Pickering emulsion-based biomimetic microreactors

Abstract

Pickering emulsions are dispersions of two immiscible liquids stabilized by surface-active colloidal nano-/microparticles. Their compartmentalized structures closely resemble the characteristics of cellular and subcellular systems, enabling the development of biomimetic microreactors that enhance catalytic processes. By enlarging interfacial areas while effectively partitioning reactants into their preferred phases, Pickering emulsion-based microreactors improve kinetic parameters and prevent unwanted interactions. The adaptability of Pickering emulsions is further augmented through modifications to the properties and composition of the particle emulsifiers, rendering them multifunctional and facilitating efficient reactions between immiscible phases, such as oil and water, especially when the emulsifiers themselves act as catalysts. This review summarizes recent advances in Pickering emulsion-based biomimetic microreactors, focusing on the versatile choice of various particles, design principles, and their applications in facilitating biphasic catalysis in a biomimetic way. We also discuss the challenges and future perspectives for further refining these microreactors for enhanced biphasic catalytic processes.

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Xiao Xu

Xiao Xu received her bachelor's degree from the Qufu Normal University in 2018. She is currently a PhD candidate at Fuzhou University in Prof. Zhaowei Chen's group. Her research interests mainly focus on Pickering emulsions and biocatalysis.

Min Zhou

Min Zhou received her bachelor's degree from Hunan Agricultural University in 2023 and is pursuing her master's degree at Fuzhou University in Prof. Huanghao Yang's group. Her research interest focuses on multifunctional nanomaterials in cancer therapy and bioimaging.

Ting Wu

Ting Wu received her bachelor's degree from the Qufu Normal University in 2020. She is currently a PhD candidate at Fuzhou University in Prof. Zhaowei Chen's group. Her research interests mainly focus on Pickering emulsions and artificial cells.

Zhaowei Chen

Zhaowei Chen is currently a Professor in the College of Chemistry at Fuzhou University and an adjunct Professor in the College of Pharmaceutical Sciences at Zhejiang University. He received his B.E. degree at Northwestern Polytechnical University in 2010, and PhD degree in Inorganic Chemistry and Chemical Biology at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2016. Then, he worked as a Postdoc Researcher sequentially in the Joint Department of Biomedical Engineering at the UNC and NCSU, Argonne National Laboratory, and University of California-Los Angeles. His group focuses on the design and synthesis of bioinspired and biomimetic materials for biomedical engineering and biocatalysis.

Huanghao Yang

Huanghao Yang is a Professor in the College of Chemistry at Fuzhou University and a fellow of the Royal Society of Chemistry. He received his PhD degree from Xiamen University in 2002 and engaged in postdoctoral research at Hong Kong University of Science and Technology (2002–2004). He joined Fuzhou University in 2008 as a Min Jiang Scholar Professor. Prof. Yang's research interests mainly focus on nanotechnology, bioanalytical techniques, and cancer therapy. He has published more than 300 high-impact papers, and has an H-index of 106.

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1. Introduction

Cells are the fundamental structural and functional units of living organisms, where hundreds to thousands of biochemical reactions are contemporarily occurring to sustain life.¹ The presence of organelles further enhances the efficiency and orchestration of these intracellular processes.² The superior catalytic performance of cells is attributed to their hierarchical micro- and nano-interfaces and confined spaces, which provide ample attachment sites for biocatalytic units (enzymes) and create specific reaction niches for biological metabolism.³ These structural features enable biocatalytic reactions to take place in a specific microenvironment under precise regulatory mechanisms.⁴ Drawing inspiration from the hierarchically organized cellular and subcellular systems found in nature, researchers have developed diverse biomimetic microreactors. These artificial microscale reaction systems are engineered to structurally emulate the compartmentalized and selectively permeative architectures of biological cells, while functionally facilitating specific chemical/biochemical reactions or processes within confined microenvironments.^5,6 These include micelles,⁷ vesicles,^5,8 liposomes,^9,10 polymersomes,^11,12 coacervates,^13,14 and Pickering emulsion droplets,^15,16 all designed to regulate mass transfer and improve catalytic efficiency.

