Nanoassembly of Block Copolymer Micelle and Graphene Oxide to

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Nanoassembly of Block Copolymer Micelle and Graphene Oxide to Multilayer Coatings Jinkee Hong,^,† Yong Soo Kang,‡,* and Sang Wook Kang§,* †

School of Chemical & Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea WCU Program Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea § Department of Chemistry, Sangmyung University, Seoul 110-743, Republic of Korea ‡

ABSTRACT: We report on the preparation and characterization of multidimensional nano-objects nanoassembly in film surface coatings. 2D reduced graphene oxide (rGO) sheets are assembled with 0D block copolymer micelles (BCM)s through electrostatic layer-by-layer assembly. The BCM/rGO paired nanoassembly surface coatings were formed via sequential adsorption of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) aqueous solutionsand modified rGO sheets with carboxylic acids groups, and the process was monitored by UV-vis spectroscopy. SEM and AFM were used to investigate surface morphology and the growth of multilayer coatings as a function of the number of bilayers. In addition, encapsulated hydrophobic fluorescence organic dyes were imaged via confocal microscopy. Evenly distributed BCMs on coatings were successfully obtained. We expect that this demonstration offers a new route to introduce nanoassembly of two different dimensional-shaped nano-objects in surface coatings for many industrial uses and makes building more complex, functional multicomponent, and multidimensional structures possible for use in nano-objects incorporated various multicoating applications.

1. INTRODUCTION Recently, functional multilayer structures have attracted big attention as smart coatings in order to present the solution on several interfacial matters. Importance of well-controlled structure of functional objects into surface coatings from micro- to nanoscopic scales is related to the preparation of multifunctional films. To endow desirable functionalities into the coatings, a variety of nano-objects have been suggested for the preparation of the functional films. Block copolymer micelles (BCM) have attracted considerable attention due to their various potential applications as candidates for targeting, sensing, diagnosis, or surface modification.1-8 In particular, BCMs are considered ideal candidates for 0D spherical objects possessing physical/chemical stability and mechanical flexibility, which may be used as nanocontainers.9,10 Zhang and coworkers have reported that BCMs with incorporated hydrophobic photochromic properties can be assembled into multilayer films via a layer-by-layer (LbL) assembly method. Furthermore, they showed that the prepared polyelectrolyte/block copolymer micelle thin films are useful for photoswitching materials.11,12 In addition, graphene, well-known as a monolayer carbon sheet consisting of hexagonal sp2-hybridized carbons, has attracted great attention in recent years, as applied in various devices, because of its extraordinary electrical and mechanical properties.13-18 Graphene is considered as a particularly promising candidate for 2D nano-objects, such as nanocomposite materials possessing superior thermal, mechanical, and electrochemical properties. Numerous methods have been reported for generating nanohybrids by employing various functionalities, with a view toward generating synergies by combining the advantages of various nano-objects.19-24 Among the advantages of various functionalized nano-objects, which endow enhanced properties for various applications, wellorganized nanohybrid structures may play a key role in facilitating r 2011 American Chemical Society

object assembly. Preparing nanoassembly of functional nano-objects to produce a well-organized nanohybrid structure has been one of thegreat challenges over the past few decades in the area of multifunctional thin films due to their potential applications in electronic or biomedical devices. Various approaches to prepare nano-objects with incorporated coatings have mainly focused on 0- and 1D nanoobject thin films to take full advantage of their morphological or inherent functions. However, in our knowledge, there are no reports of surface coatings based on using both 0D micelles and 2D graphene sheets. Herein, we report a novel way to prepare nanoassembly of 0D BCMs and 2D reduced graphene oxide (rGO) as a surface coating for composite multilayer films. Layer-by-layer (LbL) assembly was used to prepare organized nanocomposite multilayer films with a highly ordered structure.25-28 The advantage of this method is that it enables the preparation of nanocomposite multilayer films with tailored film thickness and allows for the incorporation of multidimensional nano-objects with nanosize thickness control over the film.29-32 Additionally, various nano-objects can be inserted into nanocomposite multilayer films via complementary interactions, such as electrostatic, hydrogen-bonding, or covalent interactions.33-37 In our study, 0D, positively charged BCMs and 2D, negatively charged rGO were used to prepare BCM/rGO multilayer coatings. 2. EXPERIMENTAL SECTION 2.1. Materials. Graphene oxide was synthesized from gra-

