Nanopatterns of Phospholipid Assemblies on Carbon Nanotubes: A

Subscriber access provided by UNIV OF SCIENCES PHILADELPHIA

C: Physical Processes in Nanomaterials and Nanostructures

Nanopatterns of Phospholipid Assemblies on Carbon Nanotubes: A Molecular Dynamics Simulation Study Qing-Yan Wu, Wende Tian, and Yu-qiang Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10875 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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 26 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

Nanopatterns of Phospholipid Assemblies on Carbon Nanotubes: A Molecular Dynamics Simulation Study Qing-Yan Wu,† Wen-de Tian,∗,‡ and Yu-qiang Ma∗,†,‡ National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China, and Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China E-mail: [email protected]; [email protected]

∗ To

whom correspondence should be addressed University ‡ Soochow University † Nanjing

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 The self-assembly of amphiphiles on the surface of carbon nanotubes is an effective strategy for the dispersion of carbon nanotubes in aqueous environment. However, the underlying mechanism of the self-assembly of amphiphiles on carbon nanotubes remains elusive. In this article, extensive coarse-grained molecular dynamics simulations were performed to investigate the self-assembly of single-tailed phospholipid lysophosphatidylcholine on carbon nanotubes. The simulations present a cornucopia of polymorphic nanopatterns of amphiphile assemblies on carbon nanotubes with different amphiphile concentrations and carbon nanotube diameters. We further explore the mechanism for the formation of different nanopatterns. The results reveal that three factors—the reasonable packing of hydrophilic headgroups and hydrophobic chains of amphiphiles, the arrangements of amphiphiles, and the amount of amphiphiles adsorbed on carbon nanotubes, strongly influence the pattern formation. Additional simulations of armchair carbon nanotubes further verified and supplemented these inferences. This work provides new understandings to guide the improvement in the dispersion of carbon nanotubes and has implications on the exploitation of novel nanomaterials with complex structures.

2


Page 2 of 26

Page 3 of 26 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


Introduction The remarkable physical and chemical properties of carbon nanotubes (CNTs) make them desirable nanomaterials for a wide range of potential application. 1–4 Due to the hydrophobicity of CNTs, they tend to aggregate into bundles in aqueous environment and this hinders their application efficiency. Covalent or noncovalent modification of CNTs can effectively increase the solubility of CNTs. While covalent modification has been shown to result in structural defect and decrease its mechanical and electrical performances, 5,6 noncovalent modification is a preferred strategy for maintaining the structural integrity of CNTs. Recently this successful strategy has been broadly used to disperse CNTs. It has been proved that noncovalent conjugation of amphiphiles such as lipids, surfactants and polymers on CNTs can improve the solubility of CNTs in aqueous environment. These amphiphiles self-assembling on CNTs exhibit polymorphic structures. Understanding how amphiphiles self-assemble on the surface of CNTs is essential to guide the dispersion of CNTs. Currently there are several structures proposed experimentally for CNTs solubilization with amphiphiles. For example, CNTs adsorbed with surfactants in half-cylindrical micelles (hemimicelles), 7 CNTs encased in cylindrical micelles, 8,9 and CNTs randomly adsorbed with surfactants. 10 Beside, some experimental studies suggest that the morphology of surfactants such as sodium dodecyl sulfate (SDS) is concentration dependent. 11 Recent atomistic simulations found that the morphologies of amphiphile aggregates on CNTs are influenced by many factors. For example, Xu et al. and Tummala et al. showed that the amphiphile aggregates are influenced by the surfactant concentration and the CNT diameter. 12–15 Suttipong et al. and Calvaresi et al. found that the molecular structure of amphiphile such as the length of surfactant affects their packing on CNTs . 16,17 In addition, single-tailed phospholipids such as lysophosphatidylcholine (LPC) and lysophosphatidylglycerol are found to form aggregates that wrap around CNTs, and LPC can greatly increase the solubility of CNTs. 18,19 By using atomistic simulations, Qiao et al. found that LPCs self-assembled into "crests" that wrap CNTs. 20 For atomistic simulation studies, the systems were relatively small due to limited computational resources. Using coarse-grained simulations, 3



the time and space scales of systems can be largely increased, thus more distinguishable structures of LPC aggregates such as helical morphologies can be observed. 21,22 Beside CNT diameters and amphiphile concentrations, Ma¨att ¨ a¨ et al. found that lipid spontaneous curvature influences the assembled structures. 22 Moreover, Wallace et al. suggested that the chirality of CNTs affects the wrapping angle of LPCs on CNTs. 23 Recent simulations indicated that the morphology of amphiphile aggregates is controlled by multiple factors, however, how specific factors affect the formation of different morphologies is not unambiguously determined. Therefore, systematic studies are needed to uncover the underlying mechanism of the assembly of amphiphiles on CNTs. More importantly, it is necessary to explore the mechanisms of the formation of polymorphology in order to correctly predict the nanopatterns at different conditions to further exploit the applications of CNTs and develop novel nanomaterials with complex structures. In this work, by performing large-scale coarse-grained molecular dynamics simulations, the self-assembly of LPC molecules on CNTs, which have been proved experimentally to separate CNTs more efficiently than other common amphiphiles, 18,19 were studied with different LPC concentrations and CNT diameters. We found polymorphic transition of LPC assemblies on CNTs with the variation of LPC concentrations and CNT diameters, some morphologies have not been observed in previous experiments or simulation studies. For example, for CNTs of 2 nm diameter, LPC molecules self-assemble into irregular aggregates, spiral morphology and monolayers that completely cover the surfaces of CNTs with the increase of LPC concentrations. Further we probe the formation mechanisms of different morphologies, mainly including straight, spiral, ring-like, complete cover and incomplete cover morphologies.

