Bioinspired Multivalent Ion Responsive Nanopore with Ultrahigh Ion

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Bioinspired Multivalent Ion Responsive Nanopore with Ultrahigh Ion Current Rectification Zhong-Qiu Li, Yang Wang, Zeng-Qiang Wu, Ming-Yang Wu, and Xing-Hua Xia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02279 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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The Journal of Physical Chemistry

Bioinspired Multivalent Ion Responsive Nanopore with Ultrahigh Ion Current Rectification Zhong-Qiu Li, Yang Wang, Zeng-Qiang Wu,* Ming-Yang Wu, Xing-Hua Xia*

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China

ACS Paragon Plus Environment

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ABSTRACT: Ion transport in bio-processes and can be regulated by various external stimulations. Multivalent ion as one of the stimulators shows a great ability in tuning the ion transport properties, while the mechanism is yet unclear. Here, inspired by this multivalent ion involved process, we propose a multivalent ion responsive symmetric hourglass polycarbonate nanopore. Addition of multivalent ion triggers formation of an abrupt bipolar junction inside the nanopore and an ultrahigh ion current rectification ratio higher than 650 can be achieved. The ion transport property can be reversibly and significantly regulated via the reversible adsorption/desorption of multivalent ions on nanopore surface. This study provides the fundamentals to the understanding of biological process and proposes an effective strategy to build smart nanofluidic devices. Multivalency depicts the multivalent interaction between complementary counterparts. It is widely existed in nature and plays important roles in biological processes like cell interaction and transmembrane ion transportation.1-3 The multivalency involved ion transport in biological nanopores is of great interest because it directly correlates with cell function and signal transduction. For instance, Kir2.1 potassium channel can be blocked by multivalent ions (like Mg2+)4 and the ion selectivity of OmpF channel (outer membrane


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porin of E. coli) is reversed in concentrated solutions of divalent cations.5 Despite these special phenomena have been discovered for decades, the ion transport mechanism in these processes is yet unclear. Bioinspired artificial nanopore is more processable and stable when compared with biological nanopores.6,7 It has been widely used to explore the ion transport mechanism in biological nanopores, for example, multivalent-ion manipulated ion transport has been studied in conical nanopores.8 Further investigation of biological processes with bioinspired artificial nanopores will help us understand the fundamentals and develop high performance nanofluidic devices.

Charge inversion is one of the typical phenomena resulted from multivalent interaction. It occurs at the Stern layer when counterions in the electric double layer exceed the amount needed to compensate the surface charge.9,10 This charge inversion phenomenon has been studied in nanofluidic devices and it can reverse the ion selectivity of the nanopore,11-14 for example, the selectivity of a polyamine functionalized conical nanopore changes from anion into cation in high-concentration phosphate solutions.15


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With controlled local charge inversion, it is possible to construct a bipolar junction inside a nanopore and significantly tune the ion transport property. Here, to mimic and better understand the functions of multivalency involved ion transport system, we present a multivalent ion responsive symmetric hourglass polycarbonate (PC) nanopore (Figure 1a). Upon addition of multivalent ions, an abrupt bipolar junction will be established owing to the synergistic effect of spatially confined ion diffusion and charge inversion inside the nanopore, which results in an ultrahigh ion current rectification (ICR). The influences of ion valence, ionic concentration and the diameter of the small tip are investigated and a theoretical model is established to explain the experimental results. This multivalent ion stimulated nanofluidic device would help the understanding of multivalency involved bio-processes and shows great potential in constructing “smart” systems.

EXPERIMENTAL SECTION


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Materials. The PC membranes (12 μm thick, with multi-ion tracks in the center) were purchased from Wuwei Kejin Xinfa Technology Co., Ltd. (China). Fluo-8 (Ca2+ fluorescent probe) was purchased from AAT Bioquest Inc. (America). Other analytical grade chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All solutions were prepared with deionized water (18.2 MΩ cm -1, Milli Q).

