DOI: 10.1002/chem.201301655

An Amino-Acid-Based Self-Healing Hydrogel: Modulation of the Self-Healing Properties by Incorporating Carbon-Based Nanomaterials

Subhasish Roy, Abhishek Baral, and Arindam Banerjee*[a]

Introduction

Low-molecular-weight molecular gels are an important areaof soft-materials research.[1] Hydrogels are of special impor-tance, owing to their high water content and various applica-tions in biomedical and bio-engineering research.[2] Hamleyand co-workers have reported the formation peptide-basedhydrogels with a functional entity, such as RGD peptide.[1b]

Bhattacharya and co-workers have developed fatty-acid�amine- and lithocholic-acid�organic-amine-based two-com-ponent hydrogels.[1m,n] Adams and co-workers have reported9-fluorenylmethoxycarbonyl (Fmoc)-protected amino-acid-based hydrogels by using glucono-d-lactone as a pH trig-ger.[1o] The UV-light-triggered hydrogelation of small pep-tide has also been reported by the same group.[1u] Interest-ingly, peptide- and amino-acid-based hydrogels are exten-sively used for various applications, including tissue engi-neering, drug delivery, and the formation of metal nanopar-ticles and silver nanoclusters.[1d,3] However, there are fewreports on amino-acid/peptide-based thixotropic hydro-gels.[3d, 4]

Self-healing materials belong to a unique class of smartmaterials that have a structurally incorporated ability torepair the damage that is caused by mechanical usage over a

period of time. Self-healing properties is a common phe-nomenon in living organisms. However, they are also occa-sionally found in inanimate systems. It is remarkable to findthat two pieces of hydrogels “heal” spontaneously over aperiod of time after keeping them in contact. Self-healing[5]

materials are endowed with the property of being able torepair by themselves after physical/chemical/mechanicaldamage to fully or partially recover their original properties.There are a few examples of polymer- and/or nanocompo-site-hydrogel-based self-healing materials[5a–c] and some ex-amples of low-molecular-weight self-healing organogelshave been reported.[5a,d] Thus, the design and formation ofself-repairing material are highly desirable because they ef-fectively increase the lifetime of a functional material.[6] Thedevelopment of supramolecular smart soft materials thatcan self-repair has been of great recent significance.[7] Inspite of the usefulness of self-healing soft materials for ex-ploring some natural processes, including nerve-fiber regen-eration and muscle thixotropy, there is a lack of small-mole-cule-based self-healing soft materials that can operate asmodel systems for exploring such processes.

For this purpose, low-molecular-weight hydrogels(LMHGs) are attractive and challenging candidate materialsbecause they are sometimes endowed with thixotropic andself-healing properties.[8] Therefore, there is a need to designand construct small-molecule-based hydrogels that can ex-hibit self-healing properties and also to be able to modulatethis function. Most of the LMHGs are highly sensitive to-wards mechanical stress or strain. Typically, these gels irre-versibly release bind solvent molecules in their self-assem-bled network system upon the application of a definiteamount of stress or strain.[2] These gels act as a solid suspen-sion that loses its original elastic properties after the remov-

Abstract: An amino-acid-based (11-(4-(pyrene-1-yl)butanamido)undecanoicacid) self-repairing hydrogel is report-ed. The native hydrogel, as well ashybrid hydrogels, have been thoroughlycharacterized by using various micro-scopic techniques, including transmis-sion electron microscopy (TEM),atomic force microscopy (AFM),Raman spectroscopy, fluorescencespectroscopy, FTIR spectroscopy,X-ray diffraction, and by using rheolog-

ical experiments. The native hydrogelexhibited interesting fluorescence prop-erties, as well as a self-healing property.Interestingly, the self-healing, thixotro-py, and stiffness of the native hydrogelcan be successfully modulated by incor-porating carbon-based nanomaterials,

including graphene, pristine single-walled carbon nanotubes (Pr-SWCNTs), and both graphene and Pr-SWCNTs, within the native gel system.The self-recovery time of the gel wasshortened by the inclusion of reducedgraphene oxide (RGO), Pr-SWCNTs,or both RGO and Pr-SWCNTs. More-over, hybrid gels that contained RGOand/or Pr-SWCNTs exhibited interest-ing semiconducting behavior.

Keywords: amino acids · gels ·nanostructures · self-healing · thixo-tropy

[a] S. Roy, A. Baral, Prof. A. BanerjeeDepartment of Biological ChemistryIndian Association for The Cultivation of ScienceJadavpur, Kolkata, 700032 (India)Fax: (+ 91) 33-2473-2805E-mail : bcab@iacs.res.in

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201301655.