phite powder (45 μm, Sigma Aldrich) by a modified Hummer’s

Received: November 4, 2010 Accepted: January 18, 2011 Revised: December 21, 2010 Published: February 18, 2011 3095

dx.doi.org/10.1021/ie1022282 | Ind. Eng. Chem. Res. 2011, 50, 3095–3099

Industrial & Engineering Chemistry Research

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Figure 1. (a) Schematic representation of electrostatic interaction based layer-by-layer assembly of block copolymer micelles and reduced graphene oxide. (b) Photograph images of (PS-b-P4VP/rGO)n (n = 6, 14, 22, 38 and 46) multilayer films as a function of bilayer number. (c) UV-vis spectra of (PS-b-P4VP/rGO)n with increasing the bilayer number (n) from 6 to 46 at pH 4/6. The absorption peak at 254 nm originates from the pyridine groups in PS-b-P4VP micelles. (d) The absorbance at 254 nm grows linearly with increasing bilayer numbers of (PS-b-P4VP/rGO)n.

method and was reduced, according to previous reports.38,39 Polystyrene(Mw = 17K)-block-poly(4-vinylpyridine)(Mw = 49K) (PS-bP4VP) block copolymer was purchased from Polymer Source and was used as received without any purification. 2.2. Film Construction. Layer-by-layer multilayer films were assembled with a programmable Carl Zeiss slide stainer. Films were constructed on a silicon wafer and quartz glass, which were treated initially with a Piranha solution (sulfuric acid/hydrogen peroxide = 70/30 v/v%). Negative charge modification was performed by heating at 70 C for 30 min in a 5:1:1 vol% mixture of water, hydrogen peroxide (H2O2), and a 29% ammonia solution (RCA solution). The substrate was first dipped into a positively charged 1 mg/mL PS-b-P4VP BCM dispersed aqueous solution (pH 4) for 10 min, followed by three rinsing steps with pH-adjusted water for 1 min each. Afterward, the substrate was dipped into a negatively charged 1 wt % rGO solution (pH 6.0) for 10 min, and the same rinsing steps as described above were repeated. 2.3. Characterization Equipments. The ζ-potentials of BCMs and rGO were measured using an electrophoretic light scattering spectrophotometer (ELS-8000). The morphologies of the (BCM/ rGO)n multilayer coatings were examined using a field-emission SEM (JEOL JSM-7401F) and atomic force microscopy (AFM) in the tapping mode (Nanoscope IIIa, Digital Instruments). The thickness and refractive indices of multilayer films on Si-wafers were measured by ellipsometry (Gartner Scientific Corp., L2W15S830) with 632.8 nm He-Ne laser light. UV-vis spectra were taken with a PerkinElmer Lambda 35 UV-vis spectrometer. The pyridine groups of PS-b-P4VP show an absorbance band centered at 254 nm. Confocal microscope images of (BCM/rGO)n coatings were taken using confocal fluorescence microscopy (Zeiss, LSM 5 Pascal).

