Ion Transport Traversing Bioinspired Ion Channels at Bionic Interface

Article pubs.acs.org/JPCC

Ion Transport Traversing Bioinspired Ion Channels at Bionic Interface Hong Xia, Dongdong Qin, Xibin Zhou, Xiuhui Liu, and Xiaoquan Lu* Key Laboratory of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, 730070, China S Supporting Information *

ABSTRACT: Ion transport is especially crucial in normal body function, which is regulated by specialized ion channels. In this report, the simple hydrophilic alumina nanochannel is constructed at liquid/liquid (L/L) interface to simulate veritably and compactly complex cross-channel ion transfer processes of living systems in a similar physiological saline environment. The selectivity and regulation that are known for being two important characteristics of ion channels were achieved due to controllable electrosurface properties of alumina nanochannel. This channel shows good selectivity for ion with low electronegativity. The regulatory role of ion channel in confined space was achieved by varying diameters of nanochannel and lengths of ions travel path. In addition, a new theory based on the Randles-Sevcik equation is proposed for evaluation of cross-channel ion transfer in this article for the first time. The ingenious design strategy is demonstrated to be a useful means for investigating the complex cross-channel ion transport.

1. INTRODUCTION Biologic cells continuously need to exchange ions with the exterior environment to keep balance of electrolyte. These processes are facilitated by ion channels, which play important roles in cell physiology regulation of many critical cellular functions, including cardiac action potentials, epithelial electrolyte transport, and neurotransmitter release.1,2 Since the concept of ion channel was first proposed, many studies have been performed about this topic, such as selectivity3,4 and the energetics and dynamics of ion permeation in the channel.5−11 However, understanding of these processes is still far from complete although many models have been set up to study these systems. For example, some new materials such as carbon nanotubes, supramolecular channels, and synthetic film with nanopore structure8,12−16 have been applied to describe ion channels. Many methods including spectroscopy,17 molecular dynamics simulations,9 and biochemical and electrophysiological methods2 have been extensively used to research structure of ion channels and ion transport. However, there are few reports about electrochemical methods especially scanning electrochemical microscopy in studying ion channels. What’s more, the SECM technique has exhibited superiority to determine the kinetics of ion transfer through ion channels, contrasting with other methods. Recently, fabrication of artificial nanochannels is increasingly attracting researchers’attentions. Compared with biological counterparts, artificial nanochannels offer greater flexibility in terms of shape and size, superior robustness, and surface properties that can be tuned depending on the desired function.18−20 The porous anodic alumina films (AAO) possess uniform cylindrical pore sizes, high pore density, good stability © 2013 American Chemical Society

and easy tailoring, which are known for being important membrane attributes.21 These advantages make it possible for the AAO to be used in liquid/liquid (L/L) interface to study ion transfer through ion channels. Moreover, L/L interface is the simplest and most promising model for understanding charge-transfer process in biological systems. The advantages of this model include simple device and easy operation, as well as fast mass-transfer rate, negligibly small resistive potential drop, low double-layer charging current, and simple steady-state measurements by reducing the liquid/liquid interface to the nanometer range.22,23 In nature, almost every life cycle involves the process of cross-channel ion transport. In general, it is more complex and more important for living organisms in regulating ion transport. Thus, there has been significant interest in the development of bionic channels that may provide a highly efficient means to simulate the complex processes in living systems. In this study, the simple hydrophilic nanochannel is used as a prototype to permit rigorous exploration of ion transport through ion channel. It is noteworthy that this design strategy can take advantage of the electrosurface properties of alumina nanochannel to achieve both selectivity and regulation of ion transport through nanochannel at the same time. The model constructed here has the high similarity with biological membranes. Furthermore, because of the differences between ion transfer through channels and at micro-L/L interface, the traditional theoretical treatment is not suitable to evaluate the maximum peak current of ion transfer reactions at AAO Received: August 2, 2013 Revised: October 19, 2013 Published: October 21, 2013 23522

