Dispersing Carbon-Based Nanomaterials in Aqueous Phase by

Oct 7, 2013 - Graphene oxides (GO) can be considered as polyelectrolytes with surfactant-like characteristics. On one hand, due to the electrical repu...
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Dispersing Carbon-Based Nanomaterials in Aqueous Phase by Graphene Oxides Yilun Li, Juan Yang,* Qinghua Zhao, and Yan Li* Beijing National Laboratory for Molecular Sciences, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Graphene oxides (GO) can be considered as polyelectrolytes with surfactant-like characteristics. On one hand, due to the electrical repulsion between the negatively charged ionized edges, GO exhibits great water solubility; on the other hand, its hydrophobic central plane retains the potential of strong π−π interaction with other conjugated sp2 network structures. Therefore, it is expected that GO can serve as an excellent dispersing agent for dispersion of various carbon-based nanomaterials in aqueous phase. Here we report a systematic study of dispersing various carbon-based nanomaterials, including SWNTs, C60, and graphene, by aqueous GO. The GO-dispersed allcarbon nanocomposites are characterized using various spectroscopic methods and electron microscopies, and their stabilities are tested. Compared to other dispersing agents, the GO concentration is much lower than the concentrations of other dispersing agents used when similar contents of carbon-based nanomaterials are dispersed. Involving only simple ultrasonication and centrifugation processes, GO dispersion thus offers an easy manipulation for large-scale solution-dispersed all-carbon nanocomposites.

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central plane consisting of unoxidized sp2 benzene rings. The electrical repulsion between the negatively charged ionized edges makes GO water-soluble, yet the potential of strong π−π interaction between the hydrophobic central plane and other conjugated sp2 network structures is retained. Therefore, it is expected that aqueous GO can serve as an excellent dispersing agent for effective dispersion of various carbon-based nanomaterials.30−32 In addition, GO itself is also a carbon-based nanomaterial; thus, all-carbon nanocomposites can be formed without introducing additional materials. The dispersion of MWNTs,33 SWNTs,29 and GN34 by GO have been reported previously, and a water-processable stable colloidal C60/ SWNT/GO dispersion has also been achieved for thin film processing and for applications in organic optoelectronic devices.11,12 In this work, a systematic study of dispersing various carbonbased nanomaterials, including SWNTs, C60, and graphene, by aqueous GO is reported, and the stabilities of these GOdispersed all-carbon nanocomposites are tested. Both spectroscopic methods and electron microscopies are utilized to characterize the GO-dispersed nanocomposites. Comparing to other dispersing agents, the GO concentration is much lower than the concentrations of other dispersing agents used when similar contents of carbon-based nanomaterials are dispersed.

INTRODUCTION Carbon-based nanomaterials, including fullerenes, carbon nanotubes (both multiwalled, MWNTs, and single-walled, SWNTs), and graphene (GN), have drawn special attention for their unique structures, extraordinary physical and chemical properties, and various potential applications in drug delivery,1−4 biosensors,5−8 and photovoltaics.9−12 Because of the poor solubility of these carbon-based nanomaterials in both aqueous and organic environments, effective dispersion is strongly needed before any further applications can be applied. Generally, the dispersion methods have two approaches: by covalent interaction13−18 and by noncovalent interaction.19−26 As the covalent interaction requires modification or surface functionalization of the conjugated sp2 network structure, which in turn severely alters the properties of the nanomaterials, the noncovalent approach is more promising, and a variety of methods have already been developed, such as surfactant adsorption,19,20 DNA21,22 or polymer wrapping,23,24 π−π stacking with aromatic molecules,25,26 etc. However, the limitations lie in low dispersed material content, high price and high concentration requirement of the dispersing agents, lack of biocompatibility, and so on. Graphene oxides (GO), which can be easily produced in large scale from the cheap material graphite by Hummer’s methods27 and are reported to exhibit very good biocompatibility,28 can be considered as polyelectrolytes with surfactantlike characteristics.29,30 GOs are hydrophilic at the edges due to the ionizable carboxylic acid groups and hydrophobic on the © XXXX American Chemical Society