Model and Methods We use coarse-grained molecular dynamics simulations to study the self-assembly of LPC molecules on CNTs. The MARTINI coarse-grained force field was used to describe every component of the

4


Page 4 of 26

Page 5 of 26 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


system. Recently it has been used to investigated various biomolecules such as lipids and peptides, 24–29 and it is also suitable to investigate the self-assembly of surfactants. 30 In MARTINI force field, four non-hydrogen atoms were mapped to one interaction center. 31–33 The mapping of LPC molecule is directly taken from MARTINI lipid force field, as shown in Figure 1A. For achiral zigzag CNTs, the topology and the bead type to represent carbon atoms have been given in previous studies. 21,23 In our model, the distance between adjacent beads is 2.84 Å, and the hexangular symmetry of CNTs is preserved. We also constructed armchair CNTs. The mappings of CNTs are shown in Figure 1B.

Figure 1: (A) Coarse-graining mapping of the atomistic LPC model. The hydrophilic headgroup of LPC molecule is represented by coarse-grain units Q0, Qa, P1 and Na, and the hydrophobic chain is modeled by C1. (B) Coarse-graining mappings of zigzag and armchair CNTs. The coarsegrained structures of CNTs are overlay on its corresponding atomistic model. We constructed CNTs of length ∼31 nm with different diameters of about 1 nm, 2 nm, 4 nm, 6 nm and 8 nm. 34,35 A CNT was in the center of systems during simulations. The sizes of systems are, for example, ∼14 × 14 × 35 nm3 for CNTs of 2 nm diameter. LPCs were randomly distributed in systems. We simulated systems with LPC concentrations of 0.03 mol/L, 0.05 mol/L, 0.1 mol/L, 0.2 mol/L, 0.36 mol/L and 0.54 mol/L, and for the system of CNTs with 2 nm diameter, for example, there are 222 LPC molecules in the system of 0.05 mol/L. Sufficient coarse-grained waters were added to systems. In this system, water and amphiphiles can exchange between the interior of CNTs and outer bulk solutions. Considering the CNTs are immersed in bulk solutions in reality, therefore the system is more realistic than closed ones in which molecule exchanges 5



are not considered. 36 The structures of simulations are converged, for example, we have tested the stability and repeatability of the interesting spiral morphology, and some simulations run in replicate with different initial configurations, as described in later section. It was found that the effective time is four times bigger than the simulation time due to large particles used in MARTINI force field, 31 however, the time mentioned in this article is the simulation time. Simulations were performed 3 µs for each system. The integration time was set to 10 fs. The Lennard-Jones and the Coulomb interaction were shifted to zero between 0.9 to 1.2 nm and 0 to 1.2 nm, respectively. The CNT, LPC and water were separately coupled to velocity rescaling thermostat 37 at temperature 305 K with a coupling time of 1 ps. The pressure was maintained at 1 bar by applying Parrinello–Rahman pressure control 38 with isotropic pressure coupling with a time constant of 4 ps. The periodic boundary conditions were applied in all three directions. All simulations were performed by Gromacs simulation package, version 4.5. 39,40

Results and Discussion The polymorphic nanopatterns of LPC assemblies with the variation of CNT diameters and LPC concentrations are shown in Figure 2, and representative structures are shown in Figure 3. Below, we first analyze the microstructures of these morphologies and then explore the formation mechanisms of different morphologies. At very low concentrations of 0.03 mol/L, LPCs self-assemble into irregular small aggregates on CNTs of 1 and 2 nm diameters, as shown in Figure 3a and d. We can imagine that on bigger diameter CNTs, LPCs will also self-assemble into irregular domain (data not shown) at this concentration. In addition, we observed that on CNTs of 1 nm diameter LPCs tend to form aggregates in which LPCs accumulate on the surface of CNTs even at very low concentrations of 0.03 mol/L (Figure 3a), while on CNTs of 2 nm diameter LPCs only assemble into single-layer aggregates (Figure 3d). We infer that the arrangements of LPCs situating along the line of hexagonal lattices of CNTs (discussed later) will result in large bending energies for thin CNTs of 1 nm diameter, and

