doi:10.1246/cl.2013.189 Published on the web February 2, 2013
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Solubilization of Chemically Reduced Graphene Oxide Using Coffee Catechol Jang Mi Lee,1 Geumbi Jeong,1 Mi Yeon Lee,1 So Yeon Kim,1 Young Ho Park,1 Sung Young Park,2 Sang Chun Kim,*1 Byung-Gak Min,*1 and Insik In*1 1 Department of Polymer Science and Engineering, Korea National University of Transportation, Chungju 380-702, Korea 2 Department of Chemical and Biological Engineering, Korea National University of Transportation, Chungju 380-702, Korea (Received November 8, 2012; CL-121129; E-mail:
[email protected])
The idea of soluble graphene is based on the fact that certain functionalization of chemically reduced graphene oxide (RGO) can significantly hinder the strong van der Waals interaction between individual RGO plates in solution.1 While either covalent2 or noncovalent chemistries3,4 have been successfully utilized to provide soluble RGO by incorporating charges, small molecules, polymers, or biomolecules on each RGO plate, most of these approaches have been based on “chemical reduction” of graphene oxide (GO) with the use of chemical reducing agents such as sodium borohydride, hydrazine, L-ascorbic acid, etc. Recent novel approaches using “catechol-induced reduction and functionalization” have unveiled both simultaneous reduction and surface functionalization methods to provide soluble RGO without using any chemical reducing agent.5 In this approach, naturally occurring catechols such as dopa, dopamine, and norepinephrine have successfully produced soluble RGO through initial anchoring on GO plates and simultaneous reducing of GO plates to RGO plates simply in alkaline condition.6 Because small amounts of catechol occur naturally in fruits and vegetables, certain catechol-containing food or beverages are potential candidates for producing soluble RGO through “catechol-induced reduction and functionalization.” Recent success of green tea catechol for the formulation of soluble RGO prompts us to investigate another catechol-rich beverage, “coffee” for the solubilization of RGO.7 Chlorogenic acid (CA), an ester of caffeic acid and quinic acid, was selected as model coffee catechol because of it natural abundance in daily drinkable coffee.8 Because catechol can induce reduction of GO only under basic conditions, all experiments were performed in basic buffer solution (10 mM Tris, pH 8.5). At first, 1.00 mg of CA was solubilized in 20 mL of buffer solution and then mixed with another 20 mL of buffer solution containing 1.00 mg of GO. At room temperature, there was no indication of the reduction of GO. Heating CA/GO mixture (1:1 mass ratio) solution at 40 °C initiated the reduction of GO to RGO, which is evident from the occurrence of darkblack color after 24 h (Figure 1b). This prompted us to test real coffee drink for the preparation of soluble RGO. Two milliliters Chem. Lett. 2013, 42, 189190
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Stable aqueous dispersion of chemically reduced graphene oxide (RGO) was prepared through simple “catechol-induced reduction and functionalization” with a coffee catechol, chlorogenic acid (CA) or real Americano coffee. Prepared CA/RGO or coffee/RGO assembly was optically clear and stable for more than 6 months. This coffee catechol based approach for the formulation of soluble RGO can be easily extended to other “green chemistry” where catechol-rich fruits such as wine or beverages such as berry juice can be used for the formulation of soluble graphene.
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Wavelength/nm Figure 1. a) UVvis spectra of a) CA/GO and CA/RGO solutions, and b) coffee/GO and coffee/RGO solutions (the insets are photos of GO and RGO solutions, respectively). of Americano coffee having 1.00 mg of CA (from Caffé Bene Co.)9 was filtered through filter paper and mixed with 18 mL of buffer solution and then mixed with 20 mL of buffer solution containing 1.00 mg of GO. Again, heating of this coffee/GO mixture solution at 40 °C for 6 h produced dark-black colored coffee/RGO solution (Figure 1a). Increased optical absorption in UVvis spectra of both CA/RGO and coffee/RGO solutions after reduction clearly demonstrated that both reduction and recovery of ³ conjugation is successful through “catecholinduced reduction and functionalization” without chemical reducing. To elucidate the detailed structural features of produced RGO from both experiments, FT-IR and Raman spectroscopy of filtered RGO films were examined. In FT-IR spectra, the distinct carboxy C=O stretching peak of GO at 1728.5 cm¹1 completely disappeared after reduction with either CA or coffee (Figure 2a). This confirms that the reduction of GO using coffee catechol is almost completed. Also hydroxy CO stretching peaks of GO at 1399 and 1064 cm¹1 were significantly reduced in both RGO films. Raman spectra of both RGO films also confirmed that the reduction of GO to RGO is completed by using CA or coffee (Figure 2b). Both RGO films showed much increased D band intensities at 1338 cm¹1 compared with G band intensities at 1602 cm¹1.