3. RESULTS AND DISCUSSION Part a of Figure 1 shows the overall process of assemblythe various nano-objects, that is the 0D block copolymer micelles

(BCM) and 2D reduced graphene oxide (rGO) sheets. The multilayer films were prepared by alternate assembly of cationic BCM and negatively charged rGO based on electrostatic interactions. For layer-by-layer (LbL) assembly, PS-b-P4VP BCM solutions adjusted to pH 4 were prepared based on a previous report that suggested this is the condition required for the deposition of relatively thicker, denser structures because of weaker electrostatic repulsion between groups, driven by the fact that 42% of the pyridine groups in PS-b-P4VP are protonated at these conditions. The zeta-potentials of PS-bP4VP BCM at pH 4 and rGO at pH 6 were 25.1 ( 3.9 mV and -39.8 ( 6.5 mV, respectively. On the basis of the electrostatic interactions between BCM-NH3 þ and rGOCOO-, nanoassembly of 0D BCMs (PS-b-P4VP) and 2D reduced graphene oxide sheet (rGO) was successfully performed. Taking full advantage of BCM as a nanocontainer, water-insoluble coumarin 30, which is a well-known fluorescence dye, was incorporated into the hydrophobic PS cores of PS-b-P4VP (pH 4). Part b of Figure 1 shows representative images of (BCM/ rGO)n multilayer thin films on glass substrates prepared from pH 4/6, where films have a characteristic color corresponding to their number of bilayers. The films became yellow with increasing thickness and appeared dark yellow at greater than 22 bilayers due to the encapsulated coumarin dyes. UV-vis absorption spectra of the (BCM/rGO)n multilayer films were monitored for different numbers of bilayers. The linear growth of multilayers by the spectrum of pyridine groups in PS-b-P4VP micelles shows absorption at 254 nm (parts c and d of Figure 1). Figure 2 shows SEM images of 5-, 10-, 18-, 26-, 34-, and 46bilayer BCM/rGO multilayer films, which reflect the influence of bilayer number (n) on the surface morphology. Small, spherical features, which dominate for (BCM/rGO)5 or 10, suggest that the individual sheets of rGO are rarely coated onto evenly distributed 3096

dx.doi.org/10.1021/ie1022282 |Ind. Eng. Chem. Res. 2011, 50, 3095–3099

Industrial & Engineering Chemistry Research BCMs in the initial multilayer film. This is attributable to the morphological difference between 0D BCMs and 2D rGO sheets when selectively adsorbing to oppositely charged objects. However, these multilayers regularly increase with increasing bilayer number. Therefore, the multilayer morphology of the film with 26 bilayers was well covered by both BCMs and rGO sheets. In this case, the film thickness of the multilayer films measured by ellipsometry changes from 47.3 nm for 10 bilayers to 231.4 nm for 46 bilayers.

Figure 2. SEM images of (PS-b-P4VP/rGO)n multilayer films formed at pH 4/6 as a function of bilayer number (n): (a) n = 5, (b) n = 10, (c) n = 18, (d) n = 26, (e) n = 34, and (f) n = 46.

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The surface morphology of nanoassembly of multidimensional nano-objects was also observed by atomic force microscopy (AFM). Surface rms roughnesses (Rq) were determined to be 13.5 and 25.2 nm for AFM images over 3 3 mm and 5 5 mm regions in Figure 3. The surface morphology of the multilayer films strongly depends on the spherical curvature of PSb-P4VP BCMs (17.4 ( 2.4 nm diameter under dry conditions), as shown in Figure 3. To demonstrate optical functionality in the nanoassemblycoatings, hydrophobic organic dye (Coumarin 30 with a PL emission peak at 480 nm, bluish green in color) was incorporated into the hydrophobic PS cores of PS-b-P4VP micelles. Hydrophobic Coumarin 30 is uniformly entrapped within the PS cores of PS-b-P4VP micelles by self-diffusion because of the more energetically favorable interactions between the dye and the PS blocks. Images of fluorescence dye encapsulated nanoassembly of (PS-b-P4VP/rGO)46 coating are shown in Figure 4 for various magnification images. These results clearly imply that various hydrophobic materials can be incorporated into the BCMs without any need for chemical modifications to enable dispersion in water. Furthermore, it should be emphasized at this point that the fluorescence efficiency of BCM-encapsulated fluorescence material is much higher than that of chemically modified material.

