Selectively Enhanced Ion Transport in Graphene Oxide Membrane

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Functional Nanostructured Materials (including low-D carbon)

Selectively enhanced ion transport in graphene oxide membrane/PET conical nanopore system Yuhua Dong, Yaxiong Cheng, Guo-Heng Xu, Hongwei Cheng, Ke-Jing Huang, Jinglai Duan, Dan Mo, Jian Zeng, Jing Bai, Youmei Sun, Jie Liu, and Huijun Yao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01071 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Selectively Enhanced Ion Transport in Graphene Oxide Membrane/PET Conical Nanopore System Yuhua Dong1, 2, Yaxiong Cheng1, 2, Guoheng Xu1, 3, Hongwei Cheng1, 2, Kejing Huang1, 2, Jinglai Duan1, 2, Dan Mo1,2, Jian Zeng1,2, Jing Bai1,2, Youmei Sun1,2, Jie Liu1,2,* , Huijun Yao1,2,* 1 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, China * Corresponding authors at: Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, P.R. China. Tel.: +86 931 4969334; fax: +86 931 4969334. E-mail addresses: [email protected] (J. Liu), [email protected] (H. Yao) ABSTRACT:

Graphene oxide (GO) has become a promising 2D material in many areas, such as gas separation, seawater desalination, antibacterial materials and so on due to its abundant oxygen-containing functional groups and excellent dispersibility in various solvents. The graphene oxide membrane (GOM), a laminar and channel-rich structure assembled by stacked GO nanosheets, served as a kind of precise and ultrafast separation material has attracted widespread attention in membrane separation field. In order to break the trade-off between ion permeability and ion selectivity of separation membrane based on GOM, GOM/conical nanopore system is obtained by spin-coating ultrathin GOM on PET conical nanopore which possesses ion rectification property. Comparing to pure PET conical nanopore, the existence of GOM not only enhances the cation conductance but also makes the ion rectification ratio increase from 4.6 to 238.0 in KCl solution. Assisted by COMSOL simulation, it is proved that the GOM can absorb large amount of cations and act as cation source to improve the ion selectivity and rectification effect of GOM/conical nanopore system. Finally, the chemical stability of GOM/conical nanopore is also investigated and the corresponding results reveal that the GOM/conical nanopore system can perform the ion rectification behavior in a wider pH range than pure PET conical nanopore. The presented findings demonstrate the great 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

potential applications of GOM/conical nanopore system in ionic logic circuits and sensor systems. Key words: Ion irradiation, Graphene oxide membrane, Conical nanopore, Ionic current rectification, Ion selectivity 1. INTRODUCTION Graphene oxide (GO) in the form of partially oxidized graphene has attracted great attention as a new functional material since its atomically thin thickness, twodimensional structure, high strength and chemical stability1. Owing to the large number of oxygen-containing functionalization groups, GO can be easily dispersed in water by sonication to form stable and homogeneous GO suspensions, which are easy to fabricate ultrathin, high-flux and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution2. And more importantly, the GO laminates are hopefully to realize industrial-scale production3. Up to now, several methods, such as drop-casting 4-5,

spin-coating6, vacuum filtration7-9 and so on, have been adopted to produce GO

membranes (GOMs), which possess excellent mechanical strength due to the strong interlayer hydrogen bonds between adjacent GO sheets that hold all the layers tightly. On the basis of the above advantages, GOMs show great potential in a variety of applications, including water desalination and purification2, 9, gas and ion separation7, supercapacitors10 and lithium batteries11. In gas and ion separation area, there is a major challenge greatly impedes the development of separation membranes, which is the general trade-off between permeability and selectivity12-13. In order to solve this problem, some approaches or strategies, such as mixed-matrix membranes, biological membranes and metal-organic frame (MOF)

14-17,

are developed. However, most of these methods are complicated,

time-consuming, and high-cost, which makes them difficult to scale-up and actual application. Recently, based on isotope labelling technology, it is proved that liquid water can afford an ultrafast permeation speed through GOMs and the ion diffusion speed is even slightly faster than water molecule’s because the interaction between ion and GO sheet has an effect on the accelerated ion transportation18. Meanwhile, GOMs also exhibit the high H2/CO2 separation selectivity up to 3400 which is one or two 2


Page 2 of 24

Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


orders of magnitude higher than that of microporous membrane 7. The interlayer spacing between graphene oxide sheets (a sheet is a single flake inside the membrane) can be adjusted with Ångström precision by physical confinement19 and cationic control4, and the corresponding well controlled GOMs could exhibit good ion or salt rejection properties. Consequently, it is proved that GOMs have the ability to separate different ions in solutions with a relatively fast speed based on the physical size effect of nanocapillaries and the diverse interactions between ions and GOMs5. In order to break the trade-off between ion permeability and ion selectivity of GOM, a facile, time-saving approach that ultrathin GOM is spin-coated on PET conical nanopore which can not only support the GOM but also possess ion rectification effect (Figure 1). The prepared GOM/PET conical nanopore exhibits both enhanced cation transport properties and higher rectification ratio than those of pure PET conical nanopore. It is also proved that the GOM/PET conical nanopore can perform ion rectification ability in a wider pH range. 2. MATERIALS AND METHODS Preparation of conical nanopore in PET membrane: The Polyethylene terephthalate (PET) membrane with thickness of 12 μm was irradiated with 12.5 MeV/u 181Ta31+ ions at the Heavy Ion Research Facility in Lanzhou (HIRFL), China. In order to get single conical nanopore in PET membrane, single ion irradiation was essential to be carried out firstly. For this purpose, 1 mm thick metal mask with 0.5 mm aperture in the center was placed in front of PET membrane during irradiation to reduce the ion flux and pre-determine the ion irradiation position. And a scintillation radiation detector was fixed behind the PET membrane to detect the ions which penetrated through the PET membrane. Once one ion was detected by the detector, the ion beam would be switched off immediately to finish the single ion irradiation. After ion irradiation, the PET membrane underwent an asymmetric chemical etching in a home-made two-compartment electrolytic cell. In order to get the desired conical nanopore, the etching solution we chose was 5 M NaOH solution, and the 3



