A New Graphene Derivative: Hydroxylated Graphene with Excellent

Apr 7, 2016 - (10) The resultant hydroxyl-functionalized graphene showed a good hydrophilicity, electroactivity and biocompatibility to human retinal ...
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A New Graphene Derivative: Hydroxylated Graphene with Excellent Biocompatibility Jing Sun,†,‡ Yuan Deng,†,§ Jipeng Li,‡,§ Gang Wang,‡ Peng He,‡ Suyun Tian,‡,⊥ Xiuming Bu,‡ Zengfeng Di,‡ Siwei Yang,*,‡ Guqiao Ding,*,‡,∥ and Xiaoming Xie‡,⊥ ‡

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai 20050, China § Department of Ophthalmology, Shanghai Ninth People’s Hospital, Shanghai JiaoTong University School of Medicine, Shanghai 20011, China ⊥ School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, China ∥ Shanghai SIMBATT Energy Co., Shanghai 201821, China S Supporting Information *

ABSTRACT: Graphene derivatives (such as graphene oxide and hydrogenated graphene) have been widely investigated because of their excellent properties. Here, we report large-scale (kilogram scale) synthesis of a new unique graphene derivative: hydroxylated graphene (G-OH). The exclusive existence form of oxygen-containing groups in G-OH is hydroxyl, which was verified by spectral characterization and quantitative halogenating reaction. It is very interesting that both the wettability and electrical conductivity show reversible change in halogenating and hydrolysis reaction cycles, which demonstrates the versatility of G-OH. Most importantly, the hydrophilicity and weak inductive nature of G-OH provides a well microenvironment for the cells adhesion and proliferation. On G-OH paper, rat adipose tissue-derived stromal cells exhibited a typical fibroblast-like shape with high rate of increase and survival after 3 day of incubation. This G-OH paper with good mechanical property is expected to be a new biomaterial for bone, vessel and skin regeneration. KEYWORDS: graphene derivative, hydroxylated graphene, biomaterial, reversible, regeneration

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INTRODUCTION Diverse kinds of graphene derivatives have been developed.1−3 Typical graphene derivatives mainly contain two types: graphene oxide (GO)1,3 and hydrogenated graphene (GH).2 Because of the functional groups in these graphene derivatives, they show more diversified functions.1−3 With a large amount of oxygen-containing groups (such as hydroxy, aldehyde groups, carboxyl groups, and epoxy groups), GO can be welldissolved in water,3−5 and it can be widely used in the aqueousphase synthesis of graphene-based materials.4,6,7 It also can be easily transformed into reduced graphene oxide via hydrothermal reduction, vapor phase reduction, or thermal recovery.4,6 On the other hand, with unique band structure, GH can shows ferromagnetism in theory and displays tunable band gaps depending on the extent of hydrogenation,8 and is a possible candidate for hydrogen storage media9 because of their ability to undergo reversible hydrogenation.2 © 2016 American Chemical Society

Recently, hydroxyl-functionalized graphene has appeared in researcher’s view.10,11 Dai et al. used solid KOH powder and graphite flakes to prepare hydroxyl-functionalized graphene in one step using ball milling in solid state.10 The resultant hydroxyl-functionalized graphene showed a good hydrophilicity, electroactivity and biocompatibility to human retinal pigment epithelium cells.10 These results implied the enormous potentiality of hydroxyl-functionalized graphene. However, it is worth noting that it still have substantial (about 30% of total oxygen-containing functional groups) other oxygen-containing functional groups (typically C−O−C).10 The nonexclusive existence form of oxygen-containing groups makes it hard to evaluate the effect of hydroxy in the physical and chemical Received: February 17, 2016 Accepted: April 7, 2016 Published: April 7, 2016 10226

DOI: 10.1021/acsami.6b02032 ACS Appl. Mater. Interfaces 2016, 8, 10226−10233

Research Article

ACS Applied Materials & Interfaces

is reversible between G-OH and G-I through halogenating and hydrolysis reaction cycles. The excellent hydrophilia, with the zeta potential ca. −50 mV, and inductive nature of G-OH promises its biomedical application. G-OH papers via the routine filtration technique were directly used as effective biologically active substrate. This G-OH paper with good mechanical property is expected to be a new biomaterial, which may be used in bone, vessel, and skin regeneration.

