Graphene Oxide-Based Supramolecular Hydrogels for Making Nanohybrid Systems with Au Nanoparticles

Published: December 01, 2011

r 2011 American Chemical Society 1460 dx.doi.org/10.1021/la203498j | Langmuir 2012, 28, 1460–1469

ARTICLE

pubs.acs.org/Langmuir

Graphene Oxide-Based Supramolecular Hydrogels for MakingNanohybrid Systems with Au NanoparticlesBimalendu 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

’ INTRODUCTION

Supramolecular gels belong to a fascinating class of softmaterials in which a large number of solvent molecules areimmobilized by the network structure provided by the assembledgelator molecules.1 These gelator molecules are generally basedon small organic molecules.1,2 A variety of organic moleculesincluding amides, peptides, ureas, saccharides, nucleobases,molecules with long alkyl chains, steroid derivatives, and othershave been found to be low-molecular-weight gels.2 Some of thesesupramolecular gels have been used to construct carbon nano-tube (CNT)-3 and graphene (G)-4containing hybrid nanoma-terials. It has been demonstrated that a gel�CNT nanohybridsystem can be made successfully by incorporating single-walledCNTs into the organogels obtained from alanine and all-transtri(phenylenevinylene) bis-aldoxime-based gelators.3a,c Cur-rently, graphene-5 and graphene oxide-6based hydrogels are anemerging area of nanomaterial research. Graphene oxide (GO) isan important building block for constructing various functionalmaterials.7 However, less attention has been paid to the self-assembling behavior of GO sheets. Shi and co-workers havemade an outstanding contribution to GO-based functional hydro-gels having various applications.6a�d They have reported the 3Dself-assembly of 2D reduced graphene oxide sheets into hydrogelsusing a one-step hydrothermal strategy, and these gels haveexhibited excellent mechanical, thermal, and electrochemicalproperties.5a They have also developed graphene oxide/DNAcomposite hydrogels,6c graphene oxide/hemoglobin compositehydrogels,6b and graphene oxide/poly(vinyl alcohol) composite

hydrogels.6d A recent report includes the demonstration of agraphene-based aerogel that exhibits high electrical conductivityand a large internal surface area.5b

There are many examples of the preparation of G/GO-basednanohybrid systems. Recently, different nanohybrid systems in-cluding G/GO inorganic nanoparticles (NPs) have been exten-sively 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 oxideNPs,8d,12 graphene�semiconductor NPs,13 graphene oxide�metalNPs,8f,g,14�16 and graphene oxide�metal oxide NP17 composites.Kamat and co-workers have made an outstanding contribution tothe investigation of graphene�metal/metal oxide NP compos-ites.8a,c,9m,11a,13d Recently, Ruoff and co-workers have developed areduced graphene oxide/Fe2O3 nanocomposite as a high-perfor-mance anodematerial for lithium ionbatteries.12a Lin and co-workershave developed different graphene�NP hybrids for various appli-cations including formic acid oxidation11c and the detection oforganophosphate pesticides.8e,d Graphene�AuNP hybrids havebeen obtained by applying a common reducing agent—hydrazine—to reduce both GO and HAuCl4.

9d El-Shall and co-workers havedeveloped amethod to prepare graphene�metalNPhybrid systemsin either aqueous or organic media.11b This process is assisted by the

Received: September 6, 2011Revised: November 30, 2011

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 supramolecularstable hydrogels. These hydrogels have been characterized by field-emission scanning electron micro-scopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) analysis, Raman spectros-copy, 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 aGO-based hydrogel show a very highG0 value (6.058� 104 Pa),indicating the rigid, solid-like behavior of this gel. One of these hydrogels (GO-tryptophan) has beensuccessfully utilized for the in situ synthesis and stabilization of Au nanoparticles (Au NPs) within thehydrogel matrix without the presence of any other external reducing and stabilizing agents to make Au NPscontaining the GO-based nanohybrid material. The Au NPs containing the hybrid hydrogel has beencharacterized by using UV/vis spectroscopy, X-ray diffraction (XRD), and transmission electron micro-scopy (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 theGO-based nanohybridmaterial within the hydrogelmatrix. The significance ofthis method is the in situ reduction of gold salt within the gel phase, and this helps to decorate the nascently formed Au NPs almosthomogeneously and uniformly on the surface of the GO nanosheets within the gel matrix.

