Redox-Responsive Pickering Emulsions Based on Silica

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Redox-responsive Pickering Emulsions Based on Silica Nanoparticles and Electrochemical Active Fluorescent Molecule Qiuyan Jiang, Ning Sun, Qiuhong Li, Weimeng Si, Jiao Li, Aixiang Li, Zengli Gao, Weiwei Wang, and Jiarui Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00250 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 14, 2019

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Redox-responsive Pickering Emulsions Based on Silica Nanoparticles and Electrochemical Active Fluorescent Molecule Qiuyan Jiang, Ning Sun, Qiuhong Li*, Weimeng Si, Jiao Li, Aixiang Li, Zengli Gao, Weiwei Wang, Jiarui Wang School of Materials Science and Engineering, Shandong University of Technology, Zibo, Shandong, 255049, P.R. China

ABSTRACT

In this paper, we report a novel redox responsive water-in-oil Pickering emulsion stabilized by negatively charged silica nanoparticles in combination with a trace amount of redox switchable fluorescent molecule ferrocene azine (FcA), in which ferrocene serves as a redox-sensitive group, and anthryl unit serves as a fluorescence emission center. By alternately adding oxidant and reducing agent at a moderate condition, the amphiphilicity of silica nanoparticles changes because of the adsorption of Fc+A and the desorption of FcA on the silica surface. On one hand, the stability of emulsions can be transformed between stable and unstable at ambient temperature via redox trigger, and the regulation process can be cycled at least three times. On the other hand, the fluorescent intensity of FcA molecule can be regulated by redox stimuli, thus the change in

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fluorescent behavior of the emulsion droplets is observed upon redox cycles, which makes it useful in the fluorescent label of stimuli-responsive Pickering emulsions. This work provides a deep understanding of regulation mechanism of Pickering emulsions upon redox stimuli and opens the new way for in-situ fluorescent label of stimulus-responsive Pickering emulsions without introducing additional fluorescent molecule.

INTRODUCTION Emulsions are a thermodynamically unstable dispersion system in which liquid is dispersed as a very small droplet in another immiscible liquid.1 In order to maintain the stability of the emulsions, it is necessary to add a stabilizer, that is, a surfactant or a surface-active polymer. The emulsions using solid colloidal particles as stabilizer are called Pickering emulsions.2, 3 The solid particles adsorbed at the oil/water interface form an irreversible mechanical barrier film to prevent aggregation between the droplets and improve emulsion stability.4 Compared with the traditional surfactant-stabilized emulsions, Pickering emulsions have many advantages, such as small dosage, low toxicity and environmental friendliness. Therefore, studies on Pickering emulsions are important in the fields of food, cosmetics, coatings, medicine, materials science and biotechnology, etc.5-10 Many factors can affect the stability of Pickering emulsions, such as solid particles, oil phase type, pH value and aqueous electrolyte.11-14 Among them, the surface wettability,15 particle size,16 concentration of solid particles17 are important factors affecting the stability of emulsions. The solid particles used to stabilize Pickering emulsions include CaCO3,18 SiO2,19 clay particles,20 layered double hydroxide,21 polystyrene microspheres,22 etc. However, in addition to some synthetic polymer particles, most natural solid particles do not have a strong surface activity and cannot be used to prepare stable Pickering emulsions system alone. Therefore, how


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to change surface activity of solid particles has been an important topic. As we all know, the surface activity of solid particles depends on the surface wettability, which can be changed by interacting with suitable amphiphiles with opposite charge. That is to say, the surface activity can be controlled by selecting appropriate molecular structure and concentration of the amphiphile. Sugita et al.23 changed the aggregation state of silica in the dispersion medium by pre-adsorbing hydroxypropyl methylcellulose (HPMC) on the surface of nano-silica to modify the nano-silica particles successfully, which could be used to stabilize the silicone oil/water system. However, in some practical applications, the requirements for emulsion stability are different according to different purpose. For example, in fuel production, oil transfer, and emulsion polymerization, the emulsion requires only temporary stabilization and subsequent demulsification. Therefore, the Pickering emulsions responding to various stimuli have received considerable attention in recent years.24-28 Various triggers such as light irradiation,29, 30 pH,31-33 CO2,34-36 temperature,37 magnetic field,38, 39 and specific ion concentration40 have been reported. Among these triggers, CO2 is widely studied because of its low cost, environment friendly and easily removed from the system.34 Cui et al.41 used negatively charged silica nanoparticles to mix with trace surfactant 4-butyl-4-azobenzene hydrogencarbonate (AZOB4) and obtained dualstimulation responsive stabilizer, which could be used to prepare Pickering emulsions responding to CO2/N2 and light dual stimuli. But the modulation of emulsions often occurs at high temperature, which limited the application of CO2-responsive system.42 Afterwards, they studied the switchable Pickering emulsion stabilized by silica nanoparticles hydrophobized in situ with a conventional cationic surfactant,43 where the emulsion stabilization-destabilization was recycled just by alternatively adding anionic or cationic surfactants. However, it is difficult to be demulsified when it is used in some areas such as drug delivery and oil transport. As a stimuli


