Article pubs.acs.org/JPCC
Single-Layer Graphenes Functionalized with Polyurea: Architectural Control and Biomolecule Reactivity Raymond L. D. Whitby,*,† Alina V. Korobeinyk,† Vladimir M. Gun’ko,‡ Daniel B. Wright,† Gennaro Dichello,† Lauren C. Smith,† Takahiro Fukuda,§ Toru Maekawa,§ Julian R. Thorpe,∥ and Sergey V. Mikhalovsky†,⊥ †
Nanoscience & Nanotechnology Group, Faculty of Science and Engineering, University of Brighton, Brighton, BN2 4GJ, United Kingdom ‡ Chuiko Institute of Surface Chemistry, 17 General Naumov Street, 03164 Kiev, Ukraine § Bio-Nano Electronics Research Centre, Toyo University, 2100 Kujirai, Kawagoe, Saitama 350-8585, Japan ∥ Sussex Centre for Advanced Microscopy, Life Sciences, University of Sussex, Falmer, Brighton, BN1 9QG, United Kingdom ⊥ School of Engineering, Nazarbayev University, 53, Kabanbay Batyr Ave., Astana 010000, Kazakhstan ABSTRACT: The nondestructive, covalent reactivity of single-layer graphene oxide (SLGO) and hydrazine-reduced graphene oxide (rGO) in relation to its 3-dimensional geometry has been previously considered for various chemical reactions. However, the capability of the modified system to undergo additional chemistry is now demonstrated through an in-situ polycondensation reaction resulting in various linear or hyperbranched condensed polymers [e.g., polyureas, polyurethanes, and poly(urea−urethane)-bonded graphenes]. The use of aliphatic diisocyanates as the anchor molecule initially forms star-like clusters of SLGO and rGO, and on in-situ polycondensation reaction with aliphatic diamines, the underlying graphene architecture is further modified into scroll-like domains with extensive intersheet bridging. The use of aromatic isocyanates as bridging molecules keeps the graphene structure flat and is maintained throughout the polycondensation reaction with aromatic diamines. Critical point drying of the graphene− polymer composites shows that changes to the architecture of the composite occur in the solution phase and not through surface tension effects on drying. According to TGA analysis, the aliphatic systems have higher grafted polymer weight proportions of polyurea than the aromatic counterparts and the rGO systems are found to be greater than the SLGO composites. In all experiments, the external surface of the graphene−polyurea macrostructure is demonstrated to be reactive toward biomolecules such as ferritin and is therefore useful toward a solution chemistry development of morphology-controlled graphene-based bionano applications.
1. INTRODUCTION
SLGO can be chemically treated to reduce the acidic oxygen groups (rGO) using, for example, hydrazine and to partially restore electrical conductivity, but where in-sheet erosion has occurred through the SLGO formation conditions, the lattice will remain fractured. Moreover, it was shown that hydrazine reduction does not completely eliminate acidic oxygen groups, only decreasing their number by around 20%, and further chemical treatment of rGO for covalent functionalization and cross-linking purposes, e.g. using high-temperature reflux with thionyl chloride, causes the graphene sheet degradation into smaller fragments and loss of a greater number of acidic oxygen groups (by around 60%) when compared with similar treatment directly on SLGO. Ultimately, this indicates that
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Single-layer graphene (SLG) is a monolayer of sp carbon in a honeycomb lattice that exhibits remarkable electrical conductivity.1,2 A typical method of SLG production involves mechanical peeling of single layers from highly crystalline graphite; however, this methodology does not yet lend itself to industrial scaling and alternative routes have been reported, which include thermal expansion and chemical oxidative exfoliation, i.e. variations on the Hummers method.3,4 SLG produced through this route generally becomes oxidized (SLGO), which often leads to smaller graphene sheets and also introduces a number of oxygen-containing groups, i.e., carboxyl, lactone, and phenol, that directly affect its properties through increase of electrical resistivity.5 This approach is scalable and therefore a preferred and less-expensive production method for large quantities of graphene. © XXXX American Chemical Society
Received: March 4, 2013 Revised: May 10, 2013
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in methanol with spin centrifuging and washing twice with methanol, and finally dried from acetone in vacuo. SLGO was also refluxed in hydrazine solution for 12 h, according to a previous procedure. Typically, SLGO (50 mg) was dispersed in water (15 mL) and hydrazine solution (15 mL of 35 wt % in water) and refluxed under nitrogen (12 h). rGO was purified according to the same procedure for SLGO. 2.2. SLGO Electrostatic Interaction with Ferritin. Purified SLGO and rGO (20 mg) were dispersed into deionized water (20 mL) to obtain a stable suspension, and an excess of ferritin was added (horse spleen, 5.1 mmol) and stirred (24 h, 20 °C) to ascertain electrostatic interaction between SLGO and rGO and ferritin. The mixture was centrifuged, and the top liquid layer was removed through careful decantation. Deionized water was added to the sample and sonicated before reapplying centrifiguation. SLGO−ferritin was dispersed in methanol and dropped onto a TEM grid for analysis. 