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Photothermal breaking of emulsions stabilized with graphene Matthew Quinn, Khu Vu, Stephen Madden, and Shannon M Notley ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00737 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 12, 2016

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ACS Applied Materials & Interfaces

Photothermal breaking of emulsions stabilized with graphene Matthew D. J. Quinn, Khu Vu, Stephen Madden and Shannon M. Notley* Matthew D. J. Quinn Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra 0200 Australia Dr Khu Vu, Assoc. Prof. Stephen Madden Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra 0200 Australia Assoc. Prof. Shannon M. Notley Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra 0200 Australia E-mail: [email protected] KEYWORDS photothermal, graphene, particle stabilized emulsion, near infrared, 2D, emulsion breaking ABSTRACT: Pristine graphene particles prepared using an aqueous phase exfoliation technique have been used to promote the stabilization of emulsions through adsorption at the oil-water interface. Highly localized phase separation of these ultra-stable emulsions could however be induced through photothermal heating of the graphene particles at the interface exposed to near infrared light. The graphene wettability, which is a key determinant in preventing droplet coalescence was altered through the adsorption of nonionic block copolymer surfactants. Varying the aqueous solution conditions influenced the hydration of the hydrophilic component of the surfactant providing a further opportunity to alter the overall particle wettability and hence stability of the emulsion. In this way, highly stable o/w emulsions were produced with decane however w/o emulsions were formed with toluene as the oil phase.

Introduction Dispersions of an immiscible liquid in a continuous phase of another liquid, or emulsions, are common in many cosmetics,1 food , medical2-4 and industrial applications. For macroemulsions where the droplet size is typically greater than 1 μm, both oil in water (o/w) and water in oil (w/o) types are possible. Surfactants are usually added in order to reduce the interfacial tension (the energetic cost of creating oil-water surface area) between the oil and water phases that results in significantly improved stability of the dispersed liquid droplets against phase separation. The type of emulsion depends largely on the properties of the surfactant or emulsifier used as well as the relative proportion of oil and water in addition to the preparation method. Macroemulsions are not thermodynamically stable, due in large part to the high interfacial area. Adsorption and desorption of amphiphilic emulsifiers such as surfactants at the oil-water interface is dynamic which eventually results in the formation of necks between closely associated droplets. The energy of removing a surfactant monomer from the interface is approximately the same as the thermal energy of the system. Increased stability of the emulsion can be achieved however by using particles to form so called Pickering emulsions. These particles typi-

cally have intermediate wettability (contact angle with water ~ 90°) and their size means that the energy required to remove them from the interface is orders of magnitude greater than the thermal energy.5, 6 Hence Pickering emulsions are stable over greater time periods than emulsions formed by using simple surfactants. Particles of different morphologies, sizes and chemistries have been employed in the stabilization of emulsions7, 8 with an increased focus on particulate carbon materials such as nanotubes9-11 and graphene oxide.12-16 A typical feature is to tune the relative wettability using adjustments to pH which influences surface potential of particles with weakly ionizable groups.17 A recent study used graphene oxide in order to improve the stability of o/w emulsions with the pH of the solution highly influential for the formation of the particle stabilized emulsion.16 The use of graphene oxide or similar atomically thin crystalline materials is attractive for a number of reasons but perhaps the most important is the vast surface area to volume ratio.18-20 Furthermore, graphene and analogous single layered 2D particles such as MoS2 have interesting electronic, optical, thermal and mechanical properties.21-23 Aside from the large interfacial area, 2D particles also strongly interact with incident light as the molecularly thin sheets allow all atoms to participate in absorption, which is distinct from the corresponding 3D materials.24

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Graphene was produced using the liquid phase exfoliation technique with the assistance of added surfactant. Typically a 1 % w/w suspension of graphite in de-ionized water (Milli-Q water, Millipore) was prepared. The suspension is prepared with the appropriate concentration of the non-ionic surfactant in order to maintain the surface tension of the aqueous phase within the range of 40 - 42 mJm-2 during sonication. This surface tension has been shown to be the optimum for efficient exfoliation. The surfactants employed for these experiments have varying lengths of the hydrophobic and hydrophilic blocks. The PPO segments are adsorbed to the hydrophobic planes of the 2D sheets with the PEO regions being exposed into solution. It is the interchain repulsion between these extended PEO segments of the surfactants that drives the steric repulsion providing the suspension with the impressive stabilization.

