One-step synthesis of porous graphene-based hydrogels containing oil droplets for drug delivery

Abstract

Three-dimensional graphene hydrogels with a porous network have attractive potential in electrode materials, catalysis, and drug delivery. A series of porous graphene-based hydrogels containing oil droplets is fabricated on the basis of Pickering emulsions co-stabilized by graphene oxide and Fe₃O₄ nanoparticles. The volume ratio of water to oil and the concentration of stabilizers have significant effects on Pickering emulsion stability. A hierarchical pore structure is observed in the hybrid hydrogels prepared by in situ reduction of graphene oxide. Owing to the inner oil droplets distributing in the hydrogels which can be loaded with oil-soluble materials, the obtained reduced graphene oxide hydrogels show sustained release of hydrophobic drugs, which makes them a candidate for drug delivery.

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1. Introduction

In recent years, considerable attention has been paid to graphene because of its excellent mechanical, electrical, photovoltaic, and optical properties.^1–6 Because of its one-atom thickness and two-dimensional plane, graphene possesses potential applications in energy-storage devices, electrodes, polymer composites, photoluminescence and low noise amplifiers.^7–10 Generally, graphene sheets can be synthesized through several chemical methods like chemical vapor deposition, micromechanical exfoliation, epitaxial growth, and the creation of colloidal suspensions.^11–15 Among various graphene products, graphene gels have attracted much interest all over the world as a result of their versatile functionalities. To date, different kinds of graphene-based hydrogels have been prepared such as reduced graphene oxide (RGO) film by gel coating,¹⁶ poly(N-isopropylacrylamide) (PNIPAAm) microgels mixed with graphene,¹⁷ and gels by in situ gelation of graphene oxide (GO) nanosheets.¹⁸ Contributing to the abundant oxygen functional groups, GO sheets can be dispersed well in water and are easily processable.^19,20 Therefore, some research groups have begun to focus on the assembly of three-dimensional (3D) graphene-based hydrogels by reducing GO back to hydrophobic graphene under heating or in the presence of reducing agents, such as NaHSO₃, Na₂S, vitamin C (VC), hydroquinone, which provides us an innovative method to produce monolithic graphene-based hydrogels. Furthermore, by means of embedding inorganic nanoparticles,²¹ mixing polymers with graphene,²² or forming interpenetrating network,²³ RGO hydrogels can be endowed with tremendous properties such as magnetism,²⁴ pH or temperature sensitivity,²¹ photoelectricity, and bi-compatibility.²⁵

Pickering emulsions are solid particle-stabilized emulsions without any molecular surfactants, and the study on them becomes more and more popular recently due to their excellent stability and applications.^26,27 Many kinds of inorganic or organic nanoparticles have been used to prepare Pickering emulsions,^28,29 including GO sheets³⁰ and Fe₃O₄.^31,32 Among numerous GO hybrids, GO/Fe₃O₄ hybrids are intensely studied. For example, Yu et al.³³ used a ferrocene-decorated graphene oxide sheets for formation of graphene oxide gel via the π-stacked supramolecular self-assembly. However, GO/Fe₃O₄ hybrids for Pickering emulsion stabilization is barely studied so far, and the work on the preparation of graphene hydrogels by emulsion templating is also limited, much less using Pickering emulsions. Although Song et al.³⁴ produced styrene-in-water Pickering emulsions stabilized by GO nanosheets with the ratio of water to oil of 20 : 1, the emulsions were not uniform enough. Generally, the interactions between GO and other polymers or particles is needed to stabilize Pickering emulsions,³⁵ because GO nanosheets are a little too hydrophilic to stabilize non-aromatic oil in the aqueous phase solely. In fact, the type of oil is crucial for the formation of the GO stabilized Pickering emulsions, and aromatic solvents with substituent groups or multiple benzene rings better stabilized the emulsions than other solvents.³⁶ The applications of GO stabilized Pickering emulsions are also studied in various fields, such as enhanced supercapacity GO covered polyaniline nanoparticles.³⁷ Recently, a few research groups have explored graphene gels for biomedical applications, like drug loading and delivery,^38,39 but the work on the preparation of 3D RGO hydrogels by Pickering emulsions is still scarce.

