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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2025 , Vol. 37 >Issue 7: 1074 - 1090

DOI: https://doi.org/10.7536/PC240720

Review

Multiple-Responsive Pickering Emulsion Interfacial Catalysis and Its Application

  • Mingxia Zhang ,
  • Heng Zhang ,
  • Anguo Ying , *
Expand
  • School of Chemistry and Chemical Engineering， Qufu Normal University， Qufu 273165， China
*

Received date: 2024-07-29

  Revised date: 2024-12-01

  Online published: 2025-02-25

Supported by

the National Natural Science Foundation of China(22278243)

Fold

Abstract

In recent years， Pickering emulsions have attracted substantial attention owing to their facile preparation and superior stability. Pickering emulsions are emulsions stabilized by solid particles that are far more stable than conventional emulsions. Solid particles， acting as the core part of the emulsion system， play an important role in the preparation and application of Pickering emulsions. Here， this review concentrates on the impact of various single stimulus responses （pH， temperature， carbon dioxide， redox， light irradiation， magnetic fields） and multiplexed stimulus responses on the stability and performance of Pickering emulsion systems. Additionally， it highlights the latest research and advancements concerning the application of Pickering emulsion systems in a multitude of reactions， such as oxidation reaction， reduction reaction， hydrolysis reaction， condensation reaction， esterification transesterification reaction， and cascade reaction.

Contents

1 Introduction

2 Responsive Pickering emulsion

2.1 pH-responsive

2.2 Temperature-responsive

2.3 CO2-responsive

2.4 Ox/Red-responsive

2.5 Light-responsive

2.6 Magnetoresponsive

2.7 Multiresponsive

3 Application

3.1 Pickering emulsion in oxidation reactions

3.2 Pickering emulsion in reduction reactions

3.3 Pickering emulsion in hydrolysis reactions

3.4 Pickering emulsion in condensation reactions

3.5 Pickering emulsion in esterification transesterification reactions

3.6 Pickering emulsion in cascade reactions

4 Conclusion and outlook

Key words： Pickering emulsions; wettability; stimulus-responsiveness; interfacial catalysis; reactive application

Cite this article

Mingxia Zhang , Heng Zhang , Anguo Ying . Multiple-Responsive Pickering Emulsion Interfacial Catalysis and Its Application[J]. Progress in Chemistry, 2025 , 37(7) : 1074 -1090 . DOI: 10.7536/PC240720

1 Introduction

An emulsion is a system composed of two immiscible liquids, where one liquid is dispersed within the other. When interfacial tension exists between these two components, the phenomenon known as emulsification occurs. Due to the high surface energy of the two immiscible phases in an emulsion system, it is generally considered thermodynamically unstable. Therefore, emulsion formation typically requires the presence of surfactants or solid particles; emulsions stabilized by solid particles are commonly referred to as Pickering emulsions. In recent years, Pickering emulsions have become the preferred choice for many researchers due to their advantages, including low toxicity, cost-effectiveness, and ease of recovery[1-2].
As early as the 20th century, with the pioneering work of Ramsden[3]and Pickering[4], research on Pickering emulsions was initiated. Compared to traditional surfactant-stabilized emulsions, Pickering emulsions exhibit superior resistance to deformation. This is because solid colloidal particles possess partial wetting characteristics[5], enabling solid particles to undergo irreversible adsorption at the interface between two immiscible liquids[6-7]. In the early stages, due to limited choices of available materials, Pickering emulsions did not receive widespread attention in practical applications. With advancements in materials and technology, researchers have begun exploring the use of particles with tunable surface wettability to stabilize Pickering emulsions. In the process of stabilizing Pickering emulsion systems, the size of adsorbed particles, the interfacial tension of the emulsion, and the wetting characteristics are all critical factors. The contact angle of solid particles at the oil-water interface determines the type of Pickering emulsion—either O/W or W/O[8](Figure 1Figure 1). Hydrophilic particles (θ < 90°) tend to form O/W emulsions, while hydrophobic particles (θ > 90°), under monolayer conditions, preferentially stabilize W/O droplets. When Pickering emulsions are stabilized through multilayer structures, the contact angles for O/W and W/O emulsions fall within the ranges of 15° < θ < 120° and 50.7° < θ < 160°, respectively[9].
图1 表示颗粒接触角与Pickering乳液类型（O/W或W/O）之间关系的示意图

Fig.1 Scheme representing the relationship between the particles contact angle and the type of Pickering emulsions （O/W or W/O）

Pickering emulsions have attracted widespread attention in multiple fields, including food, cosmetics, oil extraction, and drug delivery[2,10-25,88]. In terms of applications, the long-term stability of emulsions is particularly crucial; however, in special cases such as oil extraction[26], liquid-phase multiphase catalysis[27,89], and emulsion polymerization[28,91], they only need to exhibit transient stability. Therefore, it is especially important to design Pickering emulsions with intelligent responsiveness to external stimuli—responsive Pickering emulsions. To date, various triggers have been used to create responsive emulsions, and their systems are mainly divided into two categories. Many systems can be switched by a single triggering mechanism, including pH[29-30], temperature[31], carbon dioxide/nitrogen (CO2/N2)[32-33], redox[34], light irradiation[35-36], and magnetic fields[37]. Other systems rely on dual triggering mechanisms, which may include pH-magnetic field[38], pH-ionic strength[39], pH-temperature[40], CO2-magnetic field[41], and magnetic field-redox[42]combinations (Figure 2).
图2 刺激响应性Pickering乳液的响应类型

Fig.2 Response types of stimulus-responsive Pickering emulsions

2 Responsive Pickering emulsion

2.1 pH response

Currently, pH-responsive Pickering emulsions have attracted widespread attention (Table 1). This response can be achieved simply by protonation/deprotonation of functional groups in the emulsifier molecules under different pH conditions, thereby altering the wettability of the emulsifier and regulating the stability of the emulsion. By alternately adding acid or base into the system, controlling the pH is both simple and easy to implement. Although this leads to salt formation and accumulation throughout the system, these salts do not pose a hazard to the environment. A range of materials have been explored as pH-responsive Pickering emulsifiers, including unmodified nanoparticles[43-44], non-covalently[45,90]and covalently[30]surface-functionalized nanoparticles, as well as self-assembled micelles[46].
表1 pH响应性Pickering乳液系统

