CIS-01437; No of Pages 8 Advances in Colloid and Interface Science xxx (2014) xxx–xxx

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Aqueous processing of graphene–polymer hybrid thin film nano-composites and gels Shannon M. Notley a,⁎, Drew R. Evans b a b

Dept of Chemistry and Biotechnology, Faculty of Engineering, Science and Technology, Swinburne University of Technology, Hawthorn, VIC 3122, Australia Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Australia

a r t i c l e

i n f o

Available online xxxx Keywords: Graphene Conducting polymer Thin films Nano-composite Hydrogel

a b s t r a c t Research into the structure, properties and applications of graphene has moved at a tremendous pace over the past few years. This review describes one aspect of this research, that of the incorporation of graphene particles with a range of polymers to create novel hybrid materials with increased functionality such as improved conductance, increased strength and introduced biocompatibility or cytotoxicity. This review focuses on dispersing graphene in polymer matrices, both insulating and conducting. Additionally, a brief discussion of carbon based platelet production methods is given in order to provide context on the subsequent use of this family of materials such as graphene, graphene oxide (GO) and reduced graphene oxide (rGO) incorporated into polymeric thin films. © 2014 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of graphene . . . . . . . . . . . . . . . . . . . . Functionalization of graphene particles . . . . . . . . . . . . . . Thin film preparation and applications . . . . . . . . . . . . . . 4.1. Surface functionalization using layer-by-layer approach . . . 4.2. In situ vapor phase polymerization . . . . . . . . . . . . 4.3. Bio-compatibilization of surfaces for biomedical applications 4.4. Filtration preparation methods . . . . . . . . . . . . . . 5. 3D applications: gels and hydrogels . . . . . . . . . . . . . . . 6. Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction The properties and uses of graphene have been intensively studied over the past decade [1,2]. This novel material has captured the imagination of scientists and technologists alike. Graphene gained increased prominence following the award of the 2009 Nobel Prize in Physics to Geim and Novoselov for their pioneering work on elucidating structure and function and spawned a global industry in incorporating this material into applications as diverse as electronics, health, water and energy [3,4].

⁎ Corresponding author. Tel.: +61 3 92148635. E-mail address: [email protected] (S.M. Notley).

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Graphene is a 2 dimensional form of carbon only one atom thick and linked hexagonally in a lattice. The origin of many of graphene's interesting properties is due to the unique spatial and bonding arrangement of atoms through sp2 hybridization of all of the carbon–carbon bonds across the sheet. This leads to extraordinary electron mobility [5] as well as intrinsic material strength and thermal properties. Importantly, these properties of graphene are significantly different to those of the stacked 3 dimensional form of carbon, graphite. Hence interest in exploiting graphene is high due to the relatively low cost of the bulk material as well as accessibility [6,7]. This, however, is balanced against the current challenges of scalability of production methods of the graphene. Research continues unabated though as these challenges are increasingly being met with expansion of the technology into new high volume applications.

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Please cite this article as: Notley SM, Evans DR, Aqueous processing of graphene–polymer hybrid thin film nano-composites and gels, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.04.006