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method without adding other chemicals, light irradiation has attracted wide attention, but the high transparency of the system is required.44 In order to expand the application of emulsions, other trigger methods need to be explored, such as electrochemical stimuli, which is helpful to achieve remote control of emulsions under specific conditions. However, the electrochemical demulsification method reported so far is mainly applicable to water-in-oil (W/O) emulsions and can only be used under higher electric field conditions (above 2000 V.cm-1). Herein, we report a novel redox-responsive Pickering emulsion stabilized by negatively charged SiO2 nanoparticles in combination with a trace amount of redox switchable fluorescent ferrocene azine molecule (FcA, Scheme 1, synthesized with a modified facile route

45, 46).

The

oxidized fluorescent molecule (Fc+A, Scheme 1) can be combined with SiO2 nanoparticles by electrostatic interaction to enhance hydrophobicity. By alternately adding oxidant and reducing agent, the fluorescent intensity of FcA molecule can be modulated. At the same time, the stability of emulsions changes between stable and unstable at ambient temperature by redox stimuli. Moreover, the results obtained by confocal laser scanning microscopy (CLSM) show that SiO2 nanoparticles modified by Fc+A molecule adsorb at oil-water interface to stabilize emulsions. And a reversible change in the fluorescence intensity of droplets can occur by adding redox species alternatively.

Scheme 1. Chemical structure of ferrocene azine (FcA) and its redox stimuli response. EXPERIMENTAL SECTION


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Materials. Acetylferrocene was purchased from Aladdin. Ethyl silicate and hydrazine hydrate (80%) were purchased from Sinopharm Chemicals and Sigma-Aldrich, St. Louis (USA) respectively. The other chemical reagents were of analytics grade. Water used in the experiment was triply distilled. All reagents were used as received without further purification. The SiO2 nanoparticles particles were synthesized according to the classic stöber method.47 Synthesis and characterization of FcA. About 2 mmol acetylferrocene-hydrazone prepared according to the procedures reported in the literature46 was dissolved in CHCl3 (20 mL) in a dry three-necked flask, and an anhydrous ethanol solution containing nonanalaldehyde (3 mmol) was added drop wise under magnetic stirring, then refluxed under a nitrogen atmosphere for 6 h. The solution was cooled to room temperature. The resulting FcA solution was placed in the refrigerator at 5 °C for 48 h and the orange-yellow particles were obtained. 1HNMR spectra were recorded in CDCl3 using a 400MHz Bruker Avance-400 spectrometer. 1HNMR (400 MHz, CDCl3) characterization of FcA: δ=2.22 (s, 3H), δ=4.21 (s, 5H), 4.29 (t, 2H), 4.39 (t, 2H), δ=7.56 (m, 4H), 8.08 (d, 2H), 8.61 (s, 1H), 8.79(d, 2H), 10.14 (s, 1H). Preparation and characterization of Fc+A modified SiO2 Particles. The oxidized form of FcA (Fc+A) was obtained by adding a desired amount of oxidant equal to half the molar amount of FcA, hydrogen peroxide (H2O2). Different concentrations of Fc+A molecule and SiO2 (0.06 g) were ultrasonically dispersed in distilled water (10 mL) and was stirred for 12 h (500 r/min). After a further 12 h without stirring, the Fc+A modified SiO2 particles were obtained by centrifugation. The reduction process could be initiated with hydrazine hydrate (NH2-NH2). Fourier-transform infrared (FTIR) spectra were recorded on a Bruker-500 spectrometer (Nicolet 5700) in the range of 4000-400 cm-1. The morphology of the samples and distribution of the elements were characterized by scanning electron microscopy (SEM, FEI Sirion 200, USA).