Table 1 pH-responsive Pickering emulsion systems

Particles Modifiers Ref
Ludox CL nanoparticles - 43
Graphene oxide nanoribbons（GONR） - 44
Sodium caseinate（NaCas） - 50
Carboxymethyl starch（CMS）/xanthan gum（XG） combinations - 51
Silica nanoparticles（NPs） - 52
Dextran Acetalated dextran （Ace-dextran） 29
4-formylbenzoic acid （FA） and 12-aminolauric acid （AA） - 30
PAH （polyallylamine hydrochloride）-BA （benzaldehyde） - 53
Alumina nanoparticles - 54
Lignin/chitosan nanoparticles （Lig/Chi NPs） - 55
Nanogels （nGels） N-（3-aminopropyl）-methacrylamide （NIPAM-co-APMA）and N-（1，1-dimethyl-3-oxobutyl）-acrylamide （NIPAM-co-DAA） 56
Silica nanoparticles（NPs） trimethoxysilylpropyldiethylenetriamine and n-octyl-
trimethoxysilane		 57
Fe3O4 nanoparticles（NPs） AA-co-MMA 58
Carboxymethyl maize starch （CMS） 2-（dimethylamine） ethyl methacrylate （DMAEMA） 59
Many unmodified nanoparticles exhibit pH-responsive characteristics. For example, Guo et al.[47]used chitosan nanoparticles as a pH-responsive Pickering emulsifier. In fact, chitosan itself, without hydrophobic modification, is not a good emulsifier. However, due to the presence of free amino groups on its main chain, it possesses pH-tunable sol-gel transition properties. Chitosan is considered a potential pH-responsive polymer because, under lower pH conditions, the chitosan amines become protonated and carry positive charges, transforming it into a water-soluble cationic polymer. Conversely, at higher pH levels, these amines undergo deprotonation, causing the polymer to lose its charge and become insoluble. Since the differing water solubility of chitosan nanoparticles at various pH levels forms the basis for the system's pH responsiveness, these chitosan nanoparticles can be utilized as stabilizers for Pickering emulsions. Studies have shown that at pH values of 3.0 or 4.5, most chitosan molecules are protonated and dissolve individually in aqueous solutions, forming limited chitosan aggregate particles (Figure 3). At pH 6, there is an equilibrium between protonation and deprotonation of chitosan, leading to the formation of sufficient nano-sized aggregate particles that provide some stability to Pickering emulsions. When the pH is below 6.0, acidification of the aqueous phase causes the chitosan nanoparticles to dissolve in water, thereby accelerating demulsification. As the pH increases from 3.0 to 6.0, the average size of the emulsion droplets decreases. Additionally, Pickering emulsions with different oil-to-water ratios and nanoparticle concentrations remain stable for up to 45 days and exhibit low water separation rates.
图3 壳聚糖的pH 响应性组装形成胶体颗粒[47]

Fig.3 pH-Responsive assembly of chitosan to form colloid particles[47]

Although certain nanoparticles exhibit pH-responsive surface properties, the surface functionalization process is a method that enables non-responsive particles to display a broader range of pH responsiveness. Qiu et al.[45](Figure 4)modified alkaline lignin by grafting quaternized amino groups, and in the presence of SiO2, prepared stable pH-responsive O/W Pickering emulsions. Studies have shown that when the pH is between 3 and 4, the modified lignin is adsorbed onto the surface of SiO2 via electrostatic interactions, thereby achieving in situ hydrophobization of SiO2 and forming a stable Pickering emulsion. However, at pH > 4, due to electrostatic repulsion between SiO2 particles and the modified lignin, only a very small amount of modified lignin is adsorbed onto the SiO2 particles, resulting in an unstable emulsion. By adjusting the pH of the aqueous phase, reversible emulsification and demulsification can be achieved; after five cycles, there was no significant change in droplet size or stability.
图4 AML@SiO2 化合物稳定的 Pickering 乳液的 pH 响应机制[45]

Fig.4 pH-responsive mechanism of Pickering emulsions stabilized by AML@SiO2 compounds[45]

Binks et al[30]reported the novel pH-responsive compound (FA-AA) constructed via dynamic covalent bonds, as shown in Figure 5. Studies have demonstrated that under alkaline conditions (pH>10), 12-aminolauric acid (AA) and 4-formylbenzoic acid (FA) undergo a nucleophilic addition reaction in water to form a Schiff base compound (FA-AA). However, under acidic conditions (pH<4.1), the FA-AA molecule decomposes to produce 12-aminolauric acid hydrochloride (H+AA), which can independently stabilize conventional O/W emulsions and also combine with negatively charged silica nanoparticles to stabilize Pickering O/W emulsions.
图5 通过形成动态共价键构建新型表面活性剂FA-AA示意图

Fig.5 Schematic diagram of the construction of a novel surfactant FA-AA by forming dynamic covalent bonds

Pickering emulsifiers derived from self-assembled structures or cross-linked microgels have also demonstrated the ability to stabilize or destabilize emulsions in response to pH changes[48-49]. Thomas et al.[46]successfully synthesized polyacrylamide-based nanogels with varying degrees of cross-linking and charge using free-radical suspension polymerization. The study found that O/W Pickering emulsions stabilized by polyacrylamide-based nanogels could rapidly demulsify in alkaline solutions, while exhibiting remarkable stability under saline and acidic conditions. When pH < 7, the polyacrylamide nanogels carry a neutral charge, and become negatively charged when pH > 9. Size distribution analysis and emulsion state studies indicated that these nanogels contribute to the formation of stable Pickering emulsions.

2.2 Temperature response

Typically, temperature-triggered functional groups are grafted onto the surfaces of polymers and inorganic particles. Compared to adding chemicals or altering the pH of the system, destabilizing emulsions using temperature changes does not directly affect the chemical composition of the system. Table 2summarizes several temperature-responsive Pickering emulsion systems.
表2 温度响应Pickering乳液

Table 2 Thermo-responsive Pickering emulsions

Particles Modifiers Ref
PNIPAM microgel - 60
PNIPAM and PNIPMAM microgels - 61
Silica nanoparticles PNIPAM 62
PNIPAM and DEX microgels - 63
POEMA microgels - 64
Nardello-Rataj et al.[31]prepared a series of polyethylene glycol (PEG)-functionalized silica nanoparticles with different molecular weights (M W =200, 400, 550, 2000, 5000 g·mol-1) via a one-step synthesis method, and successfully used them to prepare temperature-responsive Pickering emulsions. The study found that when SiO2@mPEG550 formed temperature-responsive Pickering emulsions with different oil phases, these emulsions exhibited significant temperature-triggered instability as the temperature was gradually increased from room temperature to 80 ℃. However, no instability was observed in emulsions stabilized by longer PEG/mPEG chains (M W =2000 and 5000 g·mol-1). This difference may be attributed to changes in the distribution of PEG/mPEG chains at the oil-water interface (Figure 6).
图6 温度诱导的乳液去稳定的方法示意图[31]

Fig. 6 Schematic diagram of the method for temperature-induced emulsion destabilization[31]

Another versatile approach is the graft modification of particulate polymers. Fu et al.[65]reported modifying lignin by grafting poly(ethylene glycol) methyl ether methacrylate (PEGME), followed by self-assembly of the modified lignin into microspheres, which were then applied to temperature-responsive lignin-based Pickering emulsions. Due to the temperature responsiveness of the grafted polymer, the particle size of the modified lignin microspheres varied with temperature, thereby influencing the radius of the emulsion droplets. The study found that when the temperature increased from 25 ℃ to 70 ℃, the average particle size of the lignin microspheres decreased from 159 nm to 141 nm. Further increases in temperature could lead to aggregation of the lignin microspheres and destabilization of the Pickering emulsion. After four thermal cycles, the prepared Pickering emulsion showed no significant changes, indicating that the lignin-based Pickering emulsion exhibits excellent thermal cycling stability.

