Electrostatic-Charge- and Electric-FieldInduced Smart Gating for Water Transportation Kai Xiao,† Yahong Zhou,‡ Xiang-Yu Kong,‡ Ganhua Xie,† Pei Li,† Zhen Zhang,† Liping Wen,*,‡ and Lei Jiang*,‡ †
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Institute of Chemistry, and Key Laboratory of Bioinspired Smart Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China
‡
S Supporting Information *
ABSTRACT: Regulating and controlling the transport of water across nanochannels is of great importance in both fundamental research and practical applications because it is difficult to externally control water flow through nanochannels as in biological channels. To date, only a few hydrophobic nanochannels controlling the transport of water have been reported, all of which use exotic hydrophobic molecules. However, the effect of electrostatic charges, which plays an indispensable role in membrane proteins and dominates the energetics of water permeation across aquaporins, has not gained enough attention to control water transport through a solid-state nanochannel/nanopore. Here, we report electrostatic-charge-induced water gating of a single ion track-etched sub-10 nm channel. This system can directly realize the gating transition between an open, conductive state and a closed, nonconductive state by regulating the surface charge density through a process that involves alternating capillary evaporation and capillary condensation. Compared to the introduction of exotic hydrophobic molecules, water gating controlled by electrostatic charges is simple, convenient, and effective. Such a system anticipates potential applications including desalination, controllable valves, and drug delivery systems. KEYWORDS: nanochannel, nanopore, gating, water transportation, ion transportation
W
nucleation sites for bubbles, which can eventually span the cross section of the channel and block the water flow.4,6,13 However, the effect of electrostatic charges, which plays an indispensable role in the biological water channels and dominates the energetics of water permeation across aquaporins,14,15 has not been studied well to control water transport in solid-state nanochannels as biological aquaporins. Therefore, it is important and valuable to study the relation between the conductive/nonconductive state and the surface charge of the solid-state gating. Here, we report electrostatic-charge- and electric-field-induced smart gating not only for ions but also for water by measuring the ionic current through it (Figure 1A). Such a system can directly achieve the gating transition between an open, conductive state and a closed, nonconductive state by regulating the surface charge density and external electric field through a process that involves vapor−liquid phase transitions (Figure 1B).
ater is a prerequisite composition in organisms, and the transportation of water across cell membranes is the foundation of many physiological processes.1 For instance, the renal water conservation process is associated with water absorption and release, while regulation of body temperature can be achieved by redistributing water between tissues with remarkable speed and accuracy through water channels. Thus, it is essential to understand and characterize the transportation behavior of water in nanochannels. Furthermore, controlling water transport across a nanochannel is of great importance for seawater desalination, controllable valves, and drug delivery systems.2−5 In biological water channels, the wetting/dewetting process based on the hydrophobic lining of the channel walls was recognized as a key mechanism.6−8 With regard to solid-state water gating, many mechanisms have been experimentally and theoretically investigated.4,5,9,10 One such study revealed that the hydrophobic surface is necessary for spontaneous dewetting in nanochannel membrane filters.11,12 Specifically, hydrophobic patches existing in the nanochannels are thought to act as © 2016 American Chemical Society
Received: August 23, 2016 Accepted: September 20, 2016 Published: September 20, 2016 9703
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Figure 1. Smart gating is based on the pH of the electrolyte solution, which can regulate the surface charge density of the nanochannel. (A) Side view of the gating schematic diagram. (B) Top: Space-filling models of conductive and nonconductive states. Bottom: Mechanism of water evaporation and condensation. (C) Top view scanning electron microscopy image of a sub-10 nm channel. (D) Reversible switching of the nanochannel between nonconductive (pH 2.8 and 7) and conductive (pH 10) states at 1 V. (E−G) Confocal microscopy images of the membrane with multi-nanochannels exposed to fluorescent dye solution (sulfonated rhodamine) with different pH (2.8, 7, and 10) and cartoon drawing of electrolyte solution in the nanochannel.
