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High-Concentration Aqueous Dispersions of Graphene Using Nonionic, Biocompatible Block Copolymers Jung-Woo T. Seo,† Alexander A. Green,†,‡ Alexander L. Antaris, and Mark C. Hersam* Department of Materials Science and Engineering, Department of Chemistry, and Department of Medicine, Northwestern University Evanston, Illinois 60208-3108, United States
bS Supporting Information ABSTRACT: The ability to disperse pristine graphene at high concentrations in aqueous solutions is an enabling step for large-scale processing and emerging biomedical applications. Herein we demonstrate that nonionic, biocompatible block copolymers are able to produce graphene dispersions with concentrations exceeding 0.07 mg mL1 via sonication and centrifugation, resulting in optical densities above 4 OD cm1 in the visible and nearinfrared regions of the electromagnetic spectrum. The dispersion efficiency of graphene using Pluronic and Tetronic block copolymers varies substantially depending on the lengths of their hydrophilic and hydrophobic domains, with the best of these copolymers sharing similar domain molecular weight ratios and comparable overall molecular weights. This study presents a new class of biocompatible dispersing agents for graphene in aqueous solution, thus suggesting a facile route to employ graphene in biomedical sensing, imaging, and therapeutic applications. SECTION: Nanoparticles and Nanostructures
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raphene exhibits a number of exceptional properties that make it a promising material for use in biological systems. Its high surface area, hydrophobicity, and nanometer-scale thickness can be exploited to deliver low-solubility drugs to cells, target tumors, and enable biological imaging.13 Furthermore, the strong near-infrared optical absorption of graphene provides a pathway to eliminating malignant cells through photothermal ablation.3 An enabling step in these applications is the development of methods to suspend graphene at high concentrations in aqueous solutions using biocompatible dispersing agents. Prior work has shown that stable suspensions of graphene oxide can be readily produced in water and in a number of organic solvents.46 This chemically modified graphene can subsequently be reduced to regain some of the properties of pristine graphene while being stabilized in aqueous solution with biocompatible polymers.1,7 Although high concentrations of reduced graphene oxide can be obtained using this approach, harsh chemical treatments are typically employed to both oxidize and reduce the graphene, which complicates processing, reduces compatibility with living systems, and raises concerns over its long-term environmental impact. Alternatively, stable pristine graphene dispersions can be obtained directly from pristine graphite sources using organic solvents,811 superacids,12 and aqueous solutions containing amphiphilic surfactants.1316 Whereas these approaches obviate the need for aggressive chemical functionalization, the use of organic solvents, superacids, and ionic surfactants for dispersion generally precludes their use in biological systems. Moreover, only a limited number of these systems have been shown to exfoliate pristine graphene at reasonably high concentrations.9,1113,15 Consequently, there exists a continued need to expand the library of dispersing agents capable of r 2011 American Chemical Society
efficiently exfoliating and stabilizing pristine graphene in aqueous solution. Ideally, these dispersing agents should be nonionic and biocompatible and support high graphene concentrations to enable their introduction into living systems for biomedical applications. In this Letter, we evaluate a set of nonionic biocompatible block copolymers, Pluronics and Tetronics, for their ability to disperse pristine graphene in aqueous solutions. The resulting graphene suspensions possess concentrations exceeding 0.07 mg mL1, which correspond to optical densities exceeding 4 OD cm1 in the visible and near-infrared regions of the electromagnetic spectrum. Scanning electron and atomic force microscopy (AFM) indicate that the suspended graphene nanoplatelets possess lateral dimensions of several hundred nanometers and thicknesses ranging from 1 to 10 graphene layers. A comprehensive survey of 19 different Pluronic and Tetronic copolymers quantifies the effect of the hydrophobic and hydrophilic domain size on the concentration and defect density of the suspended graphene nanosheets. Pluronics and Tetronics are commercially available nonionic, amphiphilic block copolymers containing hydrophobic polypropylene oxide (PPO) and hydrophilic polyethylene oxide (PEO) domains. Pluronics are linear molecules consisting of a central PPO region flanked on either end by PEO domains of equal length (Figure 1A, Supporting Information Figure S-1). In contrast, Tetronics are cross-shaped molecules containing a central ethylenediamine linker tethered to four identical diblock copolymer segments (Figure 1B, Supporting Information Figure S-1). These diblock Received: March 16, 2011 Accepted: April 8, 2011 Published: April 12, 2011 1004
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Figure 1. Schematic illustrations of the interaction of (A) Pluronic and (B) Tetronic block copolymers with graphene nanoplatelets.
