Ionic Current Rectification by Laminated Bipolar Silica Isoporous

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Ionic Current Rectification by Laminated Bipolar Silica Isoporous Membrane Fei Yan, Lina Yao, Qian Yang, Kexin Chen, and Bin Su Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04639 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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Analytical Chemistry

Fei Yan,||,† Lina Yao,|| Qian Yang, Kexin Chen and Bin Su* Institute of Analytical Chemistry, Department of Chemistry, Zhejiang University, Hangzhou, 310058, China ABSTRACT: Ionic current rectification (ICR) is one of interesting characteristics displayed by nanochannels with asymmetric geometry, ionic concentration or charge distribution, which has been utilized for the development of chemical sensors and biosensors. Herein we report the ICR phenomenon observed with ultrathin silica isoporous membrane (SIM), which was prepared by laminating two layers of SIM with opposite charges and different pore dimeters, designated as bipolar SIM (bp-SIM). The negatively charged layer, called as n-SIM, was 86-nm-thick and consisted of channels with a diameter of 2–3 nm. The positively charged layer with a thickness of 59 nm, termed as p-SIM, was comprised of channels of 4.5–5.5 nm in diameter. They were primarily grown on the solid surface using the Stöber-solution and biphasic-stratification growth approaches, respectively, and then exfoliated to obtain perforated structures by the polymer-protected chemical etching and transfer method. The negative charges of n-SIM and positive ones of p-SIM were generated by the deprotonation of pristine surface silanol and post-modified ammonium groups, respectively. Neither n-SIM nor p-SIM alone displays the ICR characteristic, because of their symmetric structure and uniform charge distribution. When laminating two of them, an apparent ICR characteristic was observed for the bp-SIM with a typical diode-like currentvoltage response. This behavior was rationalized to arise from the asymmetric charge distribution on two layers by finite element simulations. Considering the facile preparation and diverse surface functionalities, as well as its uniform and highly porous structure, the bp-SIM provides an attractive platform for designing ICR-based sensors.

Ionic current rectification (ICR) is a unique ion transport characteristic displayed by biological and artificial solid-state nanochannels, which has attracted considerable research attention recently.1-5 It arises from the asymmetric transport of cations and anions under different potential polarities, displaying phenomenologically a nonlinear current-potential (I–V) response. In general, ICR is generated for artificial nanochannels with broken factors, including asymmetric geometry, asymmetric ion concentration, asymmetric surface charge distribution or the combination of them.6-10 Nanochannels with asymmetric surface charge distribution, known as bipolar nanochannels, refer to those carrying both positive and negative charges on different parts of channel surface. ICR by bipolar nanochannels have shown great potential in the fields of sensors and energy conversion.11 Many theory and simulation works have been carried out to rationalize ion transport and ICR in bipolar nanochannels.12-26 Fabrication of new bipolar nanochannels with ICR property and tunable surface functionalities is of great interest in extending their sensing, biomimetic and energy conversion applications. In general, the preparation of bipolar nanochannel falls into two categories, including asymmetric surface modification and exploitation of heterogeneous materials. The former method includes direct asymmetric modification with functional molecules,27,28 packing suspended nanoparticle crystal with different groups,29 atom transfer radical polymerization30 and atomic layer deposition.31 But this method is limited to nanochannels with a high aspect ratio that is difficult to be precisely controlled. The latter one is to construct heterogeneous membranes by physical or chemical laminating oppositely charged

materials, such as block copolymer (BCP)/anodic alumina (Al2O3),32 BCP/polyethylene terephthalate (PET),33 mesoporous carbon/Al2O3,34 Al2O3/SiO2,35,36 and two layers of PET,37 BCP,38 or Al2O3.39 The thickness of these bipolar nanochannel membranes is mostly on the micrometer scale. To the best of our knowledge, ICR by ultrathin bipolar nanochannels has been less studied. One of primary objectives of studying ICR is to understand the rectified ion transport across ion channels on biomembranes, which have a thickness of a few nanometers. Wu et al. have fabricated bipolar Al2O3/SiO2 heterogeneous nanochannel structure with a thickness of ca. 400 nm.40 However, the fabrication process involved the use of reactive ion etching and pattern transfer that either are expensive or need harsh condition. In this work, ultrathin silica isoporous membrane with asymmetric surface charges (designated as bipolar SIM, bpSIM) was prepared by simply laminating two perforated layers of SIM with opposite surface charges and different pore sizes together. Because the pore size of both layers is within the Debye screening length, they are highly permselective toward oppositely charged ions, respectively, that is, selectively favor the transport of counter-ions but block that of co-ions. Therefore, asymmetric permselectivity of two layers in bp-SIM led to the rectified ion transport, generating a diode-like current signal in response to the externally biased voltage. Scheme 1 shows the preparation of ultrathin and perforated bp-SIM consisting of one layer carrying negative surface charges, designated as n-SIM, and the other bearing positive surface charges, designated as p-SIM. To prepare p-SIM, the membrane was firstly grown on the indium tin oxide (ITO) 2