A key characteristic of biomimetic microreactors is the inner cavity, which can be used to encapsulate substrates or immobilize (bio-)catalysts.¹⁷ Among various approaches, Pickering emulsions stand out as a prevalent framework for constructing microreactors, which can be straightforwardly formed through assembling colloidal particles at the interface between oil and water phases.¹⁸ Each Pickering emulsion droplet can be regarded as a miniaturized microreactor, effectively isolating reaction substances within the internal cavity or external continuous phase, thereby minimizing unwanted reactions and significantly enhancing kinetic parameters.¹⁹ Moreover, by adjusting the physicochemical properties and composition of the particle emulsifiers, Pickering emulsions can be tailored to achieve multifunctionalities, including on-demand emulsion revision and destabilization.²⁰ The large oil–water interface area provided by Pickering emulsions is particularly advantageous for mass transfer in biphasic reactions; in particular, when particle emulsifiers also serve as catalysts, the maximized oil-catalyst–water interface further accelerates reaction rates.²¹ Given their straightforward manufacturing and structural versatility, Pickering emulsions present tremendous opportunities for advancing biphasic catalysis.²²

In this review, we delve into recent advances in biomimetic microreactors constructed from Pickering emulsions for biphasic catalytic applications (Fig. 1). We begin by discussing the versatile selection of colloidal particles for formulating Pickering emulsions. Next, we outline the design principles of Pickering emulsion-based biomimetic microreactors, including compartmentalized structures, selective permeability, directed anchoring of biocatalysts, and holistic emulsion design. Following this, we highlight the application of these biomimetic microreactors in bioinspired catalysis. Finally, we discuss the current challenges to facilitate the design and application of Pickering emulsion-based biomimetic microreactors in industrial catalysis.

Fig. 1 Schematic illustration of Pickering emulsion-based biomimetic microreactors constructed with various colloidal particles for biomimetic catalysis (drawn using Figdraw).

2. Versatile choice of colloidal emulsifiers

In the early 20th century, Ramsden and Pickering successively discovered that colloidal particles could absorb at the oil–water interface to form stable emulsions, and they systematically explored the mechanisms behind emulsion formation.^23,24 Consequently, emulsions stabilized by colloidal particles are referred to as Pickering emulsions. Compared to traditional surfactant-stabilized emulsions, Pickering emulsions offer numerous advantages, including high resistance to coalescence, tunable properties, low toxicity, and flexibility in particle material selection.¹⁹ The performance of Pickering emulsions, such as stability, emulsion types, and morphology, largely depends on the surface characteristics of the colloidal emulsifiers.²⁵ Generally, hydrophilic particles with contact angles (between the solid and the oil–water interface) less than 90° facilitate the formation of stable oil-in-water (O/W) emulsions and hydrophobic particles with contact angles greater than 90° tend to stabilize water-in-oil (W/O) emulsions. Only particles with an appropriate wettability can effectively stabilize emulsions; excessively hydrophilic or hydrophobic particles tend to detach from the interface. Additionally, chemical crosslinking of the colloidal particles can create microcapsules with robust, particle-stacked, porous surfaces, which are known as colloidosomes.²⁶ Therefore, the selection of appropriate particles is crucial for constructing stable and functional Pickering emulsions. We will discuss in detail several categories of particles that have been employed for this purpose.

2.1. Inorganic particles

Inorganic particles were the first emulsifiers utilized for the preparation and stabilization of Pickering emulsions.²⁷ Among various inorganic materials, silica (SiO₂) nanoparticles have emerged as a preferred choice due to their cost-effectiveness, excellent resistance to acidic and alkaline conditions, surface adjustability, and controllable size and structure. However, SiO₂ nanoparticles synthesized through conventional methods typically possess a high number of hydroxyl (SiOH) groups on their surface, making them too hydrophilic to stabilize emulsions.²⁸ To address this challenge, Binks and Rodrigues proposed a method that combined di-chain cationic surfactants with negatively charged SiO₂ nanoparticles, which allowed the surfactants to modify the wettability of the nanoparticles through electrostatic binding (Fig. 2a).²⁹ By varying the amounts of surfactant used, they successfully produced both O/W and W/O emulsions. In addition to physically adsorbing surfactants, chemical grafting of long chain silanes onto SiO₂ nanoparticle surfaces can also be adopted to modulate the hydrophobicity. By adjusting the amounts of silane coupling agents, SiO₂ nanoparticles with varying hydrophobicity (SiOH content ranging from 100% to 15%) were prepared and employed in the preparation of both O/W and W/O Pickering emulsions.³⁰