© 2013 The Chemical Society of Japan
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Figure 2. a) FT-IR and b) Raman spectra of GO and filtered RGO films using either CA or coffee. The exfoliated individual RGO plates were clearly observed both in atomic force microscopy (AFM) and transmission electron microscopy (TEM). AFM images of both CA/RGO and coffee/RGO assemblies showed layer thickness of around 3.2 nm and layer width around 1 ¯m for individual assembly, confirming that coffee catechols are actually functionalized on the surface of RGO plates because the typical thickness of GO or graphene plates is less than 1 nm (Figure 3a). TEM image again showed single layered structure of individual RGO plates after functionalization with coffee catechol (Figure 3b). All these results show that coffee catechol simultaneously reduces and functionalizes GO. This is coincident with the high dispersion stability of both CA/RGO and coffee/RGO assembly solutions with optical clarity for more than 6 months. In summary, stable RGO solutions were successfully prepared from the simultaneous reduction and functionalization of GO with either CA or real coffee. This coffee catechol based approach for the soluble RGO can be easily extended to other “green chemistry” where catechol-rich fruits such as wine or beverages can be used for the formulation of soluble graphene. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (No. 2012-0004806), a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea (No. 1415120175), Fusion Research R&D Program from the Ministry of Knowledge Economy (MKE, 2012), and a grant from the Academic Research Program of Korea National University of Transportation in 2012. Chem. Lett. 2013, 42, 189190
um Figure 3. a) AFM and b) TEM image of RGO plates with the use of coffee catechol. References and Notes 1 a) D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H. B. Dommett, G. Evmenenko, S. T. Nguyen, R. S. Ruoff, Nature 2007, 448, 457. b) H. A. Becerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, Y. Chen, ACS Nano 2008, 2, 463. c) D. R. Dreyer, S. Park, C. W. Bielawski, R. S. Ruoff, Chem. Soc. Rev. 2010, 39, 228. 2 a) S. Stankovich, R. D. Piner, X. Chen, N. Wu, S. T. Nguyen, R. S. Ruoff, J. Mater. Chem. 2006, 16, 155. b) X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang, F. Zhang, Adv. Mater. 2008, 20, 4490. c) D. Li, M. B. Müller, S. Gilje, R. B. Kaner, G. G. Wallace, Nat. Nanotechnol. 2008, 3, 101. 3 a) Y. Xu, H. Bai, G. Lu, C. Li, G. Shi, J. Am. Chem. Soc. 2008, 130, 5856. b) S.-Z. Zu, B.-H. Han, J. Phys. Chem. C 2009, 113, 13651. 4 a) S. Yoon, I. In, Chem. Lett. 2010, 39, 1160. b) S. Yoon, I. In, J. Mater. Sci. 2011, 46, 1316. c) D. Y. Lee, S. Yoon, Y. J. Oh, S. Y. Park, I. In, Macromol. Chem. Phys. 2011, 212, 336. d) D. Y. Lee, Z. Khatun, J.-H. Lee, Y.-k. Lee, I. In, Biomacromolecules 2011, 12, 336. e) J. Y. Lee, I. In, Chem. Lett. 2012, 41, 127. f) M. Y. Yeo, S. Y. Park, I. In, Chem. Lett. 2012, 41, 197. 5 H. Lee, S. M. Dellatore, W. M. Miller, P. B. Messersmith, Science 2007, 318, 426. 6 a) S. M. Kang, S. Park, D. Kim, S. Y. Park, R. S. Ruoff, H. Lee, Adv. Funct. Mater. 2011, 21, 108. b) L. Q. Xu, W. J. Yang, K.-G. Neoh, E.-T. Kang, G. D. Fu, Macromolecules 2010, 43, 8336. 7 Y. Wang, Z. Shi, J. Yin, ACS Appl. Mater. Interfaces 2011, 3, 1127. 8 M. R. Olthof, P. C. H. Hollman, M. B. Katan, J. Nutr. 2001, 131, 66. 9 Further information for used Americano coffee is also available on the Caffé Bene Web site, http://caffebene.co.kr/index.php.
© 2013 The Chemical Society of Japan
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