DOI: 10.1002/chem.201101360

Pyrene-Containing Peptide-Based Fluorescent Organogels: Inclusion ofGraphene into the Organogel

Bimalendu Adhikari, Jayanta Nanda, and Arindam Banerjee*[a]

Introduction

Low-molecular-weight supramolecular gels[1] belong to animportant class of soft materials with a wide range of appli-cations. Various kinds of small organic molecules have beeninvestigated as low-molecular-weight gelators, such asamides,[2] peptides,[3] ureas,[4] saccharides,[5] nucleobase,[6]

molecules with long alkyl chains,[7] steroid derivatives,[8] andothers.[9] The self-assembly of small organic molecules playsan important role in gelation by using specific intermolecu-lar non-covalent interactions, including hydrogen bonding,p–p stacking, van der Waals forces, hydrophobic interac-tions, charge-transfer interactions, and others.[1–9] In particu-lar, smart fluorescent organogels are of much interest due totheir potential applications including organic light emittingdiodes,[10] photovoltaic cells,[11] light-harvesting systems,[12]

templates for making nanomaterials,[13] and sensing abili-ties.[14] Fluorescent organogelators based on various aromat-

ic building blocks, including oligo(phenylenevinyl-ACHTUNGTRENNUNGene)s,[12b,d,15] oligothiophenes,[16] fulvalenes,[17] perylenebisim ACHTUNGTRENNUNGides,[11, 12c,18] naphthalimides,[8c,19] anthracene,[20] naptha-nyl,[14b, 21] pyrenyl,[8d,22] and other p-conjugated molecules[23]

have been studied in recent years. Pyrene, which is an im-portant moiety with excellent fluorescence properties, hasbeen used as a good attachment for making fluorescent ge-lators.[22] Some of these pyrene-containing fluorescent gelshave potential applications in sensing important biomolecu-les[22d] and color-tunable materials.[22c] Our research work isinvolved in searching for the peptide-based molecules thatcan form supramolecular gels.[24] In the course of our investi-gation on the self-assembly of peptides, we found in thisstudy that the N-terminally pyrene-attached tripeptide,namely, Py-Phe-Phe-Ala-OMe (Scheme 1) can form stabletransparent fluorescent organogels in various organic sol-vents including benzene, toluene, xylene, chlorobenzene, o-dichlorobenzene (ODCB), bromobenzene, chloroform, and

Abstract: The N-terminally pyrene-conjugated oligopeptide, Py-Phe-Phe-Ala-OMe, (Py= pyrene 1-butyryl acyl)forms transparent, stable, supramolec-ular fluorescent organogels in variousorganic solvents. One of these organo-gels was thoroughly studied using vari-ous techniques including transmissionelectron microscopy (TEM), field-emis-sion scanning electron microscopy(FESEM), atomic force microscopy(AFM), Fourier-transform infrared(FTIR) spectroscopy, photolumininis-cence (PL) spectroscopy, and rheology.Unfunctionalized and non-oxidizedgraphene was successfully incorporatedinto this fluorescent organogel in o-di-chlorobenzene (ODCB) to form a

stable hybrid organogel. Graphene iswell dispersed into the gel medium byusing non-covalent p–p stacking inter-actions with the pyrene-conjugated ge-lator peptide. In the presence of gra-phene, the minimum gelation concen-tration (mgc) of the hybrid organogelwas lowered significantly. This suggeststhat there is a favorable interaction be-tween the graphene and the gelatorpeptide within the hybrid organogelsystem. This hybrid organogel wascharacterized using TEM, AFM, FTIR,PL, and rheological studies. The TEM

study of graphene-containing hybrid or-ganogel revealed the presence of bothgraphene sheets and entangled gelnanofibers. The AFM study indicatedthe presence of 3 to 4 layers in exfoliat-ed graphene in ODCB and the pres-ence of both graphene nanosheets andthe network of gel nanofibers in thehybrid gel system. The rheological in-vestigation suggested that the flow ofthe hybrid organogel had become moreresistant towards the applied angularfrequency upon the incorporation ofgraphene into the organogel. Thehybrid gel is about seven times morerigid than that of the native gel.Keywords: fluorescence · gels ·

graphene · peptides · rheology

[a] B. Adhikari,+ J. Nanda,+ Prof. A. BanerjeeDepartment of Biological ChemistryIndian Association for the Cultivation of ScienceJadavpur, Kolkata 700 032 (India)Fax: (+91) 332473-2805E-mail : bcab@iacs.res.in

[+] These two authors contributed equally to this work.