Fig. 2 Inorganic particles for stabilizing Pickering emulsions. (a) Left to right and down, optical microscopy images of dodecane–water (1 : 1) emulsions stabilized by SiO₂ nanoparticles with different initial concentrations of di-C₁₀DMAB surfactant, optical microscopy image of O/W dodecane–water (1 : 1) emulsions at a surfactant concentration of 0.1 mM, and cryo-scanning electron microscope (cryo-SEM) images of a particle interfacial arrangement of oil droplets using 1 mM and 50 mM surfactants. Reproduced with permission from ref. 29. Copyright 2007 Wiley. (b) Magnetic response of amine-functionalized Fe₃O₄ magnetic nanoparticle-labeled liquid crystal double emulsion with (up) radial and (down) monopolar organizations: (i) side-view microscopy images, (ii) confocal microscopy images, (iii) polarized-light optical microscopy images, and (iv) schematic representations. Reproduced with permission from ref. 31. Copyright 2020 American Chemical Society. (c) Left: TEM images of the obtained SPIO (left) and FS (right) nanoparticles and corresponding submicron colloidosomes with open-shells. (d) Schematic diagram of the synthesis process based on self-assembly and characteristics of a nanoporous bifunctional submicron colloidosome. Reproduced with permission from ref. 32. Copyright 2015 Wiley.

Furthermore, a variety of inorganic colloidal nanoparticles, including carbon, metallic oxide, noble metal and hybrid nanoparticles, have also demonstrated excellent performance in stabilizing Pickering emulsions.²⁷ Among these, carbon-based nanomaterials such as mesoporous carbon spheres, carbon nanotubes, graphene oxide (GO) and graphitic carbon nitride (g-C₃N₄) nanosheets serve as effective stabilizers due to their thermal stability, intrinsic hydrophobicity, environmental friendliness, and ease of surface modification.³³ Emulsions stabilized by these carbon nanomaterials exhibit enhanced stability and improved performance in applications such as water treatment, catalysis, and energy conversion.

Functional nanoparticles possess unique physicochemical properties that can trigger supplementary effects in Pickering emulsions at specific times and locations through non-invasive methods, like using simple external stimuli like magnetic fields and light.¹⁹ For instance, Swager and colleagues demonstrated controlled orientation changes and rotational movement of emulsion droplets using Fe₃O₄ nanoparticles as colloidal emulsifiers, effectively manipulating droplet behavior under an external magnetic field (Fig. 2b).³¹ Titanium dioxide (TiO₂) nanoparticles, known for their ability to absorb and convert ultraviolet light into chemical energy, are excellent candidates for constructing Pickering emulsions designed for biphasic photocatalytic applications.³⁴ Additionally, the unique optical and catalytic properties of gold nanoparticles facilitate the creation of surface-accessible plasmonic Pickering emulsions, which are well-suited for surface-enhanced Raman scattering-based analysis and interfacial catalysis.³⁵

Binary and multicomponent mixtures of nanoparticles, exhibiting distinct properties such as opposing charges or different functional moieties, can generate synergistic effects that enhance oil–water interface stability beyond the capabilities of single-type nanoparticles.³⁶ In a pioneering study, Rodrigues and colleagues demonstrated the formation of stable O/W Pickering emulsions through the co-stabilization by binary nanoparticles with opposite charges, eliminating the need for additional particle surface modifications.³⁷ This finding underscored the potential of binary particle systems to enhance emulsion stability. Building on this concept, Rezwan's group further expanded the functionality of Pickering emulsions by combining superparamagnetic iron oxide nanoparticles with fluorescent SiO₂ nanoparticles (Fig. 2c and d).³² This innovative approach enabled the creation of Pickering emulsions that exhibited both superparamagnetism and fluorescence, simplifying the preparation process and allowing the resulting colloidosomes to inherit desirable properties from the binary-stabilized emulsion template. The integration of nanoparticles with different materials and functionalities broadens the range of properties of the resulting Pickering emulsions.

The versatility in the design and synthesis of inorganic nanoparticles has a profound effect on the production and properties of Pickering emulsions. Increasing research efforts are focused on controlling and manipulating the physical and chemical properties of nanoparticles, including surface modifications with various functional groups. This customization enables the development of responsive and highly stable Pickering emulsion systems tailored for specific applications.

