Functionalized

Article pubs.acs.org/Langmuir

Reinforcement of a Sugar-Based Bolaamphiphile/Functionalized Graphene Oxide Composite Gel: Rheological and Electrochemical Properties Ji Ha Lee,† Junho Ahn,† Mitsutoshi Masuda,‡ Justyn Jaworski,§ and Jong Hwa Jung*,† †

Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Korea Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562 Japan § Department of Chemical Engineering, Hanyang University, Seoul 133-791 Korea ‡

S Supporting Information *

ABSTRACT: A sugar-based bolaamphiphile/graphene oxide composite hydrogel has been prepared using simple mixing. Unlike the corresponding sugar-based native gel, the composite gel exhibits a fibrillar structure with a 10−20 nm fiber diameter. The composite gel forms an interdigitated bilayer structure incorporating intermolecular hydrogen-bonding interactions. The composite gel formation did not change the beneficial electrical properties of graphene offering the potential for integration of this new material into electronic systems. Interestingly, the mechanical and electrochemical properties of the composite gel are both dramatically enhanced when compared to the native gel, thereby reflecting that the functionalized graphene oxide layers are efficiently intercalated within the composite gel structure.

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INTRODUCTION Graphene is a well-defined two-dimensional structure consisting of carbon atoms. It has received a great deal of attention because of its unique electronic, thermal, and mechanical properties. Recent work has demonstrated that self-assembly is a powerful technique for constructing hierarchical graphene oxide-based nanomaterials with novel functions.1−11 In particular, graphene oxide can easily be functionalized through both covalent and noncovalent bonding, thereby making graphene oxide an important building block for the synthesis of new materials. Very recently, graphene-based hydrogels/or composites have been investigated by several groups for the purpose of producing 2D macroassemblies, which have a variety of possible applications including drug delivery,12 tissue scaffolds,13 bionic nanocomposites,14 and supercapacitors.15−18 For example, the Shi group has recently developed graphene oxide/ DNA composite gels, graphene oxide/hemoglobin composite gels,19 and graphene oxide/poly(vinyl alcohol) composite hydrogels.7 A clear need for the creation of gel-based homogeneous functional hybrid systems still exists in current advanced nanomaterials research. A particularly strong requirement of graphene/graphene oxide composite gels is to generate materials with incremental improvements in terms of physical, chemical, mechanical, and/or material properties when compared to the corresponding native gel matrix. For example, if a higher electrochemical capacity could be introduced into © XXXX American Chemical Society

sugar-based gels, the resulting gel material may be applicable to electrochemical sensing or even drug delivery, given their high expectation for biocompatibility. In this work, we show a novel material usage of graphene oxide as a functionalized intercalating component of composite gels. We find that fibrillar graphene structures form within the composite providing not only impressive improvements in the mechanical strength of the hydrogels but also introducing useful electrical properties analogous to reduced graphene oxide. In fact, this work reveals that composite hydrogels can make use of the beneficial electrical properties of graphene which was also found to greatly enhance the mechanical strength. Here, we provide the first example of a sugar-based bolaamphiphile/βglucopyranoside-functionalized graphene oxide composite hydrogel (SB/G-GO); furthermore, we present a detailed investigation of the rheological properties of this material.

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EXPERIMENTAL SECTION

Instruments. 1H and 13C NMR spectra were measured on a Bruker ARX 300 apparatus. IR spectra were obtained for KBr pellets, over the range of 400−4000 cm−1, with a Shimadzu FT-IR 8400S instrument, and mass spectra were obtained by a JEOL JMS-700 mass spectrometer. The absorption spectra of the samples were obtained Received: July 4, 2013 Revised: September 23, 2013

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Scheme 1. Preparation of β-Glucopyranoside-Functionalized Graphene Oxide (Abbreviated as G-GO)