2.3. SLGO and Carbodiimide Cross-Linking with Ferritin. SLGO and rGO (50 mg) were dispersed in methanol (50 mL), and then 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDAC, 1 mg) was added in sufficiently low quantity to ensure that SLGO did not immediately separate from solution.22,23 However, rGO sediments in methanol quickly on standing. The mixture was stirred (20 °C for 2 h), and then N-hydroxysuccinimide (NHS, 2 mg) was added and the mixture stirred (for another 2 h). The mixture was spincentrifuged and washed with methanol to remove excess EDAC and NHS before being dispersed in methanol. Ferritin (∼0.1 mmol) was separated from aqueous solution and dispersed into DMSO before adding to the SLGO-NHS sample and then stirred (8 h). The mixture was spin-centrifuged and washed with water to separate SLGO−ferritin from free ferritin, though no free ferritin was observed. Samples were separated from aqueous solution with methanol washes and then dropped onto TEM grids from acetone. 2.4. SLGO or rGO, Diisocyanate, and Ferritin. Typically, dried SLGO or rGO (50 mg) was dispersed in dry dimethylformamide (DMF) (50 mL) to which was added either excess 1,6-diisocyanatohexane (hexamethylene diisocyanate, HDI) (6.2 mmol) or 2,4-diisocyanato-1-methylbenzene (toluene diisocyanate, TDI) (6.9 mmol). The mixture was refluxed under nitrogen (90 °C, 4−8 h). SLGO/rGOisocyanate (∼20 mg) was isolated under centrifugation and repetitive washing in dry DMF to ensure removal of the free diisocyanate. SLGO-isocyanate (the HDI or the TDI system) was dispersed in dry DMF, and the temperature lowered to 0− 2 °C. DMF was added to ferritin (∼0.1 mmol), spincentrifuged to separate the ferritin from aqueous solution, dispersed in DMSO, and added to the SLGO-isocyanate and stirred (12 h), allowing the temperature to reach ambient (20 °C). The mixture was spin-centrifuged and washed with water to separate SLGO-isocyanate−ferritin from free ferritin, though no free ferritin was observed. Samples were separated from aqueous solution with methanol washes and then dropped onto TEM grids from acetone. 2.5. SLGO or rGO, Diisocyanate, Diamine, and Ferritin. The procedure in section 2.4 was repeated, isolating SLGOisocyanate (3) and (4) in dry DMF. Either HDI and diaminooctane (DAO) or TDI and p-phenylenediamine (PDA) were added in equal molar proportions (6 mmol of each) at 0−4 °C and stirred (4 h), allowing the temperature to reach ambient (20 °C). Excess diisocyanate and diamines were
the graphene structure has been significantly weakened by the chemical reduction step. Oxygen-containing groups on SLGO provide chemical anchor points that can be exploited through cross-linking reactions, leading to polymer composite additives,6 drug delivery vehicles,7 heavy metal sensors,8 and magnetic composites.9 However, their very presence also leads to conformational changes of SLGO that can be induced through changes in pH, salinity, or temperature of the supporting solution10 or through chemical functionalization with species such as diisocyanates, where aliphatic and aromatic diisocyanates lead to markedly different graphene structures.11 When using aliphatic diisocyanates, the graphene sheets self-assembly into star-like formations whereas use of aromatic diisocyanates maintains a flat-sheet state. It was deduced that the former promotes intrasheet bonding, leading to a self-agglomerated state, and the latter facilitates intersheet bonding, leading to a multisheet agglomerated state. When incorporated into a polymer matrix, the final composite exhibited a marked difference in electrical conductivity, which was dependent on the graphene geometry and may also influence mechanical performance.11 The covalent attachment of proteins to carbon nanomaterials has been of significant interest in the production of sensors, drug-delivery composites, and improved cell−nanomaterial interaction where the nanoscale effects and geometry exert a positive impact on composite properties and performance.12,13 In particular, ferritin is a preferred protein from an analytical point of view of the attachment process as it possesses 4500 iron atoms at its core within a hollow spherical shell and therefore exhibits a dark contrast in transmission electron microscope (TEM) imaging or enables facile detection through EDX scanning analysis.14 Moreover, ferritin has reduction− oxidation capability and when combined with a conductive substrate, e.g. carbon nanotubes, forms a composite suitable for biosensing, biofuel electrodes, and bioapplications.15−17 The combination of nanosized materials with specific proteins has generated composites with improved performance, whether for nerve cell growth using nerve growth factors immobilized on carbon nanotubes or for cancer cell targeted therapy using the high specificity of water-soluble carbon nanotubes with monoclonal antibodies.18,19 However, the intrinsic architecture of carbon nanotubes is inflexible to change through conventional chemical treatment, whereas graphene sheets have been shown to be susceptible to modification.20,21 Therein, it is reasoned that graphenes may prove a useful nanoscale system in its ability to be architecturally shaped to provide better interface at the nano-bio level and for improving performance in the final application, whether as sensors, drug or biomolecule interaction, or cell or tissue growth support. Herein, we report the control of graphene architecture using a facile chemical strategy and the successful cross-linking with ferritin, which will pave the way for other proteins and biomolecules to be covalently bound to the graphene structure.
