Gated Water Transport through Graphene ... - ACS Publications

Subscriber access provided by GRIFFITH UNIVERSITY

Article

Gated Water Transport Through Graphene Nanochannels: From Ionic Coulomb Blockade to Electroosmotic Pump Wen Li, Wensen Wang, Yingnan Zhang, Youguo Yan, Caili Dai, and Jun Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05374 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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 free 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 accessible to all readers and 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.

The Journal of Physical Chemistry C 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 23

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

The Journal of Physical Chemistry

Gated Water Transport through Graphene Nanochannels: From Ionic Coulomb Blockade to Electroosmotic Pump Wen Lia,b,c,†, Wensen Wanga,b,†, Yingnan Zhanga,b, Youguo Yana,b, Caili Daid,* and Jun Zhanga,b,*

a College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, People’s Republic of China b Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong (China University of Petroleum), Qingdao, Shandong 266580, People’s Republic of China c Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, United States d School of Petroleum Engineering, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China

1

ACS Paragon Plus Environment


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

ABSTRACT: Understanding and controlling water or ion transport in nanochannels plays an important role in further unravelling the transport mechanism of biological membrane channels and designing functional nanofluidic devices. In this work, molecular dynamics simulations were conducted to investigate water and ion transport in graphene nanochannels. Similar to electron coulomb blockade phenomenon observed in quantum dots, we discovered an ionic coulomb blockade phenomenon in our graphene nanochannels, and another two ion transport modes were also proposed to rationalize the observed phenomena under different electric field intensities. Furthermore, based on this blockade phenomenon we found that the Open and Closed state of the graphene nanochannels for water transport could be switched according to external electric field intensities, and electroosmotic flow could further enhance the water transport. These findings might have potential applications in designing and fabricating controllable valves in lab-on-chip nanodevices.

2


Page 2 of 23

Page 3 of 23

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. INTRODUCTION Water or ion transport through cell membrane channels plays a key role in understanding of many biological activities.1-3 Therefore, in the past decades water permeation in biological membrane channels such as Aquaporin and glycerol facilitator (GlpF) have been widely studied.4-8 These biological channels facilitate fast water or ion transport and are usually gated, which could switch between Open and Closed states according to external stimuli.9-11 Therefore, inspired by these functional biological nanochannels many efforts have been devoted to designing artificial nanochannels to mimic the functions of biological nanochannels for applications in molecular separation, desalination, energy conversion, molecular sensing, and related fields.12-16 Recently, carbon nanotube and nanoporous graphene have shown great potential in mimicking biological channels due to their excellent performances as nanofluidic channels. In 2001, single-walled carbon nanotubes (SWNTs) were designed as a water channel by Hummer et al.17 to mimic biological nanochannels. From then on, using molecular dynamics (MD) simulations, great research interests have been initiated to study the transport mechanism of water or ion in CNTs and design bio-inspired artificial nanochannels with distinctive functions. Up to now, many novel applications18 have been realized by controlling water or ion transport in CNTs through electric field,19 mechanical stress20-21 and the structure outside the CNTs.22-24 Furthermore, by controlling the pore sizes, single or multi-layer nanoporous graphene could facilitate fast water transport meanwhile reject ions, and therefore, efficient seawater desalination has been achieved.25-27 Apart from above methods, electroosmotic flow provides another effective way to control water or ion flow in nanochannels.28 When ionic solution is confined in a charged nanochannel, an electrical double layer (EDL) could be formed near the channel surface.29 In an external electric field, water 3