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glass using the biphasic-stratification method (see Scheme S1 and supporting information for details).41 It was subsequently exfoliated to obtain perforated structure using the method reported recently,42 which involved the sequential operations of poly(methyl methacrylate) (PMMA)-protected chemical etching, transfer onto silicon nitride (SiN) window, dissolution of PMMA and surface modification with N-trimethoxysilylpropyl-N,N,N-trimethylammonium (TMA+) (see Scheme S2 and supporting information for details). As shown in the top-right corner of Scheme 1, thus obtained structure was designated as p-SIM/SiN, consisting of perforated nanochannel array with a positively charged surface positioned on the top of

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a single micrometer-sized SiN pore. Given the pristine SIM surface was negatively charged due to deprotonation of silanol groups, the post-modification of TMA+ converted the surface charge state from negative to positive. As for n-SIM, it was primarily grown on the ITO glass using the Stöber-solution growth approach.43 It was exfoliated from the ITO surface using the same approach as described above. More details can be found in Schemes S3, S4 and supporting information. Finally, n-SIM was overlaid onto the p-SIM/SiN. Two layers of SIM can form chemical bonding at the interface to obtain stable bp-SIM/SiN, as shown in Scheme 1.

Scheme 1. Illustration of the preparation of bp-SIM by chemical etching of electrode-supported SIM under protection with poly(methyl methacrylate) (PMMA), mechanical supporting of positively charged SIM (p-SIM) with porous silicon nitride (SiN) and overlaying negatively charged SIM (n-SIM).

As characterized by electron microscopy (see images in Figure S1, supporting information), both n-SIM and p-SIM possess straight and closely packed nanochannels. n-SIM is 86nm-thick and consists of channels with a diameter of 2–3 nm. p-SIM with a thickness of 59 nm is comprised of channels of 4.55.5 nm in diameter. Note that the channel diameter of pSIM was estimated by subtracting the molecular size of TMA+.44,45 Figure S2 shows the top-view SEM images of bare SiN and SiN-supported SIM, namely p-SIM/SiN, n-SIM/SiN and bp-SIM/SiN. As revealed in Figure S2a, a single pore of 6 m in diameter was found in the center of bare SiN. After supporting SIM, the 6-m-diameter pore of SiN was fully covered without any cracks but remained visible, due to ultrasmall thickness of SIM.

Figure 1. CVs of the bp-SIM/ITO (solid curve) and bare ITO (dashed curve) electrode in 1 mM KCl (a) and 0.5 M KCl (b) containing 0.2 mM Ru(NH3)63+ (yellow curve) and Ru(CN)64 (green curve). The scan rate was 50 mV s1 and the electrode area was fixed as 0.25 cm2. (c) Illustration of the permselective transport of Ru(NH3)63+ and Ru(CN)64 at the bp-SIM/ITO electrode at a low (left) and high (right) ionic strength.

The ion selectivity and permeability of SIM were examined by cyclic voltammetry (CV) using two oppositely charged 2


