pH and Magnetism Dual-Responsive Pickering Emulsion Stabilized by Dynamic Covalent Fe3O4 Nanoparticles

Herein, we describe pH and magnetism dual-responsive liquid paraffin-in-water Pickering emulsion stabilized by dynamic covalent Fe3O4 (DC-Fe3O4) nanoparticles. On one hand, the Pickerinfigureg emulsions are sensitive to pH variations, and efficient demulsification can be achieved by regulating the pH between 10 and 2 within 30 min. The dynamic imine bond in DC-Fe3O4 can be reversibly formed and decomposed, resulting in a pH-controlled amphiphilicity. The Pickering emulsion can be reversibly switched between stable and unstable states by pH at least three times. On the other hand, the magnetic Fe3O4 core of DC-Fe3O4 allowed rapid separation of the oil droplets from Pickering emulsions under an external magnetic field within 40 s, which was a good extraction system for purifying the aqueous solution contaminated by rhodamine B. The dual responsiveness enables Pickering emulsions to have better control of their stability and to be applied more broadly.


Introduction
Pickering emulsions, which were stabilized by solid particles, have gained much attention thanks to the low toxicity and long-term stability [1,2]. The outstanding stability of Pickering emulsions is required in the field of cosmetic formulations, food storage, and so on [3]. However, in some fields like heterogeneous catalysis [4,5], emulsion polymerization [6], and oil transportation and recovery [7][8][9], temporary stabilization is required. Chemical or physical methods are used in the industry to achieve demulsification, which are often energy-intensive or require extra complex additives, leading to increased economic and environmental costs [10,11]. Thus, stimuli-responsive Pickering emulsions are desired in the above applications because the stability of the emulsions can be easily controlled by external stimuli [12].

Synthesis of DC-Fe 3 O 4 Nanoparticles
Fe 3 O 4 was synthesized using the co-precipitation method [35]. FeSO 4 solution with a concentration of 0.5 M and FeCl 3 solution with a concentration of 1 M were prepared using 0.2 M HCl (aq), respectively, as solvent. During preparation, 400 mL of 1.5 M NaOH (aq) was poured into the three flasks and heated to 82 • C, and then the mixture of FeSO 4 solution (40 mL, 0.5 M) and FeCl 3 solution (40 mL, 1 M) was added to the three flasks dropwise within 30 min under the atmosphere of N 2 at 82 • C. As soon as the mixture turned to black, it was allowed to cool down to 25 • C under mechanical stirring. Then, the resultant black nanoparticles were washed with ethanol four times with the help of a magnet. The obtained product Fe 3   To prepare dynamic covalent Fe3O4 (DC-Fe3O4), Fe3O4-NH2 (1.0 g) and benzaldehyde were mixed in methanol (20 mL) under mechanical stirring for 1 h; the molar ratio of -NH2 and -CHO was 1. A schematic illustration of the synthesis of DC-Fe3O4 is presented in Figure 1b. The product was collected and then washed with ethanol four times with the help of a magnet. The product DC-Fe3O4 nanoparticles were dried under vacuum for 16 h.

Preparation of Pickering Emulsions
For Pickering emulsions' preparation, water, DC-Fe3O4 nanoparticles, and liquid paraffin were placed into a glass bottle. The concentration of DC-Fe3O4 varied from 0.1 to 2.0 wt%. The water and liquid paraffin were in an equal volume ratio. Then, the mixture was homogenized using a JY88-IIN sonicator with a 6 mm probe at 50 W twelve times (5 s, 5 s off) to obtain Pickering emulsions. The Pickering emulsions were kept at room temperature for 1 month to observe the storage stability.

Characterization of DC-Fe3O4 Nanoparticles
DC-Fe3O4 nanoparticles were dispersed in water at a concentration of 0.05 wt%. Approximately 20 μL of the above DC-Fe3O4 dispersion was loaded onto a copper grid. The DC-Fe3O4 sample was allowed to dry and imaged using a transmission electron microscope (Hitachi HT7700).
The composition of the Fe3O4, Fe3O4-NH2, and DC-Fe3O4 was investigated by FTIR spectrometer (Bruker Optics, Germany). The morphology of DC-Fe3O4 was characterized using TEM (Hitachi HT7700). For solid/air/water three-phase contact angle measurement, samples of the Fe3O4, Fe3O4-NH2, and DC-Fe3O4 were compressed into films.
For three-phase (solid/air/water) contact angle measurements, samples of the Fe3O4, Fe3O4-NH2, and DC-Fe3O4 nanoparticles were compressed into films using a table press (Shimadzu Press). The contact angle was measured using the contact angle goniometer (DataPhysics Instruments GmbH, Filderstadt, Germany). The contact angle was recorded by placing a droplet of water with a volume of 3.0 μL onto the film in the air. When the water was in contact with the film, the image of the morphology of the water droplet on