2.2. Polymer particles

Polymer particles, like polystyrene (PS) latex particles and microgels, are composed of long chains or three-dimensional networks formed through the interweaving or chemical crosslinking of monomers.³⁸ Their potential in stabilizing Pickering emulsions is primarily determined by their ability to irreversibly adsorb at the oil–water interface. The viscoelastic nature of polymer particles, especially microgels, allows them to deform and stretch at the interface, forming a robust steric barrier that prevents droplet coalescence. Additionally, microgels can spread and interpenetrate at the interface, further enhancing emulsion stability by creating a thick, long-lasting viscoelastic layer.³⁹

PS is a widely used polymer known for its chemical stability, ease of synthesis and ability to be functionalized with various chemical groups. PS microspheres can be synthesized with a high degree of monodispersity, which is crucial for creating uniform Pickering emulsion droplets. Moreover, PS microspheres are mechanically robust, which means they can withstand the physical stresses encountered during Pickering emulsion preparation and use, such as stirring or shearing forces. This robustness helps to maintain the integrity of the Pickering emulsion over time and under various conditions. Nagayama et al. pioneered the preparation of Pickering emulsions using PS microparticles with sulfate and amidine surface groups, utilizing 1-octanol as the oil phase.⁴⁰ Following this, Lumsdon and colleagues prepared W/O Pickering emulsions using spherical, monodispersed PS microparticles of different sizes, with cyclohexane as the continuous phase.⁴¹ Weitz et al. later created colloidosomes from water-in-decalin Pickering emulsions stabilized by PS microspheres, where the particles were locked together through sintering at ∼105 °C, addition of oppositely charged polymers, or van der Waals forces (Fig. 3).⁴²

Fig. 3 Construction of colloidosomes with PS microspheres. (a) SEM image of the colloidosomes. Panels (b) and (c) show magnified views of (a) and (b), respectively. Reproduced with permission from ref. 42. Copyright 2002 American Association for the Advancement of Science.

Microgels are particularly effective Pickering emulsifiers due to their exceptional deformability and stimulus responsiveness.⁴³ Generally, the deformability of microgels inversely correlates with their cross-linking density: highly cross-linked microgels are stiffer and less deformable than those with lower cross-linking. Research studies by Schmitt, Ravaine and Richtering showed that Pickering emulsions stabilized by highly deformable microgels were more stable than those using less deformable counterparts.^44–46 This increased stability was attributed to the larger adsorption cross-sectional area of highly deformable microgels at the oil–water interface, which reduced interfacial energy and enhanced resistance to coalescence. The deformability of microgels can also be regulated by various external stimuli, such as temperature, pH, and light, which can induce volume phase transitions. For instance, poly-N-isopropyl acrylamide (PNIPAAm) microgels are known for their thermal responsiveness,⁴⁷ exhibiting a lower critical solution temperature around 32 °C. Above this temperature, the microgels collapse and shrink; below it, they swell in water. Pioneering studies by Ngai and others have thoroughly explored the use of PNIPAAm microgels in forming temperature-responsive Pickering emulsions.^48–50 By adjusting the temperature, these emulsions can be triggered to rapidly and completely emulsify or demulsify. Furthermore, microgels produced with other functional comonomers, such as methacrylic acid⁵¹ and photo-switchable molecules like spiropyran, endowed the resulting Pickering emulsions with pH or light responsiveness (Fig. 4a and b).⁵²

Fig. 4 Examples of Pickering emulsions stabilized by microgels. (a) Scheme of PNIPAAm microgel-stabilized Pickering emulsion and confocal laser scanning microscopy images of Pickering emulsions stabilized by ordinary microgels (upper right) and pH-expanded soft microgels (low right). Reproduced with permission from ref. 51. Copyright 2016 Elsevier. (b) Schematic diagram of the preparation process of PNIPAM-co-MAA microgel stabilized W/O Pickering emulsions. Reproduced with permission from ref. 52. Copyright 2022 Royal Society of Chemistry.

2.3. Biological particles

Biological particles represent a compelling resource for the preparation and stabilization of Pickering emulsions. Extensive research has demonstrated that biomaterials such as proteins, bacteria, and viruses can serve as effective Pickering stabilizers in various biologically related applications.⁵³ The non-toxic nature of these biological particle-based Pickering emulsions is crucial for scenarios involving direct contact with living organisms or for use in drug delivery systems,⁵⁴ thereby minimizing the risk of adverse side effects. The inherent biocompatibility of these biomaterials ensures that the resulting Pickering emulsions can interact with biological systems without eliciting adverse reactions.

2.3.1. Protein. Proteins have long been regarded as exceptional building blocks for the development of micro- and nano-sized particles due to their diverse sources, natural non-toxicity, ease of extraction, and excellent biocompatibility.⁵⁵ The amphiphilic nature of proteins, characterized by both hydrophilic and hydrophobic domains, facilitates effective adsorption at the oil–water interface, leading to a reduction in interfacial tension. By forming a physical barrier between oil droplets and inducing repulsive forces, such as steric and electrostatic interactions, protein nanoparticles stabilize emulsions, preventing the aggregation and separation of oil droplets.