segments consist of a PEO and PPO domain with the hydrophobic segment covalently bound to the nitrogen atoms of the linker. Importantly, the sizes of the hydrophobic and hydrophilic blocks of both Pluronics and Tetronics can be tuned independently, thus enabling a large number of possible copolymers to be tested for their effectiveness in dispersing graphene. Both copolymers are conveniently named following the relative composition of their polymer blocks. The names of Pluronics begin with a letter that designates their state at room temperature (flake, paste, or liquid), followed by a set of two or three digits. The last of these digits multiplied by 10 denotes the percentage by weight of the PEO block, whereas the earlier digits multiplied by 300 correspond to the approximate average molecular weight of the PPO block. For example, Pluronic F68 exists in flake form at room temperature, consists of 80% PEO by molecular weight, and contains a PPO block with approximate molecular weight of 1800 Da. Tetronics follow a similar naming convention in which the last digit of their name multiplied by 10 designates the percentage by weight of their hydrophilic segments, whereas the earlier digits multiplied by 45 provide the approximate molecular weight of the PPO block. In graphene suspensions, the hydrophobic PPO segments are expected to interact strongly with the graphene faces leaving the hydrophilic PEO chains free to interface with other nearby PEO chains and the surrounding aqueous environment (Figure 1). These copolymers have previously been used with carbon nanomaterials in a number of different contexts. For example, Zu and Han demonstrated that graphene oxide can be stabilized by Pluronics during its reduction with hydrazine.7 Both Pluronics and Tetronics have also been used to disperse single-walled carbon nanotubes (SWCNTs)17 and enable separation according to their physical and electronic structure.18,19 Furthermore,
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Figure 2. (A) Photographs of aqueous graphene dispersions in Pluronics L64 and F77 and Tetronics 904 and 1107. (B) Optical absorbance spectra of the copolymer-graphene dispersions shown in panel A. (C) Graphene concentration map for Pluronics and Tetronics. Colored circles and squares represent the actual experimental graphene concentrations obtained for the Pluronic and Tetronic copolymers, respectively, whereas the underlying color map was obtained by averaging a moving window over the experimental Pluronic data.
Pluronic-SWCNT dispersions have been shown to be biocompatible in animal models.20,21 To prepare the graphene dispersions, we combined 0.6 g of natural graphite flakes (Asbury Carbons, 3061 graphite) with 8 mL of 1% w/v aqueous solution containing the block copolymer. (See the Supporting Information for full experimental details.) A horn ultrasonicator was used to exfoliate graphene directly from the graphite flakes through cavitation. The sonicated mixture was subsequently centrifuged to remove any poorly dispersed graphitic material. Figure 2A displays the graphene suspensions obtained using four different copolymers that possess differing degrees of dispersion efficiency. Herein we use the term dispersion efficiency to describe the capacity of the block copolymer to produce graphene dispersions with high concentrations. As a result, this parameter is a function of the block copolymer’s exfoliation efficiency (i.e., its ability to tease apart neighboring graphene sheets) and stabilization efficiency (i.e., its capacity for preventing individualized graphene sheets from reaggregating once exfoliated). The above results show that small-molecular-weight Pluronics having predominantly hydrophobic composition, such as L64 and L62, were the least effective dispersing agents. In contrast, other copolymers, such as Pluronic F77 and Tetronic 1107, yielded dark black graphene dispersions. To quantify the dispersion efficiency, the optical absorbance of the graphene suspensions was measured in the ultraviolet, visible, and near-infrared regions of the electromagnetic spectrum (Figure 2B). Those graphene dispersions with measurable 1005
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The Journal of Physical Chemistry Letters optical absorbance displayed a strong peak at ∼268 nm arising from the π-plasmon resonance commonly observed in graphitic materials.22 For longer wavelengths, the absorption spectrum is featureless out to the near-infrared with a monotonic decrease in intensity with increasing wavelength. For the Pluronic L64 dispersion, the optical absorption of graphene was barely detectable, whereas the optical absorption increased progressively in the order: Tetronic 904, Pluronic F77, and Tetronic 1107. To better understand the effect of PPO and PEO chain lengths, we tested the dispersion efficiency for a set of 14 different Pluronic and 5 different Tetronic block copolymers. Graphene concentrations were determined from optical absorbance measurements using Beer’s Law based on an extinction coefficient of 6600 L g1 m1.15 This extinction coefficient is to our knowledge the highest reported for graphene and was chosen to establish conservative lower bounds for the graphene concentrations dispersed by the block copolymers. (Experimental optical density values are tabulated in the Supporting Information Table S-2.) Figure 2C summarizes the experimental data, plotting the resulting graphene loadings of all tested copolymers as a function of their hydrophilic and hydrophobic molecular weights. Colored circles and squares are used