Graphene also absorbs light across a broad spectrum. This includes in the near infrared (NIR) region for pristine graphene or highly reduced graphene oxide due to the highly conjugated nature of extended π bonded network.25, 26 Hence, graphene shows great promise in biomedical applications where exposure to NIR radiation induces photothermal heating. Top down methods for the production of graphene particles are of most relevance for the subsequent use in the preparation of particle stabilized emulsions. These methods include techniques such as intercalation, chemical exfoliation or oxidation of graphite with subsequent reduction after exfoliation.27 These produce 2D particles with varying sizes and defect levels in suspensions but can be scaled to provide large quantities. Another promising method for the high yield production of graphene and other atomic crystalline materials is aqueous phase exfoliation using sonication in the presence of surfactants.28-30 Typically a surfactant is added to the aqueous phase to reduce the energy of cohesion of water to that of the van der Waals bonded solid. Shear is imparted through sonication in order to exfoliate the particles. The surfactant also adsorbs onto the graphene particle surface inhibiting re-aggregation.31 Ionic surfactants achieve this through increasing the particulate surface potential whereas polymeric surfactants prevent flocculation through steric repulsion. This method produces graphene with only edge defects with lateral dimensions typically less than 1 μm.31, 32

Characterization of particles A JEOL 2100F transmission electron microscope (TEM) was used to image the exfoliated graphene sheets (Figure 1). Particles were deposited onto holey carbon TEM grids by vacuum suctioning 100 µL of suspension and dried for 24 hours prior to imaging.

Whilst inhibiting phase separation is important, many applications such as in drug delivery may benefit from controlled breaking of emulsions on action of any external stimulus.33, 34

Figure 1. The transmission electron microscopy images collected show several exfoliated graphene sheets, highlighting both the extent of exfoliation as well as the quantity of sheets that can be easily produced via the liquid exfoliation method.

Here, graphene stabilized emulsions are demonstrated along with localized phase separation due to photothermal heating on absorption of NIR light by the interfacial graphene particles.

The particles produced using the liquid phase exfoliation method were characterized using Raman spectroscopy using an alpha 300A Raman system from WITEC with laser excitation at 532 nm. The particles were initially added drop wise to a 0.22 µm pore size alumina filter (Whatman) prior to measurement. A minimum of 20 individual particles were measured and the data presented here is representative of the exfoliated material (Figure 2).

Experimental Section Materials Natural graphite flakes were purchased from Sigma Aldrich and used without purification. In the liquid phase exfoliation method for producing single layer graphene, the addition of surfactant aids in both lowering the interfacial energy of the liquid, whilst additionally preventing reaggregation upon exfoliation. Furthermore, by choosing surfactants with varying hydrophiliclipophilic balance, o/w or w/o emulsions could be prepared. Three different triblock copolymers were used in this study: F127, P123 and L64 (see Supplementary Table 1 for surfactant composition data). All were purchased from Sigma Aldrich Australia and used without further purification. All suspensions and emulsions were prepared using Milli Q water. The emulsification was performed using two different oils: decane and toluene that were of analytical grade.

Figure 2. Raman spectra were collected to confirm exfoliation of liquid exfoliated graphene sheets. The shape of the 2D peak and the

Preparation of suspensions of exfoliated graphene

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positioning of the G peak both indicate highly exfoliated sample toward monolayer graphene. The D peak is expected to be present for the Raman spectra collected for sheets of this size as the laser diameter is larger than the sheets and will therefore detect the SP3 hybridized carbon atoms of the sheet edges. The ratio of the intensities of the 2D to the G peak is approximately 1:2, again indicating highly exfoliated graphene.

phene lattice structure, an inherent fluorescence of the particles is observed. Visualization of the graphene sheets within the emulsions was performed using a FV10i Fluoview (Olympus, Japan) confocal laser scanning microscope (CSLM). Preparation of emulsions

The exfoliated graphene particles were characterized using UV-visible-NIR spectrophotometry that was carried out on a UV-310PC (Shimadzu)(Figure 3). Prior to measurement, the samples were diluted using Milli-Q water. The spectra were recorded using quartz cuvettes in the wavelength range of 200 nm to 1300 nm. The concentration of particles in suspension used herein was determined from the extinction coefficient of 1390 L g-1 m-1 @ 660 nm for graphene.30