dx.doi.org/10.1021/jp407760m | J. Phys. Chem. C 2013, 117, 23522−23528

The Journal of Physical Chemistry C

Article

was filled with x M KCl or x M NaCl aqueous solution from the back by using a small syringe. The droplet electrodes were for the cyclic voltammograms, namely a drop of a certain volume of aqueous solution containing KCl or NaCl, K4Fe(CN)6, and K3Fe(CN)6 was transferred to the surface of a freshly polished platinum disk electrode (d = 2 mm) with a small syringe. Then, the electrode was turned over and immersed immediately into an organic phase solvent (of a certain volume) containing supporting electrolyte. The approach curves (iT−d curves) were obtained by moving the tip toward the bottom L/L interface and recording the tip current (iT) as the function of the tip/interface separation distance (d). The coordinate of the L/L interface (d = 0) was decided from the sharp increase in iT that occurred when the tip touched the bottom DCE/W interface. All experiments were performed at room temperature (20 ± 2 °C).

modified L/L interface. With this motivation, a new theory based on the Randles-Sevcik equation for the evaluation and identification of cross-channel ion transfer is proposed in the present study. The results obtained here may offer not only some substantial information for understanding natural ion channels but also theoretical principle for calculating kinetic constants of ion transfer reactions at porous materials modified L/L interface.

2. EXPERIMENTAL SECTION 2. 1. Materials. Potassium chloride (KCl, AR), sodium chloride (NaCl, AR), potassium ferrocyanide (K4Fe(CN)6, AR), potassium ferricyanide (K3Fe(CN)6, AR), and 1,2dichloroethane (DCE, 99.8%, AR) were purchased from Beijing Chemical, China. Tetrabutylammonium tetraphenylborate (TBATPB, 99%, Aldrich) and tetramethyammonium chloride (TMACl, 99%, Aldrich) were used as received. Naphtho-15-crown-5 (N15C5) was synthesized by Pedersen’s method and recrystallized three times from heptane.24 The AAO templates were left in water solution for about 12 h to ensure complete wetting. The organic phase was washed several times with deionized water before use. The aqueous solutions were prepared with deionized water by a Milli-Q system (MilliQ, Millipore Corp.). Special precautions were taken for dealing with DCE and other hazardous chemicals. The pHs were adjusted with dilute KOH. 2.2. Apparatus and Electrochemical Measurements. A CHI-900 electrochemical workstation (CH Instruments Inc.) connected to a personal computer was used for all electrochemical measurements. Cyclic voltammograms were recorded using a three-electrode potentiostat. Nanopipets employed as the SECM tips were made from borosilicate glass capillaries (o.d. = 2.0 mm, i.d. = 1.16 mm, L = 10 cm) using a laser-based puller (made by our laboratory). Aqueous solutions were infused into the pipets from the back with a small syringe (5 μL). As shown in Scheme 1, a three-electrode setup was employed in the experiments. A nanopipet was used as the SECM tip, a Ag wire coated with AgTPBCl served as the reference electrode, and a Pt wire served as the counter electrode. A Ag/AgCl electrode were inserted into the aqueous phase inside the nanopipe as the reference electrode. The tip

3. RESULTS AND DISCUSSION 3.1. Mechanism of Ion Transfer through the Ion Channels. The mechanism of ion transfer facilitated by N15C5 was verified to involve the transfer by interfacial complexation (TIC) at the tip and transfer by interfacial dissociation (TID) at the bottom of W/DCE interface.25 That is kb

M+(w) + N15C5(DCE) → [M(N15C5)x ]+ (DCE) (at the tip) kf

[M(N15C5)x ]+ (DCE) → M+(w) + N15C5(DCE) (at the ITIES)

where M+ is K+ and Na+. The following electrochemical cell (cell I) was employed to investigate ion transfer through ion channels facilitated by N15C5. Pt|1mMK4Fe(CN)6 + 1mMK3Fe(CN)6 + xMMCl(W) || 5mMTBATPB + 1mMN15C5(DCE)|AgTPBCl|Ag (cell I)