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

molecules in the EDL can be dragged by moving ions. This generates electroosmotic flow. However, when the channel sizes approach the dimensions of hydrated ions, the hydration effects or steric interactions will be crucial to determine the ion or water transport behavior,30-32 and the corresponding electroosmotic behavior will also be changed significantly. Actually, Feng, et al. recently reported measurements of ionic transport through a single sub-nm pore junction.33 Their results showed that ionic current was suppressed at low voltage, while the I-V curves showed normal ohmic responses at high voltage. An ionic Coulomb blockade similar to electronic Coulomb blockade observed in quantum dots was proposed to rationalize their results at low voltage. However, if this ionic Coulomb blockade occurs in an ultra-narrow and long nanochannel, it will be of great interest to understand the corresponding blockade mechanism and electroosmotic behavior. In addition, based on the principle of this ionic Coulomb blockade phenomenon, we assume that pressure induced water transport in this kind of channel might be turned between On and Off states according to the applied electric field intensities. In contrast to mechanical switches, this might provide a new way to design nanofluidic switching devices without moving parts. To realize this assumption, in this study we devised a series of graphene nanochannels with different pore sizes and channel lengths which were controlled by the number of graphene layers. Using MD method, electric field driven ion transport in these channels were first simulated to prove whether or not there was ionic Coulomb blockade phenomenon in these devised channels. Then, pressures induced water transport in these channels were studied under different electric field intensities to evaluate whether or not the electric field could turn the On and Off states of these channels for water transport. 2. COMPUTATIONAL DETAILS 4


Page 4 of 23

Page 5 of 23

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


Figure 1(top panel) shows the devised graphene flakes with nanopores in them. The pore was constructed by deleting carbon atoms, and the dangling carbon atoms were saturated by hydrogen atoms.25 Three nearly circular pores with different diameters were built, and the dimeters were roughly measured through the blue circles. Then, different layers of the devised graphene flakes with interlayer distance of 3.4 Å were stacked to form graphene nanochannels with different lengths. In this work, 3, 9 and 15 layers of graphene flakes were used and the corresponding length was 0.68 nm, 2.72 nm, and 4.76 nm, respectively. Figure 1(bottom panel) shows one typical system for pressure and/or electric field (0-0.4 V/nm) driven water and/or ion transport. The system dimension ranges from 3.2 nm ×3.4 nm ×6.2 nm to 3.2 nm ×3.4 nm ×10.2 nm depending on the channel lengths. The ionic concentration (NaCl) was set as 0.5 mol/L. Periodic boundary conditions were applied in all the X, Y and Z directions. During the simulation, the carbon atoms at the edges of the each graphene flake were fixed and other atoms could move freely.

Figure 1. (Top panel) The designed graphene nanopores with different pore sizes (diameters of Pore-I: 4.91 Å; Pore-II: 6.37 Å; Pore-III: 7.42 Å). (Bottom panel) The built system for electric field and/or pressure induced ionic solution transport. The yellow and blue balls denote Na+ and Cl-, respectively. The water is shown as transparent blue.

For pressure induced water transport, external forces were applied to selected atoms in the water 5



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 6 of 23

reservoir34-35 to produce pressure differences between the graphene channels. Constant forces (f) were applied to the oxygen atoms of water molecules in the Z direction within approximately 0.5 nm of the left and right boundaries of the system (yellow regions in Figure 1(bottom panel)) to generate steady water flow through the channel. The pressure difference (∆P) was calculated by ∆P = nf/A, where n was the number of water molecules within the selected 1 nm region and A was the cross-section area of the system. In our studies, relatively large pressures of 0-120 MPa were used to obtain accurate statistical results because of the limited simulation time.36 The water or ions were recorded to pass through the channel once they were dragged out of the rightmost graphene flake (Figure 1(bottom panel)). All the MD simulations were conducted by NAMD software.37 CHARMM27 force fields38 were using to describe the interatomic interactions, and TIP3P model was used to describe water molecules.39 Table 1 shows the atomic charges and Lennard-Jones parameters using in our MD simulations. The atom charges of graphene were obtained from Ref. 25.25 The cutoff distance for vdW interactions was set as 12 Å, and the switching distance for nonbonded interactions was set to 8 Å. The long range coulomb interactions were calculated using particle mesh Ewald summation method.40 The simulations were conducted in NVT ensemble at T = 300 K with time step of 2 fs. The Langevin dynamics with a damping constant of 1 ps-1 were used to control the temperature.