2.3 CO2response

Among various stimuli, CO2is a unique stimulus for emulsion control, as CO2is an abundant resource of biological metabolism and possesses advantages such as environmental friendliness and biocompatibility. When dissolved in water, CO2can combine with water molecules to form carbonic acid, creating a weakly acidic solution. CO2can also be easily removed from water by bubbling inert gases such as N2 [66]. Compared to pH adjustment, the addition and removal of CO2is a non-cumulative process. Therefore, CO2/N2-responsive surfactants have attracted widespread attention.
Zeng et al[32]reported a novel method for rapidly CO2/N2-responsive switchable Pickering emulsions, which are fast-switchable Pickering emulsions stabilized by modified surface-active alumina nanoparticles. By adsorbing trace amounts of superamphiphiles onto the surface of alumina nanoparticles, in-situ hydrophobization was employed to prepare switchable surface-active alumina nanoparticles. The superamphiphiles were formed by mixing anionic fatty acid (oleic acid) and cationic amine (Jeffamine D-230) at a 1∶1 molar ratio. The study found that upon introduction of CO2into the emulsion, the emulsion exhibited a rapid response within 30 seconds, with stable Pickering emulsions spontaneously breaking and phase separation occurring. Subsequently, after purging with N2for 10 minutes, the emulsion could be re-emulsified into a stable Pickering emulsion. Zhang et al[33]investigated a CO2-switchable high internal phase Pickering emulsion stabilized by silica nanoparticles functionalized with N, N-dimethyl-N-dodecylamine (C12A). The study revealed that under relatively mild conditions, alternating bubbling of CO2and N2in the emulsion system allowed C12A to undergo reversible conversion between cationic and nonionic states, adsorbing onto or desorbing from the particle surface (Figure 7). Thus, this high internal phase Pickering emulsion can be reversibly "opened" and "closed" using CO2/N2.
图7 N，N-二甲基-N-十二烷基胺（C12A）的非表面活性和表面活性形式之间的可逆转化

Fig.7 Reversible transformation between non-surface-active and surface-active forms of N，N-dimethyl-N-dodecyl amine （C12A）

Jiang et al[67]prepared CO2/N2switchable Pickering emulsions using alumina nanoparticles and a novel CO2-switchable surfactant (sodium 11-(N, N-dimethylamino)undecanoate, NCOONa). The study found that bubbling CO2not only enables the Pickering emulsion to demulsify within 30 minutes, but also triggers the transformation of this novel surfactant into a more hydrophilic zwitterionic state (N+COONa). Furthermore, the aqueous phase containing the surfactant (NCOONa) and alumina particles can be recovered and reused at room temperature by bubbling N2to remove CO2, without contaminating the oil phase. Under ambient conditions, CO2/N2triggering allows for more than six cycles of demulsification/re-stabilization (Figure 8).
图8 CO2可转换的NCOONa表面活性剂和具有可回收水相的CO2响应性Pickering乳液示意图[67]

Fig.8 Schematic illustration of the CO2-switchable NCOONa surfactant and the CO2-responsive Pickering emulsion with a recyclable aqueous phase[67]

Pei et al.[68]Using amine-functionalized quaternary ammonium salt surfactants as stabilizers, CO2or N2was bubbled into the system at 25 ℃to achieve switching behavior in a CO2/N2-responsive emulsion system. This surfactant (N+-C n-N (n=14 or 16)) is a typical cationic surfactant in neutral or alkaline aqueous media. The study found that after bubbling CO2at 25 ℃, the surfactant (N+-C n-N (n=14 or 16)) transformed into a Bola form (N+-C n-NH+(n=14 and 16)). Due to the protonation of the tertiary amine groups, the surfactant's solubility in water increased, leading to demulsification of the O/W emulsion. When N2was introduced at 25 ℃, the Bola-type surfactant reverted to its conventional form, and bubbling N2resulted in the regeneration of N+-C n-N (n=14 or 16) (Figure 9). Subsequently, these emulsions could switch between stable and unstable states for several cycles. Unlike other convertible surfactants, N+-C n-N (n=14 or 16) did not transfer to the oil phase after demulsification but returned to the aqueous phase along with nanoparticles, allowing the entire aqueous phase to be reused.
图9 表面活性剂N+-Cn-N（n=14或16）对CO2/N2刺激反应的机制[68]

Fig.9 The mechanism of CO2/N2 stimulation response of surfactant N+-Cn-N （n=14 or 16）[68]

2.4 Redox response

Redox responsiveness is typically induced by redox initiators. Yu et al.[34]reported a redox-responsive Pickering emulsion stabilized jointly by aluminum oxide nanoparticles and a sodium selenocarboxylic acid surfactant, sodium hexadecyl selenocarboxylate (C10-Se-C10·(COONa)2) (Figure 10). Specifically, the hydrophobic selenide group of C10-Se-C10·(COONa)2is oxidized to a hydrophilic selenide structure (C10-Se-C10·(COONa)2-Ox) via a redox reaction, causing the surfactant to transform from a bola-shaped form to a pseudo-trimeric form with three amphiphilic groups. Before oxidation, C10-Se-C10·(COONa)2adsorbs onto the surface of aluminum oxide particles through electrostatic interactions, endowing the particles with surface activity and enabling their irreversible adsorption at the oil-water interface. After oxidation, most of the hydrophilic C10-Se-C10·(COONa)2-Ox surfactant remains adsorbed onto the particles via its carboxyl groups; however, the enhanced hydrophilicity of the particles causes them to desorb from the oil-water interface back into the aqueous phase. Furthermore, the aqueous phase containing the oxidized surfactant and aluminum oxide particles can be reactivated by reduction with Na2SO3, which facilitates the recycling of the Pickering emulsifier. The oil phase can be easily separated without surfactant contamination, and the aqueous phase containing nanoparticles and surfactants can be recovered and reused for approximately 10 cycles or more.
图10 含硒表面活性剂和含氧化铝纳米颗粒的氧化还原反应Pickering乳液示意图[34]

Fig.10 Schematic illustration of selenium-containing surfactant and the redox-responsive Pickering emulsion containing alumina nanoparticles[34]