1461 dx.doi.org/10.1021/la203498j |Langmuir 2012, 28, 1460–1469

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microwave irradiation of GO and metal salts in the presence ofvarious reducing agents.11b The study of GO�metal NPs has beenrelatively less studied than the graphene�metal NP nanohybrid.Zhang and co-workers have reported the synthesis of Ag nanopar-ticles on single-layer GO and reduced graphene oxide (RGO)surfaces using heat treatment.15c In another method, a GO�AuNP nanohybrid system has been obtained using either heattreatment14b or by stirring the GO solution with externally preparedAu NPs.8g Some of these GO�AuNP nanohybrids have showncatalytic properties.8g,14b However, to the best of our knowledge,there is no report on the in situ synthesis of Au nanoparticles withintheGO-based supramolecular hydrogelmatrix tomake a nanohybridsystem in which AuNPs are almost uniformly distributed on GOnanosheets.

In this study, GO-based supramolecular hydrogels have beenformed in the presence of a small amount of amino acid or anucleoside. In these hydrogels, GO sheets form a network andamino acids/nucleosides act as physical cross-linking agents.Moreover, one of these GO-based reported hydrogels (theGO-tryptophan hydrogel) has been used for the in situ prepara-tion of Au NPs within the hydrogel matrix to make a nanohybridsystem. Au3+ has been bioreduced by the tryptophan within thehydrogel without any external reducing agents. This is a con-venient way to prepare the GO-based nanohybridmaterial withinthe hydrogel matrix using a straightforward one-step “greenchemical”method. The elegance of this method is in utilizing thetryptophan-containing GO -based hydrogel for the in situ re-duction of the Au3+ salt and the concomitant stabilization of AuNPs within the gel system so that the nascently formed Au NPscan be homogeneously and uniformly distributed on the surfacesof the GO nanosheets to make a hybrid gel.

’EXPERIMENTAL SECTION

Synthesis of Graphene Oxide. Graphene oxide was synthesizedfrom natural graphite powder (<30 μm) by a modified Hummersmethod.18a In a typical method, 0.5 g of graphite powder was dispersedin 20 mL of concentrated H2SO4 and then 0.25 g of sodium nitrate(NaNO3) was added and the mixture was cooled to 0 �C. Next, 1.5 g ofKMnO4 was slowly added to this mixture so that the temperature was<20 �C during these KMnO4 addition steps. Then the solution wasmixed well and transferred to a 35 �C water bath and stirred for 30 min.A brownish-gray paste was formed. The temperature was raised to 90 �Cduring the addition of 30 mL of water, and this temperature wasmaintained for 15 min. Then the whole solution was mixed with80 mL of warm water, and to this solution 0.5 mL of 30% H2O2 wasadded, which reduces the residual permanganate. The warm solutionwas filtered and washed thoroughly with warm water three to four times.The solid was dispersed in 100 mL of distilled water by sonication, andthe solution was centrifuged at 3000 rpm for 15 min. The filtrate wasresuspended using sonication and again centrifuged at 20 000 rpm. Theresuspension/centrifugation process was repeated several times. Finally,centrifuged viscous graphene oxide (GO) was collected and used toprepare the hydrogel after proper dilution with water.

