key: cord-0701910-xhhwlzi4
authors: Jiang, Hang; Sheng, Yifeng; Ngai, To
title: Pickering Emulsions: Versatility of Colloidal Particles and Recent Applications
date: 2020-05-08
journal: Curr Opin Colloid Interface Sci
DOI: 10.1016/j.cocis.2020.04.010
sha: e31d9a3908dd64d2a926440f6d7633534ada7151
doc_id: 701910
cord_uid: xhhwlzi4

Abstract The versatility of colloidal particles endows the particle-stabilized or Pickering emulsions with unique features and can potentially enable the fabrication of a wide variety of derived materials. We review the evolution and breakthroughs in the research on the use of colloidal particles for the stabilization of Pickering emulsions in recent years for the particle categories of inorganic particles, polymer-based particles, and food-grade particles. Moreover, based on the latest works, several emulsions stabilized by the featured particles and their derived functional materials, including enzyme immobilized emulsifiers for interfacial catalysis, 2D colloidal materials stabilized emulsions as templates for porous materials, and Pickering emulsions as adjuvant formulations, are also summarized. Finally, we point out the gaps in the current research on the applications of Pickering emulsions, and suggest future directions for the design of particulate stabilizers and preparation methods for Pickering emulsions and their derived materials.

An emulsion is a system consisting of two immiscible liquids where one of the liquids is dispersed in the other. Due to the high surface energy of the interface between the two immiscible phases, emulsions are thermodynamically unstable. Conventional emulsions are stabilized by surfactants or amphiphilic polymers that reduce the oil-water interfacial tension and form a molecular film around the liquid droplets. In addition to molecular emulsifiers, solid particles can also serve as emulsion stabilizers.

Particle-stabilized emulsions known as "Pickering emulsions" were first observed and described by Ramsden and Pickering in the early 20 th century [1, 2] . In recent decades, benefiting from the rapid development of particle synthesis techniques and discovery of new colloids with tunable surface properties, the applications of Pickering emulsions and derivative materials have attracted increasing attention. Although conventional surfactant-stabilized emulsions have been successfully and extensively used in various fields such as crude oil recovery, food, paints and coatings, and water treatment, the large-scale use of surfactants in industrial applications or personal care products is not cost-effective and in some cases may cause adverse effects such as irritation and hemolytic behavior. Therefore, the use of colloidal particles may be a promising alternative to the use of surfactants, motivating the recent trend in the studies of Pickering emulsions of seeking to enable the commercial manufacturing and practical use of these materials.

Pickering emulsions are often considered to be highly stable due to the nearly irreversible interfacial adsorption of particulate stabilizers [3] , and the stabilization mechanism of Pickering emulsions has been explored by many researchers [3, 4] . Due to the high energy of particle desorption from the interface, the resultant superior review. Unlike the method of using the sacrificial seed template particles, the preparation of capsules or colloidosomes from Pickering emulsions does not require the subsequent removal of the template particles, and furthermore, cargos can be encapsulated inside the particles in advance. [5] Recently, Binks et al. [6] has reviewed the methods and applications of Pickering emulsion templated capsules and discussed the typical examples of the research studies conducted in recent years.

Among the various applications of Pickering emulsions and their derivative materials, particle stabilizers are particularly significant because they not only serve as the emulsifiers for the emulsion stabilization but can also be equipped with desired functionalities such as oxidation resistance, UV protection, environmental responsiveness, and even electromagnetic properties. The widely varying characteristics of these particles enable their use in a wide range of applications, and the interfacial assembly of these particles by Pickering emulsions offers a new opportunity for more effective exploitation of their performance.