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al of the external applied stress or strain.[2] However, thixo-tropic gels can break up into a viscous solution under an ex-ternally applied mechanical stress or strain and they canregain their previous elastic properties automatically uponthe release of the applied stress or strain.[7b] If a self-healinghydrogel is endowed with electroactive properties, it can beused for the recovery of nerve damage.[5c] Wang and co-workers, Aida and co-workers, Pochan and co-workers,Yang and co-workers, and Feng and co-workers have madesignificant contributions to the field of thixotropic, self-heal-ing, and shear-thinning hydrogels.[5c,7b, 8–11] 11-Aminoundeca-noic-acid-containing gels have also been reported in the lit-erature.[12] However, to the best of our knowledge, there areno reports of amino-acid-based thixotropic, self-healing,low-molecular-weight superhydrogel systems or of the mod-ulation of self-healing properties by incorporating carbon-based nanomaterials.

In this study, an amino-acid-containing amide-based mole-cule, 11-(4-(pyrene-1-yl)butanamido)undecanoic acid, wasfound to form a hydrogel (Figure 1) at pH 13.4 at room tem-perature. It was found that the minimum gelation concentra-

tion that was required to form the hydrogel was 0.06 % w/v.Interestingly, gelator 1 formed a transparent, fluorescent,thixotropic, self-healing superhydrogel in basic medium(Figure 1).

The gelator molecule has a p surface that can interactwith graphene and pristine single-walled carbon nanotubes(Pr-SWCNTs) through p�p interactions. Reduced grapheneoxide (RGO) and Pr-SWCNTs were separately incorporatedinto the native hydrogel of gelator 1 to form semiconduct-ing, thixotropic, self-healing hybrid gels (Figure 1). Gra-

phene, Pr-SWCNTs, and both graphene and Pr-SWCNTscould be simultaneously incorporated into the native gel byutilizing p�p interactions between the pyrene moiety of thenative gelator and the p surface of the graphene and/or Pr-SWCNTs.

Results and Discussion

Formation and characterization of the hydrogels : First, aknown amount of the hydrogelator was placed in a glassvial. Then, a 1 m aqueous solution of NaOH (0.1 mL) and50 mm phosphate buffer (0.9 mL, pH 7.46) were added andthe mixture was heated to obtain clear solution to make atotal of 1 mL of the hydrogel. Then, the temperature of theglass vial that contained gelator solution was allowed to coolback to room temperature. The hydrogelation was con-firmed by inverting the glass vial.

Formation of RGO- and Pr-SWCNT-containing hydrogels :RGO was synthesized according to our previously reported

procedure.[13] First, a suspensionof the synthesized RGO inwater was mixed with 50 mm

phosphate buffer (pH 7.46) anda solution of the gelator in 1 m

aqueous NaOH (50 mm phos-phate buffer/1m NaOH 9:1,v/v). Then, the combined solu-tion was tip-sonicated for10 min at 40 W. The resultingblack solution was heated at80 8C for 1–2 min and sonicatedin an ultrasound bath for 30 s.Then, the solution was left tostand at room temperature toform a gel. The gel formationwas confirmed by using the “in-verted test tube” method. Thesame procedure was used tomake the Pr-SWCNT-contain-ing hybrid hydrogel, and theratio of gelator to RGO/SWCNTs was 10:1 (w/w). Thesehydrogels were stable at roomtemperature. We also checkedthe dispersions of RGO and Pr-

SWCNT in solution; both RGO and the Pr-SWCNTs afford-ed black, clear solutions with no suspended particles visible.Recently, Mezzenga and co-workers demonstrated the dis-persion of carbon nanotubes and graphene by using amyloidfibrils (from proteins) to make nanohybrid materials.[14] Thedispersion of RGO and Pr-SWCNTs was achieved owing tothe presence of p moieties within the gelator, which inter-acted with the p wall of the SWCNTs and with the p planeof the RGO. These hybrid hydrogels were thoroughly char-acterized by using various microscopic techniques (TEM,

Figure 1. Macroscopic views of native and hybrid hydrogels: a) before, and b) after exposure to UV light at365 nm. Top: molecular structure and photograph of the native gel (gelator 1).

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AFM, field-emission SEM, and fluorescence microscopy),rheometry, Raman spectroscopy, and X-ray diffraction.