solvent was the mixture of water and methanol with volume ratio of 1:1. The 0.1 M HCl solution was filled into one compartment as stopping solution to neutralize the etchant when it penetrated the PET membrane from another compartment. Meanwhile, a voltage of 0.1 V was applied on two Pt electrodes which were immersed in etching solution and stopping solution by Keithley 6482 picoammeter/voltage source. During the etching process, the transmembrane ionic current was recorded and the etching process would be stopped once it reached 2 nA. Subsequently, the compartment filled with NaOH solution was emptied and rinsed with HCl in order to remove the residual etchant on the membrane completely. Finally, both compartments were rinsed with DI water at least 5 times. The wide end of conical nanopore facing to etchant was referred as “base” with the diameter of 1.2 μm, and the narrow end was referred as “tip” in the later experiments with the size of around 6.2 nm20. Preparation of GOM/PET conical nanopore: The graphite oxide powders were purchased from Nanjing XFNANO Materials Tech Co., Ltd and used directly without further purification. A certain amount of graphite oxide powders were dissolved in DI water by constantly magnetic stirring for 20 hours (500 rpm, 20 ℃ ) to obtain the 10 mg/mL GO suspension. Accordingly, a 4-hour ultrasonication treatment was used to exfoliate graphite oxide into few-layer or monolayer graphene oxide nanosheets, resulting in a stable GO solution. The asprepared solution was centrifuged at 5000 rpm for 10 min to eliminate the lager GO sheets and the supernatant was collected for next spin-coating procedure. To obtain GOM/PET conical nanopore, the as-prepared PET membrane with single conical nanopore was adopted as the spin-coating substrate. During the spin-coating step, the PET membrane was placed on the spin coater (model KW-4A/5) with the tip side of conical nanopore facing up, and 0.4 mL prepared GO solution was dropped onto the surface by dropper. The GO solution was spin-coated at the speed of 1300 rpm for 48 s and dried in oven at 80℃ for 45 min. Measurement: The ion transport measurement of pure PET conical nanopore and GOM/PET 4


Page 4 of 24

Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conical nanopore was carried out by Agilent B2902A. Two Ag/AgCl electrodes were used to apply the voltage bias and record the corresponding ionic current. The voltage applied between electrodes was varied from -4 V to +4 V. The rectification ratio was defined as the ratio of the current amplitude at negative voltage (-4V) to the current value at positive voltage (+4V), and the applied electric filed direction from base to tip was set as positive. COMSOL simulation: In our COMSOL simulation, GOM/PET conical nanopore structure was simplified as conical PET nanopore connecting with cylindrical GO nanochannel as shown in SI Figure S2 and 10 mol/L KCl solution used for simulation. At steady state, the current can be described by PNP-NS equation

21-22.The

ion flux is governed by the general

multi-ionic mass transport Nernst-Planck (NP) equation: 𝐷𝑖

𝑵𝒊 = 𝒖𝑐𝑖 ― 𝐷𝑖∇𝑐𝑖 ― 𝑧𝑖𝑅𝑇𝐹𝑐𝑖∇𝜑

, i=1, 2

(1)

At steady state, the equation has the following condition ∇ ∙ 𝑵𝒊 = 0

(2)

The electric potential is provided by the Possion equation ―ε𝜀𝑟∇2𝜑 = 𝜌𝑒

(3)

𝜌𝑒 = 𝐹(𝑐1𝑧1 + 𝑐2𝑧2) represents the space charge density. The electroosmotic flow in the nanopore is also considered, and the fluid motion is modeled by the Navier-Stokes equation with the continuity equation ―∇𝑝 + 𝜇∇2𝒖 ―𝐹(𝑐1𝑧1 + 𝑐2𝑧2)∇𝜑 = 0

(4)

∇∙𝒖=0

(5)

The 𝑵𝒊, 𝒖, 𝑐𝑖, 𝐷𝑖, and 𝑧𝑖 represent the ionic flux, the fluid velocity, the concentration, the diffusion coefficient and the valence of the ith ionic species. 𝜀𝑟, 𝑅, 𝑇, 𝐹, 𝜇 and C0 are the relative dielectric constant, universal gas constant, absolute temperature, Faraday constant, fluid viscosity and concentration of bulk solution respectively. The physical parameters used in this study are set as follows: 𝜀𝑟=80, 𝑇 =300 K, D(K+)=1.95 × 10-9 m2/s, D(Cl-)=2.03 × 10-9 m2/s, 𝜇=1 × 10-3 Pa ∙ s, 𝜌=1 × 103 kg/m3 and C0=10 mol/m3. The tip radius (Rt), base radius (Rb) and the length (L) of 5



PET conical nanopore are chosen as Rb=30 nm, Rt=5 nm, LR=b=200 nm. The radius of GO nanochannel (RGO) and length (LGO) are 2 nm and 20nm, respectively. The boundary conditions in equations (1)-(5) are listed in SI Table 1. 3. RESULTS AND DISCUSSIONS To confirm the morphology and quality of the GO sheets which are used as basic blocks in assembling GOM, the dispersed GO solution for spin-coating was transferred to SiO2/Si substrate and TEM grid for AFM and TEM analysis, respectively. Figure 2a supplies the AFM image of GO sheets on SiO2/Si substrate and reveals that the asprepared GO sheets possess a lateral size of around 1.5 μm. The corresponding height profile (Figure 2b) shows the thickness of GO sheet is around 1.5 nm which is larger than the thickness of single layer graphene sheet (0.34 nm) because of the attachment of oxygen functional groups23 and the surface roughness of the substrate we used. The typical TEM images of GO sheets are presented in Figure 2c and d with different magnifications. From Figure 2c, it can be found that the GO sheets with size up to 5 μm is still integrated without any tear or broken area. Meanwhile, the GO sheets utilized in our experiment also possess the single layer features as shown in Figure 2d which is consistent with the AFM results. The good structure integrality of the GO sheet can ensure that the ions and water molecules transport in interlayers of GOM with low resistance and high speed. In order to inspect the structure homogeneity of prepared GOM, Raman analysis is adopted and the results from five different testing points are shown in Figure 3a. Two obvious peaks located at 1350 cm-1 and 1600 cm-1 can be found and named as D peak and G peak24, respectively. The D peak is caused by the breathing mode of sp2 carbon rings and activated by defects3 and G peak is ascribed to the bond stretching of sp2 carbon pairs. The structural imperfection of GOM in our experiment is ascribed to the attachment of oxygen based functional groups in the carbon basal plane3. And it is worth noting that the average intensity ratio of D peak to G peak is about 0.94, which indicates the GOMs in this work are highly oxidized and have abundant oxygen groups or defects. Normally, the presence of defects would blunt the electrical properties and 6


