Synthesis and Characterization of Graphene Oxide–Polystyrene

May 11, 2017 - IFT data showing the effects of added electrolyte are presented in Figure 1f with olive oil and Figure 1g with toluene. As with .... As...
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Synthesis and Characterization of Graphene Oxide−Polystyrene Composite Capsules with Aqueous Cargo via a Water−Oil−Water Multiple Emulsion Templating Route Muthana Ali,†,‡ Thomas M. McCoy,† Ian R. McKinnon,† Mainak Majumder,§ and Rico F. Tabor*,† †

School of Chemistry, and §Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australia ‡ Department of Chemistry, University of Karbala, Karbala 56001, Iraq ABSTRACT: Graphene oxide/polystyrene (GO/PS) nanocomposite capsules containing a two-compartment cargo have been successfully fabricated using a Pickering emulsion strategy. Highly purified GO sheets with typically micrometer-scale lateral dimensions and amphiphilic characteristics were prepared from the oxidation reaction of graphite with concomitant exfoliation of the graphite structure. These GO sheets were employed as a stabilizer for oil-in-water emulsions where the oil phase comprised toluene or olive oil. The stability and morphology of the emulsions were extensively studied as a function of different parameters including GO concentration, aqueous phase pH, ultrasonication time, effects of added electrolytes and stability to dilution. In selected conditions, the olive oil emulsions showed spontaneous formation of multiple w/o/w emulsions with high stability, whereas toluene formed simple o/w emulsions of lower overall stability. Olive oil emulsions were therefore used to prepare capsules templated from emulsion droplets by surrounding the oil phase with a GO/PS shell. The GO sheets, emulsions and composite capsules were characterized using a variety of physical and spectroscopic techniques in order to unravel the interactions responsible for capsule formation. The ability of the capsules to control the release of a model active agent in the form of a hydrophilic dye was explored, and release kinetics were monitored using UV−visible spectroscopy to obtain rate parameters. The composite capsules showed promising sustained release properties, with release rates 11× lower than the precursor GO-stabilized multiple emulsion droplets. KEYWORDS: graphene oxide, multiple emulsions, encapsulation, release kinetics, Pickering emulsions, composite capsules, emulsion templating

1. INTRODUCTION Graphene oxide (GO) is being exploited by researchers in a vast array of applications because of its unique two-dimensional amphiphilic structure, combined with excellent mechanical and optical properties.1,2 This has resulted in potential uses for GO in biomedical technology,3,4 drug delivery,5 adsorption,6 and catalysis.7 The ability of GO to locate and adsorb at a variety of interfaces has been experimentally8 and thermodynamically investigated.9 Because of its amphiphilicity, GO can easily locate at the interface between two immiscible liquids in an emulsion system, although it is yet unclear whether its behavior is closest to that of a classical surfactant or particle, displaying properties of both.10 By preparing functionalized GO composites, it has been shown that further diversity can be introduced that enables these materials to act as a stabilizer for Pickering emulsions11 that have a variety of applications. The affinity of GO for liquid−liquid interfaces can be controlled by adjusting the aqueous phase pH value, and it is seen that at very low pH, GO is most surface-active and shows particle-like properties,10,12 making it a good candidate for Pickering (or Ramsden) emulsion formation, which is an emulsion stabilized by solid particles.13 In most emulsification experiments, GO tends to form oil-in-water (o/w) emulsions © 2017 American Chemical Society

due to its dispersibility in water (i.e., the Bancroft rule), though it can be wetted by both oil and water. Emulsions may be classified as single (e.g., o/w or w/o) or multiple emulsions. In the latter type, a single emulsion is emulsified in an immiscible liquid, resulting in, for example, water droplets inside oil droplets that are themselves dispersed in a continuous water phase (designated w/o/w). Multiple emulsions often result from the combination of different stabilizers (either molecular or particulate), although they can be formed with only one stabilizer by a phase inversion of the emulsion.14,15 Because of its unique structure, GO can readily form composites with many diverse materials such as polymers16,17 and metal nanoparticles.18,19 GO-based composite materials with polymers and nanomaterials have attracted much interest from researchers in various applications including chemical catalysis,20 pollutant separation,21,22 coating applications23 and drug delivery.24 The formation of core−shell capsules based on PS/GO composite materials has been achieved using several strategies. Received: February 23, 2017 Accepted: May 11, 2017 Published: May 11, 2017 18187