Herein, we used a one-step hydrothermal method to produce the porous graphene-based hydrogels using paraffin-in-water Pickering emulsions templates as shown in Fig. 1. RGO hydrogels containing oil droplets were in situ formed by reducing GO in the presence of VC at 90 °C. Fe₃O₄ nanoparticles were introduced into the GO aqueous solution. Both of GO nanosheets and Fe₃O₄ nanoparticles acted as the Pickering emulsion stabilizers. The addition of Fe₃O₄ nanoparticles can not only stabilize the emulsion templates, but also endow the obtained hydrogels with magnetic properties, which is not achievable by using conventional molecular surfactant. In order to load drugs, relatively healthy liquid paraffin was chosen as oil phase in our experiments. Moreover, 1-phenylazo-2-naphthalenol (PN) and ibuprofen (IBU) acting as hydrophobic drug models were loaded in the oil droplets within the graphene-based hydrogels. The release behaviors of PN and IBU from the hydrogels were investigated. Given their special structure, we believe that the porous graphene-based hydrogels will play a significant role in various fields like drug delivery, water purification and other applications.

Fig. 1 Illustration of preparation of graphene-based hydrogels containing oil droplets with drugs by paraffin-in-water emulsions stabilized by GO/Fe₃O₄ stabilizer.

2. Experimental section

2.1. Materials

GO was prepared from purified natural graphite (Shanghai Colloid Chemical Plant, China) by the modified Hummer's method.^40,41 Fe₃O₄ nanoparticles were prepared by the precipitation oxidation method.⁴² Liquid paraffin, paraffin wax, n-hexane, VC, and PN were of analytical grade and were purchased from Tianjin Damao chemical reagent factory (China). IBU was purchased from Sigma-Aldrich and used without further purification. Other reagents were guaranteed to be of analytical grade and used as received. Water used in all experiments was purified by deionization and filtration with a Millipore purification apparatus to a resistivity higher than 18.0 MΩ cm.

2.2. Preparation of Pickering emulsions and RGO hydrogels

A series of liquid paraffin-in-water (oil-in-water, O/W) Pickering emulsions stabilized by GO nanosheets and Fe₃O₄ nanoparticles was prepared as shown in Table 1. All the aqueous volumes were fixed at 4 mL in the preparation of emulsions. In a typical process, taking the preparation of sample S6 in Table 1 for example: first, 4 mL of aqueous dispersion containing GO (5 mg mL⁻¹), Fe₃O₄ (1.5 mg mL⁻¹), and VC (0.1 g) was mixed homogeneously by ultrasonication (100 W, 40 kHz) for 5 min. Then 4 mL of liquid paraffin was added to the above aqueous dispersion and the mixture was emulsified with a high speed shearing machine (8000 rpm, 2 min). The obtained emulsion was heated at 90 °C for 2 h without stirring. Finally, the as-prepared RGO hydrogel was washed by deionized water and anhydrous ethanol to remove the residual components.

Table 1 Parameters for O/W Pickering emulsions

Sample	GO (mg mL^−1)	Fe3O4 (mg mL^−1)	O : W (v/v)	Droplet size (μm)
S1	5	0	0 : 1	—
S2	5	0	1 : 2	113 ± 35
S3	5	0	1 : 1	119 ± 33
S4	5	0.5	1 : 1	115 ± 35
S5	5	1.0	1 : 1	118 ± 35
S6	5	1.5	1 : 1	118 ± 34
S7	5	2.0	1 : 1	121 ± 36
S8	1	1.5	1 : 1	108 ± 35
S9	2	1.5	1 : 1	115 ± 38
S10	5	1.5	1 : 2	110 ± 36
S11	0	1.5	1 : 1	99 ± 38

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2.3. Preparation of drug-loaded RGO hydrogels

PN or IBU-loaded graphene-based hydrogels were prepared in the same process except that PN or IBU were dissolved in liquid paraffin with a concentration of 1 mg mL⁻¹ before emulsification. The as-prepared RGO hydrogels were washed by deionized water several times until the washing liquid exhibited negligible UV absorbance emission of PN or IBU molecules, in order to ensure the removal of all the PN or IBU molecules that failed to be entrapped by the RGO hydrogels.