Fabrication and Characterization. The conical and hourglass nanopores were fabricated by asymmetric and symmetric chemical etching of the PC membrane according to a previously reported method.16,17 For conical nanopore, the PC membrane was etched only from one side by using 9 M NaOH and the other side was in contacted with an acidic solution (1 M HCOOH+1 M KCl). For fabrication of hourglass nanopore, the PC membrane was simultaneously etched from both sides using 9 M NaOH. Two Pt electrodes were used to apply a transmembrane voltage (1 V) to monitor the etching process. After etching for a certain time, the basic solution was neutralized using a solution of 1 M HCOOH+1 M KCl, and the etching process was stopped. Afterwards, the membrane was stored in ultrapure water prior to use. The base diameter of the conical


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nanopore was characterized by a scanning electron microscopy (SEM, S-4800, Hitachi) and the diameter of the tip was obtained by using an electrochemical method. Electrochemical Measurement. For ion transport studies, the membrane was mounted between two half-cells and the IV curve was recorded by a Keithley 6487 picoammeter. A scanning voltage from -1 to +1 V was applied across the membrane by using two Ag/AgCl electrodes. The electrode at multivalent-counterion side was grounded in all experiments. The concentration of all the solutions, if not mentioned, was 10 mM and all the experiments were performed at room temperature. Mass Transport Experiment. CaCl2 (10 mM) and Fluo-8 (1 mg/mL) were utilized to investigate the diffusion of Ca2+ through the hourglass nanopore. The experiment was conducted on a homemade device. Ca2+ and Fluo-8 were filled in the feed and permeate cells, respectively. Once Ca2+ diffuses across the tip, it reacts with Fluo-8, generating a fluorescent signal at ~520 nm, which was recorded by a laser scanning confocal microscope (TCS SP5, Leica). The excitation wavelength was set at 488 nm.


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RESULTS AND DISCUSSION

To confirm the multivalent ion adsorption induced charge inversion on PC materials surface, we first investigated the ion transport property of a conical nanopore. The conical nanopore shows a base-diameter of 378 nm on one side of the membrane and a tipdiameter of 0.46 nm on the other side (Figure S1 and Table S1). The chemical etching of the ion-tracked PC membrane produces carboxyl groups on the surface of the nanopore.16,17 These carboxyl groups dissociate in neutral solutions and carry negative charges, which endows the membrane with cation selectivity and ICR property.18,19 The current-voltage (IV) curves of this nanopore were measured in KCl and FeCl3 solutions, respectively (Figure S2). In KCl solution, larger currents occur at positive voltages; however, larger currents are observed at negative voltages in FeCl3 solution, showing a reversed ICR effect. The change in rectification direction demonstrates that the surface charge of the conical nanopore experiences a reversion (from negative charge to positive charge), which is caused by the strong interaction between Fe3+ and the negatively charged surface functional group (-COO-).


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The hourglass nanopore has a symmetric structure. The diameter of the base at both sides of the membrane is 208 nm and the diameter of the tip embedded inside the membrane is calculated to be 0.91 nm (Figure S1 and Table S1). The ion transport of this nanopore was investigated by placing different types of electrolyte solutions on each side of the membrane. ICR in four different electrolyte combinations (KCl/KCl, KCl/K3Fe(CN)6, FeCl3/KCl and FeCl3/K3Fe(CN)6) was studied (Figure 1b and c). There displays only a tiny ICR ratio (~2) under the symmetric electrolyte condition (KCl/KCl), which is attributed to the imperfect pore structure.20,21 After replacing the KCl solution with K3Fe(CN)6 in one side, forming KCl/K3Fe(CN)6, a slightly increased ICR ratio is observed owing to the enhanced potential difference across the nanopore caused by K+ concentration gradient.22,23 A great increase of ICR ratio (~80) is observed when FeCl3 is introduced into the system (FeCl3/KCl). In this case, the surface charge of the PC nanopore will be inverted by Fe3+ adsorption, resulting in positively charged half-nanopore and establishment of a bipolar structure. The ICR ratio can be further improved to ~300 under the