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Samples were dried directly onto carbon tape and allowed to dry overnight before being sprayed by gold. Transmission electron microscopy (TEM) observation was performed using a Tecnai G2 F20 S-TWIN (FEI, USA) with an accelerating voltage of 100 kV. Preparation and characterization of Pickering Emulsions. About 0.3 wt % of Fc+A modified SiO2 particles were ultrasonically dispersed in distilled water (4 mL), and then toluene (6 mL) was added to the dispersion. The oil/water mixture was then homogenized using an IKA Ultra-Turrax T-18 homogenizer for 3 minutes (10000 r/min). And all the obtaining Pickering emulsions were stored for at least seven days to observe their stability. The emulsion type was identified by conductivity measurement. Micrographs for emulsion droplets were observed by a polarized optical microscope (POM) with a CCD camera (Panasonic Super Dynamic II WVCP460). The freshly prepared Pickering emulsions were dropped in a transparent glass container and observed by confocal laser scanning microscope (CLSM Leica TCS-SP2). The red fluorescent channel was excited at 382 nm. Redox-induced Emulsification and Demulsification. Emulsions stabilized by 0.3 wt % of Fc+A modified SiO2 particles were demulsified by adding a desired amount of NH2-NH2 followed by hand shaking or sonication. The separated oil-water mixture was restabilized after adding a desired amount of H2O2 followed by homogenization at 10000 rpm for 3 minutes. This process was repeated several times as needed. Measurements. Zeta potential. The SiO2 and Fc+A modified SiO2 particles were ultrasonically dispersed in distilled water, respectively. Then the zeta potentials were measured at 25°C using Zeta instrument (LCS-901). Contact angle. The dispersion of 0.3 wt % SiO2 nanoparticles in distilled water was dropped on the surface of glass slide to from a coating. After freeze-drying, the glass slide were


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immersing into toluene solutions of FcA. Then the contact angles of pure water drops as well as water drops containing H2O2 or NH2-NH2 on the coating of SiO2 nanoparticles were measured by an optical contact angle measuring device (Kruss DSA 25, Germany). Droplets size distribution. According to the optical micrographs of emulsions, the droplet average diameter (Dn) and surface-average particle diameter (D3,2), volume-average particle diameter (D4,3) of emulsions at different particle concentrations were estimated by

𝐷𝑛 =

∑ 𝐷𝑖

(1)

𝑁 ∑𝐷3𝑖

𝐷3,2 = ∑𝐷2

(2)

𝑖

∑𝐷4𝑖

𝐷4,3 = ∑𝐷3

(3)

𝑖

Where N is the number of droplets (N≥200).[48] Partitioning extent of Fc+A between water and toluene. 4 mL toluene and 6 mL water with 4.30 mg Fc+A dissolved in water were mixed together in a glass vessel followed by stirring for more than 12 h. After further settling for 12 hours, the water phase was removed through a valve at the bottom into a beaker. The water was evaporated by heating close to 100 °C. Then the sample was dissolved in THF after drying at 110 °C for further 12 h. Then, the fluorescence intensity of Fc+A in THF was measured on a fluorescence spectrometer (FL4500) with a quartz cell (1×1 cm). The Fc+A concentration was obtained according to the standard curve of Fc+A in THF. [49] RESULTS AND DISCUSSION a. Characterization and Analysis of Fc+A Modified SiO2 Particles. The morphology of SiO2 and Fc+A modified SiO2 particles can be characterized by SEM and TEM. As can be seen from Figure 1, SiO2 nanoparticles with a uniform size and smooth surface are observed, and no


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aggregation occurrs (Figure 1a, 1c). However, after the fluorescent molecules Fc+A adsorb to the surface of the negatively charged SiO2 by electrostatic interaction, the adhesion of particles is observed (Figure 1b, 1d). Furthermore, EDS images (Figure 1e) also show that the Fe, N and C elements are almost homogenously distributed on the SiO2 particles surfaces. These imply the successful modification of SiO2 particles surface by Fc+A molecules.

Figure 1. SEM and TEM images of SiO2 (a, c), Fc+A modified SiO2 (b, d) and EDS element distribution of C, Fe, N, O, and Si for Fc+A modified SiO2 nanoparticles (e). The FTIR spectrum can also be used to verify the formation of Fc+A modified SiO2 particles. Figure 2 shows the FTIR spectrum of FcA, SiO2 and Fc+A modified SiO2 particles. It can be seen from curve a that the peaks at 3092 cm-1, 2921 cm-1, 1467 cm-1, and 825 cm-1 are attributed to infrared characteristic peaks of ferrocene. The stretching vibration absorption peak of -C=Nand the N-N infrared absorption peak appears at 1598 cm-1 and 1002 cm-1 respectively. In curve b, 953 cm-1 and 1107 cm-1 are attributed to the stretching vibration of Si-OH and antisymmetric stretching vibration peak of Si-O-Si, and the peak at 799 cm-1 is the symmetric stretching vibration peak of Si-O-Si. The infrared spectrum of Fc+A modified SiO2 is shown in Figure 2c, indicating that Fc+A molecule is successfully adsorbed on the surface of the SiO2 particles.


