Characterization of Interfaces between Graphene Films and Support

Aug 27, 2013 - structure at the interface between graphene films and support substrates in ... support substrates on graphene properties in liquid env...
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Characterization of Interfaces between Graphene Films and Support Substrates by Observation of Lipid Membrane Formation Kenji Yamazaki,* Syunsuke Kunii, and Toshio Ogino* Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan ABSTRACT: Chemical properties of graphene film surfaces are investigated by observing lipid membrane formation on them in liquid environment for bioapplications of graphene. It is found that water molecules form a layered structure at the interface between graphene films and support substrates in liquid. Lipid monolayer membranes are selectively formed on the graphene film surfaces above the hydrophilic domains of the sapphire surface that is phase-separated into hydrophilic and hydrophobic domains. This means that the hydrophilicity/hydrophobicity of the support substrate is permeable through the graphene film to the graphene surface not only in air but also in liquid environment. Controllability of the graphene film properties by the substrate surface will be useful for application of graphene to biotechnology where liquid environment is required.

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exist at the interface between the graphene film and the substrate, has been one of the hot topics for several years. In catalytic chemical vapor deposition using metal substrates, grown graphene films are transferred from the metal substrates to an insulating substrate using a polymer film as a transient support. In this process, one of the serious problems is contamination from the residual polymer after its removal.26−30 All of the above-mentioned reports indicate the importance of the physical and chemical control of the graphene/substrate interfaces. For bioapplications of graphene films, therefore, effects of the support substrates on graphene properties in liquid environment should be revealed in addition to the air environment. We used lipid vesicles to investigate the properties of graphene surfaces in liquid environment. Lipid membranes can be formed by fusion and rupture of lipid vesicles, and their formation process is sensitive to hydrophilic/hydrophobic interactions between the vesicles and the substrate surfaces.31 Therefore, lipid membranes formed on solid substrates are a good system to demonstrate the physical and chemical properties of graphene surfaces influenced by the support substrates. To investigate effects of the substrate surfaces on formation of supported lipid bilayers, various surfaces, such as chemical-modification-patterned surfaces and step-aligned single-crystalline surfaces, were used in the vesicle fusion method or the self-spreading of lipid molecules. 32−34 Interactions between lipid membranes and graphene surfaces have also been reported.15,35−38 In this paper, we focus on effects of sapphire substrates in liquid environment on the properties of attached graphene

INTRODUCTION Graphene, a single layer of carbon atoms, has widely attracted attention1,2 owing to its potential applications, such as mechanical devices using its unique two-dimensional structures, electronics using its extremely high carrier mobility, and optoelectronics using its high transparency.3−5 Recently, much interest has been devoted to hybrid devices consisting of graphene and biomolecules because graphene has many suitable properties for bioapplications.6 In particular, detections of pH,7,8 enzymes and products of enzymatic reactions,9−11 protein molecules,7,12 DNA hybridization,13,14 and cell activities15−17 are promising applications of graphene and its composite materials. Biomolecules keep their activities only in aqueous environment, and target molecules are detected through their adsorption or immobilization to the graphene surfaces supported by the substrates in liquid environment. Therefore, to reveal properties of graphene film surfaces in liquid environment is one of the crucial issues for development of their bioapplications. Properties of graphene films have been studied mainly in air, and it has been widely recognized that the graphene films are strongly affected by the solid support substrates. SiO2-covered Si substrates are one of the widely used materials for the electronic device applications, but their surface roughness deforms the graphene films and limits the performance of graphene devices.18−20 Surfaces of single crystalline substrates, such as hexagonal boron nitride (h-BN) and sapphire, are atomically flat21,22 and form good interfaces with graphene films, which achieve higher carrier mobilities23 than the SiO2/Si systems. They are also used in selective etching of the graphene films controlled by the atomic arrangement of the substrate surfaces.24,25 Graphene film surfaces are also influenced by chemical interactions with the support substrates. Especially, chemical doping from impurities, such as water and gases that © 2013 American Chemical Society

Received: May 6, 2013 Revised: August 12, 2013 Published: August 27, 2013 18913

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films by observing lipid membrane formations. Atomically flat terraces with morphologically controlled atomic steps can be obtained on sapphire surfaces in liquid environment. Moreover, we have already investigated protein adsorption and lipid bilayer membrane formation on the physically and chemically controlled sapphire substrates.34,39,40 Therefore, sapphire substrate is a suitable material to reveal the influences of the support surfaces.