Interactions of pristine SWCNTs and RGO with the gelator :Zeta-potential measurements were carried out to investigatethe dispersion of RGO and Pr-SWCNTs within the hydroge-lator solution in aqueous media. The zeta potential for thedispersion of RGO was �50.4 mV, whilst the zeta potentialfor the dispersion of pristine SWCNTs, the value was�47.6 mV in aqueous media in presence of gelator mole-cules. A higher magnitude (typically �25 mV) of the zetapotential implies that the colloid system exhibits improvedstability against coagulation.[15] High zeta potentials for thedispersions of RGO and Pr-SWCNTs in the presence of thegelator in aqueous media indicate good dispersion and sta-bility of these carbon-based nanomaterials in water in thepresence of gelator molecules.

Thermal study of the hydrogels : The gel-melting tempera-tures (Tgel) were calculated by heating the gels in an electri-cally controlled digital water bath at a heating rate of 1 8Cevery 5 min by using the vial-inversion method. These Tgel

values were plotted against the concentration of the gelatormolecules (Figure 2). Initially, Tgel increases sharply andthen more slowly until it reaches the saturation point.

Fluorescence study : The hydrogelator (Figure 1) contains afluorescence-active pyrene moiety. It is interesting to notethat its hydrogelation behavior can be monitored by usingfluorescence spectroscopy. The fluorescence of the gelatormolecules in the solution state and the gel state are differ-ent. In the sol state, the hydrogelator showed a fluores-cence-emission peak at 417 nm, along with two small humpsat 447 and 485 nm. However, in the gel state, the fluores-cence-emission peak was observed at 485 nm (SupportingInformation, Figure S4). Interestingly, after the formation ofthe hydrogel, the fluorescence-emission peak was shiftedfrom 417 nm to 485 nm. This red-shifting clearly shows theaggregation of the hydrogelator molecules to form the hy-drogel.[16] The emission peak at 485 nm is due to the forma-tion of an excimer complex that involves pyrene moieties.[16]

Raman spectroscopic analysis : Raman spectroscopic analysis(Supporting Information, Figure S5) was performed by usingvery dilute solutions of gelator 1 that contained Pr-

SWCNTs, RGO, and both Pr-SWCNTs and RGO. The solu-tion of gelator 1 that contained Pr-SWCNTs showed a peakat 1587 cm�1, which corresponded to the tangential mode ofthe CNTs. The solution of gelator 1 that contained RGOshowed peaks at 1337 and 1607 cm�1, which corresponded tothe D and G bands. However, the solution of gelator 1 thatcontained both RGO and Pr-SWCNTs showed peaks at1343 and 1589 cm�1, which corresponded to the D andG bands of graphene and the tangential mode of Pr-SWCNTs.

Microscopy studies : Figure 3 a shows the morphology of thenative hydrogel, which indicates the formation of a networkof nanofibers. The TEM image (Figure 3 a) shows that the

widths of these nanofibers are within the range 15–40 nm.The AFM image (Figure 4 a) shows that the widths of thesefibers are within the range 2–4 nm. Figure 3 b and Figure 4 bshow the presence of two distinctly different nanoscopic ar-chitectures, nanofibrils (owing to native gelator 1) and nano-sheets (owing to the RGO), in the hybrid gel system. Fig-ure 3 c shows the morphology of the SWCNT-containinghybrid gels, which indicates the presence of both bent andthin nanotubes (obtained from the SWCNTs), as well as rel-atively wide nanofibers (obtained from the native gelators).Interestingly, these two types of architectures are different,as evident from the widths and natures of these two nano-features in their corresponding TEM images (Figure 3 c andthe Supporting Information, Figure S6). The widths of thenative gelator fibers are within the range 22–38 nm, whilstthe widths of the SWCNTs are 15–25 nm. Figures 3 d and 4 cshow the presence of three different nanofeatures (nano-tubes, nanofibers, and nanosheets) in a trihybrid gel thatconsisted of gelator 1, SWCNTs, and RGO. The incorpora-tion of both SWCNTs and RGO within a native hydrogel

Figure 2. Tgel profile of the native hydrogel as the gelator concentration isvaried.

Figure 3. a) TEM image of the native xerogel. b) TEM image of theRGO-containing hybrid xerogel. c) TEM image of the Pr-SWCNTs-con-taining hybrid xerogel. d) TEM image of the hybrid xerogel that con-tained pristine SWCNTs and RGO.