to represent the actual experimental graphene concentrations obtained for the Pluronic and Tetronic copolymers, respectively, whereas the underlying color map was determined by averaging a moving window over the experimental Pluronic data. (See the Supporting Information.) In addition, the PEO and PPO molecular weights of the Tetronic polymers are plotted at half their actual values because Tetronics can be viewed as a pair of Pluronic chains connected at their midpoints. Inspection of these results reveals two principal trends in the dispersion efficiency of the Pluronic family. First, graphene nanoplatelets are more efficiently exfoliated as the molecular weight of the PEO block size increases. Similar to effects observed with carbon nanotubes,18 it is likely that Pluronics having short PEO segments do not provide sufficient steric hindrance to prevent nearby graphene sheets from interacting and ultimately aggregating with one another in solution. This behavior is in agreement with previous results reported by Smith and coworkers where graphene flakes were stabilized in aqueous solutions by steric potential barriers provided by nonionic surfactants such as Triton X-100.16 This study indicated that the potential barrier can be maximized by intensifying the interaction of the hydrophilic group of nonionic surfactants with water and neighboring hydrophilic segments, which is akin to increasing the length of the copolymer PEO domains. Second, the Pluronic copolymers sharing the same percentage molecular weight of PEO exhibit dispersion efficiencies that peak at a particular overall molecular weight. This effect is most clearly observed in Pluronics F38, F68, F88, F98, and F108 in Figure 2C, which all possess 80% PEO molecular weight. This phenomenon likely arises as a result of two countervailing forces. On the one hand, the hydrophobic domain of the copolymer must be large enough to interface strongly with the graphene to separate it from its neighbors. On the other hand, copolymers having very high molecular weights are too bulky to intercalate between graphene layers for efficient exfoliation. The above trends lead to a graphene dispersion “sweet spot” encompassing Pluronics F68, F77, and F87. Because there are fewer members of the Tetronic copolymer family, the survey of their dispersion efficiency as a function of both PEO and PPO molecular weights is more limited. Nevertheless, important observations can be made, including the fact
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Figure 3. (A,B) SEM images of restacked graphene films produced using (A) Pluronic F77 and (B) Tetronic 1107. (C,D) AFM images of graphene nanoplatelets in (C) Pluronic F77 and (D) Tetronic 1107 deposited on SiO2. (D, bottom) AFM line profiles of graphene nanoplatelets. Scale bars: (AC) 500 and (D) 250 nm.
that Tetronics 1107 and 1307 are found to be the most effective dispersing agents of all the copolymers studied. Despite their morphological differences compared with Pluronics, these Tetronics possess structures that fall within the optimal molecular weight window established by the Pluronics. The higher dispersion efficiencies measured overall for the Tetronics suggest that their ethylenediamine cores exhibit increased affinity for the graphene surface and promote exfoliation. Previous studies employing aminated dispersing agents to exfoliate carbon nanotubes have demonstrated increased affinity of amine groups for metallic carbon nanotubes.23,24 Similar interactions with zero bandgap graphene nanoplatelets could explain the increased affinity of Tetronics. Interestingly, Tetronic 304, which is the smallest molecular weight copolymer tested, displayed dispersion efficiencies comparable to much higher molecular weight copolymers such as Pluronic F88 and Tetronic 908. (See the Supporting Information Table S-2.) The PEO and PPO molecular weights of Tetronic 304 place it well below the range of the molecular weights of the other Pluronic and Tetronic copolymers studied. Its comparatively high dispersion efficiency may result from a low barrier to intercalation during initial exfoliation, which successfully compensates for the reduced stabilization efficiency provided by its short PEO blocks, and/or fundamentally different dispersion behavior for block copolymers in this low-molecular-weight range. Thin films of restacked graphene were prepared from the graphenecopolymer dispersions using vacuum filtration. Following the transfer of these films to SiO2, the graphene nanoplatelets were imaged using scanning electron microscopy (SEM). Representative SEM images of the graphene films obtained from Pluronic F77 and Tetronic 1107 are shown in Figure 3A,B. As illustrated in these images, the graphene nanosheets are deposited at random orientations in the plane parallel to the filtration membrane. The graphene nanoplatelets exhibit a wide distribution of surface areas, with most sheets having lateral dimensions of a few hundred nanometers. SEM measurements of graphene samples prepared from other copolymers showed similar 1006
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Figure 4. (A) Raman spectra at a 514 nm excitation wavelength obtained from restacked graphene films produced using Pluronic F77 and Tetronics 904 and 1107. (B) Graphene D/G ratio map for Pluronics and Tetronics. Colored circles and squares represent the actual experimental D/G ratios obtained for the Pluronic and Tetronic copolymers, respectively, while the underlying color map was obtained by averaging a moving window over the experimental Pluronic data.