6


Page 6 of 26

Page 7 of 26 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 2: Phase diagram of LPC assemblies on zigzag CNTs. The morphologies of LPC assemblies include irregular, straight, spiral, ring-like, complete cover and incomplete cover morphologies. the arrangements of LPCs parallel to the axes of CNTs will also give rise to a large potential energy, therefore LPCs tend to adsorb to LPC aggregates that already adsorbed on CNTs rather than the surface of CNTs. With the increase of LPC concentration, these aggregates fuse together and form single-layer ribbons in which the whole hydrophobic chains of LPC molecules are adsorbed on the surface of CNTs, or form hemimicelles on the surface of CNTs. In the following paragraphs we will discuss these nanopatterns. On CNTs of 1 nm diameter, the hemimicelles are straight or there is a small angle between the orientations of hemimicelles and axes of CNTs, as shown in Figure 3b and c. Here we consider they are both straight patterns. The hydrophobic chains of LPC molecules adsorb to the surface of CNTs and form the core of hemimicelles, and hydrophilic headgroups outside contact with water. We infer that one reason that causes the formation of straight pattern is the packing of LPCs. Hydrophobic chains of LPC molecules occupy certain areas of the surface of CNTs, meanwhile, hydrophilic headgroups also need some space to stretch. The straight pattern provides more spaces than other patterns. If the hemimicelle slant with the axes of CNTs (Figure 4), considering S1 + S2 > S and the widths of hemimicelles are rarely changed, the space between the boundaries of the hydrophobic parts of hemimicelles of the spiral morphology is smaller than that of straight hemimicelles. Hence there will be less space for hydrophilic headgroups and LPCs would over-

7



Figure 3: The structures marked in Figure 1. The two pictures of each subgraph show the assemblies of whole LPCs and hydrophobic parts of LPC molecules on CNTs at the ends of simulations, respectively. The water is not shown for clarity. The hydrophilic headgroups and hydrophobic chains of LPC molecules are shown in red and yellow, respectively, and the CNT in cyan. To see structures more clearly, the right side of (b) and (c) show their structures from another viewpoint. The right side of (g) shows the cross section of its left structure. The right picture of (h) shows another resulting morphology of this system (the same system run a different time). The snapshots are made by VMD software. 41

8


Page 8 of 26

Page 9 of 26 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: Schematic representation of straight (A) and spiral (B) hemimicelles on 1 nm diameter CNTs. The bottom hexagonal lattices depicts the CNT that are cut open from one line on the surface of CNT that parallel to the axis of CNTs. The regions colored light blue represent hydrophobic parts of hemimicelles. crowd on the surface of CNTs, which is energetically unfavorable. Thus the straight pattern was adopted. For CNTs of 2 nm diameter, ribbons and hemimicelles on the surface of CNTs show spiral features. Except single chiral helixes, there are also morphologies that contain two chirality (for example, the system of 0.1 mol/L concentration). The formation processes are depicted in Figure 5. For the two chirality helix system (Figure 5A), at 350 ns, a short helix forms in the upper part of the CNT. Meanwhile, an opposite chiral short helix appears in the lower part of the CNT. As simulation goes on, the upper helix contacts with the lower helix at 430 ns. However, its chirality was not changed in the sequential time, and a helix consisting of two chirality forms. In Figure 5B, there are several inclined aggregates with different chirality on the surface of the CNT at 80 ns. These aggregates adjust their shapes and positions continuously and at 230 ns they connected with each other. Finally, they self-assemble into a single chirality helix. For the spiral morphologies at relative low concentration of 0.05 mol/L, the simulation run 3 times with different initial configurations and resulting structures are all spiral morphology (date not shown). After examining the arrangements of LPCs that form ribbons, we found that the tail (hydrophobic chain) of LPC molecule situated along the line of hexagonal lattices of CNTs, as 9



Figure 5: The formation processes of (A) single chirality helix and (B) two chirality helix morphologies on CNTs of 2 nm diameter. The hydrophobic parts of LPC molecules are shown in yellow, and the CNT in cyan.

Figure 6: The arrangements of LPC tails of 0.05 and 0.36 mol/L systems on zigzag CNTs with 2 nm diameter. Each different LPC is shown in different color. For 0.36 mol/L system, only beads of LPCs tails that contact with the surface of CNTs are shown. 10


Page 10 of 26

Page 11 of 26 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


shown in the left side of Figure 6. This phenomenon is in accordance with previous studies. 7,23 The tail of LPC molecule tends to tilt on the surface of CNTs with preferred angles. In zigzag CNTs, if the arrangement of LPC tail is perpendicular to the axis of CNT, it will cause the increase of bending energies of LPCs. The probability distribution of angles between the LPCs and the axis of CNTs for the system of 0.05 mol/L is shown in Figure 7 (the computational method of angle has been given in ref. 20, because the hydrophilic headgroups of LPC molecules stretch to bulk with random orientations, the angle was measured by calculating the angle between the vector that originates from the last bead of the tail to the first bead of the tail and the vector of the axis of CNTs), and this illustrates that LPC tails distribute obliquely on the surface of CNTs. Note that the angle distribution of peak value is bigger than that of atomistic simulations in ref. 22. This is because the coarse-grained model is somewhat different from the atomistic model, and the arrangements of alkyl chains of LPC molecules on CNTs with minimum potential energies are not entirely the same as that of atomistic model. There are more orientations that correspond to minimum potential energies in atomistic model, 42 hence LPCs could adopt other arrangements that are different from coarse-grained model. Although the angle distribution is not very accuracy, the simulations qualitatively reproduce the phenomenons of experimental studies, 7,8,18 and the morphologies are in agreement with previous coarse-grained simulations. 21–23 We also measured the diffusion coefficient of LPC molecules on CNTs in the last 1µs of 3µs simulations, and D = 0.0056 ± 0.0007 nm2 /ns. Although LPC molecules move slowly on the surface of CNTs (D = 0.0825 ± 0.0037 nm2 /ns for LPC in bulk solutions), they are not in solid state. For a certain LPC that situates along the line of hexagonal lattices of CNTs, due to hydrophobic effect, the second LPC, especially the hydrophobic chains of the LPC molecule could adopt four kind of possible distributions, as shown in Figure 8. In Figure 8A and B, the second LPC is parallel with or lower a little than the first one. If the second LPC is higher than or further lower than the first one (Figure 8C and D), this will result in the increase of potential energy. We also simulated small systems with two LPCs on the surface of CNTs with 2 nm diameter. The total potential energy of two LPCs of Figure 8A and B are -595.17 and -596.26 kJ/mol, respectively, and Figure 8C and D are -586.96