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system is an interesting event in terms of the alignment andoccurrence of different carbon-based nanomaterials, that is,nanotubes and nanosheets, within the gel-based hybridsystem. There have only been a few reports on the inclusionof both SWCNTs and graphene within a single gel net-work.[17] AFM images (Figure 4 c and d, and the SupportingInformation, Figure S7) also revealed the presence of threedifferent nanostructures within the hybrid hydrogel matrix.

X-ray diffraction analysis : The X-ray diffraction pattern ofthe xerogel is shown in the Supporting Information, Fig-ure S8. A sharp peak at 2q= 20.068, which corresponds to ad value of 4.42 �, indicates the presence of hydrogen-bond-ing within the self-assembled hydrogel. Interestingly, thepeak at 2q=25.658, which corresponds to d=3.48 �, sug-gests the presence of p�p stacking interactions within thepyrene p planes of the hydrogelator molecules in the assem-bled gel state.[3b] From the X-ray diffraction analysis, it isevident that the gelator molecules are self-assembledthrough hydrogen-bonding interactions between the amidemoieties (see the FTIR section in the Supporting Informa-tion) and p�p stacking interactions. By combining the FTIRanalysis (see the Supporting Information) with the X-raydiffraction study, we can conclude that not only intermolec-ular hydrogen-bonding and ionic interactions, but also p�p

stacking interactions are the driving force for hydrogelation.The X-ray diffraction pattern of the hydrogelator in the wet

gel state is different from that in xerogel state. Two broadpeaks (Supporting Information, Figure S8) that are centeredat 2q=13.258 and 2q=30.458 are due to significant scatter-ing from the solvent molecules (water) that are entrappedwithin the fibrillar network structure of the hydrogel.[18]

Therefore, it is very difficult to correlate the data that areobtained from the wet gel state with the self-assembly pat-tern of hydrogel formation that is obtained from the X-raydiffraction pattern of the xerogel state.[18] The peak at 2q=

25.698, which corresponds to d=3.46 �, for the RGO-con-taining hybrid hydrogel, the peak at 2q=25.478, which cor-responds to d= 3.49 �, for the Pr-SWCNT-containing hybridhydrogel, and the peak at 2q=25.578, which corresponds tod= 3.46 �, for the hybrid hydrogel that contained bothRGO and Pr-SWCNT (Supporting Information, Figure S8)suggest the presence of p�p stacking interactions of thepyrene p planes of the hydrogelator molecules with thep planes of the graphene, as well as with the p walls of thePr-SWCNTs, in the assembled hybrid gel state.

Rheological study : Figure 5 shows plots of G’ and G’’ versusangular frequency (w). G’ and G’’ do not vary significantlywithin the applied range of angular frequency (w) and G’>G’’ indicates the presence of a stable and stiff gel-phase ma-terial. Interestingly, the stiffness of the native gel increasedupon the incorporation of Pr-SWCNTs, RGO, and both Pr-SWCNTs and RGO into the hybrid gel system. From the

Figure 4. a) AFM image of the native xerogel. b) AFM image of the RGO-containing hybrid xerogel. c) AFM image of the hybrid xerogel that containedPr-SWCNTs and RGO. d) Magnified view of (c). e) Height profile of the RGO nanosheet. (A color version of this figure is available in the SupportingInformation.)

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Supporting Information, Table S2, the order of the rigidityof the gels is as follows: Hybrid gel that contained gelator 1and RGO>hybrid gel with gelator 1 and Pr-SWCNTs>hybrid gel that contained gelator 1, Pr-SWCNTs, andRGO>native gel that contained gelator 1. There are a fewreports on the augmentation of stiffness of the gel upon theinclusion of carbon-based nanomaterial(s).[19]

Step-strain experiment : A simple step-strain experiment[20]

was performed in several steps: First, the gel was subjectedto a constant strain at 0.1 % (step 1). Then, the strain was in-creased from 0.1 % to 10 % and kept at 10 % strain for afew minutes to completely break the gel (step 2). Finally,the strain was decreased from 10 % to 0.1 % and kept at0.1 % for a few minutes to observe the gel-restoration kinet-ics (step 3, Figure 6). The angular frequency was kept con-

stant at 1 rad s�1 throughout the experiment. When the con-stant strain was 10 %, the loss modulus was larger than thestorage modulus (G’’>G’), thus indicating the sol-likenature of the soft material during the second step. Thenative hydrogel regained 93 % of its original strength andthe RGO-containing hybrid hydrogel regained 100 % of itsoriginal strength (initial G’ at a constant strain of 0.1 %)during the last step within a few minutes after decreasing

the strain from 10 to 0.1 %. The recovery time was estimat-ed to be 3 min (TR1) for the native hydrogel, whilst the com-plete recovery of the hybrid gel required only 1 min 40 s(TR2). The recovery time was calculated from the point ofwithdrawal of 10 % strain, after the complete breakage ofthe gel, until the point that the gel phase was repaired againand restored its 100 % (or maximum) strength in the thirdstep of the experiment.

