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A Bioinspired Multifunctional Heterogeneous Membrane with Ultrahigh Ionic Rectification and Highly Efficient Selective Ionic Gating Zhen Zhang, Xiang-Yu Kong, Kai Xiao, Ganhua Xie, Qian Liu, Ye Tian, Huacheng Zhang, Jie Ma, Liping Wen,* and Lei Jiang* Various asymmetric biological ion channels embedded in cell membranes with multiple functions have gained wide attention. Examples include bacterial outer membrane protein F (OmpF porin) (Figure 1a, left), which is a multifunctional channel with ionic selectivity,[1] ionic gating,[2] and ionic rectification properties.[3] These functions are realized cooperatively mainly through the selective tuning of the ionization states of pH-sensitive residues along the channel via different pH stimuli. Thus, OmpF porin can adapt to complicated and changeable pH environments and plays critical roles in life processes, including maintaining intracellular acidity, keeping osmotic balance, participating in ion exchange, etc.[4] Several protein-based nanodevices have important applications in materials science; however, they are not fully compatible with the application requirements because the lipid membranes are difficult to establish. For practical applications, bioinspired heterogeneous membranes, which generally refer to the composite porous membrane formed by the hybridization of two functional membranes with different chemical composition, have drawn enormous research attention because of their simplicity and potential applications in mimicking various functions of biological ion channels. Recent advancements concerning heterogeneous membranes have been achieved through utilizing

Z. Zhang, K. Xiao, G. Xie, Dr. Y. Tian, Prof. L. Jiang Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected] Dr. X.-Y. Kong, Dr. H. Zhang, Dr. J. Ma, Prof. L. Wen, Prof. L. Jiang Laboratory of Bioinspired Smart Interfacial Science Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected]; [email protected] Q. Liu Beijing Key Laboratory of Energy Conversion and Storage Materials College of Chemistry Key Laboratory of Theoretical and Computational Photochemistry Ministry of Education Beijing Normal University Beijing 100875, P. R. China

DOI: 10.1002/adma.201503668

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inorganic materials such as silicon dioxide, titanium dioxide, and mesoporous carbon.[5] Nevertheless, the as-prepared heterogeneous membranes generally only involve a monotonic functionality. In view of the critical roles of multifunctionality in living organisms, achieving various functions in a single piece of membrane is highly desirable. Compared with commonly used inorganic materials with monotonic functional groups, amphiphilic block copolymer (BCP) membranes[6] exhibit superior properties because of their diverse and multifunctional macromolecular blocks. These mobile, stable, and ductile macromolecular groups with different responsiveness can act as smart gatekeepers that manage and constrain the flow of ionic species through the nanoconfined environment. With respect to maintaining high mechanical strength and promoting the functionality of the fabricated membranes, organic/inorganic hybrid heterogeneous membranes represent an ideal candidate. Inspired by the multifunctionality of OmpF here, we prepared a multifunctional heterogeneous membrane by combining an asymmetric block copolymer membrane, polystyrene-b-poly(4-vinylpyridine) (PS48000-b-P4VP20300), and a porous anodic alumina (PAA) membrane (Figure 1a, right). Despite the structure of each channel unit is not nearly as refined as that of the OmpF, the system can realize the multifunctions as what OmpF does in nature. The as-prepared membrane possesses an ultrahigh ionic rectification, with a rectification ratio of ≈489; it also exhibits highly efficient regulatable anion-selective or cation-selective gating implemented through tuning the pH-responsive surface groups, similar to the operating principle of OmpF porins. This porous heterogeneous membrane might have potential applications in the fields of energy conversion system (including concentration cell[5d,7] and fuel cell[8] and filtration system (such as the selective separation of proteins[9] or viruses.[10] Figure 1b shows a schematic of the heterogeneous membrane. The channel diameter of the PAA membrane (bottom layer) is ≈80 nm. The as-prepared BCP membrane (top layer) is composed of a thin layer (≈100 nm in thickness) of hexagonally packed cylindrical channels (diameter, ≈22 nm) above a 1.1 µm thick disordered network-like layer (Figure S1 and S2, Supporting Information). The green segments represent the porous matrix formed by the major component, PS, whereas the blue segments refer to the channels formed by the minor component, P4VP. The formation of the porous membrane includes a microphase separation process that is guided by the solvent gradient (Figure S3, Supporting Information).[11] Figure 1c shows the pH-responsive property of the BCP nanochannels. When