Considering the fact that colloidal particles are the key component in the formation of Pickering emulsions, and that a wide range of particles such as inorganic particles, polymer particles, and food-grade particles have been found to stabilize Pickering emulsions, in this review we will first concisely review the various types of particle-stabilized Pickering emulsions, with several important recent studies chosen as the typical illustrative examples. Additionally, with the explosive increase in the study of functional materials, such as 2D materials and biomedical materials, many of these materials have been proven to greatly decrease the oil-water interfacial energy, and the combination of Pickering emulsions and these new materials is becoming an increasingly popular research topic compared to the traditional applications of Pickering emulsions. In this review, we will also survey the recently reported novel applications of Pickering emulsions and the functional materials stabilized by these novel particles, including interfacial bio-catalysis, biomedical adjuvants, and foam materials.

Inorganic particles were the first colloidal particles studied in the preparation and stabilization of Pickering emulsions, and are still the most studied colloidal particles for this purpose. Due to the high stability of inorganic particles, many Pickering emulsions stabilized by inorganic particles have been explored in the past decades.

Among these, silica colloidal particles have been the most popular materials because of their good resistance to acidic and basic environments, easy surface modification, and controllable size and structure.

Monodispersed silica particles can be synthesized by the Stöber method [7] , and the diameter of the silica particles can be controlled in the range from tens of nanometers to several micrometers. In 1988, Levine and co-workers successfully fabricated an oil-in-water (o/w) emulsion stabilized by hydrophilic silica nanoparticles with the diameter of ~572 nm, and found that a monolayer of silica nanoparticles was formed at the oil-water interface. [8] Since the Pickering emulsion is actually formed by the assembly of colloidal particles at the liquid-liquid interface, the size of the emulsion droplet is inevitably limited by the particulate stabilizers used, and hence the majority of the reported silica-stabilized Pickering emulsions are in the micrometer size range, hindering their use in a wide variety of medical-related applications, and particularly in the applications that require cell penetration. and then chemically modified the silica nanoparticles to make them hydrophobic in order to stabilize the water-in-oil (w/o) Pickering emulsions. It was found that the size of the obtained emulsion droplets could be reduced through the decrease in the diameter of the silica stabilizers, and the diameter of the emulsion droplets can even be smaller than 500 nm when 50 nm silica nanoparticles were used as the emulsifiers.

By using the submicron Pickering emulsion as the template, tetraethyl orthosilicate (TEOS) was introduced into the oil phase and a silica shell was formed at the oil-water interface, simultaneously fixing the adsorbed silica nanoparticles. The Furthermore, many other inorganic colloidal particle including nonmetallic oxide particles, magnetic particles and elemental particles exhibit good performance for the stabilization of Pickering emulsions. For example, laponite clay consists of disc-like particles, and a toluene-in-water Pickering emulsion was successfully prepared using these particles by Ashby and Binks approximately 20 years ago [13] . In another work, Kikuchi and co-workers fabricated a calcium carbonate (CaCO 3 ) capsule templated from the Pickering emulsion stabilized by calcium carbonate [14] , and showed that the CaCO 3 capsule is soluble under acidic conditions. ( Figure 1C ) Additionally, carbon-based particles such as carbon quantum dots [15] (Figure 1D ), carbon nanotubes [16, 17] , and graphene oxide particles [18] have been shown to be excellent materials for use as Pickering stabilizers for applications in water purification, biosensors and electrochemistry related fields. On the other hand, metal salts have been rarely used in Pickering emulsions. In the two example of such studies, Chen and coworkers synthesized highly uniform zirconium phosphate (α-ZrP) nanoparticles to produce α-ZrP nanodisk-stabilized o/w emulsions [19] (Figure 1E ), and Liu et al. nanoparticles can help to screen UV radiation and enhance the photocatalytic performance. In addition to these two examples, many other inorganic particles such as gold nanospheres have been used as the stabilizers for Pickering emulsions [23] .

With the ongoing progress in the development of particle synthesis, the focus of particle design and synthesis studies has advanced beyond obtaining particles with a uniform and controlled size, and currently, an increasing research effort is devoted to the control of the particle structure, morphology, and functionality, particularly for the well-known classical colloidal particles such as silica, TiO 2 , and gold nanoparticles.