Figure 4. Confocal fluorescence images of (BCM/rGO)46 multilayer film at (a) relatively low (100 times) and (b) high (400 times) magnifications. (c) Fluorescence image of (BCM/rGO)46 multilayer film coated glass substrate (side view).

Figure 3. (a), (c) 2D and (b), (d) 3D AFM height images of (BCM/rGO)n as a function of bilayer (n): (a), (b) n = 5 and (c), (d) n = 46 multilayers prepared by the layer-by-layer assembly method. 3097



4. CONCLUSIONS We have demonstrated the nanoassembly of 0D block copolymer micelles (BCM) and 2D reduced grapheneoxide sheets in multilayer coatings through a layer-by-layer (LbL) assembly method. The nanopacked BCMs and rGO sheets in multilayer coatings showed linear growth of thickness from 47.3 nm, for 10 bilayers to 231.4 nm for 46 bilayers. (BCM/rGO)n nanopackaged multilayer coatings were formed via sequential adsorption of PS-b-P4VP BCM aqueous solution and rGO sheets modified with carboxylic acids groups; this process was monitored by UV-vis spectroscopy. SEM and AFM analysis were used to investigate surfacemorphology and to analyze the growth of multilayer films as a function of the number of bilayers. In addition, encapsulated hydrophobic fluorescence organic dyes were imaged via confocal microscopy. Evenly distributed BCMs on coatings were successfully produced. We regard our simple approach has a significant step forward in nanoassemblyof multidimensional nano-objects in surface coatings based on both 0D BCMs and 2D rGO. Considering the broad applications of both nano-objects (i.e., micelles, graphene-sheets) and nanosized assembly in surface coatings, the approach introduced in this report presents new possibilities for novel, nano-objects incorporated film engineering systems. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: þ82 2 2287 5362 (S.W.K.), E-mail: [email protected]. Tel.: þ82 2 2220 2336 (Y.S.K.). Present Addresses ^

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, United States.

’ ACKNOWLEDGMENT This work was supported by the Korea Center for Artificial Photosynthesis (KCAP) located in Sogang University funded by the Ministry of Education, Science, and Technology (MEST) through the National Research Foundation of Korea (NRF2009-C1AAA001-2009-0093879), and WCU (World Class University) program through the National Research Foundation funded by the Ministry of Education, Science and Technology (R31-2008-000-10092). ’ REFERENCES (1) Ma, D. W.; Wang, Y. Z.; Wu, J.; Zhao, Y. C.; Ming, M.; Ding, J. D. Amphiphilic Block Copolymers Significantly Influence Functions of Bacteriorhodopsin in Water. Soft Matter 2010, 26, 4920. (2) Addison, T.; Cayre, O. J.; Biggs, S.; Armes, S. P.; York, D. Polymeric Microcapsules Assembled from a Cationic/Zwitterionic Pair of Responsive Block Copolymer Micelles. Langmuir 2010, 26, 6281. (3) Chen, H. T.; Kim, S. W.; Li, L.; Wang, S. Y.; Park, K.; Cheng, J. X. Release of Hydrophobic Molecules from Polymer Micelles into Cell Membranes Revealed by Forster Resonance Energy Transfer Imaging. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 6596. (4) Hirano, A.; Iijima, M.; Emoto, K.; Nagasaki, Y.; Kataoka, K. Multi-layered Nanoball as High Performance Permselective Membrane. Mater. Sci. Eng., C 2004, 24, 761. (5) Ergican, E.; Gecol, H. Nonlinear Two-Phase Equilibrium Model for the Binding of Arsenic Anions to Cationic Micelles. J. Membr. Sci. 2008, 325, 69. (6) Fang, Y. Y.; Zeng, G. M.; Huang, J. H.; Liu, J. X.; Xu, X. M.; Xu, K.; Qu, Y. H. Micellar-Enhanced Ultrafiltration of Cadmium Ions with Anionic-Nonionic Surfactants. J. Membr. Sci. 2008, 320, 514.

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Nanoassembly of Block Copolymer Micelle and Graphene Oxide to

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