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COMMUNICATION Figure 1. Bioinspired multifunctional heterogeneous membrane. a) Schematic of the biological multifunctional OmpF porin (left) with ionic selectivity, ionic gating, and ionic rectification properties driven by external pH stimuli, and the bioinspired heterogeneous membrane (right) composed of a block copolymer membrane and a PAA membrane exhibiting similar functions. b) Schematic of the heterogeneous membrane. c) Reversible pH-responsive behavior of the BCP nanochannels. d) Liquid-phase AFM images of the BCP membrane immersed in 10 × 10−3 M KCl solutions with pH values varying between 3.0 and 11.0. e) The pI of alumina is ≈8–9, and its charge depends on the environmental pH.

the P4VP chains (pKa ≈ 5.2) on the inner walls were exposed to solutions with pH levels below 5.2, they exhibited a swollen, positively charged, and hydrophilic state; otherwise, they exhibited a collapsed, neutral, and hydrophobic state.[12] The conformational changes of the P4VP chains were confirmed by liquid-phase atomic force microscopy (AFM) characterization (Figure 1d). When the BCP membrane was immersed in an acidic solution (pH 3.0), the porous morphology disappeared as the swollen P4VP chains filled the channels. When the same BCP membrane was placed in an alkaline solution (pH 11.0), the porous morphology appeared as the P4VP chains transformed into collapsed states. When the sample was returned to the acidic solution (pH 3.0), the porous morphology disappeared again, indicating that the BCP nanochannels could reversibly close and open in response to pH stimuli. Moreover, these morphology changes were in good agreement with the cryo-SEM images (Figure S4, Supporting Information). Furthermore, the inherent charged property of P4VP chains under different pH conditions was studied using fluorescence characterization (Figure S5, Supporting Information). Notably, the network-like bottom layer also exhibits a pH-responsive property even though it does not have defined structures.[13] In addition to the BCP membrane, the PAA membrane can also respond to external pH stimuli. The isoelectric point (pI)

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of alumina is ≈8–9 and the hydroxyl groups on the inner surface of the hydrophilic channel can be positively or negatively charged, depending on the external pH (Figure 1e).[14] In our experiments, we exerted symmetric/asymmetric pH stimuli to regulate the charge and wettability states of the two parts of the heterogeneous membrane.[15] Clearly, the PAA membrane used here not only served as a substrate material but also coupled with the BCP membrane to form a charge heterojunction. The ionic transport property of the heterogeneous membrane was examined by current–voltage (I–V) measurements (Figure S7, Supporting Information). Figure 2a shows the I–V curve of the heterogeneous membrane, which was recorded with the membrane immersed in a 10 × 10−3 M KCl solution, integrated with asymmetric pH 3.0/11.0 stimuli (BCP side: pH 3.0). The curve shows apparent diode-like behavior. The ionic current on the positive bias is so small that it almost approaches the x-axis; however, the current greatly increases when the bias is reversed. This diode-like behavior closely resembles the rectifying mechanism of OmpF[3] in an asymmetric pH environment and can be quantified by the current rectification ratio (I−2V/I+2V). The rectification ratio of the heterogeneous membrane is 489 ± 50, which is the highest value reported in an ionic rectifying system.[16] The heterogeneous membrane in this condition holds an opposite charge distribution with



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Figure 2. Ultrahigh ionic rectification. a) I–V curve of the heterogeneous membrane recorded in 10 × 10−3 M KCl solutions integrated with asymmetric pH stimuli of 11.0 (PAA side)/3.0 (BCP side) stimuli. b) Histogram of the rectification ratio calculated from the I–V curves recorded in the same conditions. The separate BCP and PAA membranes were used for comparison. c) Concentration dependence of ionic conductance. The transmembrane ionic conductance (circle) deviated from bulk behavior (dashed line) when the concentration was less than 1 M, indicating charge-governed ionic transport. d) The effect of electrolyte (KCl) concentration on the rectification ratio.