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Figure 1. (a) Scheme of a bipolar hourglass nanopore achieved by multivalent ion stimulated local charge inversion. (b) IV curves of the hourglass nanopore under the electrolyte conditions of KCl/KCl, KCl/K3Fe(CN)6, FeCl3/KCl and FeCl3/K3Fe(CN)6. The concentration of all these solutions is 10 mM. (c) Calculated ICR ratios from the IV curves. (d) “On” (at +1 V) and “off” (at -1 V) currents under different electrolyte conditions, data extracted from IV curves. (e) Simulated ion concentration profiles of four ions (Fe3+, Cl-,


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K+ and Fe(CN)63-) in the nanopore with tip diameter of 1 nm in 10 mM FeCl3/K3Fe(CN)6. The transmembrane voltage is set as +1 V and -1 V, (f) Reversible switch in ICR ratios of the nanopore diode under alternate electrolyte conditions of KCl/K3Fe(CN)6 and FeCl3/K3Fe(CN)6.

electrolyte condition of FeCl3/K3Fe(CN)6, which is much larger than the values of most reported nanofluidic diodes.24-29 As can be seen from Figure 1d, the “off” current at negative voltages decreases to a tiny value, while the “on” current at positive voltages remains almost unchanged, resulting in considerable ICR ratio. Further finite element method simulation shows that K+ and Cl- are the major charge carriers in the nanopore and will be prominently enriched at the tip under positive voltages. However, at negative voltages, ion depletion region appears in the nanopore for all the ions (Figure 1e). This is because that, at negative voltages, K+ and Cl- ions are driven away from nanopore by electromigration, while Fe3+ and Fe(CN)63- ions have difficulty in overcoming the potential barrier to enter the nanopore owing to the strong electrostatic repulsion with the pore entrance. It is noteworthy that high valence of ions used here might induce a difference


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in transport numbers and enlarge current difference between the “on” and “off” states, which results in an increased ICR ratio.30 The total junction potential at the Ag/AgCl electrodes, which is induced by the asymmetric electrolyte condition, is calculated to be small enough that can be ignored (Table S2).31 The possible formation of deposits (e.g., Prussian blue) from the Fe3+-Fe(CN)63- combination can be ruled out (Figure S3). It is observed that the ICR ratio gradually increases within 20 min and finally reaches a plateau (Figure S4), indicating that the Fe3+ adsorption process occurs slowly to attain a steady state. In addition, the ICR ratio of this hourglass nanopore displays an excellent reversibility when cycling the electrolyte at one side of the nanopore between KCl and FeCl3 while keeping the electrolyte at another side of nanopore as K3Fe(CN)6), which can be explained by the reversible adsorption/desorption properties of the multivalent ions on the negatively charged surface (Figure 1f). These results demonstrate the highly efficient regulation of ion transport in a symmetric nanopore achieved by multivalent ion-stimulated local charge inversion.


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Figure 2. IV curves of an hourglass nanopore with different concentrations of FeCl3/K3Fe(CN)6: (a) 1 M~1 mM, (b) 4 mM~100 mM. (c) Calculated ICR ratios in different concentrations of FeCl3/K3Fe(CN)6. (d) “On” (at +1 V) and “off” (at -1 V) currents in different concentrations of FeCl3/K3Fe(CN)6. (e) IV curves of the hourglass nanopore in electrolyte solutions of counterions with different valences. Monovalent: Li+, Na+; divalent: Mg2+, Ca2+; trivalent: Fe3+, La3+. The concentrations of all these solutions are 10 mM. (f) Calculated ICR ratios in electrolyte solutions of counterions with different valences.