These particles have been synthesized to obtain various shapes and engineered structures, or modified with different functional groups via surface chemistry to meet the needs of various applications. The breakthroughs in particle synthesis have also strongly influenced the fabrication of Pickering emulsions, leading to the realization of novel and interesting Pickering emulsion systems. It is expected that this trend will continue in the future.

The inorganic particles used for Pickering emulsions have been mainly elemental particles, as well as the salts and oxides of inorganic elements, and therefore they are usually rigid and difficult to deform. By contrast, polymer colloidal particles are composed of three-dimensional polymer networks that are entangled and chemically crosslinked. Polystyrene (PS) particles are a classical example of such polymer particles, and were used by Dinsmore et al. to fabricate a "colloidosome" capsule that was templated from an o/w Pickering emulsion stabilized by monodispersed PS spheres. [24] The colloidosome structure was fabricated from the emulsion with the full coverage of the PS spheres at the oil droplet. Although the PS spheres were polymer-based, they still behaved as rigid particles due to the high glass transition temperatures (T g ) of the polystyrene polymer. Recently, Douliez et al. [25] reported a hydrogelled colloidosome that was derived from a water-in-water (w/w) Pickering emulsion stabilized by amine-modified polystyrene latex beads. The internal phase of the w/w emulsion was an aqueous liquid filled with dextran, while the continuous phase consisted of a polyethylene oxide (PEG)-enriched solution. After the gelation of the core of the prepared w/w Pickering emulsions, the obtained colloidosome with molecularly crowded interiors displayed reversible swelling, as shown in Figure 2A .

Unlike the w/o emulsions, this w/w colloidosome allowed the uptake and exclusion of macromolecular payloads, making it a novel Pickering system for microencapsulation.

Responsive microgels [26] [27] [28] are a new type of particulate emulsifiers that have been used to stabilize Pickering emulsions. [29] Similar to polystyrene spheres, microgels are polymer-based colloidal particles. However, the microgels are more flexible than the rigid PS particles, and are very hydrophilic and thus can easily absorb water within the polymer networks. Moreover, microgels can switch between the states of swelling and collapse, with the switching triggered by the changes in the environmental conditions such as temperature [30, 31] and pH [32, 33] . In the swollen state, the microgel is soft and highly deformable, and the polymer chains are stretched.

On the other hand, the collapsed microgel resembles a rigid particle due to the shrinkage of polymer chains. [34] In the synthesis of microgels, functional and responsive groups are introduced in order to endow the microgels with environmentally-dependent behavior. For the conventional rigid particle stabilized Pickering emulsions, the stabilization mechanism is explained well by the high desorption energy from the interface that can be calculated from the contact angle and the oil-water interfacial tension. However, the stabilization mechanism underlying the preparation of the microgel-stabilized emulsions is far from being completely understood. [35, 36] Compared to the rigid colloids, the microgels are soft, fuzzy, and deformable [26] so that the concept of the contact angle may be not applicable to the microgels at the oil-water interface. In the emulsification process, the microgel is more similar to a polymer emulsifier because it itself is composed of polymers. Thus, the stabilization mechanism may also be explained by the ability of the microgel to reduce the oil-water interfacial tension. A previous study has indicated that soft particles reduced the oil-water interfacial tension more effectively than rigid particles. [37] Moreover, the soft particles also exhibited significantly higher interfacial activity at the oil-water interface than the rigid particles, and a clear correlation between the deformability of the microgels and the emulsions stability arising from the core-shell structure of the microgels was demonstrated [38] . by gelatin particles, [46] and subsequently, numerous edible particle stabilized HIPEs were reported. In Table 1 , for reference, we summarized the studies of edible particles used for the stabilization of Pickering HIPEs reported in the literature in the last two years. 