Received: June 25, 2013 Revised: September 10, 2013

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Figure 1. (a) Photographs of GO only, SWNT−GO, and the same SWNT−GO dispersion after settling for 7 days. (b) Optical absorption spectra of SWNT−GO and GO only. (c) Photographs of SWNT−GO, before and after adding 0.2 M 1:1 NaCl solution and settling for 5 h. (d) Optical absorption spectra of SWNT−GO dispersion settled for 0, 1, 3, 5, and 7 days. The spectra are offset manually for clarity. The sediments was of negligible amount, and the supernatant was further diluted into different GO concentrations, denoted as the predispersed GO. A different amount of NaCl was added to predispersed GO, and the solution was equilibrated for 8 h. The zeta potential of the solutions was then measured by zetaPLAS (Brookhaven Instrument Corp.), and the results are given in Supporting Information Figure S1. Sample Dispersion. In a typical experiment, SWNTs, C60, or graphene of ∼0.20 mg was added to 5 mL of 0.1 mg/g predispersed GO; the mixture was ultrasonicated in ice−water bath at 400 W for 90 min and then centrifuged at 6950g for 30 min. The resulting supernatants, denoted as SWNT−GO, C60−GO, or GN−GO, were collected for further characterizations. The blank control “GO only” solution was prepared with ultrasonication and centrifugation in a similar way only without the addition of SWNTs, C60, or graphene. The SWNT−IL dispersion was prepared by carefully grinding different amounts of SWNTs in an agate mortar and pestle with 0.5 mL of [BMIM]PF6 for 30 min, and the mixture was then washed off from the mortar and pestle by 9.5 mL of pure [BMIM]PF6. The resulting homogeneous solution was collected for further characterizations. The SWNT−SDS dispersion was prepared by mixing ∼0.20 mg of SWNTs with 5 mL of 1 wt % SDS solution. The mixture was ultrasonicated in ice−water bath at 400 W for 60 min and then centrifuged at 6950g for 30 min. The resulting supernatant was collected for further characterizations. Characterization. The optical absorption spectra were collected in a 1.0 cm path length cuvette with a PerkinElmer Lambda 950 spectrophotometer. The Raman spectra were acquired on a HORIBA Jobin Yvon LabRAM ARAMIS spectrometer with excitation laser of 532 nm. The photoluminescence spectra were collected on a HORIBA Jobin Yvon Nanolog-3 spectrofluorometer equipped with a 450 W xenon arc lamp and a liquid nitrogen cooled InGaAs detector. An 830 nm filter was set in front of the detector to cut off the Rayleigh scattering. The scanning electron microscopy (SEM) images were taken on a S4800 (Hitachi Co.). The transmission electron microscopy (TEM)

Involving only simple ultrasonication and centrifugation processes, GO dispersion thus offers an easy manipulation for large-scale solution-dispersed all-carbon nanocomposites.

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EXPERIMENTAL SECTION

Materials. SWNTs produced by decomposition of CO on cobalt− molybdenum catalyst (CoMoCAT, 704113) were purchased from Sigma-Aldrich, Inc. The tube diameter is 1.0 ± 0.3 nm, tube length is 400−2300 nm (mode 800 nm), and aspect ratio is about 1000, reported by Sigma-Aldrich, Inc. from optical absorbance and atomic force microscope (AFM). The graphite flakes (332461) were also purchased from Sigma-Aldrich, Inc. The C60 (99.9%) sample was produced using the arc discharge method, purchased from Puyang Yongxin Fullerene Technology Co., Ltd., China. Graphene was produced by the arc discharge method described elsewhere.35 The surfactant sodium dodecyl sulfate (SDS) and the ionic liquid (IL) 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) were purchased from Acros Organics, Inc., and Henan Lihua Pharmaceutical Co. Ltd., China, respectively. Preparation of Aqueous GO. The aqueous GO was prepared using the improved Hummers’ method15 with minor modification. Briefly, a 9:1 mixture of concentrated H2SO4/H3PO4 (63/7 mL) was added to a mixture of graphite flakes (0.5 mg) and KMnO4 (3 g), and the resulting mixture was then stirred under 50 °C for 12 h. The products were cooled to room temperature and poured onto ice (200 mL) with 30% H2O2 (3 mL). Plenty of water was added into the dispersion after the solution turned bright brown, and no gas was released. The dispersion was then centrifuged at 12400g for 30 min followed by repeated washing with deionized water to yield the original GO solution. 1.028 g of original GO was dried under 90 °C, and the residue GO solid was weighed to be 0.011 g, giving an original GO concentration of 10.5 mg/g. The as-prepared original GO was first diluted into 0.5 mg/g, then ultrasonicated in a tip sonicator (JY92-2D, Xin Zhi Co.) in ice−water bath at 400 W for 60 min, and finally centrifuged at 12000g for 30 min. B