a fragmentary fluorescence signal existed in the nanochannel that could be explained by the process of alternating capillary evaporation and capillary condensation, which also proved the gating was in a transition state (Figure 1F). At pH 10, the intense fluorescence signal observed in the nanochannels indicated that the gating was opened drastically (Figure 1G). These results confirmed that this smart gating could cause the conduction of water under high density electrostatic charges while rejecting water from the channel under low density electrostatic charges (Supporting Information Figure S6), which verified the inherent feature of the electrostatic-charge-induced gating properties. Similar with the hydrophobic channels with a threshold voltage,4,21 the electrostatic-charge-induced smart gating of a single sub-10 nm channel can be switched from a nonconductive state to a conductive state only when the applied electric field was sufficiently strong. As discussed above, the gating was closed tightly at pH 2.8, even when the voltage was as high as 8 V (Figure 2A), which also indicated that nanobubbles existed in the channel in this condition.22 It was completely opened at pH 10, even at a low external voltage (1 V), and exhibited increasing current as the voltage increased (Figure 2B). By contrast, the gating was in a transition state at pH 7, and the conductive and nonconductive states could be tuned by regulating the external voltage. As shown in Figure 2C, the gating was initially closed but could be opened at 1 V due to the hysteresis of the voltage response. Remarkably, excellent reproducibility of the gating and almost instantaneous switching were demonstrated at 1 V after standing for 0.5 h because there was an equilibrium between water evaporation and condensation in the nanochannel. It is worth noting that the gating could not be opened at 1 V after standing for 1 h because the existing nanobubbles grow up slowly with the water evaporation. However, the gating could be opened instantaneously and remain in a stable conductive state
RESULTS AND DISCUSSION In this experiment, single nanochannels with a diameter from ∼3 to ∼400 nm were prepared in a 12 μm thick polyethylene terephthalate (PET) membrane using a well-developed tracketching technique (Supporting Information Figures S1−S3).16−18 As a result of etching, the channel surface contained carboxyl groups at a density of ∼1 nm−2, which endows the surface with a regulated charge property by the electrolyte solution.19,20 We found that a single nanochannel with a sub-10 nm diameter (Figure 1C) exhibited electrostatic-charge-induced gating properties in 0.1 M KCl solutions (pH 2.8, 7, and 10) and that reversible processes were observed as switches between conductive and nonconductive states (Figure 1D). At pH 2.8, the inner surface was electroneutral, and thus, the nonconductive, closed state was observed. After the pH 2.8 electrolyte solution was changed to pH 7 electrolyte solution, weakly negative charges were introduced onto the nanochannel surface, whereas the closed state was retained. Nevertheless, a stable ionic current of approximately 100 pA was measured when the pH 7 electrolyte solution was changed to pH 10 electrolyte solution, in which condition more negative charges were introduced onto the nanochannel. These switches between conductive and nonconductive states occurred as the electrostatic charges were changed, indicating that the smart gating possessed excellent reversibility and stability (Supporting Information Figures S4 and S5). When exposed to fluorescent dye solution (sulfonated rhodamine, pKa = 1.8) at different pH values (pH 2.8, 7, and 10), the confocal microscopy images of the membrane with multinanochannels provided further evidence of the smart gating. At pH 2.8, there was no fluorescence signal in the nanochannels (Figure 1E) because the fluorescent dye solution could not enter the nanochannels ascribed to the existing of nanobubbles. At pH 7, 9704
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Figure 2. Typical examples of smart gating regulated by voltage at various pH values. (A) At pH 2.8, the nanochannel is electroneutral and cannot be infiltrated, even at a high external voltage (8 V). (B) At pH 10, the nanochannel is densely negatively charged and conductive at a low external voltage (1 V). (C) At pH 7, the nanochannel is negatively charged with a low density and is in a critical state, which can be switched reversibly between conductive and nonconductive states depending on voltage.
Figure 3. Different effects of the electrostatic charge with varied diameters. (A) Different effects of the electrostatic charge. (B) Transmembrane ionic conductance deviates substantially from the bulk value, indicating a surface-charge-governed ion transport of the gating with the sub-10 nm channel. (C) Critical state of the smart gating can be realized at pH 10 when the diameter is approximately 3 ± 0.6 nm. (D) Critical state can be realized at pH 7 when the diameter is approximately 7 ± 1.1 nm. (E) Critical state can be realized at pH 2.8 when the diameter is approximately 10 ± 1.8 nm.