2. EXPERIMENT 2.1. SLGO Purification and rGO. SLGO was obtained from CheapTubes Inc. and purified by sonicating in distilled water and adjusting the pH to alkali and back to acid with centrifugation conducted at each stage to ensure samples were free of fulvic acids caused by the acid erosion of the graphene planes during preparation. The final sample was washed with deionized water until a neutral pH was achieved, then sonicated B
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removed, and then ferritin was added as per section 2.4 to yield SLGO−polyurea−ferritin. Similar procedures were followed for rGO. 2.6. Electron Microscopy. For TEM observation, samples were dispensed onto holey carbon film-coated support grids and examined on a Hitachi 7100 TEM at 100 kV accelerating voltage. Images were acquired digitally with an axially mounted (2K × 2K pixel) Gatan Ultrascan 1000 CCD camera. For scanning EM (SEM), samples (in methanol) were transferred into porous pots and critical point dried from liquid CO2 and then subsequently examined (uncoated) on a Leica Leo S420 SEM at 10 kV with a probe current of 10 pA.
3. RESULTS AND DISCUSSION 3.1. Chemical Reactivity of Different Graphene Architecture. Purified SLGO22,24 has been used before for a number of carbodiimide-mediated synthesis routes in order to control its macrostructure. However, cross-linking to ferritin has not been reported. The control experiments investigated the electrostatic interaction between SLGO and ferritin in deionized water, which revealed that SLGO remained as individual flat sheets, and no presence of ferritin was observed through TEM imaging (Figure 1a). Previous use of a carbodiimide, EDAC, followed by NHS, on SLGO (in dimethyl sulfoxide, DMSO) resulted in collapse of the sheet structure through intrasheet bonding as well as “salting out” of solution;22 therefore, the concentration of EDAC (0.01 mmol) and NHS (0.01 mmol) was lowered so that SLGO remained as a stable suspension over several hours. For similar experiments with rGO, the graphene system did not result in a stable suspension, but the same concentration of carbodiimide was used. Excess reactants were removed prior to contact with ferritin, which itself was extracted into DMSO to avoid the possibility of unwanted condensation reactions of the carbodiimide. Excess ferritin was removed in washing with water. TEM images revealed that ferritin was present across the surface of SLGO, heavily clustered toward the edges of the sheet and becoming more spaced toward the center of each sheet (Figure 1b). SLGO largely remains in a planar conformation; however, folding and collapse of the sheet is observed at the outer extremities of several sheets, potentially indicating that either limited intrasheet cross-linking through the carbodiimide reaction or geometric rearrangement of the graphene sheet due to addition of EDAC occurred. Despite the reduction of oxygen-containing groups through hydrazine reduction of SLGO to generate rGO, previous work showed using Boehm titration analysis that only around 25% of the oxygen groups were removed, leaving 75% available for reactions, which were tested in line with the SLGO reactions. rGO shows a similar pattern of ferritin attachment using carbodiimide coupling, although the sheets of rGO are agglomerated and highly fragmented due to the conditions of the hydrazine reflux.22 3.2. Polyurea−Graphene Composites. Isocyanate functionalization of graphene has been reported before,25,26 but demonstration of subsequent postfunctionalization has been limited.11 Aliphatic diisocyanate, e.g. 1,6-diisocyanatohexane (HDI), leads to partial or full collapse of the SLGO structure into star-like formations through intrasheet bonding, whereas aromatic diisocyanate, e.g. 2,4-diisocyanato-1-methylbenzene (TDI), leads to stacking of the SLGO sheets but remains in a flat conformation due to surface adsorption and/or inhibited intrasheet connectivity. It was found that after diisocyanate
Figure 1. Transmission electron microscopy images of (a) electrostatic interaction between SLGO and ferritin revealing an absence of ferritin attachment, (b) carbodiimide cross-linking of ferritin to SLGO with limited intrasheet bonding at sheet edges, (c) aliphatic diisocyanate cross-linking of ferritin to SLGO with the onset of intrasheet bonding and deposition of ferritin across the surface (insert), (d) aromatic diisocyanate cross-linking of ferritin to SLGO with intersheet bonding and distribution of ferritin (inset), (e) web structure and (f) scrolling structure of an intrasheet bonded SLGO−aliphatic polyurea composite, (g) distribution of ferritin across the surface of an SLGO−polyurea scroll exhibiting ferritin particles available on the external surface (inset), (h) the planar, agglomerated structure of SLGO−aromatic polyurea composite, and (g) distribution of ferritin particles across the surface and at the edges of SLGO−polyurea (inset).