Preparation of Pickering Emulsions
For Pickering emulsions' preparation, water, DC-Fe 3 O 4 nanoparticles, and liquid paraffin were placed into a glass bottle. The concentration of DC-Fe 3 O 4 varied from 0.1 to 2.0 wt%. The water and liquid paraffin were in an equal volume ratio. Then, the mixture was homogenized using a JY88-IIN sonicator with a 6 mm probe at 50 W twelve times (5 s, 5 s off) to obtain Pickering emulsions. The Pickering emulsions were kept at room temperature for 1 month to observe the storage stability.

Characterization of DC-Fe 3 O 4 Nanoparticles
DC-Fe 3 O 4 nanoparticles were dispersed in water at a concentration of 0.05 wt%. Approximately 20 µL of the above DC-Fe 3 O 4 dispersion was loaded onto a copper grid. The DC-Fe 3 O 4 sample was allowed to dry and imaged using a transmission electron microscope (Hitachi HT7700).
The composition of the Fe 3 O 4 , Fe 3 O4-NH 2 , and DC-Fe 3 O 4 was investigated by FTIR spectrometer (Bruker Optics, Germany). The morphology of DC-Fe 3 O 4 was characterized using TEM (Hitachi HT7700). For solid/air/water three-phase contact angle measurement, samples of the Fe 3 O 4 , Fe 3 O 4 -NH 2 , and DC-Fe 3 O 4 were compressed into films.
For three-phase (solid/air/water) contact angle measurements, samples of the Fe 3 O 4 , Fe 3 O 4 -NH 2 , and DC-Fe 3 O 4 nanoparticles were compressed into films using a table press (Shimadzu Press). The contact angle was measured using the contact angle goniometer (DataPhysics Instruments GmbH, Filderstadt, Germany). The contact angle was recorded by placing a droplet of water with a volume of 3.0 µL onto the film in the air. When the water was in contact with the film, the image of the morphology of the water droplet on the film surface was recorded and later analyzed by the software Photoshop to obtain the contact angle.

Characterization of Pickering Emulsions
To conform the emulsion type, liquid paraffin was stained with Nile red. The Pickering emulsion droplets were detected with a confocal fluorescence microscope (Carl Zeiss, Oberkochen, Germany). The micrographs of the Pickering emulsions were observed with an A1Pol optical microscope (ZEISS, Oberkochen, Germany).

pH Modulation of Pickering Emulsion
The pH modulation (between pH 10 and 2) of the Pickering emulsion was achieved by alternately adding 1 M HCl or 1 M NaOH, followed by stirring for 30 s and standing for 30 min.

Magnetic Modulation of Pickering Emulsion
The magnetism-responsive character of the Pickering emulsion was carried out by applying an NdFeB permanent magnet (Jiangsu Lingxi Magnetic Industry Co., Suzhou, China) with a size of 60 × 40 × 5 mm.

Extraction of RhB from the Aqueous Solution
The RhB-polluted aqueous solution (4 mg/L) was prepared by dissolving RhB in water. Pickering emulsion (1 mL) was added to RhB-polluted aqueous solution (5 mL) to extract RhB. After standing for 20 min, the purified water was separated over a magnet and poured out. Prior to and after extraction, the concentration of RhB was determined using a UV/Vis spectrophotometer at 553 nm. According to the following equation, the extraction efficiency (E) can be calculated: where C 0 (mg/L) and C e (mg/L) are the concentrations of RhB aqueous solution before and after extraction, respectively. In addition, Pickering emulsion could be obtained again by washing with water, and the extraction process could be repeated at least three times.