Thixotropic behavior : There are only a few examples oflow-molecular-weight functional hydrogels with moldable,thixotropic, and self-healing properties.[8] This hydrogelshowed thixotropic properties at 1 % w/v. First, the hydrogelwas formed and then the gel was broken by applying me-chanical shaking (shaking time: 30 s to 1 min). The self-as-sembled, viscous solution was remarkably transformed intoa gel within 7 min on standing and complete withdrawal ofthe mechanical stress (Figure 7). Interestingly, the gel-recov-

ery time was successfully tuned by incorporating carbon-based nanomaterials into the native gel to form a hybridsystem. It is evident from the gel-recovery-time experimentthat the reformation of the hydrogel is significantly short-ened from 7 min to 2.48 min by the incorporation of bothSWCNTs and RGO within the native hydrogel (Figure 7and the Supporting Information, Table S3). The gel-recoverytime was also shortened from 7 min to 3.25 min and 3 minby the individual inclusion of RGO and Pr-SWCNTs, re-spectively, within the native gel. A probable reason for thefaster gel recovery in the hybrid gels is due to the presenceof more particles per unit volume that are moving and inter-acting with themselves than that in the native gel. The im-proved physical properties are likely due to the additionalcross-linking points that are provided by the p�p stacking

Figure 5. Plots of the angular frequency (w) versus the storage modulus(G’) and loss modulus (G’’) of different hydrogels.

Figure 6. a) Plots of the storage modulus (G’) and the loss modulus (G’’)of the native hydrogel versus time at different step strains. b) Plots of thestorage modulus (G’) and the loss modulus (G’’) of the RGO-containinghybrid hydrogel versus time at different step strains.

Figure 7. a) Thixotropic recovery of the native hydrogel, and b) thixo-tropic recovery of the RGO-containing hybrid hydrogel.

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interactions in the RGO and Pr-SWCNTs. It is interestingto note that, per unit time and per unit volume in thebroken gel (self-assembled sol state), the number of inter-acting particles follows the order: Hybrid gel with gelator 1,RGO, and Pr-SWCNTs>hybrid gel with gelator 1 and Pr-SWCNTs�hybrid gel with gelator 1 and RGO>native gelwith gelator 1. This result explains why self-healing is fastestin the hybrid gel with RGO and Pr-SWCNTs and is slowestin the native gel.

Self-healing behavior : This native hydrogel also showed aninteresting self-healing property. The hydrogel was madewithin a glass vial (volume: 1.5 mL) in two different colors:One part of the gel was in its natural color, whilst the otherpart of the gel was turned to a pink color by mixing rhoda-mine B within the hydrogel. Then, these two different slicesof gel was kept in contact with each other by applying out-side support (Figure 8). After resting for 20 min, these two

differently colored gel slices were joined together (Figure 8).The joined slice did not separate on pulling the gel slicefrom two different ends up to a certain limit and one couldeven hold the gel slice vertically by hand. This healing be-havior of the hydrogel did not result from the sticky natureof the material and the self-healed material could withstandsome pressure if pulled from both ends (Figure 9). The geldid not break by lifting the slice. This experiment was alsoperformed in a reverse manner. The self-healing experimentwas also performed by using the RGO-containing hybrid hy-drogel (Figure 10). This hybrid gel exhibited self-healing be-havior and it took just under 10 min to heal. One interesting

feature of this study was that the minimum gelation concen-tration and the thixotropic and self-healing properties of thehydrogel were nicely modulated by the incorporation ofcarbon-based nanomaterials (graphene, Pr-SWCNTs, orboth graphene and Pr-SWCNTs) within the native hydrogelsystem.

Moldability : The hydrogel that was derived from 11-(4-(pyrene-1-yl)butanamido)undecanoic acid was found to be afreestanding hydrogel at 1 % w/v. The hydrogel materialcould be kept on any surface without the presence of anycontainer and, interestingly, this hydrogel was found to bemoldable, that is, it took the shape of the container in whichit was present and it could be molded to give any type ofshape as required (Figure 8).