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201101360. Scheme 1. Chemical structure of pyrene-conjugated gelator peptide.

� 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2011, 17, 11488 – 1149611488

dichloromethane. These gels are thermoreversible in natureand show strong blue-emitting fluorescence.

Recently, graphene has attracted a significant amount ofresearch interest all over the world on account of its excel-lent physical, chemical, and material properties and its lowcost.[25] Studies of graphene-based nanocomposite materialshave drawn significant attention because of their excellentproperties and applications.[26] However, dispersion of gra-phene still remains a challenging issue. Noncovalent func-tionalization by using dispersants has provided a simple andeffective route to solve this problem. Until now, variouspolymers, surfactants, aromatic-residue-containing mole-cules, and biomolecules (e.g., DNA, amino acid) have beenused to facilitate the dispersion of graphene.[27] Supramolec-ular gel-phase materials can provide fibrillar network struc-tures that contain porous pockets filled with many solventmolecules and they can accommodate various nanomaterialhosts, including nanoparticles, nanotubes, and others.[28]

There are several reports in the literature about investigat-ing the incorporation of carbon nanotubes (CNTs) in hydro-gels, organogels, and polymeric matrices.[22b, 29] Stupp and co-workers have reported the inclusion of CNTs into peptide-amphiphile-based hydrogel.[29f] Rao and Bhattacharya et al.have prepared organogel–CNT composites in which single-walled CNTs (SWCNTs) are successfully incorporated intoorganogels obtained from l-alanine[29c] and all-trans tri(p-phenylenevinylene) bis-aldoxime[29a]-based gelators. Ajaya-ghosh and co-workers have reported that pristine SWCNTscan accelerate the organogel formation obtained fromoligo(p-phenylenevinylene).[29d] Very recently, Shi and co-workers have developed graphene oxide/DNA compositehydrogels, graphene oxide/hemoglobin composite hydrogels,and graphene oxide/poly(vinyl alcohol) composite hydro-gel.[30] They have also shown the gelation of only grapheneoxide[31a] and self-assembled graphene hydrogel by using ahydrothermal method.[31b] Han and co-workers have demon-strated the dispersion of graphene sheets using copolymersand the formation of supramolecular hydrogel.[32] However,none of these above-mentioned examples show the incorpo-ration of graphene into the fluorescent gel. In this study, wehave demonstrated that a pyrene-conjugated peptide-basedfluorescent organogel acts as an excellent host for the incor-poration of graphene to make a graphene-containing fluo-rescent hybrid organogel system. Furthermore, the hybridgel is more rigid than the native gel.

Results and Discussion

Formation and characterization of the organogel : Supra-molecular gelation is a process in which a large volume ofsolvent molecules are immobilized within the three-dimen-sional network structure provided by the self-association ofthe gelator molecules; this leads to the formation of visco-ACHTUNGTRENNUNGelastic semi-solid soft materials. The N-terminally pyrene-conjugated tripeptide Py-Phe-Phe-Ala-OMe acts as an ex-cellent supramolecular organogelator and this gelator mole-

cule is self-associating in various organic solvents to formgels under suitable conditions. It forms transparent gels inbenzene, toluene, xylene, chlorobenzene, o-dichlorobenzene(ODCB), bromobenzene, chloroform, and dichloromethane.The gelation ability of the gelator molecule is summarizedin Table 1. It is evident from the table that the values of

minimum gelation concentrations are lower than 1 % (w/v)for most of the tested solvents. The gelator peptide is readilysoluble in organic solvents with increasing temperature.These hot solutions are allowed to cool to room tempera-ture followed by sonication to obtain gels within a few sec-onds. It can be noted that in the absence of sonication thegel formation was delayed and even sometimes (at a lowconcentration of the gelator) failed to form any type of gel.This indicates that sonication has a definite role in assistingthe gelator peptide to form gels. There are some reports inthe literature about sonication-induced gel formation.[6b, 9e]