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Over the past decade, the major theme for many researchers in materials science has been to understand the properties of graphene as well as to develop methods for its large scale production. While novel uses are regularly found, immediate goals for graphene incorporation into applications have centered on the electronics and materials industries. In these uses, the physical, mechanical and electrical properties of graphene are ideal. Functional electronics applications include supercapacitors, opto-electronic devices and displays, electrodes and high electron mobility transistors. Current market drivers are predominantly in composites, coatings and thin films [8]. In these uses, graphene can add functionality or be used to improve mechanical properties. The physical structure of graphene in terms of the ultra-high aspect ratio due to the atomic thickness of the sheets as well as the highest known tensile strength [9] makes it a desirable load distributing and bearing component in composite materials. Graphene has a moderate surface energy (partially water wetting) making compatibilization with matrix polymeric materials somewhat challenging. However methods for the production of graphene with engineered soft matter functionality may assist in overcoming this issue [10]. Herein, methods for producing graphene will be presented; topdown methods [11] that exfoliate graphene from graphite or its variants. These top-down methods produce graphene sheets that are dispersed in a liquid making them amenable to incorporation within a polymer thin film. To achieve the ultimate incorporation into the polymer films, the graphene itself may require further functionalization. The application of graphene to create hybrid thin films and gels will be briefly discussed, giving an overview of some popular directions currently being explored in the open literature; layer-by-layer approach to functionalize a surface, in-situ vapor phase polymerization of composite thin films, and the creation of graphene based surfaces for biomedical applications. 2. Preparation of graphene Just as the list of applications for graphene expands, the means by which to produce graphene does too. Of particular interest to graphene–polymer hybrids are the graphene production methods that yield “pristine” graphene or graphene oxide which may be subsequently reduced, be it single or multilayer, dispersed in a liquid. These methods to produce graphene dispersions are gaining increasing popularity due to their amenability for large scale production [11–13]. From a very simple view point, large 3 dimensional volumes of graphite or graphite oxide are processed in a liquid in order to “break down” the volume into individual or multilayers of graphene. When graphene oxide is employed, there exist additional processing steps to reduce the material back to graphene however there are both advantages and disadvantages to these types of approaches [14–16]. When the graphene is directly exfoliated from bulk graphite, the graphene layers that are produced have very large surface areas which are exposed to the host liquid in which it is dispersed. While this large surface area provides great advantage in certain applications, it does prove disadvantageous when trying to stabilize the graphene against re-aggregation in a dispersion. Two means to stabilize the large surface area, thus inhibiting aggregation of the graphene, are to use electrostatic and/or steric forces between neighboring graphene particles. While the use of such forces to stabilize particles (nano- or colloidal in size) in a dispersion is reasonably well understood and widely applied in practice, their use with graphene proves to be a challenge. Pure graphene is a 2 dimensional network of carbon atoms, which theoretically does not have any defects, be they physical or chemical. This means that the surface of the graphene only possesses sp2 hybridized carbon atoms, and lacks “active” sites where a charge could reside or a surfactant/polymer may strongly adsorb through electrostatic means. In practice, the presence of a non-negligible amount of defects, particularly at the edge of the graphene sheet allows for these “active” sites to be present,

