Sonication Exfoliation of Defect-Free Graphene in Aqueous Silk

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Sonication Exfoliation of Defect-Free Graphene in Aqueous Silk Nanofiber Solutions Hongmei Zhuo, Xiaoyi Zhang, Lili Wang, Qiang Lu, and David L Kaplan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02644 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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ACS Sustainable Chemistry & Engineering

Sonication Exfoliation of Defect-Free Graphene in Aqueous Silk Nanofiber Solutions

Hongmei Zhuoa,#, Xiaoyi Zhangb,#, Lili Wangb, Qiang Lua,b,*, David L Kaplanc

a

College of Chemistry, Chemical Engineering and Materials Science & Collaborative

Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, People’s Republic of China b

National Engineering Laboratory for Modern Silk, Soochow University, Suzhou

215123, People’s Republic of China c

Department of Biomedical Engineering, Tufts University, Medford, Massachusetts

02155, United States

Keywords: Graphene; Silk Nanofiber; Exfoliation; Mass Production; Biomedical Application

Corresponding author: Qiang Lu, Tel: (+86)-512-67061649; E-mail: [email protected] Postal Address: 199 Renai Road, Soochow University, Suzhou, China, 215123.

1 ACS Paragon Plus Environment

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ABSTRACT: Green processes to exfoliate graphene would be advantageous but challenged by issues such as stability and dispersibility. Here, stable silk fibroin nanofibers (SFNs) with high crystallinity (β-sheet content) and high net negative charge, were endowed with hydrophobic features yet excellent water dispersibility. These nanofibers were used to exfoliate graphite into graphene in aqueous solutions. A high concentration (1.92 mg mL-1) and yield (approaching 20%) were achieved for the exfoliated graphene dispersions using low initial graphite concentrations via sonication treatment. This approach was superior to most protein-assisted exfoliation processes reported to date and the graphene obtained in the process exhibited fewer defects, good conductivity and cell compatibility. The graphene prepared by this new process provided an option for SF-graphene composite materials and devices where defect-free graphene with improved performance through the introduction of SFNs into the preparation strategies will bring desired revolution in graphene materials.


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INTRODUCTION Graphene possesses unique characteristics of superlative mechanical properties, high thermal conductivity and electrical conductivity, and good gas impermeability, and has attracted extensive interest worldwide since its discovery in 2004.1-2 These outstanding properties endows graphene with a promising future as the core of many cutting-edge devices related to electronics, energy storage, and photonics, and also for potential applications in catalysis, biomedicine and composite materials.3-5 Among the graphene material families, defect-free pristine graphene materials show higher Young's modulus (1.0 TPa), higher fracture strength (130 GPa) and faster electron transfer rate (200,000 cm2 v−1 s−1),4-6 making them more suitable for various applications such as mechanical enhancement, electronic and optical devices. Scalable production of high-quality graphene via simple, low-cost, efficient and reproducible approaches is a prerequisite for realizing the potential for this remarkable material. Many methods have been developed to form 2D graphene-based materials,6-8 however, the demand remains to evolve better strategies for the mass production of defect-free graphene materials for both 2D and 3D material systems.

Among the current approaches explored, sonication assisted liquid-phase exfoliation (SLPE) is one of the most widely used strategies due to its distinct advantages, such as ease of processing, simplicity and versatility.9-12 Compared to chemical exfoliation, SLPE can produce defect-free graphene in colloidal dispersions, making it easy to further process into various states or hybrid systems suitable for many practical applications.1,13-14 Even though SLPE suffers from several inherent limitations, including long sonication times and low exfoliation yields,15,16 it is still extremely attractive and suitable in many advanced fields, especially in biological and



biomedical applications.17

The increased demand for such materials in biomedical fields helps to stimulate studies to develop biomolecular-assisted aqueous exfoliation of graphite into graphene. These exfoliation processes have significant environmental and economic benefits over similar methods involving amphiphilic surfactants and organic solvents, and also endow the exfoliated graphene with improved biocompatibility and potential functionalization with biomolecules.18 A wide variety of biomolecules, including proteins,19-21 peptides,22 polysaccharides23,24 and nucleocides/DNA3,18,25,26 have been used to achieve exfoliation and aqueous dispersions of defect-free graphene. Subsequent studies revealed the promising potential for these materials in a wide-range of fields in biomedicine,27 energy storage,28 and for composite materials.1,29 However, most of the biomolecules used in the processing are destroyed under the ultrasonic treatment, resulting in the loss of their biological activity. The negative effect of the biomolecules on electrical and mechanical performance of the graphenes also has to be considered in some applications. Moreover, the high cost of purified biomolecules for the processing is an additional shortcoming,22-24 which limits the commercial use of the prepared graphene. Therefore, there is a significant need to identify and develop new biomolecular stabilizers which are low cost, stable and biocompatible, and able to produce high yields of graphene.