Table 1. The atomic charges and Lennard-Jones parameters used in our simulations Elements

ε (Kcal/mol)

σ (Å)

q (e)

2

C(sp )

0.0700

3.9848

0

CCH

0.0460

2.9850

0.115

HCH

0.0301

2.4200

-0.115

Hw

0.0300

2.7164

0.417

Ow

0.1521

3.5364

-0.834

6


Page 7 of 23

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


Na+

0.0469

2.8215

1.0

Cl-

0.1500

5.5400

-1.0

3. RESULTS AND DISCUSSION 3.1. Electric field driven ion transport. 3.1.1. The effect of channel lengths. The Pore-II graphene flake in Figure 1(top panel) was selected to study the effects of channel lengths on ionic flow behavior under different electric field intensities. 50 ns simulation was conducted for each system. The first 10 ns was used to stabilize ionic flow, and the last 40 ns was used for data analysis. Figure 2 shows the number of passed ions through the channels under different electric fields. During our simulations, we observed that the channel could selective transport anion (Cl- ion) even at high electric field of 0.4 V/nm. This might be due to the Coulomb coupling between the passing ion and the positive charged hydrogen atoms attached to the nanochannel rims.41 From Figure 2, the number of passed ions increases nearly linear with the electric field intensity, and the shorter channel shows larger ion flux because of its shorter ion path lengths and lower energy barriers.

Figure 2. The number of ions passed through the graphene channels with different lengths at electric fields from 0 to 0.4 V/nm.

However, detailed observation shows that for the channel with 15 layers of graphene flakes the

7



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

number of passed ions is suppressed from 0 to 0.1 V/nm, and is nonlinear with electric field increase between 0-0.1 V/nm and 0.1-0.4 V/nm. From the simulation trajectories, the Cl- ion could enter the channel at electric fields from 0-0.1 V/nm, and move in the channel back and forth. But it could not pass through the channel during the 40 ns simulation. To unravel the reason, the electric potential profiles along the center of the 3, 9 and 15-layer channels in Figure 2 were calculated through the electrostatic potential integration of the atomic charges on the channels, shown in Figure 3. From Figure 3, longer channels show deeper and broader potential wells, which favors ion trapping in it. However, from the simulation trajectories of the 3-layer and 9-layer models, no ion could be trapped in the 3-layer channel even without electric field, and the ion in the 9-layer channel could be easily dragged out of the channel at a small electric filed. This is because the potential well is too small and narrow for vibrating ion originated from its intrinsic thermal fluctuation to be trapped. Furthermore, the average time for one ion transport through the 9-layer channel is much longer than that of the 3-layer channel due to its deeper and broader potential well and longer channel. For the 15-layer channel, it has the deepest and broadest potential well among the three channels. The entered ion could be trapped in the channel until the electric field force is large enough to drag the trapped ion out of the channel. Therefore, the deep and broad characteristics of potential well all together contribute to the ionic trapping phenomenon.

8


Page 8 of 23

Page 9 of 23

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


Figure 3. Distributions of the intrinsic electrostatic potential along the axis of our graphene nanochannels. The regions between the dashed lines represent the left and right boundaries of the channels.

We also observed some interesting ion transport phenomena in the 15-layer channel. At 0 V/nm, only one Cl- ion could be trapped in this channel. This is because the potential well is not deep and broad enough to accommodate two ions trapping in it at the same time. The first entered Cl- ion would create a strong coulomb repulsion preventing other Cl- ions to flow into the channel. We also recorded the time evolution of the number of Cl- ions in this channel under different electric field intensities (Figure 4). Figure 4a shows that even at electric field of 0.05 V/nm there is still only one Cl- ion trapping in the channel during the whole simulation, and the Cl- ion outside the channel could not enter the channel as well (see Movie S1 in Supporting Information (SI)). This indicates that on the one hand, the applied electric field force could not drag the trapped Cl- ion out of the channel, and on the other hand, the electric field force also could not overcome the coulomb repulsion interactions created by the trapped Cl- ion to allow other Cl- ions to enter the channel . This phenomenon is similar to electron Coulomb blockade observed in a quantum dot.42 In this work, we also called this phenomenon as ionic coulomb blockade analogy to Ref. 33.33

9



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

Figure 4. Time evolution of the number of ions present in the graphene channel (Pore-II and 15-layer) under various electric fields. The right figures in (a) and (b) show the diagram for ions in the graphene channels. The atoms in the hydration shell of the trapped ions are shown as VDW balls.