2.4. Preparation of anhydrous hydrogels

For the anhydrous gel S1 without liquid paraffin, the hydrogel S1 was freeze-dried by a vacuum freeze dryer to remove the adsorbed water. Owing to the low freezing point, paraffin oil could not be pumped out through vacuum exhaust pipe. So we prepared the S10 anhydrous hydrogels with solid paraffin substituting for liquid paraffin. Simply, we prepared paraffin wax-in-water emulsion S10 at 70 °C because paraffin wax was liquid phase at this temperature, while paraffin wax can solidify into solid again at room temperature. Then the S10 emulsion was transformed to the hydrogel after heating for 2 h at 90 °C. The hydrogel with paraffin wax was freeze-dried by a vacuum freeze dryer.

2.5. Conductivity of RGO hydrogels

The conductivity of RGO hydrogels was calculated according to eqn (1):

The cross sectional area (S) of the hydrogel was 1 cm² and the voltage (U) was fixed at 10 V. Current (A) was measured by a electrochemical workstation (CHI 660C, China). Briefly, 1 cm² of hydrogel was clamped by two platinum electrodes, whose length (L) was measured previously. Then the electrodes were connected with electrochemical workstation and linear fits of the data between 0 and 100 s yielded current A of each hydrogel. According to the value of A, L and eqn (1), we can calculate the conductivity of the RGO hydrogels.

2.6. Release study

PN and IBU as the drug models were loaded into the hydrogels by being dispersed in paraffin initially. The concentration of PN and IBU in paraffin was fixed at a constant of 1 mg mL⁻¹. To study drug release, the hydrogel S6 and S10 acting as the drug carrier were immersed in 150 mL of different solution (hexane, acetone, or different pH phosphate buffered saline), which were chosen as the external phases. The external solution was gently stirred by magnetic stirrer at room temperature and the temperature of PBS was controlled with a thermostatic water bath. The released PN and ibuprofen concentrations were analyzed by a UV-Vis spectrometer at the wavelength of 478 and 222 nm, respectively. The external solution of 3 mL was taken at predetermined time, and poured back after UV analysis. According to the calibration curves of the drugs in the different solutions, we calculated the percentage of released drugs from the hydrogels. Every experimental point of drug release was tested for three times (n = 3).

2.7. Characterization

Photographs of vials containing emulsions were taken with a Canon digital camera (IXUS 9515). Pickering emulsion droplets were observed with an optical microscope (Carl Zeiss, Germany). The ultrasonication equipment and the high speed shearing machine are KQ 218 (China) and IKA T25 (Germany). Anhydrous hydrogels were dried by a vacuum freeze dryer (LGJ-10, China). The size and Zeta potential of GO and Fe₃O₄ were measured using a Zetasizer Nano ZS90 instrument (Malvern, UK). The morphology of the RGO hydrogels was examined by a scanning electron microscope (SEM, Zeiss EVO18). For the SEM characterization, a small piece of anhydrous hydrogel was taken and stuck by conducting tapes on a copper sheet. All of the above were sputter-coated with a thin overlayer of gold to prevent sample charging effects. The Raman spectra were recorded on a spectrometer (JYH800UV) equipped with an optical microscope at room temperature. For the characterization of Raman, a pinch of anhydrous hydrogel was grinded into power and characterized by spectrometer. A Multimode Nanoscope IIIa atomic force microscope (AFM) was used to examine the surface characteristics of GO. Ultraviolet (UV) absorbance of PN and IBU was recorded with a Hitachi U-3010 ultraviolet and visible spectrophotometer.

3. Results and discussion

3.1. Preparation of Pickering emulsions and graphene-based hydrogels

The typical AFM image of single sheets of prepared GO is shown in Fig. 2. The average thickness of the GO sheets was about 1 ± 0.5 nm and the lateral dimension ranged from hundreds of nanometers to micrometers. The thickness was somewhat larger than 0.8 nm predicted by theory,⁴³ which has been observed also by the other GO AFM study.^43,44 This can be assigned to individual graphitic sheets bearing oxygen-containing groups on both faces.

Fig. 2 (a) AFM image of GO and (b) height profile along the line indicated in A.

