Graphene Oxide-Based Supramolecular Hydrogels for Making

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Graphene Oxide-Based Supramolecular Hydrogels for Making Nanohybrid Systems with Au Nanoparticles Bimalendu Adhikari, Abhijit Biswas, and Arindam Banerjee* Department of Biological Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

bS Supporting Information ABSTRACT: In the presence of a small amount of a proteinous amino acid (arginine/tryptophan/ histidine) or a nucleoside (adenosine/guanosine/cytidine), graphene oxide (GO) forms supramolecular stable hydrogels. These hydrogels have been characterized by field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) analysis, Raman spectroscopy, and rheology. The morphology of the hydrogel reveals the presence of nanofibers and nanosheets. This suggests the supramolecular aggregation of GO in the presence of an amino acid/nucleoside. Rheological studies of arginine containing a GO-based hydrogel show a very high G0 value (6.058  104 Pa), indicating the rigid, solid-like behavior of this gel. One of these hydrogels (GO-tryptophan) has been successfully utilized for the in situ synthesis and stabilization of Au nanoparticles (Au NPs) within the hydrogel matrix without the presence of any other external reducing and stabilizing agents to make Au NPs containing the GO-based nanohybrid material. The Au NPs containing the hybrid hydrogel has been characterized by using UV/vis spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). In this study, gold salt (Au3+) has been bioreduced by the tryptophan within the hydrogel. This is a facile “green chemical” method of preparing the GO-based nanohybrid material within the hydrogel matrix. The significance of this method is the in situ reduction of gold salt within the gel phase, and this helps to decorate the nascently formed Au NPs almost homogeneously and uniformly on the surface of the GO nanosheets within the gel matrix.

’ INTRODUCTION Supramolecular gels belong to a fascinating class of soft materials in which a large number of solvent molecules are immobilized by the network structure provided by the assembled gelator molecules.1 These gelator molecules are generally based on small organic molecules.1,2 A variety of organic molecules including amides, peptides, ureas, saccharides, nucleobases, molecules with long alkyl chains, steroid derivatives, and others have been found to be low-molecular-weight gels.2 Some of these supramolecular gels have been used to construct carbon nanotube (CNT)-3 and graphene (G)-4containing hybrid nanomaterials. It has been demonstrated that a gel
CNT nanohybrid system can be made successfully by incorporating single-walled CNTs into the organogels obtained from alanine and all-trans tri(phenylenevinylene) bis-aldoxime-based gelators.3a,c Currently, graphene-5 and graphene oxide-6based hydrogels are an emerging area of nanomaterial research. Graphene oxide (GO) is an important building block for constructing various functional materials.7 However, less attention has been paid to the selfassembling behavior of GO sheets. Shi and co-workers have made an outstanding contribution to GO-based functional hydrogels having various applications.6a
d They have reported the 3D self-assembly of 2D reduced graphene oxide sheets into hydrogels using a one-step hydrothermal strategy, and these gels have exhibited excellent mechanical, thermal, and electrochemical properties.5a They have also developed graphene oxide/DNA composite hydrogels,6c graphene oxide/hemoglobin composite hydrogels,6b and graphene oxide/poly(vinyl alcohol) composite r 2011 American Chemical Society

hydrogels.6d A recent report includes the demonstration of a graphene-based aerogel that exhibits high electrical conductivity and a large internal surface area.5b There are many examples of the preparation of G/GO-based nanohybrid systems. Recently, different nanohybrid systems including G/GO inorganic nanoparticles (NPs) have been extensively studied because of their various applications in catalysis, energy conversion, fuel cells, chemical sensors, and other fields.8 These nanohybrid systems may be classified into different groups, including graphene
metal NPs8e,i,9
11 graphene
metal oxide NPs,8d,12 graphene
semiconductor NPs,13 graphene oxide
metal NPs,8f,g,14
16 and graphene oxide
metal oxide NP17 composites. Kamat and co-workers have made an outstanding contribution to the investigation of graphene
metal/metal oxide NP composites.8a,c,9m,11a,13d Recently, Ruoff and co-workers have developed a reduced graphene oxide/Fe2O3 nanocomposite as a high-performance anode material for lithium ion batteries.12a Lin and co-workers have developed different graphene
NP hybrids for various applications including formic acid oxidation11c and the detection of organophosphate pesticides.8e,d Graphene
AuNP hybrids have been obtained by applying a common reducing agent—hydrazine— to reduce both GO and HAuCl4.9d El-Shall and co-workers have developed a method to prepare graphene
metal NP hybrid systems in either aqueous or organic media.11b This process is assisted by the Received: September 6, 2011 Revised: November 30, 2011 Published: December 01, 2011 1460