11



and -587.66 kJ/mol, respectively. Therefore LPCs would adopt the distributions of Figure 8A or B. When more LPCs attach to the LPC aggregate, an inclined ribbon would form. This ribbon twines around CNTs and form single helix structures.

Figure 7: Probability distribution of angles between LPC tails and the axes of zigzag CNTs with 2 nm diameter in 0.05 mol/L system. In order to manifest the inference that the inclined arrangements of LPC tails cause the wrapping of ribbons on CNTs at relative low concentrations, we constructed new systems of armchair CNTs (Figure 1B) with 2 nm diameter. In armchair CNTs, LPC tails should also tend to situate along the line of hexagonal lattice of CNTs, as well as zigzag CNTs. The arrangements that LPC tails parallel to the axes of CNTs correspond to the minimum bending energy of LPCs, other arrangements will result in the increase of bending energy. At low concentrations of 0.05 mol/L, as shown in the left side of Figure 9, the LPC assemblies form ring-like morphologies on the surface of armchair CNTs of 2 nm diameter. After carefully examining the arrangements of LPC molecules, as expected, we found that LPC tails tend to be parallel to the axes of CNTs, as shown in the left side of Figure 10. The probability distribution of angles is shown in Figure 11, and the angles of the armchair CNT system tend to distribute at smaller angle regions compare with that of zigzag CNTs (Figure 7). Comparing with the patterns of armchair and zigzag CNT systems, we infer that the morphological transition of LPC assemblies from spiral to ring-like is due to the structure alteration of CNTs from zigzag to armchair. The structure alteration of CNTs only results in different arrangements of LPCs on the surface of CNTs, especially the hydrophobic chains of 12


Page 12 of 26

Page 13 of 26 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: Schematic representation of different distributions of two LPC molecules on the surface of CNTs. The hydrophobic tails and hydrophilic headgroups are colored black and red, respectively.

Figure 9: The structures of LPCs on armchair CNTs of 2 and 4 nm diameters with 0.05 and 0.36 mol/L concentrations. The color scheme is the same as Figure 3. LPC molecules. Therefore, at low concentrations where LPCs aggregate to ribbons, the inclined arrangement of LPC tail is a significant factor for the formation of spiral pattern. However, at 0.36 mol/L, the LPCs form spiral morphology, as shown in the left side of Figure 9. In addition, we found that LPC tails of hemimicelles that contact with CNTs (at least three beads out of the four of the LPC tail contacting to the surface of CNT is defined as contacting with CNTs) are not parallel to the axis of CNTs, as shown in the left side of Figure 10, and the probability distribution of angles (Figure 11) shows that more angles distribute at higher angle regions compared with the system of

13



Figure 10: The arrangements of LPC tails on armchair CNTs of 2 and 4 nm diameters with concentrations of 0.05 and 0.36 mol/L. armchair CNTs of 0.05 mol/L. This is because the hemimicelle has an intrinsic curvature along its extension direction. 22 When the curvature of CNTs is smaller than the intrinsic curvature of the hemimicelle, the hemimicelle would wrap around CNTs to obtain a larger curvature radius (this will be further discussed later). Besides, the wrapping of hemimicelles affect the orientation of LPC tails that contact with the surface of CNTs by prompting them to adopt other arrangements, as shown in the left side of Figure 10. Therefore, we propose that at higher concentrations that LPCs form hemimicelles, the intrinsic curvature of hemimicelles is also an important factor for the formation of spiral morphology.

Figure 11: Probability distribution of angles between LPCs and the axes of 2 nm diameter armchair CNTs in systems of 0.05 and 0.36 mol/L concentrations. For CNTs of 4 nm diameter, the spiral morphologies of ribbons and hemimicelles are different 14