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Figure 2. FTIR spectrum of (a) FcA , (b) SiO2 and (c) Fc+A modified SiO2. The bare SiO2 nanoparticles are highly negatively charged, as indicated by a zeta potential, ζ = −43.6 ± 0.2 mV for 0.1 wt % particles dispersed in pure water at 25 °C. The zeta potential of Fc+A modified silica nanoparticles in pure water as a function of Fc+A concentration is shown in Figure 3. As can be seen, with the increase of Fc+A concentration, the zeta potential becomes positive gradually, indicating adsorption of Fc+A molecules on SiO2 particle surfaces.50, 51 And the zeta potential increases to 0 V when Fc+A concentration is about 8 mmol.L-1, which is slightly lower than its cmc (9.91 mmol.L-1, Figure S1) determined by conductivity measurement. The further increase of Fc+A concentration (higher than 9.91 mmol.L-1) will cause the micellization of Fc+A, which leads to the electrical inversion of SiO2 nanoparticles. That is to say, the zeta potential becomes positive. When Fc+A concentration increases to 10 mmol.L-1, the potential reaches 4.17 V and no obvious increase can be observed with the further increase of Fc+A concentration, indicating that the adsorption of Fc+A molecule at the particle-water interface is saturated. Therefore, the concentration 10 mmol/L for Fc+A is chosen to modify SiO2 nanoparticles in the following experiments.


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Figure 3. Zeta potential of Fc+A modified SiO2 dispersed in distilled water. The electrochemical behavior of FcA were evaluated in 0.1 mol.L-1 NaCl solution by cyclic voltammetry (Figure S2(a)). The reversible oxidation and reduction wave corresponds to the change of the ferrocenyl group between hydrophobic Fc and hydrophilic Fc+, indicating the excellent electrochemical reversibility of FcA moleule. Thus, the stability of emulsions stabilized by Fc+A modified SiO2 particles can be modulated by redox stimuli based on the change of amphiphilictity, which can be supported by the measurements of contact angle of pure water drops as well as water drops containing oxidant H2O2 and reducing agent NH2-NH2 on the coating of SiO2 nanoparticles in toluene solution of FcA. As seen in Figure 4, the contact angle increases from 7° (pure water drops) to 30° for water drops containing H2O2, indicating the hydrophobic modification of SiO2 nanoparticles by Fc+A molecule. Then the contact angle decreases dramatically to less than 10° for water drops containing NH2-NH2 because of the disappearance of electrostatic interaction between SiO2 nanoparticles and FcA molecule. Therefore, the amphiphilictity of Fc+A molecule modified SiO2 particles can be modulated by


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alternatively adding oxidant and reducing agent, and the transition process can be cycled at least three times. In addition, in FcA molecule, the existence of the electron-donating ferrocene leads to the quench of the excited states of the anthryl unit, as shown in the steady-state fluorescent spectra (Figure S2 (b)) because of the electron-transfer from electron-donor ferrocene moiety to electron-receptor anthracene group.52 However, the fluorescence intensity of anthracene group is restored after the oxidation of Fc to Fc+ because of the reduction of the electron density in Fc+, which makes it useful in controllable fluorescent “on-off” switch and can be used as a probe to observe emulsification/demulsification process.

Figure 4. Contact angles change of pure water drops and water drops containing H2O2 and NH2-NH2 on the coating of SiO2 nanoparticles in toluene solutions of FcA for three cycles. b. Formation and Characterization of Pickering Emulsions. In the initial experiment, the ability of FcA and Fc+A molecule to stabilize toluene-in-water (6:4) emulsions is investigated respectively, and no stable Pickering emulsion was obtained for bare SiO2 particles and SiO2 mixed with FcA (Figure S3). However, when the particles are hydrophobized by the addition of Fc+A, very stable emulsions to coalescence are obtained. The conductivity measurement