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Langmuir microwave irradiation of GO and metal salts in the presence of various reducing agents.11b The study of GO
metal NPs has been relatively less studied than the graphene
metal NP nanohybrid. Zhang and co-workers have reported the synthesis of Ag nanoparticles on single-layer GO and reduced graphene oxide (RGO) surfaces using heat treatment.15c In another method, a GO
Au NP nanohybrid system has been obtained using either heat treatment14b or by stirring the GO solution with externally prepared Au NPs.8g Some of these GO
AuNP nanohybrids have shown catalytic properties.8g,14b However, to the best of our knowledge, there is no report on the in situ synthesis of Au nanoparticles within the GO-based supramolecular hydrogel matrix to make a nanohybrid system in which AuNPs are almost uniformly distributed on GO nanosheets. In this study, GO-based supramolecular hydrogels have been formed in the presence of a small amount of amino acid or a nucleoside. In these hydrogels, GO sheets form a network and amino acids/nucleosides act as physical cross-linking agents. Moreover, one of these GO-based reported hydrogels (the GO-tryptophan hydrogel) has been used for the in situ preparation of Au NPs within the hydrogel matrix to make a nanohybrid system. Au3+ has been bioreduced by the tryptophan within the hydrogel without any external reducing agents. This is a convenient way to prepare the GO-based nanohybrid material within the hydrogel matrix using a straightforward one-step “green chemical” method. The elegance of this method is in utilizing the tryptophan-containing GO -based hydrogel for the in situ reduction of the Au3+ salt and the concomitant stabilization of Au NPs within the gel system so that the nascently formed Au NPs can be homogeneously and uniformly distributed on the surfaces of the GO nanosheets to make a hybrid gel.

’ EXPERIMENTAL SECTION Synthesis of Graphene Oxide. Graphene oxide was synthesized from natural graphite powder ( 2. The strength/rigidity of these GObased hydrogels has been explained on the basis of basicity or pKa values of the nitrogen- (N-)containing functionalities of the binder (amino acid/nucleoside). The arginine has a pKa of 12.48 for the side-chain guanidinium group, and it is positively charged under neutral, acidic, and even the most basic conditions. This provides a strong basic chemical property to the arginine. Moreover, the positive charge of the guanidinium group is highly delocalized, and this enables the possibility of forming multiple hydrogen bonds and an electrostatic attraction with the COOH group of the GO sheet. However, pKa values of the amino functionality in adenine and the indole NH in tryptophan are 4.1 and
3.6, respectively. Therefore, the pKa values of side-chain N-containing functionalities follow the order arginine (1) > adenosine (3) > tryptophan (2), which is the same order for basicity, 1 > 3 > 2. Therefore, it can be stated that the strength of the acid
base-type electrostatic attraction between the COOH group of GO and nitrogen-containing functionalities of the binder follows the order 1 > 3 > 2. A possible correlation can

a blue shift in the diffraction peak from 2θ = 10.63 (d spacing 8.31) to 2θ = 9.57 (d spacing 9.23). This is because the absorption of adenosine on GO sheets has induced a slight lengthening of the GO interplanar spacing from 7.82 to 9.23 Å. Raman Spectroscopy. Raman spectroscopy provides a powerful tool for characterizing the carbon-based materials. Figure 5 represents the Raman spectrum of the dried hydrogel (GO-adenosine) sample. Two fundamental vibrations were observed at 1590 and 1339 cm
1 corresponding to the G and D bands of graphene oxide, respectively. It can be noted that the G band corresponds to the first-order scattering of the E2g mode of sp2 C atoms and the D band corresponds to the A1g-symmetry mode. The G-band peak observed for the GO-based hydrogel (1590 cm
1) is shifted toward longer wavenumber compared to that of raw graphite (1580 cm
1).19 This is due to the presence of isolated double bonds in GO that resonate at frequencies higher than that of the G band of graphite.19 Therefore, our results