facilitating a surface charge or the binding of steric stabilizers. For sufficient surface charge to occur for electrostatic stabilization the common pathway is to convert graphite to graphene oxide, exfoliate the layers, and then reduce this back to graphene. Given that the reduction process does not fully reduce the material to pure graphene, this intermediate material is commonly referred to as reduced graphene oxide (rGO). On the other hand, by using surfactant assisted direct exfoliation of graphene (labeled as SA-G), a material that is stable in an aqueous suspension is generated that is much closer to that of pure graphene however removal of the surfactant post-production can be difficult. The use of rGO or SA-G in a dispersion for the creation of graphene– polymer hybrids will greatly depend on the end application in mind where a compromise between the ease of fabricating the hybrid and the final performance and properties of the hybrid needs to be weighed. Few methods currently exist for the large scale production of chemically unmodified or “pristine” graphene sheets. One route which shows some potential for scale up is liquid phase exfoliation of graphite in an appropriate solvent system [17]. Such a method has been demonstrated in aqueous systems in the presence of surfactants as well as in organic solvents [18,19] and ionic liquids [20,21]. In most cases, the concentration of graphene in the liquid is relatively low, typically of the order of less 100–500 ppm by mass. Liquid phase exfoliation is a simple process whereby the cohesive energy of the graphite is matched to the cohesive energy of the liquid [22,23]. Hence, there is no significant energy barrier to split the sheets apart under moderate sheer conditions imparted through sonication. Typically, the required surface tension is ~ 41 mJ/m2. This optimum surface energy can be achieved simply by judicious choice of solvent of which organic exemplars include N-methylpyrrolidone (NMP) and N,N-Dimethylacetamide (DMA) [17]. Alternatively, graphene production can be performed under aqueous conditions in the presence of a surfactant to reduce the surface tension to the appropriate level for efficient exfoliation [22,24–30]. The use of surfactants also increases the stability of the suspensions as adsorption to the graphene surface provides an extra repulsive barrier against aggregation. Fig. 1 shows typical platelet graphene particles generated using surfactant assisted aqueous based exfoliation. The presence of a single layer material is confirmed by Raman spectroscopy and TEM imaging. In organic systems and ionic liquids, the exfoliation of graphite to graphene is self-limiting as the particles are themselves surface active which thereby reduces the interfacial energy. Similarly, in aqueous systems, adsorption of surfactant molecules to the graphene–water interface depletes the surfactant from the liquid phase leading to an increase in interfacial tension; again a self-limiting process. Recently it was demonstrated that the concentration could be increased further simply by replenishing the surfactant continuously resulting in a concentration of graphene in suspension up to 20 times greater [22]. Interestingly, any surfactant may be used as long as the surface tension can be tuned to the exfoliation window at 41 mJ/m2. Ionic surfactants such as the cationic hexadecyltrimethylammonium bromide and anionic surfactants such as sodium dodecylsulfate and sodium cholate have been extensively used [10]. Furthermore, non-ionic surfactants such as the block co-polymers of polyethylene oxide (PEO) and polypropylene oxide (PPO) have also been used [22,31]. These polymeric type surfactants are irreversibly adsorbed giving rise to highly stable suspensions, particularly at elevated ionic strength and over a range of pH however surfactant removal is hence non-trivial. The graphene particles produced using liquid phase exfoliation typically retain the extended conjugation across the sheet. Under certain conditions such as high sonication power, defects may be introduced. Also, the edges of the graphene particles are stabilized by oxygen containing moieties so there is some small amount of negative charge of these particles in solvents with a high dielectric constant such as water. The sonication procedure also produces a range of thicknesses of the particles, however the majority of particles are single or have few layers.

Please cite this article as: Notley SM, Evans DR, Aqueous processing of graphene–polymer hybrid thin film nano-composites and gels, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.04.006

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Fig. 1. Left: TEM image of exfoliated graphene particles, middle: Raman spectra of natural graphite (top) and exfoliated graphene particles (bottom) demonstrating the offset of the 2D peak at 2685 cm−1 used to confirm the presence of single layer graphene material and right: graphene dispersion in water. Reprinted with permission from Langmuir 2012, 28, 14110–14113. Copyright 2012 American Chemical Society.

The proportion of single layer graphene can be preferentially enriched through controlled centrifugation [32]. Liquid phase exfoliation, particularly in aqueous conditions, is a promising technique for functionalizing graphene particles with surfactants or polymers. As described above, Pluronic type polymeric surfactants (block co-polymers of PEO and PPO) can be simply used. These block co-polymers are able to impart varied wettability depending on the lengths of the hydrophilic and hydrophobic blocks. Other polymers have also been used to functionalize graphene exfoliated in this manner. These include polyelectrolytes such as polystyrene sulfonate [33], polyethylene imine [28], polyacrylic acid [28] and DNA [34]. Graphene– polymer hybrid sheets can subsequently be used as a building block in the preparation of thin film applications. 3. Functionalization of graphene particles For improved interaction between the graphene and the polymer within the hybrid film, the graphene itself may require functionalization. This functionalization provides chemical groups on the surface of the graphene which can strongly and positively interact with the polymer matrix, thus improving the properties of the final graphene–polymer hybrid film. For example, functionalized graphene sheets (FGSs) have been incorporated into PMMA, with the thermal and mechanical properties of the hybrid film (100 μm thick) being enhanced at FGS loadings as low as 0.05 wt.% [35]. In this case the FGS was produced by rapid thermal expansion of fully oxidized graphite oxide; thus resulting in graphene oxide (the oxide is considered as the functionalization of the graphene). The concept of functionalizing graphene is an interesting one, as any modification of the graphene chemistry necessarily means that pristine graphene is no longer the product. This redirection from pristine graphene must also result in a deterioration of the properties of the graphene-based material itself; thus a trade-off is observed between the properties of the graphene-based material and its (positive or negative) interaction with the host matrix. Another point to note is that the functionalization of the graphene may lead to changes in its dispersion quality in solution. This dispersion quality in the solvent will almost certainly define the level of dispersion quality of the graphenebased material within the final hybrid film. In addition to the oxidation of graphene described above, graphene may be doped with nitrogen or boron, or functionalized by adsorbing material to its surface. For doping of graphene sheets amenable to solution processing, two techniques presented in the scientific literature are (i) doping via arc discharge of carbon electrodes in a nitrogen/boron environment [36] and (ii) plasma treatment of exfoliated graphene oxide in a nitrogen environment [37]. Despite formation of these nitrogen or boron doped graphene-like materials, to the best of our knowledge these forms of graphene have not been incorporated into graphene– polymer hybrid films. Functionalization by adsorbing material to the graphene surface not only changes the interfacial and adhesion properties of the graphene,