Beginning with the pioneering work of Royer and colleagues,⁵⁶ protein nanoparticles have been extensively studied as emulsifiers for creating emulsion droplets. For instance, Velikov et al. demonstrated that nanoparticles derived from water-insoluble zein could effectively stabilize Pickering emulsions with droplet sizes ranging from 100 to 200 μm.⁵⁷ Additionally, Yamashita et al. successfully prepared ferritin-stabilized Pickering emulsions without the use of surfactants, employing n-dodecane, toluene, and vegetable oil as the oil phases.⁵⁸

Further advancements included the synthesis of protein–polymer conjugates through the chemical grafting of polymers onto proteins. For instance, conjugation of ferritin with poly-N-isopropyl acrylamide (PNIPAAm) has been shown to greatly enhance its surface activity, effectively reducing interfacial tension and hence facilitating the formation of Pickering emulsions at lower concentrations.^59,60 Concurrently, Huang et al. synthesized bovine serum albumin (BSA)-based nanoconjugates by coupling mercaptothiazoline-activated PNIPAAm with the interfacial primary amines of cationized BSA (BSA-NH₂) (Fig. 5a).⁶¹ They constructed proteinosomes through interfacial assembly at the Pickering interface using the resulting BSA-NH₂/PNIPAAm nanoconjugates.

Fig. 5 Examples of Pickering emulsions stabilized by biological particles. (a) Schematic illustration of the preparation of BSA-NH₂/PNIPAAm nanoconjugates to stabilize Pickering emulsions and proteinosomes. Reproduced with permission from ref. 61. Copyright 2013 Springer Nature. (b) Schematic illustration of bacterial imprinting at the interface via Pickering emulsion polymerization. Reproduced with permission from ref. 62. Copyright 2014 Wiley. (c) Schematic illustration of the preparation of individual encapsuled bacterium to stabilize Pickering emulsions and interfacial biocatalysis. Reproduced with permission from ref. 63. Copyright 2015 Wiley. (d) Schematic illustration of TMVCP programming self-assembly at oil–water interfaces for the construction of robust capsules and TEM images of TMVCP stabilized Pickering emulsion. Scale bars represent 200 nm (left) and 100 nm (right). Reproduced with permission from ref. 64. Copyright 2017 American Chemical Society.

These protein-stabilized Pickering emulsions have demonstrated suitability for a variety of applications, including cargo encapsulation and release, enzyme-catalyzed reactions, and facilitating selective permeability.^61,65–67 In this context, the utilization of protein or protein-derived particles has significantly advanced the design and development of Pickering emulsifiers, enabling their application in the construction of biomimetic models and biocatalysis.

2.3.2. Bacteria. Recent studies have highlighted the potential of certain microorganisms to adhere to and pack at oil–water interfaces, enabling the stabilization of Pickering emulsions.^68–70 Gray et al. were among the first to demonstrate that bacteria could stabilize emulsions through the formation of films at the interface,⁷¹ induced by interactions between bacteria. This ability to attach to the interface is largely attributed to the hydrophobic interactions generated by the bacteria and their surface characteristics, including surface charge, structural features, and functional groups.⁷²

Given the presence of negative charges on bacterial cell walls, which can render them overly hydrophilic for effective stabilization of Pickering emulsions, it is advantageous to enhance emulsion stability through various modifications to the cell wall. For example, Wongkongkatep et al. successfully coated bacteria with positively charged chitosan, creating a bacteria–chitosan network that enhanced bacterial surface hydrophobicity.⁷³ The resulting particles were effective in stabilizing O/W Pickering emulsions, and this modification could be adapted to utilize different bacterial strains. Moreover, Ye et al. reported the use of bacteria-pre-polymer complexes to stabilize Pickering emulsions, which served as templates for bacterial imprinting, facilitating selective bacterial recognition (Fig. 5b).⁶²

Besides coating with organic matter, biomineralization techniques have also been employed to modify bacterial surfaces. Chen et al. immobilized individual bacteria using calcium phosphate doped with magnetic nanoparticles, resulting in O/W Pickering emulsions that significantly improved the tolerance of bacteria against organic solvents (Fig. 5c).⁶³ Ali et al. explored the aforementioned binary strategy for creating bacteria-based Pickering emulsions, demonstrating that the addition of hydrophilic SiO₂ nanoparticles (Aerosil R974) greatly enhanced the emulsification efficiency hydrophobic bacteria (Mycobacterium sp.).⁷⁴ These findings emphasize the potential of modulating bacterial surface hydrophobicity to meet the demands for effective emulsifiers, providing a flexible approach to enhance emulsion stability and functionality.