with approximately 100 pA at 2 V because of the sufficient driving force supplied by the external electric field. Furthermore, the
gating was nonconductive at 1 V and the initial 400 s at 2 V after standing for 2 h but could be opened abruptly after a hysteresis 9705
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in which condition the channel surface was fully negatively charged. With regard to the nanochannel with a diameter of ∼7 nm (Figure 3D), the gating was closed at pH 2.8 while exhibiting hysteretic conductivity and finally stabilized at ∼40 pA at pH 7, as discussed above in detail. With regard to the nanochannel with a diameter of ∼10 nm (Figure 3E), the gating opened at pH 2.8, and bursts of large-amplitude current signals of several hundred picoamperes could be observed in the initial ∼300 s. Eventually, the gating stabilized in the closed state because of water evaporation. However, the gating was opened with ∼40 pA current after the low density surface charge was introduced (electrolyte solution pH 7) and exhibited a step current as the channel surface charge density increased (electrolyte solution pH 10). For the gating with a diameter exceeding 10 nm, it was physically conductive under any conditions (Supporting Information Figure S10). These results indicated that a small nanochannel diameter, with a correspondingly large ratio of surface area to volume, was necessary for the smart gating. There have been several possible mechanisms proposed to explain these discrete fluctuations, all of which may be accurate for the specific system in study: one is complete wetting/ dewetting of the nanochannels (e.g., the presence of nanobubbles),4,27 and another one is the blockage by a charged ion or molecule.28,29 In this system, the nanochannel is predicted to exhibit a nanobubble in pH 2.8 because the current is zero and cannot be affected by the electric field in the channel, which has been proven by Figures 1E, 2A, and S9. To investigate this further, we performed ion current measurements of the nanochannel in 0.1 M HCl, in which condition the channel surface was electroneutral. The results confirmed that no ion current was observed in 0.1 M HCl (Supporting Information Figure S11), which also provides evidence that the nonconducting state corresponds to a nanochannel either completely or in part filled with water vapor and not liquid water because it is difficult to prevent protons from flowing through even a single file of water as occurs in aquaporins (Figure 4A). In this system, the calculated contact angles on a concave surface with sub-10 nm channel diameter are approximately 20° larger than that on a planar surface because of the curvature effect (Supporting Information Figures S12 and S13), and this effect will be even more significant for a smaller system.30−32 Hence, at pH 2.8, for the single sub-10 nm nanochannel, a hydrophobiclike behavior of nonconductivity can be observed (Figure 4A).
period and then maintained a steady value of approximately 100 pA, indicating the conductivity of the gating. The aforementioned experimental results showed that the gating was in a critical state because of the vapor−liquid equilibrium in the nanochannel. It is also important to note that the gating could not stay opened constantly regardless of the external voltage after standing for 12 h. In this state, the nanochannel was completely filled with vapor and was stabilized in the nonconductive state because of the complete water evaporation.10 The nanobubbles eventually span the cross section of the channel and block the water flow. Although the gating exhibited delayed voltage responsiveness because the voltage response enabling the ionic transport did not always induce kinetic opening and closing as quickly as electrostatic-chargeinduced gating (Supporting Information Figures S7 and S8), it could be harnessed to build devices that resemble conventional electronic circuits based on the voltage response properties. The dimension of biological water channels has been recognized as a major determinant for realizing the water channel functions.23,24 Analogously, this factor is also important for the electrostatic-charge- and electric-field-induced smart gating. Actually, the electrostatic charge could influence the ion conduction effectively only with an appropriate diameter (ion conduction effective area), and electrostatic-charge-induced smart gating can be realized only in the sub-10 nm nanochannel (gating effective area, Figure 3A). For the nanochannel with a diameter of ∼7 nm (Figure 3B), the transmembrane ionic conductance at pH 7 (green ball) deviated remarkably from the bulk value (green dashed line) at ∼0.001 M, whereas the conductance at pH 10 (purple ball) deviated from the bulk value (purple dashed line) at ∼0.1 M. Meanwhile, the conductance exhibited no obvious differences between pH 7 and pH 10 at high KCl concentration (above 0.1 M), whereas an obvious parallel conductance plateau was observed at low KCl concentration (below 0.1 M). This behavior could be well-explained by chargegoverned ionic transport because the surface charge could be modified by changing the electrolyte pH (Supporting Information Figure S9).25,26 Furthermore, the gating character can be realized under different pH values with varied diameters. As shown in Figure 3C, the nanochannel with a diameter of ∼3 nm could not be opened and exhibited an unsteady state at pH values of 2.8 and 7. Eventually, this channel was conductive up to 40 pA at pH 10,