attachment, ferritin (in DMSO) could be added to the suspension to bind to any pendant isocyanate groups. TEM images revealed that the SLGO conformation as a result of the diisocyanate reaction is retained, in that the addition of ferritin C
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Figure 2. Scanning electron microscopy images of critical point dried (a) SLGO−aliphatic polyurea, (b) SLGO−aromatic polyurea, (c) rGO− aliphatic polyurea, and (d) rGO−aromatic polyurea, which reveal the combined effect of the initial diisocyanate modification and the final in-situ polycondensation reaction on the graphene structure (scale bars = 10 μm).
does not further impact the physicochemical environment leading to change in shape. Aliphatic SLGO−HDI revealed collapse of the graphene structure (Figure 1c) and ferritin molecules are observed to decorate the surfaces of SLGO (Figure 1c inset). Therefore, not all HDI molecules added lead to intrasheet bonding and internalization of the dangling isocyanate molecule, thus leaving free isocyanate groups (with the other anchored to an acidic group on the SLGO surface) available for further reaction, as demonstrated with the presence of ferritin molecules. Aromatic SLGO−TDI retained flat conformations of graphene sheets (Figure 1d) with extensive coverage of the SLGO surfaces with ferritin molecules (Figure 1d inset), indicating dangling isocyanate groups are present for covalent reactions. The SLGO structure can be further controlled through addition of a diamine to the diisocyanate reaction, after reflux, to generate polyurea. Herein, aliphatic or aromatic bridging in the diamine and diisocyanate is maintained for these experiments. Typically, either HDI and diaminooctane (DAO) or TDI and p-phenylenediamine (PDA) were added in equal molar proportions to preformed SLGO−HDI and SLGO− TDI, respectively. In the case of the aliphatic polyurea (HDI−DAO) system, the star-like SLGO−HDI reacts with DAO to form polymer chains that interlink with other SLGO−HDI molecules. TEM shows extensive scrolling of SLGO with webbed areas of SLGO stretching beyond each scroll, which also often appear in turn to be partially scrolled into frond-like projections (Figure 1e). Both the scrolls and the webbing overlap and cross-link with their neighbors yielding an extensive SLGO−polymer composite array (Figure 1f). In several areas it is observed that the SLGO composites have fractured and torn (Figure 1e, over the central hole of the carbon supporting film, the frond-like projections are stretched and some are broken). Herein, it is surmised that part of the macrostructure is formed on drying the SLGO composite on the TEM grid through evaporation of the solvent at room temperature. The presence of holes (Figure 1f) is deemed to occur through a combination of partial overlapping SLGO scrolls and the surface tension forces exerted on the SLGO−polymer composite interface with
neighboring molecules while the solvent evaporates, thus pulling SLGO sheets into a tighter structure. After addition of ferritin to the SLGO−aliphatic urea composite, the SLGO surface was extensively decorated with ferritin either as isolated molecules or in larger clusters (Figure 1g). Collapse and scrolling of the rGO−aliphatic polyurea system also occurred, whereas rGO−aromatic polyurea did not. It appears that even after reduction of surface groups and the erosion of the graphene lattice, rGO is still a high flexible structure capable of intrasheet and intersheet bridging leading to changes in its structure. To ascertain the degree with which drying may have exerted surface tension forces upon the composites during preparation for TEM analysis, leading to changes in graphene structure, all graphene−polyurea samples were synthesized and kept in solvent (after purification) to avoid atmospheric drying out. The solvent was replaced through centrifugation and decanting and then redispersed into methanol. The samples were finally centrifuged in methanol, and the supernatant was removed. The samples were kept wet and transferred, within porous pots, to a critical point drier apparatus and then dried from liquid carbon dioxide to avoid exertion of surface tension under solvent evaporation. Under SEM analysis it is possible to see the stark contrast in structure of the graphene composites according to their functionality (Figure 2). SLGO−aliphatic polyurea reveals fine, wire-like projections, corresponding to the scroll-like processes observed in TEM (Figure 2a). It is also evident that not all the graphene−polymer composites are scrolled, but there are areas which remain open. In the overlap of adjacent sheets, pseudopores are evident. Conversely, SLGO−aromatic polyurea does not feature the wire-like projections but appears to possess flat sheets, presumably extensively agglomerated, but with rough edges that are consistent with the limited folding observed in some TEM images (Figure 2b). A number of graphene−polymer composites do appear to have collapsed, which is atypical of the SLGO system. rGO−aliphatic polyurea is similar to that of the SLGO composite, where the graphene− polymer structure has collapsed into numerous fine wire-like projections and is more extensive than its SLGO counterpart. D