properties of graphene. Meanwhile, these current reported preparation methods are facing the problem of low output (milligram scale), which makes it difficult for real applications. Thus, it is necessary to develop a new approach for the large scale synthesis of hydroxylated graphene with only hydroxy groups. Here, we report large scale (kilogram scale) synthesis of hydroxylated graphene (G-OH), with exclusive existence form of oxygen-containing groups of hydroxy. The iodo-graphene (G-I) was obtained via hydrothermal reaction in KI aqueous solution, which directly confirms pure hydroxyl in our G-OH. The G-I showed large contact angle of 73−82°, whereas the contact angle of G-OH is 19.1°, and has excellent electrical conductivity with sheet resistance 10−20 ohm □−1, whereas that of G-OH is > 1 × 107 ohm □−1. As shown in Scheme 1, it

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EXPERIMENTAL SECTION

Chemicals. All the chemicals were purchased from Aladdin (Shanghai, China) and used as received without further purification. The water used throughout all experiments was purified using a Millipore system. Synthesis. The synthesis approach of G-OH can be divided into two steps. First, the precursor is obtained as follows: Typically, 10.0 g of graphite was mixed with 20.0 mL of H2SO4 (98 wt.%) and 1.0 gof KMnO4 at room temperature. The mixture was stirred for 2 h at 25 °C. The obtained precursor was washed by deionized water and H2O2 (30 wt %). The yield of the precursor is 87% calculated from graphite. Second, the precursor was dispersed into 2.0 M NaOH with the mass concentration of 2.0 mg mL−1. The dispersion liquid was sheared and emulsified by high pressure disrupter at 207 MPa for 2 h. The G-OH was obtained after filtration and washing. The yield and output of GOH is 81% (calculated from graphite) and 8 g, respectively. Characterization Methods. TEM was carried out on a Hitachi H8100 EM (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. FESEM was performed on a JEOL JSM-4700S equipped with an EDS INCA X-Max at 30 kV. AFM data were obtained in a Bruker Dimension Icon with a Nanoscope 8.15 in tapping mode. FTIR was recorded by a Bruker Vertex 70v FT-IR spectrometer under vacuum (1 × 107 ohm □−1, respectively). Moreover, both wettability and electrical conductivity show no obvious change after 12 halogenating and hydrolysis reaction cycles. Large-Scale Synthesis. It is worth noting that our approach is suitable for large-scale synthesis of G-OH. Typically, 1.0 kg of graphite was mixed with 2.0 L of H2SO4 (98 wt %) and 0.1 kg of KMnO4 at room temperature. The mixture was stirred for 2 h at 25 °C. The obtained precursor was washed by deionized water and H2O2 (30 wt %). The yield of the precursor is 84% calculated from graphite. Moreover, the hydroxylation process was carried out on high pressure disrupter (which is suitable for continuous processing). The precursor was dispersed into 2.0 M NaOH with the mass concentration of 2.0 mg mL−1. The dispersion liquid was sheared and emulsified by high pressure disrupter at 207 MPa for 2 h. The G-OH was obtained after filtration and washing. The yield and output of G-OH is 79% (calculated from graphite) and 0.79 kg, respectively. Table 1 lists the output of previously reported preparation methods for hydroxylated (or hydroxyl-functionalized) carbon materials. It is clear that the output of our approach is more than 2 orders of magnitude higher than that of others, which indicates our approach shows obvious advantage over other methods in practical applications.