DOI: 10.1021/acsami.7b02576 ACS Appl. Mater. Interfaces 2017, 9, 18187−18198

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) AFM image of GO sheets prepared on glass using spin coating. The height profile in (b) corresponds to the line in (a). (c and d) show the effect of added electrolytes on GO zeta potential for different concentrations of NaCl and CaCl2 at pH 1 and pH 5 respectively. (e) Interfacial tension (IFT) between different concentrations of aqueous GO dispersions and olive oil or toluene. (f) IFT between olive oil and 1 mg/mL aqueous GO dispersion at different concentrations of NaCl and CaCl2. (g) IFT of toluene and 1 mg/mL aqueous GO dispersion at different concentrations of NaCl and CaCl2. (h, i) Images showing the bulk stability of GO in various solution conditions, as a function of (h) added salt and (i) pH adjustment. The GO remains fully dispersed in 0.1 M NaCl and at a pH value of 5, but begins to flocculate in 0.1 M CaCl2 and at pH 1 and 11.

The efficacy of the capsule system can be logically divided into two key aspects: control (i.e., the ability to capture and transport the loaded material) and release of the loaded materials. Many studies have been conducted into controlling the release of drugs in biological systems as a function of different physical parameters, including redox reactions,29 temperature,30 and pH.31 In addition, drug release has been studied for capsules that can be addressed by external release stimuli, such as near-infrared light,32 ultrasound energy,33and electric fields.34 In this work, we demonstrate a simple, one-pot method to fabricate a capsule system based on olive oil multiple emulsions (w/o/w) decorated with a PS/GO composite material as a shell, via Pickering emulsion formulation. First, emulsions stabilized by GO dispersed in water (as the continuous phase) with toluene and olive oil (dispersed phase) are optimized as a function of different parameters, including pH, ultrasonication time, effects of added electrolytes, stability to dilution, and overall GO concentration. The emulsions and capsules produced are explored using a variety of techniques, including optical microscopy, atomic force microscopy, and FT-IR spectroscopy. It was found that olive oil with GO spontaneously formed multiple emulsions (w/o/w) and was therefore investigated as a template for the fabrication of GO/PS encapsulation systems for delivery of water-soluble materials. The capsules thus formed were examined for their loading and delivery characteristics using a water-soluble dye.

These include deposition of amphiphilic GO sheets onto colloidal PS particles using a layer-by-layer technique (LBL),25 coating PS particles with GO sheets by aerosol spray pyrolysis26 and Pickering emulsion polymerization,27 which is considered a more environmentally friendly method for polymer particle production.17 Using this emulsion polymerization method, Thickett and Zetterlund used GO sheets as a Pickering stabilizer to polymerize styrene and prepare PS/GO composite hollow sphere capsules.27 Liu et al.17 also prepared PS/GO nanocomposites using a Pickering emulsion strategy, and showed that the particle size of the nanocomposite could be controlled to some extent by the nature of the GO nanosheets. They also found that in addition to acting as a stabilizer, GO could be dispersed throughout the PS to act as an effective nanofiller. GO-nanocomposites have also been used as a shell stabilizer in core−shell capsule systems. Rajendra et al. fabricated capsules using a composite comprising GO/poly(allylamine hydrochloride), and successfully utilized them for encapsulation of multiple drugs.16 They found that the core−shell capsules exhibited lower permeability than anticipated for several positively charged molecules (dyes, drugs, and especially proteins) because of adsorption to the walls of the capsules. Zhang et al. fabricated a versatile and superelastic capsule using an oil-in-water emulsion based PS/graphene composite. This capsule showed unique mechanical properties and superhydrophobic properties, making it a promising material for multifunctional applications.28 Micrometer-sized hollow capsules have also been achieved from composite graphene oxide systems with small molecule cross-linkers using a Pickering emulsion strategy.27 It is suggested that such microcapsules represent an excellent material for encapsulation technology, with the principle demonstrated by showing the loading of gold nanoparticles.28