distributions of flake areas. (See the Supporting Information Figure S-2.) The exfoliated graphene was also deposited onto SiO2-capped silicon wafers and imaged with AFM to assess nanoplatelet thickness (Figure 3C,D). The graphene thicknesses obtained from these measurements range from 1 to 4 nm, which is consistent with graphene nanoplatelets having 1 to ∼10 layers. The lateral dimensions of the graphene nanosheets in the AFM images range between ∼50 nm and several hundred nanometers. Although the relatively small lateral areas of the graphene in these dispersions present challenges for their use in some highperformance electronic applications, their dimensions are comparable to graphene nanoplatelets produced using ionic surfactants under similar sonication conditions that have demonstrated competitive electronic conductivity in thin film form.13 Because sonication is known to reduce the size of solution-processed graphene, it is likely that the dimensions of copolymer-stabilized graphene can be increased by employing gentler sonication conditions over longer periods of time.15 However, larger area graphene sheets may actually be an impediment to biological applications by increasing cytotoxicity21,25 and inhibiting cellular uptake,26 thus suggesting that the relatively small area graphene studied here may possess advantages for biomedical applications. The thin films of graphene nanoplatelets were also characterized using Raman spectroscopy. The Raman spectra from the samples at a 514 nm excitation wavelength display three dominant peaks, G, 2D (or G0 ), and D, commonly observed in graphene as well as the D0 peak visible as a high-frequency shoulder to the G band (Figure 4A).27 The 2D peak of the graphene samples is adequately
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described by a single Lorentzian, which is consistent with graphene sheets restacked with random interlayer registration.28 The defectrelated D and D0 peaks are significant in all copolymer-dispersed graphene samples. These defects are present at the edges of the small graphene nanoplatelets and are likely introduced to the graphene basal plane during horn ultrasonication. To assess statistically the variations in defect density as a function of copolymer composition, we acquired Raman spectra from the films at a minimum of eight different locations. The G, D, D0 , and 2D peaks of the resulting spectra were fit to single Lorentzian lineshapes. (See the Supporting Information Figure S-3.) Analysis of these data revealed a general trend of increasing defect density (D/G ratio) of the graphene platelets for Pluronic copolymers of increasing molecular weight having hydrophilic domains larger than 3 kDa (Figure 4B). The observed molecular weight dependence may be due to steric effects that hinder exfoliation by the bulkier, high-molecularweight copolymers, which in turn lead to higher energies applied to the graphene as it is exfoliated. In contrast, the Tetronic dispersed graphene did not exhibit a correlation between molecular weight and defect density. These dispersing agents displayed lower defect densities overall, which can likely be understood by the improved exfoliation efficiency provided by their amine centers. In conclusion, we have studied a set of nonionic, biocompatible block copolymers capable of dispersing pristine graphene at high concentrations in aqueous solution. The best of these copolymers, Pluronics F68, F77, F87 and Tetronics 1107 and 1307, readily produce graphene suspensions with optical densities exceeding 4 OD cm1 from the visible to the near-infrared, corresponding to graphene concentrations exceeding 0.07 mg mL1. The ease of processing and high dispersion efficiency of these copolymers establishes them as prime candidates for enabling graphene in biomedical applications, particularly where the low cost and high surface area of graphene provide it with distinct advantages over competing nanomaterials. Moreover, the structural diversity of these Pluronic and Tetronic copolymers provides tunable surface interactions that suggest their use in methods to separate graphene sheets according to their physical and electronic structure6,13 and in the synthesis of graphenenanoparticle composite materials,29 whose performance in catalysis30 and energy conversion31 may be improved by using pristine graphene scaffolds.
’ ASSOCIATED CONTENT
bS
Supporting Information. Graphene dispersion procedures, optical absorbance data, and additional SEM images. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses ‡
Wyss Institute for Biologically Inspired Engineering, Harvard University, Boston, Massachusetts 02215, United States.