At electric field of 0.1 V/nm (Figure 4(b)), two Cl- ions could stay inside the channel at the same time from 22 ns to 24 ns. This is because the increased electric field force (Felec) could overcome the coulomb repulsion interactions originated from the first trapped Cl- ion. Once the second anion enters the channel, the coulomb repulsion forces (Fcoul) between the two entered ions would provide another driving force for the first trapped Cl- ion escaping from the channel (see the right figure of Figure 4b and Movie S2 in SI). Therefore, the two entered anions simultaneous stay in the channel only for a short time. We define this ion transport mode as knock-on ion transport mode (Mk).43-44 This mode corresponds to the number of ions in the channel changing as 1-2-1. At electric field of 0.2 V/nm (Figure 4c), the above knock-on transport mode becomes more prominent. In addition, the electric field could also drive the ion passing through the channel directly (see Movie S3 in SI). This mode is defined as electric field driven ion transport mode (Me). This corresponds to the number of ions in the channel changing as 0-1-0 in Figure 4c. Furthermore, the ratio of the Me/Mk will increase with electric field intensities increase. 10


Page 10 of 23

Page 11 of 23

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


3.1.2. The effect of pore sizes. Three pore sizes (Figure 1(top panel)) with length of 15 stacked graphene flakes were selected to study the effects of pore sizes on electric field driven ionic flow. We also recorded the number of passed ions through these channels, shown in Figure 5. For Pore-I channel, no ions could pass through it. From its dynamic trajectory, this channel could only accommodate single file water chain in it due to its small channel diameter. Moreover, the naked and hydrated radii of Cl- ion are 0.181 and 0.332 nm, respectively.45 Thus, Cl- ion almost needs to be completely dehydrated prior to enter the Pore-I channel, which will generate a very large energy barriers for ion transport. As a result, this channel is impermeable for ion transport under our adopted electric field magnitude. For Pore-II channel, the passed ion could carry part of its hydrated water molecules into the channel because its channel diameter is comparable with the hydrated diameter of Cl- ion. Movie S4 shows the ion transport process under electric field of 0.05 V/nm. In this movie, the atoms within 3.32 Å (hydrated radius of Cl- ion) of the passed ion are shown as VDW balls. From this movie, the positive charged hydrogen atoms at the channel rims (marked as purple VDW balls in Movie S4) could participate in the ionic hydration. The purple atoms in the right figure of Figure 4a also show an instantaneous configuration of the hydrated hydrogen atoms at the channel rim. However, these hydrogen atoms could not move along with the passing ion. As a result, when the ion moves inside the channel, the hydration shell of this ion will undergo breaking and rebuilding process. This process will generate an energy barrier for ion transport. Therefore, the ion could be trapped in this channel at small electric fields (Pore-II in Figure 5).

11



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

Figure 5. The number of ions passed through the graphene channels with different pore sizes at electric fields from 0 to 0.4 V/nm.

However, for Pore-III channel the pore diameter is larger than the hydration diameter of Cl- ion. Therefore, the ion could carry most of its hydrated water molecules passing through the channel. This is consistent with our calculation of the average number of water molecules in the first hydration shell of Cl- ion46 when the ion is in (5.2) and out (5.6) the channel. Movie S5 also shows that the hydrogen atoms at the channel rims could only participate in its ion hydration occasionally, and thus the energy barriers suppressing ion transport are very small. So, the passing ion could transport through the channel even at a small electric field.