2.5 Light response

Light, as a clean and efficient stimulus-responsive method, has attracted considerable attention because it does not introduce foreign substances into the emulsion or damage the emulsion system, and it is easy to implement, allowing for remote and precise delivery in both time and space. Therefore, light responsiveness has become a favorable stimulus condition. Li et al.[35]developed a light-controlled method to regulate the amphiphilicity of Pickering emulsifiers by employing supramolecular self-assemblies of alginate grafted with β-cyclodextrin and azobenzene derivatives as photoactivated emulsifiers. Specifically, through the photoisomerization between β-cyclodextrin and azobenzene derivatives, reversible transformations between hydrophilicity and hydrophobicity can be achieved. By adjusting the external ultraviolet irradiation conditions, the amphiphilic balance at the oil-water interface can be regulated, effectively disrupting the stability of O/W emulsions and thus achieving oil-water separation in Pickering emulsions. Xin et al.[36]prepared a novel light-responsive emulsion by introducing copper nanoclusters modified with azo groups, Cu3(azopz)3(Cu3, Hazopz = (3,5-dimethyl-4-(p-phenyldiazole)-1H-pyrazole)). Cu3 was dissolved in the oil phase of an oil-in-water emulsion stabilized by the surfactant APG0810. The reversible self-assembly of Cu3 was controlled by ultraviolet and visible light, thereby realizing reversible demulsification and emulsification behaviors (Figure 11). The study found that under 365 nm ultraviolet irradiation, Cu3 self-assembled into fibrous structures in the toluene phase, disrupting the emulsion stability and causing demulsification. After demulsification, the emulsion was irradiated with visible light and then homogenized, allowing the fibers at the interface to disassemble and restoring the emulsion state to its original condition. Furthermore, the fibers self-assembled at the interface after demulsification could be collected and reused.
图11 Cu3光响应自组装诱导APG0810/Cu3乳液可逆相分离示意图[36]

Fig. 11 Schematic diagram of reversible phase separation of APG0810/Cu3 emulsion induced by light-responsive self-assembly of Cu3[36]

Evans et al[69]prepared light-responsive Pickering emulsions using azobenzene-modified SiO2nanoparticles. The surface hydrophobicity of the particles was adjusted by varying the length of the carbon spacers used for azobenzene attachment on the particle surface. For most hydrophobic particles, stable emulsions were not formed in the natural trans state. However, when these systems were irradiated with ultraviolet light, emulsification occurred because the grafted azo molecules converted to the more hydrophilic cis state. Additionally, when selected emulsions were exposed to ultraviolet or blue light, they exhibited a reversible transition between the aqueous and oil phases, a phenomenon that could be repeatedly cycled. Li et al[70]successfully prepared a novel light-responsive water-in-toluene emulsion stabilized by TiO2nanoparticles with the same charge combined with low-concentration Rhodamine B (RhB) molecules. In this system, RhB molecules adsorbed at the oil-water interface, while the nanoparticles dispersed in the aqueous phase (Figure 12). The study found that under ultraviolet light irradiation, the high catalytic activity of TiO2nanoparticles photodegraded the RhB molecules, destabilizing the emulsion. Furthermore, by adding Fe3O4nanoparticles, the transformation from the novel emulsion to a Pickering emulsion could be achieved, providing a new pathway.
图12 带正电荷的TiO2纳米粒子与RhB复合稳定的新型乳液[70]

Fig.12 Novel emulsions stabilized by positively charged TiO2 nanoparticles complexed with RhB[70]

Zheng et al.[71]reported an innovative UV/visible-light-responsive Pickering interfacial biocatalytic system stabilized by functionalized immobilized enzyme particles. Specifically, spiropyran (SP-COOH) was grafted onto the surface of amino-modified hollow mesoporous silica (HMSS-N) nanospheres via covalent bonding to create modified particles (HMSS-SP). These modified particles were then used as photocatalytic emulsifiers for lipase CL (CL@HMSS-SP) emulsification (Figure 13).The study found that introducing UV or visible light could alter the wettability of the emulsifier surface, thereby easily enabling demulsification or emulsification of O/W emulsions.
图13 螺旋吡喃（SP-COOH）通过共价键偶联在氨基修饰的中空介孔硅球（HMSS-N）表面，并用于固定化脂肪酶CL（CL@HMSS-SP）形成光响应PIB体系的示意图[71]

Fig. 13 Schematic illustration of the coupling of spiropyran （SP-COOH） on the surface of the amino-modified hollow mesoporous silica spheres （HMSS-N） via covalent bonding and used to immobilize lipase CL （CL@HMSS-SP） to form light-responsive PIB system[71]

2.6 Magnetic Response

Magnetic response typically involves the introduction of an external magnetic field to induce emulsion breaking, and the magnetic nanoparticles used in this process can be repeatedly recovered and reused multiple times. Magnetic responsiveness has garnered widespread attention due to its minimally invasive nature, simple operation, and recyclability. Magnetic Fe3O4nanoparticles combine the dual advantages of nanoparticles and magnetic particles. Zhang et al.[37]prepared a magnetic Fe3O4/ZIF-8 composite material using a solvent-free method. This composite serves as an excellent lipase carrier for Pickering emulsion catalysis. Studies have shown that Fe3O4@ZIF-8@lipase exhibits high catalytic activity and stability in ester hydrolysis. Furthermore, leveraging the interfacial activation effect of lipase, along with the high surface activity and biocompatibility of magnetically responsive nanoparticles, allows for the rapid formation of Pickering emulsions and controlled hydrolysis. In this interfacial biocatalytic system, the catalytic efficiency of Fe3O4@ZIF-8@lipase is 2.48 times higher than that of free lipase and demonstrates strong tolerance toward organic media. Therefore, the simplicity of preparation and multifunctional catalytic properties make this system promising for potential applications in biocatalytic organic synthesis.

2.7 Multiple responses

In recent years, there has been considerable interest in Pickering emulsion systems that respond to more than one stimulus. This is because the combination of multiple stimuli is particularly advantageous for reactions, and it can broaden the controllable range or enhance the precision of the system. Table 3lists several multi-responsive Pickering emulsion systems.
表3 多重响应 Pickering 乳液体系

Table 3 Multi-responsive Pickering emulsion systems

Stimuli Particles Modifiers Ref
pH/magnetic Fe3O4 nanoparticle SiO2 and chitosan 38
pH/Thermo Chitosan （CS） poly （N-isopropylacrylamide） （PNIPAM） 72
pH/light Polymeric supra amphiphilic assembly （Alg-β-CD/AzoC12） - 73
pH/ionic strength poly（4-vinylpyridine） particles（P4VP） - 39
pH/magnetic Fe3O4 nanoparticle P（AA-co-MMA） segments 59
Magnetic/redox Fe3O4 nanoparticle 3-pyridyl-5-ferrocenyl-2-pyr-azoline （PFP） 42
Sun et al.[38]reported a novel core-shell structured Pickering emulsifier Fe3O4@SiO2@CS, which exhibits dual pH/magnetic responsiveness and can be used for controlled emulsification and demulsification with liquid paraffin as the oil phase. The study found that when the pH is between 3 and 11, chitosan adheres to the surface of Fe3O4@SiO2, forming Fe3O4@SiO2@CS, thereby endowing the Pickering emulsion with dual pH/magnetic responsiveness. Under a magnetic field of 0.4 T, the composite emulsifier nanoparticles can rapidly demulsify stable paraffin oil-in-water emulsion systems. When pH ≤ 2, the emulsion is stabilized by chitosan segments and Fe3O4@SiO2particles separately, showing no stimulus responsiveness, and the system can form a stable emulsion. Moreover, the cyclic responsiveness under pH 3~11 can be repeated up to 5 times. This novel dual-responsive system provides an effective and simple method for optimizing emulsifier performance (Figure 14).
图14 （a） 水溶液中不同pH值下Fe3O4@SiO2@CS颗粒上壳聚糖变化的示意图[38]；（b） pH值在3~11 时乳化和破乳机制中pH/磁场响应的示意图[38]