Page 6 of 24

Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


restrict the application of GOM in many fields, but for the case of separation membranes, it may be an advantage because more holes mean more permeation pathways and higher ion and water flux. Meanwhile, the thickness of prepared GOM is confirmed to be only around 30 nm by AFM as shown in Figure 3b. According to rough estimates, the prepared GOM in our experiment contains only 15 layer GO sheets. Since the GOM we made is ultrathin as confirmed by AFM, the continuity of GOM would become extremely important when used in ion sieving and separation. The porous substrate has been thought as an ideal candidate to support the GOM and observe its integrality and continuity

1, 20.

Here, a porous PET membrane with pore density of

1×108 cm-2 and pore size of 500 nm was chosen as substrate. After the GOM being spin-coated on the porous PET membrane with the same procedure and parameters as described in experiment part, the corresponding morphology of prepared GOM was characterized by SEM. As shown in Figure 4a and b, the GOM exhibits good integrality and continuity at low and high magnification and there is no any broken area can be found in the whole visual field (More SEM images of porous PET membrane with and without spin-coated GOM are presented in SI Figure S3). For evaluating the ion transport behaviors in ultrathin GOM more precisely, the PET membrane with single conical nanopore to support GOM was obtained with single heavy ion irradiation technology and asymmetric chemical etching method. Although the ion transport properties in single conical nanopore of PET membrane have been widely investigated

25-27,

it is important to mention that each conical nanopore even

with the same tip diameter would show different transport behaviors28. For good comparison, the detailed ion transport properties of single PET conical nanopore was studied before GO membrane being spin-coated on. The typical I-V curves of KCl solution with different concentrations are shown in Figure 5a and all the nonlinear curves show obvious rectification behaviors due to the formation of the potential in the tip area29 and the diffusion double layer (DDL) on the conical pore surface with negative charge originated from the deprotonated carboxyls28,

30-31.

Because of the

existence of negative charge and DDL on the conical pore surface, the PET conical 7



nanopore presents cation selectivity properties in the form of exhibiting higher conductance when the current (mainly contributed by cations) enters the cone tip at negative voltage while presenting lower conductance when the current enters the cone base at positive voltage

32

as shown in Figure 5b. The rectification ratio as defined

previously is used to evaluate the rectifying ability of conical nanopore. In Figure 5c, the rectification ratio of PET conical nanopore (obtained at the voltage of 4 V) as a function of KCl concentration is depicted. It can be seen that the rectification ratio reaches its maximum value of 4.6 in the case of 0.4 M KCl and the trend (rectification ratio vs. KCl concentration) is similar to other reports33. The same I-V measurements were carried out after GOM being spin-coated on exactly the same PET substrate with single conical nanopore to investigate the influence of GOM on ion transport properties. From Figure 6a, it can be found that the GOM/PET conical nanopore structure also exhibits the same ion rectification effect as pure PET conical nanopore does (Figure 5a). While comparing to pure single PET conical nanopore further and carefully, the ion conductance at positive voltage range is depressed when the GO membrane was laid on the PET membrane as we expected because of the blocking effect of GO membrane to ions (Figure 5b and Figure 6b). However, the conductance of GOM/conical nanopore is largely enhanced while negative voltage being applied surprisingly. According to previous discussion, the conductance is mainly contributed by cations at negative voltage, which means that the existence of GOM enhances the cation transport instead of preventing it. As a result, the GOM can enhance the conductance at negative voltage and depress the conductance at positive voltage, which will magnify the ion rectification effect as a result as shown in Figure 6c. All the rectification ratios of GOM/PET conical nanopore for different KCl concentration increase obviously and the maximum value reaches to 238.0 at the concentration of 0.01 M, which is 50 times higher than the maximum value of 4.6 for pure PET conical nanopore (shown in Figure 5c). So, the enhanced cation transport and high rectification ratio of GOM/PET conical nanopore system is beneficial to designing and fabricating nanofluidic systems, such as nanofluidic diode34. 8


Page 8 of 24

Page 9 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In order to confirm the enhanced cation transport properties and rectification effect of GOM/PET conical nanopore system further, LiCl solution with different concentrations was also investigated by conducting the same I-V measurements in both pure PET conical nanopore and GOM/PET conical nanopore. For pure PET conical nanopore, it shows relatively weak rectification effect (comparing to KCl solution) and the maximum rectification ratio of 4.2 can be obtained as depicted in Figure 7a and b. After GOM being spin-coated on pure PET conical nanopore, as shown in Figure 7c, the positive ionic current is also obviously depressed while negative ionic current being enhanced like in KCl solution. The corresponding maximum rectification ratio of GOM/PET conical nanopore in LiCl solution increases to 151.8, which is 36 times higher than the value of pure PET conical nanopore (Figure 7b and d). Besides monovalent cations, the GOM/PET conical nanopore system also exhibits the enhanced cation transport properties and ion rectification effect in the solution of MgCl2 (SI Figure S5) and BaCl2 (SI Figure S6). From Figure 5 to 7, it can be clearly confirmed that both the conductance at negative voltage and the rectification effect are greatly enhanced since the GOM is spin-coated on single PET conical nanopore. According to our analysis, there are two possible reasons can be considered to explain the selectively enhanced ion transport properties after GOM being coated. One is the possible size enlargement of PET conical nanopore during GOM being spin-coated and another is GOM itself. In order to check the first hypothesis, the GOM was spin-coated on PET membrane with nanopores and the pore diameters were examined before and after GOM coating. For insuring the accuracy of statistic data, the porous PET membrane with pore density of 5 × 107 ions/cm2 and pore size of around 280 nm is used and no more than 5% variation in size happened before and after spin-coating GOM (SI Figure S1). The small pore size variation could not cause so great impact on the ion transport properties of PET nanopore (rectification ratio was magnified by a factor of 50) and should be ignored in our experiment. As for GOM itself, the individual GO sheets have two types of regions: functionalized (oxidized) and pristine. The former oxidized regions due to the 9