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covalent modification of the graphene surface and the in-situ polycondensation growth of polyurea. When aromatic diisocyanate and diamine were used, SLGO was found predominantly in flat and aggregated structures (Figure 1h). This extensive stacking was not found in TEM preparation in any of the other experiments (or using unfunctionalized SLGO) and is therefore deemed not to be due to overloading of the support grid. Typically 3−8 sheets are found per structure and do not exhibit a morphology that has been consistently observed with intrasheet bonding using carbodiimides and aliphatic diisocyanates. Similar to SLGO− TDI−ferritin, ferritin molecules are observed decorating the surfaces of SLGO. Herein, the formation of polyurea by addition of aromatic diamine does not impact the overall structure of SLGO, and therefore the initial treatment of SLGO with a diisocyanate is the critical step in the chemical control of SLGO morphology. Likewise, rGO modification with an aromatic system follows a similar trend at maintaining planar structures upon isocyanate modification and is not affected by polyurea grafting. FTIR of SLGO−aromatic−polyurea (not shown) shows a similar evolution of the spectra to that of the aliphatic system and is consistent with the modification of the graphene system with polyurea. Likewise, rGO−aliphatic and aromatic isocyanates and polyurea reveal FTIR spectra matching that of SLGO; however, free isocyanate groups at 2262 cm−1 were observed in the isocyanate and the polyurea samples. TGA was used to evaluate the thermal stability in air of the graphene system when transformed from SLGO (and rGO) to the isocyanate to the final polyurea structure. SLGO was airdried at room temperature to avoid any collapse of the structure under heating,10 but residual water was detected when heated up to 100 °C and the TGA curve was corrected (Figure 4a). Around 35 wt % mass loss was recorded from 100 to 230 °C, with a dTG maximum peak around 212 °C and is followed by a gradual mass loss of around 25 wt % up to 520 °C. These indicate the loss of oxygen-containing functional groups from the graphene structure, presumably from highly defective and less defective sites, respectively.27 The major mass loss in the system occurs between 520 and 570 °C, accounting for around 45 wt %, which reflects oxidation of the carbon system and the eventual complete destruction of the graphene lattice. When SLGO is modified with isocyanates, the mass loss profile is subtly altered compared with the pure SLGO system (Figure 4a). For the SLGO−aliphatic isocyanate system, an additional peak arises in the dTG curves between 220 and 420 °C. The 35 wt % mass loss is attributed to the presence of isocyanate groups as well as the loss of oxygen-containing groups on highly defective sites. Finally, after in-situ polycondensation, the presence of polyurea contributes a further two peaks in the dTG curves at 388 and 463 °C and suppression of the sharp intense peak at 600 °C (Figure 4c). The first corresponds to the loss of the polyurea, which is consistent with the oxidative degradation profile of pure polyurea.28 By 440 °C, there is only 5 wt % residual mass of pure polyurea. The second may be coupled to the charring of the polyurea providing a temporary coating to the oxygencontaining groups on less defective sites. The 600 °C peak in the dTG is reflective of the graphene lattice undergoing oxidation and the lowering of intensity due to lower mass fraction of graphene within the composite. In the aromatic system an additional peak in the TGA appears at 509 °C for
This is likely to be indicative of greater reactivity during the isocyanate and polycondensation reactions of the rGO system over that of SLGO. Finally, rGO−aromatic polyurea reveals a macroscale agglomeration of agglomerated graphene−polymer composites sheets. The system appears to remain flat, with no evidence of star or wire-like projections. The overall sheet size frequently appear smaller than those of SLGO due to the etching of hydrazine during their preparation. It can be deduced that the architecture of graphene is transformed in solution by the chemical processes and is not the sole result of surface tension from solvent drying in preparation for TEM. The SEM images feature numerous aspects that correlate well to those with the TEM images, though, as described above, some surface tension has been exerted, through drying from the low surface tension solvent acetone, in the TEM samples leading to damage of the web-like fronds (Figure 1e). The molecular composition of the resulting SLGO−polyurea (and rGO−polyurea) composites was confirmed by FTIR measurements. For SLGO (Figure 3i), the main adsorption
Figure 3. FTIR of (a) SLGO, (b) SLGO−aliphatic isocyanate, and (c) SLGO−aliphatic polyurea.