Sonication-induced dissolution of compounds with stronghydrogen-bonding functionality can result in their rapid ag-gregation under non-equilibrium conditions, and this leadsto the deposition of fibrillar aggregates without providingany time for full crystallization to take place.[9e] The mini-mum gelation concentration (mgc) varies from 0.48 to2.23 % (w/v) depending on the nature of the solvents. Thelowest mgc (0.48 %, w/v) is observed in ODCB solvent,which suggests that the gelator peptide most efficiently pro-duces an organogel in ODCB.

The reported organogel was melted upon heating andturned into a gel upon cooling followed by sonication;hence the organogel is thermoreversible in nature. The �tubeinversion� method was used to measure the Tgel of the gel-phase material.[24a] The temperature at which the gel-to-soltransition occurs is called the gel-melting temperature (Tgel)and it increases with an increase in concentration of the ge-lator peptides (Figure 1) until the plateau region is reached.This indicates that the formation of a supramolecular gelnetwork is essentially complete at the higher-concentrationregion. Photographs of the organogel in OCDB under expo-

Table 1. Gelation behaviors of Py-Phe-Phe-Ala-OMe in different organicsolvents.

Solvent Observations (mgc)[a]

benzene G (0.85)toluene G (0.63)o-xylene G (0.73)m-xylene G (0.77)p-xylene G (0.69)mesitylene G (0.71)chlorobenzene G (0.53)o-dichlorobenzene G (0.48)bromobenzene G (0.59)chloroform G (1.78)dichloromethane G (2.23)methanol Sethanol S

[a] S= solution; G= gel. Minimum gelation concentration (mgc) is pre-sented in parenthesis [% (w/v)].

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sure to daylight and under exposure to UV light (excitationat 365 nm) are shown in the Figure 2. The gel appears tohave a yellowish-brown color in daylight. However, it emits

a bright blue color under UV light at an excitation of365 nm. This suggests that the organogel exhibits strongfluorescence properties.

Incorporation of graphene into the organogel : To make thegraphene-incorporated hybrid organogel, we first preparedunfunctionalized and non-oxidized graphene nanosheets inODCB by using sonication. This procedure was reportedpreviously by another research group.[33] To prepare the gra-phene-incorporated hybrid organogel, we used the organo-gel in ODCB solvent. First, a gelator peptide was dissolvedin a graphene/ODCB (graphene dispersed in ODCB) stocksolution with heating. When the well-dispersed solution wascooled down to room temperature and was followed bysonication for a few seconds, it produced graphene-contain-ing stable hybrid organogel in ODCB (Figure 2c). Thechoice of ODCB as a solvent for the preparation of gra-phene-incorporated hybrid organogel was done intentional-ly. Firstly, ODCB, being aromatic, is able to interact with ex-

foliated graphene through p–p stacking interactions. Sec-ondly, the gelator peptide can produce organogel most effi-ciently with a lower mgc in ODCB than in other tested or-ganic solvents. Interestingly, in this procedure, sonicationplays a dual role: 1) to assist gel formation and 2) to dis-perse the graphene in ODCB solvent.

The presence of an aromatic pyrene moiety in the gelatorpeptide can strongly interact with the large aromatic surfaceof graphene by using non-covalent p–p stacking interactions.Interestingly, in this study, the presence of graphene assiststhe gel-formation ability of the pyrene-containing gelatorpeptide in OCDB. It can be easily followed from the plot ofminimum gelation concentration versus the percentage ofgraphene within the hybrid gel (Figure S1 in the SupportingInformation). From this plot, it is evident that the mgc ofthe organogel gradually decreases with an increasing con-centration of graphene within the hybrid organogel. This in-dicates that there is a favorable interaction (for gelation) be-tween graphene and gelator peptides within the hybrid orga-nogel system.