formation of carboxylic groups would exhibit negative charge properties and provide the active adsorption sites for positively charged salt ions. As a result, the large specific negative surface area contributed to more capacitive cation onto the GO sheets and formed a highly cation concentrated solution (close to the saturation)35 in and around the GOM, which also results in stronger electrostatic repulsion between the capacitive cation on the GOM and counter anion in solution36,37. Besides this, the oxidized regions in GO sheets can also act as spacers that keep adjacent sheets apart and also prevent GOM from being dissolved35. In solution, the spacers can help water and cations to intercalate between GO sheets, whereas pristine regions provide a network of capillaries that allow nearly frictionless flow of a layer of correlated water, similar to the case of water transport through carbon nanotubes35. When a bias was applied on the layered GOM, concentrated hydrated cation with size smaller than the GOM nanochannel can be driven to permeate through the interconnected nanochannels formed between GO nanosheets. Cations will follow a tortuous path primarily over the hydrophobic pristine regions rather than the hydrophilic oxidized region of GO38 at a speed orders of magnitude faster than that would occur through simple diffusion 35 and the corresponding cation current (conductance at negative voltage) will be strongly enhanced. Based on above analysis, the GOM itself is the main reason for the selective enhanced ion transport in GOM/conical nanopore system. Besides, for the existence of negative charge (contributed by deprotonated carboxyls)

28

and the diffusion double layer (DDL) on the conical nanopore wall and

PET membrane surface, the PET conical nanopore presents cation selectivity properties in the form of exhibiting high conductance (“on” state) when the current (dominated by cation) enters the cone tip at negative voltage while presenting low conductance (“off” state) when the current enters the cone base at positive voltage20 as shown in Figure 5. In our experiments, the GOM was laid on the tip side of the conical nanopore, once negative voltage was applied, the concentrated cations in and round GOM will be driven to enter the cone tip and largely enhance the ionic current. On the contrary, for the case of the positive bias, fewer cations can enter the cone base and lead to the low 10


Page 10 of 24

Page 11 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conductance. But for the existence of negatively charged GOM, the anions are prevented to enter the cone tip from GOM side and can not donate to the negative current any more. So, when GOM combined with pure PET conical nanopore on the tip side, the current through the GOM/PET conical nanopore will be largely enhanced at negative voltage and depressed at positive voltage. All of these make the GOM/PET conical nanopore possess excellent fluidic diode function and exhibit higher rectification ratios as shown in Figure 6c and 7d. In order to confirm the assumption about the mechanism of enhanced ion transport in GOM/PET conical nanopore, a simplified model for GOM/PET conical nanopore was established (SI Figure S2) and COMSOL simulation was used to describe the ion concentration profile. The KCl solution with 0.01 mol/L was selected for simulation and the boundary conditions were set as SI table 1. Figure 8 presents the simulation results of average cation and anion concentration distribution along the conical nanopore’s axis. It is revealed that the K+ ions are enriched in GOM area (near the tip) comparing with the pure PET conical nanopore at positive bias (Figure 8a). Conversely, for Cl- ions, the concentration decreased sharply in GOM and PET conical nanopore because of the negatively charged GOM sheets and PET conical nanopore wall (Figure 8b). In GOM/PET conical nanopore, the mass transport is also limited by the geometry, while the outer space (GOM) near the tip area can provide more ions to transport due to the larger access angle39 and make the ionic current being mainly dominated by the ions which flowed from tip to base. At positive bias, the GOM can decrease the anion concentration further in the tip area because of the depletion effect and thereby diminish the ionic current magnitude (Figure 5a and Figure 6a). In the case of opposite (negative) voltage, more cations will be pushed to the conical nanopore from GOM area and contribute to ionic current as shown in Figure 8c. Meanwhile, more anions can be attracted in GOM and tip area in order to keep charge balance simultaneous (Figure 8d). According to our previous discussion, cation selectivity of the interface gives rise to build-up of high- and low-conductance states in the GOM/PET conical nanopore at negative and positive applied voltage (Figure 5b and Figure 6b), respectively. This is 11



the way similar to that of classical ionic current rectification (ICR), but the accumulation/depletion of ions occurs not only inside the conical nanopore but also in the GOM layers. Thus, ion concentration enhancement/depletion in GOM layers gives rise to the selective ion transport and enhanced ionic current rectification effect. What’s more, the enhanced cationic selectivity of GOM/conical nanopore is also confirmed by experiments in asymmetric electrolyte conditions. In this case, we can measure separately the currents carried by cations or anions to compare the ionic transport properties in nanopore. And the results indicate that the cationic selectivity of GOM/PET system is also greatly enhanced than that in pure PET conical nanopore (SI Figure S4). As known, the chemical stability and mechanical stability are critical for the functional membrane used in practical applications. In our whole experiment, the spin coated GOM exhibit good mechanical stability and there is no disintegrate and crumble observed40-41. Subsequently, the ion transport properties of KCl solution with pH value from 1 to 10 were further investigated in order to study the chemical stability of GOM/PET conical nanopore. The corresponding I-V curves of pure PET conical nanopore and GOM/PET conical nanopore are shown in Figure 9a and 9b, respectively. The concentration of KCl solution used here is 0.2 M. With pH value decreasing from 10 to 1, a transition from cation selectivity (higher ionic current at negative voltage) to anion selectivity (higher ionic current at positive voltage) appears in both cases of with and without GOM for PET conical nanopore. In order to confirm this phenomenon, the KCl solution with concentration of 0.02 M and 0.002 M were also investigated and similar transition was observed (SI Figure S7 a-d). In order to evaluate the stability of GOM/PET conical nanopore system in detail, the relationship between rectification ratio and pH value are also plotted in Figure 9c and d. For pure PET nanopore, the transition point of ion selectivity appears at pH=3 in the case of 0.2 M KCl solution, which is consistent with our previous report20. Once the pure PET nanopore being covered with GOM, the transition point moves to pH=2. In other two cases (0.02 M and 0.002 M KCl solution), the transition point in GOM/PET conical nanopore also shifts 12