3.1.3. Electro-osmosis. We also discuss the electroosmotic flow of water induced by the electric field driven ion transport. In order to present the electroosmotic flow, the average number of passed water molecules induced by one passed ion were calculated by R=Nwater/Nion, shown in Figure 6. From Figure 6a, the electroosmotic flow is larger for longer channel. This is because the longer channel accommodates more water molecules in it, and thus the passing ion could push or drag more water molecules through the channel. From Figure 6b, no electroosmotic phenomenon is observed in Pore-I channel because no ion could pass through this channel. For Pore-II and Pore-III, similar 12


Page 12 of 23

Page 13 of 23

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


electroosmotic capability is observed. This indicates that the passing ion could only drag or push these water molecules within definite region around it, and thus larger pore size (for ultra-narrow channels) has negligible contribution to the electroosmotic performance. Furthermore, from Figure 6a and b, the electric field intensities adopted in this work also have a minor influence on the electroosmotic performance.

Figure 6. The rate of electroosmotic flow of water in channels with different lengths (a) and pore sizes (b).

3.2. Pressure induced water transport. From above discussion, we observed an ionic coulomb blockade phenomenon in the graphene channels composed of 15 stacked Pore-II graphene flakes at electric fields smaller than 0.2 V/nm. Based on this phenomenon, we inferred that the trapped ion might block the channel for water flow. To prove this, we further simulated pressures induced water transport in this kind of graphene channel. For comparison, pure water and NaCl water solution were used, respectively. 20 ns simulation was conducted for each system. The first 5 ns was used to form stable water flow, and the last 15 ns was used for data analysis. The number of passed water molecules at different pressures are shown in Figure 7. For the pure water system, the passed water molecules increase linearly with the pressures increase. However, for the NaCl water solution system nearly no water molecules could pass through the graphene channel 13



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

when pressures are lower than 120 MPa. Moreover, only small amount of water molecules could pass through the channel even at 200 MPa. Through observation of the simulation trajectories, one Cl- ion was trapped in the channel for the NaCl water solution system during the whole simulation. These results demonstrate the trapped ion could block the channel and suppress water transport effectively.

Figure 7. The number of passed water molecules through the graphene channel (Pore-II and 15-layer) at different pressures with and without NaCl in the solution.

3.3. Electric field gated graphene nanochannel for water transport. From Figure 2, the ionic coulomb blockade phenomenon only occurred at small electric fields. At large electric field, the electric field force could drag the trapped ion out of the channel. Moreover, from Figure 7, the trapped ion could prevent water flow at a broad range of pressures. Based on these results, the electric field should have the ability to turn the Open and Closed states of the graphene channel for water transport by controlling the ionic coulomb blockade phenomenon. Therefore, to prove this, we simulated pressure (40 MPa) induced water flow in the graphene channel (Pore-II and 15 staked graphene flakes) under different electric field intensities. 20 ns simulation was conducted for each system, and the number of passed water molecules and ions during the last 15 ns was recorded in 14


Page 14 of 23

Page 15 of 23

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


Figure 8. Without external electric field, for pure water system, the number of passed water molecules is 31. However, when NaCl presents in the solution, no water could pass through the channel because of the ionic coulomb blockade. The water flow is weakened significantly even at electric field of 0.1 V/nm compared with the pure water system. But when electric field is larger than 0.1 V/nm, the water flow will be enhanced greatly. Considering the results in Figure 6, the electroosmotic flow should be responsible for the enhancement of water flow. This is because the trapped ion could escape from the potential well of the graphene channel when the electric field is over than 0.1 V/nm (Figure 2). Then, the passing ions would drag the water molecules in its hydration shell through the channel. Considering the electro-osmosis ability of 15-layer channel in Figure 6a, the number of passed water molecules for 0.2-0.4 V/nm systems in Figure 8 is a result of the combination of the pressure and the electroosmotic flow. In brief, the Open and Closed states for water transport in the graphene channel could be switched by external electric fields. At small electric field, the ion could be trapped in the channel and then block water transport. At large electric field, the trapped ion could escape from the channel and then electroosmotic flow would further enhance the water transport. In this work, we only studied the ion/water transport in graphene channels composed of perfectly aligned graphene nanopores. For practical application, mismatched graphene nanopores are more prevalent. Then the effective ion passage aperture will be decreased and the ion transport path will be increased. This will result in a smaller ion or water flow, and this point will also be considered in our following studies.

