Research Note pubs.acs.org/IECR
Ethylenediamine Modified Graphene and Its Chemically Responsive Supramolecular Hydrogels Shanshan Wang, Jun Wang, Wenfeng Zhang, Junyi Ji, Yang Li, Guoliang Zhang, Fengbao Zhang, and Xiaobin Fan* State Key Laboratory of Chemical Engineering, Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, China S Supporting Information *
ABSTRACT: In this study, reduced graphene oxide was functionalized with ethylenediamine (EDA) by a simple one-pot process, using a lithium ethylenediamine derivative as a nucleophilic reagent at 50 °C for 10 h. The presence of EDA chains in graphene was identified by Fourier transform infrared spectroscopy (FTIR), electronic energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and zeta potential. By using the simple amidation reaction of EDA modified graphene and subsequent host−guest interactions between β-cyclodextrins (β-CDs) and polymers carrying guest moieties, we also synthesized a new type of graphene-based β-CD supramolecular hydrogels. The obtained hydrogels exhibited the gel-to-sol transition by adding a competitive guest sodium adamantine carboxylate (AdCNa) or competitive host α-CD. graphene.22 Unfortunately, the grafting ratios of amines are relatively low, although many of these methods are conducted under harsh reaction conditions. Therefore, more accommodating, efficient, and scaleable approaches are still desirable. In this study, we develop an easy and efficient strategy to synthesize high-density ethylenediamine functionalized graphene (EDA-G) by interaction of the graphene with the lithium ethylenediamine derivative. Although lithium metal dissolved in EDA has been employed to prepare hydrogenated fullerenes23 and hydrogenated carbon nanotubes24 by Benkeser hydrogenation, to the best of our knowledge, this is the first report on the reaction of graphene and the lithium ethylenediamine derivative. By utilizing the amidation reaction and the host− guest interactions between CDs and polymer side chains, we also demonstrate a new type of graphene-based CD supramolecular hydrogels with chemically responsive properties (Scheme 1).
1. INTRODUCTION Supramolecular hydrogels1 are an attractive field of soft materials self-assembled by gelator molecules in water through noncovalent interactions including hydrogen bonding, π−π stacking, and host−guest recognition. They have great potential applications in sensors,2,3 tissue engineering4,5and drug delivery,6,7 etc. Recently, there is a significant growing interest in the development of graphene hydrogels,8−10 especially graphene-based cyclodextrin supramolecular hydrogels that combine the unique properties of graphene11−14 with the supramolecular recognition capabilities of the cyclodextrin (CD).15 In previous report, Zu et al.16 prepared a stable aqueous graphene sheet via the chemical reduction of exfoliated graphite oxides in the presence of Pluronic copolymer [poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer, PEO-b-PPO-b-PEO]. Then, using the penetration of PEO chains into the cyclodextrin cavities, they synthesized graphene-based cyclodextrin supramolecular hydrogels. Later, Liu et al.17 demonstrated the formation of supramolecular hybrid hydrogels from β-CD modified graphene oxide with block co-polymers poly(N,N-dimethylacrylamide)-b-poly(N-isopropylacrylamide) (AZO-PDMA-b-PNIPAM). However, there are few reports about the synthesis of graphene-based cyclodextrin using graphene as the chemical precursor. The extreme insolubility of graphene in water18 is the prime obstacle in incorporating it within supramolecular hydrogels. Chemical functionalization of graphene is regarded as a promising pathway to overcome this problem.19 The use of polyamines (especially ethylenediamine, EDA) to modify graphene has received considerable attention, because they not only can improve the solubility of graphene, but also serve as useful precursors for further functionalization. Various strategies are employed to prepare EDA-functionalized graphene, such as the nucleophilic ring opening reaction of graphene oxide20,21 and the substitution reaction of fluorinated © XXXX American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Synthesis of Ethylenediamine-Modified Graphene. Reduced graphene oxide (RGO) was prepared and purified following the procedure developed by Rodney S. Ruoff.25 Typically, in a nitrogen-purged four-necked roundbottomed flask (250 mL), 70 mg of lithium was dissolved in 80 mL of dry ethylenediamine (EDA) at 30 °C for 30 min until the blue color of the solution disappeared. Then, 80 mg of RGO was introduced into the above solution under vigorous stirring. After a reaction for 10 h at 50 °C, the system was quenched by bubbling air for 1 h. The suspension was filtered and washed with copious ethanol and distilled water three times. A black solid was obtained after freeze-drying. Received: April 8, 2014 Revised: July 8, 2014 Accepted: August 7, 2014