When the concentration of the redox couple in the aqueous phase was in excess, the platinum−water interface was unpolarizable and acted as a reference/counter electrode for the aqueous phase. The current was then limited by the ion transfer rate through ion channels. 3.2. Effect of Electrosurface Properties of Ion Channels on K+ and Na+ Transfer. Alumina nanochannel is an amphoteric material whose surface electrical properties depend on solution pH.26 For example, below pH of 6.5, alumina is protonated and has positively charged surface (AlOH2+). Above pH of 6.5, the alumina has an overall negatively charged surface (AlO−). Under the pH conditions in our study, the ion channel surface is negatively charged and attracts cations, while anions are repelled. Therefore, an ion confined-diffusion region is formed near a charged surface in contact with an electrolyte. The surface electrical properties affect significantly diffusion behavior of charged species across these charged membranes.27,28 Scheme 2 showed the distinct diffusion behavior of ions across ion channels. Diffusion of ions through the charged ion channels could be divided into two different contributions, confined diffusion region and free diffusion region. Thus, the total transfer rate of ions in nanochannels varies with the thickness of confined diffusion region, which is determined by the ionic strength of

Scheme 1. (a) Ions through the Ion Channels; (b) Simple Model for Ions through the Ion Channels; (c) Model for a Three-Electrode To Investigate Ion Transport Traversing the Ion Channels by SECM; (d) Scanning Electron Micrographs of AAO

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Scheme 2. The Model of Diffusion of Target Ions Across an Alumina Nanochannel

Figure 1. Cyclic voltammograms for (a) K+ and (b) Na+ transfer facilitated by N15C5 through ion channels. The ionic strength of K+ vary from 0.01 to 0.05 M, respectively, and the concentration of N15C5 is 1 mM. Sweep rate is 50 mV/s.

the electrolyte. The cyclic voltammograms (CVs) for K+ and Na+ transfer through the ion channels at the W/DCE interface with different ionic strengths are shown in Figure 1. It was obvious that the peak currents of ion transfer increased and the half-wave potentials shifted negatively with increasing concentration of alkali metal ions from 0.01 to 0.05 M. The results were in good agreement with the theoretical simulation developed by Stewart et al.29 The voltammetric behavior obeys the Randles-Sevcik equation30−33 −3

i p = 0.4463 × 10

×n

3/2 3/2

F

−1/2

A(RT )

rD =

e ∑i cizi2

(3)

where e is the electron charge, c is the concentration, z is the valence, ε0 is the dielectric permittivity of vacuum, ε is the dielectric constant of the solvent, kB is the Boltzmann constant, and T is the absolute temperature. In this manner, the RandlesSevcik equation must be modified as follows 1/2 i p = 0.4463 × 10−3 × n3/2F 3/2(RT )−1/2 mπ (rp − rD)2 D M +

1/2 + 1/2 DM + cM v

c M+v1/2

(1)

(4)

where m is the number of nanochannels of AAO membrane exposed at interface. It can be simply calculated by using the area fraction34

where ip is the anodic peak current, A is the area of the microL/L interface, and DM+ is the diffusion coefficient of the transferred species; the other parameters have their normal definitions. Nevertheless, ion transfer through channels is different from that at micro-L/L interface, because effective transport area is not the area of the micro-L/L interface. The area of AAO exposed at interface includes cell wall and nanochannel. Furthermore, as depicted in Scheme 2, the cross section of nanochannel was divided into two different regions. Here the area of free transport region can be expressed as a = π (rp − rs)2

εε0kBT 2

m=

AP π 4

( )d2

(5)

where Ap is the area fraction of pores (here is equal to the area of micro-L/L interface), and d is the average pore diameter. The kinetic parameters of transfer reactions could be evaluated by the three-point method, which could be found from the literature to give rise to values ΔE1/4 and ΔE3/4.35 The standard rate constant (k0) is