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indicates the formation of oil-in-water (O/W) emulsions. Figure 5 shows the micrographs of the emulsions stabilized by 0.3 wt% SiO2 nanoparticles modified by Fc+A at different concentrations. It can be seen that no stable emulsion is observed when Fc+A concentration is lower than 5 mmol.L-1 and the average droplet diameter of emulsions decreases with increasing Fc+A concentration, which implies that these droplets are stabilized mainly by Fc+A modified SiO2 nanoparticles. At the same time, the effect of emulsifier concentration on the stability of emulsions is studied. Figure 6 exhibits the optical micrographs of emulsions stabilized by Fc+A modified SiO2 particles with different mass fractions at fixed oil-water ratio (6:4). As can be seen, the size of droplets reduces with the increasing emulsifier concentration, indicating the increase of stability to creaming of the emulsions. The amount of particles coating the droplets can be quantified further by measuring the drop size distributions at the different particle concentrations. The average diameter D and D3,2, D4,3 of droplet at different particle concentrations obtained by optical micrographs are listed in Table 1. The interfacial area S available in the emulsions and the adsorbed amount F of emulsifier particles per unit area at O/W interface can be calculated according to the following equations and the results are also listed in table 1.

𝑆=

6V(𝐷3,2)2 (𝐷4,3)3

F=

𝑚 𝑆

(4) (5)

where V is the volume (mL) of toluene in emulsions, and m is the mass (g) of emulsifier in emulsions. As can be seen, the interfacial area S available in the emulsion increases with the increasing emulsifier concentration. At the same time, the adsorbed amount of emulsifier particles F per unit area also increases, indicating the area that the available emulsifier can


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occupy increases, which further proves the influence of packing efficiency of the particles adsorbed at oil-water interface.

Figure 5 Photographs and optical micrographs of toluene-in-water (1:1) emulsions stabilized by a mixture of 0.16 wt % SiO2 and Fc+A at different concentrations (from a to f): 0, 2.5, 5, 8, 10, and 15 mmol.L-1.

Figure 6. Optical micrographs of O/W emulsions stabilized by Fc+A modified SiO2 nanoparticles at different mass fractions: (a) 0.1 wt %, (b) 0.3 wt %, (c) 0.5 wt %, (d) 0.7 wt %.


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Table 1 The size distributions and characterization of emulsions stabilized by Fc+A modified SiO2 nanoparticles at different concentrations. Concentration (%)

Dn (μm)

D3,2 (μm)

D4,3 (μm)

S (m2)

F (g/m2)

0.1

58.02

67.23

70.47

0.45

0.018

0.3

25.22

29.20

30.82

1.03

0.026

0.5

19.25

22.40

23.51

1.37

0.033

0.7

15.19

17.44

18.21

1.78

0.035

c. Emulsification and Demulsification Cycling of Pickering Emulsions. Based on the redox and fluorescence switch properties of FcA molecule, the stability of emulsions stabilized by Fc+A modified SiO2 nanoparticles can be modulated by adding redox species alternatively, and the obtained emulsions can be studied by CLSM (Figure 7). As can be seen from Figure 7a, strong red fluorescent spherical aggregates with a red circle in the periphery can be observed, indicating that Fc+A modified SiO2 nanoparticles adsorb at the oil-water interface to stabilize emulsions, which provides strong evidence for the formation mechanism of Pickering emulsions. However it should be noted in Figure 7a that the background and the droplets are also red in color, indicating that there is fluorescence from the external (continuous) water phase of the emulsions as well as from the oil droplets (internal phase), which means that some of the fluorescent molecules may have partitioned into the water. In order to verify this, the partitioning extent of Fc+A between water and toluene was quantified by determining the amount of Fc+A in water phase according to fluorescence standard curve of Fc+A in THF (Figure S4). The data shows that the mass of Fc+A remaining in the water phase is 5.41×10-2 mg (with 4.30 mg Fc+A initially), corresponding to a partition percentage of 1.26%. Therefore, when 0.3 wt % Fc+A modified SiO2 nanoparticles are dispersed in water to prepare stable emulsions, most particles


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adsorb at toluene-water interface because of their amphiphilicity, but some of Fc+A molecules may dissolve in external water phase and internal toluene phase as monomers, which is consistent with the result obtained by Figure 7a. At the same time, the destabilization of the emulsions can be achieved when the reducing agent NH2-NH2 is added into the emulsions followed by hand shaking or sonication, which can also be verified by CLSM and OM measurement (Figure 7b, Figure 8b). Then, the addition of the oxidizing agent H2O2 will cause the

reformation

of

emulsions

after

homogenization

(Figure

8c,

cycle

2).

