (wherein R is approximately 12 and wt : ww = 10 : 200 µm), the depletion and accumulation of ions were observed in the transition region when the negative and positive potential was applied, respectively.


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Figure 5. (a) I–V curves obtained by varying the electrolyte concentration under fixed conditions wt : ww = 10 : 200 µm and Dn = 240 nm. (b) I–V characteristics by changing the size of nanoparticle (Dn) with fixed conditions of wt : ww = 10 : 200 µm and KCl concentration = 10 mM.


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(c) I–V characteristics by changing ww with fixed conditions of wt = 10 µm, KCl concentration = 10 mM, and Dn = 240 nm.


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Table 1. Maximum currents and rectification ratios from reported nanofluidic diodes (homogeneously charged inner walls)

author,Ref. year

voltage bias

maximum current

[absolute value]

[absolute value]

KCl concentration

frec

Asymmetric nanoscale-geometry with homogeneously charged inner wall Umehara et al.,5 25 mM 2006

1V

Ali et al.,8 2010

1V

100 mM

Zhang et al.,9 100 mM 2012

5V

Gamble et al.,3 100 mM 2014

4V

6 nA

4

(11 nA)**

(5)**

3 nA

14

(100 nA)*

(7)*

12 nA

30

(1190 nA)*

(15)*

80 nA

7

(760 nA)*

(12)*

Asymmetric bath concentrations with homogeneously charged inner wall Cheng et al.,44 0.1 mM || 10 mM 2007 0.1 mM || 100 mM

*

5V

5V

4 nA

2

(240 nA)***

(33)***

10 nA

3.5

(1190 nA)****

(15)****

Measured values from proposed device under same conditions as reported values

**

Measured values from proposed device at ±1 V and 10 mM KCl

***


****



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High Current Ionic Diode Using Homogeneously ... - ACS Publications

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