Characterization of Dynamic Covalent Fe 3 O 4
The preparation of DC-Fe 3 O 4 nanoparticles through DIB formation between amino-Fe 3 O 4 (Fe 3 O 4 -NH 2 ) and benzaldehyde is presented in Figure 1. FTIR was performed to prove the successful fabrication of DC-Fe 3 O 4 nanoparticles. As shown in Figure 2, the bond at 580 cm −1 in the Fe 3 O 4 FTIR spectra was the characteristic peak of the Fe-O functional groups. Just like the peak presented in Fe 3 O 4 , the Fe-O absorption peak (580 cm −1 ) also appears in the spectra of  Figure S1). The TEM image shows that the DC-Fe 3 O 4 nanoparticles have a nearly spherical shape with a mean diameter of about 17 nm. The DC-Fe 3 O 4 nanoparticles exhibit an instant magnetic response to the external magnetic field and can be separated completely from their liquid dispersions within 30 min. When the magnetic field is removed, the DC-Fe 3 O 4 nanoparticles can be dispersed again by shaking. The stabilization of two immiscible phases was related to the wettability of particles at their interface, which was measured by contact angle [36][37][38]. Therefore, contact angle measurement was carried out for Fe3O4, Fe3O4-NH2, and DC-Fe3O4, respectively. Fe3O4 was found to have a contact angle of roughly 17° (Figure 3a), which was too hydrophilic for stabilizing Pickering emulsions; a similar result was reported by Sun et al. [9]. When Fe3O4 nanoparticles were modified by APTES, Fe3O4-NH2 nanoparticles also exhibit a hydrophilic character with a contact angle of about 25° (Figure 3b), which was also too hydrophilic for stabilizing Pickering emulsions. By modifying Fe3O4-NH2 nanoparticles with relatively hydrophobic benzaldehyde through DIB formation, the contact angle of modified Fe3O4 nanoparticles (DC-Fe3O4) increased to about 48° ( Figure 3c). The improved amphiphilicity of DC-Fe3O4 is desirable to the formation of stable Pickering emulsions. It is well-known that the particle contact angle determines the type of Pickering emulsion [39,40]. If the contact angle of particles is less than 90°, they are located preferentially in the water phase, and the resulting curvature favors O/W Pickering emulsions. In contrast, if the contact angle exceeds 90°, the particles will reside primarily in the oil phase, which will lead to W/O Pickering emulsions [41]. Hence, amphiphilic DC-Fe3O4 nanoparticles with a contact angle of about 48° are expected to prepare O/W Pickering emulsions.
The amphiphilicity of DC-Fe3O4 can be further proved by the dispersion behavior of DC-Fe3O4 on the liquid paraffin-water two phases. The DC-Fe3O4 nanoparticles straddle on the liquid paraffin-water interface instead of the aqueous phase even after shaking ( Figures S2 and 4a), indicating the amphipathic nature of the DC-Fe3O4 nanoparticles, consistent with the contact angle measurement. The stabilization of two immiscible phases was related to the wettability of particles at their interface, which was measured by contact angle [36][37][38]. Therefore, contact angle measurement was carried out for Fe 3 (Figure 3c). The improved amphiphilicity of DC-Fe 3 O 4 is desirable to the formation of stable Pickering emulsions. It is well-known that the particle contact angle determines the type of Pickering emulsion [39,40]. If the contact angle of particles is less than 90 • , they are located preferentially in the water phase, and the resulting curvature favors O/W Pickering emulsions. In contrast, if the contact angle exceeds 90 • , the particles will reside primarily in the oil phase, which will lead to W/O Pickering emulsions [41]. Hence, amphiphilic DC-Fe 3 O 4 nanoparticles with a contact angle of about 48 • are expected to prepare O/W Pickering emulsions.

Preparation of Pickering Emulsions Stabilized by DC-Fe3O4 Nanoparticles
The liquid paraffin-in-water Pickering emulsion could not be stabilized by Fe Fe3O4-NH2 alone for 30 min at the particle concentration of 1.0 wt% (a rather high co tration in Pickering emulsion preparation, Figure S3). The reason that Fe3O4 or Fe3O nanoparticles could not be used as the Pickering emulsifier might be attributed to th hydrophilicity of Fe3O4 and Fe3O4-NH2, as proven by the result of the contact angle ( 3).
Based on the discussion of the contact angle, we presume that the amphiphil Fe3O4 nanoparticles with a contact angle of about 48° are anticipated to prepare O/W ering emulsions. The ability of DC-Fe3O4 nanoparticles to stabilize the Pickering sions was investigated; the preparation process of Pickering emulsions is presen Figure 4. In the control experiment, we have proved that no emulsion was prepared out DC-Fe3O4 nanoparticles ( Figure S4). With 0.1 wt% DC-Fe3O4 nanoparticles, no