Graphene-containing hybrid organogel is also thermore-versible in nature. Hybrid organogel looks blackish colorunder daylight (Figure 2) and it exhibits a bluish color withrelatively lower intensity (relative to that of the native gel)under exposure to UV light. This suggests that the presenceof graphene significantly diminished the fluorescence inten-sity of the organogel.

Morphological study

Native organogel : Morphologies of the reported organogelswere examined by using field-emission scanning electron mi-croscopy (FESEM), transmission electron microscopy(TEM), and atomic force microscopy (AFM). The FESEMimages of xerogels obtained from various organic solventsare shown in Figure 3. All images reveal the entanglednanofibrillar structure of these organogels. Widths of the gelfibers obtained from ODCB are within the range of 30–35 nm, and these fibrils are highly cross-linked. Figure 4aand b reveal the TEM images of the dried gel-phase materi-al obtained from ODCB. Widths of these fibrils are 20–25 nm, and each fiber is a few micrometers in length. Itshould be noted that the width of the gel fiber is muchhigher than that of the molecular dimension of the single ge-lator peptide. This indicates that several molecular chainsare self-assembled to form the supramolecular fibers. A tap-ping-mode AFM image of the organogel is shown in Fig-ure 4d, and this clearly indicates the presence of an entan-gled network structure of the one-dimensional helical nano-fiber. Figure 4d reveals that the widths of these helical fibersare within the range of 25–30 nm. The morphology of asingle helical fiber is twisted with uniform right-handed biasalong the long axis of the fiber. The pitch of one of thesehelical nanofibers was determined to be 40 nm. The helicalpitch of these fibers remains uniform along the fiber andthis indicates that the underlying forces that cause helicalfiber formation are uniform throughout the fiber structure.

Figure 1. The change of Tgel with respect to gelator concentration (w/v) inODCB.

Figure 2. Photographs of organogel in OCDB a) under exposure to day-light, b) under exposure to UV light (excitation at 365 nm), and c) gra-phene-containing hybrid organogel under exposure to daylight

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In all these morphological experiments, the formation ofuniform supramolecular aggregates (i.e., entangled fibrillarstructure) suggests that the pyrene-attached peptide mole-cules are self-assembled in a hierarchical fashion to form aone-dimensional supramolecular polymer through cross-

linked fibers to form a larger network of superstructures inan organogel.[1] The fluorescence microscopic image of thedried organogel obtained from the solvent ODCB is shownin Figure 3d, and it shows the blue-emitting fiberlike aggre-gates.

Graphene-containing hybrid organogel : To investigate themorphology of the graphene-incorporated hybrid system,TEM experiments were performed by using the hybrid gelin ODCB. The result is shown in Figure 5; this reveals the

presence of both graphene nanosheets and cross-linked gelnanofibers within the hybrid organogel system. An entan-gled peptidic nanofibrillar structure is maintained within thehybrid organogel system. Thin graphene nanosheets are ob-servable, and in some cases nanofibrils are on the surface ofthe graphene nanosheet (Figure 5c). This indicates that thehybrid organogel is a true nanocomposite system and gra-phene sheets are interacting well with the peptidic gel nano-fibers within the hybrid system (Figure 5). Widths of the gelnanofibers are in the range of 20–25 nm, and this width issimilar to that of the native organogel. This suggests that in-corporation of graphene nanosheets does not disturb thenanofibrillar network structure within hybrid system. Thepresence of graphene sheet was confirmed by selected-areaelectron diffraction (SAED) experiments on the graphenesheet. The result of SAED study is presented in Figure 5d;it shows several concentric rings. This data indicates thepresence of multilayer graphene sheets within the hybrid

Figure 3. FESEM images of organogels in various organic solvents:a) ODCB, b) toluene, and c) o-xylene. d) Fluorescence microscopic imageof the organogel in ODCB (scale bars= 1 mm in a and b; scale bar=

100 nm in c).

Figure 4. a) TEM image of dried organogel obtained from ODCB; b) en-larged version of (a). c) TEM image of dried organogel obtained from o-xylene. d) AFM image of dried organogel obtained from ODCB. Insetshows the right-handed helicity of a single fiber.