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RESULT AND DISCUSSION Characterization of GO. Graphene oxide sheets were synthesized from natural graphite using a modified Hummers method.35 The oxidation process induces the amphiphilicity of the sheets through the incorporation of hydroxyl, epoxy, and carboxylic acid groups into their structure, whereby the GO 18188

DOI: 10.1021/acsami.7b02576 ACS Appl. Mater. Interfaces 2017, 9, 18187−18198

Research Article

ACS Applied Materials & Interfaces

Figure 2. Photographs and optical microscopy images of emulsions stabilized by GO sheets prepared using different GO concentrations at a 1:1 volume ratio of oil/aqueous GO dispersion: (a) photographs of as-prepared emulsion samples, (b−e) micrographs of the emulsion samples at GO concentrations of (b) 0.5, (c) 1, (d) 2, and (e) 3 mg/mL.

potential. The pH effect on GO dispersions can be observed in Figure 1i, which shows that aggregation of the GO occurs at very low and very high pH values, likely due to the protonation of the carboxylic acid groups and high ionic strength, respectively. It has been shown previously that GO has the capacity to serve as an amphiphilic stabilizer, because of the differences in hydrophobicity between the periphery functional groups and the basal plane.8 Conventionally, the effectiveness of a surfactant is assessed by its ability to reduce the interfacial tension (IFT) between two immiscible fluids. Some evidence for spontaneous adsorption of GO at air−water and oil−water interfaces has been shown, including surface pressure effects.12 It has been found that the surface tension of aqueous GO dispersions in very basic conditions (pH 14) was very high, indicating that the GO sheets are behaving poorly as surfactants and partition preferentially into the bulk aqueous phase, although at these extreme conditions, it is likely that the GO is reduced and chemically altered.10 To gain more insight into the ability of GO to act as a surfactant and assemble at the oil−water interfaces of interest in this study, tensiometry measurements were performed using toluene and olive oil as the continuous phase. The interfacial tension was then measured between each oil and an aqueous GO dispersion as a function of the GO concentration, electrolyte type and electrolyte concentration; these results are shown in Figure 1e, f, g. In each case, data were gathered for several hundred seconds in order to ensure that equilibrium had been reached. The equilibrium IFT values at different GO concentrations showed a marginal decrease in the IFT value from 27 mN/m for olive oil and water to a minimum of 23 mN/m with 1 mg/mL of GO present (Figure 1e). Curiously, the IFT increased as more GO was added, although none of the results differ particularly significantly from the GO-free interfacial tension. From these results, we can conclude that GO sheets show little evidence of spontaneous adsorption at the olive oil−water interface, indicating that GO is not behaving as a molecular surfactant in these systems. The same trend in interfacial tension is observed using toluene (Figure 1e). This suggests that the emulsions stabilized by GO that are prepared below are of the Pickering/ Ramsden (particle-stabilized) form, as hypothesized previously.10,27

sheets can be readily dispersed in water with high stability. From a colloidal perspective, this means that they compare favorably to pristine graphene which cannot be dispersed in water without the aid of stabilizing agents and vigorous sonication. Atomic force microscopy (AFM) imaging of the GO sheets when spin-coated onto a glass slide show their apparent 2D structure, and are concordant with the expected thickness of approximately 1 nm (Figure 1a, b). The product thus formed appears to be exclusively monolayer GO. The behavior of GO sheets dispersed in water can be influenced by the presence of electrolyte in the solution, and we anticipate that this should have significant effects on the propensity of the GO sheets to adsorb at interfaces and act as a stabilizer. Because of the highly charged nature of GO, purportedly resulting from the carboxylate functionalities, GO is susceptible to charge screening by the addition of salt.36 Zeta potential measurements, shown in Figure 1b, c, were performed to investigate the effects of two simple salts (NaCl and CaCl2) on the electrokinetic behavior of GO, and the zeta potential was found to be highly dependent on the type and concentration of the electrolyte. The results showed that the magnitude of the zeta potential decreased as the electrolyte concentration increased for both CaCl2 and NaCl, indicating a decrease in the surface charge on the colloids. Coagulation of the sheets could then be observed at the higher salt concentrations (Figure 1h). The results also showed that the divalent Ca2+ cation was much more efficient at causing aggregation of the GO sheets (Figure 1h), which is consistent with the Schulze-Hardy rule.37 From Figure 1d, it can be seen that in the presence of increasing concentrations of calcium chloride, the magnitude of the zeta potential values rapidly decrease from around −50 to −10 mV as the CaCl2 concentration increased from 0.1 mM to 100 mM, whereas over the same concentration range for NaCl, this decrease was much less pronounced (−50 to −32 mV). At pH 1, where the extent of deprotonation of the carboxylic acid functionalities on GO is considerably lower (Figure 1c), the zeta potential is only −30 mV at the lowest salt concentrations (0.1 mM), and decreases in magnitude to around −25 mV or −10 mV for sodium chloride and calcium chloride, respectively. It is also likely that the higher effective ionic strength at pH 1 when compared to pH 5 contributes to counterion adsorption/ pairing, and this may also decrease the magnitude of the zeta 18189