Page 14 of 26

Page 15 of 26 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 12: Schematic representation of the ideal model in which LPC ribbons are on the surface of CNTs. The regions colored light blue represent hydrophobic parts of ribbons. from that of CNTs of 2 nm diameter. For example, the number of coils of hemimicelles in the systems of CNTs with 2 nm diameter is different from that of 4 nm diameter, as shown in Figure 3f and i. Moreover, at low concentrations of 0.05 mol/L, LPCs can form both spiral and ring ribbons on CNTs of 4 nm diameter, as shown in Figure 3h, but on CNTs with 2 nm diameters only the spiral ribbons were observed (Figure 3e). This phenomenon of low concentrations could be explained in a simple way. We assume that in an ideal model, LPCs are arranged orderly with each other, especially the hydrophobic parts of LPC molecules. The hydrophilic parts of adjacent ribbons lean close with each other; otherwise more water will contact to the surface of CNTs. One example that is similar to this model is the system of 2 nm diameter CNTs of 0.05 mol/L concentration, as shown in Figure 6. We set the lengths of the hydrophobic chain and the hydrophilic part of LPC molecule on the surface of CNTs as l1 and l2 , respectively. The angle between the hydrophobic parts of ribbons and the tangential direction of the circumference of CNTs is θ , and the circumference of CNTs is S, as shown in Figure 12. We can get the period of helix l p = S tan θ , also, l p = (ls + lr )/ cos θ , in which ls is the separation distance between the hydrophobic parts of two ribbons, and lr the width of hydrophobic parts of ribbons. Then S tan θ = (ls + lr )/ cos θ . In this model, the preferred angle between the axes of CNTs and the orientation that LPC situating along the line of hexagonal lattices of CNTs is 30◦ , thus lr = 2l1 sin(60◦ + θ ) and ls = 2l2 sin(60◦ + θ ). 15



Then we get

√ 3 cot θ =

S l1 +l2

− 1, θ decreases monotonically with increasing S. Therefore, for

low concentration of 0.05 mol/L, the θ of CNTs with 4 nm diameter is smaller than that of CNTs with 2 nm diameter. We infer that one ribbon with smaller θ have more probability to fuse with adjacent ribbons above or below this one, in this way two ribbons will fuse together and form one single ribbon that are relative perpendicular to the axis of CNTs. Thus the ring-like morphology appears. For bigger CNTs of 6 and 8 nm diameter, according to above equations, the ribbons will be less inclined than that of 4 nm diameter CNTs. Thus the ribbons are more possible to fuse with adjacent ribbons and form ring-like morphologies on the surface of CNTs, as shown in Figure 3k. For the different numbers of coils of hemimicelles on CNTs of 2 nm and 4 nm diameter at higher concentrations of 0.36 mol/L, however, the reason is different from that of low concentrations, though the arrangements of LPCs that contact with CNTs are similar to that of LPCs of 0.05 mol/L system (Figure 6). The factor of the intrinsic curvature of hemimicelles should be considered. 22 For a hemimicelle with a certain intrinsic curvature, we set the radius of intrinsic curvature as Rh . As mentioned above, if a hemimicelle formed on a CNT that its radius is smaller than Rh , the hemimicelle should wrap around the CNT in order to obtain a larger curvature radius. The radius of curvature of a curve that wrap around a cylinder is Rc = R/ cos θ , in which R is the radius of the cylinder. 22,43 Bring Rh into the equation, we get cos θ = R/Rh . When R increases, θ will decreases, as well as the period of helix (l p = 2πR tan θ = 2πRh sin θ ). Hence there are more coils on CNTs of 4 nm diameter. For hemimicelles on bigger CNTs of 6 and 8 nm diameter, as shown in Figure 3l, they form ring-like morphologies. For CNTs whose radius exceeds the intrinsic curvature radius of hemimicelles, because the curvature radius of spiral morphology is bigger than that of ring-like morphology, so that the hemimicelle is favor to form ring-like morphology on the surface of CNT. We also simulated armchair CNTs with 4 nm diameter to further test the two factors that cause the formation of spiral morphologies: the inclined arrangements of LPC tails and the intrinsic curvatures of hemimicelles. As shown in the right side of Figure 9, at low concentrations of 0.05 mol/L, LPCs present a ring-like morphology on the surface of CNTs. The arrangements of LPC

16


Page 16 of 26

Page 17 of 26 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 13: Probability distributions of angles between LPCs and the axes of armchair CNTs with 4 nm diameter in systems of 0.05 and 0.36 mol/L concentrations. tails are shown in the right side of Figure 10, obviously they tend to situate along the line of the hexagonal lattice of armchair CNTs, as well as that of armchair CNTs with 2 nm diameter. The distribution of angles (Figure 13) shows that the angles between LPCs and the axes of CNTs are very small, indicating that LPCs are mostly parallel to the axes of CNTs. However, at 0.36 mol/L, unlike spiral morphology on armchair CNTs with 2 nm diameter, LPCs assemblies display ringlike morphologies (the right side of Figure 9). In addition, on armchair CNTs with 4 nm diameter, if the intrinsic curvature of hemimicelles drive them to form spiral morphology, the LPCs that contact with the surface of CNTs would tilt as well as that of armchair CNTs with 2 nm diameter. However, the arrangements of these LPCs shown in the right side of Figure 10 depict that the LPC tails are mostly parallel to the axis of armchair CNT. The angle distribution of 0.36 mol/L system is also similar to that of 0.05 mol/L system (Figure 13). As mentioned above, θ of hemimicelles of CNT with 4 nm diameters is smaller than that of 2 nm diameter. Hence the corresponding smaller "driven force" which makes hemimicelles form spiral morphology can not overcome the increase of bending energies that result from tilting LPCs that contact with the surface of armchair CNTs. Therefore hemimicelles adopt ring-like morphology on armchair CNTs of 4 nm diameter. At the high concentration of 0.54 mol/L, for CNTs of 1 and 2 nm diameters, the CNTs were entirely covered by a monolayer of LPCs, as shown in Figure 3g. Hydrophilic headgroups of LPC molecules form the outer surface and hydrophobic chains contact with the surface of CNTs. The 17