negatively charged PAA channels (hydrophilic) and positively charged BCP channels (hydrophilic). When no bias was present, the K+ and Cl− were predominantly enriched in the PAA and BCP sides, respectively, because of their different charge polarities. Under positive bias, both types of ions would migrate outward from the PAA/BCP junction, which eventually created an ion depletion zone. This depletion zone caused an energy barrier across the junction, which resulted in low conductance. On the contrary, the negative bias caused ion enrichment in the junction, which resulted in a high ion current.[17] The insets in Figure 2a show a concise graphical explanation using an individual heterogeneous channel unit. In order to facilitate discussion, the nanochannel of the BCP membrane was simplified to a cylinder of constant diameter (≈22 nm) with surface charges distributed over the entire width of the membrane.[9,15] When the heterogeneous membrane was replaced by separate PAA and BCP membranes, the rectification ratio greatly decreased to 1.9 and 2.3, respectively (Figure 2b). The effect of the KCl concentration on the ionic conductance and on the rectification ratio of the heterogeneous membrane in this asymmetric pH condition was also tested. The measured transmembrane ionic conductance (Figure 2c, circle) deviated from bulk values (dashed line) when the KCl concentration was less than 1 M, indicating charge-governed ionic transport inside the channel of the heterogeneous membrane.[18] The

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rectification ratio achieved maximum values of ≈500 at intermediate concentrations ranging from 1 × 10−3 to 10 × 10−3 M, whereas the ratio greatly decreased in the regions of low and high KCl concentrations (Figure 2d). This variation trend in the ratio is consistent with previous theoretical predications[19] and experimental demonstrations.[20] The ultrahigh ionic rectification is supported by a numerical simulation based on Poisson and Nernst–Planck (PNP) equations.[21] The total length of all the simulated models was uniformly set to 4000 nm (Figure S9, Supporting Information).[5d] The concentration profile of the positively charged BCP channels (channel width, 22 nm) was homogeneous irrespective of the bias polarity (Figure 3a,1). The absolute values of the current density along the axial line also did not show any obvious changes even though the bias polarity was reversed (Figure 3b, red squares and purple circles), implying that they did not rectify. Remarkably, after a negatively charged PAA channel (channel width, 80 nm) was coupled to form an asymmetric heterojunction, the negative currents (Figure 3c, red squares) increased sharply and the positive currents (Figure 3c, purple circles) substantially decreased and leveled off close to the x-axis. This contrast was caused by the remarkable ion enrichment at the negative bias and ion depletion at the positive bias at the PAA/BCP interface (Figure 3a,2). The calculated rectification ratio was 325. Moreover, the rectification ratio varied as


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COMMUNICATION Figure 3. Numerical simulations of the ultrahigh ionic rectification (in 10 × 10−3 M KCl). a) The calculated ion concentration profiles of four 2D models reveal that the ultrahigh ionic rectification is induced by the remarkable ion enrichment at negative bias and ion depletion at positive bias. b) The current density distribution along the axial line of the separate BCP (model 1) and PAA (model 4) channels. c) The current density distribution along the axial line of the asymmetric heterojunction (models 2 and 3). The position x = 0 nm refers to the BCP side.

a function of the PAA channel length (Figure S10, Supporting Information). When the length percentage of the PAA channel reached 80%, the rectification ratio reached the maximum value of 454, accompanied with visibly greater ion enrichment (Figure 3a,3) and larger negative currents (Figure 3c, blue triangles). The electrical potential distribution in Figure S11 (Supporting Information) also helped us understand the ion enrichment and depletion phenomena. When the length percentage of the PAA channel reached 100%, switching of the voltage bias did not affect the corresponding concentration profile (Figure 3a,4) or the current absolute values (Figure 3b, blue triangles and yellow stars), indicating that the negatively charged PAA channel also did not rectify. The formation of an asymmetric electrostatic heterojunction was the key reason for the observed ultrahigh ionic rectification.[22] Notably, the asymmetric chemical composition also accounts for the ultrahigh rectification.[23] The effect of the BCP channel diameter on the ionic current rectification was also simulated (Figure S12, Supporting Information). When the diameter of the BCP nanochannel was 15 nm, the highest achieved rectification ratio reached 550. If the experimental measured ionic current of the heterogeneous membrane is normalized by the estimated number of channels, the calculated ionic current is ≈0.016 pA (−2 V), which is smaller than the reported single channel systems.[24] The reduced ionic current can be ascribed to many factors. The most important one is their difference in ionic diffusion mechanism into the channel.[25] In the interior of the nanochannel, the ionic transport is governed by surface charge