Preparation of Pickering Emulsions Stabilized by DC-Fe 3 O 4 Nanoparticles
The liquid paraffin-in-water Pickering emulsion could not be stabilized by Fe 3 O 4 or Fe 3 O 4 -NH 2 alone for 30 min at the particle concentration of 1.0 wt% (a rather high concentration in Pickering emulsion preparation, Figure S3). The reason that  (Figure 4b). The CLSM measurement shows that the labeled liquid paraffin is surrounded by the unlabeled water ( Figure 6), indicating that O/W Pickering emulsion was formed.
The morphology of Pickering emulsions was observed by optical microscopy (Figure 7a-e). As shown in Figure 7a (Figure 7f). This was because more particles were able to stabilize larger interfaces at a constant oil-water ratio, which is a common feature for Pickering emulsions [10,17]. In this study, it was found that the size of Pickering emulsion droplets can be easily adjusted by choosing a suitable DC-Fe 3 O 4 nanoparticle concentration.
geneous Pickering emulsion could be prepared because of insufficient DC-Fe3O4 nano ticles on the oil-water interface ( Figure 5). Satisfactorily, when the DC-Fe3O4 nanopar concentrations were equal to or exceeding 0.25 wt%, stable Pickering emulsions were pared ( Figure 5), which might due to the effective adsorption of amphiphilic DC-F nanoparticles at the liquid paraffin-water interface (Figure 4b). The CLSM measurem shows that the labeled liquid paraffin is surrounded by the unlabeled water (Figur indicating that O/W Pickering emulsion was formed.   ticles on the oil-water interface ( Figure 5). Satisfactorily, when the DC-Fe3O4 nanoparticle concentrations were equal to or exceeding 0.25 wt%, stable Pickering emulsions were prepared ( Figure 5), which might due to the effective adsorption of amphiphilic DC-Fe3O4 nanoparticles at the liquid paraffin-water interface (Figure 4b). The CLSM measurement shows that the labeled liquid paraffin is surrounded by the unlabeled water ( Figure 6), indicating that O/W Pickering emulsion was formed.   tions: the mean droplet sizes of Pickering emulsions with a DC-Fe3O4 nanoparticle concentration of 0.25, 0.5, 1.0, 1.5, and 2.0 wt% were 107, 22, 18, 13, and 8 μm, respectively (Figure 7f). This was because more particles were able to stabilize larger interfaces at a constant oil-water ratio, which is a common feature for Pickering emulsions [10,17]. In this study, it was found that the size of Pickering emulsion droplets can be easily adjusted by choosing a suitable DC-Fe3O4 nanoparticle concentration.  The Pickering emulsions showed almost no change in appearance after one month of storage, and no clear oil phase could be observed (Figure 5a,b), indicating the high stability of Pickering emulsions. Meanwhile, even after 1 month of storage, the droplet size of the Pickering emulsions remained nearly unchanged (Figure 8), further confirming their long-term stability. It was found that Pickering emulsions were stable for a long time because of the irreversible adsorption of DC-Fe 3 O 4 nanoparticles at the O/W interface.
The Pickering emulsions showed almost no change in appearance after one month of storage, and no clear oil phase could be observed (Figure 5a,b), indicating the high stability of Pickering emulsions. Meanwhile, even after 1 month of storage, the droplet size of the Pickering emulsions remained nearly unchanged (Figure 8), further confirming their long-term stability. It was found that Pickering emulsions were stable for a long time because of the irreversible adsorption of DC-Fe3O4 nanoparticles at the O/W interface.