Figure 14: The number density profiles of hydrophilic headgroups and hydrophobic tails of LPC molecules along the radial direction from the surface of CNTs with (A) 1 nm and (B) 2 nm diameter. density profiles of hydrophilic headgroups and hydrophobic chains are shown in Figure 14A and B, respectively. To adsorb more LPCs on CNTs and reduce the exposure of LPC tails to water, LPCs adopt this kind of morphology. The result is in accordance with previous studies. 8,9,23 For CNTs with 4 nm diameter, however, the surface of CNTs is not completely covered by LPCs, as shown in Figure 3j. There are a few areas that are not covered by LPCs. To verify whether the CNT can adsorb more LPCs, we separated the CNTs and LPCs that adsorbed on the surface of CNTs from the system and put them into a new system with 0.54 mol/L LPCs and simulated for 3µs. The CNT also did not adsorb enough LPCs that could completely cover the surface of the CNT (data not shown). We infer that the reason is the crowed packing of hydrophilic headgroups of LPC molecules. The shape of a LPC can be approximated as a cone-like configuration. In CNTs of 1 nm and 2 nm diameter, there are sufficient areas for hydrophilic headgroups of LPC molecules. Using Sh = 2π(R + l)L/n, in which l is the distance between hydrophilic headgroups

18


Page 18 of 26

Page 19 of 26 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 surface of CNTs, L is the length of CNTs and n is the amount of adsorbed LPCs, the average areas of hydrophilic headgroups Sh of 1 nm and 2 nm diameter CNTs are 0.621 and 0.542 nm2 , respectively. For CNTs of bigger diameters, if LPCs form a monolayer on the surface of CNTs, the density of the hydrophobic chains of LPC molecules has to be similar to that of CNTs of 1 nm or 2 nm diameter; otherwise the water would enter the hydrophobic parts of LPC assemblies and this will lead to the increase of potential energies. The CNT of 4 nm diameter has to adsorb about 1660 LPCs to satisfy this density of hydrophobic chains of monolayer. However, the corresponding density of hydrophilic headgroups of the monolayer will be larger than that of CNTs of 1 nm and 2 nm diameter, and the average area of hydrophilic headgroups Sh decreases to 0.482 nm2 . This will lead to the increases of free energy and surface energy, hence the CNT with 4 nm diameter is unable to adsorb so much LPCs. Actually, in the simulation, the CNT of 4 nm diameter of 0.54 mol/L adsorbed 1576 LPCs, therefore a few blank areas exist on CNTs. Beside, the contour of boundaries of LPC assemblies around these blank areas seems like that of hemimicelles, and in this way the area for hydrophilic headgroups is also increased. For bigger CNTs of 6 nm and 8 nm diameters, as shown in Figure 3m, the LPC assemblies also not completely covered CNT surfaces as well as the CNT of 4 nm diameter. Some studies observed that amphiphiles formed multilayered structures on CNTs. 14,16 However, in this work, LPC molecules assemble into single-layer ribbons or hemimicelles, even at very high concentrations, they form completely cover and incompletely cover morphologies.

Conclusions In this work, we performed extensive coarse-grained simulations to explore self-assembled morphologies on CNTs at different LPC concentrations and CNT diameters. Moreover, we uncovered the underlying mechanism of morphology formation. The distinct polymorphic nanopatterns were given in this work, among which the straight pattern, ring-like pattern and incomplete cover of LPCs on CNTs have not been observed in previous experiment or simulation studies. Through

19



examining the microstructure of LPC assemblies, the formation of straight pattern on thin CNTs and the formation of helical patterns of ribbons and hemimicelles on bigger diameter CNTs were explained. In addition, using a simple ideal model, we elaborated the different helical patterns on CNTs of 2 and 4 nm diameters as well as ring-like patterns on CNTs of bigger diameters at low LPC concentrations. We also constructed armchair CNTs and the simulation results are in accordance with our inferences from simulations of zigzag CNTs. More importantly, the simulations of armchair CNTs reveal that for hemimicelles, the interplay between the arrangements of LPCs contacting with CNTs and the intrinsic curvature of hemimicelles determine their patterns. Further, we discussed the formation of the patterns of complete and incomplete cover of LPCs on CNTs. In summary, the reasonable packing of hydrophobic chains and hydrophilic headgroups of LPC aggregates, the arrangements of LPCs and the amount of adsorbed LPCs are significant factors that lead to the formation of different patterns. These factors were strongly affected by CNT diameters, the chirality of CNTs and LPC concentrations. The rich morphologies and the mechanisms suggested in this work provide insights to better understand the adsorption of amphiphiles on CNTs and guide the dispersion of CNTs. This work also gives a reference to the self-assembly of other amphiphiles, moreover, facilitates the design of novel CNTs-based nanodevices with complex structures.