band around 3367 cm−1 relates to the stretching vibrations of the O−H group. Stretching of the carboxylic (CO) groups appears at 1730 and 1630 cm−1, and C−O stretching vibrations occur at 1225 and 1044 cm−1. When modified with isocyanate (Figure 3ii), peaks associated with pendant isocyanate groups between 2250 and 2270 cm−1 are not present; however, two peaks at 2933 and 2856 cm−1 appear. This is indicative of the termination of free isocyanate groups, possibly through intrasheet and, to a limited extent, intersheet bridging of oxygen-containing groups. In assessing the chemical reactivity of SLGO−isocyanate and SLGO−polyurea samples, ferritin was successfully added, which implies that reactive isocyanate groups were still available even if not readily detected after the sample was prepared for FTIR measurements. The peak around 3300 cm−1 has broadened and intensified compared with SLGO, indicating the presence of N−H stretching in the urethane bond. When in-situ polycondensation was carried out, the FTIR reveals peaks consistent with polyurea (Figure 3iii). Each sample was washed extensively in solvent in order to remove any free, unattached polymer from the graphene surface, and the FTIR spectra appear consistent with the E
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For rGO−isocyanate, the dTG peak at around 330 °C has slightly increased in intensity, corresponding to the presence and loss of isocyanate groups. For rGO−polyurea, the TGA profile changes due to the presence of polyurea, with additional peaks appearing in the dTG at 246, 385, 456, and 536 °C along with the suppression of the graphene oxidation peak around 600 °C. It is possible to estimate the percentage weight of polyurea within the sample by assuming the system’s components oxidize according to their separate TGA profiles. Naturally, error will be present due to any “synergistic” influence provided by the composite, as potentially seen with the coating of the remaining system from polyurea char, which could account for up to 3 wt %. From analyzing the residual mass at 440 °C in all samples, it shows that the SLGO−aliphatic polyurea, SLGO− aromatic polyurea, rGO−aliphatic polyurea, and rGO− aromatic polyurea have a grafted polymer weight proportion ( f wt%) of 14, 8, 30, and 27%, respectively. When comparing between the aliphatic and aromatic systems, it is possible that the aliphatic polycondensation provides less steric hindrance than its aromatic counterpart at the surface of the graphene. Conversely, a flat graphene sheet should provide a better platform for polymerization than a scrolled or collapsed graphene sheet. Moreover, the rGO systems have a higher grafter polymer weight proportion than the SLGO counterparts, which may be due to the superior reactivity of reduced oxygen-containing groups (i.e., alcohols converted from carboxylic groups) after hydrazine reflux. Polymer-functionalized SLGO and rGO were dispersed into solvents to determine the stability of dispersion. It was found the DMF and CHCl3 gave the longest dispersions, compared with THF and CH3CN; however, separation was achieved between 30 min and 2 h. The redispersibility of dried composites into solvents was difficult and required aggressive treatments. The poor solubility of SLGO and rGO polyurea composites meant that it was not possible to obtain an NMR spectrum. 3.3. Modeling. Semiempirical PM7 calculations29 of SLGO and either 1,6-diisocyanatohexane (HDI) or 2,4-diisocyanato-1methylbenzene (TDI) (Figures 5a and 5b, respectively) show that the structure with two embedded 1,6-diisocyanatohexane molecules is more strongly twisted than the structure with three embedded 2,4-diisocyanato-1-methylbenzene molecules. How-
Figure 4. TGA profiles (solid line) and dTGA (dashed line) of (a) SLGO, (b) SLGO−aliphatic isocyanate, and (c) SLGO−aliphatic polyurea in air with a heating rate of 10 °C min−1.
SLGO−isocyanate, and a broad peak is present at 562 °C in SLGO−polyurea. The TGA profile for rGO (not included) reveals a 20 wt % mass loss between 100 and 517 °C (a small peak in the dTG is centered at 330 °C), compared with around 45 wt % for SLGO. Herein, the hydrazine reduction of the SLGO system has removed oxygen-containing groups from the more defective areas and according to TEM images has broken the lattice, fragmenting the graphene sheets into smaller domains. The reduction of the remaining oxygen-containing groups may only occur by around 25%; however, the TGA profile indicates greater oxidative stability of the residual oxygen-containing groups compared with those on the SLGO system.