A
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lithium. Later, careful experiments (see the Supporting Information) revealed that lithium could first react with EDA, forming the monolithium amide derivative (Li−EDA). Then, Li−EDA as a nucleophilic reagent attacked the CC of graphene, resulting in the formation of ethylenediamine covalently bonded graphene (EDA-G). In order to eliminate the physical absorption of ethylenediamine, EDA-G was washed thoroughly with extensive solvent. The as-obtained EDA-G could be well-dispersed in water, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), because of the hydrophilicity of primary amines (see Figure S1 in the Supporting Information). High-resolution SEM of EDA-G showed more wrinkled structures than RGO (see Figure S2 in the Supporting Information). FTIR spectra of RGO and EDA−G are shown in Figure 1. The FTIR spectrum of RGO exhibited a strong absorption
Scheme 1. Illustration for the Preparation of Ethylenediamine Functionalized Graphene (EDA-G) and Graphene-Based Supramolecular Hydrogels
2.2. Preparation of Graphene-Based β-Cyclodextrin Supramolecular Hydrogels. 2.2.1. Synthesis of β-Cyclodextrin Functionalized Graphene. Fifty milligrams (50 mg) of carboxylated β-cyclodextrin (β-CD−COOH) was dissolved in 10 mL of Morpholineethanesulfonic acid (MES) solution (pH 5.5), followed by the addition of 335 mg of N-(3(dimethylamino)propyl)-N-ethylcarbodiimide hydrochloride (EDC) and 150 mg of N-hydroxysuccinimide (NHS). After sonication for 30 min, 40 mg of EDA-G was added and the solution was stirred for 24 h at room temperature. The β-CD− G product was isolated and purified by repeated centrifugation and washing with H2O. 2.2.2. Preparation of Supramolecular Hydrogel. In order to prepare graphene supramolecular hydrogel, poly(acrylic acid) carrying 2 mol % of dodecyl group (PAA2) was used as a guest polymer. PAA2 was obtained according to the previous paper.26 In a typical experiment, 10 mg of PAA2 was dissolved in 2 mL of 0.1 M NaOH, followed by adding 4 mg of β-CD−G. The mixture was ultrasonicated for 10 min and then kept still for certain time at room temperature to form hydrogel. The apparent formation of the hydrogel was examined by a tube inversion method. 2.3. Characterization. The samples were characterized by Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet Nexus FTIR), electronic energy loss spectroscopy (EELS, Philips Tecnai G2 F20), X-ray photoelectron spectroscopy (XPS) (Perkin−Elmer, PHI 1600 spectrometer), zeta potential (Nano ZS), Raman spectroscopy (NT-MDT NTEGRA Spectra), and scanning electron microscopy (SEM) (Hitachi S4800).
Figure 1. Fourier transform infrared (FTIR) spectra of reduced graphene oxide (RGO) and ethylenediamine functionalized graphene (EDA-G).
band at 1620 cm−1, because of the CC stretching. After the reaction, however, the peak intensity of CC weakened, and a new band emerged at 1236 cm−1, corresponding to the C−N characteristic stretching vibrations. In addition, the presence of strong N−H stretching and bending vibrations at 3166 and 1572 cm−1, associated with −CH2 stretching bands (2919 and 2858 cm−1) demonstrated the successful functionalization of EDA onto graphene. This result was further confirmed by electron energy loss spectroscopy (EELS). As shown in Figure 2a, the N K-edge σ* resonance at 405.2 eV in the EELS spectrum of EDA-G could be easily observed, indicating the attachment of EDA. Both the RGO and EDA-G displayed two characteristic peaks with energy losses of 287.6 and 297.7 eV, which were assigned to C K-edge π* resonance and σ* resonance, respectively. Based on the intensity ratio of π* and σ* peaks, it could be roughly estimated as the change of sp2 hybridized carbon amounts. In our study, Iπ*/σ* decreased from 0.58 to 0.45 after EDA modification, suggesting that the introduction of EDA disrupted the conjugated structure of sp2 hybridized carbon atoms and formed abundant sp3 carbon on graphene. This conclusion was consistent with the Raman spectra analysis of EDA-G (see Figure S3 in the Supporting Information). EELS mapping (Figure 2e) showed that the elemental nitrogen was homogeneously distributed on the entire surface of the obtained EDA-G.
3. RESULTS AND DISCUSSION The study was initiated by an interesting finding in the Benkeser hydrogenation using lithium metal and EDA as a powerful reducing agent. In a control experiment, we surprisingly observed that a stable graphene suspension could be prepared by simply changing the adding orders of RGO and B
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Figure 2. (a) EELS spectra of RGO and EDA-G, N K-edge EELS spectra of EDA−G [(a), inset]. (b) EELS−TEM image of EDA-G, and corresponding quantitative EELS element mapping of (c) C, (d) O, and (e) N.