Current–voltage (I–V) measurements : It was interesting tomeasure the electrical properties of the carbon-nanomateri-als-containing hybrid hydrogels because all of these gelsshowed self-healing properties. Electrical measurements[21]

were performed by using xerogel pellets of the RGO-con-taining hybrid gel, the Pr-SWCNTs-containing hybrid gel,and the hybrid gel that contained both RGO and Pr-SWCNTs. The I–V profiles of the native gel and of all ofthese hybrid gels are shown in the Supporting Information,Figure S9. These I–V profiles indicate the electroactive andsemiconducting nature of each of these hybrid hydrogels.

Conclusion

An amino-acid-based (11-(4-(pyrene-1-yl)butanamido)unde-canoic acid), self-repairing hydrogel has been discoveredunder basic pH conditions. Interestingly, the self-healing,thixotropy, and stiffness of the native hydrogel could be suc-cessfully modulated by incorporating carbon-based nanoma-terials, including graphene, Pr-SWCNTs, and both grapheneand Pr-SWCNTs, within the native gel system. The self-re-covery time of the gel was shortened by the inclusion ofRGO, Pr-SWCNTs, or both RGO and Pr-SWCNTs. More-over, hybrid gels that contained RGO and/or Pr-SWCNTsexhibited interesting semiconducting behavior (see the I–Vstudy above and the Supporting Information, Figure S9).

Figure 8. A bridge that is constructed by connecting together two blocksof native hydrogel can be horizontally suspended and held vertically. (Acolor version of this figure is available in the Supporting Information.)

Figure 9. Pulling of the gel slice from two different ends shows the intact-ness of the united gel slice after self-healing. (A color version of thisfigure is available in the Supporting Information.)

Figure 10. Two blocks of the RGO-containing hybrid hydrogel can jointogether under support.

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The injectable and self-healing nature of the native hydrogelholds promise for the development of new advanced materi-als, the properties of which can be modulated by the incor-poration of carbon-based nanomaterials or metal or semi-conducting nanoparticles.

Experimental Section

Materials : 1-pyrenebutyric acid, 11-aminoundecanoic acid, rhodamine B,and graphite powder were purchased from Aldrich. HOBt, DCC, sodiumdihydrogen phosphate, disodium hydrogen phosphate, NaOH, MeOH,silica gel (100–200 mesh), Et2O, petroleum ether, EtOAc, DMF, sodiumnitrite, potassium permanganate, conc. H2SO4, H2O2, and ascorbic acidwere purchased from SRL (India). Single-walled carbon nanotubes werepurchased from Cheap Tube (US). The water used in all experiments wasof Millipore Milli-Q grade.

Synthesis of 11-(4-(pyrene-1-yl)butanamido)undecanoic acid methylester : 1-Pyrene butyric acid (2.88 g, 10 mmol) was dissolved in dry DMF(5 mL). Then, 1-hydroxybenzotriazole (HOBt·H2O, 1.53 g, 10 mmol) wasadded to the solution and the reaction mixture was cooled in an ice-water bath. 11-Amino undecanoic acid methyl ester was isolated by neu-tralization of the corresponding methyl ester hydrochloride (3.66 g,17 mmol), extraction with EtOAc, and concentration to 10 mL. Then, theextract was added to the reaction mixture, followed immediately by N,N’-dicyclohexylcarbodiimide (DCC, 2.06 g, 10 mmol). The reaction mixturewas allowed to warm to RT and stirred for 48 h. From the reaction mix-ture, dicyclohexyl urea (DCU) was filtered off. The organic layer wassuccessively washed with 1m aqueous HCl (3 � 50 mL), brine (2 � 50 mL),a 1m aqueous solution of sodium carbonate (3 � 50 mL), and brine (2 �50 mL), dried over anhydrous sodium sulfate, and evaporated in vacuo.The residue was subjected to column chromatography (petroleum ether/EtOAc) to yield the pure product (4.12 g, 8.48 mmol, 85% yield).1H NMR (300 MHz, CDCl3): d =8.28 (d, J =9 Hz, 1 H; pyrene-H), 8.15(d, J=7.8 Hz, 2H; pyrene-H), 8.09 (d, J =8.4 Hz, 2H; pyrene-H), 8.02–8.01 (m, 3 H; pyrene-H), 7.84 (d, J=7.8 Hz, 1H; pyrene-H), 5.41 (br s,1H; amide-NH), 3.65 (s, 3 H; OCH3), 3.40–3.37 (m, 2H; CH2), 3.23–3.17(m, CH2; 2H), 2.29–2.17 (m, 6H; 3 CH2), 1.60–1.56 (m, 2H; CH2), 1.44–1.40 (m, 2H; CH2), 1.23 ppm (br s, 12H; 6CH2); HRMS: m/z calcd (%)for C32H39NO3: 485.66; found: 486.2427 [C24H46N2O5+H]+, 508.2255[C24H46N2O5+Na]+; elemental analysis calcd (%) for C32H39NO3 (485.66):C 79.14, H 8.09, N 2.88; found: C 78.87, H 8.46, N 3.00.