Colloidal particles are the essential components in the formation of a Pickering emulsion, and generally, the materials, texture and the functionality of the particles determine the specific applications of the prepared emulsions. While the previous section discussed the recent development of different types of particles and the corresponding Pickering emulsions, in this section, we will review several emulsions stabilized by functional particles that have been applied in recent years in the popular fields such as catalysis, biomedicine, and advanced materials.

Pickering emulsions are an ideal platform for biphasic catalysis due to their enhanced stability, large interfacial area, facile product separation, and easy catalyst recycling. Figure 3E ). This work provides an approach for amplifying the interfacial catalysis by Pickering emulsions that has great potential for industrial use.

A number of biomedical applications related to Pickering emulsions have been reported based on the good stability, high payload capacity, and most importantly the biocompatibility of the particle stabilizers. Therefore, many naturally derived particles and biodegradable particles, including chitosan, poly(lactic-co-glycolic acid) (PLGA), gelatin, and protein-based particles have been applied for emulsion stabilization. [74] While surfactant-stabilized emulsions have been explored as adjuvants for vaccine formulations [75] , Pioneering works on the use of Pickering emulsions in biomedical applications have also extensively focused on drug encapsulation and delivery (such as topical drug delivery and oral drug delivery) [77, 78] , bio-imaging [79] , and stimuli-responsive materials [80] . For further information regarding biomedical-relevant applications, the readers are referred to the latest review on the biomedical applications of Pickering emulsions. [74] 3.3 Functional materials

The 21 th century is the century of materials, and research on advanced materials including nanomaterials, biomedical materials, and energy materials has been extremely popular in recent years. In particular, porous foam materials have been shown to be promising materials for use in a variety of fields. As a pristine three-dimensional structure, Pickering emulsion is a natural template for the fabrication of hierarchically structured foam materials. Although many methods can be used for producing porous foam, such as freezing-drying, bubbling, and particle scaffolds. [82, 83] In recent years, the applications of Pickering stabilized foams for cell culture have been more focused on the concept of green chemistry, because many protein-based colloidal particles are suitable for the preparation of Pickering HIPEs, and the polymerization or cross-linking can be achieved in a milder manner. [84] In more practical and engineering applications, HIPE templated foams have been widely used for oil/water separation, oil capture, and water treatment. Since the internal volume fraction of the HIPE is very high, the templated foam material displays a low density after liquid removal. When the primary emulsion is of the w/o type, polymerization of the oil liquid can always obtain a hydrophobic scaffold foam.

One of the advantages of such foam is that it can freely float on water, and the large voids inside the foam can absorb a large amount of oil, endowing these foams with great potential for the treatment of marine oil spills. In recent years, many works have reported on the innovative use of HIPEs-templated foams in water treatment [85] , oil capture [86, 87] , oil/water separation [88] , and CO 2 or pH-switchable oil/water absorption [88, 89] . However, the majority of the above mentioned emulsion-templated foams are derived from surfactant-stabilized emulsions. While

Pickering-type polyHIPEs have been reported for more than a decade [90, 91] , the use Currently, the study of the preparation and applications of 2D materials is one of the hottest areas in science and technology research. Due to their outstanding conductivity, unique electronic structure, and large surface area, 2D materials are considered to have a very high potential for applications in the electrochemistry and energy fields. [92] However, the performance of 2D materials is severely hindered by their tendency to aggregate or stack, with the solutions of this issue including the generation of inter-layer space, creation of hierarchical structures, and assembly of the 2D material flakes into a 3D macroscopic structure. [93] Thus, the use of Pickering emulsions is an approach for the interfacial assembly of 2D materials.

As the oldest and most intensely investigated 2D material, graphene oxide (GO) has attracted considerable attention since the first discovery of graphene in 2004. [94] Due to its amphiphilic nature and extraordinary interface chemistry properties, GO acts as a sheet-structure colloidal particle for the fabrication of Pickering emulsions, while the GO sheets can assemble in a 3D structure at the liquid-liquid interface. [ Figure 4C ) Similarly, Russell et al. [107] demonstrated that cooperative assembly of amine-functionalized polyhedral oligomeric silsesquioxane (POSS-NH 2 ) and Ti 3 C 2 T x -Mxene sheets improved the interfacial activity at the water-oil interface. The obtained w/o emulsion was concentrated by centrifugation to fabricate the MXene aerogel, and it was shown that the aerogel has a macroporous structure and low density, and can be potentially used for oil absorption and electromagnetic interference (EMI) shielding ( Figure 4D ).