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(Figure 2c),[26] while in the solution ranges near the entrance and exit of each nanochannel, the ion motion is diffusion controlled.[27] When half of the pore center-to-center distance (rc) is larger than the diffusion layer thickness, the spherical diffusion layer near each channel entrance will not overlap, and each channel behaves independently. Otherwise, the diffusion layer of each channel will overlap resulting in a linear diffusion into the channels, which will lead to a large decrease of ion flux to an individual channel in comparison to an isolated channel.[28] In our systems, the BCP membrane pores are densely packed (Figure 1d), and half of the pore center-tocenter distance (rc ≈ 40 nm) is much smaller than the diffusion layer thickness which is in the hundred nanometers range.[29] Thus, the diffusion layers will heavily overlap and the linear diffusion is dominant, which would be the reason for the reduced ionic current. In addition to ultrahigh ionic rectification, the heterogeneous membrane also exhibited highly efficient selective ionic gating. When the heterogeneous membrane was placed in an alkaline solution of pH 11.0 (Figure 4a, triangles), a low ionic conductance (2 V, 0.15 µA) was obtained. Here, low conductance is defined as the “OFF” state, whereas high conductance is defined as the “ON” state. In this condition, the PAA channel was negatively charged and hydrophilic, whereas the BCP channel was neutral and hydrophobic (Figure S13, Supporting Information). In this case, the wettability difference between the PAA and BCP channels formed an obvious wettability gradient. In a nanoconfined region where a wettability gradient exists, ions are generally believed to permeate from the



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Figure 4. Regulatable selective ionic gating. a) Anion gating. The I–V curves of the heterogeneous membrane when pH value of the electrolyte solution was increased from 3.0 to 11.0. The anode faced the BCP side. b) Explanation of the anion-gating behavior. c) Cation gating. The I–V curves of the heterogeneous membrane when the pH value of the PAA side was increased from 3.0 to 11.0 (the pH value of the BCP side was fixed at 11.0). The anode faced the PAA side. d) Explanation of the cation-gating behavior. All the electrolyte solutions were 10 × 10−3 M KCl.

hydrophilic zone to the hydrophobic zone.[22,30] With respect to the direction of the applied voltage, the transmembrane transport of cations (K+) was required to overcome the unfavorable wettability gradient; thus, the anions (Cl−) became the major current carriers under this condition. The low ionic conductance is ascribed to the effect of the negative surface charge of the PAA channel, which hindered the anion (Cl−) transport (Figure 4b,1).[31] When the pH was reduced to 7.0 (Figure 4a, squares), the current increased to 1.5 µA (2 V), implying that the ionic gate was in the “ON” state. The inversion of the surface charge polarity of the PAA channel was responsible for the increased ionic current (Figure 4b,2). Furthermore, we also demonstrated that the pH could tune and amplify the electronic readout in the “ON” state. When the pH value was reduced to 3.0 (Figure 4a, circles), the current increased substantially (2 V, 8.3 µA). The transition of the BCP channel from hydrophobicity to hydrophilicity was the key reason for the substantial increase of the ionic current (Figure 4b,3). Notably, this selective pH-gated ionic transport is due to the cooperative effect of the charge and the wettability gradient. The heterogeneous membrane also exhibits an efficient cation-gating property. The pH value in the BCP side was fixed to 11.0. When the pH in the PAA side was 3.0 (Figure 4c, circle), the ionic gate was in the “OFF” state (2 V, 0.048 µA). Under this condition, an obvious wettability gradient was also present between the PAA channel (positively charged, hydrophilic) and the BCP channel (neutral, hydrophobic). In this case, the cations (K+) became the major current carriers. Similarly, the positive surface charge polarity of the PAA channel restricted the transport of the cations (K+) which would result in the “OFF”