Page 12 of 24

Page 13 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to lower pH value while comparing with pure PET nanopore. As a result, the GOM/PET conical nanopore exhibits the cation selectivity and higher rectification ratio in a wider pH value than that of pure PET conical nanopore. As known, the ionization status in the solution will be changed and charges on the surface of PET conical nanopore will be redistributed correspondingly while regulating the pH value of solution, which results in pH-responsive ion rectification effects 42. Because isoelectric point (IEP) for track etched PET is 3.8, the surface charge state of PET conical nanopore will be converted from negative to positive when immersed in the solution with pH lower than 3.8 and the ion selectivity is also converted from cation selective to anion selective. However, the IEP of the graphene oxide is similar to 1.743, which will lower the transition point and make the GOM/PET conical nanopore system can work in a wider pH range. 4. CONCLUSIONS In conclusion, the graphene oxide membrane (GOM) /PET conical nanopore structure is fabricated on PET conical nanopore with spin-coating method and the corresponding ion transport properties are investigated. It is confirmed that the existence of GOM enhances the cation transport but depresses the anion, which gives rise to enhanced ion rectification effect. Combining COMSOL simulation, it is proved that the GOM can collect cations from reservoir and act as cation source for selective ion transportation in PET conical nanopore. It is also confirmed that the GOM/PET conical nanopore can display good selective ion transport features in a wider pH range than pure PET conical nanopore because of the chemical stability of graphene oxide. Finally, we believe that the combination of GOM and conical nanopore is a facile design which not only can magnify the ion rectification effect of conical nanopore but also can selectively increase the ionic current, which makes the GOM/PET conical nanopore hold great potential in nanofludic sensor design and high-efficiency separation membrane production. ASSOCIATED CONTENT Supporting Information 13



The Supporting Information is available free of charge on the ACS Publications website

at DOI:XX.XX.XX Figure S1. The variation of nanopore diameter in PET membrane before and after GOM spin-coating. Figure S2. Schematic representation of the PET conical nanopore and cylindrical GO nanochannel which contact with two identical reservoirs. Figure S3. The SEM images of porous PET membrane before and after spin-coating GOM with different magnifications. Figure S4. The I-V curves and ionic current ratios of PET and GOM/PET nanopores in asymmetric condition. Figure S5. The ion transport properties of MgCl2 electrolyte solution in pure PET conical nanopore and GOM/PET conical nanopore. Figure S6. The ion transport properties of BaCl2 electrolyte solution in pure PET conical nanopore and GOM/PET conical nanopore. Figure S7. The I-V curves of PET conical nanopore and GOM/PET conical nanopore with KCl solution at different pH conditions. Table S1. The boundary conditions of segments in Figure S2. AUTHOR INFORMATION Corresponding Authors *(Jie Liu) E-mail: [email protected] *(Huijun Yao) E-mail: [email protected] Notes The authors declare no competing financial interest. ORCID Huijun Yao: 0000-0002-9684-8254 ACKNOWLEDGEMENTS The authors gratefully acknowledge the finance support from National Nature Science Foundation of China (Grant Nos. 11575261, 11575260, 11775279 and 11505243). We would like to thank the members of the Materials Research Department 14


Page 14 of 24

Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and the accelerator staff of HIRFL of IMP for preparation and irradiation of PET membrane. REFERENCES 1.

Wang, L.; Williams, C. M.; Boutilier, M. S. H.; Kidambi, P. R.; Karnik, R., Single-Layer Graphene

Membranes Withstand Ultrahigh Applied Pressure. Nano Letters 2017, 17 (5), 3081-3088. 2.

Han, Y.; Xu, Z.; Gao, C., Ultrathin Graphene Nanofiltration Membrane for Water Purification.

Advanced Functional Materials 2013, 23 (29), 3693-3700. 3.

Pei, S.; Wei, Q.; Huang, K.; Cheng, H.-M.; Ren, W., Green Synthesis of Graphene Gxide by Seconds

Timescale Water Electrolytic Oxidation. Nature Communications 2018, 9 , 145. 4.

Chen, L.; Shi, G. S.; Shen, J.; Peng, B. Q.; Zhang, B. W.; Wang, Y. Z.; Bian, F. G.; Wang, J. J.; Li, D. Y.;

Qian, Z.; Xu, G.; Liu, G. P.; Zeng, J. R.; Zhang, L. J.; Yang, Y. Z.; Zhou, G. Q.; Wu, M. H.; Jin, W. Q.; Li, J. Y.; Fang, H. P., Ion Sieving in Graphene Oxide Membranes via Cationic Control of Interlayer Spacing. Nature 2017, 550, 7676. 5.

Sun, P. Z.; Zhu, M.; Wang, K. L.; Zhong, M. L.; Wei, J. Q.; Wu, D. H.; Xu, Z. P.; Zhu, H. W., Selective

Ion Penetration of Graphene Oxide Membranes. Acs Nano 2013, 7 (1), 428-437. 6.

Robinson, J. T.; Perkins, F. K.; Snow, E. S.; Wei, Z.; Sheehan, P. E., Reduced Graphene Oxide

Molecular Sensors. Nano Letters 2008, 8 (10), 3137-3140. 7.

Li, H.; Song, Z. N.; Zhang, X. J.; Huang, Y.; Li, S. G.; Mao, Y. T.; Ploehn, H. J.; Bao, Y.; Yu, M., Ultrathin,

Molecular-Sieving Graphene Oxide Membranes for Selective Hydrogen Separation. Science 2013, 342 (6154), 95-98. 8.