DOI: 10.1021/acsami.7b02576 ACS Appl. Mater. Interfaces 2017, 9, 18187−18198

Research Article

ACS Applied Materials & Interfaces

Figure 3. Histograms showing droplet size distributions in olive oil and toluene emulsions stabilized by GO sheets, prepared from different starting GO concentrations at a 1:1 volume ratio of oil/aqueous GO dispersion.

Figure 2 shows photographs and optical microscopy images of emulsions stabilized by GO sheets that have been prepared using different GO concentrations. All emulsions were prepared using a 1:1 volume ratio of oil to aqueous GO dispersion, where the oil phase comprises olive oil or toluene. As GO concentration was increased, the size of the emulsion droplets decreased, whereas the emulsion stability increased, indicating that the presence of additional GO enhances stabilization of the droplet interfaces. The decreasing droplet size with additional GO may suggest that it is behaving as a limiting stabilizer−i.e. that in order to stabilize more oil−water interface, more GO is required. The obtained droplet size distributions for emulsions with different starting GO concentrations are shown in Figure 3. These data confirm the trends seen using optical microscopy, that increasing GO concentration results in smaller droplet sizes for a given mixing time, indicating that GO is likely acting as a limiting interfacial stabilizer. Interestingly, the formation of multiple emulsions (i.e., waterin-oil-in-water, w/o/w) was observed when olive oil was used as

As mentioned previously, the dispersion behavior of GO is strongly affected by the addition of salt, and this has also been seen by Tang et al.11 using zeta potential measurements. IFT data showing the effects of added electrolyte are presented in Figure 1f with olive oil and Figure 1g with toluene. As with zeta potential, the IFT values are influenced more by CaCl2 than NaCl, and this is the case for both olive oil and toluene. Emulsions Stabilized by GO Sheets. The apparent level of adsorption of graphene oxide at the oil−water interface depends strongly on a variety of factors including the preparation conditions for the emulsions and the mechanism of emulsification. Although graphene oxide stabilized oil-in-water emulsions have been studied in some depth previously,11,38 here we provide observations and measurements pertinent to the oils that are the focus of this study. In this section, we present intensive information on the properties of GO-stabilized emulsions across a range of experimental conditions including the GO concentration, pH, oil, electrolyte type and concentration, volume fraction, and ultrasonication time. 18190

DOI: 10.1021/acsami.7b02576 ACS Appl. Mater. Interfaces 2017, 9, 18187−18198

Research Article

ACS Applied Materials & Interfaces

Figure 4. Fluorescence and transmission optical micrographs showing multiple emulsion formation for olive oil and single emulsion formation for toluene with aqueous GO solutions; (a) fluorescence image of an olive oil emulsion, (b) optical microscopy image of an olive oil emulsion, (c) fluorescence image of a toluene emulsion, (d) optical microscopy image of a toluene emulsion. All samples were prepared with 1 mg/mL GO at pH 1.

emulsions, as the pH was increased, the emulsion droplet size also increased while the emulsion stability significantly decreased, with the majority of emulsion droplets unstable to coalescence at pH 11. This is likely a result of the emulsions becoming too strongly charged as a result of carboxyl deprotonation, hence the GO sheets preferentially exist in the bulk water solution instead of the oil/water interface. In contrast, when olive oil was used, the droplet size decreased significantly and emulsion stability increased when increasing the pH, which appears to reflect the more complex chemical composition of olive oil. Olive oil comprises primarily triglycerides of long chain fatty acids (with the majority of chains being oleic acid, a singly cis-unsaturated C18 hydrocarbon). The oil also contains as minor components (generally 10 mm below the layer, capturing only signal from dye that had been released into the bulk subnatant aqueous phase. The UV−visible absorbance at a fixed wavelength of 532 nm was monitored as a function of time, and converted to an absolute dye concentration using a calibration curve.

present) is only 110 min, whereas with 20% PS present in the shell, the release half-life is 47 h. The total amount released is evidently limited by the low volume fraction of the internal water droplets and the starting concentration of the dye used during the encapsulation step, and thus higher loadings could be achieved by modulating either of these variables.