using a UV−vis spectrophotometer (Hitachi U-2900). AFM imaging was performed in a noncontact mode under the following conditions: scan rate of 1.0 Hz; PPP-NCHR 10 M cantilever (Park systems). The electrochemical impedance spectra (ESI) were collected using a CHI 760D potentiostat (CH Instruments Inc., Shanghai, China) in the frequency range of 10 mHz to 100 kHz with a 10 mV ac amplitude. Cyclic voltammetry measurements were performed using a BASi C3 EF-1085 cell stand with samples coated onto a polished GC electrode in 1.0 M KCl solution with a carbon counter electrode and Ag/AgCl reference electrode. Circular dichroism (CD) spectra were measured on a JASCO J-715 spectrophotometer (cell diameter 10 mm) at 25 °C. Characterization. The specimens were examined with a JEOL JEM-2010 transmission electron microscope operating at 200 kV using an accelerating voltage of 100 kV and a 16 mm working distance. Scanning electron micrographs of the samples were taken with a field emission scanning electron microscope (FE-SEM, Philips XL30 S FEG). The SEM accelerating voltage was 5−15 kV, and the emission current was 10 μA. The SB/G-GO composite gel and native gel 1 were freeze-dried to observe the SEM image and XRD pattern in vacuum at −5 °C. For XRD measurements, the freeze-dried sample was measured with a Rigaku type 4037 diffractometer using graded d-space elliptical side-by-side multilayer optics, monochromated Cu Kα radiation (40 kV, 30 mA), and an R-Axis IV imaging plate. The typical exposure time was 10 min with a 150 mm camera length. Freeze-dried samples were dried to constant weight and then loaded into capillary tubes. UV−vis absorption spectra of the sugar-based bolaamphiphile/graphene oxide composite hydrogels were observed at room temperature in the range of 200−800 nm. UV−vis absorption spectra of the sugar-based

bolaamphiphile/functionalized graphene oxide composite hydrogels were observed at various ratios (0.1−0.5) of bolaamphiphile 1:functionalized graphene oxide. Rheological Measurements. Rheological properties were observed on freshly prepared sugar-based bolaamphiphile/functionalized graphene oxide composite hydrogels using a controlled stress rheometer (AR-2000ex, TA Instruments Ltd., New Castle, DE). A cone-type geometry of 40 mm diameter was employed throughout. The dynamic oscillatory measurement was performed at a frequency of 1.0 rad s−1. The following tests were conducted: increased amplitude of oscillation up to 100% apparent strain on shear, frequency sweeps at 25 °C (from 0.1 to 100 rad s−1). Unidirectional shear routines were performed at 25 °C covering a shear-rate regime between 10−1 and 103 s−1. Mechanical spectroscopy routines were completed with transient measurements. Preparation of Composite Gels. The sugar-based bolaamphiphile 1 was dissolved by heating and ultrasonication in aqueous solution at pH =7.0. To this solution was added a small volume of GGO in aqueous solution in varying concentrations from 0.1 to 0.5 by weight with respect to the concentration of the sugar-based bolaamphiphile 1. Preparation of the Glass Carbon Electrode by SB/G-GO Composite Gel. The glass carbon electrode (GCE) was polished on chamois leather with 0.05 M alumina slurry and then washed by sonication in water and absolute ethanol. The cleaned GCE was allowed to dry at room temperature. 60 μL of SB/G-GO composite gel was dropped at the treated GCEs surface and then dried at room B

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Figure 1. (A) Wide-scan XPS spectra of (a) r-GO and (b) G-GO. Deconvolution of the C 1s signal for (B) r-GO and (C) G-GO. temperature for 12 h. After rinsing thoroughly with pure water, electrochemical measurement was performed. Preparation of Sugar-Functionalized Graphene Oxide (GGO) via Covalent Bonding. Graphene oxide (GO) and reduced graphene oxide (rGO) were prepared by a previously reported method (see Supporting Information).20 The azobenzocarboylicdiazonium salt was prepared by the following procedure.21,22 Compound 4 (900 mg) and sodium hydroxide (280 mg, 7 mmol) were added to water (80 mL). Following this, sodium nitrite (526 mg, 7.6 mmol) was added slowly to the solution, and the temperature was maintained at 0−5 °C. This solution was added quickly to HCl (5 mL, 20%, 19.2 mmol), and the resulting red solution was stirred for 1 h. The preparation of G-GO was performed by sonicating 300 mg of reduced GO (r-GO) dispersed in 1.0 wt % aqueous sodium dodecylbenzenesulfonate (SDBS) surfactant. The diazonium salt solution was added to r-GO solution in an ice bath under stirring, and the mixture was maintained in ice bath at 0−5 °C for ∼4 h. Next, the reaction mixture was stirred at room temperature for 4 h. Finally, the resulting suspension was filtered using a 0.2 μm polyamide membrane and was washed with water and then with ethanol, DMF, and acetone.