Xu, W. W. L.; Fang, C.; Zhou, F. L.; Song, Z. N.; Liu, Q. L.; Qiao, R.; Yu, M., Self-Assembly: A Facile

Way of Forming Ultrathin, High-Performance Graphene Oxide Membranes for Water Purification. Nano Letters 2017, 17 (5), 2928-2933. 9.

Ma, F. Y.; Li, Z.; Zhao, H. G.; Geng, Y. Y.; Zhou, W.; Li, Q. N.; Zhang, L., Potential Application of

Graphene Oxide Membranes for Removal of Cs(I) and Sr(II) from High Level-Liquid Waste. Separation and Purification Technology 2017, 188, 523-529. 10. Lin, J. H., The Anionic Surfactant/Ionic Liquids Intercalated Reduced Graphene Oxide for Highperformance Supercapacitors. Nanoscale Research Letters 2018, 13, 215. 11. Chao, Y.; Jalili, R.; Ge, Y.; Wang, C.; Zheng, T.; Shu, K.; Wallace, G. G., Self-Assembly of Flexible FreeStanding 3D Porous MoS2-Reduced Graphene Oxide Structure for High-Performance Lithium-Ion Batteries. Advanced Functional Materials 2017, 27, 22. 12. Werber, J. R.; Osuji, C. O.; Elimelech, M., Materials for Next-generation Desalination and Water Purification Membranes. Nature Reviews Materials 2016, 1 (5), 1-16. 13. Hu, R. R.; Zhang, R. J.; He, Y. J.; Zhao, G. K.; Zhu, H. W., Graphene Oxide-in-polymer Nanofiltration Membranes with Enhanced Permeability by Interfacial Polymerization. Journal of Membrane Science 2018, 564, 813-819. 14. Seoane, B.; Coronas, J.; Gascon, I.; Etxeberria Benavides, M.; Karvan, O.; Caro, J.; Kapteijn, F.; Gascon, J., Metal-organic Framework Based Mixed Matrix Membranes: A Solution for Highly Efficient CO2 Capture? Chemical Society Reviews 2015, 44 (8), 2421-2454. 15. Koenig, S. P.; Wang, L.; Pellegrino, J.; Bunch, J. S., Selective Molecular Sieving Through Porous Graphene. Nature Nanotechnology 2012, 7 (11), 728-732. 16. Surwade, S. P.; Smirnov, S. N.; Vlassiouk, I. V.; Unocic, R. R.; Veith, G. M.; Dai, S.; Mahurin, S. M., 15



Water Desalination Using Nanoporous Single-layer Graphene. Nature Nanotechnology 2015, 10 (5), 459-464. 17. Bachman, J. E.; Smith, Z. P.; Li, T.; Xu, T.; Long, J. R., Enhanced Ethylene Separation and Plasticization Resistance in Polymer Membranes Incorporating Metal-organic Framework Nanocrystals. Nature Materials 2016, 15 (8), 845-849. 18. Sun, P. Z.; Liu, H.; Wang, K. L.; Zhong, M. L.; Wu, D. H.; Zhu, H. W., Ultrafast Liquid Water Transport Through Graphene-based Nanochannels Measured by Isotope Labelling. Chemical Communications 2015, 51 (15), 3251-3254. 19. Abraham, J.; Vasu, K. S.; Williams, C. D.; Gopinadhan, K.; Su, Y.; Cherian, C. T.; Dix, J.; Prestat, E.; Haigh, S. J.; Grigorieva, I. V.; Carbone, P.; Geim, A. K.; Nair, R. R., Tunable Sieving of Ions Using Graphene Oxide Membranes. Nature Nanotechnology 2017, 12 (6), 546-551. 20. Yao, H.; Zeng, J.; Zhai, P.; Li, Z.; Cheng, Y.; Liu, J.; Mo, D.; Duan, J.; Wang, L.; Sun, Y.; Liu, J., Large Rectification Effect of Single Graphene Nanopore Supported by PET Membrane. ACS Applied Materials & Interfaces 2017, 9, 11000-11008. 21. Hsu, J. P.; Wu, H. H.; Lin, C. Y.; Tseng, S., Ion Current Rectification Behavior of Bioinspired Nanopores Having a pH-Tunable Zwitterionic Surface. Analytical Chemistry 2017, 89 (7), 3952-3958. 22. Ai, Y.; Zhang, M.; Joo, S. W.; Cheney, M. A.; Qian, S., Effects of Electroosmotic Flow on Ionic Current Rectification in Conical Nanopores. Journal of Physical Chemistry C 2010, 114 (9), 3883-3890. 23. Peng, L.; Xu, Z.; Liu, Z.; Wei, Y.; Sun, H.; Li, Z.; Zhao, X.; Gao, C., An Iron-based Green Approach to 1-h Production of Single-layer Graphene Oxide. Nature Communications 2015, 6 , 5716. 24. Zeng, J.; Liu, J.; Zhang, S. X.; Zhai, P. F.; Yao, H. J.; Duan, J. L.; Guo, H.; Hou, M. D.; Sun, Y. M., Irradiation Effects of Graphene and Thin Layer Graphite Induced by Swift Heavy Ions. Chinese Physics B 2015, 24 (8), 086103. 25. Zielinska, K.; Gapeeva, A. R.; Orelovich, O. L.; Apel, P. Y., Diode-like Properties of Single- and Multipore Asymmetric Track Membranes. Nuclear Instruments & Methods in Physics Research Section BBeam Interactions with Materials and Atoms 2014, 326, 131-134. 26. Kovarik, M. L.; Zhou, K.; Jacobson, S. C., Effect of Conical Nanopore Diameter on Ion Current Rectification. Journal of Physical Chemistry B 2009, 113 (49), 15960-15966. 27. Liu, Q.; Wang, Y.; Guo, W.; Ji, H.; Xue, J.; Ouyang, Q., Asymmetric Properties of Ion Transport in a Charged Conical Nanopore. Physical Review E 2007, 75 (5), 051201. 28. Gamble, T.; Decker, K.; Plett, T. S.; Pevarnik, M.; Pietschmann, J.-F.; Vlassiouk, I.; Aksimentiev, A.; Siwy, Z. S., Rectification of Ion Current in Nanopores Depends on the Type of Monovalent Cations: Experiments and Modeling. Journal of Physical Chemistry C 2014, 118 (18), 9809-9819. 29. Ramirez, P.; Apel, P. Y.; Cervera, J.; Mafe, S., Pore Structure and Function of Synthetic Nanopores with Fixed Charges: Tip Shape and Rectification Properties. Nanotechnology 2008, 19 (31), 315707. 30. Schiedt, B.; Healy, K.; Morrison, A. P.; Neumann, R.; Siwy, Z., Transport of Ions and Biomolecules through Single Asymmetric Nanopores in Polymer Films. Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions with Materials and Atoms 2005, 236, 109-116. 31. Siwy, Z.; Gu, Y.; Spohr, H. A.; Baur, D.; Wolf-Reber, A.; Spohr, R.; Apel, P.; Korchev, Y. E., Rectification and Voltage Gating of Ion Currents in a Nanofabricated Pore. Europhysics Letters 2002, 60 (3), 349-355. 32. O'Hern, S. C.; Boutilier, M. S. H.; Idrobo, J. C.; Song, Y.; Kong, J.; Laoui, T.; Atieh, M.; Karnik, R., Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes. Nano Letters 2014, 14 (3), 1234-1241. 16