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RESULTS We show AFM topographies and frictional force images of the sapphire surfaces in air after deposition of graphene films in Figure 1. These images were taken by the contact mode, and

EXPERIMENTAL SECTION

We deposited graphene films to sapphire surfaces by the mechanical exfoliation method.2 We used three morphologically different (0001) sapphire substrates to support graphene films. One is a surface that has no specific atomic step arrangement, which we refer to as a randomly stepped (0001) surface. The second is a surface that exhibits straight-step/flatterrace structures, which we refer to as a single-stepped (0001) surface. The other is a surface that has bunched steps accompanied by crossing steps, which we refer to as a crossstepped (0001) surface. The cross-stepped surface has two domains with different chemical states. These step/terrace structures can be prepared by the annealing conditions and miscut orientation of the substrate tilted from the crystallographically low-index orientation. Single-stepped and crossstepped sapphire surfaces were prepared by annealing at 1000 °C for 3 h or at 1400 °C for 3 h in air, respectively. We had reported the phase separation on single crystalline sapphire (0001) surfaces with cross-steps in detail.39 These sapphire substrates were sonicated in pure water for 5 min and cleaned with an H2SO4−H2O2 mixing solution at 90 °C for 10 min to remove organic contamination. Then, they were again sonicated in pure water for 5 min. After graphene deposition to the substrates, we formed lipid membranes on the partially graphene-deposited sapphire surfaces by the vesicle fusion method.32,41 Lipid membranes were formed using 1,2-dioleoyl-sn-glycero3-phosphocholine (DOPC) dissolved in chloroform. The lipid solution was vacuum-dried to remove the solvent and obtain dry lipid films. The lipid films were added to a buffer solution that consisted of 10 mM N-2 hydroxyethylpiperazine-N′-2ethanesulfonic acid (HEPES), 150 mM KCl, and 1 mM CaCl2. The pH value of the buffer solution was regulated to be 7.4 using KOH solution. The lipid solution was warmed up above the gel−liquid crystal transition temperature (−20 °C for DOPC). Large multilamellar spherical vesicles were obtained by stirring this lipid solution. We prepared large unilamellar vesicles by sonicating the multilamellar vesicles using a bathtype sonicator. To facilitate the vesicle rupture on the substrate surface, the lipid solution temperature was kept above the phase transition temperature (at 50 °C) during the incubation for 60 min. We observed the results of graphene deposition and formation of lipid membranes on the sapphire surfaces by the atomic force microscopy (AFM) in both air and liquid environment. We used Si3N4 cantilevers with a force constant of 0.08 N/m for the contact mode measurement in air. In the liquid environment, we used the cyclic contact mode of AFM using Si cantilevers with a force constant of 1.6 N/m. In the observation in liquid environment, we used the buffer solution that were used for the formation of lipid membranes.

Figure 1. AFM images in air of sapphire surfaces after the graphene deposition: (a, b) a randomly stepped surface, (c, d) a single-stepped surface, (e, f) a cross-stepped surface, (a, c, e) topographies, and (b, d, f) frictional force images.

the topographies and frictional force images were simultaneously obtained. In the frictional force images, graphene surfaces are observed as darker regions than the sapphire substrates. Since contrasts of frictional force are generated by a difference in the magnitude of torsion of the AFM tip, they are determined by the capillary force between the adsorbed water layers on the AFM tip and the sample surfaces. So the larger frictional force corresponds to more hydrophilic surfaces. In our experiments, a sapphire substrate surface is more hydrophilic than the graphene films. The cross-stepped surface consists of two domains: the circular domains are relatively hydrophobic, and their outer areas are hydrophilic, as shown in the previous reports.39,40 But, even the hydrophobic domains are relatively more hydrophilic than the graphene surfaces, as shown in Figure 1f. Figure 2 shows morphologies of graphene films on the single-stepped sapphire surface by AFM observation (a) in air by contact mode and (b) in a buffer solution by cyclic contact mode. The height of graphene films on the sapphire surface was about 0.3 nm, and that of the atomic steps on the sapphire substrate was about 0.2 nm in air. However, the height of the same graphene films was 1.0 nm in the buffer solution though the observed step height was the same, 0.2 nm. Moreover, differences in the height of few-layer-graphene films with different layer numbers were observed to be from 0.2 to 0.3 nm per a single layer both in air and in liquid. The atomic steps on the sapphire substrates were clearly observed on the graphene film surfaces after the graphene deposition. Figure 2c shows an 18914