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To understand the ion transport mechanism, IV curves of the hourglass nanopore in different electrolyte concentrations of FeCl3/K3Fe(CN)6 were measured. As shown in Figure 2a-c, the ICR ratio merely changes at ultra-low concentrations of 1 M and 10 M. With increasing the concentration to 100 M and 1 mM, the ICR ratio grows slightly. Further increase in electrolyte concentration results in a sharp rise in ICR ratio, and a significant value of 650 is obtained at 100 mM. These results indicate that the surface charge is partially (or fully) screened by Fe3+ at low concentrations (100 M~1 mM), and only a small ICR ratio appears. Charge inversion occurs with concentration higher than 1 mM, which results in a bipolar structure and dramatically increases the ICR ratio. As shown in Figure 2d, the “on” current increases gradually with the concentration, while the “off” current experiences a rapid decrease at concentrations higher than 1 mM. This indicates a significant change in surface charge at higher concentration range. If we only consider electrostatic interaction in this system (ignore hydration effects and specific chemical interactions), the charge-inversion concentration is supposed to be only valence-dependent, and it is about 1 mM for trivalent counterion,32 which is in consistent with our result.


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The valence of the counterions has a great influence on the ion transport properties inside the nanopore because it directly correlates with the charge inversion process. We measured the IV curves of the hourglass nanopore with different-valence counterions (Figure 2e and f). The ICR ratio increases with the valence of the counterions, which is mainly attributed to the difference in interactions between the charged surface and counterions. For monovalent counterions, the surface of the nanopore remains negatively charged due to its weak interaction with counterions,10 leading to a non-polar structure. In the case of divalent counterions, half of the nanopore becomes partially zero-charged since the charge-inversion concentration is larger than 100 mM,33 leading to a unipolar structure. For trivalent counterions, half of the nanopore becomes positively charged due to charge inversion effect, forming a bipolar structure. These results confirm the importance of counterions valence in triggering charge inversion and achieving high ICR ratio in a symmetric nanopore. The influence of tip diameter on the ion transport was further investigated. By controlling the etching time during fabrication, tips with diameters of 7.8 nm (PC-l) and


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0.91 nm (PC-s) are obtained (Figure S1 and Table S1). As shown in Figure 3a and b, the current value of PC-l is

Figure 3. IV curves of hourglass nanopores with tip diameters of 0.91 nm (a) and 7.8 nm (b) under the electrolyte condition of FeCl3/K3Fe(CN)6. The concentrations of FeCl3 and K3Fe(CN)6 are 10 mM. (c) ICR ratios calculated from the IV curves. (d) Fluorescence intensity-time curves of Ca2+-Fluo-8 compound at permeate side of PC-l and PC-s. CaCl2 concentration at feed side is 10 mM and Fluo-8 concentration at permeate side is 1 mg/mL. (e) Simulated Fe3+ concentration-time curves at permeate side of PC-l and PC-s. The tip


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diameters of PC-l and PC-s are set as 8 nm and 1 nm, respectively. The concentration of FeCl3 at feed side is 10 mM. (f) Simulated concentration distribution of Fe3+ along the axis of the nanopore after 10-min diffusion. The tip location is set as x=0.

three orders of magnitude larger than that of PC-s. However, the PC-l shows much smaller ICR ratio as compared to PC-s (Figure 3c), since the electric double layer is not yet effectively overlapped for PC-l under the electrolyte condition (thickness of electric double layer: 2.5 nm). In addition, Fe3+ ions can diffuse easily through the tip of PC-l which breaks the abrupt bipolar junction. This is further confirmed by monitoring the ion (Ca2+) diffusion process using a fluorescent method (Experimental section, Figure S5). For PC-l, Ca2+ ions can diffuse easily through the tip to react with the fluorescent probe Fluo-8 at permeate side as evidenced by the gradual increase of fluorescent intensity. However, this phenomenon is not observed for PC-s (Figure 3d). This spatially confined diffusion process for divalent Ca2+ ions should be observable to trivalent Fe3+ ions. As shown by the finite element method simulations (Figure 3e), Fe3+ can barely diffuse across the small tip of PC-s. The distribution of Fe3+ concentration along the axis of the nanopore after 10-


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min diffusion (Figure 3f) shows that there is a steep decrease in concentration at the tip (x=0) for PC-s, while there is a gentler change for PC-l. This is mainly caused by the limited diffusion in the nanoconfined space and the electrostatic repulsion between Fe3+ and the charge inversed tip. With further decreased tip diameter (

Bioinspired Multivalent Ion Responsive Nanopore with Ultrahigh Ion

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