pH-Responsive Behavior of the Pickering Emulsions
On the basis of the above discussions, we can conclude that stable Pickering emulsions were obtained using the amphiphilic DC-Fe3O4 nanoparticles as a stabilizer. Considering the dynamic character of DIB to pH [42][43][44][45][46], the amphiphilicity of DC-Fe3O4 nanoparticles may be changed by transforming pH. To verify this, contact angle measurement for the DC-Fe3O4 film with acidic water (pH 2) was conducted. The contact angle of the DC-Fe3O4 nanoparticle film decreased from about 48° to about 25° after adding water droplets with a pH of 2 ( Figure S5), indicating the dissociation of amphiphilicity DC-Fe3O4 into hydrophilic Fe3O4-NH2 and benzaldehyde in the acidic environment. That is to say, the amphiphilicity of DC-Fe3O4 nanoparticles could be changed by changing the pH.
In light of the change in amphiphilicity of DC-Fe3O4 nanoparticles when the pH is changed, we speculated that the pH could be used to adjust the stability of the obtained Pickering emulsions. To validate this hypothesis, liquid paraffin-in-water Pickering emulsion stabilized by 1.0 wt% DC-Fe3O4 nanoparticles was used as the representative sample. At pH 10, DC-Fe3O4 nanoparticles can be used as a stabilizer to prepare O/W Pickering emulsions because of the effective adsorption of amphiphilic DC-Fe3O4 at the liquid paraffin-water interface (Figure 9a and Scheme 1a). By decreasing the pH from 10 to 2, complete phase separation was achieved within 30 min (Figure 9b and Scheme 1b). At pH 2, the amphiphilic DC-Fe3O4 nanoparticles decomposed into hydrophilic Fe3O4-NH2 and inactive benzaldehyde, both of which were desorbed from the oil-water interface, causing demulsification (Figure 9b and Scheme 1b). The hydrophilic Fe3O4-NH2 nanoparticles participated in the aqueous phase (Scheme 1b). Additionally, benzaldehyde is surface-inactive and cannot stabilize emulsions effectively, as reported by our previous study [17]. We estimate that about 53% of the benzaldehyde migrates into the liquid paraffin phase based on the UV/Vis results (Scheme 1b). Moreover, after increasing the pH from 2 to 10, stable Pickering emulsion was reformed after re-sonication because of the re-formation of amphiphilic DC-Fe3O4 nanoparticles through DBI formation between hydrophilic Fe3O4-NH2 and surface inactive benzaldehyde (Figure 9a and Scheme 1a). Furthermore, pH-induced

pH-Responsive Behavior of the Pickering Emulsions
On the basis of the above discussions, we can conclude that stable Pickering emulsions were obtained using the amphiphilic DC-Fe 3 O 4 nanoparticles as a stabilizer. Considering the dynamic character of DIB to pH [42][43][44][45][46], the amphiphilicity of DC-Fe 3 O 4 nanoparticles may be changed by transforming pH. To verify this, contact angle measurement for the DC-Fe 3 O 4 film with acidic water (pH 2) was conducted. The contact angle of the DC-Fe 3 O 4 nanoparticle film decreased from about 48 • to about 25 • after adding water droplets with a pH of 2 ( Figure S5), indicating the dissociation of amphiphilicity DC-Fe 3 O 4 into hydrophilic Fe 3 O 4 -NH 2 and benzaldehyde in the acidic environment. That is to say, the amphiphilicity of DC-Fe 3 O 4 nanoparticles could be changed by changing the pH.
In light of the change in amphiphilicity of DC-Fe 3 O 4 nanoparticles when the pH is changed, we speculated that the pH could be used to adjust the stability of the obtained Pickering emulsions. To validate this hypothesis, liquid paraffin-in-water Pickering emulsion stabilized by 1.0 wt% DC-Fe 3 O 4 nanoparticles was used as the representative sample. At pH 10, DC-Fe 3 O 4 nanoparticles can be used as a stabilizer to prepare O/W Pickering emulsions because of the effective adsorption of amphiphilic DC-Fe 3 O 4 at the liquid paraffin-water interface (Figure 9a and Scheme 1a). By decreasing the pH from 10 to 2, complete phase separation was achieved within 30 min (Figure 9b and Scheme 1b). At pH 2, the amphiphilic DC-Fe 3 O 4 nanoparticles decomposed into hydrophilic Fe 3 O 4 -NH 2 and inactive benzaldehyde, both of which were desorbed from the oil-water interface, causing demulsification (Figure 9b and Scheme 1b). The hydrophilic Fe 3 O 4 -NH 2 nanoparticles participated in the aqueous phase (Scheme 1b). Additionally, benzaldehyde is surface-inactive and cannot stabilize emulsions effectively, as reported by our previous study [17]. We estimate that about 53% of the benzaldehyde migrates into the liquid paraffin phase based on the UV/Vis results (Scheme 1b). Moreover, after increasing the pH from 2 to 10, stable Pickering emulsion was reformed after re-sonication because of the re-formation of amphiphilic DC-Fe 3 O 4 nanoparticles through DBI formation between hydrophilic Fe 3 O 4 -NH 2 and surface inactive benzaldehyde (Figure 9a and Scheme 1a). Furthermore, pH-induced reversible emulsification and demulsification can be repeated up to three times without loss of efficacy (Figure 9a,b and Scheme 1a,b). Additionally, after three emulsification and demulsification cycles, the size of a newly formed Pickering emulsion shows a negligible increase compared with the original Pickering emulsion (13.1 µm vs. 13.5 µm, Figure S6). Emulsification and demulsification of the Pickering emulsion are determined by the pH responsiveness of the imine bond. reversible emulsification and demulsification can be repeated up to three times wi loss of efficacy (Figure 9a,b and Scheme 1a,b). Additionally, after three emulsificatio demulsification cycles, the size of a newly formed Pickering emulsion shows a negl increase compared with the original Pickering emulsion (13.1 μm vs. 13.5 μm, Figur Emulsification and demulsification of the Pickering emulsion are determined by th responsiveness of the imine bond.