15



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

Figure 8. The number of ions and water molecules passed through the graphene channel (Pore-II and 15-layer) under different electric field intensities. All the systems were simulated under external pressure of 40 MPa. The leftmost system shows the pure water system, and other systems were simulated in NaCl solution with concentration of 0.5 mol/L.

4. CONCLUSIONS In this work, MD simulations were conducted to investigate ion and water transport in multilayer graphene nanochannels with different channel lengths and pore sizes. Similar to electron coulomb blockade observed in quantum dots, we found an ionic coulomb blockade phenomenon in the multilayer graphene nanochannels. Because of the existence of potential wells, the passing ion could be trapped in the channels. The trapped ion could escape from the potential well when the electric field is over than 0.1 V/nm. Based on this ionic blockade phenomenon, we proposed that the Open and Closed states for pressure induced water transport in the graphene channels could be switched by the external electric fields. We hope that this method will be helpful for future development of novel nanaofluidic devices based on multilayer nanoporous graphene.

ASSOCIATED CONTENT 16


Page 16 of 23

Page 17 of 23

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


Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Movie S1: ionic coulomb blockade phenomenon; Movie S2: knock-on ion transport mode; Movie S3: electric field driven ion transport mode. Movie S4: ion transport in Pore-II channel; Movie S5: ion transport in Pore-III channel; AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Author Contributions

†W.L. and W.W. contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by National Basic Research Program of China (2015CB250904), National Natural Science Foundation of China (51302321), Climb Taishan Scholar Program in Shandong Province (tspd20161004), Fundamental Research Funds for the Central Universities (15CX05049A, 15CX08003A, 14CX02222A) and Key Laboratory of Tectonics and Petroleum Resources (TPR-2016-16).

17



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

REFERENCES (1) Murata, K.; Mitsuoka, K.; Hirai, T.; Walz, T. Structural Determinants of Water Permeation through Aquaporin-1. Nature 2000, 407, 599-605. (2) Agre, P. Aquaporin Water Channels (Nobel Lecture). Angew. Chem., Int. Ed. 2004, 43, 4278-4290. (3) Miyazawa, A.; Fujiyoshi, Y.; Unwin, N. Structure and Gating Mechanism of the Acetylcholine Receptor Pore. Nature 2003, 423, 949-955. (4) Gravelle, S.; Joly, L.; Detcheverry, F.; Ybert, C.; Cottin-Bizonne, C.; Bocquet, L. Optimizing Water Permeability through the Hourglass Shape of Aquaporins. Proc. Natl. Acad. Sci. 2013, 110, 16367-16372. (5) de Groot, B. L.; Grubmüller, H. Water Permeation across Biological Membranes: Mechanism and Dynamics of Aquaporin-1 and GlpF. Science 2001, 294, 2353-2357. (6) de Groot, B. L.; Grubmüller, H. The Dynamics and Energetics of Water Permeation and Proton Exclusion in Aquaporins. Curr. Opin. Struct. Biol. 2005, 15, 176-183. (7) Zhu, F.; Tajkhorshid, E.; Schulten, K. Theory and Simulation of Water Permeation in Aquaporin-1. Biophys. J. 2004, 86, 50-57. (8) Borgnia, M. J.; Agre, P. Reconstitution and Functional Comparison of Purified GlpF and AqpZ, the Glycerol and Water Channels from Escherichia Coli. Proc. Natl. Acad. Sci. 2001, 98, 2888-2893. (9) Tajkhorshid, E.; Nollert, P.; Jensen, M. Ø.; Miercke, L. J.; O'connell, J.; Stroud, R. M.; Schulten, K. Control of the Selectivity of the Aquaporin Water Channel Family by Global Orientational Tuning.

Science 2002, 296, 525-530. (10) Vidossich, P.; Cascella, M.; Carloni, P. Dynamics and Energetics of Water Permeation through 18


Page 18 of 23

Page 19 of 23

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


the Aquaporin Channel. Proteins: Struct., Funct., Bioinf. 2004, 55, 924-931. (11) Wan, X.; Steudle, E.; Hartung, W. Gating of Water Channels (Aquaporins) in Cortical Cells of Young Corn Roots by Mechanical Stimuli (Pressure Pulses): Effects of ABA and of HgCl2. J. Exp.