The presence of huge amount of oxygen-containing functional groups makes GO highly hydrophilic. Therefore, it is easy to fully exfoliate GO into monolayer nanosheets in water by ultrasonication. After ultrasonication, GO nanosheets were about 10 μm in diameter in the aqueous dispersion and its Zeta potential was −38.3 mV (Fig. S1†). The basal planes of the carbon networks and the three kinds of oxygen-containing functional groups that are on those planes endow the GO sheets with both hydrophilic and hydrophobic properties. So according to the Zeta potential of GO and its unique physico-chemical characteristics, GO sheets could behave like a colloidal surfactant and form stable Pickering emulsions. However, to date, all studied GO-stabilized O/W Pickering emulsions were only stable in the high ratio of water to oil, specially using non-aromatic oil as oil phase.^34,36 In a Pickering emulsion, the decrease in the free energy is related to the size of the particles, the liquid–liquid and liquid-particle interfacial tension. Too hydrophilic or hydrophobic particles cannot lead to stable Pickering emulsions.^45,46 When fixing the concentration of emulsifiers in water, with the bigger ratio of water to oil, more stable O/W Pickering emulsions would be. However, with the co-stabilizing effect of GO nanosheets and Fe₃O₄ nanoparticles, we produced the paraffin-in-water emulsions with the water to oil ratios of 2 : 1 and 1 : 1 (see Table 1). Fig. 3 shows the microphotograph of the S3 emulsion, which is without Fe₃O₄ nanoparticles. The image revealed that all of the emulsions droplets were spherical with an average size of 100 μm. Those emulsions could maintain stable for a long time without droplet-breaking. However, the emulsions would experience a phase inversion, from O/W to water-in-oil (W/O) once the volume fraction of the oil phase was above 50%. Furthermore, there were still many GO sheets dispersed in water phase rather than acting as particle emulsifiers.

Fig. 3 Optical photographs of Pickering emulsions S3.

More detailed evidences can be found in Fig. S3,† photos of the GO aqueous dispersion and the typical Pickering emulsions. Without using Fe₃O₄ as the co-stabilizer, excess water at the bottom of the vials (S2 and S3) was black. It indicated that there were still many free GO nanosheets existing in water phase and they could not totally act as stabilizers. We also can find that the volume of the emulsions increased with increasing the oil to water ratio (S2 and S3, S10 and S6). Compared to the emulsion S2, the water phase at the bottom of S10 was clear, whose ingredient was the same as S2 except for using Fe₃O₄ as the co-stabilizer. Similar phenomenon could be also found in S3 and S6. Furthermore, different concentrations of Fe₃O₄ were studied from 0.5 to 2 mg mL⁻¹ (S4–S7). We found the water phase became clear when the concentration of Fe₃O₄ exceeded 1 mg mL⁻¹ (Fig. S4†), indicating that GO and Fe₃O₄ had a synergy effect on the stabilization of the emulsions when the concentration of Fe₃O₄ reached 1 mg mL⁻¹. As shown in Table 1, the size of emulsion droplets was almost 115 μm except S11. The droplets of emulsion S11 were the smallest where no GO sheets were used. This was in accordance with the normal droplet size of Fe₃O₄ stabilized Pickering emulsions. However, coalescence would occur only one day after the emulsion preparation. Furthermore, the size of droplets increased from 90 to 156 μm. Since Fe₃O₄ nanoparticles were prepared by the precipitation oxidation method with ammonium hydroxide, there should be some amino groups on its surface. While individual GO sheets possessed oxygen-containing groups on both faces, like –COOH or –OH groups. These functional groups may have interaction forces which make GO and Fe₃O₄ form complexes and endow them synergy effect.⁴⁷ Therefore both GO nanosheets and Fe₃O₄ nanoparticles can act as stable particles simultaneously, which resulted to the bottom of vial becoming transparent. Detailed characterization about the interaction of GO and Fe₃O₄ is shown in Fig. S5 and S6.†

The images of the prepared RGO hydrogels are shown in Fig. S3b.† Compared with the bare RGO hydrogel (S1), the volumes of other hydrogels containing oil droplets expanded several times. The concentration of GO sheets also had an important effect on the hydrogel formation. When the concentration of GO was fixed at 1 mg mL⁻¹, the hydrogel could not be formed after heating. It is well known that GO can be reduced to graphene with a 3D architecture driven by the hydrophobic and π–π stacking interaction of the conjugated structures of graphene.⁴⁸ During reduction, graphene sheets generated from GO aggregates due to its increasing hydrophobicity. So graphene sheets could not touch each other and consequently assemble to the hydrogel in the reducing process if the concentration of GO was too low. Only when the concentration of GO nanosheets was bigger than 2 mg mL⁻¹, the 3D hydrogels could be obtained (Fig. S7a and b†).