2.3.3. Virus. In addition to the proteins and bacteria, some viruses exhibit a tendency to self-assemble at flat liquid–liquid interfaces, forming closely packed, ordered structures that stabilize Pickering emulsions.⁷⁵ Early studies utilized cowpea mosaic virus (CPMV),⁷⁶ turnip yellow mosaic virus (TYMV),⁷⁷ and tobacco mosaic virus (TMV)⁷⁸ to create Pickering emulsions with perfluorodecalin as the oil phase. CPMV and TYMV are both icosahedrally symmetrical spherical viruses, whereas TMV is a rod-shaped helical virus. However, the stability of these Pickering emulsions is constrained by the geometric characteristics of the nanorods. Advancing this research, Niu and colleagues developed long-term stable Pickering emulsions using TMV-like nanorods through a programmed self-assembly of TMV coat proteins (TMVCP) (Fig. 5d).⁶⁴ Their approach involved the initial self-assembly of amphiphilic TMVCP at the oil–water interface, followed by a pH-induced assembly process that transformed the TMVCP into larger nanorods, thereby increasing particle size and interfacial stability. Subsequent cross-linking facilitated the formation of robust capsules composed of TMV-like nanorods, which hold promising potential for applications in drug delivery and virus recognition.

Overall, the stability, wettability and surface chemical design of particles play a crucial role in determining the characteristics and specific applications of the resulting Pickering emulsions (Table 1).⁷⁹ Inorganic particles, such as SiO₂, carbon, and metal oxide nanoparticles, have attracted significant attention due to their diverse sources and superior chemical and mechanical properties, which have spurred extensive research into their applications in Pickering emulsions. Concurrently, polymer particles, including PS particles and microgels, are highly effective for emulsion stabilization, thanks to their tunable deformability and customizable functionalities. Moreover, the recent exploration of biological nanoparticles, such as proteins, bacteria, and viruses, as stabilizers has highlighted their advantageous interfacial activity, biocompatibility, and biocatalytic properties, broadening the spectrum of available stabilizers. This expanding diversity of particle types and functionalities is poised to drive innovative applications of Pickering emulsions across various fields.

Table 1 Comparative analysis of colloidal emulsifiers

Particle type	Stability	Reactivity	Toxicity	Emulsion type	Emulsion responsiveness	Ref.
SiO2	High chemical stability	Low reactivity (surface hydroxyl groups for functionalization)	Low toxicity	O/W or W/O	None	29 and 30
Carbon nanotubes	High mechanical and thermal stability	Low reactivity	Low toxicity	O/W	None	33
GO	Good stability	Reactive with oxygen groups	Low toxicity	O/W	None	33
g-C3N4	Good thermal and chemical stability	Photocatalytic reactivity	Low toxicity	O/W	None	33
Fe3O4	Good stability	Low reactivity	Low toxicity with coating	O/W	Magnetic responsive	31
TiO2	High chemical stability	Photocatalytic reactivity	Cytotoxicity (under UV light)	O/W	None	34
PS	Good stability	Chemically inert	Low toxicity	O/W	None	40–42
Microgels	Stability depends on crosslinking degree and environmental conditions	Reactive with crosslinking agents	Low toxicity	O/W or W/O	pH/thermal responsive	48–50
Zein	Degrades under high humidity	Reactive with crosslinking agents	Non-toxic and good biocompatibility	O/W	pH responsive	57
BSA-NH2/PNIPAAm	Good stability	Reactive with crosslinking agents	Good biocompatibility	O/W	Thermal responsive	61
Bacteria	Good stability	Reactive with positively charged groups	Pathogenic strains toxic	O/W	None	62, 63 and 73
Virus	Good stability	Reactive with crosslinking agents	Pathogenic (varies by type)	O/W	None	64

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3. Design principles of biomimetic microreactors

Cells have evolved a diverse array of organelles that are spatially segregated from one another, providing stable environments for simultaneous execution of various biochemical reactions involving incompatible substances.⁸⁰ This intricate organization allows for sophisticated resource allocation. Inspired by these cellular structures and functions, researchers attempt to develop biomimetic microreactors that facilitate biochemical reactions within confined spaces, thereby enhancing the efficiency and selectivity of catalytic processes. Pickering emulsions, which form membranous compartments through the adsorption of solid nanoparticles at the oil–water interface, serve as a powerful tool in this endeavour.¹⁵ With their versatility in choice of colloidal emulsifiers (discussed above), compartmentalized structure, selective permeability, and flexibility in enzyme immobilization, Pickering droplets offer a versatile platform for cellular biomimetics and biocatalysis. To provide a comprehensive understanding of these principles, this section is organized into three distinct subsections, each presenting an in-depth discussion supported by numerous examples.