but also allows for it to be dispersed in a range of solvents for use in wet processing methods to prepare graphene–polymer hybrid films. The adsorbing materials interact with the graphene through either covalent or non-covalent interactions; in the context of covalent bonding it becomes apparent that pristine graphene lacks the “active” sites for such binding to occur. For example, the hydroxyl and carboxyl groups on the graphene surface can be reacted with SOCl2 and then longchain aliphatic amines to create amide functionalization of the graphene [38]. This then leads to the improved dispersion of the graphene in nonpolar solvents; where the nonpolar solvent may be desirable for the fabrication of the subsequent hybrid film. Non-covalent interaction between adsorbing molecules and the graphene is investigated primarily from the view point of dispersion quality in aqueous or other polar solvents. The discussion of which was covered in the section on preparation of graphene above. Of all the functionalizing methods currently described, oxidation of the graphene and non-covalent adsorption of molecules present as the most popular for exploiting graphene and graphene-like materials in polymer hybrid films. Already graphene oxide has been employed in the fabrication of a variety of hybrid films, with the primary goal to improve the processability and mechanical properties of the polymer (adjusting glass transition temperatures, Young's modulus, and tensile strength) and including interfacial polymerization techniques [11,39]. Incorporation of nanoparticles has been another strategy [40]. Alternatively, the non-covalently bound molecules, such as non-ionic surfactants, directly influence the dispersion quality of the graphene in solution. More importantly, the graphene material is relatively much closer to the pristine state than thermally or chemically rGO. This method has a twofold benefit to the resulting hybrid film; better dispersion quality of the graphene during film fabrication, and incorporation of a more desirable form of graphene (closer to the properties of pristine graphene). A recent example of this uses a triphenylene based molecule to stabilize few layer graphene in water, which is then incorporated into poly(vinyl alcohol) [41]. In this example the electrical conductivity, Young's modulus, yield strength and hardness all show enhancement as the graphene volume fraction is varied to at most only a few percent. Dramatic enhancement of the hybrid properties at such low loading levels is another key advantage of employing graphene to create the hybrid materials. 4. Thin film preparation and applications 4.1. Surface functionalization using layer-by-layer approach A simple, flexible and facile method for the modification of surfaces is through the sequential layer-by-layer (lbl) deposition of polymers and particles onto a solid substrate. This technique pioneered by Decher [42], Caruso [43,44] and others [45–52] gained popularity more than a decade ago due to the enhanced functionality that could be imparted to surfaces through simple adsorption. The basis for the assembly of

Please cite this article as: Notley SM, Evans DR, Aqueous processing of graphene–polymer hybrid thin film nano-composites and gels, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.04.006