ca. 284.2 eV, along with an O 1s peak at ca. 532 eV (Figure S4).14−16 The O/C atomic ratio is 0.17. The high O/C atomic ratio indicates the abundant oxygen-containing function groups in G-OH thus obtained. Figure 2a shows well-fitted C 1s spectrum of G-OH. The peaks located at 284.98 and 287.00 eV correspond to the signals of C−C/CC and C−O bonding models, respectively.6,17 O 1s XPS spectrum (Figure 2b) shows the singlet at 533.4 eV which indicates the exclusive existence form of oxygen-containing groups is hydroxy (C-OH groups, Figure 2b inset).6 FESEM and corresponding EDS mapping images (Figure 2c−e) shows the element distribution of C and O intuitively. The EDS results indicate the uniform distribution of O on G-OH sheets. Moreover, the FTIR spectrum (Figure S5) of G-OH doubly confirmed the XPS results. The broad peak located at 3431 cm−1 corresponding to the signals of COH groups.18 The exclusive existence form of oxygen-containing groups (C-OH groups) in G-OH can be due to the hydroxylation progress under mechanochemical reactions involve highly reactive centers generated by the mechanical energy imparted to the reaction system. Wang et al. reported preparation of dumbbell-shaped C120 from C60 with KOH by means of nucleophilic addition.19 In our approach, graphite was preoxidated by KMnO4 in acid environment to get the precursor (oxidized graphite, G(O)n), and it further reacted with OH− to form G(O)n−(OH)m. The resulting products were hydrolysis to give G-OH, which is illustrated in Figure 2f. Meanwhile, the precursor sheets were exfoliated synchronously under high pressure shearing (207 MPa). We further used quantitative halogenating reaction to confirm the present of C−OH groups because of the hydroxyl groups are easy substituted by I− (Figure 2g).19 G-I can be obtained when G-OH is hydrothermal treated in KI aqueous solution (3.5 mM) under 120 °C for 24 h. As shown in Figure S6, XPS survey spectrum for G-I shows a C 1s peak at ca. 284.2 eV, along with a I 3d5/2 peak at ca. 621 eV.20,21 The I/C atomic ratio of G-I is 0.17 which is same with the O/C atomic ratio of G-OH.22 Meanwhile, no signal of O can be observed in the XPS survey spectrum for G-I. This indicates all C−OH groups were replaced by C−I groups. C 1s and I 3d spectra (Figure 2h and (i) of the G-I indicates the presence of a C−C/CC (284.98 eV) and C−I (294.88 eV) bonding model.23 FESEM and corresponding EDS mapping images (Figure 2j−l) shows the element distribution of C and I intuitively. The EDS results indicate the uniform distribution of I on G-OH sheets. Moreover, the broad peak located at 3431 cm−1 also 10229

DOI: 10.1021/acsami.6b02032 ACS Appl. Mater. Interfaces 2016, 8, 10226−10233

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excellent water dispersibility of G-OH. Figure 4b inset shows the digital photograph of 5.0 mL of homogeneous G-OH aqueous dispersion (0.5 mg mL−1), which stockpiled for 1 and 30 days. The G-OH aqueous dispersion exhibits the Tyndall effect when a laser beam is passing through, suggesting the uniform graphene dispersion in water. The concentration could be controlled by varying the final volume of the dispersions. Dispersions with concentration ranging from 0.1 to 5.0 mg mL−1 were prepared. No macroscopic aggregates were observed in as-prepared samples of all above concentrations. Zeta potential of this solution was measured to be ca. −50 mV in average, indicating the formation of highly negative charged surface. Moreover, due to the exclusive existence form of oxygen-containing groups, the G-OH shows much better water dispersibility than previously reported hydroxyl-functionalized graphene. The stable G-OH aqueous dispersion indicates it can be directly spray coated on hydrophilicity substrates and get uniform G-OH coat. Figure 4c shows the digital photograph of G-OH plated circuit on paper. The corresponding SEM image (Figure S8) shows stretched graphene morphology, which is quite different from those aggregated rGO sheets dispersed in water (Figure S9). It can be inferred that all sheets dispersed in water are freely outstretched, not crumpled. It is worth noting that the G-OH coat shows high sheet resistance (>1 × 107 ohm □−1) because of the abundant C−OH groups.

Table 1. Brief Summary of the Output of Various Synthetic Methods for Hydroxylated (Or Hydroxyl-Functionalized) Carbon Materials product

method

output (g/batch)

ref

G-OH Hy-Ga [60]Fullerols Hy-CNTb Hy-CNTb

high pressure disrupter ball milling ball milling hydrothermal reaction ball milling

>790 0.01 0.062 2.5 0.2

this work 10 18 19 20

a Hydroxyl-functionalized graphene. bHydroxyl-functionalized carbon nanotube.