As a natural protein, silk fibroin (SF) has been widely studied as a functional material in a broad range of applications, from tissue engineering and drug release to electronic and optical device realms due to it’s biocompatibility, outstanding mechanical properties, and control of nanostructures, conformations and surface properties.30-34


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This protein is available in large quantities and can be purified via eco-friendly processes. The rich active functional chemistry groups on SF provide plenty of choices to achieve diverse functionality towards different applications.35-37 All of these features make SF suitable and potentially superior to the currently used biomolecules as high-performance stabilizers during graphene processing. However, due to the gelation of SF under ultrasonic treatment, silk fibroin-assisted exfoliation of graphene is a challenge yet to be solved. Recently, SF nanofibers (SFNs) were prepared through a controllable assembly process in aqueous solutions.38 These nanofibers were mainly composed of β-sheet structures, resulting in high hydrophobicity suitable for graphene exfoliation and also stable enough to resist gelation under ultrasonic treatment. Considering that the nanofibers avoid the inherent drawback of SF molecules without the compromise of the advantages listed above, the SFNs were recently used to exfoliate graphite via turbulence-assisted shear exfoliation and the prepared graphene dispersions with high concentrations suggested the feasibility of obtaining graphene materials by a SF-assisted exfoliation processes.39 However, edge type defects formed for the shear induced exfoliated graphene, resulted in inferior performance. Thus, alternative methods are still required to prepare defect-free graphene for use in biomedical fields.

Herein, we explored the exfoliation of graphene using SFNs and low initial graphite concentrations to produce nearly defect-free graphene aqueous dispersions. With the inclusion of SFNs, the concentration of exfoliated graphene reached 1.92 mg ml-1 with

the

production

yield

approaching

20%,

superior

to

most

other

biomolecule-assisted SLPE systems.22-24 The quality of the generated graphene was systematically studied in terms of water stability, lateral dimensions, number of layers



and defects. The separation of graphene and SFNs was carried out by simple centrifugation to obtain graphene with little SFNs, significantly improving the versatility of the system for further downstream processing related to materials fabrication. MATERIALS AND METHODS SFNs Solution System Fabrication. Fresh silk fibroin solutions were treated by a slow concentration-dilution process reported previously.34 The solutions (6 wt%) were slowly concentrated to about 20 wt% over 24 h at 60oC to form metastable nanoparticles, and then diluted to below 2 wt% with distilled water. The diluted silk solution was incubated for about 24 h at 60oC to induce nanofiber hydrogel formation. Exfoliation of Graphite. Graphite powder (purity>99%, 300 mesh) was purchased from Qingdao (Haida Inc., China). Graphite powder of 100 mg was dispersed in 10 mL of the silk nanofiber aqueous solution (5mg mL-1, pH 7) at 4oC. The mixture was then sonicated for 1 h by a high-probe sonicator. The probe tip was located 1 cm below the liquid surface. Ultrasonic power was 195 W and working pulse was 5s/2s. In order to suppress the temperature increase during sonication, the vessels were kept cold in an ice bath. After sonication, the dispersions were kept for 24 h to precipitate unexfoliated graphite particles and then were centrifuged at 1,500 rpm for 30 min to further remove thick flakes. According to the typical methods reported previously,18-20 the concentration of graphene suspension was determined by measuring the absorbance at 660 nm (A660) (Fig S1).19 The suspension could also be further centrifuged at 14,000 rpm for 20 min to remove free SFNs and re-dispersed in water with sonication treatment. The centrifuged graphene was termed C-graphene. Characterization of the Graphene Dispersions. The morphology of the samples was examined by Scanning Electron Microscopy (SEM, S-4800, Hitachi, Tokyo, 6 ACS Paragon Plus Environment