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Langmuir be made between the strength/rigidity of the hydrogel and the strength of the acid
base-type electrostatic attraction between GO sheets and the corresponding binder. Therefore, it can be concluded that the stronger the acid
base interaction, the greater the strength/rigidity of the hydrogel. Therefore, the strength/rigidity of these hydrogels also follows the order 1 > 3 > 2. The G0 value for the arginine-containing gel (6.058  104 Pa) is higher than those of conventional self-assembled hydrogels1 and comparable to those of various chemically cross-linked polymer hydrogels.20 Although the GO hydrogels contain about 98% w/v water, their mechanical properties is still impressive. This is probably due to the following reasons. The mechanical stiffness of GO itself is high because of the presence of polyaromatic domains in the basal plane of GO. Moreover, during gel formation GO sheets are interacting favorably with the binder (amino acid/nucleoside) to form an extended structure, and this structure self-assembles to form a robust 3D network structure in a gel-phase material. The cross-linked nanofibrous 3D network structure is evident from FE-SEM and AFM images (Figures 2 and 3). The gelation behavior and the mechanical strength of the resulting hydrogel depend on the concentrations of GO and amino acid/nucleoside in their respective mixtures. It was observed that an increase in the concentration of gelators (GO and arginine) can improve the mechanical strength of the resulting hydrogel (Figure S3 in the Supporting Information). It is evident from the concentration-dependent rheological study that the storage modulus (G0 value) of the gel-phase material is increased considerably as the gelator concentration is increased from 1.5 to 2.0% w/v. This happens because of the fact that the enhancement of the robust 3D network structure in the gel phase occurs with an increase in the concentration of gelators by increasing the cross-linking sites between GO sheets and binders (amino acid/ nucleoside).5a With further increases in the concentration of gelators from 2.0 to 2.50% w/v (via 2.25% w/v), the storage modulous (G0 value) is not increased significantly (Figure S3a in the Supporting Information). This suggests that with further increases in the concentration of gelators to up to 2.50% w/v the cross-linking network structure is almost saturated. A plot of storage modulus values (G0 ) against the corresponding concentration of gelators suggests that the G0 value is almost saturated in the higher-concentration region (Figure S3b in the Supporting Information). Tentative Model for Gel Formation. On the basis of morphological studies (using FE-SEM and AFM experiments) and XRD and rheological studies, a tentative model for hydrogel formation from the GO sheet and a binder amino acid (arginine) has been proposed in Scheme 2. The structural model of GO shows the presence of hydrophobic polyaromatic domains in its basal plane and hydrophilic hydroxyl and carboxylic acid groups along the edges. The arginine molecule can act as a binder between GO sheets through multiple hydrogen bonding/acid
base-type electrostatic attraction, and as a result of that, an extended layer-type structure has been formed (Scheme 2). This extended layer structure is further self-assembled using noncovalent interactions to form a robust 3D network structure containing nanosheets and nanofibers, and it is evident from morphological studies. The noncovalent interactions are favorable because of the larger contacting area between GO sheets.6a This extended layer -type structure may be twisted, folded, and rolled to some extent to form the fibrous structure. However, the exact reason and the origin forming this type of structure are yet to be explored. Water molecules can be immobilized