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Figure 2. (a) Optical absorption spectra of SWNT−GO and GO only, with arrows pointing to the S22 peak used for RPH calculation. (b) RPH calculation for SWNT−GO. images were taken on a JEM-2100F (JEOL Co.) using TEM grid with micrometer-sized holes.

transferring. Both approaches are not applicable for measuring the SWNT concentrations in SWNT−GO dispersion. Recently, we have developed a quantitative method for measuring the chirality abundance of SWNTs dispersed in ILs by optical absorption spectra.36 As SWNTs can be dispersed in ILs at relatively high contents without any precipitation and absolutely no centrifugation involved during sample preparation, the SWNT concentrations thus can be readily calculated from the precisely weighed SWNT masses. We believe the calibration curve derived from the SWNT−IL dispersions is still suitable for our SWNT−GO dispersion since SWNTs dispersed in both IL and GO are in forms of fine bundles, which can be proven by optical absorption spectra and TEM characterizations discussed later. Moreover, the exact amount of dispersed SWNTs in SWNT−GO is not vital in this case as far as the relative amount of SWNTs can be compared under various disperse conditions. However, as the background signals of SWNT−GO and SWNT−IL differ from each other and depend greatly on ultrasonication conditions,38 this variation needs to be deducted from the absorbance for quantification purposes. Therefore, we use the indexing relative peak height (RPH), as defined in Figure 2, as the quantification parameter to calculate the dispersed SWNT concentrations. The RPH in SWNT−GO is obtained by subtracting the linear background baseline drawn according to the absorbance at the valleys of 630 and 718 nm, where nearly no absorbance of SWNTs is observed, from the absorbance at 668 nm, which corresponds to the most significant band in the S22 range and is less likely to be affected by the environment than bands in S11. For RPH in SWNT−IL, the corresponding S22 band peaks at 662 nm and thus the absorbance at 662 nm are used instead (Figure S2a,b). As shown in the calibration line in Figure S3c, a series of SWNT− IL dispersions with SWNT concentrations of 0.050 63, 0.021 84, 0.007 832, and 0.001 125 mg/g give RPH values of 0.0828, 0.0359, 0.0129, and 0.001 90, respectively, resulting in a calibration function of RPH = 1.63 × [SWNTs]. The fitting is excellent with R2 = 0.999 99. The dispersed SWNT concentration in the selected SWNT−GO dispersion in Figure 2 is then calculated to be 0.018 mg/g. In order to achieve the optimal dispersion efficiency with the highest SWNT concentration, optimization of various disperse parameters has been conducted. Specifically, the ultrasonication time and the concentration of GO are optimized while the amount of input SWNTs (0.20 mg) and the ultrasonication power (400 W) are fixed in the optimization. With the readily established quantification method toward the dispersed