Graphene suspension (2 mL) was added to the oil (2 mL) followed by agitation by hand for approximately 30 seconds. In order to test the influence of salt concentration, increasing amounts of NaCl was added to the aqueous phase prior to emulsification. The samples were left to stand in a glass vial for 3 days prior to image capture. In some cases, the emulsions were stable for more than 6 months. The electrical conductivity of the emulsion was subsequently measured in order to determine whether o/w or w/o emulsions were formed. Emulsion stability analysis Emulsion sets and their respective blanks were prepared and left to stand with regular images take to observe the rate of emulsion destabilization. Optical microscopy was performed in order to determine the size distribution of the emulsions prepared over time. A small drop of the emulsion to be measured was placed onto a microscope slide with a coverslip placed on top. After 60 seconds waiting period images were taken and the drop size was measured using ImageJ software. The average of 3 drops per emulsion sample was used.

Figure 3. The absorbance scan above shows the characteristic absorption peak for graphene at 270 nm but importantly the strong broad band absorption allowing wavelengths from the ultra violet region all the way through the infrared to be absorbed. The absorbance @ 660 nm = 1.32 and is used in conjunction with the extinction coefficient of 1390 L g-1 m-1 to determine the concentration of the graphene solution employed. The smaller peak at 205 nm is due to the surfactant absorption.

Breaking of emulsions Photothermal heating of water and the aqueous graphene suspension with a concentration of 0.1 mg/mL was first investigated prior to experiments with emulsions. 2 mL samples were placed in cuvettes and irradiated with two different NIR lasers (808 nm @ 500 mW, and 980 nm @ 200 mW) for a period of 10 minutes. A thermocouple measured the temperature of the sample at a height of approximately 12 mm above the incident laser beam. Photothermal heating of the emulsions in the absence and presence of graphene was subsequently undertaken using the NIR lasers. Graphene is highly efficient at converting incident light irradiation into heat due to strong absorption in the NIR region as demonstrated by the UV-visible-NIR spectra shown in Figure 3.

The zeta potential of the exfoliated nanoparticles stabilized with non-ionic surfactant was determined 90Plus Particle Size Analyzer from Brookhaven Instruments Corporation (Figure S1). The measurements were performed in Milli-Q water (pH 6.5 unless otherwise stated). Typically 5 measurements of zeta potential using 4 separate samples were undertaken. The value reported here is an average of these 20 measurements. The particle size distribution was also measured using this instrument. These measurements are based on dynamic light scattering (DLS) with an equivalent sphere diameter reported here. It is clear from direct TEM imaging though, that the particles have a very high aspect ratio and hence the utility of the light scattering approach to determine particle size is questionable. However the equivalent sphere derived from DLS can provide a guide to the effective size of the graphene particles that may be interfacially active under certain solution conditions (Figure S2).

Results and Discussion The exfoliated graphene particles were thoroughly characterized prior to their use in the emulsion stabilization studies. A high proportion of single and few-layer material was observed as has been previously shown for the same preparation method for exfoliated graphene.28 Figure 1 shows a typical TEM image of the exfoliated particles demonstrating that the surfactant assisted exfoliation method with continuous surfactant addition efficiently produces particles with a single unit cell thickness.

Confocal microscopy was used to characterize the emulsion structure. In particular, confocal microscopy is useful to demonstrate that the graphene particles are at the oil-water interface (Figure S3). Due to the extended conjugation of SP2 hybridized carbon atoms in the gra-

The exfoliation down to single and few layers of graphene is also confirmed from the Raman spectra shown in Figure 2 with the three major peaks at 1335 cm-1, 1587 cm-1

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and 2675 cm-1 corresponding to the D, G and 2D peaks. The 2D peak is shifted to lower wavenumber than bulk graphite and shows a single symmetric peak indicative of single layer graphene.35, 36 Furthermore, the D peak intensity is relatively low in comparison to the G peak indicating graphene sheets with few defects. As the particles themselves are much smaller in lateral dimensions than the laser spot, it is likely that the defects are located on the edges. The intensity ratio of the G and 2D peaks is approximately 1:2 indicating a high proportion of single layer material.37 Strong absorbance across all wavelengths in the UV-visible-NIR spectrum (also shown in Figure 3) for the exfoliated graphene also confirms the extended conjugation of the graphene sheets with the peak at 270 nm also suggesting little to no defects are present in basal plane. Importantly, these graphene sheets absorb light in the NIR region necessary for the subsequent photothermal studies with determined extinction coefficients of 1257 and 1311 Lg-1m-1 at 808 nm and 980 nm respectively.