Page 16 of 24

Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


33. Apel, P. Y.; Ramirez, P.; Blonskaya, I. V.; Orelovitch, O. L.; Sartowska, B. A., Accurate Characterization of Single Track-etched, Conical Nanopores. Physical Chemistry Chemical Physics : PCCP 2014, 16 (29), 15214-15223. 34. Karnik, R.; Duan, C. H.; Castelino, K.; Daiguji, H.; Majumdar, A., Rectification of Ionic Current in a Nanofluidic Diode. Nano Letters 2007, 7 (3), 547-551. 35. Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R., Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes. Science 2014, 343 (6172), 752-754. 36. Hu, C. Z.; Liu, Z. T.; Lu, X. L.; Sun, J. Q.; Liu, H. J.; Qu, J. H., Enhancement of the Donnan Effect through Capacitive Ion Increase Using An Electroconductive rGO-CNT Nanofiltration Membrane. Journal of Materials Chemistry A 2018, 6 (11), 4737-4745. 37. Gogotsi, Y., Moving Ions Confined Between Graphene Sheets. Nature Nanotechnology 2018, 13 (8), 625-627. 38. Mi, B. X., Graphene Oxide Membranes for Ionic and Molecular Sieving. Science 2014, 343 (6172), 740-742. 39. McKelvey, K.; Kinnear, S. L.; Perry, D.; Momotenko, D.; Unwin, P. R., Surface Charge Mapping with a Nanopipette. Journal of the American Chemical Society 2014, 136 (39), 13735-13744. 40. Xiong, R.; Hu, K. S.; Zhang, S. D.; Lu, C. H.; Tsukruk, V. V., Ultrastrong Freestanding Graphene Oxide Nanomembranes with Surface-Enhanced Raman Scattering Functionality by Solvent Assisted SingleComponent Layer-by-Layer Assembly. Acs Nano 2016, 10 (7), 6702-6715. 41. Xiong, R.; Kim, H. S.; Zhang, S.; Kim, S.; Korolovych, V. F.; Ma, R.; Yingling, Y. G.; Lu, C.; Tsukruk, V. V., Template-Guided Assembly of Silk Fibroin on Cellulose Nanofibers for Robust Nanostructures with Ultrafast Water Transport. Acs Nano 2017, 11 (12), 12008-12019. 42. Wang, Z. W.; Fan, X.; Wang, Q. Q.; Hou, S. N.; Wang, H. M.; Zhai, J.; Meng, X. M., pH- and lightregulated Ion Transport in Hourglass Shaped Al2O3 Nanochannels Patterned with N719 and APTES. Rsc Advances 2016, 6 (68), 63652-63659. 43. Hu, X. B.; Yu, Y.; Wang, Y. Q.; Zhou, J. E.; Song, L., Separating Nano Graphene Oxide From the Residual Strong-acid Filtrate of the Modified Hummers Method with Alkaline Solution. Applied Surface Science 2015, 329, 83-86.

17



Figures

Figure 1. The schematic diagram of the preparation processes of GOM/PET conical nanopore

(a) (b) 1.5 1.0

Height (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

0.5 0.0 -0.5 -1.0 -1.5 -2.0 0.0

0.5

1.0

1.5

2.0

Position (m)

Figure 2. (a) AFM image of single piece of graphene oxide; (b) the corresponding height profile of graphene oxide from (a); (c) and (d) TEM image of graphene oxide with low and high magnification, respectively.

18


Page 19 of 24

(a) 18000

D

G

(b)

(1)

14400 10800 7200 17400

(2)

14500 11600

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8700 13800

(3)

11500 9200 6900 13800

(4)

11500 9200 6900 13500

(5)

10800 8100 5400 1000

2000

3000

0

4000

Wave Number (cm-1)

2

4 μm

6

8

Figure 3. (a) The Raman results from 5 different testing points of GOM to confirm the structure homogeneity. The average intensity ratio of D peak to G peak is about 0.94, which means that the GOM is highly oxidized and has abundant oxygen groups or defects; (b) AFM image and corresponding height profile of prepared GOM. The height of prepared GOM is around 30 nm.

Figure 4. The SEM images of spin-coated GOM on porous PET. (a) and (b) top view of GOM/porous PET for observing the integrality and continuity with low and high magnification, respectively.