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RESULTS AND DISCUSSION Dispersion of SWNTs by Aqueous GO. Figure 1a shows the photographs of blank control GO only, SWNT−GO, and the same SWNT−GO dispersion after settling for 7 days, and Figure 1b illustrates the corresponding optical absorption spectra. Both the obvious color difference between GO only and SWNT−GO dispersion and the presence of SWNT optical bands (S11 800−1300 nm, S22 550−800 nm, and M11 450−600 nm for CoMoCAT SWNTs) have proven the successful dispersion of SWNTs by aqueous GO solution. The asprepared SWNT−GO dispersion is stable for several months. After settling for 7 days at room temperature, no sediment is observed and no significant decrease in the optical absorption spectra is found (Figure 1d). Meanwhile, a typical as-prepared SWNT−GO dispersion is first lyophilized, followed by stored at room temperature for 5 days, and then redispersed in nanowater by mild water-bath sonication for 20 min. The SWNT−GO dispersion is successfully retrieved. Although not as stable as the original, the redispersed sample remains stable without any precipitation for 12 h. In addition, when NaCl solution (0.2 M, 1:1) is added to the as-prepared SWNT−GO dispersion, all dispersed SWNTs and GO precipitate quickly within 5 h, and the supernatant becomes colorless (Figure 1c). This can be explained by the significantly weakened electrical repulsion between the GO sheets with the addition of electrolytes. The aggregation and precipitation of GO sheets brings down the interacted SWNTs simultaneously. In order to determine the SWNT contents in SWNT−GO dispersion and to compare the dispersion efficiency under various disperse conditions, it is necessary to develop a quantification method to measure the dispersed SWNT concentrations. The commonly used approaches in literatures are all based on the establishment of a calibration curve according to the optical absorbance of SWNTs at a certain absorption wavelength. The SWNT concentrations can be calculated either directly from the weighed amount of original SWNT samples29,36 or by subtracting the weight of the dispersing agents from the total dry weight of the supernatant after centrifugation.37 The obvious limitation of the former approach is that the amount of SWNTs precipitated after centrifugation is neglected, which clearly causes overestimation in dispersed SWNT concentration. The disadvantage of the latter lies in the complicated procedures, requirement of large sample amounts, and the sample loss during drying and C

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Figure 3. (a) SWNT concentration as a function of ultrasonication time, dashed curve showing an exponential fit with R2 = 0.98445. (b) SWNT concentration as a function of GO concentration, dashed curve showing a polynomial fit.

SWNTs concentration, we are now able to quantitatively compare the dispersion efficiency of samples prepared with different dispersion parameters. Figure 3a shows the dispersed SWNT concentration as a function of ultrasonication time at 400 W. As can been seen, the SWNT concentration tends to increase rapidly during the first 90 min, and the increase slows down after that. This trend can be fitted well with an exponential function and can be understood by the decrease of GO size with increased ultrasonication time. As mentioned earlier, the π−π interaction between the hydrophobic polyaromatic central plane of GO and the sidewalls of SWNTs play an important role in dispersing SWNTs by aqueous GO solution. With longer ultrasonication time, the GO sheets become smaller and more flexible. Therefore, the π−π interaction is enhanced, resulting in more stable dispersion and higher dispersed SWNT contents. As a result, the optimal ultrasonication time is selected to be 90 min, where the SWNT concentration is already high and only limited improvement could be expected with further extended ultrasonication time. In Figure 3b, the dispersed SWNT concentration is plotted as a function of GO concentration. At low GO concentration, the dispersed SWNT concentration increases with increasing GO content due to more SWNT−GO interactions. However, at high GO concentration, further increase in GO content leads to decreasing SWNT concentration because of the predominant GO−GO interactions. It is found that the SWNT concentration is optimal with a GO concentration of 0.10 mg/g among the four prepared SWNT−GO dispersions. Using the optimized ultrasonication time of 90 min and GO concentration of 0.10 mg/g, we now increase the amount of input SWNT masses from 0.2 to 0.4 mg, and consequently the dispersed SWNT concentration is increased to 0.020 mg/g, close to the reported SWNT concentration in SDS dispersion.39 Comparing SWNT−GO dispersion to the classic dispersions by various surfactants, the concentration of the dispersing agent GO is about 2 orders of magnitude lower than the concentration of other dispersing agents, such as SDS, sodium cholate (NaC), or sodium dodecyl benzenesulfonate (SDBS), as listed in Table 1, when similar dispersed SWNT concentrations are achieved. Therefore, the SWNT−GO dispersion shows an obvious advantage of effectively dispersing SWNTs with low dispersing agent concentration. Such an advantage is important in the application of large-scale solutiondispersed SWNTs, as the influence of the dispersing agent could be minimized. It is worthy to mention that although