Bot. 2004, 55, 411-422. (12) Li, W.; Bell, N. A.; Hernandez-Ainsa, S.; Thacker, V. V.; Thackray, A. M.; Bujdoso, R.; Keyser, U. F. Single Protein Molecule Detection by Glass Nanopores. ACS Nano 2013, 7, 4129-4134. (13) Duan, R.; Xia, F.; Jiang, L. Constructing Tunable Nanopores and Their Application in Drug Delivery. ACS Nano 2013, 7, 8344-8349. (14) Duan, C.; Majumdar, A. Anomalous Ion Transport in 2-nm Hydrophilic Nanochannels. Nature

Nanotechnol. 2010, 5, 848-852. (15) Azamat, J. Functionalized Graphene Nanosheet as a Membrane for Water Desalination Using Applied Electric Fields: Insights from Molecular Dynamics Simulations. J. Phys. Chem. C 2016, 120, 23883-23891. (16) Sun, P.; Zheng, F.; Wang, K.; Zhong, M.; Wu, D.; Zhu, H. Electro-and Magneto-Modulated Ion Transport through Graphene Oxide Membranes. Sci. Rep. 2014, 4, 6798. (17) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Water Conduction through the Hydrophobic Channel of a Carbon Nanotube. Nature 2001, 414, 188-190. (18) Thomas, M.; Corry, B.; Hilder, T. A. What Have We Learnt about the Mechanisms of Rapid Water Transport, Ion Rejection and Selectivity in Nanopores from Molecular Simulation? Small 2014,

10, 1453-1465. (19) Su, J.; Guo, H. Control of Unidirectional Transport of Single-File Water Molecules through Carbon Nanotubes in an Electric Field. ACS Nano 2010, 5, 351-359. 19



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

(20) He, Z.; Corry, B.; Lu, X.; Zhou, J. A Mechanical Nanogate Based on a Carbon Nanotube for Reversible Control of Ion Conduction. Nanoscale 2014, 6, 3686-3694. (21) Wan, R.; Li, J.; Lu, H.; Fang, H. Controllable Water Channel Gating of Nanometer Dimensions.

J. Am. Chem. Soc. 2005, 127, 7166-7170. (22) Gong, X.; Li, J.; Zhang, H.; Wan, R.; Lu, H.; Wang, S.; Fang, H. Enhancement of Water Permeation across a Nanochannel by the Structure Outside the Channel. Phys. Rev. Lett. 2008, 101, 257801. (23) Kou, J.; Yao, J.; Lu, H.; Zhang, B.; Li, A.; Sun, Z.; Zhang, J.; Fang, Y.; Wu, F.; Fan, J. Electromanipulating Water Flow in Nanochannels. Angew. Chem., Int. Ed. 2015, 127, 2381-2385. (24) Li, J.; Gong, X.; Lu, H.; Li, D.; Fang, H.; Zhou, R. Electrostatic Gating of a Nanometer Water Channel. Proc. Natl. Acad. Sci. 2007, 104, 3687-3692. (25) Cohen-Tanugi, D.; Grossman, J. C. Water Desalination across Nanoporous Graphene. Nano Lett. 2012, 12, 3602-3608. (26)Surwade, S. P.; Smirnov, S. N.; Vlassiouk, I. V.; Unocic, R. R.; Veith, G. M.; Dai, S.; Mahurin, S. M. Water Desalination Using Nanoporous Single-Layer Graphene. Nature Nanotechnol. 2015, 10, 459-464. (27) Cohen-Tanugi, D.; Lin, L.-C.; Grossman, J. C. Multilayer Nanoporous Graphene Membranes for Water Desalination. Nano Lett. 2016, 16, 1027-1033. (28) Zambrano, H. A.; Vásquez, N.; Wagemann, E. Wall Embedded Electrodes to Modify Electroosmotic Flow in Silica Nanoslits. Phys. Chem. Chem. Phys. 2016, 18, 1202-1211. (29) Qiao, R.; Aluru, N. Charge Inversion and Flow Reversal in a Nanochannel Electro-Osmotic Flow. Phys. Rev. Lett. 2004, 92, 198301. 20