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consists of graphene height (about 1.0 nm) and the membrane one, the membrane height is estimated to be about 2.3 nm on graphene films. We clearly observed the boundaries of the graphene films with different layer numbers even after the lipid deposition. On the other hand, the atomic steps on the sapphire substrate became less sharp through the lipid membranes. In the case of the cross-stepped sapphire surface, lipid membranes selectively form above the hydrophilic domains of the original surfaces. The height difference between the membrane-covered and noncovered areas is 2.75 nm, as observed in the cross-sectional view along line B shown in Figure 3b. Since this value consists of the height difference between the hydrophobic and hydrophilic domains of the sapphire surface (the latter is about 0.2 nm higher) and the membrane one, the membrane height is estimated to be about 2.5 nm. Lipid membranes were hardly formed on the surfaces of thick graphite with a hundred nanometers deposited on the sapphire substrates.

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Figure 2. Heights of single layer graphene films on a single-stepped sapphire substrates in air and in aqueous environment. (a) AFM topographic image in air and (b) that in a buffer solution. (c) Observation of interfacial water between graphene flakes and the sapphire substrate in air. (d) Comparison of the heights of single layer graphene films and atomic steps of the sapphire substrate.

DISCUSSION First, we discuss apparent heights of graphene films deposited on the sapphire surfaces when they are measured by AFM in a buffer solution. When graphene films are tightly attached to a sapphire surface in air, the height of a single layer graphene film almost equals to the interlayer distance of graphite.21 As shown in Figure 2, the height of graphene films measured in the buffer solution is larger than that measured in air, whereas the height of the steps of the sapphire substrate is independent of the environment of the AFM measurement. Therefore, the increase in height of graphene films is not attributed to a difference in the condition of the AFM measurement but a difference in attachment strength of the graphene film to the substrate. Interfacial water layers are often observed in air when graphene films are attached to hydrophilic substrates such as mica and sapphire.27,42 We have reported on the interfacial water structure between the graphene films and the sapphire surfaces based on the AFM measurement in air.27 According to our previous study, the interfacial water exhibits a layered structure that consists of single or multilayer of structural water and isolated islands depending on the humidity at graphene deposition. In air or vacuum, the water molecules can evaporate from the graphene edges, and the graphene edges are closely attached to the substrate surface without water layers. As a result, the interfacial water molecules are confined by the graphene films. In contrast, much water molecules exist around the graphene film edges in liquid environment. The water molecules can get into the interface between the graphene films and the sapphire surfaces and lift the graphene edges. When a single layer of graphene films is deposited at high humidity in air, its height is approximately 0.78 nm. In Figure 2b, the apparent height of a single layer graphene film on the singlestepped sapphire surface in liquid is 1.0 nm, which almost equals to sum of the height of a graphene film in air (0.3 nm) and two-layer interfacial water thickness (0.72 nm).27 We generally observe smoother surfaces for the graphene films in a solution than that in air probably because the interfacial water layers are more uniform in a solution with a unit thickness that is uniquely determined by the interaction of the sapphire substrates and the graphene films. The height difference between double-layer and single-layer graphene films is about 0.3 nm, which means that there is no interfacial water layer between the graphene layers, at least at their edge, owing to high hydrophobicity of graphene. The step structure of the

AFM image of a graphene film observed in air by the cyclic contact mode, where interfacial water layers are clearly observed. Note that the interfacial water is confined in the graphene films and that the graphene edges are directly attached to the sapphire surface. The observed heights of the graphene films attached to the sapphire surfaces are depicted with the step heights in Figure 2d. Figure 3 shows morphology of lipid membranes deposited on the graphene films supported by the randomly stepped and

Figure 3. AFM topographies and cross sections of lipid membranes formed on graphene films that are supported by (a) the randomly stepped surface and (b) the cross-stepped surface. These images were taken by the cyclic contact mode in the buffer solution.

cross-stepped sapphire substrates. Lipid membranes uniformly formed on the graphene surfaces supported by the randomly stepped sapphire surfaces, as shown in Figure 3a. We observed formation of lipid membranes on the graphene surfaces whose layer number was less than about 10. The height difference between the bare sapphire surface and the membrane-covered graphene surface is 3.33 nm, as observed in the cross-sectional view along the line (A) shown in Figure 3a. Since this value 18915