Synthesis of 11-(4-(pyrene-1-yl)butanamido)undecanoic acid : To a solu-tion of 11-(4-(pyrene-1-yl)butanamido)undecanoic acid methyl ester(3.88 g, 8 mmol) in MeOH (100 mL) was added 2 m aqueous NaOH(8 mL) and the progress of the saponification reaction was monitored bythin layer chromatography (TLC). The reaction mixture was stirred and,after 10 h, MeOH was removed under vacuum and the residue was takenup in water (50 mL) and washed with Et2O (2 � 50 mL). Then, the pHvalue of the aqueous layer was adjusted to pH 2 by the dropwise additionof 1m HCl and the layer was extracted with EtOAc (3 � 50 mL). Thecombined extract was dried over anhydrous sodium sulfate and evaporat-ed under vacuum to yield a white solid.

Yield: 3.01 g (6.4 mmol, 80%); 1H NMR (300 MHz, [D6]DMSO): d=

11.93 (br s, 1 H; COOH), 8.35 (d, J =9 Hz, 1 H; pyrene-H), 8.26–8.21 (m,2H; pyrene-H), 8.20–8.17 (m, 2H; pyrene-H), 8.01–8.01 (m, 3 H; pyrene-H), 7.91 (d, J =7.8 Hz, 1H; pyrene-H), 7.77 (br s, 1 H, amide-NH), 3.32–3.26 (m, 6 H; 3CH2), 3.07–3.01 (m, 2H; CH2), 2.23–2.09 (m, 4 H; 2CH2),2.04–1.97 (m, 2H; CH2), 1.14–1.38 (m, 6 H; 3 CH2), 1.19–1.16 ppm (m,6H; 3 CH2); 13C NMR (75 MHz, [D6]DMSO): d =174.4 (COOH), 171.6(amide C=O), 136.5–123.4 (aromatic carbon atoms), 35.0–24.4 ppm (ali-phatic carbon atoms); HRMS: m/z calcd (%) for C31H37NO3: 471.63;found: 472.3484 [C31H37NO3+H]+, 494.3411 (major peak)[C31H37NO3+Na]+; elemental analysis calcd (%) for C31H37NO3 (471.63):C 78.95, H 7.91, N 2.97; found: C 78.81, H 8.27, N 3.13.

Fluorescence spectroscopy : The fluorescence spectra were recorded on aPerkin–Elmer LS55 Fluorescence Spectrometer. The gel samples wereplaced in a quartz cell (path length: 1 cm) and excited at 388 nm. Theemission scans were recorded from 398–600 nm by using a slit width of2.5 nm for both excitation and emission slits in sol and gel states.

Fluorescence microscopy : The gel-phase material was placed onto a glassmicroscope slide and examined under a fluorescence microscope (OLIM-PUS BX-61) at � 40 magnification.

TCSPC study : TCSPC measurements were performed on a Horiba JobinYvon IBH instrument with an MCP PMT Hamamatsu R3809 detector;all data were fitted by using Data Station v2.3. The fluorescence-decaytime were fitted by using the Equation (1), where b is the correction forthe baseline, n is the number of discrete emissive species, and ai and ti

are the pre-exponential factor and the excited-state fluorescence lifetimeof the ith component, respectively.

PðtÞ ¼ bXn

¼1ai expð�t=tiÞ ð1Þ

Transmission electron microscopy (TEM) study : The morphologies of thenative and hybrid hydrogels were investigated by using a transmissionelectron microscope. The samples were prepared by depositing a drop ofthe highly diluted gel-phase materials onto a TEM grid (300 mesh Cugrid) that was coated with Formvar and carbon film. Then, the grid wasdried under vacuum at 30 8C for two days. Images were recorded on aJEOL electron microscope at an accelerating voltage of 200 kV.