3.1. Compartmentalized structures

Compartmentalized structures refer to a structural characteristic of biological cells that organizes and separates macromolecules, metabolites, and biochemical pathways through spatial isolation.⁸¹ Pickering emulsions, composed of numerous stable droplets, function as microscale compartments that effectively replicate these compartmentalized structures observed in biological systems. The first documented use of Pickering emulsions as compartmentalized systems was by Nagayama et al. in 1996.⁴⁰ The compartmentalized structures formed by Pickering emulsions offer several advantages.

First of all, Pickering emulsions provide well-defined microchambers that can be used to encapsulate catalytically active guests while allowing for the separation of incompatible components. For instance, Keating and colleagues demonstrated the encapsulation of enzymes within liposome-stabilized Pickering emulsions, which served as microscale bioreactors for ribozyme cleavage reactions using hammerhead ribozymes.⁸² Further advancements in compartmentalized structures have been achieved through the lamination of Pickering emulsions, as described by Yang et al.⁸³ In this approach, incompatible reagents were effectively separated and compartmentalized within the water droplets of the emulsion, preventing mutual destruction while emulating the compartmentalization found in living systems.

Furthermore, the increased oil–water interfacial area of Pickering emulsion-based compartmentalized structures facilitates mass transfer between the inner and outer phases. Yang et al. investigated enzymatic reactions occurring in W/O Pickering emulsions without stirring, where enzymes were compartmentalized in the aqueous phase while organic substrates were dissolved in the surrounding oil phase (Fig. 6a).⁸⁴ Their findings demonstrated that the Pickering emulsion system facilitated efficient enzymatic reactions through molecular self-diffusion, a phenomenon attributed to the reduced diffusion distances afforded by the larger interfacial area.^16,85,86

Fig. 6 Compartmentalized structures formed by Pickering emulsions. (a) Schematic illustration of the compartmentalized structure of Flow Pickering emulsion and interfacial biocatalysis. Reproduced with permission from ref. 84. Copyright 2016 American Chemical Society. (b) Scheme of the preparation of polymersome-stabilized Pickering emulsions. Reproduced with permission from ref. 87. Copyright 2012 Wiley. (c) Schematic illustration of biphasic biocatalytic Pickering emulsion droplets controlled by near infrared/visible light with reversible conversion capability. Reproduced with permission from ref. 88. Copyright 2014 American Chemical Society. (d) Schematic diagram of the formation processes of Pickering emulsions with magnetic response. Reproduced with permission from ref. 89. Copyright 2018 Royal Society of Chemistry.

Additionally, Pickering emulsions built with elaborately designed colloidal particles can gain precise spatiotemporal control over biochemical reactions. An early study by van Hest and colleagues highlighted the use of a Pickering emulsion system with distinct compartments to significantly enhance enzyme catalytic performance (Fig. 6b).⁸⁷ They achieved precise control over enzyme localization and specific activity by encapsulating them either in the aqueous phase or within the lumen of polymer vesicle stabilizers. Subsequent research has explored controlled biochemical transformations using Pickering emulsions with stimulus-responsive properties. For example, Chen et al. developed a light-controlled Pickering catalytic system by conjugating photochromic spiropyran with upconversion nanophosphors to form interfacially active nanoparticles (Fig. 6c).⁸⁸ These nanoparticles could switch between hydrophilic and hydrophobic states when exposed to near-infrared and visible light, respectively, facilitating the inversion of the emulsion and thereby allowing for remote control over the biphasic biocatalysis. In another innovative study, Song et al. proposed that natural magnetotactic bacteria, which contain specialized organelles called magnetosomes (intracellular structures made of magnetic nanoparticles), could function as miniature stirrers within Pickering emulsion droplets (Fig. 6d).⁸⁹ When subjected to an external magnetic field, these nanoscale stirring agents rotated within the water droplets, significantly accelerating mass transfer and increasing the reaction rate.

Overall, these Pickering emulsion-based compartmentalized structures open new avenues for the construction of biomimetic microreactors. Structurally, Pickering emulsions provide a confined space for active units, further isolating them from the surrounding environment. Concurrently, Pickering emulsions significantly increase the contact area between oil and water phases and reduce mass transfer resistance, substantially enhancing reaction efficiency.