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within the obtained 3D network structure to form a hydrogel. In Scheme 2B-ii, the distance between two extended layer structures is 9.23 Å, and this is evident from the XRD study, which suggests that GO sheets have induced a slight lengthening of the GO interplanar spacing from 7.82 (for pure GO) to 9.23 Å after the interaction with binder molecules (amino acid/nucleoside) within the gel system. From the rheological study, it is evident that an increase in the concentration of gelators (GO and arginine) creates an enhancement in the mechanical strength (rigidity) of the resulting hydrogel. This is consistent with our proposed model that indicates the increase in the number of cross-linking sites in the 3D network gel structure with an increase in gelator concentration.5a Au Nanoparticles within the GO-Tryptophan Hydrogel. The preparation of nanoparticles within the gel matrix is a growing area of current research.21,22 Wet gels have a lot of free space among the 3D cross-linked network system, and this provides a wonderful opportunity for the nucleation and growth of nanoparticles. Stability, longevity, and the regulated growth of nanoparticles can be attained with ease within the gel matrix. Though there are many reports on the entrapment of preprepared21 metal nanoparticles within the small-organic-moleculebased supramolecular gel, only a few examples exist for the in situ synthesis22 of nanoparticles within the small-organic-moleculebased supramolecular gels, where the ingredients of gels can be used for the actual reaction medium for syntheses of nanoparticles. However, there are several examples of the synthesis of metal NPs on the surface on graphene or graphene oxide to make nanohybrid systems. The procedure for making graphene
metal NP composites is to mix up GO and the respective metal salt solution (AgNO3, HAuCl4, or H2PtCl6) and then add a reducing agent such as NaBH4 to the mixture. The chemical reduction process reduces both GO and metal ions simultaneously. To the best of our knowledge, there is no report on the in situ synthesis of Au nanoparticles within the GO-based supramolecular hydrogel matrix. Herein, we have successfully demonstrated the in situ and green synthesis of Au nanoparticles within a tryptophan-containing GO-based hydrogel. It is well reported that the redox-active tryptophan moiety can reduce Au3+ to form Au nanoparticles.23 In this study, we have successfully utilized the tryptophan moiety (gelator) of the gel for the in situ reduction of Au3+ within the hydrogel matrix without the requirement of any external reducing and stabilizing agents. One advantage of this method is that a weak reducing agent such as tryptophan can reduce Au3+ to metallic Au0 nanoparticles. However, it is unable to reduce graphene oxide to graphene. As a result, Au NPs containing the GO-tryptophan-based hybrid hydrogel were obtained instead of a composite comprising reduced graphene oxide, tryptophan, and AuNPs together. To get a confirmation of the reduction inability of tryptophan toward GO to graphene, an experiment was performed. For this purpose, we have allowed the mixture of GO and tryptophan to stand for more than 48 h at room temperature. No color change from brownish (for GO) to dark black (for graphene) was observed (until 48 h) after the addition of tryptophan to an aqueous dispersion of GO. Even tryptophan cannot reduce GO in the presence ammonia and/or heat. The ability of tryptophan to reduce Au3+ to metallic Au0 and the inability of tryptophan to reduce GO to graphene can also be explained in light of the reduction potential values of these half reactions. The reduction potential (E0red) values are as follows: +1.5 V for Au3+/Au,
0.87 V for GO/G (GO stands for 1466

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Figure 7. (i) UV
vis spectra of (a) the formation of Au NPs within the GO-tryptophan gel, (b) the mixture of GO and gold salt, and (c) the mixture of tryptophan and gold salt. (ii) XRD patterns of the formation of Au NPs within the GO-tryptophan gel: (a) the GO-tryptophan gel alone and (b) the GOtryptophan hybrid gel after the formation of Au NPs. (iii) TEM images of (a) the GO-tryptophan hybrid gel containing Au nanoparticles and (b) an enlarged portion of image a with high resolution showing the almost uniform fabrication of Au nanoparticles (NPs) on GO sheets.

graphene oxide and G stands for graphene), and +1.015 V for tryptophan.23,24 From these above-mentioned values it can be stated that tryphophan can reduce Au3+ to metallic Au0 and it does not have a sufficient reduction potential to reduce the graphene oxide to the graphene. Detailed experimental proof and calculations based on the reduction potential have been provided in the Supporting Information. Our method is facile, and it involves the reduction of Au3+ using a green chemical approach because no toxic reducing agent (such as borohydride or hydrazine) or toxic stabilizing agent is required for this procedure. The formation and characterization of Au nanoparticles within the gel have been established using UV
vis absorption spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). We have investigated the spontaneous reduction of Au3+ by using UV
vis absorption spectroscopy, and it is shown in Figure 7i. Figure 7ia exhibits the absorption spectra of a mixture (GO + tryptophan + HAuCl4). The presence of a surface plasmon resonance band centered at 530 nm suggests

the formation of Au nanoparticles within the hydrogel matrix. We believe that Au3+ has been reduced by the tryptophan molecule rather than by GO to establish that we have separately stirred an aqueous HAuCl4 solution with GO and tryptophan. It has been observed that a surface plasmon resonance band centered at 530 nm has been observed only in the case of tryptophan (Figure 7ib,ic). This indicates that the reduction of Au3+ occurs via the tryptophan molecule. The XRD pattern for the hydrogel
Au nanoparticles composite has shown diffraction peaks at 2θ = 38.2, 44.3, 64.7, 77.5, and 81.7, all of which are consistent with those for Au nanoparticles (Figure 7ii). These diffraction peaks correspond to the (111), (200), (220), (311), and (222) Miller indices of Au, respectively. To investigate the morphology of the Au NPs containing a hybrid gel, a TEM experiment has been performed using the Au NP-containing hybrid gel. The TEM images are shown in Figure 7iii, and these reveal the uniform decoration of Au NPs on the GO nanosheets based within the hybrid gel system. The sizes of the Au nanoparticles have been determined from this TEM image, and they were found to be within 25
30 nm. In this 1467