Figure 5. Geometry of two small SLGO sheets interacting with (a) two 1,6-diisocyanatohexane and (b) three 2,4-diisocyanato-1-methylbenzene molecules are optimized using the PM7 (MOPAC 2012) method. F
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Figure 6. Geometry of two SLGO sheets (∼4000 atoms) interacting with (a, b) 2,4-diisocyanato-1-methylbenzene or (c, d) 1,6-diisocyanatohexane molecules optimized using the CharMM force field.
nanoscale system, with their ability to be architecturally shaped to provide better interfaces at the nano-bio level and thus improving performance in their final applications, whether as sensors, drug or biomolecule interaction, or cell or tissue growth support.
ever, the difference is not very large because of the relatively small models used. It is noted that the reactivity of two diisocyanate molecules enveloped in SLGO is different for TDI but the same for the aliphatic HDI, which is deemed to be the controlling factor for structural changes of the SLGO sheet. In the case of larger models (Figure 6), the thickness of the two-layer model with adsorbed aromatic TDI molecules (Figure 6a,b) is 0.6 nm thinner (middle part of the system) than that of the model with adsorbed aliphatic HDI molecules (Figure 6c,d). This occurs despite a larger number (10, Figure 6b) of the TDI molecules in this region than the number of the HDI molecules (7, Figure 6d). In other words, the adsorption of TDI molecules provides the formation of a more planar sandwich structure with two SLGO sheets than in the case of adsorbed HDI molecules. As a consequence, aliphatic HDI would be “free” to chemically bond to anchor sites on the SLGO sheet, potentially leading to intrasheet bonding and enabling greater curvature of the sheet if two bonding sites at distance are connected, whereas aromatic TDI would be adsorbed to unmodified (unfunctionalized) areas of SLGO, leaving the molecules in a near planar orientation, promoting van der Waals attractive forces between adjacent sheets, rather than intersheet (or intrasheet) bonding.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (R.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Marie-Curie Industry-Academia Partnerships and Pathways programme (FP7-PEOPLE-IAPP-2009-251429UNCOS) and the FP7-PEOPLE-IRSES (project No 230790, Compositum).
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REFERENCES
(1) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6, 183−191. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666−669. (3) Hummers, W.; Offeman, R. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (4) Park, S.; Ruoff, R. S. Chemical methods for the production of graphenes. Nat. Nanotechnol. 2009, 4, 217−224. (5) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007, 45, 1558−1565. (6) Satti, A.; Larpent, P.; Gun’ko, Y. Improvement of mechanical properties of graphene oxide/poly(allylamine) composites by chemical crosslinking. Carbon 2010, 48, 3376−3381. (7) Liu, Z.; Robinson, J. T.; Sun, X.; Dai, H. PEGylated nanographene oxide for delivery of water-insoluble cancer drugs. J. Am. Chem. Soc. 2008, 130, 10876−10877. (8) Zhang, T.; Cheng, Z.; Wang, Y.; Li, Z.; Wang, C.; Li, Y.; Fang, Y. Self-assembled 1-octadecanethiol monolayers on graphene for mercury detection. Nano Lett. 2010, 10, 4738−4741.
4. CONCLUSION SLGO and rGO have been modified to facilitate in-situ polycondensation reactions with diisocyanates and diamines. The nature of each species has been shown to control the graphene composite architecture in solution, such that aliphatic polyurea forms scroll-like domains and aromatic polyurea maintains flat structures. The grafted polymer weight proportion is higher for aliphatic than for aromatic and also higher for rGO than for SLGO systems. The flexibility of SLGO and rGO and their chemical anchor sites can be exploited to construct 3-dimensional systems where the external-facing surface is available for further covalent chemical cross-linking with biomolecules, as demonstrated by the covalent bonding of ferritin to the different SLGO architecture. Therein, it is reasoned that graphenes may prove a useful G