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state (Figure 4d,1).[31a,31b] When the pH in the PAA side was increased to 7.0 (Figure 4c, squares), the ionic gate switched to the “ON” state. As the pH was increased, the deprotonation process increased the amount of neutral hydroxyl groups in the PAA channel, resulting in a “less positive” surface. The increase in the ionic current (2 V, 0.7 µA) is ascribed to the weakening of the charge screening effect (Figure 4d,2). Remarkably, a significant increase in the ionic current (2 V, 3.1 µA) was also observed when the pH was changed to 11.0 (Figure 4c, triangles). This greatly increased ionic current is ascribed to the inversion of the surface charge polarity of the PAA channel from positive to negative, which promotes the cation (K+) transport (Figure 4d,3). Notably, the influence of the wettability is greater than that of the charge, as indicated by this greatly increased ionic current still being lower than that measured when the PAA channel is in a hydrophilic state (Figure 4a, circles). The chemical actuation of the anion or cation gate from the “OFF” to the “ON” state promoted an ≈98.5% increase in the channel conductance (Figure S14, Supporting Information), which is larger than that observed in OmpF, i.e., an ≈85% increase in channel conductance when opening their channels.[32] In summary, we have demonstrated a bioinspired multifunctional heterogeneous membrane capable of achieving ultrahigh ionic rectification and highly efficient cation-selective gating and anion-selective gating. These functions are implemented through selective tuning of the pH-responsive surface groups via different pH stimuli, and the multifunction is simultaneously and cooperatively dominated by the charge and the wettability. We expect that greater levels of multifunctionality can


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www.advmat.de www.MaterialsViews.com Natural Science Foundation (Grant Nos. 21171171, 21434003, 91427303, 21201170, 91127025, and 21421061), and the Key Research Program of the Chinese Academy of Sciences (KJZD-EW-M03).

Experimental Section

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Materials: The PAA membrane was purchased from Hefei Pu-Yuan Nano Technology, Ltd. PS48000-b-P4VP20300 (Mw/Mn = 1.13) was purchased from Polymer Source, Inc. (Canada). Cellulose acetate was purchased from Sigma-Aldrich. Other chemicals were analyticalgrade reagents and all solutions were prepared using Milli-Q water (18.2 MΩ cm). Membrane Fabrication: To fabricate the heterogeneous membrane, a solution of BCP molecules in dichloromethane was spin-coated onto a porous alumina membrane, thereby forming an asymmetric membrane via microphase separation (Figure S2, Supporting Information). To prevent the BCP solution from penetrating into the alumina pores, cellulose acetate was filled into the PAA membrane beforehand and could be dissolved using acetone after the heterogeneous membrane was successfully prepared (details are given in the Supporting Information). Current–Voltage Recordings: The ionic transport property of the heterogeneous membrane was examined by I–V measurements using a commercial Keithley 6487 picoammeter (Keithley Instruments, Cleveland, OH). The electrochemical testing setup is shown in Figure S7 (Supporting Information). The heterogeneous membrane was mounted between the two chambers of the conductivity cell. Both halves of the cell were filled with KCl solutions. The pH of the KCl solutions was adjusted using 1 M HCl/KOH. A scanning voltage varying from −2 V to +2 V was applied through Ag/AgCl electrodes as the transmembrane potential. The process and conditions of all the measurements mentioned in this article were the same unless otherwise specified. Numerical Simulation: The ultrahigh ionic rectification property was systematically analyzed on the basis of coupled PNP equations. The model simulations were carried out with the commercial finiteelement software package COMSOL (version 4.4) Multiphysics using the “electrostatics (Poisson equation)” and “Nernst–Planck without Electroneutrality” modules. The simulation model used to analyze the ultrahigh ionic rectification phenomenon is shown in Figure S9 (Supporting Information). To obtain an affordable computation scale, we used a 22 nm wide cylindrical nanochannel array to simulate the nanochannels inside the BCP membrane and an 80 nm wide cylindrical nanochannel to simulate the nanochannel of the PAA membrane. Two reservoirs were added because the magnitude of the entrance or exit mass transfer resistance could influence the overall transport of ions, and this influence could not be neglected. The corresponding ion concentration, electrical potential, and current distributions in the fluid were obtained by solving the PNP equations.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the National Research Fund for Fundamental Key Projects (Grant Nos. 2011CB935702 and 2011CB935703), National

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be implemented by optimizing the composition of the used BCP, thereby opening new applications in energy conversion, filtration, desalination, etc. Moreover, these results also provide a basic platform to imitate and assemble various ion transport functions in living organisms and move one further step toward the development of “smart” multifunctional heterogeneous membrane systems for real-world applications.

Received: July 29, 2015 Revised: August 27, 2015 Published online: November 9, 2015



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