Author Contributions †
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These authors contributed equally to this work. dx.doi.org/10.1021/jz2003556 |J. Phys. Chem. Lett. 2011, 2, 1004–1008
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’ ACKNOWLEDGMENT This work was supported by the National Science Foundation (DMR-0520513, EEC-0647560, and DMR-1006391), the Army Research Office (ARO W911NF-05-1-0177), and the Nanoelectronics Research Initiative. A Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship (A.A.G.) is also acknowledged. SEM was performed in the NUANCE facility at Northwestern University, which is supported by the NSF-NSEC, NSF-MRSEC, Keck Foundation, and State of Illinois. This research also utilized instruments at the Center for Nanoscale Materials at Argonne National Laboratory. The Center for Nanoscale Materials is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. ’ REFERENCES (1) Liu, Z.; Robinson, J. T.; Sun, X. M.; Dai, H. J. PEGylated Nanographene Oxide for Delivery of Water-Insoluble Cancer Drugs. J. Am. Chem. Soc. 2008, 130, 10876–10877. (2) Sun, X. M.; Liu, Z.; Welsher, K.; Robinson, J. T.; Goodwin, A.; Zaric, S.; Dai, H. J. Nano-Graphene Oxide for Cellular Imaging and Drug Delivery. Nano Res. 2008, 1, 203–212. (3) Yang, K.; Zhang, S. A.; Zhang, G. X.; Sun, X. M.; Lee, S. T.; Liu, Z. A. Graphene in Mice: Ultrahigh In Vivo Tumor Uptake and Efficient Photothermal Therapy. Nano Lett. 2010, 10, 3318–3323. (4) Park, S.; An, J. H.; Jung, I. W.; Piner, R. D.; An, S. J.; Li, X. S.; Velamakanni, A.; Ruoff, R. S. Colloidal Suspensions of Highly Reduced Graphene Oxide in a Wide Variety of Organic Solvents. Nano Lett. 2009, 9, 1593–1597. (5) Rao, C. N. R.; Sood, A. K.; Voggu, R.; Subrahmanyam, K. S. Some Novel Attributes of Graphene. J. Phys. Chem. Lett. 2010, 1, 572–580. (6) Green, A. A.; Hersam, M. C. Emerging Methods for Producing Monodisperse Graphene Dispersions. J. Phys. Chem. Lett. 2010, 1, 544–549. (7) Zu, S. Z.; Han, B. H. Aqueous Dispersion of Graphene Sheets Stabilized by Pluronic Copolymers: Formation of Supramolecular Hydrogel. J. Phys. Chem. C 2009, 113, 13651–13657. (8) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z. Y.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y. K.;HighYield Production of Graphene by Liquid-Phase Exfoliation of Graphite. Nat. Nanotechnol. 2008, 3, 563–568. (9) Khan, U.; O’Neill, A.; Lotya, M.; De, S.; Coleman, J. N. HighConcentration Solvent Exfoliation of Graphene. Small 2010, 6, 864–871. (10) Hamilton, C. E.; Lomeda, J. R.; Sun, Z. Z.; Tour, J. M.; Barron, A. R. High-Yield Organic Dispersions of Unfunctionalized Graphene. Nano Lett. 2009, 9, 3460–3462. (11) Liang, Y. T.; Hersam, M. C. Highly Concentrated Graphene Solutions via Polymer Enhanced Solvent Exfoliation and Iterative Solvent Exchange. J. Am. Chem. Soc. 2010, 132, 17661–17663. (12) Behabtu, N.; Lomeda, J. R.; Green, M. J.; Higginbotham, A. L.; Sinitskii, A.; Kosynkin, D. V.; Tsentalovich, D.; Parra-Vasquez, A. N. G.; Schmidt, J.; Kesselman, E.;Spontaneous High-Concentration Dispersions and Liquid Crystals of Graphene. Nat. Nanotechnol. 2010, 5, 406–411. (13) Green, A. A.; Hersam, M. C. Solution Phase Production of Graphene with Controlled Thickness via Density Differentiation. Nano Lett. 2009, 9, 4031–4036. (14) Lotya, M.; Hernandez, Y.; King, P. J.; Smith, R. J.; Nicolosi, V.; Karlsson, L. S.; Blighe, F. M.; De, S.; Wang, Z. M.; McGovern, I. T.; Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions. J. Am. Chem. Soc. 2009, 131, 3611–3620. (15) Lotya, M.; King, P. J.; Khan, U.; De, S.; Coleman, J. N. HighConcentration, Surfactant-Stabilized Graphene Dispersions. ACS Nano 2010, 4, 3155–3162. (16) Smith, R. J.; Lotya, M.; Coleman, J. N. The Importance of Repulsive Potential Barriers for the Dispersion of Graphene Using Surfactants. New J. Phys. 2010, 12, 125008.
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