In addition, the interfacial behavior and jamming of the Ti 3 C 2 T x nanosheets at the oil-water interface was examined and described by Zettl et al.

[108] It is believed that the development of 2D-material-stabilized Pickering emulsions and derived functional materials will be a new trend in colloidal and material science.

Pickering emulsions have received considerable attention in recent years, and the applications of Pickering emulsions have been extended to a variety of fields, including material science, engineering, biochemistry, food, and cosmetics. This review briefly describes the various applications derived from Pickering emulsions focusing on several different categories based on the particle stabilizers comprised by inorganic particles, polymer particles, food-grade particles, and several composite particles and newly-discovered 2D material particles. Due to the recent breakthroughs in particle synthesis, the functionalities of the particle stabilizers endow the as-prepared Pickering emulsions with unique features that are unavailable in conventional surfactant-stabilized emulsions, and many studies have demonstrated the importance of these Pickering emulsions in interfacial catalysis, biomedicine, drug delivery, functional materials and other fields. We believe that the design of engineered colloidal particles will be a popular topic of future research. issues. Finally, we believe that Pickering emulsification is an efficient approach for the assembly of 2D materials at the interface for the construction of 3D functional foams that is envisioned for enabling the applications of the Pickering emulsion derived materials in the fields of electronics, energy, and even wearable devices.

Nothing declared.

Cxcvi.-emulsions

Separation of solids in the surface-layers of solutions and 'suspensions' (observations on surface-membranes, bubbles, emulsions, and mechanical coagulation).-preliminary account

Particles as surfactants-similarities and differences

Electrostatics at the oil-water interface, stability, and order in emulsions and colloids

Colloidosomes: Versatile microcapsules in perspective

Capsules from Pickering emulsion templates

Controlled growth of monodisperse silica spheres in the micron size range

Stabilization of emulsions by fine particles i. Partitioning of particles between continuous phase and oil/water interface. Colloids and Surfaces

All-silica submicrometer colloidosomes for cargo protection and tunable release

Submicron w/o Pickering emulsions and templated colloidosomes solely stabilized by silica nanoparticles are fabricated, with the adjustable shell thickness, and tunable release of encapsulated cargos

Tuning the interfacial activity of mesoporous silicas for biphasic interface catalysis reactions

Mesoporous silica stabilized Pickering emulsions can switch from o/w to w/o, and were proven to be highly stable against coalescence even at high-salinity conditions, also catalyst can be encapsulated in the pores of silica for promoting catalytic reaction without stirring

Construction of biocatalytic colloidosome using lipase-containing dendritic mesoporous silica nanospheres for enhanced enzyme catalysis

A MSNs-stabilized Pickering emulsion platform for interfacial enzymatic catalysis was obtained by using mesoporous silica with large pore size, and lipase B was successfully carried in the MSNs for interfacial catalysis

Novel fabrication of stable Pickering emulsion and latex by hollow silica nanoparticles

Pickering emulsions stabilised by laponite clay particles

Fabrication of hybrid capsules via CaCO 3 crystallization on degradable coacervate droplets

Inverse Pickering emulsions stabilized by carbon quantum dots: Influencing factors and their application as templates

Multiwalled carbon nanotubes at the interface of Pickering emulsions

Stable Pickering emulsions using multi-walled carbon nanotubes of varying wettability

Ionic liquid-containing Pickering emulsions stabilized by graphene oxide-based surfactants

Microwave-assisted rapid synthesis of hexagonal α-zirconium phosphate nanodisks as a Pickering emulsion stabilizer