The successful formation of exfoliated graphene oxide (GO) in waterhas been confirmed using a UV�vis absorption spectroscopy and atomicforce microscopic (AFM) study. In the UV�vis absorption spectrum,the presence of two peaks at 230 and 300 nm suggests the formation ofgraphene oxide (Figure S1 in the Supporting Information). An AFMstudy of GO indicates that GO sheets have an average thickness of1.138 nm (Figure S2 in the Supporting Information). This suggests thatgraphite powder has been completely exfoliated to a single layer ofGO18b and this as-prepared GO has been used for gelation studies.

Preparation of the GO-Based Hydrogel. A stock dispersion ofviscous GO (concentration 22.06 mg/mL) in water has been used toprepare the hydrogel. The concentration of GO was known by freeze-drying the GO/water. A stock solution of amino acid or nucleoside(concentration 5 mg/mL) was also prepared by dissolving these inwater. In a typical experiment, 0.5 mL of GO and 0.05mL of amino acid/nucleoside were mixed; these two solutions (GO and amino acid/nucleoside) were then brought to the required volume, sonicated, andheated, followed by cooling to produce the hydrogel.Synthesis of AuNPswithin theGO-TryptophanGel. First, in

an aqueous solution of GO (concentration 22.06 mg/mL, volume0.5 mL), 50 μL of an aqueous HAuCl4 solution (25 mM) was addedand mixed well. Then to this mixture the required amount of tryptophanwas added, and it was mixed well and sonicated. As a result, Au NPscontaining a hybrid hydrogel were obtained. In this study, we believe thatAu3+ was reduced in situ by the tryptophan molecules within thehydrogel.Instrumentation. Field-Emission Scanning Electron Microscopy

(FE-SEM). For the FE-SEM study, these GO-based hydrogels wereplaced on a microscope cover glass and these samples were freeze-dried.Then, the freeze-dried samples were coated with platinum. Micrographswere recorded by using an FE-SEM apparatus (JSM-6700F Jeol Scan-ning Microscope).

Transmission Electron Microscopy (TEM). The morphologies of thetryptophan-containing GO-based hydrogel and gold nanoparticles (AuNPs) containing a hybrid hydrogel were investigated via transmissionelectron microscopy (TEM). The sample was prepared by depositing adrop of the dilute gel-phase material of the corresponding compoundson a TEM grid (300 mesh Cu grid) coated with Formvar and a carbonfilm. The grid was then allowed to dry under vacuum at roomtemperature for 2 days. Images were taken by a JEOL electron micro-scope operated at an accelerating voltage of 200 kV.

Atomic Force Microscopy (AFM). The morphologies of the reportedGO-based hydrogels were investigated via tapping-mode atomic forcemicroscopy (AFM). AFM studies were conducted by placing a smallamount of a wet hydrogel on mica. The material was then allowed to dryin air first by slow evaporation and then under vacuum at roomtemperature for 2 days. Images were recorded by using an AutoprobeCP Base Unit di CP-II instrument (model no. AP-0100).

Rheology. Rheological experiments were performed using an AR2000 advanced rheometer (TA Instruments).

UV�Vis Spectroscopy. UV�vis absorption spectra of the Au3+-containing GO-tryptophan hydrogel, Au3+-containing GO solution inwater, and Au3+-containing tryptophan solution in water were recordedby using a Varian Cary 50 Bio UV/vis spectrophotometer.

X-ray Diffraction (XRD). The experiment was carried out by using adried GO-based hydrogel and Au NPs containing hybrid hydrogels byusing an X-ray diffractometer (Bruker D8 Advance) equipped with aconventional Cu Kα X-ray radiation (λ = 1.54 Å) source and a Braggdiffraction setup (Seifert 3000P).

Raman Spectroscopy. For Raman spectra, the GO-adenosine hydro-gel sample was placed on a glass slide and then dried well. It wasmeasured by irradiating with laser light at 632.81 nm in a Horiba JobinYvon instrument (LABRAM HR 800).