(2)

where rp is the radius of the nanochannel, and rs is the thickness of confined transport region. Nevertheless, the thickness of confined transport region greatly depends on the Debye length rD, which has the form

k0 = 23524

λD0 a

(6) dx.doi.org/10.1021/jp407760m | J. Phys. Chem. C 2013, 117, 23522−23528


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Figure 2. Experimental approach curves fit with theoretical values of the transfer of (a) K+ and (b) Na+ facilitated by N15C5 through ion channels. For curves from bottom to top, the ionic strengths of K+ are 0.01, 0.02, 0.03, 0.04, and 0.05 M, respectively. (c) Transfer rate constants vs the ionic strength of aqueous solution.

was expected to increase with increasing ionic strength of aqueous solution. In addition, the transport rates of K+ were higher than that of Na+ (Figure 2c), that is to say, it is easier for K+ to pass through the ion channels. Thus, alumina nanochannel shows good selectivity for K+ over Na+, which is similar to usual channels in organism. This could be elucidated by means of the following factors: (i) the enhanced confinement of motion in confined diffusion region due to the larger electronegativity of Na+ (0.93) than that of K+ (0.82), resulting in the enhancement of the interaction between the electric field and Na+, and (ii) N15C5, a neutral ionophore, exhibited better selectivity for potassium than sodium, leading to extremely larger extraction constant for K+ than that for Na+.36 As pore surface charge plays an important role in transport of ions across the nanochannels and is also affected by pH values of aqueous solutions, the effect of pH was investigated by varying pH from 6.5 to 8.0 (Figure S1, Supporting Information). Interestingly, lower current response was observed at higher pH values. Transport rate constants obtained from experimental approach curves decreased with increasing pH values. The phenomenon supports the hypothesis that the enhanced confinement of motion in the confined-diffusion region is attributed to the increased surface charge density at higher pH values. When pH values increased, surface negative charge density increased as well, which enhanced the electrostatic interaction between the electric

A family of approach curves of different ionic strengths is shown in Figure 2. These results are in good agreement with that obtained from CVs. As discussed above, the thickness of confined diffusion region affected seriously transfer behavior of ion across ion channels. In this region, the diffusion of K+ and Na+ was restricted due to the interaction between ions and surface charges contrasting with the free transport region. At low ionic strengths (low concentrations of K+ and Na+), the confined-diffusion region expands while the free-diffusion region shrinks. On the contrary, the confined-diffusion region shrinks while the free diffusion region expands in high ionic strength solution.28 Table 1 lists the calculated results under various conditions. As a result, the transport of alkali metal ions Table 1. Confined-Diffusion Region Thickness and Its Percentage of the Pore Cross Section under Different Conditions

c[M]

rD [nm]

20 nm pore % pore crosssection

0.01 0.02 0.03 0.04 0.05

3.07 2.17 1.77 1.55 1.30

51.98 38.70 32.27 28.60 24.31

c[M] 0.03

pore widenig time [min]

average pore diameter [nm]

pore diameter % pore crosssection

0 10 20 30 40

20 30 45 60 70

32.27 22.22 15.12 11.46 9.86 23525



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field and ion. Furthermore, the influence of pH was also important to thick biological membranes such as the human skin37 and the cornea,38 where differences in the permeabilities of ion were ascribed to the existence of charges within the membrane. 3.3. Effect of Size of Nanochannels. The contribution of the confined-diffusion region to the total ion transfer strongly depends on the pore size; hence, further study was undertaken to discuss the effect of pore size. AAO with different pore diameters were fabricated via soaking AAO templates in a 5 wt % phosphoric acid for different time.39 The pore-widening process can be easily achieved without changing the interpore distance, which is ideal in simulating the elasticity of ion channels in living systems. Figure 3 gives the SEM images of

that pore size of nanochannel plays an important role in the regulation of ion transport. Next, the transfer behavior of ions through the channels with various lengths of path that ions travel was investigated. AAO with the same surface morphology but different thickness were used in the experiments. The four alumina films used were fabricated in 0.5 M oxalic acid at 40 V with different anodizing time of 2, 3, 4, and 5 h.34 The surface morphology and pore characteristics were the same but the nanochannel depth was different. As shown in Figure 4, along with the anodizing time prolonged, the thickness obviously increased.