Table 1. Comparison between SWNT−GO Dispersion and Some Classic Surfactant Dispersions dispersing agents, concn (mg/g)

SWNT concn (mg/g)

ratio

GO, 0.1 SDS, 1037,39 NaC, 10037 SDBS, 1037

0.020 0.020−0.025, 0.045 0.025 0.065

5 400−500, 222 4000 154

surfactants can disperse SWNTs individually whereas most SWNTs are dispersed in bundles by aqueous GO, we do observe individually dispersed SWNTs wrapped by GO sheets. The SWNT−GO dispersion with the highest SWNT concentration of 0.020 mg/g obtained under the optimal disperse conditions is characterized using various methods, including optical absorption, Raman, fluorescence spectra, and SEM as well as TEM. Figure 4a illustrates the optical absorption spectra of this SWNT−GO dispersion as well as a SWNT−IL and a SWNT−SDS dispersion with comparable dispersed SWNT concentration. In the S11 region, both IL and GO dispersions show clearly broadened and significantly redshifted bands comparing to SDS-dispersed SWNTs. This can be explained by that SWNTs are dispersed mainly as individual tubes in SDS, but mostly as bundles in both IL and GO. In addition, it is noticed that GO-dispersed SWNTs are even more red-shifted than IL-dispersed SWNTs. This is due to the strong π−π interaction existing between GO and SWNTs, while only weak van der Waals interaction existed in SWNT−IL. In the S22 region, the band positions for all three dispersion are rather close with minor differences, which also indicates that the S22 bands of SWNTs is less likely to be affected by environment than S11. After manually subtraction of the absorption baseline, the SWNT−GO spectrum can be deconvoluted into a series of individual absorption peaks, and the corresponding assignments are labeled in Figure 4b. No significant selectivity of any diameter, semiconducting/metallic type, or chirality for GOdispersed SWNTs is observed, although diameter selectivity is reported for GO-dispersed MWNTs.33 The lack of selectivity could be attributed to the fact that the diameters of SWNTs are very small comparing to the size of GO sheets; thus, the changes in SWNT diameters have negligible effect on the π−π interaction between SWNTs and GO. In Figure 4c, Raman spectra provide additional information toward the unique optical properties of GO-dispersed SWNTs. The revealed RBM, G, and 2D bands of SWNTs in SWNT− GO confirm the existence of SWNTs in the dispersion, whereas the upshifted Raman frequencies of these bands comparing to D

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Figure 4. (a) Optical absorption spectra of SWNT−IL, SWNT−SDS, and SWNT−GO. (b) Deconvolution of the baseline subtracted optical absorption spectra of SWNT−GO. (c) Raman spectra of SWNT−GO and GO only, excited at 532 nm. (d) Emission spectra of SWNT−IL and SWNT−GO, excited at 650 nm. (e, f) TEM images of SWNT−GO obtained using TEM grid with micrometer-sized holes.

SWNTs alone verify the strong π−π interaction between SWNTs and GO. Fluorescence emission spectra of SWNT− GO and SWNT−IL dispersions with excitation at 650 nm are shown in Figure 4d. Although SWNTs are dispersed as bundles in both IL and GO dispersions, and the fluorescence of SWNTs is already significantly reduced comparing to SDS-dispersed individual SWNTs by the metallic tubes within the bundles through an energy transfer mechanism, IL-dispersed SWNTs still maintain good FL intensity. Incomplete thermalization before exciton recombination is found in IL-dispersed SWNT bundles.40 However, for GO-dispersed SWNTs, the fluorescence signal of SWNTs is completely quenched, indicating an effective energy transfer from SWNTs to nearby GO sheets and providing vital evidence for the strong π−π interaction between SWNTs and GO. According to the TEM images in Figure 4e, most SWNTs are dispersed in bundles of ∼6 nm by GO sheets. In a rough estimation, the average length and diameter of CoMoCAT