Figure 4. Scheme of the smart gating with and without an external voltage. (A) At pH 2.8, the smart gating is nonconductive. (B) At pH 7, the gating is in a critical state because of the introduced low charge negative surface charge, which can provide osmotic pressure and Maxwell stress (FC). (C) At pH 10, the gating is conductive because of the introduced high density negative surface charge. (D) At low voltage, the nanochannel is still in critical state. (E) At high voltage, the smart gating is conductive because of electrostriction. 9706
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where E is the electric-field intensity. ΔQ is the net charge of the electrolyte solution in the nanochannel and can be expressed by the Debye−Huckel theory:38,39
The process of conductive and nonconductive can be discussed by Kelvin equation33 ln
βγ νl P = − lv P0 r
(1)
ΔQ =
where vl is the molecular volume of the liquid, γlv is the surface tension of the gas−liquid interface, and 1/r is the curvature of the gas−liquid interface meniscus. On a concave surface of a cylindrical pore with radius R, the equation R = r cos θ can be used to describe the relationship between the curvature 1/r and contact angle θ. Combined with Young’s equation: γwv = γwl + γlv cos θ, eq 1 can be described as ln
β(γ − γwl)νl PR = − wv P0 R
∫S PR dS
(2)
CONCLUSIONS In summary, we reported an electrostatic-charge- and electricfield-induced smart gating not only for ions but also for water based on the single sub-10 nm channel. Compared to other existing hydrophobic water gating, we can directly achieve the transition between an open, conductive state and a closed, nonconductive state by switching the surface charge density without the assistance of exotic hydrophobic molecules. Three factors play critical roles in this system: the diameter of the nanochannel (large ratio of surface area to volume),11,43−45 the electrolyte solution pH value (which can change the surface charge density), and the external electric field. If there are adequate surface charges, the smart gating can be opened. On the contrary, the gating will be closed if the charged density is decreased because nanobubbles in the channel finally block the water flow. Furthermore, the electrostatic-charge-induced gating exhibited a voltage response property. This smart gating anticipates wide applications in electrically controllable water transport devices and drug delivery systems that function with pH response.
(3)
After negative charge was introduced onto the channel surface (Figure 4B,C), an overlapped electrical double layer is formed. Based on the nanoslit model,34,35 the distributions of osmotic pressure and Maxwell stress can be introduced and can be described as PC: ⎛ zeV ⎞2 2cosh 2κx ⎟ + 2nbkT PC = nbkT ⎜ ⎝ kT ⎠ cosh 2κh + 1
(4)
where nb is the concentration of the electrolyte solution, k is Boltzmann’s constant, κ is the inverse Debye length, and T is equal to 293 K. V is a function of surface charge density σ and can be expressed by V=
σ cosh κh κεε0 sinh κh
EXPERIMENTAL SECTION Materials and Fabrication. Polymer foils of PET (Hostaphan RN 12, Hoechst) of 12 μm thickness were irradiated at the linear accelerator UNILAC (GSI, Darmstadt), with single swift heavy ions (Au) having an energy of 11.4 MeV per nucleon. The fabrication of a single cylindrical nanochannel in a PET membrane was accomplished by symmetric etching of the damage trail of a single heavy ion which passed through this membrane. Before the chemical etching process, both sides of the polymer films were exposed to the UV light (365 nm, 20 W) for 1 h. The following are the etching and stopping solutions for the etching of PET under 60 °C: 2 M NaOH for etching, 1 M KCl + 1 M HCOOH for stopping. Then the membrane was soaked in Milli-Q water (18.2 MΩ) to remove residual salts. During etching, a potential of 1 V was applied across the membrane in order to observe the current flowing through the nascent nanochannel. The current remains zero as long as the channel is not yet etched through, and after the break through, the increase of current is observed. The etching process was stopped when the current reached a certain value. Electrical Characterization. The ionic transport properties of the nanochannel were studied by measuring ionic current through the nanochannel. Ionic current was measured by a Keithley 6430 picoammeter (Keithley Instruments, Cleveland, OH). A single cylindrical nanochannel PET membrane was mounted between two chambers of the etching cell mentioned above. Ag/AgCl electrodes were used to apply a transmembrane potential across the film. Confocal Microscopy Image Characterization. The conductive property of single cylindrical nanochannel under different pH values was studied using fluorescence characterization. The selected fluorescent dye was sulfonated rhodamine (pKa ∼ 1.8) with high photostability.