Acknowledgments This work was supported by the National Natural Science Foundation of China under grant No. 91027040 and the National Basic Research Program of China (No. 2012CB821500). Meanwhile, W.-d.T. thank NSFC with Nos. 21474074, 21674078. The computational resources provided by the High Performance Computing Center (HPCC) of Nanjing University are greatly appreciated.

20


Page 20 of 26

Page 21 of 26 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) Liu, Z.; Cai, W.; He, L.; Nakayama, N.; Chen, K.; Sun, X.; Chen, X.; Dai, H. In Vivo Biodistribution and Highly Efficient Tumour Targeting of Carbon Nanotubes in Mice. Nat. Nanotechnol. 2007, 2, 47–52. (2) Shi Kam, N. W.; Jessop, T. C.; Wender, P. A.; Dai, H. Nanotube Molecular Transporters: Internalization of Carbon Nanotube-Protein Conjugates into Mammalian Cells. J. Am. Chem. Soc. 2004, 126, 6850–6851. (3) Biercuk, M.; Llaguno, M. C.; Radosavljevic, M.; Hyun, J.; Johnson, A. T.; Fischer, J. E. Carbon Nanotube Composites for Thermal Management. Appl. Phys. Lett. 2002, 80, 2767– 2769. (4) Avouris, P.; Freitag, M.; Perebeinos, V. Carbon-Nanotube Photonics and Optoelectronics. Nat. Photonics 2008, 2, 341–350. (5) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Functionalization of Carbon Nanotubes by Electrochemical Reduction of Aryl Diazonium Salts: A Bucky Paper Electrode. J. Am. Chem. Soc. 2001, 123, 6536–6542. (6) Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’ko, Y. K. Small but Strong: A Review of the Mechanical Properties of Carbon Nanotube–Polymer Composites. Carbon 2006, 44, 1624– 1652. (7) Richard, C.; Balavoine, F.; Schultz, P.; Ebbesen, T. W.; Mioskowski, C. Supramolecular SelfAssembly of Lipid Derivatives on Carbon Nanotubes. Science 2003, 300, 775–778. (8) O’connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C. et al. Band Gap Fluorescence from Individual Single-Walled Carbon Nanotubes. Science 2002, 297, 593–596.

21



(9) Matarredona, O.; Rhoads, H.; Li, Z.; Harwell, J. H.; Balzano, L.; Resasco, D. E. Dispersion of Single-Walled Carbon Nanotubes in Aqueous Solutions of the Anionic Surfactant NaDDBS. J. Phys. Chem. B 2003, 107, 13357–13367. (10) Yurekli, K.; Mitchell, C. A.; Krishnamoorti, R. Small-Angle Neutron Scattering from Surfactant-Assisted Aqueous Dispersions of Carbon Nanotubes. J. Am. Chem. Soc. 2004, 126, 9902–9903. (11) Das, S. K.; Sengupta, S.; Velarde, L. Interfacial Surfactant Ordering in Thin Films of SDSEncapsulated Single-Walled Carbon Nanotubes. J. Phys. Chem. Lett. 2016, 7, 320–326. (12) Xu, Z.; Yang, X.; Yang, Z. A Molecular Simulation Probing of Structure and Interaction for Supramolecular Sodium Dodecyl Sulfate/Single-Wall Carbon Nanotube Assemblies. Nano Lett. 2010, 10, 985–991. (13) Tummala, N. R.; Striolo, A. SDS Surfactants on Carbon Nanotubes: Aggregate Morphology. Acs Nano 2009, 3, 595–602. (14) Tummala, N. R.; Striolo, A. Curvature Effects on the Adsorption of Aqueous SodiumDodecyl-Sulfate Surfactants on Carbonaceous Substrates: Structural Features and Counterion Dynamics. Phys. Rev. E 2009, 80, 021408. (15) Tummala, N. R.; Morrow, B. H.; Resasco, D. E.; Striolo, A. Stabilization of Aqueous Carbon Nanotube Dispersions Using Surfactants: Insights from Molecular Dynamics Simulations. ACS nano 2010, 4, 7193–7204. (16) Suttipong, M.; Tummala, N. R.; Kitiyanan, B.; Striolo, A. Role of Surfactant Molecular Structure on Self-Assembly: Aqueous SDBS on Carbon Nanotubes. J. Phys. Chem. C 2011, 115, 17286–17296. (17) Calvaresi, M.; Dallavalle, M.; Zerbetto, F. Wrapping Nanotubes with Micelles, Hemimicelles, and Cylindrical Micelles. Small 2009, 5, 2191–2198. 22