Magnetism-Responsive Behavior of the Pickering Emulsions
According to the reports, Pickering emulsion droplets stabilized by Fe3O4 nano cles can move in the direction of a magnet, or even coalesce when subjected to mag fields [9,23]. Upon exposure to a magnet, the DC-Fe3O4-coated Pickering emulsion lets showed an instantaneous response with unidirectional movement toward the ma as demonstrated in Figure 9c and Scheme 1c, which was attributed to the superpara netic property of DC-Fe3O4 (magnetic saturation value, 43 emu/g). The time need remove all of the droplets was 40 s.
Magnetism-responsive Pickering emulsions have been widely used in extractin ganic pollutants from aqueous solutions [9,34]. Here, we discuss the possibility of the magnetism-responsive Pickering emulsion to extract a pollutant. The pink color be seen in photos of the RhB solution ( Figure S7a). Nonetheless, the pink color in aqueous solution nearly disappeared after Pickering emulsion was added for 20 which suggested that RhB molecules were efficiently removed from the aqueous sol reversible emulsification and demulsification can be repeated up to three times without loss of efficacy (Figure 9a,b and Scheme 1a,b). Additionally, after three emulsification and demulsification cycles, the size of a newly formed Pickering emulsion shows a negligible increase compared with the original Pickering emulsion (13.1 μm vs. 13.5 μm, Figure S6). Emulsification and demulsification of the Pickering emulsion are determined by the pH responsiveness of the imine bond.

Magnetism-Responsive Behavior of the Pickering Emulsions
According to the reports, Pickering emulsion droplets stabilized by Fe3O4 nanoparticles can move in the direction of a magnet, or even coalesce when subjected to magnetic fields [9,23]. Upon exposure to a magnet, the DC-Fe3O4-coated Pickering emulsion droplets showed an instantaneous response with unidirectional movement toward the magnet, as demonstrated in Figure 9c and Scheme 1c, which was attributed to the superparamagnetic property of DC-Fe3O4 (magnetic saturation value, 43 emu/g). The time needed to remove all of the droplets was 40 s.
Magnetism-responsive Pickering emulsions have been widely used in extracting organic pollutants from aqueous solutions [9,34]. Here, we discuss the possibility of using the magnetism-responsive Pickering emulsion to extract a pollutant. The pink color could be seen in photos of the RhB solution ( Figure S7a). Nonetheless, the pink color in RhB aqueous solution nearly disappeared after Pickering emulsion was added for 20 min, which suggested that RhB molecules were efficiently removed from the aqueous solution