We also carried out zeta-potential measurements to evaluate the surface charge on graphene sheets. The result showed that RGO was slightly negatively charged in aqueous media (−16.8 mV), which could be due to the small amounts of residual carboxylic acid groups after reduction. However, after the reaction, EDA-G exhibited zeta-potential values at +28.5 mV under the same solvent, which was consistent with the positive charge of primary amine in water. Since the primary amine could react with many chemical reagents, this method might open an exciting opportunity to prepare different types of functional materials. To test this idea, we prepared graphene-based supramolecular hydrogels. First of all, β-CD was covalently bonded to EDA-G via the amidation between −NH2 on graphene and −COOH on the carboxylated β-CD. β-CD modified graphene was characterized by FTIR and Raman spectroscopy (see Figures S4 and S5 in the Supporting Information). Based on the fact that CD can capture a wide variety of guest compounds, we then successfully synthesized graphene hybrid supramolecular hydrogels. By using host−guest interactions between β-CDs of β-CD− G and polymers carrying guest moieties, supramolecular graphene hydrogels were prepared. In this study, poly(acrylic acid) (PAA, Mw = 300 000) carrying dodecyl groups was selected as a guest polymer, because of its good water solubility and eco-friendly behavior. According to the previous studies, PAA carrying 2 mol % of dodecyl groups (PAA2) showed Newtonian liquid behavior and was less likely to form the interpolymer aggregates,27 so PAA2 was used in this experiment. By adding β-CD−G into PAA2, the supramolecular graphene hydrogels were simply obtained. In contrast, the mixture of EDA-G and PAA2 could not form hydrogels, indicating that β-CD played a key role in the formation of graphene-based hydrogels with PAA2. In addition, EDA-G could not form gels in the presence of any fatty acid like Myristic acid, oleic acid, palmitic acid, or any type of diacids (such as adipic acid and glutaric acid). The reason may be that the fatty acid and diacid were too small to provide a suitable binding arm for the construction of 3D graphene gel. We also investigated the pH dependent gel formation tendency and found that EDA-G could form hydrogels in the alkaline pH (pH ≥8) instead of acidic pH (pH less than 4) or pH between 4 and 8. A reasonable explanation was that alkaline pH (pH ≥ 8) could increase the electrostatic repulsion of βCD−G, which was favorable for capturing the dodecyl groups of PAA2 into the β-CD cavities.
In order to obtain the chemical state and atomic ratio of each element in EDA-G, XPS was performed. The survey XPS full spectra of EDA-G showed an intense N 1s signal at 399.8 eV, as compared to that of RGO, suggesting the efficient amination of RGO. This speculation was in conjunction with the results from the N 1s and C 1s XPS spectra. As shown in Figure 3b, the N
Figure 3. (a) Full XPS spectra, and (b) the high-resolution N 1s XPS spectrum of EDA-G; also shown are the C 1s XPS spectra of (c) RGO and (d) EDA-G.
1s signal with binding energies at 399.2, 400.1, and 401.7 eV could be assigned to C−NH−C, C−NH2, and hydrogen bonding between primary amine groups, respectively. The deconvoluted C 1s spectrum for RGO also revealed a significant decrease in carbon, in the form of graphite after EDA treatment, with the appearance of an additional peak at 285.7 eV, corresponding to C−N bond. These observations indicate that EDA was bonded to the graphene successfully by nucleophilic addition. Quantitative analysis shows the element atomic ratios of C, O, and N in EDA-G were 77.75%, 13.65%, and 8.6%, respectively. Therefore, the density of the introduced amines was calculated to be 6.8 mmol g−1, which was higher than other reports.20,21 Detailed calculations can be found in Supporting Information. C
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destruction of gel structures. Another type of gel−sol transition was also observed after the addition of α-CD, because dodecyl moieties of PAA formed complexes with α-CD more easily than with β-CD. The transition process is illustrated in Scheme 2.
The formation of three-dimensional cross-linked network structures in the obtained hydrogels was confirmed by SEM (Figure 4). The mechanical strength and stability of the
Scheme 2. Illustration for the Formation of Graphene Hybrid Hydrogels and Gel to Sol Transition When Adding Competitive Guest Sodium Adamantine Carboxylate (AdCNa) or Competitive Host α-CD
Figure 4. SEM image of graphene hybrid hydrogels.
supramolecular hydrogels was investigated by rheological test (25 °C). As shown in Figure 5, the storage modulus (G′) and
4. CONCLUSIONS In summary, we have presented a simple and easily accessible strategy to prepare high-density ethylenediamine modified graphene (EDA-G). The obtained EDA-G showed excellent solubility and dispersibility in water or other organic solvent. This method might be a better choice for attaching the polyamines onto graphene. Moreover, by using EDA-G as a precursor, we synthesized a new type of graphene-based chemically responsive supramolecular hydrogels. The graphene hydrogels exhibited the gel-to-sol transition by adding a competitive guest sodium adamantine carboxylate (AdCNa) or competitive host α-cyclodextrin (α-CD). Notably, as CD can form host−guest complexes with various types of functional compounds, the strategy of forming graphene hydrogels reported here might provide insight for the design of graphene-based hydrogel with reversible responses to environmental changes, as well as expand the scopes of applications in biomedicine.