The traditional emulsions are produced by adding surfactants. However, when we prepared the emulsion using the small molecule surfactant like Tween 60 as the emulsifier, no hydrogel was formed after heating for reduction of GO (see Fig. S7c†). Hydrogen bonding may be responsible for this phenomenon: C–O–C groups in Tween 60 could interact with the oxygen containing groups of GO by H-bonding, which resulted in the failed formation of hydrogels.⁴⁸ Furthermore, we did not need to remove surfactants when using particles as the stabilizers, which also confirmed the advantages of Pickering emulsions. Additionally, adding Fe₃O₄ nanoparticles not only made full use of GO sheets, but also introduced the various functions of nanoparticles to the RGO hydrogels. So the combination of GO and Fe₃O₄ was the key to the successful formation of the RGO hydrogels.

3.2. Characterization of RGO hydrogels with oil droplets

For the characterization of the hydrogels, the RGO hydrogels were washed by acetone to remove paraffin, and then dried in oven for two days. Fourier transform infrared (FTIR) spectra of GO and RGO hydrogels (S1) are shown in Fig. S8.† In the GO spectrum, a strong and broad peak at 3683 cm⁻¹ is observed which is due to the O–H stretching vibration. The C=O stretching of COOH groups at the edges of GO sheets is observed at 1725 cm⁻¹. The absorption due to the O–H bending vibration and epoxide group vibration is observed around 1673 cm⁻¹.⁴⁹ In Fig. S8,† the FTIR spectrum of the RGO hydrogel confirmed the reduction of the GO sheets. Here the absorptions for the C=O group (1725 cm⁻¹) and O–H stretching vibration (3683 cm⁻¹) decrease obviously in intensity, and the absorption at 1481 cm⁻¹ which may be attributed to C–OH groups disappeared.

Raman spectra of the pure GO and RGO hydrogels are shown in Fig. 4. For GO and RGO, there are always two peaks appearing at about 1591 and 1329 cm⁻¹ which are known as G-band and D-band, respectively. According to Kudin et al.,⁵⁰ the G peak is the in-phase vibration of graphite lattice, while the D peak is the disorder band caused by graphite edges. Isolated double bonds of GO with more defects resonate are at higher frequencies than the G peak of graphite and graphene. The oxygen functional groups in the GO sheets can be removed by a chemical reduction, and the conjugated G-band (sp² carbon) will be re-established. However, the size of the re-established G network is smaller than the original one, which would consequently lead to an increase in the ratio of the D : G intensity of I_D/I_G. In order to point the structure difference of GO and RGO, I_D/I_G of graphene were contracted. As shown in Fig. 4, an increase in I_D/I_G varied from 1.50 to 1.76, which indicated decreasing defects inside graphene sheets and indeed confirmed the reduction of GO.

Fig. 4 Raman spectra of GO and RGO hydrogels.

Due to the oil droplet separating, the hydrogel S10 did not form the same network just as S1. We found that there were some obvious differences between the morphologies of lyophilized bare RGO gel sample (S1) and the RGO hydrogel S10 prepared from Pickering emulsion, as shown in Fig. 5. Due to the π–π stacking between GO sheets, the bare RGO hydrogel (S1) have the interconnected network with the porous structure. The pores in the RGO hydrogel were micrometers in size while the pore walls were rather thin, indicating the efficient self-assembly of graphene sheets. The network of graphene sheets could be seen more clearly in Fig. 5B, which was the magnified image of Fig. 5A. In contrast, the S10 hydrogel using paraffin wax as the oil phase showed a more loosened 3D network. Round paraffin spheres were found to distribute in the network uniformly and covered with the RGO sheets. The size of paraffin spheres were about 100 μm, even a little larger than pores in the bare RGO hydrogel (S1). Therefore RGO sheets can barely contact each other when reducing S10. As shown in Fig. 2A, the emulsions were full of oil droplets which in close arrangement and GO sheets were on the edge of S1, but a hierarchical structure system: not only 3D network existed when reducing, but also numerous oil pores formed in the hydrogels. Thus the low concentration of GO was not able to form the hydrogel under this circumstance. Also due to this reason, the hydrogels produced by the emulsion templates were softer and more fragile than the bare RGO hydrogel because of the less contact leading to the weak structure, which is what we need to improve in the future.