19



(b)

2

1.2

5

1.0

Conductance (S)

Current (A)

1

(c)

0 -1

KCl 0.001 M 0.002 M 0.004 M 0.008 M 0.01 M 0.02 M

-2 -3 -4

0.04 M 0.08 M 0.1 M 0.2 M 0.4 M 0.8 M 1M

0.04 M 0.08 M 0.1 M 0.2 M 0.4 M 0.8 M 1M

KCl 0.001 M 0.002 M 0.004 M 0.008 M 0.01 M 0.02 M

0.8

0.6

0.4

0.2

4

Rectification ratio

(a)

3

2

1

0.0 0

-5 -4

-2

0

2

-4

4

-2

0

2

4

0.001

Voltage (V)

Voltage (V)

0.01

0.1

1

Concentration (M)

Figure 5. (a) Typical I-V curves of pure PET conical nanopore with KCl electrolyte concentration from 0.001 M to 1 M and applied voltage from -4 to 4 V; (b) The corresponding ionic conductance of pure PET conical nanopore as a function of the applied voltage deduced from (a); (c) Ion rectification ratio as a function of KCl electrolyte concentration at the voltage of 4 V obtained from (a).

(a)

(c)

(b)

4

4.0

250

2

Conductance (S)

-2 -4

0.04 M 0.08 M 0.1 M 0.2 M 0.4 M 0.8 M 1M

KCl 0.001 M 0.002 M 0.004 M 0.008 M 0.01 M 0.02 M

-6 -8 -10 -12

KCl 0.001 M 0.002 M 0.004 M 0.008 M 0.01 M 0.02 M

3.0 2.5 2.0 1.5

0.04 M 0.08 M 0.1 M 0.2 M 0.4 M 0.8 M 1M

1.0

0

Voltage (V)

2

150

100

50

0

0.0

-16 -2

200

0.5

-14

-4

Rectification ratio

3.5

0

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

4

-4

-2

0

2

Voltage (V)

4

0.001

0.01

0.1

Concentration (M)

Figure 6. (a) Typical I-V curves of GOM/conical nanopore with KCl electrolyte concentration from 0.001 M to 1 M and applied voltage from -4 to 4 V; (b) The corresponding ionic conductance of GOM/conical nanopore as a function of the applied voltage obtained from (a); (c) Ion rectification ratio as a function of KCl electrolyte concentration at the voltage of 4 V deduced from (a).

20


1

Page 21 of 24

(b)

(a) 1.0

4.5

PET conical nanopore

4.0


Rectification ratio

Current (A)

0.5 0.0 -0.5

LiCl 0.001 M 0.002 M 0.004 M 0.008 M 0.01 M 0.02 M

-1.0 -1.5 -2.0

0.04 M 0.08 M 0.1 M 0.2 M 0.4 M 0.8 M 1M

3.5 3.0 2.5 2.0 1.5 1.0

-2.5

0.5

-3.0 -4

-2

0

2

0.001

4

(c) 1

0.01

0.1

1

Concentration (M)

Voltage (V)

(d) 180

GOM/PET conical nanopore

160

0


140

Rectification ratio

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1 -2

LiCl 0.001 M 0.002 M 0.004 M 0.008 M 0.01 M 0.02 M

-3 -4 -5

0.04 M 0.08 M 0.1 M 0.2 M 0.4 M 0.8 M 1M

120 100 80 60 40 20

-6

0

-7 -4

-2

0

2

4

0.001

Voltage (V)

0.01

0.1

1

Concentration (M)

Figure 7. (a) Measured I-V curves of pure PET conical nanopore performed in LiCl electrolyte solution; (b) The rectification ratio as a function of LiCl electrolyte concentration at the voltage of 4V deduced from (a); (c) I-V curves of GOM/PET conical nanopore carried out in LiCl electrolyte solution; (d) The enlarged rectification ratio of GOM/PET conical nanopore obtained from (c). The concentration of LiCl ranges from 0.001M to 1M.

21



(b)

(a)

12

GOM/PET nanopore PET conical nanopore +1V bias voltage

120

Concentration of Cl- (mol/m3)

Concentration of K+ (mol/m3)

140

100 80 60 40 20

-0.4

GOM/PET nanopore PET conical nanopore

10

+1V bias voltage

8

6

4

2

0

0 0.0

0.4

0.8

-0.4

1.2

0.0

0.4

0.8

1.2

Position (m)

Position (m)

(d)

(c) 300


250

Concentration of Cl- (mol/m3)

Concentration of K+ (mol/m3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

-1V bias voltage 200

150

100

50

-1V bias voltage 150

100

50

0

0 -0.4


200

0.0

0.4

0.8

-0.4

1.2

0.0

0.4

0.8

1.2

Position (m)

Position (m)

Figure 8. COMSOL simulation of the average ion concentration distributions along the conical nanopore’s axis. (a) and (b) present the average concentration distribution of K+ and Cl- ions at positive bias, respectively; (c) and (d) give the average concentration distribution of K+ and Cl- ions at negative bias, respectively.

22


Page 23 of 24

(a)

(b)

6

1.0



5

0.5

0.2 M KCl pH=1 pH=2 pH=3 pH=4 pH=5

3 2 1

0.0

pH=6 pH=7 pH=8 pH=9 pH=10

Current (A)

Current (A)

4

0 -1

-0.5

0.2 M KCl pH=1 pH=2 pH=3 pH=4 pH=5

-1.0 -1.5 -2.0

pH=6 pH=7 pH=8 pH=9 pH=10

-2.5

-2

-3.0

-3 -4

-2

0

2

-3.5

4

-4

Voltage (V)

-2

0

2

4

Voltage (V)

(c)

(d)

10

100


PET conical nanopore 10

1

Rectification ratio

Rectification ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

0.002 M KCl 0.02 M KCl 0.2 M KCl 0.01

1

0.002 M KCl 0.02 M KCl 0.2 M KCl

0.1

0.01

0.001

0.001

0

2

4

6

8

10

0

pH

2

4

6

8

10

pH

Figure 9 (a) and (b) typical I-V curves of pure PET conical and GOM/PET conical nanopore performed in 0.2 M KCl with pH value from 1 to 10, respectively. The current of 0.2 M KCl (pH=1) in Figure 9 (b) was divided by 5 for better comparison. (c) and (d) the rectification of pure PET conical nanopore and GOM/PET conical nanopore, respectively. The dashed line is drawn only for guiding the naked eye.

23



Table of Contents

24


Page 24 of 24

Selectively Enhanced Ion Transport in Graphene Oxide Membrane

Recommend Documents