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Analytical Chemistry redox species, namely Ru(NH3)63+ and Fe(CN)63–, as probes. Figure S3 compares CVs obtained with n-SIM/ITO and pSIM/ITO electrodes. Both Ru(NH3)63+ and Fe(CN)63– could transport to the underlying ITO electrode surface through nanochannels and subsequently be reduced/oxidized thereby to produce electrical current. In comparison with the bare ITO, at the n-SIM/ITO a comparable current magnitude was observed for Ru(NH3)63+ but a much suppressed one for Fe(CN)63– (see Figure S3a), indicating cation selectivity of nSIM (as exemplified in the left side of Figure S3c). In the case of p-SIM/ITO, an opposite phenomenon was observed, as shown in Figure S3b. The p-SIM was permselective to anions, as exemplified in the right side of Figure S3c. The permselectivity arises from the prominent overlap of electrical double layer (EDL) in the radial direction of nanochannels. In order to explore the permselectivity of bp-SIM, an ITO electrode was used as the support instead of SiN window to prepare bp-SIM/ITO. In this case, the transport of anions (cations) through the channels of bp-SIM experienced electrostatic repulsion (attraction) from top n-SIM layer and electrostatic attraction (repulsion) from bottom p-SIM layer. Intriguing electrochemical behaviors dependent on the solution ionic strength were displayed. As shown in Figure 1a, at a low ionic strength (1 mM KCl), an enhanced voltammetric response was observed for anionic Ru(CN)64– in comparison with a bare ITO, whereas the phenomenon was reversed for cationic Ru(NH3)63+. It suggests that surface charges on p-SIM determine the overall permselectivity of bp-SIM. Note that the EDL in nanochannels (both n-SIM and p-SIM) overlaps at this low ionic strength, thus possessing high permselectivity.46 Nevertheless, in the case of Ru(CN)64–, the electrostatic attraction by bottom p-SIM can eventually exceed the resistance by the top n-SIM (as exemplified in the left panel of Figure 1c). As for Ru(NH3)63+, although the top n-SIM layer exerts an electrostatic attraction, it can only access close to the laminating boundary between n-SIM and p-SIM because of electrostatic repulsion by the bottom p-SIM. Upon increasing the ionic strength (e.g., 0.5 M KCl), the current responses of two probes at the bp-SIM/ITO are similar to those at a bare ITO (as shown in Figure 1b), suggesting the loss of permselectivity due to shrinking EDL effect (as exemplified in the right panel of Figure 1c).46 Due to the asymmetric surface charge distribution in bpSIM, ionic rectification behavior was observed. The experimental device used was described in Scheme S5. bp-SIM was fixed in between two compartments containing the same concentration of KCl solution (cb). By applying voltage (V) via two Ag/AgCl electrodes, the ion transport across bp-SIM produced electric current, which was detected by picoammeter. As shown in Figure 2a, when V > 0, the electric field in the axial direction drives the transport of K+ from right to left. K+ in the orifice of n-SIM is simultaneously subject to the electrostatic attraction by channel walls of n-SIM and repulsion by positive charges of p-SIM. The same situation also applies to Cl in the reverse direction. This case ultimately leads to the accumulation of total ion concentration inside nanochannels of bp-SIM and thus a high conductance. In contrast, when V < 0, both external electrical field and surface charge on bp-SIM promote the ion depletion inside nanochannels, resulting in a low conductance. Therefore, the current magnitude obtained at positive voltages is larger than that at negative voltages, giving rise to the typical ICR behavior. Phenomenologically, it was illus-

trated as an asymmetric current–voltage (I–V) curve, as shown by the red dots in Figure 2b. For comparison, I–V curves of nSIM and p-SIM were also measured under the same condition (black and blue dots). As can be seen, only linear and symmetric responses were observed, due to the symmetric charge distribution on channel surfaces.

Figure 2. (a) Scheme of ion transport across a nanochannel inside bp-SIM with opposite charge state and different size under different polarities of applied potential. The left reservoir is grounded and V is applied to the right reservoir. cb refers to the concentration of KCl in the reservoir. (b) IV curves of n-SIM, p-SIM and bp-SIM at a concentration of 1 mM. (c) Rectification ratio of bpSIM at different concentrations of KCl. (d) Numerical simulation of total ion concentration profiles inside bp-SIM at the bias voltage of +1 V and 1 V. cb is set as 1 mM.

The rectification ratio, defined as |I(+1V)/I(–1V)|, was used to describe the efficiency of ICR by bp-SIM. We first investigated the solution pH effect on the ICR behavior. The surface charge state of n-SIM is dependent on the solution pH as the isoelectric point of silanol group is ca. 23.42 At pH 3.00, the surface charge density of n-SIM was significantly reduced or approaches zero, whereas that of p-SIM remained. In this case, the degree of surface charge asymmetry decreased, thus the total ion accumulation or depletion inside the membrane became weaker than that at pH 5.76, ultimately leading to the decrease of rectification ratio (see Figure S4). Note that n-SIM and p-SIM also have a slight asymmetry in structure, which may also contribute to the ionic current rectification. Figure 2c summarized the rectification ratio at various concentrations of KCl at pH 5.76, with corresponding I–V curves shown in Figure S5. It was maintained at ca. 3.0 in the concentration range from 104 M to 101 M and decreased slightly at 0.3 M. This decrease can be attributed to the weak permselectivity at a high electrolyte concentration. Figure S6 shows the rectification ratio of bp-SIM under different applied voltages. It can be seen that the rectification ratio decreases with increasing the applied voltage. This is because the potential barrier for co-ions at the entrances diminishes and results in the decreased ionic selectivity.47 Moreover, in comparison with thicker bipolar membranes,32-34 the rectification ratio obtained with bp-SIM is not that high. In order to understand the ion