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Langmuir study, the Au NPs almost homogenously and uniformly decorate the graphene oxide nanosheets. This may be due to the in situ reduction of the gold salt within the gel matrix by the tryptophan moiety. The presence of GO nanosheets and Au NPs has been characterized using a selected-area electron diffraction (SAED) study and energy dispersive X-ray (EDX) analysis (Figures S4
S6 in the Supporting Information).

’ CONCLUSIONS Graphene oxide-based supramolecular hydrogels have been obtained in the presence of a small number of biomolecules (amino acids/nucleosides). Morphological studies of these hydrogels indicate the presence of a network structure obtained from cross-linked fibers and nanosheets. One of these hydrogels (GO-tryptophan) has been successfully utilized for the in situ synthesis of Au NPs within the gel matrix. This is a facile and convenient green chemical approach to make a gel-based nanohybrid system, in which the Au NPs are almost uniformly fabricated on the surfaces of GO nanosheets. This method does not require an external or toxic reducing/stabilizing agent for the synthesis of Au NPs. The functional properties of this hybrid nanomaterial are yet to be explored. The as-preapred Au NPs containing the GO-tryptophan hybrid gel can be explored within the catalysis of an organic transformation using a green chemical approach. ’ ASSOCIATED CONTENT

bS

Supporting Information. Concentration-dependent rheological study of hydrogels. SAED and EDX images of AuNPs containing a hybrid gel. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: +91-332473-2805. E-mail: [email protected] .

’ ACKNOWLEDGMENT B.A. and A.B. thank the CSIR, New Delhi, India, for financial assistance. We acknowledge Mijanur Rahaman Molla of Polymer Science Unit, IACS, for the rheological measurements. We also acknowledge the support by the DST, India, Project SR/S1/OC73/2009. ’ REFERENCES (1) (a) Zhang, Y.; Kuang, Y.; Gao, Y.; Xu, B. Langmuir 2011, 27, 529–537. (b) Yan, C.; Pochan, D. J. Chem. Soc. Rev. 2010, 39, 3528–3540. (c) Adams, D. J.; Topham, P. D. Soft Matter 2010, 6, 3707–3721. (d) Banerjee, S.; Das, R. K.; Maitra, U. J. Mater. Chem. 2009, 19, 6649–6687. (e) Hirst, A. R.; Escuder, B.; Miravet, J. F.; Smith, D. K. Angew. Chem., Int. Ed. 2008, 47, 8002–8018. (2) (a) Wang, T.; Jiang, J.; Liu, Y.; Li, Z.; Liu, M. Langmuir 2010, 26, 18694–18700. (b) Suzuki, M.; Yumoto, M.; Shirai, H.; Hanabusa, K. Chem.—Eur. J. 2008, 14, 2133–2144. (c) Ma, M.; Kuang, Y.; Gao, Y.; Zhang, Y.; Gao, P.; Xu, B. J. Am. Chem. Soc. 2010, 132, 2719–2728. (d) Krysmann, M. J.; Castelletto, V.; Kelarakis, A.; Hamley, I. W.; Hule, R. A.; Pochan, D. J. Biochemistry 2008, 47, 4597–4605. (e) Buerkle, L. E.; Li, Z.; Jamieson, A. M.; Rowan, S. J. Langmuir 2009, 25, 8833–8840. (f) Mallia, V. A.; George, M.; Blair, D. L.; Weiss, R. G. Langmuir 2009, 25, 8615–8625. (g) Bag, B. G.; Dinda, S. K.; Dey, P. P.; Mallia, V. A.; Weiss, R. G. Langmuir 2009, 25, 8663–8671. (h) Ryan, D. M.;

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dx.doi.org/10.1021/la203498j |Langmuir 2012, 28, 1460–1469