SWNTs can be assumed to be 800 and 1.0 nm, respectively, and the average size of the GO sheets is about 0.1 μm2 according to the SEM image in Figure S4a. With the maximum SWNT concentration of 0.020 mg/g and GO concentration of 0.1 mg/g, the average number of SWNTs per GO sheet can be calculated to be ∼30, which is in good accordance with the ∼6 nm bundle size observed. Despite most SWNTs are dispersed in bundles by GO, Figure 4f clearly shows an individual SWNT with diameter of ∼0.7 nm is wrapped by a GO sheet, which provides direct evidence for the disperse mechanism of strong π−π interaction through GO wrapping. Additional TEM images with full resolution are provided as Figure S3a−d. It should be noted that the average GO size of 0.1 μm2 is significantly smaller than the reported up to 3 μm in the MWNT−GO dispersion,33 which might be a key factor toward the achieved effective dispersion of SWNTs. It is an interesting issue of how the length of SWNTs affects the stability of SWNT−GO dispersion. We have ultrasonicated E

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Figure 5. (a) Photographs of GO, C60−GO, and the same C60−GO dispersion after settling for 1 day. (b) Optical absorption spectra of C60−GO settled for 0, 2, 8, and 14 days. Inset: optical absorption spectra of C60−GO, GO only, and their subtraction. (c) Raman spectra of GO only, C60− GO, and C60. (d) TEM image of C60−GO.

preparation. In addition, both the ratio of absorbance at 450 and 340 nm and the red-shifts of the absorption peak in the region of 260−280 nm are used in the literature20 to indicate the degree of C60 aggregation. As is given in Figure 5b and Table 2, the A(450 nm)/A(340 nm) ratio increases from 0.45

CoMoCAT−GO dispersion for 3 h at 400 W; however, the tube length does not change significantly. We then use another SWNT sample prepared by chemical vapor deposition using Ni/Co as catalyst (NiCoCAT) instead, the original tube length of which is 5−30 μm. By varying the ultrasonication time, two NiCoCAT−GO dispersions are prepared with average tube length of 4 and 2 μm, respectively (SEM images in Figure S4b,c). It is found that NiCoCAT−GO with longer SWNT precipitates faster than the other one, and both precipitate much faster than CoMoCAT−GO. The absorption spectrum of NiCoCAT−GO with longer SWNT also shows less absorbance (Figure S4d). Therefore, we believe the length of SWNT does affect the stability of SWNT−GO, and shorter SWNTs likely give better stability. Dispersion of C60 by Aqueous GO. Assuming that a similar disperse mechanism of π−π interaction could be expected between C60 and GO, we extend the method to C60−GO dispersion. As is shown in Figure 5, both the color difference in solution and the revealed absorption peaks convince the successful dispersion of C60 by GO. The absorption peak positions at 340 and 450 nm are consistent with the reported values in the literature.41 The concentration of dispersed C60 is estimated to be about 2.1 μg/mL, based on the reported calibration curve41 of A335 = 0.00881c(C60, μg/ mL) + 0.0288. The Raman spectrum of the C60−GO dispersion shows the signatures of D- and G-band of GO and a stretching mode of C60 at 1457 cm−1. From the TEM images, it is observed that C60 are dispersed as aggregates of about 3 nm and only attached to the GO sheets. Unlike 1D SWNTs, C60 are too small to be wrapped by GO sheets. Since the π−π interaction between C60 and GO is much weaker due to the small size of C60, the stability of C60−GO dispersion is poor, with obvious precipitants after settling for 24 h after

Table 2. Optical Absorption Features of the Same C60−GO Dispersion with Different Settling Times settling time (days) A450/A340 λmax (nm)

0

2

4

8

14

0.45 266

0.45 268

0.47 270

0.50 272

0.53 274

to 0.53, and the peak position is red-shifted from 266 to 274 nm for a C60−GO sample settled for 14 days, which indicates that GO-dispersed C60 has a significant tendency to aggregate overtime, in consistent with the observed precipitants. Dispersion of Graphene by Aqueous GO. Graphene can be dispersed by aqueous GO in a similar way (Figure 6). The upshifted Raman G-band of GN−GO comparing to that of GN alone verifies the strong π−π interaction between GN and GO. Precipitants appear in GN−GO dispersion after settling for 4 days after preparation, indicating GN−GO is less stable than SWNT−GO but more stable than C60−GO. The lower stability of GN−GO dispersion than SWNT−GO can be understood by the stronger π−π interaction of GN−GN than SWNT−SWNT. It is also an interesting issue of how the relative size of GN and GO affects the stability of GN−GO dispersion. By varying the ultrasonication time, two GN−GO dispersions with different relative size are prepared, denoted as GN−GO large and GN−GO small, respectively. Both SEM and AFM images (Figure S5) show that in GN−GO large the size of GO is F