Figure 5. a) TEM image of the hybrid organogel (in ODCB) showing thepresence of both graphene nanosheets and peptidic gel nanofibrils; b)and c) show the enlarged version of (a) with increasing magnification, re-spectively. d) Diffraction pattern of the isolated graphene within thehybrid gel system.

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sample.[34] TEM studies suggest that the resulting hybrid or-ganogel preserved the basic characteristics of supramolec-ular gel including cross-linked nanofibers.

In this study, the graphene (dispersed in ODCB) used forincorporation purposes was very thin. Its thickness was de-termined by using AFM studies. The height-profile diagram(Figure 6a and Figure S2 in the Supporting Information) in-

dicates the presence of a few layers (3 to 4) of graphene.AFM studies were performed by using graphene-containinghybrid organogel; this is presented in Figure 6b. It shows thepresence of gel nanofibers on the surface of the graphenenanosheet. In the hybrid system, the widths of the gel nano-fibers are in the range of 25–30 nm. This suggests thatwidths of these fibers remain almost the same before andafter the incorporation of graphene into the organogel.

Fluorescence study : To study the molecular interactions ofthe gelator peptides in the gel phase, the fluorescence studywas carried out in gel and solution phases (Figure 7a). Thiscan offer useful information about the interaction of the ar-omatic moieties present in the gelator molecules. In solution(at very low concentration 0.005 % w/v) the gelator peptideshows a strong emission peak at 400 nm along with twoother peaks located at 378 and 417 nm, when the dilute solu-tion is excited at 335 nm. This emission is arising from thenon-aggregated pyrene-containing gelator peptide.[3e] Under

self-assembly conditions, in the gel state (in ODCB withconcentration 0.5 % w/v) they exhibit a strong excimer emis-sion peak centered at 480 nm. This indicates that pyrenylgroups of these gelator peptides are dimerized in the gelstate, and this happens due to the presence of strong p–p

stacking interactions.[3e] The observed significant redshift offluorescence emission maxima is obviously due to the pres-ence of a strong molecular aggregation in the gel state.[3e]

A time-correlated single-photon counting (TCSPC) studywas used to measure the fluorescence lifetimes of our syn-thetic gelator peptide in solution as well as in the gel state(Figure S3 in the Supporting Information). In solution andin the gel state in ODCB, average lifetimes were found tobe 28.29 and 1.98 ns, respectively. The decrease in fluores-cence lifetime value of the gel relative to that in solutionsuggests a dynamic quenching in the gel state and the exci-tons may decay through the network junctions.[35]

A fluorescence study was also performed using the gra-phene-containing hybrid organogel (Figure 7b). It is interest-ing to note that a significant amount of quenching (reducedby a factor of 14) of emission of the gel was observed afterthe inclusion of graphene into the organogel. This suggests

Figure 6. AFM images of a) graphene dispersed in ODCB showing thenanosheet structure and b) graphene-containing hybrid organogel exhib-iting the presence of gel nanofibers on the surface of graphene nano-sheets.

Figure 7. a) Fluorescence spectra of gelator peptide in solution (c) andin the gel state (*). b) Comparison of fluorescence spectra betweennative organogels (*) and graphene-containing hybrid gel (c) showingthe drastic decrease in fluorescence intensity after the incorporation ofgraphene.

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that the significant interaction between the pyrene-conjugat-ed peptide and graphene is present in the hybrid system.The quenching phenomenon is similar to a recent report byRao and co-workers. They observed that the fluorescence ofcoronene moiety was quenched in the presence of graphenein water.[27c]

FTIR spectroscopic study : To understand the intermolecularinteractions between the gelator peptides in the gel state,FTIR experiments were carried out using wet organogel inODCB and as well as with the dried organogel obtainedfrom ODCB (Figure 8). For wet organogel, the amide A

band (NH stretching) was observed as a broad peak cen-tered at 3281 cm�1; no peak was observed around 3400 cm�1.This suggests that all peptide NH groups are involved in hy-drogen-bonding interactions in the gel state. The appearanceof the C=O stretching band (amide I) at 1634 cm�1 and theNH bending (amide II) band at 1544 cm�1 indicates that a b-sheet conformation is present in the gel state.[24d] Anotherpeak at 1754 cm�1 appeared; this is characteristic of the C=

O stretching band of the ester group present in the gelatorpeptide. FTIR studies of the solvent ODCB were also per-formed to ensure the above-mentioned bands arise from thegelator peptide and not from the solvent molecule. InFigure 8, three dashed lines indicate the presence of thesebands in the gel system. They are absent in pure ODCB sol-vent. Moreover, in the dried gel state (Figure 8d), the pres-ence of bands at 3280, 1633, and 1537 cm�1 suggests thatsimilar b-sheet structure is preserved in the dried gel state.