Experimental investigation of hydrophobically modified alpha-zrp nanosheets for enhancing oil recovery in low-permeability sandstone cores

Light and magnetic dual-responsive Pickering emulsion micro-reactors

Photocatalytic degradation enhancement in Pickering emulsions stabilized by solid particles of bare TiO 2

Black gold: Plasmonic colloidosomes with broadband absorption self-assembled from monodispersed gold nanospheres by using a reverse emulsion system

Colloidosomes: Selectively permeable capsules composed of colloidal particles

Preparation of swellable hydrogel-containing colloidosomes from aqueous two-phase Pickering emulsion droplets

A hydrogelled colloidosome was derived from a water-in-water (w/w) Pickering emulsion stabilized by amine-modified polystyrene latex beads, and the obtained colloidosome with molecularly crowded interiors displayed reversible swelling, allowing the uptake and exclusion of macromolecular payloads

Functional microgels and microgel systems

The polymer/colloid duality of microgel suspensions

Nanogels and microgels: From model colloids to applications, recent developments, and future trends

Microgel particles at interfaces: Phenomena, principles, and opportunities in food sciences

H NMR investigation of thermally triggered insulin release from poly(n-isopropylacrylamide) microgels

Novel emulsions stabilized by pH and temperature sensitive microgels

Stimulus-responsive emulsifiers based on nanocomposite microgel particles

Biofunctionalized pH-responsive microgels for cancer cell targeting: Rational design

When colloidal particles become polymer coils

Microgel particles at the fluid-fluid interfaces

Responsive emulsions stabilized by stimuli-sensitive microgels: Emulsions with special non-Pickering properties

Fundamental study of emulsions stabilized by soft and rigid particles

Soft microgels as Pickering emulsion stabilisers: Role of particle deformability

Colloidal lattices of environmentally responsive microgel particles at ionic liquid-water interfaces

Core-shell microgel particles with both temperature and pH responsiveness were used for stabilizing ionic liquid (IL)-in-water Pickering emulsions, and the monolayer of the microgels adsorbed at the interface even displayed a colloidal lattice structure

High internal phase emulsions stabilized solely by microgel particles

Hydrophobized nanocomposite hydrogel microspheres as particulate stabilizers for water-in-oil emulsions

Nanocomposite microgels with adjustable hydrophobicity and surface roughness were synthesized to stabilize o/w and w/o Pickering emulsions, while both water-in-polar oil and water-in-nonpolar oil Pickering emulsions can be successfully prepared

Development and characterisation of tempered cocoa butter emulsions containing up to 60% water

Pickering emulsions for food applications: Background, trends, and challenges

Colloids in food: Ingredients, structure, and stability

Pickering emulsions in foods -opportunities and limitations

Gelatin particle-stabilized high internal phase emulsions as nutraceutical containers

High internal phase emulsions stabilized by starch nanocrystals

Fabrication of osa starch/chitosan polysaccharide-based high internal phase emulsion via altering interfacial behaviors

A stable high internal phase emulsion fabricated with osa-modified starch: An improvement in β-carotene stability and bioaccessibility

High internal phase oil-in-water Pickering emulsions stabilized by chitin nanofibrils: 3d structuring and solid foams

Fabrication of zein/pectin hybrid particle-stabilized Pickering high internal phase emulsions with robust and ordered interface architecture

Tuning particle properties to control rheological behavior of high internal phase emulsion gels stabilized by zein/tannic acid complex particles. Food Hydrocoll

Development of stable high internal phase emulsions by Pickering stabilization: Utilization of zein-propylene glycol alginate-rhamnolipid complex particles as colloidal emulsifiers

Development of antioxidant gliadin particle stabilized Pickering high internal phase emulsions (HIPEs) as oral delivery systems and the in vitro digestion fate

Fabrication and characterization of novel water-insoluble protein porous materials derived from Pickering high internal-phase emulsions stabilized by gliadin-chitosan-complex particles