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Japan), Atomic Force Microscopy (AFM, Nanoscope V, Veeco, NY, USA) and Transmission Electron Microscopy (TEM, FEI, Hillsboro, USA). The SEM was at 3kV with about a 2 uL sample were added directly onto conductive tape, dried in air, and coated with platinum for 60 s. For AFM experiments, graphene suspensions were diluted to below 0.001 wt% to show the original morphology and 2 µL of the diluted solution was spin-coated onto freshly cleaved mica surfaces. A 225 µm long silicon cantilever with a spring constant of 3 N m-1 was used in tapping mode at 0.5 Hz-1 scan rate. Samples for TEM were prepared by placing samples onto copper TEM grids and dried for 2 h in air. The samples were imaged at 200 kV. Ultraviolet-visible (UV-Vis) absorption spectra were collected over the range of 200-800 nm using a UV-Vis-NIR Spectrophotometer (Cary5000, Agilent, Santa Clara, USA). The infrared spectra (IR) were measured using a Fourier transform-infrared spectrometer (FTIR, Nicolet 6700 FT-IR, Thermo Scientific, FL, USA) where 64 scans were run in the wavenumber range 400-4000 cm-1 with a resolution of 4 cm-1. Raman spectra were recorded using a Raman spectrometer (Renishaw, 532 nm diode laser, New Mills, UK). A 532 nm laser was used with exposure time 10 s and laser power 50%. The microscope was calibrated using a typical Raman shift of Si-wafer sample at 520.7 cm−1. Graphene samples were dried on Si-wafers with ≈1 cm spot size. The ζ-potentials of suspension were characterized with a Zetasizer (Nano ZS, Malvern, Worcestershire, UK) at 25 oC. The electrical performance of the graphene materials was obtained using a CMT series JANDEL four-point probe at room temperature. After a high speed centrifugation and resuspension in water through ultrasonic treatment for 20 min, the



pure graphene suspension (1 mg mL-1) was painted onto a standard printing paper with a paintbrush and dried at 120oC for 1 min according to the protocol reported previously.40 Sheet conductivity of the paper was measured using a CMT series JANDEL four-point probe at room temperature. The conductivity was calculated with the following equation: σ=1/tRs (1) where Rs is the sheet resistance measured by a four-point probe device and t is the thin film thickness measured using a Hitachi S-4800 SEM (Japan). Simultaneously, a test with a light emitting diode, battery and the graphene sheet was performed to confirm the conductivity of the graphene materials. Stability of Graphene Suspensions. The stability of aqueous graphene dispersions was assessed at various temperatures and pH values. After incubation for 30 days, the suspensions were measured with a UV-Vis-NIR Spectrophotometer (Cary5000, Agilent, Santa Clara, USA). The change in concentration of the suspensions was calculated by comparing the values of absorbance at 660 nm. Biocompatibility of Graphene Dispersions. SFNs-graphene and centrifuged graphene were coated on the silicon wafers and cultured with bone marrow mesenchymal stem cells (BMSCs). Cells were seeded on the samples in 24-well plates (5×104 cells per well) and incubated in 2 mL Dulbecco's modified Eagle medium (DMEM, Invitrogen, Carlsbad, CA) containing 10% (v/v) fetal bovine serum (FBS, Invitrogen, Carlsbad, CA). After 2 and 7 days of incubation, the viability of cells on the coatings was assessed by live/dead staining method according to previous schedules.32 Viable (green) and dead cells (red) were counted under a fluorescence


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microscope (Olympus FV10 inverted microscope, Nagano, Japan).

Cell proliferation was analyzed by DNA content assay on days 1, 3 and 7. The DNA content was achieved using the PicoGreen DNA assay (Invitrogen, Carlsbad, CA) and measured with a BioTeK Synergy 4 spectrofluorometer (BioTeK, Winooski, VT). The excitation wavelength and emission wavelength were 480 and 530 nm, respectively. Cell morphology was evaluated using confocal microscopy. After culturing for 1, 3 and 7 days, the cells were stained with FITC-phalloidin (Sigma-Aldrich, St. Louis, MO) and DAPI (Sigma-Aldrich, St. Louis, MO) according to procedures reported previously.41 Statistical Analysis. SPSS v.18.0 software was used to perform statistical analyses. One-way ANOVA followed by the Student−Newman−Keuls test was used to compare the mean values of the data. Unless otherwise indicated, measurements are presented as means ± standard deviation.