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fects, the first relating to the surface forces between particles and the second relating to the wettability of the particles. This latter property can be highlighted particularly when observing the contrasting behavior of these suspensions in emulsions formed with different oil phases. Figure 4 shows the emulsions formed when an aqueous graphene suspension is mixed with decane as a function of increasing salt concentration. Two different surfactants were used in this instance to stabilize the graphene sheets, L64 and P123 that have HLB values of 12-18 and 7-9 respectively. In Figure 4 it can be seen that the particles with the more hydrophobic surfactant P123, adsorbed spontaneously to form an emulsion in decane through efficient immobilization at the interface. This suggests in the absence of salt the graphene-P123 particles have the intermediate wettability required to effectively stabilize the decane-water interface. The graphene-P123 particles become increasingly hydrophobic upon increasing the electrolyte concentration resulting in an emulsion with lower stability. Furthermore, at salt concentrations greater than 0.2 M, significant aggregation of the graphene sheets was apparent through the presence of a sedimentious material at the bottom of the vials. Conversely, the graphene-L64 particles are relatively hydrophilic in the absence of any added salt and favorably remain in the aqueous phase. Increasing the electrolyte again reduces the hydrophilicity of the graphene-L64 particles resulting in the formation of increasing amounts of stable emulsions. Particles that remained in the aqueous phase do however show obvious signs of increased sedimentation at high salt concentrations after a few weeks reflecting the reduced stability of these particles against aggregation.

The zeta potential of the exfoliated graphene particles was determined through electrophoretic mobility measurements. The graphene samples have a relatively low potential of –5 mV at pH 6 (Figure S1). This suggests that the graphene sheets have few or no defects in the basal plane and the charge arises from the edges. Importantly, the charge is low resulting in only mild repulsion between 2D sheets due to overlapping electrical double layers, steric effects due to the adsorbed polymeric surfactants dominate the stabilization mechanism. Hence strategies that act to reduce the steric stabilization effect are necessary so that the intimate contact required between the particles is achievable in order to stabilize the oil-water interface. The two oil phases chosen for use in the preparation of particle stabilized emulsions were toluene and decane. Toluene is poorly soluble in the aqueous phase (less than 0.1% w/w at 293K) whereas decane is insoluble. The interfacial energies of toluene-water and decane-water interfaces are 36.6 mNm-1 and 51.2 mNm-1 respectively. These interfacial tensions are of the order of the surface energy of the unmodified graphene (41 mNm-1) however surfactant adsorption can change this depending on the relative balance of hydrophilic and lipophilic components (or HLB value). Thus, the exfoliated graphene particles stabilized with various non-ionic surfactants, having differing affinities for the interface depending on the oil phase, however, the wettability of the particles can be further tuned using salt addition.

Figure 4. Decane in water particle stabilized emulsions formed from exfoliated graphene stabilized with the non-ionic surfactant L64 (top) and P123 (bottom). The salt concentration increases from left to right in the order 0.01, 0.05, 0.2, 0.5 and 1.2 M.

Oil in water (o/w) type emulsions were observed for all of the graphene-surfactant particles when the aqueous suspension was used in conjunction with decane. This was confirmed through electrical conductivity measurements of the emulsions that showed conductivities of the order of the aqueous phase. The emulsions typically prepared using these surfactant modified graphene sheets were stable for many weeks with no obvious signs of droplet coalescence. Indeed, the stability of the emulsion

Under low electrolyte conditions the hydrophilic nature of the PEO chains results in the chains being extended away from the interface and into solution. Increasing the concentration of the electrolyte in the aqueous phase results in the collapse of the PEO chain segments toward the surface of the graphene sheet. This is due in large part to the dehydration of the ethylene oxide groups that effectively reduces the hydrophilicity of the graphenepolymer particles. Hence the addition of salt has two ef-

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was much greater than those formed using just the nonionic block co-polymer surfactants (as shown in Figures 5 and 6).