Figure 4. SEM images of AAO with different second anodiztion times. (a) Second anodiztion for 2 h, (b) second anodiztion for 3 h, and (c) second anodiztion for 5 h. Parallel nanochannels are depicted in (d).

Figure 3. SEM images of AAO before and after etching in 5 wt % H3PO4 at 19 °C for different time. (a) Before the pore-widening process; (b) pore widening for 10 min at 19 °C; (c) pore widening for 20 min at 19 °C; (d) pore widening for 40 min at 19 °C, respectively. Scale bar: 100 nm.

As expected, with the decreasing of thickness of AAO, transfer current increased (Figure S4, Supporting Information). Transport rate across the film with the thinner porous layer were greater than that with a thicker layer. When the moving distance of ion in nanochannels became longer, the masstransport of ion was reduced. However, the mass-transport of ion within the nanochannel can be formulated by the NernstPlank equation

AAO after different etching time in phosphoric acid. It could be seen that the average pore size was apparently different. Here, different pore-widening time was used to represent average pore size. CVs at four different etching time (t = 10, 20, 30, and 40 min, respectively) in the pH-neutral environment just as in a natural organism showed that the current signal obtained across ion channels with large pore was stronger than that with small pore (Figure S2, Supporting Information). To obtain the ion transport rate, the experimental SECM approach curves fitted with theoretical values. In the case of other conditions keeping unchanged, kf increased with increasing pore diameters from 20 to 70 nm (Figure S3, Supporting Information). It was consistent with the results obtained from CVs. The results shown above could be ascribed to the thickness of confineddiffusion region and its percentage of the pore cross section.28 When pore size was very small, the percentage of confineddiffusion region to the pore cross section increased (Table 1). Thus, the pathway of free-diffusion of K+ or Na+ was decreased, which resulted in a decreased mass transportation of ion through the nanochannels. On the contrary, mass transportation of K+ or Na+ through nanochannels was expected to increase with increasing pore diameter, which caused a stronger current response. Hence, these observations indicate

J (x ) = − D

∂ϕ(x) ∂C(x) zF DC + Cνeo(x) − RT ∂x ∂x

(7)

where D, C, and z are the diffusion coefficient, concentration, and charge of the permeation ion, respectively. ∂C(x)/∂x is the concentration gradient at distance x, ∂ϕ(x)/∂x is the potential gradient, and νeo(x) is the electroosmotic velocity. The standard equation that describes the velocity (νeo) is as follows40 νeo =

ε0εrζ U η l

(8)

where η is the viscosity of the fluid, ζ is the zeta potential at the capillary wall, U is the voltage applied, and l is length of nanochannel. Other parameters have their normal definitions. According to eqs 7 and 8, theoretical description of the masstransport suggests that J(x) is inversely proportional to the length of nanochannel. In our experiments, decreasing transport rate was indeed observed when length of nano23526



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channel increased, which indicated that dynamics of ions transport was closely related to the nanochannel depth of AAO.

4. CONCLUSIONS The hydrophilic alumina nanochannel is used to simulate ion channel for researching complex ion transfer processes in living systems. This channel shows good selectivity for ions with low electronegativity due to a difference in electrostatic interaction between the electric field and different ions. Surface charges of nanochannel was governed by adjusting ionic strength of the electrolyte and pH of the aqueous phase. The regulatory role of ion channel in confined space was achieved by varying diameter of alumina nanochannel, which could be accomplished by soaking AAO templates in a phosphoric acid. In addition, the well-known Randles-Sevcik equation was modified in this work to fit the maximum peak current of ion transfer reactions at porous materials modified L/L interface. The L/L-AAO interface constructed here had high similarity with biological membranes, which provided a better platform for understanding ion transfer reaction through natural ion channels. It may be helpful for the applications such as chemical/biological separation and drug release.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-931-7971276. Fax: +86931-7971323. Notes

The authors declare no competing financial interest. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21175108). The author also would like to gratefully acknowledge Huixia Guo and the other members of the Professor Lu’s group for the assistance of this work.

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REFERENCES

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Ion Transport Traversing Bioinspired Ion Channels at Bionic Interface

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