The

demulsification/emulsification process can be cycled four times by adding redox species alternatively as shown in Figure 8d (cycle 3) and Figure S5 (cycle 4).

Figure 7. CLSM images of Fc+A modified SiO2 particle (0.3 wt %) stabilized emulsion droplets (a) and the emulsion after adding reducing agent (b). d. Postulated Mechanism of Emulsification/Demulsification. Herein, a reasonable mechanism for redox-responsive emulsions is proposed (Figure 9). The bare SiO2 nanoparticles cannot be used as emulsifier alone because of its hydrophilicity. The fluorescence molecules Fc+A combined with negatively charged SiO2 by electrostatic interaction increase the hydrophobicity of SiO2 nanoparticles, and the obtained Fc+A modified SiO2 nanoparticles adsorbed at the oil-


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water interface can be used as emulsifier to stabilize Pickering emulsions. The property of SiO2 nanoparticles changes between amphipathic and hydrophilic as a result of the adsorption of Fc+A and the desorption of FcA by adding redox species alternatively. Thus, a redox-stimuli responsive Pickering emulsion is obtained, and the emulsification/demulsification process can be repeated at least three times. At the same time, the fluorescence molecule Fc+A can be used as a probe to observe the transformation process based on its reversible fluorescence switching behavior

Figure 8. Photographs of redox-regulated Pickering emulsions (1) and optical micrographs of the emulsions stabilized by Fc+A modified SiO2 nanoparticles (2a), after adding reducing agent NH2NH2 (2b), after adding oxidant H2O2 (2c) and after adding H2O2 again (2d). The concentration of emulsifier was 0.3 wt %. .


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Figure 9. The structural description for redox conversion of FcA and Fc+A modified SiO2 particles (top) and strategy for emulsification and demulsification of Pickering emulsions triggered by redox (bottom). CONCLUSION In summary, we have demonstrated that a redox responsive Pickering O/W emulsions can be prepared using negatively charged SiO2 nanoparticles in combination with a trace amount of Fc+A as stabilizer. The results show that the use of the fluorescent molecule Fc+A as a modifier significantly improves the stability of the emulsion without flocculation and delamination. The CLSM measurement shows that Fc+A modified SiO2 nanoparticles adsorbed onto the oil-water interface to stabilize emulsions and Fc+A can be used as a probe to observe the emulsification/ demulsification process. Then the emulsions can be transformed between stable and unstable by adding redox species alternatively, and this process could be repeated at least three times. It is expected that these findings will potentially further extend the scope of stimuli-responsive Pickering emulsions with in-situ fluorescent label. ASSOCITATED CONTENT


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Supporting Information Figure S1-S5 showing conductivity of aqueous solutions of Fc+A, cyclic voltammogram and fluorescence spectra of FcA, photographs and corresponding optical micrographs of emulsions prepared from FcA particles, SiO2 particles mixed with FcA and Fc+A particles, standardization curve for fluorescence intensity of Fc+A, otical micrographs of the emulsions stabilized by Fc+A modified SiO2 nanoparticles after adding oxidant H2O2 and reducing agent NH2-NH2 for the fourth cycle. AUTHOR INFORMATION Corresponding Author: *(Q.H.L.)E-mail:[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are thankful for the financial support from National Natural Science Foundation Young Investigator Grant Program (Project No. 21706148) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (Project No.115156) REFERENCES [1] Becher, P. Emulsions: Theory and Practice. 2nd ed. New York: Reinhold, 1965. [2] Ramsden, W. Separation of Solids in the Surface-Layers of Solutions and Suspensions. Proc R Soc Lond 1903, 72, 156-164. [3] Pickering, S U. Emulsions. J. Chem. Soc. 1907, 91, 2001-2021.


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GRAPHIC ABSTRACT

Redox-responsive Pickering Emulsion Based on Silica Nanoparticles and Electrochemical Active Fluorescent Molecule Qiuyan Jiang, Ning Sun, Qiuhong Li*, Weimeng Si, Jiao Li, Aixiang Li, Zengli Gao, Weiwei Wang, Jiarui Wang School of Materials Science and Engineering, Shandong University of Technology, Zibo, Shandong, 255049, P.R. China The stability of Pickering emulsion stabilized by negatively charged silica nanoparticles and redox switchable fluorescent molecule ferrocene azine (FcA) can be transformed between stable and unstable at ambient temperature via redox trigger, and the emulsification and demulsification process could be repeated at least three times.


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Redox-Responsive Pickering Emulsions Based on Silica

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