dx.doi.org/10.1021/jp4022213 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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(9) Shen, J. F.; Hu, Y. Z.; Shi, M.; Li, N.; Ma, H. W.; Ye, M. X. One step synthesis of graphene oxide-magnetic nanoparticle composite. J. Phys. Chem. C 2010, 114, 1498−1503. (10) Whitby, R. L. D.; Korobeinyk, A.; Gun’ko, V. M.; Busquets, R.; Cundy, A. B.; Laszlo, K.; Skubiszewska-Zięba, J.; Leboda, R.; Tombacz, E.; Toth, I. Y.; et al. pH-driven physicochemical conformational changes of single-layer graphene oxide. Chem. Commun. 2011, 47, 9645−9647. (11) Whitby, R. L. D.; Korobeinyk, A.; Mikhalovsky, S. V.; Fukuda, T.; Maekawa, T. Morphological effects of single-layer graphene oxide in the formation of covalently bonded polypyrrole composites using intermediate diisocyanate chemistry. J. Nanopart. Res. 2011, 13, 4829− 4837. (12) Lin, Y.; Taylor, S.; Li, H. P.; Fernando, K. A. S.; Qu, L. W.; Wang, W.; Gu, L. R.; Zhou, B.; Sun, Y. P. Advances toward bioapplications of carbon nanotubes. J. Mater. Chem. 2004, 14, 527− 541. (13) Wang, J. Carbon-nanotube based electrochemical biosensors: A review. Electroanalysis 2005, 17, 7−14. (14) Wu, L.; Li, T.; Blom, D.; Zhao, J.; Ghoshroy, S.; Wang, Q. High resolution TEM imaging and analysis of the core-shell assembly of ferritin-poly 4-vinyl pyridine (P4VP). Microsc. Microanal. 2009, 15, 96−97. (15) Watt, G. D.; Frankel, R. B.; Papaefthymiou, G. C. Reduction of mammalian ferritin. Proc. Natl. Acad. Sci. U. S. A. 1985, 82, 3640− 3643. (16) Dechakiatkrai, C.; Chen, J.; Lynam, C.; Shin, K. M.; Kim, S. J.; Phanichphant, S.; Wallace, G. G. Direct ascorbic acid detection with ferritin immobilized on single-walled carbon nanotubes. Electrochem. Solid-State Lett. 2008, 11, K4−K6. (17) Shin, H. J.; Shin, K. M.; Lee, J. W.; Kwon, C. H.; Lee, S.-H.; Kim, S. I.; Jeon, J.-H.; Kim, S. J. Electrocatalytic characteristics of electrodes based on ferritin/carbon nanotube composites for biofuel cells. Sens. Actuators, B 2011, 160, 384−388. (18) Tosun, Z.; McFetridge, P. S. A composite SWNT-collagen matrix: characterization and preliminary assessment as a conductive peripheral nerve regeneration matrix. J. Neural Eng. 2010, 7, 066002. (19) McDevitt, M. R.; Chattopadhyay, D.; Kappel, B. J.; Jaggi, J. S.; Schiffman, S. R.; Antczak, C.; Njardarson, J. T.; Brentjens, R.; Scheinberg, D. A. Tumor targeting with antibody-functionalized, radiolabeled carbon nanotubes. J. Nucl. Med. 2007, 48, 1180−1189. (20) Bettinger, H. F. Experimental and computational investigations of the properties of fluorinated single-walled carbon nanotubes. ChemPhysChem 2003, 4, 1283−1289. (21) Salavagione, H. J.; Martinez, G.; Ellis, G. Recent advances in the covalent modification of graphene with polymers. Macromol. Rapid Commun. 2011, 32, 1771−1789. (22) Whitby, R. L. D.; Korobeinyk, A.; Glevatska, K. V. Morphological changes and covalent reactivity assessment of singlelayer graphene oxides under carboxylic group-targeted chemistry. Carbon 2011, 49, 722−725. (23) Liu, Q.; Shi, J.; Sun, J.; Thanh, W.; Zeng, L.; Jiang, G. Graphene and graphene oxide sheets supported on silica as versatile and highperformance adsorbents for solid-phase extraction. Angew. Chem., Int. Ed. 2011, 50, 5913−5917. (24) Rourke, J. P.; Pandey, P. A.; Moore, J. J.; Bates, M.; Kinloch, I. A.; Young, R. J.; Wilson, N. R. The real graphene oxide revealed: stripping the oxidative debris from the graphene-like sheets. Angew. Chem., Int. Ed. 2011, 50, 3173−3177. (25) Kim, H.; Mirua, Y.; Macosko, C. W. Graphene/polyurethane nanocomposites for improved gas barrier and electrical conductivity. Chem. Mater. 2010, 22, 3441−3450. (26) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplateletes. Carbon 2006, 44, 3342−3347. (27) Scheibe, B.; Borowiak-Palen, E.; Kalenczuk, R. J. Oxidation and reduction of multiwalled carbon nanotubes - preparation and characterization. Mater. Charact. 2010, 61, 185−191.
(28) Kang, J.; Erdodi, G.; Kennedy, J. P. Rendering polyureas melt processible. J. Polym. Sci., Part A 2011, 49, 2461−2467. (29) Stewart, J. J. P. MOPAC2009. Stewart Computational Chemistry, Colorado Springs, CO, http://openmopac.net/, 2008.
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