Figure 7. Emulsions formed using an aqueous suspension of F127graphene with decane (top) and toluene (bottom). The salt concentration increases from left to right in the order 0.01, 0.05, 0.2, 0.5 and 1.2 M.

Figure 5. Histogram showing the frequency of drop sizes for l64graphene stabilized emulsions (red) compared to the emulsions without the particles present (blue) after 60 minutes of emulsion preparation. Cumulative distribution is included for both the graphene containing (red) and free emulsions (blue).

Figure 7 also shows the emulsions formed with the aqueous suspension of graphene-F127 and toluene as the oil phase. Clearly there is a significant difference in the emulsions with toluene in comparison to decane. Conductivity measurements show that a water in oil (w/o) type emulsion is formed with a decreasing volume observed with increasing salt concentration. The interfacial tension between toluene and water (36.6 mJm-2) is significantly lower than that of decane and water (51.2 mJm-2) and indeed toluene has some affinity for water due to its (very) weakly polar nature. The formation of w/o emulsions suggests that the oil phase preferentially wets the graphene-F127 particles however increasing the salt concentration further dehydrates the stabilizing polyethylene oxide chains leading to increased hydrophobicity and diminished emulsion volume. The surfactants used here are able to stabilize the oilwater interface without graphene. Figure 8 shows example optical microscopy images of the droplets formed in the o/w emulsions using the neat F127 surfactant and graphene-F127 particles 1 hour after preparation. The mean size over 24 hours of the droplets is smaller for the sample with graphene indicating the improved stability afforded by the presence of particles at the decane-water interface (Figures S4 and S5). Indeed, the emulsions with graphene present were stable for a period of more than 6 months in comparison to those with simply the neat surfactant that showed earlier signs of degradation. Short term drop size distribution analysis was the most appropriate method to demonstrate the improved stability in the presence of the particles (See Table 1).

Figure 6. Frequency of drop sizes for L64-graphene stabilized emulsion (red) and the graphene free emulsion (blue) after 24 hours. The increasing mean size of the droplets over time indicates faster destabilization of the non-particle stabilized emulsion drops. Cumulative distribution is included for both the graphene containing (red) and free emulsions (blue).

Graphene particles stabilized with the very hydrophilic (HLB > 24) surfactant F127 were also used in the preparation of emulsions with different oil phases as shown in Figure 7. The graphene-F127 particles showed similar behavior to the graphene-L64 particles when forming an emulsion with decane. That is, with increasing salt concentration, a greater volume of o/w emulsion was observed. In contrast to the graphene-L64 particles, the emulsion was stable for longer with the graphene-F127 particles and no significant sedimentation was apparent over a period of weeks. This reflects the greater ability of the F127 surfactant to sterically stabilize the particles against aggregation due to the longer ethylene oxide chains extending into the aqueous solution even in the semi-dehydrated state.

Figure 8. Optical microscopy images of emulsions formed with decane and (left) aqueous graphene suspension or (right) aqueous

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surfactant solution. Droplet size distributions with and without graphene at time = 60 minutes. Day 1 2

L64-Graphene Emulsion Median Size (µm) 4.413 7.197

suspension. Figure 9 shows that heating of water by the NIR laser is minimal over a 20 minute period however the temperature of the aqueous graphene suspension increased significantly due to absorption of light and transduction to heat. It is important to note, that the concentration of graphene in the suspension is relatively low (0.1 mg/ml) and higher concentrations are likely to elicit a more dramatic heating effect as demonstrated for gold nanoparticles previously.40 Similarly, using a reduced volume of suspension would also give rise to a higher observed temperature increase.

L64 Emulsion Median Size (µm) 9.271 12.802

Table 1. Median size of L64-graphene stabilised and blank emulsions over time showing the rate of the increase in emulsion drop size over time is larger for the blank emulsion samples, indicative of a faster destabilization of the emulsion. The free energy required to remove the solid graphene particles from the interface can be estimated using equation 1 where r is the graphene particle radius, γow is the decane-water interfacial tension and θ is the contact angle. From equation 1, the maximum energy occurs when the contact angle of the particle at the three phase line is 90°. For graphene particles with an effective radius of 75 nm, the free energy is ~ 1.5 x 105 kT. Even for a significantly lower contact angle of 20°, the free energy cost is still > 500 kT. This compares to the neat surfactant where the dynamic adsorption to and desorption from the interface is of the order of the thermal energy of the system. The relative diffusion rate for surfactant monomers is also significantly higher than that of large particles making the creation of the necessary fluctuation of the adsorbed layer density more likely to induce coalescence. Equation 1:

E = π r 2γ ow (1± cosθ )

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Figure 9. Photothermal heating curves of the aqueous surfactant solution and the aqueous graphene suspension (0.1 mg/mL) upon irradiation with a 500 mW laser with a wavelength of 808 nm.