’RESULTS AND DISCUSSION

Gelation Behavior. Graphene oxide (GO) is a 2D nanoma-terial prepared from natural graphite, and recently it has beenwidely used as a precursor of chemically converted graphene.7

GO can be easily exfoliated into a stable monolayer sheet thatis colloidally dispersed in water. This happens because of thepresence of hydrophilic oxygenated functional groups (�OHand�COOH) on the surface of the GO sheet, and the electrostatic


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repulsion between GO sheets occurs because of the repulsiveinteractions between ionized carboxyl groups (Scheme 1). Thisprevents their assembly in an aqueous medium. However, thesefunctional groups (�OH and �COOH) can form hydrogenbonds with additives under the appropriate conditions, and thisleads to the assembly of GO sheets under specific conditions toform gels. In this study, a graphene oxide (GO)-based supramo-lecular hydrogel has been obtained in the presence of any of theseamino acids—arginine/tryptophan/histidine—or any of thesenucleosides—adenosine/guanosine/cytidine—(Scheme 1 andFigure 1). The minimum gelation concentrations have been

found to be 1.45% w/v for the nucleoside-containing GO gel and2.0% w/v for the amino acid-containing GO gel. This indicatesthat GO can efficiently form hydrogels under the appropriateconditions. In these hydrogels, the amino acid/nucleoside can actas a binder between GO sheets through hydrogen bonding inter-actions and water molecules can be immobilized within thenetwork obtained from the GO sheets and the binder moleculesto form a hydrogel.It can be noted that all of these amino acids and nucleosides

contain more than one nitrogen (N)-containing functionality(Scheme 1). All three amino acids, including arginine, trypto-phan, and histidine, have N-containing basic functionalities thatcan accept protons from the COOH part of the GO sheets toparticipate in acid�base-type electrostatic attraction. Moreover,nitrogen (N)-containing functionalities of amino acids canform hydrogen bonds with hydroxyl groups (�OH) of GOsheets (Scheme 2A). Similarly, each of these three nucleosides(adenosine/guanosine/cytidine) contains more than one nitro-gen (N)-containing basic functionality, and these functionalitiescan also bind with GO using hydrogen bonding and acid�baseinteractions.The typical GO/(amino acid or nucleoside) hydrogels have

been readily prepared by mixing the aqueous dispersion of GO(22.06 mg/mL) with an amino acid or a nucleoside (5 mg/mL).In a typical experiment, 0.5 mL of GO and 0.05 mL ofamino acid/nucleoside were mixed. The mixture was sonicatedand then heated to above 100 �C, followed by cooling to room

Scheme 1. Chemical Structures of Gelator Components (a) graphene oxide (GO), (b) Amino Acids, and (c) Nucleosides

Figure 1. Photographs of GO-based hydrogels in the presence of(a) tryptophan and (b) adenosine. The photographs suggest that theyare gel-phase material and stable upon the inversion of vials containing gels.


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temperature to produce a physical hydrogel. Interestingly, therequired amount of an amino acid or a nucleoside with respect toGO is very low (∼2.21%w/w). This suggests that the amino acidor nucleoside acts as a very good cross-linker between GO sheetsthrough hydrogen bonding to form a gel.Morphological Study. Field-emission scanning electron mi-

croscopy (FE-SEM) and atomic force microscopy (AFM) havebeen performed to investigate the morphologies of these GO-based reported hydrogels. For FE-SEM observation, dilute gelmaterials (1% w/v) have been placed on a microscopic glass slideand then dried under vacuum. Figure 2a,b shows the FE-SEM

images of hydrogels obtained from GO/tryptophan and GO/adenosine, respectively. FE-SEM micrographs reveal that nanos-tructured morphology is present in these supramolecular hydro-gels. All of these gels exhibit the presence of intertwined long,thin nanofibers. The average width of these fibers is found to be50 nm, and they are a few micrometers long.Figure 3 shows AFM images of these GO-based hydrogels.