RESULTS AND DISCUSSION SFNs-assisted exfoliation of graphite was performed to prepare water-dispersible graphene via a modified SLPE method (Scheme 1). A series of SLPE parameters (duration of ultrasonication and centrifugation speed) were determined according to previously reported procedures.7,42 As a control, SF solution prepared through a traditional process40 was also used to exfoliate graphite with a similar SLPE process. A solution-hydrogel transformation appeared for SF under the sonication treatment in 5 min due to beta-sheet structure formation, resulting in the failure of the exfoliation



(Fig S2A). Unlike the SF solutions, SFNs are mainly composed of β-sheet structure, thus have high stability to limit hydrogel formation under the ultrasonic process. Dark and stable dispersions of carbon materials formed after the ultrasonication of graphite powder in SFNs solution, suggesting water-dispersible graphene was achieved (Fig S2B). The beta-sheet structure, as well as the high negative charge density, endowed SFNs with hydrophobicity and good aqueous dispersibility,38 allowing the SFNs to act as a surfactant to prevent the restacking of graphene in water.

Scheme 1. Probe sonication exfoliation of graphite using SFNs. The graphite was exfoliated into graphene sheets through sonication treatment. The hydrophobic SFNs then adhered on the exfoliated graphene sheets to prevent their aggregation due to the repulsion of negative charges of the SFNs. High speed centrifugation could further remove the SFNs to form pure graphene sheets.

UV-visible absorbance at 660 nm was used to determine a suitable concentration of SFNs to produce graphene suspensions with a high concentration (Fig S1A). The maximum graphene concentration (1.92 mg ml-1) and yield (20%) were obtained with initial graphite concentration of 10 mg ml-1 and SFNs concentration of 0.5%, significantly higher to most previously reported SLPE graphene materials.13-16 The extinction coefficient of nanofibers stabilized graphene was 382 mL mg-1 m-1 which was calculated using a calibration plot (Fig S1B). These results indicate that the SFNs


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were suitable biomolecules to effectively exfoliate the graphene sheets and efficiently stabilize them in water. Further increases in graphene concentration could be pursued through optimization of the sonication parameters as well as the initial graphite concentrations.

Similar to traditional surfactants, amphiphilic SFNs could bind to the exposed graphitic surface via hydrophobic interactions, while separating the graphitic layers and dispersing them in water through the negative charged groups on the outside of the nanofibers,38 further improving their aqueous dispersibility. Therefore, the improved graphite exfoliation efficiency of the SFNs-assisted system could be ascribed to the amphiphilic property and nanofibrous structure of the SFNs, consistent with other surfactant-assisted systems.11-17 As shown in Fig S3A, the SFNs were nanofibers with diameter of 20 nm and length of about 500-2 µm. Then both SEM and AFM images indicated successful exfoliation of graphite into graphene and the adhesion of the SFNs on the formed graphene layers (Fig 1).



Figure 1. Representative microstructures of the samples: SEM images of graphite (A) and SFNs stabilized graphene (B); AFM images of SFNs stabilized graphene (C) and (D). The arrows indicated the graphene flakes.

The Zeta potential values of the graphene dispersions were about -30.6±0.49 mV, lower than that of the SFNs (-63.9±0.45 mV), but high enough to ensure the stability of the dispersion according to colloidal science,38,43 which suggests that electrostatic repulsion between SFNs can maintain a stable graphene dispersion. After storage for 1 month at room temperature, the graphene dispersion only showed a 3% decrease of original concentration, confirming stability (Fig 2A). The stability of the graphene dispersion prepared by the above sonication-centrifugation procedure was also evaluated under varying temperatures and pH conditions (Fig 2B, C). There was a slight increase in concentration of graphene (using A660) after exposure to 30-90 oC water baths for 8 h (Fig 2B), similar to that which was found with BSA-stabilized graphene dispersion systems.19 The increased absorbance was due to changes in the graphene/protein complex at higher temperature.19 The stability of SFNs stabilized graphene dispersions under different pHs was also investigated. As the pH decreased from 12 to 3, the zeta potential of the SFNs stabilized graphene varied from -37 to +20 mV, and approached 0 mV at a pH of 4.6, the isoelectric point of silk. Significant aggregation appeared at lower pH values, resulting in a significant decrease in graphene concentration in solution (Fig 2D). The results confirmed the critical function of electrostatic repulsion on the stability of SFNs-loaded graphene.


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Figure 2. (A) The stability of the SFNs-stabilized graphene when stored at room temperature; (B) The absorbance at 660 nm of graphene dispersions at elevated temperatures; (C) Zeta potentials of graphene dispersions as a function of pHs; (D) The absorbance at 660 nm of graphene dispersions at varying pHs.

Besides dispersibility, the removal of SFNs is preferred for several electrical applications due to the negative influence of SFNs on conductivity of the devices. Considering the weak physical interaction between graphene and SFNs, higher speed centrifugation (14,000 rpm) was used to remove the nanofibers. After centrifuging twice for 20 min each, the graphene precipitated and was collected after the removal of the SFNs supernatant. The centrifuged graphene materials (C-graphene) were easily dispersed in aqueous solution after sonication, but aggregated to form precipitation after 5 days due to the removal of SFNs (Fig S3B). Although the C-graphene dispersions had inferior dispersibility, the redispersion capacity of the C-graphene in aqueous solutions would facilitate applications (Fig S4). Therefore, the present system provided an effective strategy of resolving the dilemma between the dispersibility and performance of the graphene materials. Figure 3A shows typical



UV-vis absorption spectra of SFNs, SFNs-graphene and C-graphene dispersions. The typical absorption peak of SFNs (280 nm) appeared for the SFNs-graphene, but almost disappeared for the C-graphene samples, suggesting the removal of the SFNs. FT-IR spectra of the different samples further confirmed the removal of SFNs (Fig 3B). Two typical silk peaks at 1527 and 1630 cm-1 were present in the spectra of the SFNs and SFNs-stabilized graphene materials, but disappeared in the spectra of pure graphite and the C-graphene materials. SEM and TEM images further supported the IR analysis (Fig 3C, D). After the centrifugation, little nanofibers were found on the surface of the graphene. The SEM and TEM images also showed the size distribution of the graphene flakes (Fig 3E). The majority of graphene flakes had lateral dimensions in the range of 200-500 nm, although the size was lower than found in several previous studies.13-20 Yet, the size of the flakes was still suitable for many applications.

Figure 3. Composition and structure of graphene: (A) UV-vis absorption spectrum of SFNs


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aqueous solution (a), aqueous graphene dispersion stabilized with SFNs before (b) and after high speed centrifugation (c); (B) FT-IR spectra for different samples: graphite (a), C-graphene (b), SFNs-stabilized graphene (c), and SFNs (d); (C) SEM image of C-graphene; (D) TEM image of C-graphene; and (E) Lateral size distribution of graphene flakes in the TEM image.

Raman spectroscopy, a powerful tool to characterize the carbon material, was used to assess the quality of the graphene materials generated in the process. Fig 4A displays the Raman spectra of graphite and C-graphene with excitation at 532 nm, exhibiting three typical peaks at about 1345, 1580 and 2685 cm-1, corresponding to the D, G and 2D bands of graphene, respectively. The defect content is usually represented as the intensity ratio of the D band to G band (ID/IG).18-20 For chemically reduced GO materials, the ID/IG intensity ratio is usually above 0.5.44,45 Natural polymers and proteins were also used to prepare defect free graphene, which achieved lower ID/IG intensity ratio of about 0.27 for amphiphilic peptides22 and 0.1-0.6 for BSA.19 Compared to chemically reduced GO materials,44,45 and most of polymer stabilized-exfoliated graphene,11-15 a significantly lower ID/IG intensity ratio of 0.15 was achieved for the graphene obtained here, indicating small defects and quality improvement for the SFNs-assisted graphene. The shift of the 2D band was used to evaluate the layers of the graphene sheets. The movement from 2714 to 2685 cm-1 with improved symmetric shape suggested the formation of thin graphene sheets with less than five layers (Fig 4B).46,47 The larger intensity ratio of the 2D band to the G band (I2D/IG=0.5) was also used to semi-quantitatively validate the layers of the graphene sheets, confirming the presence of a few-layer structure.9,46,48 AFM images (Fig 4C) were also utilized to confirm the layers of the graphene. Typical height profiles of the graphene sheets showed a mean thickness of about 2 nm, suggesting fewer layers in the graphene flakes. All these results confirmed that the SFNs-assisted graphene materials had fewer layers and defects. 15 ACS Paragon Plus Environment


Figure 4. (A) Overall Raman spectra of exfoliated graphene and graphite; (B) Comparison of 2D peak between graphene and graphite; (C) AFM image of the purified graphene flakes; (D) Height profile corresponding to the line shown in the AFM image.