Magnetism-Responsive Behavior of the Pickering Emulsions
According to the reports, Pickering emulsion droplets stabilized by Fe 3 O 4 nanoparticles can move in the direction of a magnet, or even coalesce when subjected to magnetic fields [9,23]. Upon exposure to a magnet, the DC-Fe 3 O 4 -coated Pickering emulsion droplets showed an instantaneous response with unidirectional movement toward the magnet, as demonstrated in Figure 9c and Scheme 1c, which was attributed to the superparamagnetic property of DC-Fe 3 O 4 (magnetic saturation value, 43 emu/g). The time needed to remove all of the droplets was 40 s.
Magnetism-responsive Pickering emulsions have been widely used in extracting organic pollutants from aqueous solutions [9,34]. Here, we discuss the possibility of using the magnetism-responsive Pickering emulsion to extract a pollutant. The pink color could be seen in photos of the RhB solution ( Figure S7a). Nonetheless, the pink color in RhB aqueous solution nearly disappeared after Pickering emulsion was added for 20 min, which suggested that RhB molecules were efficiently removed from the aqueous solution ( Figure S7d). Based on the standard curve for RhB solution (Figure S8), the extraction efficiency for RhB adsorption was 97.5% ( Figure S9). A similar adsorption experiment was conducted using only DC-Fe 3 O 4 nanoparticle aqueous dispersion or liquid paraffin as an adsorbent in order to explore the adsorption mechanism. After adding DC-Fe 3 O 4 nanoparticle aqueous dispersion or liquid paraffin into RhB aqueous solution for 1 h, the pink color did not disappear ( Figure S7b,c). Compared with DC-Fe 3 O 4 nanoparticle aqueous solution or liquid paraffin, Pickering emulsions have a higher specific surface area. Therefore, the high specific surface area of Pickering emulsions played an important role in the adsorption of RhB from the water phase into the oil-water interface. Besides, by replacing DC-Fe 3 O 4 with conventional surfactant sodium lauryl polyoxyethylene ether sulfate (AES) without a phenyl ring, the pink color did not disappear either, indicating that the AES-stabilized emulsion cannot be used to adsorb the RhB from the aqueous solution ( Figure S7e). The results demonstrate that the π-π stacking between benzene in RhB and phenyl ring in DC-Fe 3 O 4 also played important roles for the adsorption of RhB into the oil-water interface. Pickering emulsions have good magnetic responsiveness and high stability, so emulsion droplets adsorbed by RhB can be regenerated upon washing and used to extract RhB again. Even after three times, the extraction efficiency was still above 78%. During extraction, the efficiency of the extraction decreased owing to the adsorption of residual RhB molecules at the droplet interface, which were difficult to remove. Pickering emulsions can potentially serve as extraction systems for dye molecules (such as RhB) because of their high efficiency through simple, yet rapid magnetic separation. Furthermore, the Pickering emulsions could be reused for extraction by washing. This simple magnetic separation method broadens the application of magnetism-responsive Pickering emulsions.

Conclusions
To summarize, this work has developed a dual-responsive oil-in-water Pickering emulsion prepared using dynamic covalent Fe 3 O 4 (DC-Fe 3 O 4 ) as a stabilizer. At pH 10, the hydrophobic functionalization with benzaldehyde through dynamic imine bond (DIB) allowed DC-Fe 3 O 4 nanoparticles to reach the oil-water interface, forming stable Pickering emulsions. By reducing the pH from 10 to 2, DBI is decomposed so that amphiphilic DC-Fe 3 O 4 breaks into highly hydrophilic Fe 3 O 4 -NH 2 and surface-inactive benzaldehyde, resulting in a phase separation of the Pickering emulsion. Besides, Pickering emulsion stabilized by DC-Fe 3 O 4 nanoparticles was also magnetically responsive. The magnetismresponsive Pickering emulsion could be used for the extraction of RhB-polluted aqueous solutions with a high extraction efficiency, and Pickering emulsion droplets can be used for extraction at least three times after washing, thanks to their magnetic responsiveness and the high stability of Pickering emulsion. The feasible and unique dual-responsiveness enrich the intelligent control of Pickering emulsions' stability and broaden the applications of Pickering emulsion in field wastewater treatments.
Supplementary Materials: The following supporting information can be downloaded at https:// www.mdpi.com/article/10.3390/nano12152587/s1, Figure S1. TEM image of DC-Fe 3 O 4 , Figure S2. Photograph of DC-Fe 3 O 4 nanoparticles partitioning at oil-water interface. The aqueous phase was stained with RbB, Figure S3. Photographs of liquid paraffin in water Pickering emulsions stabilized by 1.0 wt% Fe 3 O 4 (a), 1.0 wt% Fe 3 O 4 -NH 2 (b) at pH 10, taken 30 min after preparation, Figure S4. Photograph of the liquid paraffin and water after sonication for 2 min without DC-Fe 3 O 4 nanoparticles, Figure S5. Image of contact measurement of acidic water droplet (pH = 2) on the DC-Fe 3 O 4 film, Figure S6. Optical micrographs of initial Pickering emulsion and after 3 cycles are shown in (a) and (b), respectively. Pickering emulsion was prepared with 1.0 wt% DC-Fe 3 O 4 at pH 10. The liquid paraffin and water were in an equal volume ratio, Figure S7