Fig.14 （a） Schematic representation of chitosan changes on Fe3O4@SiO2@CS particles at different pH values in aqueous solutions[38]；（b） Schematic illustration of the pH/magnetic field response in emulsification and demulsification mechanism at pH 3~11[38]

Zhang et al.[40]successfully prepared pH- and temperature-responsive Pickering emulsions by combining hydrophilic SiO2nanoparticles with a small amount of PEO-PPO-PEO (a conventional nonionic copolymer surfactant). The stimulus responsiveness arises from the pH- and temperature-controlled adsorption of surfactant molecules onto the particle surfaces. Specifically, PEO-PPO-PEO (polyethylene oxide-polypropylene oxide-polyethylene oxide) adsorbs onto the silica surface via hydrogen bonding between PEO and silanol (SiOH) groups. Consequently, pH responsiveness can be achieved by leveraging the pH-dependent transformation characteristics of SiOH groups. Studies have shown that increasing the pH or temperature leads to desorption of PEO-PPO-PEO from the silica surface, causing the particles to regain their hydrophilic and dispersible properties and subsequently triggering demulsification. Conversely, reducing the pH or temperature allows for the reformation of stable Pickering emulsions through homogenization, and the emulsification-demulsification cycle can be repeated at least 10 times. Lu et al.[74]successfully constructed redox- and pH-responsive Pickering emulsions using redox-responsive surfactant FA-DMDA-Ox and functional SiO2nanoparticles. Research revealed that alternating additions of Na2SO3and H2O2can induce reversible emulsification and demulsification of the Pickering emulsion, demonstrating its redox-switchable behavior. Additionally, the addition of HCl to the Pickering emulsion causes the breakdown of FA-DMDA-Ox, followed by the reformation of FA-DMDA-Ox upon subsequent addition of equimolar NaOH, which is then broken down again after adding NaOH, only to reform once more upon reintroducing HCl. This indicates that the Pickering emulsion exhibits dual pH-responsive behavior (Figure 15). Furthermore, after removing the original oil, the Pickering emulsion can still be reused three times by adding fresh oil. Therefore, the FA-DMDA-Ox solution based on SiO2nanoparticles holds significant potential for broader applications in the petroleum industry.
图15 交替添加 Na2SO3 和 H2O2 或 NaOH 和 HCl 时二氧化硅纳米颗粒和 FA-DMDA-Ox 之间的氧化还原和 pH 依赖性相互作用示意图[74]

Fig.15 Redox and pH-Dependent Interaction between Silica Nanoparticles and FA-DMDA-Ox by Adding Na2SO3and H2O2 or NaOH and HCl Alternately[74]

Song et al.[41]successfully prepared Pickering emulsions by combining negatively charged Fe3O4nanoparticles with 1-dodecyl-1H-imidazole (C12mim) as a stabilizer. The stability of these emulsions can be controlled by CO2and an external magnetic field. Specifically, due to the pH sensitivity of the imidazole groups, the introduction of CO2leads to the protonation of C12mim, which then induces electrostatic attraction to modify the negatively charged Fe3O4nanoparticles, forming [C12mim]+-modified Fe3O4nanoparticles and subsequently forming Pickering emulsions. By introducing N2, CO2can be removed; thus, alternating bubbling of CO2 and N2 at room temperature allows for the adjustment of the stabilizer's surface activity, enabling a fully reversible transition between emulsification and demulsification states (Figure 16). Since no additional additives are introduced, the emulsifier can be recycled multiple times after oil phase replacement.
图16 （a） 通过交替吹入CO2和N2，C12mim和[C12mim]+之间发生可逆转化[41]； （b） CO2和Pickering乳液磁响应的图示[41]

Fig.16 （a） Reversible transformation between C12mim and [C12mim]+ by alternatively blowing CO2 and N2； （b） illustration of CO2 and magnetic response of the Pickering emulsions[41]

3 Application

3.1 Application of Pickering Emulsions in Oxidation Reactions

Organic-aqueous biphasic reactions represent an environmentally friendly chemical synthesis and transformation process. In this system, the catalyst and products are separated into two immiscible phases, allowing for easy separation. Due to its unique advantages, it has been widely applied in various oxidation reactions. Wang et al.[75]successfully prepared a novel multifunctional Pickering emulsion using a magnetic and CO2-responsive nanohybrid Fe3O4@SiO2@P(TMA-DEA). The Fe3O4@SiO2@P(TMA-DEA) was synthesized via distillation precipitation polymerization (DPP) followed by post-oxidation. The study found that the obtained Fe3O4@SiO2@P(TMA-DEA) can stabilize oil-in-water Pickering emulsions in biphasic systems and can subsequently be demulsified by bubbling CO2on the surface. Without any energy barrier, the nanohybrid can be easily captured in situ by a magnetic field within 2 minutes, demonstrating excellent recyclability. The next cycle can be initiated by purging with N2for deprotonation (Figure 17). Furthermore, in the Anelli system used for alcohol oxidation, the catalytic efficiency of the nanocatalyst was tripled compared to the unemulsified biphasic system, and after five cycles of use, its catalytic activity showed no significant decline. This stimulus-responsive system not only ensures high efficiency in liquid-liquid reactions but also enables the recovery of nanoscale Pickering interface catalysts, holding great promise in the fields of nanocatalysis and separation.
图17 用于双相反应的磁性和CO2敏感 Pickering界面催化[75]

Fig.17 Magnetic and CO2-Sensitive PIC Used in Biphasic Reaction[75]

Qiu et al.[44]By passivating the pH-responsive groups (—COOH) on the surface of graphene oxide nanoribbons (GONR) with the aid of ionic liquids (ILs), a broadly pH-stable emulsifier named GONR-IL was successfully designed. The study found that, due to the unique structure of GONR-IL, emulsions stabilized by GONR-IL exhibit high stability at pH values ranging from 2 to 10. Moreover, Pd/GONR-IL stabilized at the toluene-water interface demonstrates high emulsion catalytic activity owing to its appropriate hydrophilic/hydrophobic balance, which ensures the stability of the emulsion reaction interface during Pickering interfacial catalysis. This provides a simple method for configuring broadly pH-stable emulsifiers for Pickering emulsions and can be extended to the preparation of other emulsifiers in the future. These unique properties give it broad application prospects in the field of biphasic catalysis operating over a wide pH range.