(5)
Hence, the normal stress exerted on the gas−liquid interface due to the surface charge effect (FC) can be expressed as FC =
∫S PCdS
(6)
With a low surface charge density, the nanochannel would swing between conductive and nonconductive states, due to the equilibrium of FR and FC (Figure 4B), whereas a conductive state could be attained if the nanochannel was fully negatively charged (Figure 4C). However, the conductive state cannot be maintained if the charged density is decreased. In this condition, FC could not be sufficient to overcome FR. As a result, more liquid water evaporates to gas while little gas water condenses to liquid. The gas bubbles may occur locally in small patches, which can extend across the entire channel and disturb the flow of liquid. If FC was not sufficient to overcome FR (Figure 4B), the external voltage would play a crucial role and change the shape and position of the gas−liquid interface. The simulations confirmed that intermittent water-filling and electrostriction in hydrophobic nanochannels can be induced by increasing field strength (Figure 4D).36,37 Thus, the gating would be voltage responsive (Figure 4D,E). The electrical force (FE) could be described as FE = E × ΔQ
(8)
When the external voltage is sufficiently large, water can still permeate the channel under the influence of a strong electric field generated by an ion concentration imbalance at both ends of the channel (Figure 4E).40−42 Therefore, this electrostaticcharge-induced water gating can execute the switching between conductive and nonconductive states under the equilibrium of resistance (FR), electrostatic charge force (FC), and electrical force (FE).
Hence, for a given pore with radius R, the vapor pressure PR is the test criterion of the water gating. If the actual vapor pressure is greater than PR, water will condense in the channel; if the actual vapor pressure is less than PR, only vapor will exist in the pore. At this point (Figure 4A), the resistance (FR) can be illustrated by FR =
∫V σκe−κxdV
(7) 9707
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ACS Nano In the experiment, the cylindrical nanochannel membrane was immersed in dye solutions with different pH values beforehand, and the treated membrane was washed for 10 min in deionized water. After the preliminary process, the confocal microscopy images could be obtained by OLYMPUS FV1000-IX81. Scanning Electron Microscopy Measurement. Scanning electron microscopy measurements were captured in the field-emission mode using a Hitachi S-4800 microscope at an acceleration voltage of 5 kV.
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ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05682. Details of the experimental setup for fabrication and characterization, stability of electrostatic induction of water gating, statistic conduction, threshold voltage between conductive and nonconductive, ionic conductance under pH 7 and 10 (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors thank the Material Science Group of GSI (Darmstadt, Germany) for providing the ion-irradiated samples. This work was supported by the National Research Fund for Fundamental Key Projects (2013CB933000), the National Natural Science Foundation (21434003, 91427303, 21421061), and the Key Research Program of the Chinese Academy of Sciences (KJZD-EW-M03). REFERENCES (1) Sui, H. X.; Han, B. G.; Lee, J. K.; Walian, P.; Jap, B. K. Structural basis of Water-Specific Transport through the AQP1 Water Channel. Nature 2001, 414, 872−878. (2) Hou, X.; Hu, Y.; Grinthal, A.; Khan, M.; Aizenberg, J. Liquid-based Gating Mechanism with Tunable Multiphase Selectivity and Antifouling Behaviour. Nature 2015, 519, 70−73. (3) Rios, F.; Smirnov, S. N. pH Valve based on Hydrophobicity Switching. Chem. Mater. 2011, 23, 3601−3605. (4) Powell, M. R.; Cleary, L.; Davenport, M.; Shea, K. J.; Siwy, Z. S. Electric-Field-Induced Wetting and Dewetting in Single Hydrophobic Nanopores. Nat. Nanotechnol. 2011, 6, 798−802. (5) Gong, X.; Li, J.; Lu, H.; Wan, R.; Li, J.; Hu, J.; Fang, H. A ChargeDriven Molecular Water Pump. Nat. Nanotechnol. 2007, 2, 709−712. (6) Beckstein, O.; Sansom, M. S. P. Liquid-Vapor Oscillations of Water in Hydrophobic Nanopores. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 7063−7068. (7) Birkner, J. P.; Poolman, B.; Kocer, A. Hydrophobic Gating of Mechanosensitive Channel of Large Conductance Evidenced by SingleSubunit Resolution. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 12944− 12949. (8) Kou, J.; Yao, J.; Lu, H.; Zhang, B.; Li, A.; Sun, Z.; Zhang, J.; Fang, Y.; Wu, F.; Fan, J. Electromanipulating Water Flow in Nanochannels. Angew. Chem. 2015, 127, 2381−2385. (9) Smeets, R. M. M.; Keyser, U. F.; Wu, M. Y.; Dekker, N. H.; Dekker, C. Nanobubbles in Solid-State Nanopores. Phys. Rev. Lett. 2006, 97, 088101. 9708
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