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the multilayer system is the adsorption of the polyelectrolyte or particle and subsequent charge reversal of the substrate allowing further deposition of a material with opposite charge as shown in Fig. 2. While the majority of lbl studies involve construction through electrostatic interactions, hydrogen bonding may also be used to form assemblies. Using a lbl approach, the thickness of the layer can be simply controlled through the choice of the number of deposited bi-layers. Furthermore, the lbl technique can be seen as a form of surface engineering, where the properties of the underlying substrate such as the wettability, adhesion, friction, biocompatibility, toughness and elasticity can be modified through judicious selection of the adsorbed polymers and/or particles. Incorporating graphene into lbl films is highly attractive as the electrical, optical and thermal properties of graphene can be imparted to many solid substrates and to those with complex geometry. The physical structure of graphene platelets also lends itself to the idea of creating synthetic nacre like surfaces [53,54]. Graphene oxide and reduced graphene oxide which are negatively charged have been used in conjunction with positively charged polyelectrolytes to create lbl films [55–57]. The rGO particles still retain sufficient charge to bind together electrostatically with polycations such as PDADMAC [15]. Notley showed that multilayers could also be prepared through the use of pristine graphene particles produced through liquid phase exfoliation [29]. Edge defects provide a small amount of charge for stabilization however this could be improved through the use of a cationic polyelectrolyte adsorbed to the particle surface prior to use in deposition [29]. This work was extended to use graphene platelets stabilized using nonionic tri-block co-polymer surfactants [58]. 4.2. In situ vapor phase polymerization While much attention has been given to the enhanced mechanical properties achieved when creating graphene–polymer hybrid films, only recently have there been demonstrations of the hybrid films for other applications. Of particular interest herein is the use of graphene in nano-composite films for optical, electrical and opto-electronic applications. Initial studies in this space have focused on incorporating graphene into classic insulative polymers, such as polystyrene, which yields working devices such as transistors [59]. In recent times these investigations have broadened to encompass a much broader range of polymers in attempts to enhance device performance. The polymers of choice for such applications are those that also have the ability to conduct or transport charge; namely the inherently conducting polymers [60,61]. Methods to achieve such hybrid films of graphene and conducting polymers include filtration of the graphene and polymer from solution [62], polymerization of the polymer from a solution containing the dispersed graphene, or to electrospin the polymer from a solution containing the dispersed graphene [63].

An exciting new direction is emerging through a vapor-based method for polymerizing conducting polymers. The vapor phase polymerization (VPP) technique is a variation of chemical oxidative polymerization and is used to produce highly conductive PEDOT thin films [64]. This process involves coating a substrate with an oxidant solution and subsequently exposing the coated surface to monomer vapor. The method was first described by Muhammadi et al. [65]. They used FeCl3 or H2O2 as the oxidant to polymerize polypyrrole films by a chemical vapor deposition (CVP) process. In later studies [66–68], the technique was also applied for the polymerization of a variety other monomers including 3,4ethylenedioxythiophene (EDOT). However, the first conjugated polymer films made by VPP suffered from poor homogeneity and low conductivity compared to films made by conventional solvent processing [69]. One reason was that oxidants like FeCl3 started to crystallize upon deposition onto the substrate due to rapid solvent evaporation. The grainy oxidant products produced numerous pinhole defects into the forming polymer films. At a later stage, pioneering work by Winther-Jensen and coworkers [69,70] led to the identification of Fe(III) tosylate as an alternate oxidant for the VPP process. By using Fe(III) tosylate they were able to polymerize PEDOT films with conductivities exceeding 1000 S·cm−1, at least one order of magnitude more conductive than previously reported PEDOT films. This system was then further developed by Fabretto et al. [64,71,72] with the addition of block co-polymers of PEO and PPO to suppress oxidant crystal formation, and furthermore reducing the apparent reactivity of Fe(III) tosylate thus forming high quality PEDOT films with conductivity up to 3400 S·cm−1 [64,73]. These highly conducting films were recently demonstrated to be the first reported example of an air stable semi-metallic polymer [74], in contrast to the Fermi glass classification for conventional PEDOT doped with the anion of poly(styrene sulfonate). Despite such advances, the VPP reaction process is still not fully understood. The popular description of the overall polymerization process is presented in three main steps: 1) oxidation of monomers as they adsorb to the oxidant surface, and subsequent oxidation of oligomers, by Fe(III) tosylate, 2) recombination of the oxidized monomers and oligomers to oligomers or polymers, and 3) oxidative doping of the neutral oligomers or polymers into conductive polycations. It is believed that the oxidation of the monomer is the rate-determining step of the polymerization reaction. However, later studies [72,75] have shown that proton acceptors (for example H2O) can be used to enhance the polymerization reaction. It is assumed that such proton scavengers prevent the back-reaction of dimers and oligomers and thus, increase the overall polymerization rate. Beside the high quality of PEDOT films produced by VPP, the technique also has the advantage of being a more versatile technique compared to electrochemical polymerization. It can be used to coat complex shaped and non-conducting surfaces. In addition the process can easily be adapted to several patterning techniques like inkjet