3.2 Application of Pickering Emulsions in Reduction Reactions

Pickering emulsions are an excellent platform for interfacial catalysis. However, developing simple and effective strategies for product separation as well as catalyst and emulsifier recovery remains a challenge. Luo et al.[76]prepared multifunctional starch-based nanoparticles via grafting with [2-(dimethylamino)ethyl methacrylate] (DMAEMA) and a gelatinization-ethanol precipitation process, which were used to form pH-responsive Pickering emulsions. The study found that in-situ separation and recovery of the nanoparticles could be easily achieved by adjusting the pH value. Moreover, DMAEMA served as both a stabilizing agent and growth center for Au nanoparticles. Combined with the large interfacial area of the emulsion droplets and the reversible emulsification/demulsification cycling characteristics under pH response, the resulting Pickering emulsion was highly suitable as a catalytic microreactor, particularly in the application of hydrogenation of p-nitroanisole at the oil-water interface (Figure 18). At the same time, the product could be separated from the reaction system without remaining in the residual phase. This continuous catalyst separation and recycling system significantly improved the overall efficiency of the chemical process and simplified post-processing methods.
图18 催化 Pickering 乳液的催化机理示意图[76]

Fig. 18 Schematic diagram of the catalytic mechanism of catalyzing Pickering emulsions[76]

In addition, Wang et al.[77]reported a novel photoresponsive Pickering emulsion capable of flexibly switching between emulsification and demulsification under visible light and ultraviolet irradiation. This Pickering emulsion is composed of silica nanoparticles loaded with Pd (Pd@SM), azobenzene ionic liquid surfactant (ILS), n-octane, and water. Under ultraviolet irradiation while stirring, the initially stable Pickering emulsion becomes unstable, leading to the separation of water and oil. These photoresponsive Pickering emulsions can serve as microreactors for catalytic reactions. Catalytic hydrogenation of unsaturated hydrocarbons was carried out at room temperature and atmospheric pressure, and conversion rates were found to be above 94%. The photoresponsive Pickering emulsion system enabled efficient separation of products and recovery of the emulsifier. This strategy provides an innovative approach for developing sustainable green chemical processes. (Figure 19)
图19 （a） 苯乙烯在 O/W Pickering 乳液中于25 ℃和大气压下的催化氢化的反应式； （b） 用于催化氢化、产物分离以及ILS和催化剂回收的 Pickering乳液的光控乳化和完全相分离的示意图

Fig. 19 （a） The reaction equation for the catalytic hydrogenation of styrene in O/W Pickering emulsion at 25 ℃ and atmospheric pressure； （b） Schematic illustration of the light-switchable emulsification and complete phase separation of a Pickering emulsion for catalytic hydrogenation， product separation and recycling of the ILS and catalyst

Han et al.[78]successfully obtained Pickering emulsions stabilized by magnetite/reduced graphene oxide (MRGO) nanocomposite sheets using aggregation and electrostatic attraction. Specifically, after adding ethanol to the Pickering emulsion, it can break down into MRGO composite patches. By employing an in-situ reduction method, Pd nanoparticles were used to modify the MRGO composite material, and then the Pd-functionalized MRGO-stabilized Pickering emulsion was utilized for the catalytic oxidation of Sudan-4 and the hydrogenation of 4-nitrotoluene (4-NT). The results showed that this system can efficiently degrade Sudan-4/4-NT with high yield in the presence of H2O2/NaBH4. Moreover, the Pd-functionalized patches can be recycled up to 8 times in the same reaction, indicating that the developed Pickering emulsion holds great potential in the field of interfacial catalysis (Figure 20).
图20 （a） MRGO-Pd在乳液中进行界面催化的示意图：Pickering乳液破裂（步骤1）和MRGO-Pd重新形成Pickering乳液（步骤2），MRGO-Pd稳定Pickering乳液和界面催化（步骤3和4）[78]； （b） 使用 MRGO-Pd稳定的Pickering乳液对苏丹-4的H2O2氧化和对硝基甲苯的NaBH4氢化反应式

Fig.20 （a） Schematic diagram of MRGO-Pd to perform interfacial catalysis in emulsion： the PE rupture （step 1） and MRGO-Pd re-form Pickering emulsions （step 2）， MRGO-Pd stabilization of Pickering emulsions and interfacial catalysis （step 3 and 4）[78]； （b） H2O2 oxidation of Sudan-4 and NaBH4 hydrogenation of 4-NT using MRGO-Pd stabilized Pickering emulsion

Hong et al.[79]By jointly mediating polymerization-induced self-assembly with a binary mixture of macromolecular chain transfer agents, spherical surface-separated micelles (SSMs) were successfully prepared: pH-responsive poly(2-(dimethylamino)methyl acrylate) and hydrophobic polydimethylsiloxane. Specifically, by using SSMs as the sole emulsifier and adjusting the pH, water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) multiple emulsions were successfully produced. The results showed that the multiple emulsion microreactor increased the oil-water interface area, making it more effective for interfacial catalysis compared to conventional O/W and W/O emulsion systems. Furthermore, it was found that this pH-switchable multiple emulsion system is suitable for developing efficient and recyclable interfacial catalytic systems.
Dong and Luo et al.[80]reported a pH-responsive Pickering emulsion system stabilized by polymer-coated nanoaggregates P-Si. The study found that the nanoaggregates flocculate at low pH, enabling them to stabilize Pickering emulsions, whereas under neutral and alkaline conditions, they disperse in water, causing emulsion breakdown. Therefore, by alternately adding acid or base to the system multiple times, the system can be switched from an emulsified state to a demulsified state. Additionally, when P-Si is mixed with silica loaded with Rh (Rh-Si), the mixture exhibits the same pH-responsive behavior as P-Si alone; although Rh-Si cannot stabilize emulsions independently, it contributes to the catalytic activity of Pickering emulsions. In this Pickering emulsion system, Rh-Si and P-Si work together to stabilize the Pickering emulsion. When reactants are added to the oil phase and hydrogen gas is bubbled into the system, the system can be used for biphasic interfacial catalytic hydrogenation of olefins. Furthermore, by adding base to the system, it was observed that the emulsion breaks down and separates into two phases, allowing the oil phase containing the product to be separated. Meanwhile, the aqueous phase containing the nanocatalyst can be recovered and reused for further Pickering emulsion catalysis. This pH-responsive Pickering emulsion stabilized by polymer-coated silica nanoaggregates provides an effective method for rapid recovery of both catalytic products and catalysts (Figure 21).
图21 pH 响应性Pickering乳液的催化循环[80]