Photothermal heating and cooling experiments were also performed to assess the stability of the graphene suspensions against aggregation at elevated temperatures. Furthermore, cycling experiments allow any loss in efficiency due to potential morphological changes to be probed, as previously associated with the use of gold nanoparticles. Figure 10 shows the observed temperature changes during 10 laser irradiation heating and ambient cooling cycles. The maximum temperature of the suspension remained constant, within experimental error, for each cycle as did the rate of heating. This indicates that the pristine graphene particles in suspension are capable of acting as an efficient transducer after multiple cycles of heating and cooling in contrast to other nanoparticulate photothermal transducers. Furthermore, the data in Figure 10 demonstrates that the surfactant coating was not compromised upon the intense temperatures experienced by the graphene sheets. In addition absorbance scans were performed before and after the cycling experiments and there was no significant change observed.

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The presence of graphene at the interface clearly inhibits phase separation. At the concentrations used here of 0.1 % w/w, little change to the viscosity or density of the aqueous phase is observed so gravitational effects are minimal. The exfoliated 2D materials have a vast surface area in comparison to many other nano and micrometer sized particles that have been previously used. Indeed, the fully exfoliated surface area of graphene is 2630 m2 g-1.38 The surface area of the graphene sheets generated using the surfactant assisted ultrasonic exfoliation technique however is somewhat lower due to the presence of fewlayer material and of the order of 500-1000 m2 g-1 depending on the process parameters. Thus, much lower mass concentrations of 2D particles should be able to stabilize the oil-water interface in comparison to the 3D analogues. Confocal microscopy was used to visualize the graphene at the interface. The highly conjugated nature of the bonding within graphene gives rise to fluorescence.39 Figure S3 shows a typical confocal image of a decane in water emulsion where graphene is clearly observed at the interface of the drops. The absorption spectrum in Figure 3 shows that the graphene particles strongly absorb light with wavelengths in the UV, visible and NIR regions. Interactions with NIR radiation are of particular use for biomedical applications as tissue is relatively transparent to light of these wavelengths. The ability to generate a thermal response upon laser irradiation was probed for the aqueous graphene

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Figure 10. Cycled photothermal heating curves of the aqueous graphene suspension (0.1 mg/mL) upon irradiation with a 500 mW laser with a wavelength of 808 nm. Thermocouple tip at 16 mm from laser site. Insert shows the maximum temperature reached after 20 minutes of heating for each cycle number.

Several studies have been performed whereby the temperature changed induced by the photothermal heating of graphene oxide suspensions have been measured. Typically, a significant drop in efficiency after several cycles is observed.41, 42 The pristine graphene suspension employed herein outperformed graphene oxide likely due to the stability provided by the surfactant coating imparting a strong repulsive mechanism. Graphene-L64 particle stabilized o/w (decane/water) emulsions as well as neat surfactant stabilized o/w emulsions were then irradiated using the 808 nm laser. Figure 11 shows a time series of optical images of the emulsions with the laser impinging from the left. In the neat surfactant sample, the laser is scattered by the oil droplets nearest to the cuvette wall. No changes in the images are observed with time indicating that the heating effect in the absence of graphene at the oil-water interface is minimal. However, the images of the graphene stabilized emulsions show that the laser light is progressively propagated through the emulsion. The laser power through the emulsion was detected as a function of time and shown in Figure 12. It can be seen that upon initial irradiation the transmitted light increases quickly indicating a reduction in scattering due to localized breaking of the emulsion. As droplets coalesce, the emulsion becomes more transparent due to a reduction in scattering allowing the laser light to penetrate further into the cell and eventually out the other side. In the absence of graphene at the interface, no significant laser light intensity was detected. Figure 12 also shows an image of the graphene stabilized emulsion demonstrating the “tunnel” created by the laser light due to the localized phase separation. The spot size of the laser is approximately 0.5 mm and the phase separated region remains for more than 2 weeks due to the relatively high viscosity of the o/w emulsion. No visible signs of phase separation were observed for the neat surfactant stabilized emulsion sample.