AFM images reveal the presence of both nanosheets andnanofibers. Moreover, these fibers are cross-linked in nature.Most of these fibers are situated on the GO sheets. The width ofthese fibers varies from 45 to 55 nm, and the length of each fiber

Scheme 2. Tentative Model Showing the Formation of the Hydrogel from a Mixture of GO Sheets and Arginine (Binder)a

a In this scheme, part (A) represents a tentative molecular interaction between GO sheets and arginine using a hydrogen bonding/acid�base-typeelectrostatic attraction to form an extended layer structure. B-i shows the schematic model illustrating the interaction between GO sheets and arginine toform an extended layer-type structure. B-ii represents the assembly of this extended layer structure using noncovalent interactions. B-iii shows the finalformation of a robust 3D network structure containing cross-linked nanofibers (within the hydrogel) obtained from the further assembly of the extendedlayer structure.


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is a few micrometers. The combination of these nanofibers andnanosheets provides a network structure that may be responsiblefor gelation.X-ray Diffraction Study. Figure 4 illustrates the X-ray diffrac-

tion (XRD) patterns of the lyophilized GO/adenosine hydrogelwith varying concentrations of adenosine and the lyophilizedGO (not the gel form). Pure GO shows a diffraction peak at

2θ = 11.29 corresponding to an interplanar spacing of 7.82 Å.6d

This result indicates that GO sheets are aggregated after lyophi-lization. XRD patterns of the GO/adenosine hydrogel at differ-ent concentration ratios are different from those of pure GO.This is mainly due to the strong interaction between thesetwo components (GO and adenosine). The XRD pattern ofthe sample with an increasing concentration of adenosine shows

Figure 2. FE-SEM images showing the morphologies of GO-based hydrogels in the presence of (a) tryptophan and (b) adenosine.

Figure 3. AFM images of GO-based hydrogels in the presence of (a) tryptophan and (b) adenosine. (c) Enlarged version of image a.


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a blue shift in the diffraction peak from 2θ = 10.63 (d spacing8.31) to 2θ = 9.57 (d spacing 9.23). This is because theabsorption of adenosine on GO sheets has induced a slightlengthening 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 ob-served at 1590 and 1339 cm�1 corresponding to the G and Dbands of graphene oxide, respectively. It can be noted that the Gband corresponds to the first-order scattering of the E2g modeof sp2 C atoms and the D band corresponds to the A1g-symmetrymode. The G-band peak observed for the GO-based hydrogel(1590 cm�1) is shifted toward longer wavenumber compared tothat of raw graphite (1580 cm�1).19 This is due to the presence ofisolated double bonds in GO that resonate at frequencies higherthan that of the G band of graphite.19 Therefore, our results

suggest that GO sheets remain unchanged (not reduced) withinthe hydrogel system.Rheological Study. Viscoelastic properties of these hydrogels

were examined by measuring their rheological properties usingthese gels at a fixed concentration of 2% w/v. The variation of thestorage modulus (G0) and loss modulus (G00) were monitored asa function of the applied angular frequency under a fixed strain(0.1%) in a frequency sweep experiment. It can be mentionedthat G0 and G00 respectively denote the ability of the deformedmaterial to restore its original geometry and the propensity of amaterial to flow. For an ideal liquid, G0 = 0, and for an ideal solid,G00 = 0. For a viscoelastic material (gel), G0 is greater than G00.Figure 6 suggests that these hydrogels exhibit a weak frequencydependence over the entire frequency range tested (0.1�628rad/s). Moreover, the storage modulus (G0) is always greaterthan the loss modulus (G00) within the experimental frequencyrange. This suggests that they are effective physical hydrogels.The G0 of the hydrogel at 10 rad/s was measured to be about60 580, 11 030, and 39 360 Pa for GO hydrogels obtained from(1) arginine, (2) tryptophan, and (3) adenosine, respectively.This suggests that the strength/rigidity of these hydrogelsfollows the order 1 > 3 > 2. The strength/rigidity of these GO-based hydrogels has been explained on the basis of basicity or pKa