Fig. 21 Catalytic cycles of a pH-responsive Pickering emulsion[80]

3.3 Application of Pickering Emulsions in Hydrolysis Reactions

Pickering emulsion systems are an excellent platform for biphasic enzymatic reactions. Compared to traditional biphasic systems, Pickering emulsions provide a larger reaction interface and lower mass transfer resistance for enzymatic catalysis. Xu et al.[81]reported the construction of enzyme-immobilized starch particles forming stable Pickering high internal phase emulsions for biphasic biocatalysis. By adjusting the temperature, the Pickering high internal phase emulsion can be reversibly transformed between emulsification and demulsification (Figure 22). Specifically, highly hydrophilic starch was modified using butyl glycidyl ether (BGE) and glycidyl trimethyl ammonium chloride (GTAC), resulting in suitable wettability and enabling stabilization of water-in-oil Pickering high internal phase emulsions. For these Pickering high internal phase emulsions with on-demand demulsification properties, lipase-catalyzed hexanoic acid hexyl ester hydrolysis was used as a model reaction. The study found that Pickering high internal phase emulsions stabilized by lipase-immobilized starch particles exhibited superior biocatalytic performance compared to other systems, owing to their shorter mass transfer distances and larger specific surface areas. Moreover, the thermoresponsive Pickering high internal phase emulsions can trigger demulsification simply by changing the temperature, making the separation of products and enzyme recovery processes straightforward.
图22 由脂肪酶固定淀粉颗粒稳定的热响应性Pickering 高内相乳液（HIPEs）示意图[81]

Fig.22 Schematic of thermoresponsive Pickering high-internal-phase emulsions （HIPEs） stabilized by lipase-immobilized starch particles applied to recycle interfacial biocatalysis[81]

3.4 Application of Pickering Emulsions in Condensation Reactions

Metal-organic frameworks (MOFs), as an emerging class of crystalline porous materials, have attracted widespread attention due to their high surface area and structural diversity, demonstrating a variety of potential application scenarios. Dong et al.[82]reported a Pickering emulsifier composed of Pd nanoparticles (Pd NPs) loaded onto nano-sized metal-organic frameworks (NMOFs). Specifically, poly[2-(diethylamino)ethyl methacrylate] (PDEAEMA) was combined with UiO-66 type nanoparticles to generate the PDEAEMA-g-UiO-66 NMOF of MOF-3. Subsequently, Pd@MOF-3 containing Pd NPs was synthesized via a solution impregnation method, and a stable water-in-toluene Pickering emulsion was prepared (Figure 23). The study showed that, due to the pH-responsive properties of the obtained Pd@MOF-3, toluene droplets could be emulsified in neutral environments, whereas emulsion breakup might occur in acidic conditions. Furthermore, the research revealed that the catalytic activity of this Pickering emulsifier did not significantly decrease even after five cycles of reuse.
图23 pH 响应性 Pd@MOF-3 稳定的Pickering乳液示意图[82]

Fig. 23 Schematic illustration of the pH-responsive Pd@MOF-3-stabilized Pickering emulsion[82]

Wang et al.[83]successfully synthesized a functionalized MOF (ZIF-90/TETA) by modifying the aldehyde groups on ZIF-90 with triethylenetetramine (TETA), and used it to fabricate CO2-/N2-responsive Pickering emulsions. Studies have shown that even when the mass fraction of ZIF-90/TETA is as low as 0.25%, this functionalized MOF can still effectively emulsify the n-hexane-water system, successfully constructing high internal phase Pickering emulsions. Under ambient pressure conditions, alternating bubbling with CO2and its removal enables reversible switching between emulsification and demulsification processes. Furthermore, the CO2/N2-switchable Pickering emulsions can facilitate efficient Knoevenagel reactions, realizing a sustainable chemical process integrating chemical reaction, product separation, and MOF recovery (Figure 24).
图24 化学反应、产物分离和 MOF 回收一体化的示意图[83]

Fig.24 Illustration for the integration of chemical reaction， product separation， and MOF recycling[83]

3.5 Application of Pickering Emulsions in Esterification Transesterification Reactions

The transesterification method includes alkaline catalysis, acid catalysis, and enzymatic catalysis. In 2022, our research group[72]successfully prepared a core-shell structured P[xSPA-yDABCO]@SiO2@Fe3O4composite material with light-responsive and alkaline catalytic activities using a layer-by-layer preparation method. By immobilizing basic DABCO and light-responsive SPA onto the magnetic olefin-modified Fe3O4@SiO2composite within the core-shell structure, we successfully developed a solid alkaline ion polymer catalyst that exhibits dual light- and magnetically-responsive controllable separation properties. The study found that this catalyst can promote the formation of methanol-in-oil Pickering emulsions in soybean oil-methanol systems. Under optimized conditions of an alcohol-to-oil molar ratio of 9∶1, a catalyst dosage of 5% by mass, a reaction temperature of 60 ℃, and a reaction time of 5 hours, the biodiesel yield can reach as high as 98.2%. Furthermore, this Pickering interfacial catalyst can be recycled more than six times without noticeable deactivation. In summary, this dual-responsive Pickering interfacial catalyst provides an efficient and sustainable approach for multiphase catalytic conversion processes such as biodiesel production.
Gao et al.[84]successfully developed a pH-responsive lipase-loaded starch nanoparticle using a green composite approach. They grafted the pH-responsive monomer [2-(dimethylamino)ethyl methacrylate] (DMAEMA) onto corn starch molecules via free radical polymerization. Subsequently, by combining gelatinization-ethanol precipitation with lipase (CRL) adsorption, they fabricated DMAEMA-grafted enzyme-loaded starch nanoparticles (D-SNP@CRL) for forming pH-responsive Pickering emulsions. The study found that, due to the tunable wettability and particle size of D-SNP@CRL under different pH conditions, the prepared Pickering emulsion could serve as a recyclable microreactor for the n-butanol/vinyl acetate transesterification reaction. This was made possible by the presence of CRL, which facilitated the smooth occurrence of the transesterification reaction at the interface (Figure 25). Furthermore, this catalyst exhibited high catalytic activity and good recyclability, making enzyme-loaded starch particles a promising green and sustainable biocatalyst in Pickering interfacial systems.
图25 O/W Pickering界面催化体系中酯交换反应示意图[84]

Fig.25 Schematic diagram of the transesterification in O/W Pickering interfacial catalytic system[84]

Our research group[85]successfully prepared a Pickering interfacial catalyst with dual CO2and magnetic responsiveness using a template-free technique. By adjusting the monomer ratios, we developed two novel Pickering interfacial catalysts: Pd-p(xTEMPA-yFDABCO-zDVB)@Fe3O4and p(xTEMPA-y[FDABCO][OH]-zDVB)@Fe3O4. Studies revealed that when the Pd loading in Pd-p(xTEMPA-yFDABCO-zDVB)@Fe3O4reached 0.2053 mmol·g-1, this catalyst could catalyze the hydrogenation of unsaturated olefins under ambient temperature and pressure conditions, achieving a maximum yield of up to 99%. Additionally, by optimizing the mixing ratio of soybean oil to ethanol and adjusting the concentration of p(xTEMPA-y[FDABCO][OH]-zDVB)@Fe3O4, relatively ideal biodiesel yields could be obtained in the catalytic reaction. Furthermore, this dual-responsive Pickering interfacial catalyst could be reused for at least five cycles without significant loss of catalytic activity.