Water Dispersibility of G-OH. We further show the unique properties of G-OH, especiallly its excellent water dispersibility and good biocompatibility. The stability of the dispersions was monitored using UV−visible (UV−vis) spectroscopy. Figure 4a presented the typical UV−vis absorption spectrum of G-OH. Obviously, G-OH dispersions showed absorption peak at 250 nm, suggesting the maintenance of sp2 carbon structure. The absorbance at 660 nm was chosen as an indicator to reflect the change of G-OH content against storing time in the dispersions.24−26 The stability of five dispersions with different concentrations (0.1, 0.5, 1.0, 2.5, and 5.0 mg mL−1) was studied. As shown in Figure 4b, no significant loss in absorbance during the initial 15 days of storing indicates the

Figure 4. Stable G-OH aqueous dispersion. (a) UV−vis spectrum of G-OH aqueous dispersion. (b) Sedimentation behavior of G-OH aqueous dispersion with different concentrations (0.1, 0.5, 1.0, 2.5, and 5.0 mg mL−1). Ao and A are the absorbance (at 660 nm) of G-OH aqueous dispersion stockpiled for 0 to different days. Inset is the digital photographs of 5.0 mL homogeneous dispersions (0.5 mg mL−1) stockpiled for 1 and 30 days. (c) Digital photograph of G-OH plated circuit on paper. (d) Digital photograph of the preparation process of G-OH paper by routine filtration technique. (e, f) Photographs of the flexible G-OH paper with metallic luster. (g) FESEM image of the surface of G-OH paper. (h) Cross-section SEM image of the G-OH paper showing the uniform thickness (2.6 μm) and layered structure. (i) Contact angle of G-OH paper for water. 10230

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Figure 5. Biologically active properties of G-OH. Micrograph of (a) primary and (b) passage 2 rADSCs. Red arrows show the monoclonal cell clusters. Primary rADSCs exhibited a fibroblast-like morphology observed under a inverted phase contrast microscope. Passage 2 rADSCs remain long spindle morphology. (c) CCK-8 assay for the rADSCs proliferation on the G-OH paper. (d) FESEM image of the rADSCs proliferation on the G-OH paper in 3 days. (e) Representative fluorescence images showing the cell viability of rADSCs on G-OH paper samples after 1, 2, and 3 day of incubation, visualized by staining with a LIVE/DEAD Cell Viability Kit (Thermo Fisher Scientific). The live cells appear green, whereas the dead ones are red.

The G-OH dispersions also have advantages in the fabrication of macroscopic materials. To demonstrate this, it was directly used to prepare G-OH papers via the routine filtration technique (as shown in Figure 4d). Typically, 10.0 mL of 1.0 mg mL−1 G-OH aqueous dispersion was filtrated through a 100 nm membranes (AAO, Shangmu Technology, China), followed by drying overnight at 60 °C. The filtration of G-OH aqueous dispersion could be completed within 90−180 min, much faster than the filtration of equal volume GO dispersion.24 A free-standing and bendable G-OH paper with a diameter of ca. 4.0 cm (Figure 4e) was obtained after peeling off the membrane. This obtained G-OH paper show excellent flexibility (Figure 4f) and mechanical strength (Young’s modulus of 5.9 GPa). Figure 4g, h present surface and crosssectional SEM images of G-OH paper. Evidently, well-aligned graphene sheets in the plane of the paper were observed, indicating the parallel deposition of G-OH sheets on the filter membrane, just as GO and chemical converted graphene behave during filtration. The layered structure and uniform thickness (2.6 μm) demonstrated the outstretched and homogeneously dispersed G-OH sheets in the aqueous dispersions. The obtained G-OH paper also shows a hydrophilic nature. As shown in Figure 4i, the contact angle of G-OH paper for water is 19.1°, which is much lower than previously reported for graphene-based materials. Biologically Active Properties. Rat adipose tissue-derived stromal cells (rADSCs) are selected to evaluate the biocompatibility of G-OH paper thus formed. The primary cells isolated from subcutaneous adipose tissue of Sprague− Dawley rat were adherent to the plastic culture plates, and displayed a spindle-shaped, fibroblast-like morphology (Figure 5a). The rADSCs proliferated actively, and showed mo