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between absorbance and concentration was observed in water; this is indicative of good dispersion of G-GO. The FTIR spectra of both r-GO and G-GO are shown in Figure S2. Vibrational peaks for r-GO are present at 1718, 1678, and 1060 cm−1 for CO, C−OH, and C−O, respectively. On the other hand, new peaks for G-GO appeared at 3430, 3012, 1225, 1585, and 1435 cm−1 for the O−H stretch, the aromatic C−H ring stretch, and the C−O and CC ring stretches for the glucopyranoside group, respectively.23,24 Furthermore, the presence of elements in r-GO before and after attachment of 4 was evidenced by the photoelectron lines in the wide-scan XPS spectrum (Figure 1A). Peaks for C 1s and O 1s are observed for r-GO before attachment of 4 (Figure 1B). Deconvolution of the C 1s signal in r-GO shows the presence of −CC−/−C−C− (∼284.6 eV) and −CO− (∼288.3 eV).25 After attachment of the β-glucopyranoside group (4), the C 1s spectrum of the G-GO peaks was also deconvoluted into three components: −C−C−/−CC− at ∼284.7, C−O− C− at 286.4 eV, and −CO− at 287.0 eV. The new −C−O− C− peak for G-GO appeared at 286.4 eV (Figure 1C), indicating that it originated from 4. Figure S3 shows the atomic force microscopy (AFM) image of the G-GO sheet along with its corresponding height profile. It shows that the G-GO monolayer is regular, having a height of 1.0 nm, thus indicating that G-GO consists of a monolayer sheet. Furthermore, ca. 60 wt % of the glucopyranoside group was found to be attached onto the surface of rGO according to TGA measurements. When suspensions of various amounts of G-GO (0−10 mg/ mL) (Scheme 1) were mixed with a solution of 1 (0.5 wt %) by a few seconds of ultrasonication, we were able to obtain sugarbased bolaamphiphile/glucopyranoside-functionalized graphene oxide composite hydrogels (abbreviated as SB/G-GO) as seen in Figure 2a. On the other hand, 1 (0.5 wt %) did not form a hydrogel at the same concentration as the SB/G-GO system; however, a hydrogel was formed when twice the concentration of 1 (1.0 wt %) without G-GO was employed

RESULTS AND DISCUSSION

The preparation of β-glucopyranoside-functionalized graphene oxide (abbreviated as G-GO) is shown in Scheme 1. G-GO was characterized by UV−vis, FTIR, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The UV−vis absorption spectra of GO, r-GO, and G-GO dispersed in water are shown in Figure S1. The redshifted π−π* absorption band of r-GO at 260 nm (relative to the band for GO at 241 nm) is consistent with the partial recovery of the conjugated network. In addition, the presence of absorption at 289 nm, which is a typical wavelength for the β-glucopyranoside group attached to r-GO, strongly indicates that β-glucopyranoside exists on the surface of r-GO. Moreover, G-GO shows enhanced dispersion in water when compared to r-GO itself. The solution dispersibility of G-GO was investigated by UV−vis spectroscopy. A linear relationship C

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Figure 3. CD spectra of (a) G-GO, (b) the native gel 1 (1.0 wt %), and (c, d) the SB/G-GO composite gels; (c) 0.5:0.15, (d) 0.5:0.25.

Figure 2. Photographs of (a) SB/G-GO composite (0.5:0.15% w/w) gel and (c) the native gel 1 (1.0 wt %). SEM images of (b) SB/G-GO composite gel and (d) the native gel 1.