3.6 Application of Pickering Emulsions in Cascade Reactions

Phytosterols, as one of the most important compound types in plant-derived foods, have attracted attention due to their ability to reduce the risk of cardiovascular diseases. Zheng et al.[86]proposed a pH-switchable Pickering interfacial biocatalysis (PIB) system for the solvent-free one-pot synthesis of phytosterol esters from rice bran oil (RBO) (Figure 26a). They used amine-functionalized hollow mesoporous silica nanospheres (HMSS-N) as both emulsifiers for Pickering interfacial biocatalysis (PIB) and carriers for lipase AYS, controlling the binding/separation between fatty acids and HMSS-N by adjusting the pH of the PIB system, thus achieving switchable emulsions. The study found that the high stability of AYS@HMSS-N effectively prevented lipase deactivation during pH switching, maintaining a relative conversion rate of phytosterols at 95% even after 10 consecutive switches. Furthermore, the synthesis of phytosterol esters using different acyl donors demonstrated the robustness and adaptability of this pH-switchable PIB system. Therefore, this approach provides a powerful green platform for lipid modification and stimulus-responsive interfacial biocatalysis, offering advantages such as high efficiency, sustainability, and multiple recyclability. They[87]also reported another stimulus-responsive method for synthesizing phytosterol esters, proposing a magnetically switchable Pickering interfacial biocatalysis (PIB) system that utilizes a "one-pot" enzymatic cascade reaction to convert high-acid-value oils into high-value phytosterol ester products (Figure 26b,C). The study revealed that this magnetically regulated Pickering interfacial biocatalysis exhibits excellent catalytic activity under mild, solvent-free conditions, with a catalytic efficiency far exceeding that of traditional biphasic systems. Moreover, by using magnetic field responsiveness as an external stimulus, the enzyme can be recovered and reused in situ under an external magnetic field without the need for energy or additional chemicals. This method not only provides a sustainable interfacial biocatalytic technology but also offers a strategy for the high-value utilization of high-acid-value petroleum resources.
图26 （a） AYS@HMSS-N 稳定 pH 可切换的 PIB 的示意图，用于与RBO的“一锅法”合成植物甾醇酯（Pes）[86]；（b） AYS@MHMCS作为乳化剂和催化剂用于高酸值油一锅法合成植物甾醇酯（Pes）[87]；（c） AYS@MHMCS 催化高酸值油一锅法合成植物甾醇酯（Pes）示意图[87]

Fig.26 （a） Schematic representation of AYS@HMSS-N stabilizing pH-switchable PIB for the “one-pot” synthesis of PEs with RBO[86]； （b） AYS@MHMCS was Used as Emulsifier and Catalyst for the Synthesis of PEs From High Acid Value Oil in One-Pot Method[87]； （c） Schematic diagram of catalytic one-pot synthesis of Pes from high-acid oil by AYS@MHMCS in one pot[87]

4 Conclusion and Outlook

Compared to traditional emulsions, Pickering emulsions offer advantages such as biocompatibility and environmental friendliness, opening up opportunities for their application in industries like petroleum, food, biopharmaceuticals, and cosmetics. Depending on the specific application scenario, these emulsions may need to undergo rapid and controllable stabilization and destabilization processes. Therefore, this review primarily discusses the impact of various stimulus-responsive properties on the stability and interfacial catalytic performance of Pickering emulsion systems, and provides a comprehensive summary and overview of the synthesis routes, reaction mechanisms, and practical applications of these systems.
Numerous studies have confirmed that, under well-controlled environmental conditions, rapid and reversible regulation of the stability of oil-in-water (O/W) and water-in-oil (W/O) emulsions can be achieved, with detailed discussions on the advantages and disadvantages of various external stimuli. Taking the pH-responsive system as an example, it is simple to operate and easy to implement, making it a commonly used strategy in laboratory and small-scale applications. However, repeated use often leads to a significant accumulation of ionic strength (salinity) within the system. This accumulation not only disrupts the micellar structure of the emulsifier and reduces emulsification efficiency but may also affect the physicochemical properties of the final product, limiting the feasibility of long-term cyclic use. In contrast, temperature-responsive systems, although free from cumulative effects, face challenges in large-scale practical applications due to energy consumption issues. The CO₂-responsive system operates similarly to the pH-responsive system, both relying on changes in proton concentration in the solution for responsiveness: dissolved CO₂ forms carbonic acid (H₂CO₃), which dissociates to provide H⁺. At the same time, it shares the non-cumulative advantage similar to temperature-responsive systems. However, the primary limitation of using CO₂ as a stimulus lies in its water solubility: CO₂ dissolution in water can only produce a weakly acidic to neutral pH environment, resulting in a relatively narrow pH adjustment range, which ultimately restricts its application in more fields. Given the limitations inherent in single-stimulus responsive systems, strategies involving the synergistic combination of multiple external stimuli (such as pH/temperature, light/magnetic field, CO₂/magnetic field, etc.) are increasingly gaining attention. This multi-stimulus responsive design not only enhances the precision of emulsion state (stability/unstability) regulation through complementary effects of different stimuli but also effectively broadens the overall regulatory range of the system, demonstrating promising prospects in areas such as drug delivery, catalysis, separation engineering, and material synthesis.
Although significant progress has been made in the field of stimulus-responsive Pickering emulsions, emerging demands in practical applications continue to place higher requirements on system performance. Therefore, future developments in this field should focus on multidimensional breakthrough innovations, with key areas including: (1) stimulus-responsive intelligent systems: developing intelligent systems capable of responding to multiple environmental stimuli (such as pH, temperature, redox conditions, CO2, magnetic fields, etc.) and achieving synergistic or cascading regulation; (2) stimulus-responsive behavior of solid particles: conducting in-depth research into the adsorption, desorption, aggregation, and other behaviors of solid particles at oil/water interfaces under stimulus response, as well as their mechanisms for regulating emulsion stability; (3) integrated applications of stimulus-responsive systems: integrating stimulus-responsive behaviors with functionalities such as sensing, targeted delivery, and controlled release, thereby constructing advanced application platforms in fields including biomedicine and smart materials. Overall, future research will focus on deepening and systematizing the theoretical framework of stimulus-responsive Pickering emulsions, and further promoting and expanding their practical applications based on this foundation.

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