Figure 5. Rheological curves of PAA2 and graphene hybrid hydrogels at 25 °C.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
loss modulus (G″) of PAA2 were almost equal and approached zero. However, after mixing β-CD−G with PAA2, the storage modulus (G′) was much higher than the loss modulus (G″) and no crossing point could be observed in the experimental frequency regions (0.1−100 rad s−1), suggesting the formation of a typical soft solid-like gel.28,29 Compared the storage modulus (G′) of the graphene hydrogels with that of PAA2, we found that the mechanical strength of the obtained hydrogels enhanced greatly. In addition, both the storage modulus (G′) and loss modulus (G″) of the β-CD−G hydrogels in PAA2 increased slightly with the increase of angular frequency, which indicated that the hydrogels had excellent tolerance to external forces. The obtained supramolecular graphene hydrogels showed chemically responsive peculiarity. When sodium adamantine carboxylate (AdCNa), which is a competitive guest, was added to the hydrogels, gel−sol transition occurred. This phenomenon could be attributed to the stronger interactions between βCD and AdCNa, which resulted in the release of PAA2 and the
The details of control experiments and supplementary figures are summarized in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/. Corresponding Author
*Tel.: (+86) 22-27408778. Fax: (+86) 22-27408778. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Natural Science Funds for Excellent Young Scholars (No. 21222608), Research Fund of the National Natural Science Foundation of China (No. 21106099), Foundation for the Author of National Excellent Doctoral Dissertation of China (No. 201251), the Tianjin Natural Science Foundation (No. 11JCYBJC01700), D
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solution by ethylenediamine-reduced graphene oxide. J. Mater. Chem. 2012, 22, 5914. (22) Stine, R.; Ciszek, J. W.; Barlow, D. E.; Lee, W.-K.; Robinson, J. T.; Sheehan, P. E. High-Density Amine-Terminated Monolayers Formed on Fluorinated CVD-Grown Graphene. Langmuir 2012, 28, 7957. (23) Peera, A.; Saini, R. K.; Alemany, L. B.; Billups, W. E.; Saunders, M.; Khong, A.; Syamala, M.; Cross, R. J. Formation, Isolation, and Spectroscopic Properties of Some Isomers of C60H38, C60H40, C60H42, and C60H44Analysis of the Effect of the Different Shapes of Various Helium-Containing Hydrogenated Fullerenes on Their 3He Chemical Shifts. Eur. J. Org. Chem. 2003, 2003, 4140. (24) Tang, X.; Zhao, Y.; Jiao, Q.; Cao, Y. Hydrogenation of Multiwalled Carbon Nanotubes in Ethylenediamine. Fullerenes, Nanotubes, Carbon Nanostruct. 2010, 18, 14. (25) Zhu, Y.; Murali, S.; Stoller, M. D.; Velamakanni, A.; Piner, R. D.; Ruoff, R. S. Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors. Carbon 2010, 48, 2118. (26) Guo, X.; Abdala, A. A.; May, B. L.; Lincoln, S. F.; Khan, S. A.; Prud’Homme, R. K. Novel associative polymer networks based on cyclodextrin inclusion compounds. Macromolecules 2005, 38, 3037. (27) Wang, J.; Pham, D.-T.; Guo, X.; Li, L.; Lincoln, S. F.; Luo, Z.; Ke, H.; Zheng, L.; Prud’homme, R. K. Polymeric Networks Assembled by Adamantyl and β-Cyclodextrin Substituted Poly(acrylate)s: Host− Guest Interactions, and the Effects of Ionic Strength and Extent of Substitution. Ind. Eng. Chem. Res. 2009, 49, 609. (28) Bhattacharya, S.; Srivastava, A.; Pal, A. Modulation of viscoelastic properties of physical gels by nanoparticle doping: Influence of the nanoparticle capping agent. Angew. Chem. 2006, 118, 3000. (29) Palui, G.; Garai, A.; Nanda, J.; Nandi, A. K.; Banerjee, A. Organogels from different self-assembling new dendritic peptides: Morphology, reheology, and structural investigations. J. Phys. Chem. B 2009, 114, 1249.
and the Programme of Introducing Talents of Discipline to Universities (No. B06006).
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