ordered reflection peaks with long periods of 6.31 and 5.70 nm, respectively. These are larger than twice the extended single molecular length of 1 (2.70 nm for 1, obtained from CPK molecular modeling). There appears to be two possible molecular packing models for gel 1. As shown in Figure S5a, one is a bilayered structure that is connected by intermolecular hydrogen bonds via bridging water molecules between the layers. The other incorporates an interdigitated bilayer structure involving interaction between sugar moieties (Figure S5b). It is clear that the molecular packing structure of gel 1 would favor the latter according to our XRD observation (Figure S4). On the other hand, the d value for the SB/G-GO composite gel is longer than that for gel 1, in keeping with the intercalation of G-GO via intermolecular hydrogen-bonding interactions. The XRD observation thus provides strong evidence for intercalation of G-GO as shown in Figure S5b. We were successful in observing evidence for the presence of an intermolecular hydrogen-bonding interaction in the SB/GGO composite gel by FTIR spectroscopy (Figure S6). The −OH vibration bands of the composite gel yielded strong bands at 3438 and 3285 cm−1. On the other hand, the −OH vibration bands for G-GO were present at 3418 and 3264 cm−1, respectively. The wavenumbers of those bands are expected to be directly related to the presence or absence of hydrogenbonding interactions in the system. In the case of a transition from a free −OH to an associated −OH, the −OH bands of the SB/G-GO composite gel are shifted to higher wavenumbers compared to those for gel 1 in accord with significant hydrogen bonding being present in the SB/G-GO composite. The sol−gel transition temperature (Tgel) of SB/G-GO was measured using differential scanning calorimetry (DSC) (Figure S7) in an attempt to gain insight into the enhanced stability of the SB/G-GO composite gel. The SB/G-GO composite gel exhibited a sharp phase transition at 165 °C, which indicated an endothermic reaction. On the other hand, the sol−gel temperature of gel 1 itself was found to be 145 °C. Thus, G-GO in the SB/G-GO composite gel could well be acting as an intercalator that stabilizes the SB/G-GO composite gel. In addition, we measured the decomposition temperatures of G-GO, the native gel 1, and SB/G-GO composite gel by TGA (Figure S8). The decomposition temperature of SB/GGO composite gel is almost the same as that of the native gel 1 due to decomposition of sugar groups.

(Figure 2c). This result indicates that the gelling ability of 1 is reinforced in the presence of G-GO. In view of this, we investigated the gelation ability of 1 as a composite in the presence of an increasing amount of G-GO. Compound 1 was shown to form a gel with less than 0.5 equiv of G-GO, in keeping with the glucopyranoside moiety of G-GO being quantitatively bound to the sugar moiety in 1 by intermolecular hydrogen-bonding interactions. The morphology of the SB/G-GO composite hydrogel has been studied using scanning electron microscopy (SEM), and the result is shown in Figure 2b. The morphology of SB/G-GO is quite different from that of G-GO. The SB/G-GO shows a costructure consisting of both fibrillar (with 75−80 diameter) and sheet (with a width of 1.0−1.2 μm) arrangements. It is also quite different from that of the native gel 1 (Figure 2d) and from G-GO itself. The results suggest that G-GO is bound to the bolaamphiphilic gelator 1 by intermolecular hydrogen bonds. A discussion of the intermolecular hydrogen-bonding interaction between the glucose moiety of G-GO and 1 is presented later in the article. We also determined the CD spectra of G-GO, SB/G-GO composite gel, and native gel 1 in an attempt to detect any chiral assembly in the structures. As shown in Figure 3, the CD spectrum of G-GO is almost silent. On the other hand, the CD spectrum of SB/G-GO is strongly positive at 225 nm, indicating that gelator 1 in SB/G-GO adopts a chiral assembled structure involving intermolecular hydrogen-bonding interactions. The peak intensity of the SB/G-GO composite gel at a 1:0.5 ratio is stronger than that of the composite gel at a 1:0.3 ratio. Clearly, chiral packing between SB and G-GO is maintained in each gel. The CD spectrum of the SB/G-GO composite gel was compared to that of the native gel 1. The CD intensity of SB/G-GO composite gel is enhanced drastically compared to that for gel 1, in keeping with a wellordered chiral structure being present in this latter gel. X-ray diffraction (XRD) patterns for the SB/G-GO composite gel and gel 1 were measured in an attempt to obtain information about the molecular assembly in relation to gelation (Figure S4). The small-angle diffraction patterns for the SB/G-GO composite gel and gel 1 showed at least four D

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chemical properties to those of the reduced graphene oxide electrode material (and is superior in this regard to GO and KCl). The results indicated that the electrochemical properties of rGO are preserved in the composite material. As for the present system, similar nanocomposites with graphene have typically exhibited a characteristic rectangular curve without distinct redox peaks. Since formation of the composite gel did not markedly affect the electrochemical properties of the reduced grapheme oxide, there remains a clear potential for its future application in integrated electronic systems.26 In order to gain more information on the electrical properties of the composite gel, we utilized electrochemical impedance spectroscopy by applying a small alternating voltage across the native gel 1 as well as the SB/G-GO composite gel (1:05 wt %). The Nyquist plot in Figure S9 reveals the impedance behavior of these samples as represented characteristically by the imaginary component and real component of the impedance as obtained over various frequencies (each data point is at a different frequency with higher frequency samples representative of points on the lower left side of the curve). The native gel 1 clearly shows a rapidly vertical rising signal at lower frequencies (toward the right on the curve), indicating

To assess whether our composite material maintains the functional properties of graphene, we initially examined its conductive properties. As observed by cyclic voltammetry (Figure 4), the composite gel exhibited comparable electro-

Figure 4. Cyclic voltammograms of (a) the native gel 1, (b) rGO, and (c) the SB/G-GO composite gel.