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Figure 6. (a) Photographs and (b) optical absorption spectra of GO, GN−GO and the same GN−GO dispersion after settling for 4 days. (c) Raman spectra of GO only, GN−GO, and graphene, excited at 532 nm. (d) TEM image of GN−GO.

SWNT lengths. This material is available free of charge via the Internet at http://pubs.acs.org.

significantly larger than that of GN, whereas in GN−GO small the GO size is comparable to the GN size. As 1 nm AFM height corresponds to a monolayer GO or GN, the profile of GN−GO large indicates many locations of large monolayer GO sheets, whereas the profile of GN−GO small suggests that almost all small GO sheets are covered by GN. It is also found that the GN−GO large precipitates faster than GN−GO small; thus, small and comparable GO size with respect to GN size likely gives better GN−GO stability.

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Corresponding Authors

*E-mail: [email protected] (J.Y.). *E-mail: [email protected] (Y.L.). Tel: +86-10-62755357. Fax: +86-10-62755357.

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Notes

The authors declare no competing financial interest.

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CONCLUSIONS In summary, we have systematically studied the dispersion of various carbon-based nanomaterials by water-soluble GO through strong π−π interaction between the hydrophobic central plane of GO and the other conjugated sp2 network structures. Under similar disperse conditions, it is found that SWNT−GO is the most stable dispersion among three due to the close wrapping of SWNT bundles by GO sheets, and GN− GO is more stable than C60−GO. It is also found that shorter SWNTs is more stable than the longer ones when dispersing by GO and that comparable GN/GO size gives better stability than otherwise. It is clear GO indeed serves as an excellent dispersing agent for dispersion of various carbon-based nanomaterials in aqueous phase, with advantages such as low GO concentration, good biocompatibility, low price, easy manipulation, forming all-carbon nanocomposites, etc.

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AUTHOR INFORMATION

ACKNOWLEDGMENTS The authors thank NSFC (Projects 21005004, 21125103, 11179011, J1030413), SRFDP of China (Project 20100001120016), and MOST (Project 2011CB933003) of China for support.

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REFERENCES

(1) Bianco, A.; Kostarelos, K.; Prato, M. Applications of carbon nanotubes in drug delivery. Curr. Opin. Chem. Biol. 2005, 9, 674. (2) Sun, X.; Liu, Z.; Welsher, K.; Robinson, J. T.; Goodwin, A.; Zaric, S.; Dai, H. Nano-graphene oxide for cellular imaging and drug delivery. Nano Res. 2008, 1, 203−212. (3) Hughes, G. A. Nanostructure-mediated drug delivery. Nanomed. Nanotechnol. 2005, 1, 22−30. (4) De Jong, W. H.; Borm, P. J. Drug delivery and nanoparticles: applications and hazards. Int. J. Nanomed. 2008, 3, 133. (5) Besteman, K.; Lee, J. O.; Wiertz, F. G. M.; Heering, H. A.; Dekker, C. Enzyme-coated carbon nanotubes as single-molecule biosensors. Nano Lett. 2003, 3, 727−730. (6) Wang, J.; Musameh, M.; Lin, Y. Solubilization of carbon nanotubes by Nafion toward the preparation of amperometric biosensors. J. Am. Chem. Soc. 2003, 125, 2408−2409.

ASSOCIATED CONTENT

S Supporting Information *

Zeta potential of aqueous GO, optical absorption spectra and calibration curve of SWNT−IL, additional TEM images of SWNT−GO, and comparison of SWNT−GO with different G

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dx.doi.org/10.1021/la4024025 | Langmuir XXXX, XXX, XXX−XXX