noclonal cell cluster in passages 2 (Figure 5b), included that their great potential ability of reproductive activity. Cells proliferation was assessed using CCK-8 assay. As shown in Figure 5c, the absorbance for rADSCs on the G-OH paper is 0.24, 1.01, 1.21, and 1.25 at day 0, 1, 2, and 3, respectively. Obviously, remarkable proliferation of rADSCs can be observed after 1 day of incubation. The OD value for rADSCs on the G-OH paper at day 1 is about 4 times as much as days 0. However, the multiplication rate of rADSCs decreased after 2 day of incubation. This can be due to the contact inhibition in high cell density. SEM image (Figure 5d) shows the rADSCs attached on the surface of G-OH paper with well balanced density. The rADSCs became elongated and exhibited a typical fibroblast-like shape that illustrated the GOH paper surface provides a good microenvironment for the cell adhesion and proliferation. We further show the excellent biocompatibility of G-OH paper intuitively by biofluorescence imaging. As shown in Figure 5e, rADSCs seeded on G-OH paper exhibited excellent viability. Cell viabilities remained above 95% within the whole incubation progress. Not only that, the cells viabilities even increased during the prolonged culture time. In 3 days, cell viabilities were all greater than 99.5% and barely any dead cells were seen. This G-OH paper not only provides a living environment but also does not affect cell biofunctions. The rADSCs gathered in a cluster of living that means cells on the G-OH paper having a favorable stem cell pluripotency and multiple differentiation potentials. The cell density for rADSCs on the G-OH paper is 50, 250, 290, and 310 cells mm−2 at day 0, 1, 2, and 3, respectively. All above results indicate the G-OH paper can be used as a biologically active substrate with good biocompatibility and cytocompatibility. The excellent biological 10231