Page 22 of 26

Page 23 of 26 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


(18) Wu, Y.; Hudson, J. S.; Lu, Q.; Moore, J. M.; Mount, A. S.; Rao, A. M.; Alexov, E.; Ke, P. C. Coating Single-Walled Carbon Nanotubes with Phospholipids. J. Phys. Chem. B 2006, 110, 2475–2478. (19) Ke, P. C. Fiddling the String of Carbon Nanotubes with Amphiphiles. Phys. Chem. Chem. Phys. 2007, 9, 439–447. (20) Qiao, R.; Ke, P. C. Lipid-Carbon Nanotube Self-Assembly in Aqueous Solution. J. Am. Chem. Soc. 2006, 128, 13656–13657. (21) Lee, H.; Kim, H. Self-Assembly of Lipids and Single-Walled Carbon Nanotubes: Effects of Lipid Structure and PEGylation. J. Phys. Chem. C 2012, 116, 9327–9333. (22) Määttä, J.; Vierros, S.; Sammalkorpi, M. Controlling Carbon-Nanotube Phospholipid Solubility by Curvature-Dependent Self-Assembly. J. Phys. Chem. B 2015, 119, 4020–4032. (23) Wallace, E. J.; Sansom, M. S. Carbon Nanotube/Detergent Interactions via Coarse-Grained Molecular Dynamics. Nano Lett. 2007, 7, 1923–1928. (24) Moiset, G.; López, C. A.; Bartelds, R.; Syga, L.; Rijpkema, E.; Cukkemane, A.; Baldus, M.; Poolman, B.; Marrink, S. J. Disaccharides Impact the Lateral Organization of Lipid Membranes. J. Am. Chem. Soc. 2014, 136, 16167–16175. (25) Wu, Q.-Y.; Liang, Q. Interplay Between Curvature and Lateral Organization of Lipids and Peptides/Proteins in Model Membranes. Langmuir 2014, 30, 1116–1122. (26) Tu, C.-k.; Chen, K.; Tian, W.-d.; Ma, Y.-q. Computational Investigations of a PeptideModified Dendrimer Interacting with Lipid Membranes. Macromol. Rapid Commun. 2013, 34, 1237–1242. (27) Tian, W.-d.; Ma, Y.-q. pH-Responsive Dendrimers Interacting with Lipid Membranes. Soft Matter 2012, 8, 2627–2632.

23



(28) Liang, Q.; Wu, Q.-Y.; Wang, Z.-Y. Effect of Hydrophobic Mismatch on Domain Formation and Peptide Sorting in the Multicomponent Lipid Bilayers in the Presence of Immobilized Peptides. J. Chem. Phys. 2014, 141, 074702. (29) Tian, W.-d.; Chen, K.; Ma, Y.-q. Interaction of Fullerene Chains and a Lipid Membrane via Computer Simulations. RSC Adv. 2014, 4, 30215–30220. (30) Marrink, S. J.; Tieleman, D. P. Perspective on the Martini Model. Chem. Soc. Rev. 2013, 42, 6801–6822. (31) Marrink, S. J.; Risselada, H. J.; Yefimov, S.; Tieleman, D. P.; de Vries, A. H. The MARTINI Force Field: Coarse Grained Model for Biomolecular Simulations. J. Phys. Chem. B 2007, 111, 7812–7824. (32) Monticelli, L.; Kandasamy, S. K.; Periole, X.; Larson, R. G.; Tieleman, D. P.; Marrink, S. J. The MARTINI Coarse-Grained Force Field: Extension to Proteins. J. Chem. Theory Comput. 2008, 4, 819–834. (33) de Jong, D. H.; Singh, G.; Bennett, W. D.; Arnarez, C.; Wassenaar, T. A.; Schäfer, L. V.; Periole, X.; Tieleman, D. P.; Marrink, S. J. Improved Parameters for the Martini CoarseGrained Protein Force Field. J. Chem. Theory Comput. 2013, 9, 687–697. (34) Ajayan, P.; Ebbesen, T. Nanometre-Size Tubes of Carbon. Rep. Prog. Phys. 1997, 60, 1025– 1062. (35) Popov, V. N. Carbon Nanotubes: Properties and Application. Mater. Sci. Eng., R 2004, 43, 61–102. (36) Arai, N.; Yasuoka, K.; Zeng, X. C. Self-Assembly of Surfactants and Polymorphic Transition in Nanotubes. J. Am. Chem. Soc. 2008, 130, 7916–7920. (37) Bussi, G.; Donadio, D.; Parrinello, M. Canonical Sampling Through Velocity Rescaling. J. Chem. Phys. 2007, 126, 014101. 24


Page 24 of 26

Page 25 of 26 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


(38) Parrinello, M.; Rahman, A. Polymorphic Transitions in Single Crystals: A New Molecular Dynamics Method. J. Appl. Phys. 1981, 52, 7182–7190. (39) Hess, B.; Kutzner, C.; Van Der Spoel, D.; Lindahl, E. GROMACS 4: Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation. J. Chem. Theory Comput. 2008, 4, 435–447. (40) Van Der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. GROMACS: Fast, Flexible, and Free. J. Comput. Chem. 2005, 26, 1701–1718. (41) Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual Molecular Dynamics. J. Mol. Graphics 1996, 14, 33–38. (42) Wei, C. Radius and Chirality Dependent Conformation of Polymer Molecule at Nanotube Interface. Nano Lett. 2006, 6, 1627–1631. (43) Ni, R.; Cao, D.; Wang, W. Nanopattern of the Inner Surface of Carbon Nanotubes for SelfAssembly of Nanoparticles: A Multistep Monte Carlo Method. J. Phys. Chem. C 2007, 111, 11802–11805.

25



TOC Graphic

26


Page 26 of 26

Nanopatterns of Phospholipid Assemblies on Carbon Nanotubes: A

Recommend Documents