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CONCLUSION In this work, we have demonstrated a new, one-step encapsulation system by self-assembly of a polystyrene/graphene oxide (PS/GO) composite material at the interface of multiple emulsion droplets formed from olive oil and toluene mixtures. First, toluene and olive oil emulsions stabilized by GO were optimized as a function of various control parameters including GO concentration, solution pH, electrolyte concentration and identity, effects of dilution, and ultrasonication time. In general, multiple emulsions were produced by olive oil, and single emulsions were produced when toluene was used as the oil phase. Olive oil multiple emulsions were investigated as a facile, onestep route to prepare capsules stabilized by a PS/GO composite shell, and this was achieved by delivering GO to the interface from the aqueous phase and PS from the oil phase. This allowed successful encapsulation of small water droplets containing a cargo material within the PS/GO-coated oil drops. The capsule morphology and release properties were explored, and it was shown that these capsules could successfully delay the release of a model water-soluble cargo material, with release rates up to 11× lower than for the pure GO stabilized multiple emulsions. Overall, these results show that careful control of surface chemistry and emulsification conditions can provide a route to a facile, one-step process for forming complex, multiple-compartment capsules with two different immisicble liquids present. These capsules are stabilized by a composite PS/GO shell that offers a barrier to the release of their internal cargo. Because of the small internal water droplets within the capsules, these materials are ideal for use in circumstances where small amounts of material release are required over long (multiday) time scales, and may find use in, for example, the controlled delivery of nutrients or pesticides for agriculture, or the controlled release of biocides in water treatment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 3 9905 4558. Fax: +61 3 9905 4597. ORCID

Rico F. Tabor: 0000-0003-2926-0095 Notes

The authors declare no competing financial interest.

2. EXPERIMENTAL SECTION

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Synthesis and Purification of GO. GO was prepared from graphite powder (Sigma-Aldrich) by oxidation according to the modified Hummers method described by Marcano et al.44 Briefly, graphite powder (1 g) was dispersed in 113 mL of a concentrated mixture of sulfuric:phosphoric acids, (9:1 volume ratio) at room temperature. Under slow stirring potassium permanganate (6 g) was added slowly, after which the temperature was increased to 50 °C, and the mixture was stirred for 9 h, after which an orange/brown product was obtained. After letting the mixture cool to room temperature, the reaction mixture was poured onto 300 mL ice with approximately 1 mL of 30% H2O2. The mixture was then filtered and centrifuged at 6000 rpm for 1 h, discarding the supernatant liquid. For further purification, the GO was redispersed in ultrapure water and centrifuged at 6000 rpm for 1 h, again discarding the supernatant; the GO was then dialyzed against pure water for 1 week (cellulose dialysis tubing, molecular weight cutoff 14 kDa, SigmaAldrich), changing the water twice daily. Formulation of GO Emulsions. GO emulsions with toluene and olive oil were prepared by mixing an equal volume of oil with aqueous GO suspension to form an oil-in-water system with vigorous shaking at room temperature, followed by ultrasonication using a titanium-tipped ultrasonic horn (Branson 450 Sonifier). Other conditions of each experiment such as GO concentration, pH, ultrasonication time, and postformulation dilution were varied. The effect of electrolytes on the

ACKNOWLEDGMENTS The authors thank the Monash Centre for Atomically Thin Materials (MCATM) for supporting the facilities used in this research, and for a top-up scholarship (M.A.). This work was supported in part by the grant of an ARC Future Fellowship (R.F.T.).

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REFERENCES

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Research Article

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DOI: 10.1021/acsami.7b02576 ACS Appl. Mater. Interfaces 2017, 9, 18187−18198