The FTIR spectrum of graphene-containing hybrid wetorganogel also shows the characteristic stretching (3283,1633 cm�1) and bending (1545 cm�1) frequencies similar tothat of native wet gel (Figure 8b). This suggests that the b-

sheet structure is also preserved even after the fabricationof graphene into the hybrid organogel system.

Rheological studies : The rigidity and flow behavior of thenative organogel were determined by measuring the rheo-logical properties. In a typical frequency sweep experiment,the variation of storage modulus (G’) and loss modulus (G’’)were monitored as a function of applied frequency under aconstant strain 0.1 % at room temperature by using the or-ganogel at a concentration 1.0 % (w/v) (Figure 9). G’ repre-

sents the ability of the deformed material to restore its origi-nal geometry, and G’’ symbolizes the tendency of a materialto flow. For an ideal liquid, G’=0, and for an ideal solid,G’’= 0. For viscoelastic materials like gels, G’ is greater thanG’’, which suggests the dominant elastic behavior of thesystem. It is observed from Figure 9 that with an increase inapplied angular frequency, both G’ and G’’ exhibit very littlefrequency dependency within the experimental frequencylimit. For this organogel, G’ is an order of magnitude greater(13.98 times) than G’’ at an angular frequency 7.25 rad s�1.This suggests the existence of a soft �solid-like� gelACHTUNGTRENNUNGphase.[24e, 29a,c]

To investigate whether the rheology of the organogel wasinfluenced by the incorporation of graphene, rheologicalstudies of graphene-containing hybrid organogel (keepingthe same gelator concentration as the native gel) were alsocarried out (Figure 9). In the case of hybrid organogel, bothG’ and G’’ remained almost invariant, and G’>G’’ withinthe experimental frequency limit. This indicates that the gra-phene-containing hybrid system also behaves a �solid-like�gel material. Interestingly, upon inclusion of graphene, thestorage modulus (G’) of hybrid gel was increased by about 7times in magnitude relative to that of the native gel. Thissuggests that the graphene-containing hybrid organogel ismore rigid than native gel.[29a,c] The difference in these twomoduli (G’�G’’=DG) is considered to be a measure of thedominance of the elastic behavior of the material over itsviscous properties, and in this study (G’�G’’) is higher for

Figure 8. FTIR spectra of a) wet organogel in ODCB, b) graphene-con-taining hybrid wet organogel, c) pure ODCB solvent, and d) dried orga-nogel obtained from ODCB. In this figure, the bands that pass throughdashed lines (a), are responsible for hydrogen-bonded amide function-al groups, and the bands that pass through dotted lines (g) are respon-sible for solvent ODCB molecules.

Figure 9. Frequency dependence of the dynamic storage moduli (G’) andthe loss moduli (G’’) of native organogels (&=G’, *=G’’) in ODCB andgraphene-containing hybrid gel (~=G’, !=G’’).

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the hybrid organogel (DG =29555.8 pa) than the native or-ganogel (DG =4368.9 pa). This indicates that the elasticproperty is more dominant than the viscous nature in thecase of graphene-containing hybrid organogel relative to thenative gel.