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transport process inside ultrathin bp-SIM nanochannels, we compare the conductance of bare SiN, bp-SIM/SiN and bpSIM at various concentrations of KCl (Figure S7). Typical saturation plateau in the low concentration range was observed for bp-SIM, indicating the surface-charge-governed ion transport inside the nanochannels of bp-SIM. Finite element simulation based on coupled PoissonNernst-Planck (PNP) equations was also carried out to quantitatively estimate the ion concentration distribution and electric field inside bp-SIM. The details on the simulation model and boundary conditions were given in supporting information (See Figure S8 and Table S1). Figure S9a shows the calculated 2D concentration distribution of K+ and Cl inside bipolar nanochannels when the concentration of KCl was set to 1 mM in reservoir solutions. Figures S9b and S9c plot the concentrations of K+ and Cl, and electric field (Ez = /z) along the symmetry axis (red line shown in Figure S9b-I from position  to ) of bipolar channel under different applied voltages (1.0 V, 0 V and 1.0 V). When V = 0 V, we can clearly see the accumulation counterions, namely K+ and Cl, inside n-SIM and pSIM, respectively (Figure S9b-II), proving that bp-SIM is highly ion-permselective due to overlapped EDL at the concentration of 1 mM,46 although the total length of bp-SIM is relatively short (~150 nm). Moreover, at the entrance of nanochannels there exist apparent accumulation of counter-ions and depletion of co-ions, leading to negative overshoot of Ez (its direction is opposite to the external electric field). At the boundary between n-SIM and p-SIM, a sharp positive overshoot of Ez is observed, because the amount of accumulated K+ in n-SIM exceeds that of Cl in p-SIM, as shown in Figure S9cII. This positive overshoot turns more significant when V = 1.0 V, because a remarkable depletion of Cl occurs (see Figures S9b-I and S9c-I). On the other hand, the total ion concentration inside bp-SIM, in particular inside p-SIM, decreases obviously (as shown in Figure 2d and S9b-I), leading to a low conductance. When V = 1.0 V, both K+ and C are accumulated inside bp-SIM (as shown in Figure 2d and S9b-III), leading to the increase of total ion concentration and thus a high conductance. The positive overshoot of Ez at the boundary of nSIM and p-SIM is much smaller in this case (Figure S9c-III), because the concentration of K+ is only slightly higher than that of Cl. These calculated results support the scenario in Figure 2a and the experimentally observed ICR in IV curves (red dots in Figure 2b and Figure S5). Above results indicate that the bp-SIM has asymmetric surface charge distribution and thus obvious ion selectivity. Although the rectification ratio obtained with the present ultrathin bp-SIM is not so high as those long bipolar channels reported previously,32-34 the ultrasmall thickness of bp-SIM provides the possibility of understanding rectified ion transport across biological channels, which are only a few nanometers in thickness. On the other hand, bp-SIM on the transparent electrodes is hopeful for studying single charged molecule electrochemistry with a high resolution. Recently, Lu et al. have reported the use of ultrathin SIM (~100 nm) on ITO for capturing single redox events.48 Benefiting from the confined space of SIM, the diffusion rate of single molecule could be remarkably decreased, thereby allowing real-time imaging of single redox events with total-internal reflection fluorescence. We speculate that bp-SIM can further increase the resolution, because the ICR results obtained in this work

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suggest that charged molecules can be accumulated in the nanochannel center of bp-SIM. In summary, we have fabricated an ultrathin bp-SIM by laminating two layers with opposite charges. Due to asymmetric surface charge distribution and confined space of nanochannels, bp-SIM displays ICR phenomenon. The preparation of bp-SIM is convenient and low cost, which can be potentially used for various applications. For examples, by introducing functional groups to the channel wall, the bp-SIM can be developed for the construction of novel analytical sensors based on the variation of rectification ratio and direction. It can be also employed to pre-concentrate charged molecules inside the narrow space of nanochannels on the transparent electrode to improve the resolution of single molecule electrochemistry.

The Supporting Information is available free of charge on the ACS Publications website. Experimental details, preparation and characterization of n-SIM, p-SIM and bp-SIM, more I−V curves for various concentrations of KCl, numerical simulation for ion concentration distribution inside bp-SIM (file type, i.e., PDF) †Department of Chemistry, Zhejiang Sci-Tech University, 5 Second Avenue, Xiasha Higher Education Zone, Hangzhou, 310058, PR China

*Email: [email protected] Bin Su: 0000-0003-0115-2279 ||

F.Y. and L.Y. contributed equally to the work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest.

This work is supported by the National Natural Science Foundation of China (21575126, 21335001) and the Zhejiang Provincial Nature Science Foundation (LZ18B050001).

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Ionic Current Rectification by Laminated Bipolar Silica Isoporous

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