Atomic force microscopy (AFM): The morphologies of the highly dilutedhydrogels were investigated on an atomic force microscope in tapping-mode. AFM studies were conducted by placing a small amount of thewet hydrogel (or hydrogel-graphene/hydrogel-SWCNT hybrid or hydro-gel-graphene-SWCNT hybrid) at a very dilute concentration on a micafoil. Then, the material was allowed to first dry in air by slow evaporationand then under vacuum at RT for two days. Images were recorded on anAuto probe CP Base Unit di CP-II instrument (Model AP-0100).

NMR spectroscopy : NMR studies of all of the synthetic amino-acid de-rivatives were carried out on a Bruker ADVANCE 300 MHz spectrome-ter at RT. The concentrations of the compounds were within the range 1–10 mmol in CDCl3 or [D6]DMSO.

Mass spectrometry : Mass spectra of all of the synthetic amino-acid deriv-atives were recorded on a Qtof Micro YA263 high-resolution mass spec-trometer.

Zeta potentials : The zeta potentials of the RGO and SWCNTs that weredispersed in the hydrogel solutions were determined by using a Malvernparticle-size analyzer (Model ZES 3690 Zetasizer Nano ZS 90). For theseexperiments, a dilute RGO- and SWCNT-dispersed hybrid hydrogelsystem was used.

Raman spectroscopy : For the Raman spectroscopic analysis, samples ofRGO and the Pr-SWCNTs that contained very dilute solution of the gel-ACHTUNGTRENNUNGator (21.20 mm gelator 1) were kept in a cuvette and measured by irradiat-ing with laser light (514.5 nm, scattering angle: 908, integration time: 10 s,20 scans, 75 mW) on a Horiba Jobin Yvon instrument (Model T64000).

FTIR spectroscopy: The FTIR spectra were recorded on a Shimadzu(Japan) FTIR spectrophotometer. In the solid-state FTIR studies, thepowdered samples were mixed with KBr to prepare the thin films.

X-ray diffraction : For the X-ray diffraction analysis of the gelator, the xe-rogel was placed onto a glass slide. The analysis was carried out on anX-ray diffractometer (Bruker D8 Advance) with a parallel-beam opticsattachment. The instrument was operated at 35 kV and 30 mA by usingNi-filtered CuKa radiation and was calibrated with a standard siliconsample. For the 5–308 scans, a Lynx-Eye super-speed detector was used(scan speed: 0.3 s, step size: 0.028).

Rheology : The rheology experiments were performed on an AR 2000 ad-vanced rheometer (TA Instruments) by using the cone-plate geometry ina Peltier plate (plate diameter: 40 mm, cone angle: 48). The continuous-strain experiment was carried out by first breaking the gel after the appli-cation of a continuous strain of 0.1–50 % (Supporting Information, Fig-ure S10). After the complete rupture of the gel, as denoted by G’’>G’,gel recovery was observed at a constant strain of 0.1%. The process was

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carried out over two cycles to check the reversibility of the restorationprocess. The entire study was performed by maintaining a constant angu-lar frequency of 1 rad s�1.

I–V measurements : The conductivities of the sandwiched xerogel sam-ples were measured on an electrometer (Keithley, model 617) at 26 8C.

Acknowledgements

S.R. and A.B. gratefully acknowledge the CSIR, New Delhi (India), forfinancial assistance. We acknowledge the DST (India) for financial assis-tance. We thank Dr.Bappaditya Roy of the Department of Polymer Sci-ence unit, IACS for rheological measurements. We will also thankful toMr. Aniruddha Kundu of the Department of Polymer Science unit, IACSfor current-voltage measurements. We are also very thankful to anony-mous reviewers for their helpful comments.

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Received: April 30, 2013Published online: && &&, 0000

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A. Banerjee et al.

Hydrogels

S. Roy, A. Baral,A. Banerjee* . . . . . . . . . . . . . . . . . . &&&&—&&&&

An Amino-Acid-Based Self-HealingHydrogel: Modulation of the Self-Healing Properties by IncorporatingCarbon-Based Nanomaterials

Physician, heal thyself : An amino-acid-based (11-(4-(pyrene-1-yl)butana-mido)undecanoic acid) self-repairinghydrogel has been discovered underbasic pH conditions. Interestingly, thethixotropy, self-healing, and stiffness ofthe gel were successfully tuned byincorporating carbon-based nanomate-rials, such as reduced graphene oxide(RGO) and/or pristine single-walledcarbon nanotubes (Pr-SWCNTs).

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