Conclusion

Pyrene-conjugated, oligopeptide-based, blue-emitting trans-parent fluorescent organogels have been discovered. One ofthese organogels has been examined by using various micro-scopic and spectroscopic techniques and rheological studies.Concentration-dependent fluorescence experiments showthe excimer formation of gelator peptide in the aggregatedgel state. This fluorescent organogel in ODCB is capable toincorporate unfunctionalized and non-oxidized graphenenanosheets into the organogel system by utilizing non-cova-lent p–p stacking interactions to form a stable hybrid orga-nogel system. TEM and AFM studies clearly indicate thepresence of both kinds of morphologies: gel nanofibers andgraphene nanosheets within the hybrid organogel. Thehybrid organogel preserved the basic characteristics of thesupramolecular gel, in particular thermoreversibility, shear-thinning properties, and three-dimensional cross-linkednanofibrillar network structures. Rheological studies revealthat the storage modulus (G’) of graphene-containing hybridorganogel is seven times more rigid than that of native orga-nogel. This suggests that inclusion of graphene into an orga-nogel makes the gel a more elastic and �solid-like� soft mate-rial. Construction of graphene-containing hybrid fluorescentorganogel system may open up interesting possibilities forthe various nanotechnological applications of this type ofgraphene-based nanohybrid materials.

Experimental Section

Materials : l-Phenylalanine, l-alanine, and pyrene 1-butyric acid werepurchased from Aldrich. HOBt (1-hydroxybenzotriazole), DCC (dicyclo-hexylcarbodiimide), and all gelling solvents were purchased from SRL,India.

Synthetic procedure of gelator peptide : Peptide Py-Phe-Phe-Ala-OMewas synthesized by conventional solution-phase methodology by using aracemization free-fragment condensation strategy. In this study, first tri-peptide Boc-Phe-Phe-Ala-OMe was synthesized by using protection ofthe N terminus with the t-butyloxycarbonyl (Boc) group and the C termi-nus with methyl ester. Couplings were mediated by dicyclohexyl carbodi-ACHTUNGTRENNUNGimide/1-hydroxybenzotriazole. Then deprotection of the Boc group wasconducted by treatment with formic acid. Finally, a pyrene group was in-troduced into the N-terminally deprotected tripeptide. All the com-pounds were fully characterized by mass spectrometry, 1H NMR spectros-copy (300 MHz), and 13C NMR spectroscopy (75 MHz).

TEM study : Morphologies of the reported native organogel in ODCBand graphene-containing hybrid organogel were investigated using atransmission electron microscope. These samples were prepared throughplacing the gel-phase material on a TEM grid (300 mesh carbon-coatedCu grid). Then samples were then allowed to dry under vacuum at roomtemperature for a few hours. TEM Images were taken using a JEOLelectron microscope operated at an accelerating voltage of 200 kV.

AFM study : Morphologies of the reported native organogel, grapheneonly, and the graphene-containing hybrid organogel were examined usinga tapping-mode atomic force microscope. AFM studies were carried outby placing a small amount of sample on a mica foil. The material wasthen allowed to dry in air first by slow evaporation and then undervacuum at room temperature for a few hours. Images were recordedusing an Autoprobe CP Base Unit di CP-II instrument (model no. AP-0100).

FESEM study : FESEM experiments was performed by placing a smallportion of gel samples on a microscope cover glass. Then these sampleswere dried first in air and then in vacuum and coated with platinum.After that micrographs were recorded by using a Jeol Scanning Micro-scope JSM-6700F.

Fluorescence spectroscopy : The emission spectra were recorded using aPerkin–Elmer LS 55 fluorescence spectrometer instrument with a 1 cm-path-length quartz cell.

Fluorescence microscopy : Gel-phase material was placed on a glass mi-croscope slide, dried, and examined under a fluorescence microscope(OLIMPUS BX-61) in 40� magnification.

Rheology : Rheological experiments were performed using an AR 2000advanced rheometer (TA Instruments).

FTIR spectroscopy : FTIR spectra were recorded using a Nicolet 380FTIR spectrophotometer (Thermo Scientific) for dried gel materials witha KBr pellet technique and for wet gel material by using CaF2 cells.

TCSPC study : For TCSPC measurements, the samples were excited at340 and 405 nm for solution and gel state, respectively, using a picosec-ond NANO-LED IBH Fluorocube apparatus.

Acknowledgements

B.A. and J.N. thank the CSIR, New Delhi (India), for financial assistance.We also acknowledge the support by the DST (India), project No.SR/S1/OC-73/2009.

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Received: May 4, 2011Published online: August 29, 2011

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