One-step preparation of high internal phase emulsions using natural edible Pickering stabilizers: Gliadin nanoparticles/gum arabic. Food Hydrocoll

Formation of shelf stable Pickering high internal phase emulsions (HIPE) through the inclusion of whey protein microgels

Enhancing the viability of lactobacillus plantarum as probiotics through encapsulation with high internal phase emulsions stabilized with whey protein isolate microgels

High-internal-phase Pickering emulsions stabilized solely by peanut-protein-isolate microgel particles with multiple potential applications

Hierarchical porous protein scaffold templated from high internal phase emulsion costabilized by gelatin and gelatin nanoparticles

Casein nanogels as effective stabilizers for Pickering high internal phase emulsions

Fabrication and characterization of Pickering high internal phase emulsions (HIPEs) stabilized by chitosan-caseinophosphopeptides nanocomplexes as oral delivery vehicles. Food Hydrocoll

Nanoparticle cages for enzyme catalysis in organic media

Self-assembly of amphiphilic janus particles into monolayer capsules for enhanced enzyme catalysis in organic media

Interfacial polymerization of dopamine in a Pickering emulsion: Synthesis of cross-linkable colloidosomes and enzyme immobilization at oil/water interfaces

Enzyme-polymer conjugates as robust Pickering interfacial biocatalysts for efficient biotransformations and one-pot cascade reactions

Pickering emulsion-enhanced interfacial biocatalysis: Tailored alginate microparticles act as particulate emulsifier and enzyme carrier

Lipase enzyme was successfully immobilized in the tailored alginate microparticles, and the microparticles served as both emulsion stabilizers and catalytic sites

Inverse Pickering emulsion stabilized by binary particles with contrasting characteristics and functionality for interfacial biocatalysis

Inverse Pickering emulsions were stabilized by binary particles with contrasting characteristics, the binary particles were composed of hydrophobic silica nanoparticles and pH-responsive microgels, and enzyme was encapsulated in the microgel in a mild way for interfacial catalysis

Colloidal tectonics for tandem synergistic Pickering interfacial catalysis: Oxidative cleavage of cyclohexene oxide into adipic acid

Pickering emulsions for interfacial catalysis were fabricated by using two kinds of nanoparticles, while controlling the self-assembled behavior of the two colloidal particles, and tandem reaction was catalyzed continuously

Flow Pickering emulsion interfaces enhance catalysis efficiency and selectivity for cyclization of citronellal

Ionic liquid droplet microreactor for catalysis reactions not at equilibrium

Pickering emulsion-derived liquid-solid hybrid catalyst for bridging homogeneous and heterogeneous catalysis

The concept of a liquid-solid hybrid catalyst and utilization in continuous flow reactions was proposed

Pickering emulsion droplets hosting ionic liquid catalysts for continuous-flow cyanosilylation reaction

Recent developments in Pickering emulsions for biomedical applications

Structure-and oil type-based efficacy of emulsion adjuvants

Exploiting the pliability and lateral mobility of Pickering emulsion for enhanced vaccination

PLGA nanoparticle stabilized squalene-in-water Pickering emulsion was explored as the adjuvant system for exploiting the force-dependent deformability and lateral mobility, while the formulation displayed an enhanced antigen uptake, and promoted humoral and cellular immune responses

Characterization of Pickering o/w emulsions stabilized by silica nanoparticles and their responsiveness to in vitro digestion conditions

Pickering emulsions for dermal delivery

Plasmonic colloidosomes as three-dimensional sers platforms with enhanced surface area for multiphase sub-microliter toxin sensing

Recent studies of Pickering emulsions: Particles make the difference

Hierarchically structured composites and porous materials from soft templates: Fabrication and applications

Facile fabrication of poly(l-lactic acid)-grafted hydroxyapatite/poly(lactic-co-glycolic acid) scaffolds by Pickering high internal phase emulsion templates