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Figure 4. A conformation model of interfaces between sapphire surfaces and graphene films attached to them: (a) in air and (b) in aqueous environment.

sapphire substrate is reproduced on the graphene layers in the buffer solution as well as in air. This result indicates that the adhesion of graphene films to sapphire surfaces is still strong and thickness of the interlayer water is uniform. Figure 4 shows a model of the structure of graphene films attached to the sapphire substrates (a) in air and (b) in the buffer solution. Next, we discuss selective formation of lipid membranes on the graphene/sapphire substrates. The formation of lipid membranes is strongly influenced by the physical and chemical properties of support substrates.31 The conformation of lipid membranes strongly depends on hydrophilicity of the substrates. Generally, lipid bilayers form on the hydrophilic surfaces and its monolayers on the hydrophobic surfaces. The coverage of the lipid molecules on the substrate can be controlled by the hydrophilicity even if the same material is used.43,44 The height of lipid membranes is affected by the density of lipid molecules. The lower the density of lipid molecules was, the smaller the height of the lipid membranes.34 Since graphene is a hydrophobic material, lipid monolayers should be formed when the lipid vesicles rapture on the graphene surfaces. In Figure 3, the total thickness of the graphene film and the lipid layer is about 3.3 nm, and the lipid layer thickness is estimated to be 2.3 nm, as previously described, which is in good agreement with the theoretical height of a DOPC molecule, 2−2.5 nm. Therefore, it is concluded that lipid monolayers formed on the graphene surfaces and the lipid molecules are densely packed each other. We can clearly distinguish the difference in graphene layer number at single layer level after the formation of lipid membranes. This indicates that the individual lipid membranes form independently on every layer of the graphene films, as shown in Figure 5a. On the cross-stepped surface that consists of hydrophilic and hydrophobic domains, selectivity of formation area of lipid membranes is attributed to the

difference in surface chemical properties. In the present result shown in Figure 3b, the formation area of lipid monolayers is well explained by the difference in hydrophilicity of the original cross-stepped surface. It has been reported that the water contact angle of graphene films on the glass substrates increased by the wetting transparency effect. Surface hydrophilicity of the substrate can be permeable through the graphene films in air and affects surface hydrophilicity of even few-layer-graphene films.45 It indicates that hydrophilicity of graphene films can be regulated by the support substrate. An excessively high hydrophilic or hydrophobic surface is not suitable for formation of lipid membranes in the vesicle fusion method.43,44 Since graphite is an extremely high hydrophobic material, lipid membrane cannot be formed normally. However, lipid membranes easily form on specific graphene surfaces owing to the effect of hydrophilicity of the support substrates. In our experiments, lipid membranes hardly formed on the thick graphite films of a hundred nanometers supported by any sapphire substrate. This is interpreted by disappearance of the effect of the support substrate. In this study, we have demonstrated that the hydrophilicity/hydrophobicity of the substrate surface is permeable through graphene films. Figure 5 shows a summary of the formation model of lipid membranes on graphene films that take effects of the support substrate into account. Finally, we have to consider the possibility that the interfacial water layer plays an essential role in transferring the hydrophilicity/hydrophobicity of the substrate to the graphene surface. In other reports, the structured water affects the chemical properties of graphene films.46 Therefore, it is probable that the permeability is created by a difference in the structures of the interfacial water layer, and the hydrophilicity/hydrophobicity of the substrate is the origin of the difference in the interfacial water.

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CONCLUSION In summary, we have demonstrated that formation of lipid membranes on graphene films supported by sapphire substrates can be controlled by the hydrophilicity/hydrophobicity of the substrate surface. Lipid monolayer easily forms on graphene films that are attached to the hydrophilic area of the substrates. The effect of support substrates beneath the graphene films is one of the considerable factors for bioapplications of wellcontrolled graphene systems. We have revealed permeability of hydrophilicity of the support substrate through the graphene films in liquid environment.

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Figure 5. A formation model of lipid membranes on graphene films based on the effects of permeable hydrophilicity of the support substrates: (a) on the randomly stepped surface and (b) on the crossstepped surface.

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +81-45-339-4147 (K.Y.). 18916

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*E-mail:[email protected]; Tel +81-45-339-4147 (T.O.).

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology. The sapphire wafers were provided from Namiki Precision Jewel Co. Ltd.

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