values of the nitrogen- (N-)containing functionalities of thebinder (amino acid/nucleoside). The arginine has a pKa of12.48 for the side-chain guanidinium group, and it is positivelycharged under neutral, acidic, and even the most basic condi-tions. This provides a strong basic chemical property to thearginine. Moreover, the positive charge of the guanidinium groupis highly delocalized, and this enables the possibility of formingmultiple hydrogen bonds and an electrostatic attraction withthe COOH group of the GO sheet. However, pKa values ofthe amino functionality in adenine and the indole NH in trypto-phan are 4.1 and�3.6, respectively. Therefore, the pKa values ofside-chain N-containing functionalities follow the order arginine(1) > adenosine (3) > tryptophan (2), which is the same orderfor basicity, 1 > 3 > 2. Therefore, it can be stated that the strengthof the acid�base-type electrostatic attraction between theCOOH group of GO and nitrogen-containing functionalities ofthe binder follows the order 1 > 3 > 2. A possible correlation can

Figure 4. XRD patterns of (a) dried, pure GO and (b�d) dried GO-adenosine (GOA) hydrogels with increasing concentrations of adenosine.

Figure 5. Raman spectrum of the dried hydrogel (GO-adenosine)sample.

Figure 6. Frequency dependence of the dynamic storage modulus (G0)and the loss modulus (G00) of GO-based hydrogels in the presence ofamino acid/nucleoside as indicated.


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be made between the strength/rigidity of the hydrogel and thestrength of the acid�base-type electrostatic attraction betweenGO sheets and the corresponding binder. Therefore, it can be con-cluded that the stronger the acid�base interaction, the greater thestrength/rigidity of the hydrogel. Therefore, the strength/rigidityof 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-linkedpolymer hydrogels.20 Although the GO hydrogels contain about98% w/v water, their mechanical properties is still impressive.This is probably due to the following reasons. The mechanicalstiffness of GO itself is high because of the presence of polyaro-matic domains in the basal plane of GO. Moreover, during gelformation GO sheets are interacting favorably with the binder(amino acid/nucleoside) to form an extended structure, and thisstructure self-assembles to form a robust 3D network structure ina gel-phase material. The cross-linked nanofibrous 3D networkstructure is evident from FE-SEM and AFM images (Figures 2and 3). The gelation behavior and the mechanical strength of theresulting hydrogel depend on the concentrations of GO andamino acid/nucleoside in their respective mixtures. It was ob-served that an increase in the concentration of gelators (GO andarginine) can improve the mechanical strength of the resultinghydrogel (Figure S3 in the Supporting Information). It is evidentfrom the concentration-dependent rheological study that thestorage modulus (G0 value) of the gel-phase material is increasedconsiderably as the gelator concentration is increased from 1.5 to2.0%w/v. This happens because of the fact that the enhancementof the robust 3D network structure in the gel phase occurs withan increase in the concentration of gelators by increasing thecross-linking sites between GO sheets and binders (amino acid/nucleoside).5a With further increases in the concentration ofgelators from 2.0 to 2.50% w/v (via 2.25% w/v), the storagemodulous (G0 value) is not increased significantly (Figure S3a inthe Supporting Information). This suggests that with furtherincreases in the concentration of gelators to up to 2.50% w/v thecross-linking network structure is almost saturated. A plot ofstorage modulus values (G0) against the corresponding concen-tration of gelators suggests that theG0 value is almost saturated inthe higher-concentration region (Figure S3b in the SupportingInformation).Tentative Model for Gel Formation. On the basis of mor-

phological studies (using FE-SEM and AFM experiments) andXRD and rheological studies, a tentative model for hydrogelformation from the GO sheet and a binder amino acid (arginine)has been proposed in Scheme 2. The structural model of GOshows the presence of hydrophobic polyaromatic domains inits basal plane and hydrophilic hydroxyl and carboxylic acidgroups along the edges. The arginine molecule can act as a binderbetween GO sheets through multiple hydrogen bonding/acid�base-type electrostatic attraction, and as a result of that, anextended layer-type structure has been formed (Scheme 2). Thisextended layer structure is further self-assembled using nonco-valent interactions to form a robust 3D network structurecontaining nanosheets and nanofibers, and it is evident frommorphological studies. The noncovalent interactions are favor-able 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 structureare yet to be explored. Water molecules can be immobilized

within the obtained 3D network structure to form a hydrogel. InScheme 2B-ii, the distance between two extended layer structuresis 9.23 Å, and this is evident from the XRD study, which suggeststhat GO sheets have induced a slight lengthening of the GOinterplanar spacing from 7.82 (for pure GO) to 9.23 Å after theinteraction with binder molecules (amino acid/nucleoside) with-in the gel system. From the rheological study, it is evident that anincrease in the concentration of gelators (GO and arginine)creates an enhancement in the mechanical strength (rigidity) ofthe resulting hydrogel. This is consistent with our proposedmodel that indicates the increase in the number of cross-linkingsites in the 3D network gel structure with an increase in gelatorconcentration.5a

Au Nanoparticles within the GO-Tryptophan Hydrogel.The preparation of nanoparticles within the gel matrix is agrowing area of current research.21,22 Wet gels have a lot of freespace among the 3D cross-linked network system, and thisprovides a wonderful opportunity for the nucleation and growthof nanoparticles. Stability, longevity, and the regulated growth ofnanoparticles can be attained with ease within the gel matrix.Though there are many reports on the entrapment of pre-prepared21 metal nanoparticles within the small-organic-molecule-based supramolecular gel, only a few examples exist for the in situsynthesis22 of nanoparticles within the small-organic-molecule-based supramolecular gels, where the ingredients of gels can beused for the actual reaction medium for syntheses of nanopar-ticles. However, there are several examples of the synthesis ofmetal NPs on the surface on graphene or graphene oxide to makenanohybrid systems. The procedure for making graphene�metalNP composites is to mix up GO and the respective metal saltsolution (AgNO3, HAuCl4, or H2PtCl6) and then add a reducingagent such as NaBH4 to the mixture. The chemical reductionprocess reduces both GO and metal ions simultaneously. Tothe best of our knowledge, there is no report on the in situsynthesis of Au nanoparticles within the GO-based supramolec-ular hydrogel matrix. Herein, we have successfully demonst-rated the in situ and green synthesis of Au nanoparticles withina tryptophan-containing GO-based hydrogel. It is well re-ported that the redox-active tryptophan moiety can reduce Au3+

to form Au nanoparticles.23 In this study, we have successfullyutilized the tryptophan moiety (gelator) of the gel for the in situreduction of Au3+ within the hydrogel matrix without the require-ment of any external reducing and stabilizing agents. One advant-age of this method is that a weak reducing agent such as trypto-phan can reduceAu3+ tometallic Au0 nanoparticles. However, it isunable to reduce graphene oxide to graphene. As a result, Au NPscontaining the GO-tryptophan-based hybrid hydrogel were ob-tained instead of a composite comprising reduced graphene oxide,tryptophan, and AuNPs together. To get a confirmation of thereduction inability of tryptophan toward GO to graphene, anexperiment was performed. For this purpose, we have allowed themixture of GO and tryptophan to stand for more than 48 h atroom temperature. No color change from brownish (for GO) todark black (for graphene) was observed (until 48 h) after theaddition of tryptophan to an aqueous dispersion of GO. Eventryptophan cannot reduce GO in the presence ammonia and/orheat. The ability of tryptophan to reduce Au3+ to metallic Au0 andthe inability of tryptophan to reduce GO to graphene can alsobe explained in light of the reduction potential values of thesehalf reactions. The reduction potential (E0red) values are asfollows: +1.5 V for Au3+/Au,�0.87 V for GO/G (GO stands for


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graphene oxide and G stands for graphene), and +1.015 V fortryptophan.23,24

From these above-mentioned values it can be stated thattryphophan can reduce Au3+ to metallic Au0 and it does nothave a sufficient reduction potential to reduce the graphene oxideto the graphene. Detailed experimental proof and calculationsbased on the reduction potential have been provided in theSupporting 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 isrequired for this procedure.The formation and characterization of Au nanoparticles within

the gel have been established using UV�vis absorption spec-troscopy, X-ray diffraction (XRD), and transmission electronmicroscopy (TEM). We have investigated the spontaneousreduction of Au3+ by using UV�vis absorption spectroscopy,and it is shown in Figure 7i. Figure 7ia exhibits the absorptionspectra of a mixture (GO+ tryptophan +HAuCl4). The presenceof a surface plasmon resonance band centered at 530 nm suggests

the formation of Au nanoparticles within the hydrogel matrix.Webelieve that Au3+ has been reduced by the tryptophan moleculerather than by GO to establish that we have separately stirredan aqueous HAuCl4 solution with GO and tryptophan. It hasbeen observed that a surface plasmon resonance band centeredat 530 nm has been observed only in the case of tryptophan(Figure 7ib,ic). This indicates that the reduction of Au3+ occursvia 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 withthose for Au nanoparticles (Figure 7ii). These diffraction peakscorrespond to the (111), (200), (220), (311), and (222) Millerindices of Au, respectively.To investigate the morphology of the Au NPs containing a

hybrid gel, a TEM experiment has been performed using the AuNP-containing hybrid gel. The TEM images are shown inFigure 7iii, and these reveal the uniform decoration of Au NPson the GO nanosheets based within the hybrid gel system. Thesizes of the Au nanoparticles have been determined from thisTEM image, and they were found to be within 25�30 nm. In this

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 oftryptophan and gold salt. (ii) XRD patterns of the formation of AuNPs within the GO-tryptophan gel: (a) the GO-tryptophan gel alone and (b) the GO-tryptophan hybrid gel after the formation of Au NPs. (iii) TEM images of (a) the GO-tryptophan hybrid gel containing Au nanoparticles and (b) anenlarged portion of image a with high resolution showing the almost uniform fabrication of Au nanoparticles (NPs) on GO sheets.


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study, the Au NPs almost homogenously and uniformly decoratethe graphene oxide nanosheets. This may be due to the in situreduction of the gold salt within the gel matrix by the tryptophanmoiety. The presence of GO nanosheets and Au NPs has beencharacterized using a selected-area electron diffraction (SAED)study and energy dispersive X-ray (EDX) analysis (FiguresS4�S6 in the Supporting Information).

’CONCLUSIONS

Graphene oxide-based supramolecular hydrogels have beenobtained in the presence of a small number of biomolecules(amino acids/nucleosides). Morphological studies of these hy-drogels indicate the presence of a network structure obtainedfrom cross-linked fibers and nanosheets. One of these hydrogels(GO-tryptophan) has been successfully utilized for the in situsynthesis of Au NPs within the gel matrix. This is a facile andconvenient green chemical approach to make a gel-based nano-hybrid system, in which the Au NPs are almost uniformlyfabricated on the surfaces of GO nanosheets. This method doesnot require an external or toxic reducing/stabilizing agent for thesynthesis of Au NPs. The functional properties of this hybridnanomaterial are yet to be explored. The as-preapred Au NPscontaining the GO-tryptophan hybrid gel can be explored withinthe catalysis of an organic transformation using a green chemicalapproach.

’ASSOCIATED CONTENT

bS Supporting Information. Concentration-dependent rheo-logical study of hydrogels. SAED and EDX images of AuNPscontaining a hybrid gel. This material is available free of chargevia 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 financialassistance. We acknowledge Mijanur RahamanMolla of PolymerScience Unit, IACS, for the rheological measurements. We alsoacknowledge the support by the DST, India, Project SR/S1/OC-73/2009.

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