Fabrication of hierarchical macroporous biocompatible scaffolds by combining Pickering high internal phase emulsion templates with three-dimensional printing

Interconnected macroporous 3d scaffolds templated from gelatin nanoparticle-stabilized high internal phase emulsions for biomedical applications

Hierarchical macro and mesoporous foams synthesized by HIPEs template and interface grafted route for simultaneous removal of λ-cyhalothrin and copper ions

Continuous preparation of polyHIPE monoliths from ionomer-stabilized high internal phase emulsions (HIPEs) for efficient recovery of spilled oils

Dual-templating synthesis of compressible and superhydrophobic spongy polystyrene for oil capture

Highly porous poly(high internal phase emulsion) membranes with "open-cell" structure and CO 2 -switchable wettability used for controlled oil/water separation

Macroporous monoliths with pH-induced switchable wettability for recyclable oil separation and recovery

High internal phase emulsion templates solely stabilised by functionalised titania nanoparticles

High internal phase emulsions stabilized solely by functionalized silica particles

Recent advances in MXenes: From fundamentals to applications

3d macroscopic architectures from self-assembled MXene hydrogels

The chemistry of graphene oxide

Graphene oxide: A surfactant or particle?

Graphene oxide sheets at interfaces

Self assembly of graphene oxide at the liquid-liquid interface: A new route to the fabrication of graphene based composites

Encapsulated phase change materials stabilized by modified graphene oxide

Macroporous graphene oxide-polymer composite prepared through Pickering high internal phase emulsions

Interconnectivity of macroporous hydrogels prepared via graphene oxide-stabilized Pickering high internal phase emulsions

Hierarchical and reversible assembly of graphene oxide/polyvinyl alcohol hybrid stabilized Pickering emulsions and their templating for macroporous composite hydrogels

CO 2 /water emulsions stabilized by partially reduced graphene oxide

Carbon foams from emulsion-templated reduced graphene oxide polymer composites: Electrodes for supercapacitor devices

A green method for preparing conductive elastomer composites with interconnected graphene network via Pickering emulsion templating

Development of a highly sensitive, broad-range hierarchically structured reduced graphene oxide/polyhipe foam for pressure sensing

3D assembly of Ti 3 C 2 -Mxene directed by water/oil interfaces

An o/w Pickering HIPE was successfully prepared using CTAB-modified Ti 3 C 2 T x -MXene flakes, and the prepared emulsion acted as template for the fabrication of macroporous foam materials

Self-assembly of MXene-surfactants at liquid-liquid interfaces: From structured liquids to 3d aerogels

MXene aerogel was successfully prepared that was templated from Mxene stabilized w/o Pickering emulsion, and it was shown that the aerogel has a macroporous structure and low density, and can be potentially used for oil absorption and electromagnetic interference (EMI) shielding

Sculpting liquids with two-dimensional materials: The assembly of Ti 3 C 2 T x MXene sheets at liquid-liquid interfaces

The interfacial behavior and jamming of the Ti 3 C 2 T x nanosheets at the oil-water interface was examined and described

High internal phase emulsion with double emulsion morphology and their templated porous polymer systems

The authors gratefully acknowledge the financial support from the Hong Kong 

To NGAI, Ph.D.

: (+852) 3943 1222 : (+852) 2603 5057 Highlight:1. We review the evolution and breakthroughs in the research on the use of colloidal particles for the stabilization of Pickering emulsions in recent years for the particle categories of inorganic particles, polymer-based particles, and food-grade particles.2. We discuss recent emulsions stabilized by the featured particles and their derived functional materials, including enzyme immobilized emulsifiers for interfacial catalysis, 2D colloidal materials stabilized emulsions as templates for porous materials, and Pickering emulsions as adjuvant formulations.3. We point out the gaps in the current research on the applications of Pickering emulsions, and suggest future directions for the design of particulate stabilizers and preparation methods for Pickering emulsions and their derived materials.

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: