INTRODUCTION:

Graphene is a single carbon layer in the form of the honeycomb lattice. Graphene and its derivatives are nanoparticles with a thickness of 1-10 nm and its lateral size varied up to hundreds of nanometer. The exceptional properties of nanoscale materials are increasingly used in the medical field for diagnostics as well as therapeutics^1–3. Graphene-based materials are a promising material for drug delivery, anti-bacterial materials, and biocompatible scaffolds^4,5. As the utilization of nanotechnology is gaining in popularity, human and environmental exposure to graphene-based nanomaterials also tends to increase. It should be noted that the use of new materials can create benefits on the one hand, as well as the possible impacts on health and the environment⁶.

Since graphene has been proposed as drug delivery and scaffold material, it could enter different organs through blood or biological system. Its release in the blood causes cytotoxicity. According to Ou et al.⁷, the cytotoxicity of graphene depends on different factors: the concentration in the body, lateral layer dimension, the structure of the layer surface, interaction with other substances, pureness, and the existence of protein corona. Those characteristics of graphene are strongly affected by the synthesis process. The lateral dimension is one of the characteristics that can be controlled by the synthesis process. As a rule of thumb, Mytych and Wnuk ⁸ reported that the size of the nanoparticles can go into the cell (<100 nm), nucleus (<40 nm), and even the blood-brain barrier (<35 nm). The lateral dimension of <40nm is considered a safe boundary. The size of 100-500 nm may cause the most severe toxicity⁷. The smaller sizes are thus the better. Particle size analyzer (PSA) is a suitable instrument for measuring particle size distribution in nanoscale such as graphene. This instrument uses laser diffraction techniques to study the size distribution of a sample in the form of powder, suspension, or emulsion⁹.

Graphene’s properties require versatile and reliable synthetic routes. Several graphene synthesis methods require expensive and sophisticated processes such as epitaxial growth and chemical vapor deposition¹⁰. Another simple method of graphene synthesis is mechanical exfoliation. The exfoliation method uses mechanical power to separate graphene sheets from graphite in different dimensions with available simple equipment¹¹.

For an effective of graphene synthesis, the exfoliation process can be assisted by irradiation of the sourced graphite material using a microwave oven. Microwave irradiation has been increasingly used for both organic and inorganic syntheses^12,13. The treatment can increase the interplanar distance of the graphite layers^14–17. A microwave oven is a fast heating source^16,18. The target materials of microwave heating can be in liquid or solid form. Dipolar polarization and ionic conduction occur in liquid heating¹⁹. However, most of the microwave energy is dissipated which causes less effective energy absorption on the target material¹⁶. In carbon-solid microwave heating, the delocalization of π-electrons in the graphitic region provides semiconducting features. In this case, interfacial polarization occurs which realizes efficient and selective heating to the target material¹⁶.

Blender and ultrasonicator are mixing instruments to be used for graphite exfoliation²⁰. The graphite powder is dispersed in a solution of water and surfactant. In blender exfoliation, graphene is produced by high speed rotating blades due to exceeding the critical shear rate^21–23. Processing of large liquid volumes is possible quickly using this equipment. In sonication, micro jets and compressive shock waves induce the liquid cavitation that causes graphene dispersion. The sonication method produces more oxides and defects graphene, as evidenced by Polyakova et al.²⁴. Based on that, the longer the sonication time of graphene, the smaller the layer size.

The use of surfactants in a graphene suspension is necessary for efficient exfoliation. It creates optimum interfacial tension²⁵. The adsorption of the surfactant onto the particulate surface creates a barrier that inhibits the sheets' reaggregation. Sodium Dodecyl Sulfate (SDS) is a typical ionic surfactant to disperse graphene in water. The use of water as a liquid medium makes the cost cheaper and more environmentally friendly compared to organic solvents and ionic liquids²⁶.

In this paper, we studied graphene synthesis from graphite powder using the exfoliation method. Limited information was found about graphene synthesis that discusses obtaining a safe lateral size of graphene sheets required in the blood circulation. Therefore, this study aimed to predict a safe lateral dimension of graphene nanoparticles through exfoliation process. The results were analyzed, including the particle size change of the graphene layers.

MATERIAL AND METHODS:

Graphene Synthesis:

The conversion of graphite to graphene was performed in a multi-step process, as presented in Fig. 1. Fine natural graphite powder with the particle size of 60 mesh (>95% carbon) was used for the synthesis. 2 g of the material was put in a glass bowl and irradiated by domestic microwave (Electrolux EMM2007X, Japan) at 400 watts power. The irradiation time was set for 3 minutes with the cycle of 10 seconds on and 10 minutes off. The irradiated graphite was dispersed for the exfoliation process in the aqueous solution of 300ml water and 0.5g SDS surfactant to maintain hydrophobic material and prevent the restacking of exfoliated graphite layers. A kitchen blender (Philips HR2071, Indonesia) was used to exfoliate the suspension for 60 minutes (2 x 30’) at the maximum speed. After being left overnight, the suspension was separated from the sediment and put it into three beaker glasses. Further exfoliation was done through ultrasonic vibration using Water Bath Sonicator (Branson 3800 40 kHz, USA). The sonication times were set at 30, 60, and 120 minutes respectively. The suspension was further enriched by centrifugation by using centrifuge (Sorvall Biofuge Primo R, USA) at 7500rpm and 25^oC for 30 minutes. The obtained graphene was washed two times to remove the surfactant. The obtained graphene was collected in a watch glass and dried in ambient temperature for four days.

Characterization:

The characterization of the irradiated graphite powder was done by X-ray Diffractometer (XRD Nova SxD, Australia). The results were compared with the sourced graphite powder. The size and distribution of graphene flakes were measured by using Particle Size Analyzer (PSA) instrument (HORIBA Scientific, Japan) after blender and ultrasonication exfoliations. The images of the synthesized graphene were recorded using Transmission Electron Microscopy (TEM) model JEOL JEM-1400 (Japan) at an acceleration voltage of 150 kV. Fourier Transform Infrared (FTIR) spectra of the samples were obtained using a spectrophotometer (Shimadzu IRPrestige, Japan). The samples were prepared using a potassium bromide (KBr) mulling agent.

Fig. 1: Schematic illustration of the exfoliation method

RESULTS AND DISCUSSION:

XRD analysis:

XRD analysis showed the broader interlayer distance of graphite as shown in Fig. 2. The peak points are presented in Table 1. The XRD patterns presented that the original graphite powder has sharp diffraction peaks at 26.63° and 54.72° (2 theta). The first peak matches to the graphitic plane (0 0 2) with its interlayer spacing of 3.3477 Å. In the case of microwaved graphite, this peak is relatively lower at 26.560 (2 theta). For the corresponding peak, it exhibits the graphitic plane (0 0 2) with its interlayer spacing of 3.3480 Å. The result showed that microwave irradiation is useful to expand the interplanar spacing of the sourced graphite.

Table 1: XRD peak points of graphite and graphene

Sample	Peak	2theta	D[Å]	I/I0	FWHM
Graphite	1	26.63	3.3477	1000.00	0.1625
Graphite	2	54.72	1.6775	34.16	0.2031
Microwaved graphite	1	26.56	3.3480	1000.00	0.1625
Microwaved graphite	2	54.66	1.6793	33.6	0.2031

Fig. 2: XRD patterns of graphite before and after irradiation

The expansion of the interplanar distance of irradiated graphite was caused by weakened interaction of Van der Waals forces between molecules. For longer irradiation times, the interlayer distance of graphite shifted much larger. Similar results have been shown by Jiang et al. .²⁷ Our visual observations showed that the irradiated graphite powder was darker than the sourced graphite powder. The phase changes and restructuring of the crystallinity of the substance probably have changed 2D structure into 3D structure²⁸. The irradiated graphite powder results in more sediment compared with untreated graphite powder due to an increase in density.

FTIR analysis:

The IR spectra of graphene performed OH stretchings at 3425.58, 2924.09 and 2854.65 cm^-1 which confirmed of carboxylate functional groups. The C=C and C-O stretchings of graphene were respectively observed at 1627.92 and 1026.13 cm^-1. The sourced graphite showed peaks at 3448.72 cm^-1, 2924.09 and 2854.65 cm^-1 for OH stretchings which confirmed its carboxylate functional groups. Meanwhile the prensence of C=O and C-O stretchings were respectively showed by peaks at 1635.64 and 1033.85 cm^-1. CO stretching at 1033 cm^-1 confirmed that graphene was synthesized using the surfactant SDS which has rich oxygen structure. Furthermore, KBr was used as a mulling agent for the analysis and identified at 2337.72 cm^-1. The shift to the lower wavelength confirms a decreased layer number of graphite into graphene. A similar result has been found in Naebe’s work ²⁹. The FTIR spectra of the sourced graphite and graphene is shown in Fig. 3.

Fig. 3: FTIR spectra of graphite and graphene

TEM Observation:

Intercalation of surfactant and blender rotation and ultrasonication resulted in single and a few layers of graphene. Images of TEM observations are shown in Fig. 4. The obtained graphene structure has transparent silk-like morphology and folded layers form. Graphene nanosheets with a few hundred square nanometers were observed which look like transparent silk veil waves. Scrolled graphene nanosheets are also observed which is typical of graphene nanosheets ^30,31. Base on these TEM images, we believe that this exfoliation of graphite has demonstrated to produce monolayer and few-layer graphene.

Fig. 4: TEM images of graphene

Particle Size Analysis:

The size distributions of graphene are exhibited in Fig. 5. The sonication treatment has decreased the mean size of graphene’s lateral size: 2973,7 nm (S.D. 1057,3 nm), 2385,8 nm (S.D. 1183,6 nm), dan 795,4 nm (S.D. 401,2 nm) respectively for 30’, 60’, dan 120’ of sonication time. Fig. 6 shows that the size of 30’ sonicated graphene was mostly found between 2000-3000 nm. Meanwhile, the sonication time of 60 'and 120' produced mostly the lateral size between 1000-2000 nm and 0-1000 nm, respectively. The results showed that the mechanism of sonication has improved the dispersion and exfoliated the sheets of graphite into individual graphene flakes^32,33. The prolonged sonication time caused defects to the layer surface area ^23,34. Based on this data, we have developed a simple regression model that considers the effect of the sonication time (x) to the lateral change (y) of the graphene sheets. In this model, the intercept coefficient was 3493 and the coefficient of sonication time was -23.9. The model is expressed in the following equation: y = 3588.9 - 22.8x. Based on this model, it requires 146 min sonication time to obtain the mean size of 40 nm. However, this value may vary when using different types of equipment in the synthesis and longer sonication time is necessary to achieve the overall result < 40 nm.

Fig. 5: Frequency distribution of graphene‘s size

Fig. 6: Frequently graphene’s size found in different sonication time

The effectiveness of exfoliation using sonication is influenced by sonicator power and target material volume. According to Arao and Kubouchi³², low-power (generally less than 100 W) of sonicator leads to a low production rate of graphene sheets. In our case, the sonicator has 110 W power which is expected to perform exfoliation adequately. However, the cavitation of the water bath sonicator generator is placed at the bottom of the bath. Hence, it results in less effective cavitation behavior than one with cavitation generator direct in the vessel using a sonic tip³⁵. The volume of the sonicated suspension should be no larger than a few 100 ml to achieve a more effective result^36,37.

CONCLUSION:

In this study, graphene synthesis by mechanical exfoliation method was adapted using a blender and a water bath sonicator. Microwave irradiation has been a catalyst to shift the spacing layer distance of the 3D structure of graphite. The exfoliation process has resulted in single and a few layers of graphene successfully. PSA results evidenced that the decrease in the lateral surface of graphene layers as the effect of sonication. Future research should optimize the exfoliation time to acquire a more effective process using different types of equipment.

ACKNOWLEDGEMENT:

The authors acknowledge Product Design Lab, Department of Mechanical and Industrial Engineering, Universitas Gadjah Mada for lab equipment support, and the Indonesia Endowment Fund for Education (LPDP), Ministry of Finance of the Republic of Indonesia for funding this research through the BUDI-DN program (grant number 20161141010151).

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

REFERENCES:

1. Younis MA, Bukhari IH, Abbas Q, Bin Talib N, Shaukat S. Synthesis, importance and applications of metal oxide nanomaterials. Int J Technol. 2018;8(2): 49-57.

2. Thodeti S, Reddy SS, Vemula S. Synthesis and characterization of Copper nanoparticles by chemical reduction method. Res J Sci Technol. 2018;10(1):52-57.

3. Mahaparale SP, Kore RS. Silver Nanoparticles: Synthesis, Characterization, Application, Future Outlook. Asian J Pharm Res. 2019;9(3):181-189.

4. Shen H, Zhang L, Liu M, Zhang Z. Biomedical applications of graphene. Theranostics. 2012;2(3):283-294. doi:10.7150/thno.3642

5. Deb A, Vimala R. Graphene mediated drug delivery - A boon to cancer therapy. Res J Pharm Technol. 2017;10 (5):1571-1576. doi:10.5958/0974-360X.2017.00276.1

6. Jayandran M, Muhamed Haneefa, M Balasubramanian V. Synthesis, Characterization and Antimicrobial Activities of Turmeric Curcumin and Curcumin Stabilized Zinc Nanoparticles - A Green Approach. Res J Pharm Technol. 2015;8(4):445-451.

7. Ou L, Song B, Liang H, et al. Toxicity of graphene-family nanoparticles : a general review of the origins and mechanisms. Part Fibre Toxicol. 2016;13(57):1-24. doi:10.1186/s12989-016-0168-y

8. Mytych J, Wnuk M. Nanoparticle Technology as a Double-Edged Sword : Cytotoxic , Genotoxic and Epigenetic Effects on Living Cells. J Biomater Nanobiotechnol. 2013;4(January): 53-63. doi:http://dx.doi.org/10.4236/jbnb.2013.41008

9. Mishra S, Singh G. A Study on Particle Size Distribution Profile of Coal Combustion Residues from Thermal Power Plants of India. Asian J Res Chem. 2010;3(2):386-388.

10. Skoda M, Dudek I, Jarosz A, Szukiewicz D. Graphene: One material, many possibilities - Application difficulties in biological systems. J Nanomater. 2014;2014. doi:10.1155/2014/890246

11. Hu K, Kulkarni DD, Choi I, Tsukruk V V. Progress in Polymer Science Graphene-polymer nanocomposites for structural and functional applications. Prog Polym Sci. 2014;39(11):1934-1972. doi:10.1016/j.progpolymsci.2014.03.001

12. Raval JP, Patel H V, Patel PS, Patel NH, Desai KR. A Rapid, Convenient Microwave assisted and Conventional Synthesis of novel azetidin-2-one derivatives as Potent Antimicrobial agents. Asian J Res Chem. 2009;2(2):171-177.

13. Navale V, Mokle S, Vibhute AY, et al. Microwave-Assisted Synthesis and Antibacterial Activity of Some New Flavones and 1, 5-Benzothiazepines. Asian J Res Chem. 2009;2(4):472-475.

14. Menéndez JA, Arenillas A, Fidalgo B, Fernández Y, Zubizarreta L, Calvo EG. Microwave heating processes involving carbon materials. 2010;91(1):1-8.

15. Zhu Y, Murali S, Stoller MD, Velamakanni A, Piner RD, Ruoff RS. Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors. Carbon N Y. 2010;48(7):2118-2122. doi:10.1016/j.carbon.2010.02.001

16. Kim T, Lee J, Lee K-H. Microwave heating of carbon-based solid materials. Carbon Lett. 2014;15(1):15-24. doi:10.5714/ CL.2014.15.1.015

17. Marquardt D, Vollmer C, Thomann R, et al. The use of microwave irradiation for the easy synthesis of graphene-supported transition metal nanoparticles in ionic liquids. 2010;9:0-6. doi:10.1016/j.carbon.2010.09.066

18. Sudeep S, Tathagata D, Somila K, Jyothi Y. Microwave Assisted Synthesis of Fluoro-Pyrazole Derivatives for Antiinflammatory Activity. Res J Pharm Technol. 2011;4(3):413-419.

19. Deepali Gharge, Salve P, Raut C, Pawar K, Dhabale P. Microwave Chemistry: A Review. Asian J Res Chem. 2010;3(1):9-16.

20. Suneetha S, Reddy PBA. Investigation on Graphene Nanofluids and its Applications: A brief Literature Review. Res J Pharm Technol. 2016;9(6):655-663. doi:10.5958/0974-360X.2016.00124.4

21. Paton KR, Varrla E, Backes C, et al. Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids. Nat Mater. 2014;13(6):624-630. doi:10.1038/nmat3944

22. Yi M, Shen Z. Kitchen blender for producing high-quality few-layer graphene. Carbon N Y. 2014; 78:622-626. doi:10.1016/j.carbon.2014.07.035

23. Varrla E, Paton KR, Backes C, Harvey A, Smith RJ, Coleman JN. Turbulence-assisted shear exfoliation of graphene using household detergent and a kitchen blender. Nanoscale. 2014;6: 11810-11819. doi:10.1039/C4NR03560G

24. Stolyarova EYP, Rim KT, Eom D, et al. Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy Studies of Graphene Films Prepared by Sonication-Assisted Dispersion. 2011:6102-6108. doi:10.1021/nn1009352

25. Sham AYW, Notley SM. A review of fundamental properties and applications of polymer-graphene hybrid materials. Soft Matter. 2013;9(29):6645-6653. doi:10.1039/c3sm00092c

26. Green AA, Hersam MC. Solution Phase Production of Graphene with Controlled Thickness via Density Differentiation. 2009.

27. Jiang F, Yu Y, Wang Y, Feng A, Song L. A novel synthesis route of graphene via microwave assisted intercalation-exfoliation of graphite. Mater Lett. 2017; 200:39-42. doi:10.1016/j.matlet.2017.04.048

28. Kim T, Lee J, Lee KH. Full graphitization of amorphous carbon by microwave heating. RSC Adv. 2016;6(29):24667-24674. doi:10.1039/c6ra01989g

29. Naebe M, Wang J, Amini A, et al. Mechanical Property and Structure of Covalent Functionalised Graphene/Epoxy Nanocomposites. Sci Rep. 2014;4: 1-7. doi:10.1038/srep04375

30. Meyer JC, Geim AK, Katsnelson MI, Novoselov KS, Booth TJ, Roth S. The structure of suspended graphene sheets. 2007;446(March). doi:10.1038/nature05545

31. Hernandez Y, Nicolosi V, Lotya M, et al. High-yield production of graphene by liquid-phase exfoliation of graphite. Nat Nanotechnol. 2008;3(9):563-568. doi:10.1038/nnano.2008.215

32. Arao Y, Kubouchi M. High-rate production of few-layer graphene by high-power probe sonication. Carbon N Y. 2015; 95:802-808. doi:10.1016/j.carbon.2015.08.108

33. Barwich S, Khan U, Coleman JN. A technique to pretreat graphite which allows the rapid dispersion of defect-free graphene in solvents at high concentration. J Phys Chem C. 2013;117(37):19212-19218. doi:10.1021/jp4047006

34. Khan U, Neill AO, Lotya M, De S, Coleman JN. High-Concentration Solvent Exfoliation of Graphene. 2010;(7):864-871. doi:10.1002/smll.200902066

35. Yi M, Shen Z, Zhang X, Ma S. Vessel diameter and liquid height dependent sonication-assisted production of few-layer graphene. J Mater Sci. 2012;47 (23):8234-8244. doi:10.1007/s10853-012-6720-8

36. Smith RJ, Lotya M, Coleman JN. The importance of repulsive potential barriers for the dispersion of graphene using surfactants. New J Phys. 2010;12 (125008). doi:10.1088/1367-2630/12/12/ 125008

37. Khan U, Porwal H, Neill AO, Nawaz K, May P, Coleman JN. Solvent-Exfoliated Graphene at Extremely High Concentration. 2011:9077-9082. doi:10.1021/la201797h

Received on 27.01.2020 Modified on 19.02.2020

Research J. Pharm. and Tech. 2021; 14(1):270-274.

DOI: 10.5958/0974-360X.2021.00048.2