Fig. 2. Left: schematic showing the layer-by-layer approach pioneered by Decher adapted from [38] and right: the lbl build-up of graphene (even numbered layers) and PEI (odd number layers) on a silica substrate as measured using the quartz crystal microbalance [54] reproduced with permission from Langmuir 2014, doi: http://dx.doi.org/10.1021/la404745b. Copyright 2014 American Chemical Society.

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printing or other printing technologies where the oxidant is selectively applied onto the surface [69]. The recent advance in this polymerization process was the observation that by employing select additives within the oxidant solution, the thin film could remain liquid-like under the typical VPP conditions. It is this liquid-like property that then allows for incorporation of nanomaterials into the growing polymer. Nanomaterials, such as graphene, can be dispersed within the oxidant solution for transportation into the polymer film as it grows. This is an effective way to “grow” graphene–polymer hybrids where the polymer itself is also a conjugated material containing a pi-bonded network. Very recently attempts have been made to use graphene in the VPP process by Yang et al. [76], where exfoliated graphene (it is uncertain whether this is SA-G or rGO) was added to the oxidant solution to enable graphene–PEDOT hybrid films to be deposited on Ta2O5 substrates for capacitors as shown in Fig. 3. Demonstrating the benefit of such hybrid films, the resulting electrolyte capacitor had very low equivalent series resistance compared to electrodes coated with PEDOT only and PEDOT–graphene multilayers. Similarly, in Tung et al.'s work a thin hybrid film of rGO and PEDOT was fabricated by means of VPP as the active material in chemiresistor sensors (“electronic nose”) for volatile organic compound detection [77]. Despite these only being recent studies, they highlight the novelty that in-situ polymerization of conducting polymers in the presence of graphene brings to the field of graphene–polymer hybrid films. Significant advances in this fledgling research field are predicted to be made when the graphene itself is used as a templating material (in a similar manner to PEO–PPO–PEO copolymers) in the VPP process. 4.3. Bio-compatibilization of surfaces for biomedical applications There is continued conjecture on the biocompatibility or cytotoxicity of graphene [78]. Largely, this has arisen due to the varied nature of the material used. For example, the majority of studies in this area have

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utilized graphene oxide or reduced graphene oxide particles rather than pristine graphene particles or surfaces. Furthermore the toxicity of graphene has often been related to the size of particles or the presence of exposed edges [79,80]. There is also a strong dependence on the type of cells, for example eukaryotes or prokaryotes. Even within the prokaryotic bacterial cells, strong differences have been observed which have been related to the properties of the cell wall [81,82]. Fig. 4 demonstrates the potential bactericidal action of graphene nanosheet surfaces. The underlying compatibility of graphene-like materials with cells is of great importance as many biomedical applications have been suggested in the literature. A typical method for varying the interaction of surfaces and particles with cells has been to modify the surface with polymers. This approach has been adopted by a number of authors as described below. In order to enhance cytotoxicity, particularly in anti-bacterial applications, graphene and graphene oxide have been combined with polyelectrolytes. Some polyelectrolytes such as those which are positively charged due to quaternary ammonium groups have inherent anti-bacterial action. Hence, functionalization of graphene with such polycations has proven highly effective in inducing bactericide. Chitosan, the deacetylated form of chitin, has been used to modify graphene particles. The adsorption of chitosan improves stability in suspension and also shows strong anti-bacterial action according to Sreeprasad et al. whereas the underlying graphene oxide did not. Interestingly, combining further with lactoferrin resulted in an even greater loss in viability of the Gram-negative bacteria Escherichia coli [83]. Chitosan modified graphene films were also shown to inhibit growth of the Gram-positive bacteria Pseudomonas aeruginosa. These films also had improved thermal and mechanical (tensile) properties demonstrating that composites can be formed with a range of enhanced properties [84]. Polyvinyl-N-carbazole (PVNC) is another polymer which has been used to modify the graphene particle surface for enhancing anti-