Electrical conductivity was investigated to assess the graphene generated in this process. The exfoliated pure graphene layer was painted onto a printing (cellulose) paper to measure the electrical conductivity with a standard four-probe method.19 The thickness of conducting layers was about 20 µm, determined by measuring the cross-sectional SEM images (Fig 5A). The graphene coatings were compact without an obvious multilayer structure, suggesting strong interactions between the graphene flakes. The surface morphology of the graphene coatings (Fig 5B) indicated that the graphene flakes were homogeneously stacked to form an overlapped structure, which would promote electrical conductivity. A light emitting diode (Fig 5C, D) was tested by connecting the electrical circuit and the conductivity of the graphene coating was calculated at 1.5×103 S m-1with a resistance (Rs) of 45 Ω sq-1, superior to several


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graphene films reported previously and also our recent shear exfoliated graphene with silk fibroin as the stabilizer.9,21,49-51

Figure 5. (A) SEM cross-section image of C-graphene film; (B) Surface image of a freestanding C-graphene paper; (C) and (D) Image of exfoliated graphene film and pristine cellulose filter membranes measured by electrical conductivity test.

Protein-assisted exfoliation of graphite provides a green process to develop biocompatible carbon-based nanomaterials for biomedical applications.21 It is well recognized that the hydrophobic domains of the proteins interact with graphene while the charged regions of the protein support graphene dispersion with good stability. Compared to other proteins,18-20 SFNs have a higher negative charge density and outstanding stability without conformational changes under the sonication process. These synergistic advantages result in the superior exfoliation of graphene in aqueous solution. The biocompatibility of SFNs suggests a promising future for these 17 ACS Paragon Plus Environment


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graphenes in biomedical applications.

The coatings of SFNs-graphene and C-graphene were formed on silicon wafers and cultured with BMSCs. Based on typical requirements for graphene materials in biomedical applications, the graphene dispersions with concentration of about 0.2 mg mL-1 were coated on silicon wafers for cytocompatiblility evaluation. Similar to several studies,21,52 the cells grew well on all the coatings over 48 h (Fig S5), indicating the good cell compatibility when the used graphene concentration was below 0.2 mg mL-1. Longer cell culture times showed better cell proliferation on the SFNs-graphene

coatings

than

on

C-graphene,

possibly

due

to

excellent

biocompatibility of SFNs (Fig 6A).41 Unlike silicon wafer and C-graphene samples, stronger blue staining appeared on the SFNs-graphene coatings, suggesting the existence of silk fibroin materials since silk fibroin could be stained with DAPI staining.32,41 No significant difference appeared for the cells cultured on silicon slides and graphene materials, confirming good cytocompatibility (Fig 6B). Therefore, these results confirmed the usability of the graphene materials in biomedical fields.


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Figure 6. Cytocompatibility of graphene: (A) Confocal microscopy images of cells grew on silicon slide, SFNs-graphene and C-graphene on days 1, 3 and 7, scale bar = 150 µm; (B) BMSC proliferation behavior on different samples. *Statistically significant P < 0.05. The symbol “NS” means no significant difference.

CONCLUSIONS As a stabilizer, SFNs with a high charge density supported the exfoliation of graphene in aqueous solutions. These nanofibers resulted in high graphene yields (20%) and stable graphene dispersions using a low initial graphite concentration. The graphene had

fewer

defects,

fewer

layers,

optimized

conductivity,


and

excellent


cytocompatibility. These properties make this high quality graphene a promising biomaterial for many fields, including for biosensors, tissue engineering and drug delivery.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Images of graphite-SF hydrogel, and SFNs stabilized graphene aqueous solution obtained through sonication treatment; A/l as a function of SFNs concentration and A-CG calibration plot of Graphene stabilized by nanofibers; representative AFM images of SFNs and the stability of the C-graphene when stored at room temperature; digital photograph of graphene samples; quantification of live cells at different culture times.

AUTHOR INFORMATION Corresponding Author *Tel: (+86)-512-67061649; E-mail: [email protected]. Hongmei Zhuo and Xiaoyi Zhang have contributed equally to this paper Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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The authors thank the National Key Research and Development Program of China (2016YFE0204400), NIH (R01NS094218, R01AR070975) and the AFOSR.

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TOC Graphic

The hydrophobic SFNs as exfoliating agents achieved aqueous defect-free graphene dispersions with high concentration through sonication treatment.


Sonication Exfoliation of Defect-Free Graphene in Aqueous Silk

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