Figure 5. (A) Frequency and (B) strain sweep at a frequency of 0.01 rad s−1 or a strain of 0.1 of the native gel 1. (C, E) Frequency and (D, F) strain sweep of G′ and G″ for SB/G-GO composite gels at (C, D) 0.3 wt % and (E, F) 0.5 wt % of G-GO at a frequency of 0.01 rad s−1 or a strain of 0.1. E

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molecular building block. We expect the present study will serve as an inspiration to the design of new hierarchical and functional materials based on graphene.

capacitance behavior. In contrast, the impedance of the SB/GGO composite gel appears as a straight line. In addition, the SB/G-GO composite gel data show good electrical conductivity as well as good contact with the current collectors since no semicircular regions can be seen in the high-frequency domain.27 This promising result suggests the potential of the SB/G-Go composite gel as a possible material for electrochemical sensing applications. Rheological studies of both the composite gel and the native gel 1 were performed to probe whether rheological differences occur between these two gel types. We first used a dynamic strain sweep to determine the appropriate conditions for undertaking the subsequent examination of the gel using dynamic frequency sweep mode. As shown in Figure 5, it was observed that in the linear viscoelastic region (LVR) the storage modulus (G′) is higher than the loss modulus (G″). This is typical for formation of a gel-phase material.28−30 For the composite gel and native gel, the initial storage modulus (G′) was higher than the loss modulus (G″), in keeping with the formation of a gel-phase material, because the SB/G-GO composite gel exhibited a strong elastic-like property. The values of both G′ and G″ of the composite gel were found to dramatically increase in comparison to the native gel. This result is in agreement with G-GO being stabilized through the formation of an intercalated structure in the composite gel. We used the dynamic frequency sweep mode to study the composite gel after setting the strain amplitude at 0.1% (within the linear response region of the strain amplitude). G′ and G″ were almost constant with an increase of frequency from 0.1 to 100 rad/s (Figure 5A). The value of G′ was about 2 times larger than that for G″ over the whole range (0.1−100 rad/s), suggesting that the gel is fairly tolerant to external force. As observed on varying the dynamic strain sweep, the values of both G′ and G″ of the composite gel were ∼100 times larger than that those for the native gel (Figure 5B). The values of both G′ and G″ for SB/G-GO composite gel at 0.5 wt % were enhanced in comparison to the composite gel in the presence of 0.3 wt % of G-GO. These results reflect the stabilization of the composite gel with an almost complete network structure in the presence of 0.5 wt % of G-GO.

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ASSOCIATED CONTENT

S Supporting Information *

Information on the UV/vis and FT-IR of GO, r-GO, and GGO; AFM images of G-GO; PXRD patterns of gel 1 and SB/GGO composite gel; DSC and impedance data of native gel 1 and SB/G-GO composite gel; detailed synthesis protocols. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.H.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from NRF (2012R1A4A1027750 and 2012-002547) and the Environmental-Fusion Project (191-091-004), Korea. In addition, this work was partially supported by a grant from the NextGeneration BioGreen 21 Program (SSAC, grant PJ009041022012), Rural development Administration, Korea.

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CONCLUSIONS In conclusion, we have demonstrated the successful formation of a sugar-based bolaamphiphile/β-glucopyranoside-functionalized graphene oxide composite hydrogel. The β-glucopyranoside-functionalized graphene oxide is shown to efficiently produce a hydrogel by simple mixing with the sugar-based bolaamphiphile. This is the first example of such a sugar-based bolaamphiphile/graphene oxide composite hydrogel. Formation of the sugar-based bolaamphiphile/β-glucopyranosidefunctionalized graphene oxide composite gel dramatically increased the mechanical strength by intercalation of βglucopyranoside-functionalized graphene oxide when compared to the situation for the native gel. Basically, the evidence indicates that the sugar-based bolaamphiphile/β-glucopyranoside-functionalized graphene oxide composite gel forms an interdigitated bilayer structure. The composite gel showed good electrochemical properties. The synthesis of graphene-based composite gels appears to provide an important avenue for the development of new, molecularly defined materials in the areas of biology, medicine, and materials science. Further, the present results enable a more detailed understanding of the selfassembly behavior of functionalized graphene as a 2D F

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