Fig. 5 SEM images of the RGO hydrogels: (a and b) S1 and (c and d) S10 in different magnification.

3.3. Conductivity of RGO hydrogels

The GO monolayer behaved close-to-insulating, with differential conductivity values of 1–5 × 10⁻³ S cm⁻¹ at a bias voltage of 10 V.⁵¹ After chemical reduction, freeze-dried RGO monolayers would have a pronounced increase of conductivity, between 0.05 and 2 S cm⁻¹. However, as demonstrated in Fig. 6, the conductivity of S1 was 8.8 × 10⁻³ S cm⁻¹, which was much smaller than normal value as discussed above. It may ascribe to the bigger volume of wet RGO hydrogels we used herein than freeze-dried ones, because longer distance of RGO sheets leaded to smaller conductivity. Furthermore, the hydrogels (S2, S10, S3, S6) templated from Pickering emulsions had obviously smaller conductivity than the bare RGO hydrogel (S1), whose conductivity value were 2.6, 2.3, 1.44, and 1.98 × 10⁻³ S cm⁻¹, respectively. It was intelligible because graphene sheets touched each other more difficultly due to paraffin droplets separating, while electrons transfer only occurred when graphene sheets contacting. The conductivities of the S2 and S10 hydrogels were a little larger than the S3 and S6 hydrogels. It was because the S2 and S10 hydrogels had relatively smaller oil to water ratios which meant they had less oil phase, so that GO sheets can touch each other more easily comparing to the S6 and S10 hydrogels. Furthermore, we can find that the conductivity of the S6 hydrogel was much larger than the S3 hydrogel. The reason may also ascribe to the synergy effect which made GO nanosheets enter the emulsions with Fe₃O₄ better, resulting to closer accumulation and contact of GO sheets.

Fig. 6 Conductivity of RGO hydrogels.

3.4. Release study

Owing to the hydrophobicity of RGO and the content of the oil droplets, the as-prepared hydrogels can be applied to the lipophilic drug delivery. For the hydrogels S6 and S10, the in vitro release profiles of encapsulated PN and IBU were investigated in hexane, acetone, and phosphate buffer (PBS pH = 7.4 or 2.0), respectively. In hexane, the hydrogels maintained the cylinder shape during the entire release process. While in acetone the hydrogels flocculated gradually (Fig. S9a–d†). As shown in Fig. 7, no PN release can be detected in hexane for the S6 and S10 hydrogels, both of which kept the cylinder shape and the surrounding solution was clear all the time. However, a sustained PN release behavior can be observed when acetone was employed as the solvent. Acetone became orange as the release went on and there was much flocculation at the bottom, as shown in Fig. S9c and d.† Additionally, for the S6 hydrogel, the release of PN was faster than that of the S10 hydrogel. The difference of the release behavior was also due to the different amount of the oil phase in these two hydrogels. Owing to the looser structure mentioned above, the S6 hydrogel was weaker than the S10 hydrogel, so PN can release more easily and faster. The total release of PN in the S6 hydrogel was 78%, which was much bigger than that of the S10 hydrogel (28%). This result was in accord with the release speed because of the same reason discussed above. Release of PN was relatively rapid in the first stage followed by a gradual decrease in release rate over a study period. The initial rapid release of the drug was most likely due to the unstable exterior surface of the hydrogel, and then the release of PN was controlled by diffusion mechanism. PN continued to release from these two hydrogels at a slower rate for over 10 days. Besides, the release of hydrogels in water were also studied, the result of which was almost the same as in hexane as shown in Fig. 7.

Fig. 7 Release curves of PN. Each point on this graph represents the average of three tests (n = 3).