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(7) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906−3924. (8) Zhou, L.; Wang, Q.; Sun, Q.; Chen, X.; Kawazoe, Y.; Jena, P. Ferromagnetism in Semihydrogenated Graphene Sheet. Nano Lett. 2009, 9, 3867−3870. (9) Elias, D.; Nair, R.; Mohiuddin, T.; Morozov, S.; Blake, P.; Halsall, M.; Ferrari, A.; Boukhvalov, D.; Katsnelson, M.; Geim, A.; Novoselov, K. Control of Graphene’s Properties by Reversible Hydrogenation: Evidence for Graphane. Science 2009, 323, 610−613. (10) Yan, L.; Lin, M.; Zeng, C.; Chen, Z.; Zhao, X.; Wu, A.; Wang, Y.; Zhang, S.; Qu, J.; Dai, L.; Guo, M.; Liu, Y. Electroactive and Biocompatible Hydroxyl- functionalized Graphene by Ball Milling. J. Mater. Chem. 2012, 22, 8367−8371. (11) Lin, M.; Zou, R.; Shi, H.; Yu, S.; Li, X.; Guo, R.; Yan, L.; Li, G.; Liu, Y.; Dai, L. Ocular Biocompatibility Evaluation of Hydroxylfunctionalized Graphene. Mater. Sci. Eng., C 2015, 50, 300−308. (12) Sun, J.; Yang, S.; Wang, Z.; Shen, H.; Xu, T.; Sun, L.; Li, H.; Chen, W.; Jiang, X.; Ding, G.; Kang, Z.; Xie, X.; Jiang, M. Ultra-High Quantum Yield of Graphene Quantum Dots: Aromatic-Nitrogen Doping and Photoluminescence Mechanism. Part. Part. Syst. Charact. 2015, 32, 434−440. (13) Wang, J. Z.; Manga, K. K.; Bao, Q. L.; Loh, K. P. High-Yield Synthesis of Few-Layer Graphene Flakes through Electrochemical Expansion of Graphite in Propylene Carbonate Electrolyte. J. Am. Chem. Soc. 2011, 133, 8888−8891. (14) Zhu, C.; Yang, S.; Wang, G.; Mo, R.; He, P.; Sun, J.; Di, Z.; Kang, Z.; Yuan, N.; Ding, J.; Ding, G.; Xie, X. A New Mild, Clean and Highly Efficient Method for the Preparation of Graphene Quantum Dots without By-products. J. Mater. Chem. B 2015, 3, 6871−6876. (15) Yang, S.; Sun, J.; Zhu, C.; He, P.; Peng, Z.; Ding, G. Supramolecular Recognition Control of Polyethylene Glycol Modified N-doped Graphene Quantum Dots: Tunable Selectivity for Alkali and Alkaline-earth Metal Ions. Analyst 2016, 141, 1052−1059. (16) Zhu, C.; Yang, S.; Wang, G.; Mo, R.; He, P.; Sun, J.; Di, Z.; Yuan, N.; Ding, J.; Ding, G.; Xie, X. Negative Induction Effect of Graphite N on Graphene Quantum Dots: Tunable Band Gap Photoluminescence. J. Mater. Chem. C 2015, 3, 8810−8816. (17) Yang, S.; Sun, J.; He, P.; Deng, X.; Wang, Z.; Hu, C.; Ding, G.; Xie, X. Selenium Doped Graphene Quantum Dots as an Ultrasensitive Redox Fluorescent Switch. Chem. Mater. 2015, 27, 2004−2011. (18) Yang, S.; Sun, J.; Li, X.; Zhou, W.; Wang, Z.; He, P.; Ding, G.; Xie, X.; Kang, Z.; Jiang, M. Large-scale Fabrication of Heavy Doped Carbon Quantum Dots with Tunable-Photoluminescence and Sensitive Fluorescence Detection. J. Mater. Chem. A 2014, 2, 8660− 8667. (19) Zhang, H.; Pan, D.; Liu, Z.; Guo, F.; Zhang, D.; Zhu. Effective Mechanochemical Synthesis of Fullerols. Synth. Commun. 2003, 33, 2469−2474. (20) Kim, K.; Gossmann, A.; Winograd, N. X-ray Photoelectron Spectroscopic Studies of Palladium Oxides and the Palladium-oxygen Electrode. Anal. Chem. 1974, 46, 197−200. (21) Yang, D.; Guo, G.; Hu, J.; Wang, C.; Jiang, D. Hydrothermal Treatment to Prepare Hydroxyl Group Modified Multi-walled Carbon Nanotubes. J. Mater. Chem. 2008, 18, 350−354. (22) Liao, F.; Song, X.; Yang, S.; Hu, C.; He, L.; Yan, S.; Ding, G. Photoinduced Electron Transfer of Poly(ophenylenediamine)−Rhodamine B Copolymer Dots: Application in Ultrasensitive Detection of Nitrite in Vivo. J. Mater. Chem. A 2015, 3, 7568−7574. (23) Pan, H.; Liu, L.; Guo, Z. X.; Dai, L.; Zhang, F.; Zhu, D.; Czerw, R.; Carroll, D. L. Carbon Nanotubols from Mechanochemical Reaction. Nano Lett. 2003, 3, 29−32. (24) Lotya, M.; King, P. J.; Khan, U.; De, S.; Coleman, J. N. HighConcentration, Surfactant-Stabilized Graphene Dispersions. ACS Nano 2010, 4, 3155−3162. (25) Dikin, D.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and Characterization of Graphene Oxide Paper. Nature 2007, 448, 457− 460.

properties can be due to both good hydrophilicity property and low electrical conductivity.27 These features make it potentially attractive for biomedical applications, especially as a new biomaterial for bone, vessel, and skin regeneration.

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CONCLUSION In summary, we demonstrated the kilogram scale synthesis of a unique graphene derivative of G-OH. The exclusive existence form of oxygen-containing groups in G-OH is hydroxyl. We demonstrated the reversible transformation between G-OH and G-I, as well as the wettability and electrical conductivity. Most importantly, the hydrophilia and weak inductive nature of GOH promises advantages for biomedical systems. The G-OH paper with good mechanical property is expected to be a new biomaterials that can be used in bone, vessel, and skin repair. This work extends the graphene derivative family and opens exciting opportunities for both basic research and technological applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02032. Experimental details, HR-TEM, SEM, SAED, XPS survery, FTIR data of G-OH and G-I, and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

J.S. and Y.D. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by The National Science and Technology Major Project Fund (2011ZX02707), The Chinese Academy of Sciences (XDA02040000), Science and Technology Commission of Shanghai Municipality The Sailing Program (15YF1406800), and The National Natural Science Foundation of China (81501605).

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REFERENCES

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