Page 20 of 23

Page 21 of 23

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


(30)Joshi, R.; Carbone, P.; Wang, F.-C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H.; Geim, A. K.; Nair, R. R. Precise and Ultrafast Molecular Sieving through Graphene Oxide Membranes. Science 2014, 343, 752-754. (31) Cheng, C.; Jiang, G.; Garvey, C. J.; Wang, Y.; Simon, G. P.; Liu, J. Z.; Li, D. Ion Transport in Complex Layered Graphene-Based Membranes with Tuneable Interlayer Spacing. Science Adv. 2016,

2, e1501272. (32) Daiguji, H. Ion Transport in Nanofluidic Channels. Chem. Soc. Rev. 2010, 39, 901-911. (33) Feng, J.; Liu, K.; Graf, M.; Dumcenco, D.; Kis, A.; Di Ventra, M.; Radenovic, A. Observation of Ionic Coulomb Blockade in Nanopores. Nat. Mater. 2016, 15, 850-855. (34) Goldsmith, J.; Martens, C. C. Pressure-Induced Water Flow through Model Nanopores. Phys.

Chem. Chem. Phys. 2009, 11, 528-533. (35) Hilder, T. A.; Gordon, D.; Chung, S. H. Salt Rejection and Water Transport through Boron Nitride nanotubes. Small 2009, 5, 2183-2190. (36) Suk, M. E.; Aluru, N. Water Transport through Ultrathin Graphene. J. Phys. Chem. Lett. 2010, 1, 1590-1594. (37) Phillips, J. C.; Braun, R.; Wang, W.; Gumbart, J.; Tajkhorshid, E.; Villa, E.; Chipot, C.; Skeel, R. D.; Kale, L.; Schulten, K. Scalable Molecular Dynamics with NAMD. J. Comput. Chem. 2005, 26, 1781-1802. (38) MacKerell Jr, A. D.; Bashford, D.; Bellott, M.; Dunbrack Jr, R. L.; Evanseck, J. D.; Field, M. J.; Fischer, S.; Gao, J.; Guo, H.; Ha, S. All-Atom Empirical Potential for Molecular Modeling and Dynamics Studies of Proteins. J. Phys. Chem. B 1998, 102, 3586-3616. (39) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of 21



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

Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926-935. (40) Darden, T.; York, D.; Pedersen, L. Particle Mesh Ewald: An N⋅ log (N) Method for Ewald Sums in Large Systems. J. Chem. Phys. 1993, 98, 10089-10092. (41) Sint, K.; Wang, B.; Král, P. Selective Ion Passage through Functionalized Graphene Nanopores.

J. Am. Chem. Soc. 2008, 130, 16448-16449. (42) Beenakker, C. Theory of Coulomb-Blockade Oscillations in the Conductance of a Quantum Dot.

Phys. Rev. B 1991, 44, 1646. (43) Li, W.; Yan, Y.; Wang, M.; Král, P.; Dai, C.; Zhang, J. Correlated Rectification Transport in Ultranarrow Charged Nanocones. J. Phys. Chem. Lett. 2017, 8, 435-439. (44) Köpfer, D. A.; Song, C.; Gruene, T.; Sheldrick, G. M.; Zachariae, U.; de Groot, B. L. Ion Permeation in K+ Channels Occurs by Direct Coulomb Knock-On. Science 2014, 346, 352-355. (45) Nightingale Jr, E., Phenomenological Theory of Ion Solvation. Effective Radii of Hydrated Ions.

J. Phys. Chem. 1959, 63, 1381-1387. (46) Clementi, E.; Barsotti, R. Study of the Structure of Molecular Complexes. Coordination Numbers for Li+, Na+, K+, F− and Cl− in Water. Chem. Phys. Lett. 1978, 59, 21-25.

22


Page 22 of 23

Page 23 of 23

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


TOC Graphic

23


Gated Water Transport through Graphene ... - ACS Publications

Recommend Documents