As a widely used clinical drug, IBU has attracted many interests in recent years.⁵² Hence we further examined the IBU release behavior from the S6 and S10 hydrogels. The concentration of IBU was kept at the constant of 1 mg mL⁻¹. The experimental conditions selected were based on the composition of pharmaceutically relevant fluids such as gastric, intestinal simulated media. Herein IBU was released in PBS buffer solution (pH = 7.4 or 2.0). As shown in Fig. 8, IBU release into PBS buffer solution within 3 days for both the S6 and S10 hydrogels. Obviously, this release rate is much faster than that of PN. It can be ascribed to the different diffusion mechanism of PN and IBU. PN was diffused into acetone by dissolution of the exterior hydrogel surface. However, IBU diffused into PBS buffer solution through water connected the hydrogel and external PBS solution. Therefore, the hydrogels loaded with IBU kept their shape even release is finished (Fig. S9e–h†).

Fig. 8 Release curves of IBU-loaded RGO hydrogels in PBS (a) pH = 7.4 and (b) pH = 2.0. Each point on this graph represents the average of three tests (n = 3).

It is interesting to find that PN can release more easily from the hydrogel S6 compared to the hydrogel S10, while for IBU, the result was surprisingly inverse, no matter at pH = 7.4 or pH = 2.0 buffer solutions. The reason for this unexpected phenomenon can be explained as follows. The hydrogel S6 was prepared with a water/oil volume ratio of 1 : 1, which was smaller than that of the S10 hydrogel (2 : 1). In other words, the S10 hydrogel had more water phase. More water phase means more water channel, which makes the IBU release from the hydrogel S10 easier and more complete.

Finally we studied the pH effect on the IBU release rate and find an unusual phenomenon. The release rate of IBU in the buffer solution of pH = 2.0 was just like in the pH = 7.4 buffer solution. As is well known, the solubility of IBU in acid solution was much smaller than in neutral solution⁵³ and as a general rule, IBU would release less and more difficult in acid solution.⁵⁴ But as shown in Fig. 8, the release rate of IBU was almost the same in two PBS buffer solutions, contrarily to that expected according to the acid/base equilibrium between the non-ionized, insoluble and the ionized forms of IBU. It was reported that the release of IBU in the presence of H₂PO₄⁻ or HPO₄²⁻ increased steeply, indicating an anion exchange mechanism, electrostatic interactions being stronger in the case of phosphate anions.⁵² Herein the diffusion mechanism of IBU release was determined by anion exchange and electrostatic interaction. Because PBS buffer solution (pH = 2.0) had 0.137 mol L⁻¹ of HPO₄²⁻ and 0.2 mol L⁻¹ of PO4³⁻ anions, more than pH = 7.4 buffer solution (0.002 mol L⁻¹ of H₂PO₄⁻ and 0.01 mol L⁻¹ of HPO₄²⁻), IBU negative anions would be inclined to exchange with phosphate anions in acid PBS buffer solution. On the other hand, the solubility of IBU in the acid solution was much smaller. Taken together these two factors, the similar release curves in the different pH buffer solutions were achieved.

4. Conclusions

In this work, a series of porous graphene-based hydrogels was produced by Pickering emulsion method for the first time. Fe₃O₄ nanoparticles were adopted to cause a synergistic effect with GO to stabilize the emulsions when the concentration of Fe₃O₄ reached 1.5 mg mL⁻¹. The hierarchical pore structure of the hydrogels containing oil droplets caused the conductivity decrease comparing with the bare RGO hydrogels. It was found that the as-prepared hydrogels which were loaded with PN and IBU in paraffin had the potential functionalities for encapsulation and controlled release. The Hydrophobic surface of the hydrogels saved PN well when immersed in hexane or water, while continuous release of PN could be found in polar solvents (acetone). However, IBU could release into PBS buffer solution with different pH values due to the anion exchange mechanism. We can choose the appropriate type of the hydrogels according to our purpose and control the release rate of IBU. But the mechanical properties of the hydrogels still need to be improved in our future researches.

Acknowledgements

This work was financially supported by the National Natural Basic Research Program of China (973 Program, 2012CB821500), the National Natural Science Foundation of China (21274046), and the Natural Science Foundation of Guangdong Province (S2012020011057).

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, size and Zeta potential of GO and Fe₃O₄, optical images of different emulsions, FTIR of GO and RGO, and contact angle of aqueous dispersions. See DOI: 10.1039/c3ra44993a

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