Fig. 3. (a) Experimental scheme for creation of graphene–PEDOT hybrid films using vapor phase polymerization. (b) Resultant electrode structure of hybrid film coating a Ta substrate for capacitor applications [76]. Reproduced with permission from ACS Appl. Mater. Interfaces 2013 5, 4350–4355. Copyright 2013 American Chemical Society.

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Fig. 4. FE-SEM image of bacteria attached to graphene substrate (a: solid arrows indicate cells and unfilled arrows graphene edges) and E. coli (b before) interacting with graphene nanosheets (c after) [77]. Reproduced with permission from J. Phys Chem C, 2012, 116, 17280–17287. Copyright 2013 American Chemical Society.

bacterial properties. Previously it has been shown that PVNC is highly toxic toward bacteria and hence should prove useful in combination with graphene [85,86]. Composite films were tested against both Gram-positive and Gram-negative bacteria and demonstrated that the efficacy was greater than for simple unmodified graphene surfaces. The authors demonstrated that the colonization of the surface by the bacteria was inhibited and furthermore, the cell viability of those that attached was diminished. This suggests that not only can graphene surfaces modified with polyelectrolytes destroy bacteria but they can also prevent biofilm formation through altering the energetics of attachment. Interestingly, while such surfaces were indeed anti-bacterial, the interaction toward eukaryotic cells was substantially different. No cytotoxicity toward human fibroblast cells was observed for the graphene– PVNC thin films. Similarly, the results of a study by Some et al. showed similar behavior for graphene particles modified through the adsorption of poly-L-lysine (PLL) [87]. Graphene–PLL surface was subsequently prepared from these particles. The proliferation of human adipose derived stem cells was enhanced yet the bacterial colonization (E. coli) was shown to be inhibited. The high cationic charge of the surface was suggested as the mechanism for the reduced bacterial cell attachment. Along with the potential for creating hybrid graphene–polymer surfaces that are suitable for anti-bacterial applications, there has also been a drive to improve biocompatibility for potential use in drug delivery. Graphene is only partially wetting and hence can have a strong interaction with hydrophobic drug molecules which are otherwise insoluble in aqueous conditions [88,89]. Modification of graphene with polyethylene glycol for instance has been used to improve biocompatibility and promote selective recognition of cells [90]. 4.4. Filtration preparation methods A simple, one-step method for the preparation of polymer films incorporating graphene or graphene oxide is to simply filter a suspension of particles in the presence of the film forming polymer onto a porous membrane followed by release. Vacuum filtration leads to a strongly ordered lamellar structure with polymer chains intercalated between the graphene or graphene oxide sheets. The effective filler content can be pre-determined and very high loadings (up to 50%) are possible [91]. Brinsin et al. demonstrated the utility of this method for preparing graphene oxide reinforced polymer nanocomposites using both hydrophilic (polyvinyl alcohol) and hydrophobic (poly methylmethacrylate)