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Figure 11. Emulsion stability at t = 1, 2, 3, 16 and 60 s of exposure to 500 mW CW laser with wavelength of 808 nm in the presence of graphene (top) and in the absence of graphene (bottom). The laser light is initially only scattered at the cuvette wall however as the emulsion breaks, light scattering is progressively observed throughout the emulsion. A time lapse movie is included in the supplementary section.

non-water soluble drug delivery. The loading capabilities of emulsions are well understood and already in use for drug delivery purposes.44, 45 The laser spot size used here was relatively small giving rise to phase separation in a region of a similar scale. A greater ability to tightly target delivery of the payload can be achieved with such control over emulsion breaking. Other light sources could also be used due to the strong broad-spectrum absorbance of graphene (Figures S6 and S7 show the same experiment with a 980 nm wavelength laser). This opens up the potential for controlling the stability for a range of industrially relevant emulsion systems.

Figure 12. The transmitted light was measured showing the delayed transmission for the graphene emulsion (red), compared to the completely scattered light occurring upon the blank emulsion irradation (blue). The delayed transmission indicates the time required for the photothermal “coring” to take place and allow the light to pass through to the detector.

It is clear that graphene present at the oil water interface has a significant effect on the stability of these emulsions. Equally, localized heating of the emulsion resulting in phase separation can be induced photothermally. The oil-water interfacial tension is highly sensitive to increases in temperature with the thermal conductivity of the continuous phase (water in this case) orders of magnitude smaller compared to that of graphene. This means that the heat generated due to light absorption by the graphene particles is not efficiently dissipated through the medium leading to a rapid, highly localized temperature increase and reduction in interfacial tension. The reduced oil-water interfacial tension changes the affinity of the graphene particles for the interface resulting in destabilization and droplet coalescence.

Conclusions Excellent control over the highly localized breaking of emulsions was achieved through the use of photothermal heating of the oil-water interface. The heating was confined to small volumes through the absorption of NIR radiation by graphene used to stabilize the emulsion droplets against coalescence. The wettability of the graphene particles was tuned through the adsorption of nonionic surfactants and the use of varying aqueous solution conditions in order to extend the lifetime of the emulsions by immobilizing the graphene to the oil-water interface. These Pickering emulsions using graphene showed excellent stability under ambient conditions until the action of the external heat trigger induced through exposure to NIR light resulted in phase separation. This study hence demonstrates a potential new option for targeted breaking of emulsions in biomedical applications such as drug delivery where the delivery of the payload to confined areas is a necessity to avoid potential side effects.

The surfactants used to stabilize the graphene against aggregation in the aqueous phase and tune the overall wettability of the particles themselves, become less soluble at elevated temperature due to the further dehydration of the polyethylene oxide chains. Indeed the cloud point for L64 is 58 °C in water but is further depressed to 40 °C in the presence of 1.0 M salt.43 It is likely then that the graphene particles become more hydrophobic. Together with the change in interfacial tension, the altered wettability of the particles provide the necessary driving forces to overcome the high energy cost associated with removing a particle from the oil-water interface.

Supporting Information. Supporting information includes zeta potential and particle sizing of graphene suspensions, confocal image of particle stabilized emulsion drops, drop sizing histograms as function of time, 980 nm irradiation experiment, thermal breaking of emulsions and a time lapse movie of emulsion laser coring. This material is available free of charge via the Internet at http://pubs.acs.org.

Light with a wavelength in the NIR was used in this study to demonstrate the utility of photothermally induced phase separation of an emulsion with a view toward potential biomedical applications and in particular

AUTHOR INFORMATION

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Corresponding Author *Assoc. Prof. Shannon M. Notley, E-mail: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT SN acknowledges financial support from the Australian Research Council through the Future Fellowship Scheme.

ABBREVIATIONS w/o, water in oil; o/w, oil in water; 2D, two dimensional; TEM, transmission electron micrscopy; DLS, dynamic light scattering; HLB, hydrophile lipophile balance; NIR, Near infrared.

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