polymers with significantly enhanced mechanical properties of the prepared films aside from reduced strain at break. Indeed, vacuum filtration methods provide a potential route to artificial nacre like films with ultrahigh impact strength which may be enhanced through cross-linking [92]. Furthermore, other types of intercalating polymers have been used. This includes biomacromolecules such as lysozyme for controlled release applications [93] or polydopamine for enhanced strength [94]. Polybenzimidazole [95], carboxymethylcellulose [96] and polyethyleneimine [97] have all been used as the polymer matrix in nanocomposites incorporating graphene oxide prepared through filtration. 5. 3D applications: gels and hydrogels The interaction of materials such as graphene with polymers to form a hydrogel or organogel is worthy of discussion here although not strictly a thin film application. A hydrogel is a polymeric based material with a large amount of water, sometimes greater than 99% by mass, entrained. The polymer is gelled either through the self-assembly of small molecules with polymer chains or through the covalent cross-linking of macromolecules to form an inter-connected 3D network of soft matter. Applications of hydrogels include water remediation, tissue engineering and drug delivery. Incorporating solid phases such as graphene has attracted interest as there is the potential to modify the viscoelastic properties as well as create strong aerogels via dehydration or freeze drying. In the absence of polymers, graphene oxide hydrogels have been demonstrated by Li et al. [98,99] and Bai et al. [100]. This work was recently extended to show that these hydrogels may be used to produce improved capacitance performance [101] as well as a platform for tissue engineering [102]. Combining graphene oxide or reduced graphene oxide with polymers to form a hydrogel is also possible. A recent study by Zeng et al. incorporated rGO in poly-N-isopropylacrylamide hydrogels to influence the swelling ratio. Furthermore, as rGO has a strong absorbance across the range of incident solar radiation, temperature induced dewatering could be achieved [103]. Other polymers such as PEO or block co-polymers of PEO and PPO have also been used in conjunction with graphene particles to produce hydrogels [104]. Fig. 5 shows an example SEM image of a graphene based hydrogel demonstrating the potential for the production of materials with high surface area. Zu et al. used PEO–PPO–PEO modified graphene oxide particles in order to produce a gel in the presence of cyclodextrin [31]. The gel formed was dependent on the length of the

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Fig. 5. SEM image of graphene based hydrogel formed from particles stabilized with nonionic polymeric surfactants [29]. Reproduced with permission from J. Phys. Chem B. 2009 113, 13651–13657. Copyright 2009 American Chemical Society.

ethylene oxide chains and flow could be induced through increasing temperature. For these types of hydrogels the ethylene oxide groups dehydrate upon heating reducing the threading interaction with the cyclodextrin molecules. As with the literature surrounding hydrogels themselves, a range of composite graphene–polymer hydrogelling systems have been investigated. This includes the use of poly-N-isopropylacrylamide polymers and co-polymers such as by Liu et al. [105] or the approach adopted by Cheng where the gelation was formed through π–π stacking interactions with graphene oxide particles and low molecular weight gelling agents [106]. Others have used graphene oxide in the presence of DNA to form hydrogels with self-healing properties [107].

6. Outlook Much excitement exists around graphene being a “super material” to revolutionize a wide range of different devices and technologies owing to its performance, properties and behavior under different conditions. The ability to incorporate the graphene into polymers to form hybrid materials further opens the door to exciting new devices. Importantly, the nature of the graphene plays an important role in defining the magnitude of its behavior and that of the new hybrid. However, the way in which the graphene is fabricated impacts on its nature. When creating the hybrid materials, the fabrication method for graphene also plays a role in how the graphene integrates into or interacts with the polymer matrix. With these two points in mind, the future of graphene–polymer hybrid films will ultimately advance by improved methods of preparing graphene to keep it in a form as close as possible to pristine graphene, while still being easily processed with the polymer to form a hybrid material. This means that the hybrid materials will transition from today's research using graphene oxide in traditional polymers (PMMA, PS, etc.) to tomorrow's devices using pristine graphene in advanced polymers (PEDOT, PPy, etc.). In doing so, a new range of properties and performance will be realized, allowing for not only better performing devices of the current day, but also devices for all new applications. The breadth of application will range from biomedical devices, to water treatment, to renewable energy harvesting and storage, to opto-electronic devices, and beyond. While graphene continues to gather the greatest amount of attention, there are however a range of 2D materials or atomic crystals also with unique properties that differ significantly from the corresponding bulk material [24,108]. There is now increasing interest in the single layer forms of the transition metal dichalcogenides MoS2 and WS2 as well as materials such as germanane and the topological insulator Bi2Te3 to name but a few [109–113]. The challenges of compatibilizing these materials with polymers are similar to those of graphene and hence much of the preliminary work performed with graphene will find use in these applications.

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