3.2. Selective permeability

The selective permeability afforded by channel proteins in cellular membranes enables the selective exchange of substances between the cell and its external environment.⁹⁰ In an analogous manner, selective permeability in biomimetic microreactors allows for the preferential transport of specific molecules or molecules of specific sizes, resulting in more efficient substrate conversion and product separation in chemical reactions. The interstices between the particles assembled at the oil–water interface provide an array of pores, conferring Pickering emulsions or Pickering emulsion-templated colloidosomes with inherent semi-permeability. Notably, the size of particle-stacked pores can be easily adjusted by tuning the diameters of particles used. For instance, Saunders et al. prepared colloidosomes using pH-responsive microgels as building blocks.⁹¹ By adjusting the pH, they were able to modulate the diameter of these microgels, which in turn influenced the permeation behavior of FITC-labeled dextran polymers of different molecular weights.

Furthermore, modifying the interstices between particles with stimuli-responsive components provides further access to controlling the permeability of Pickering emulsion-templated microreactors. For instance, Mann et al. developed colloidosomes featuring electrostatically gated membrane permeability by crosslinking SiO₂ nanoparticles at the interface of Pickering emulsions with pH-responsive copolymers.⁹² The pH-dependent variations in the charge of the polymer corona facilitated the selective uptake and release of small molecules with opposite charges.

Additionally, the permeability of Pickering emulsions can also be fine-tuned by using building blocks with hierarchical porous structures,⁹³ such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) (Fig. 7), which allows for precise adjustments to the permeability of the resulting biomimetic microreactors by modifying the pore structure and size at the molecular level. Bradshaw et al. developed a microreactor using UiO-66 and Fe₃O₄ nanoparticle co-stabilized W/O Pickering emulsions, incorporating an enzyme-loaded agarose hydrogel in the inner cavity.⁹⁴ The resulting Pickering hydrogel droplets were then coated with a hierarchically structured ZIF-8 shell, imparting size selectivity for enzyme substrates. Similar strategies were employed by Yang, Wang, and others to construct microreactors exhibiting size-selective permeability and enhanced mechanical robustness.^95–98

Fig. 7 Preparation and selective permeability of a COF cross-linked porous microreactor based on Pickering emulsion. Reproduced with permission from ref. 96. Copyright 2023 Wiley.

The selective permeability of Pickering emulsion biomimetic microreactors is crucial for optimizing catalytic performance and enhancing the efficiency and selectivity of chemical reactions. By precisely tuning the permeability, it is possible to enhance the conversion of targeted substrates while enabling effective separation and purification of products. More importantly, the flexible selection of building blocks offers a powerful approach to engineering the selective permeability of the configured microreactors, which allows for selective permeability under specific environmental conditions, allowing them to 'open' or 'close' their pores in response to specific environmental stimuli. This capability enables precise control over the entry and exit of substrates and products.

3.3. Directed anchoring of biocatalytic units

Integrating catalytic functions into Pickering emulsions through biocatalyst (enzyme) entrapment is an attractive approach for fabricating biomimetic microreactors.⁹⁹ However, critical issues such as enzyme denaturation, inactivation, and complicated separation must be addressed. The mild aqueous environment provided by Pickering emulsions creates a favorable niche for maintaining the stability and activity of biocatalytic units.¹⁰⁰ Directional anchoring of biocatalytic units allows for precise positioning of the enzyme within the Pickering emulsions, enhancing reaction kinetics and selectivity. Depending on the anchoring position, Pickering emulsions can facilitate mainly two types of biocatalytic reactions: Pickering-assisted biocatalysis (PAB) and Pickering interfacial biocatalysis (PIB) (Fig. 8).¹⁹

Fig. 8 Schematic illustration of the Pickering-assisted biocatalysis (a) and Pickering interfacial biocatalysis (b).

In the PAB reaction, biocatalysts are directly dispersed in the aqueous phase, receiving maximum protection from the organic environment by a shell of colloidal particles at the oil–water interface. For example, Wang et al. utilized hydrophobic SiO₂ nanoparticles to create a W/O Pickering emulsion system, in which CalB was immobilized within the emulsion droplets.¹⁰¹ This approach significantly increased the interfacial area, enhancing mass transfer and improving enzyme accessibility to substrates. However, the recyclability of the immobilized enzymes within the Pickering emulsions over multiple cycles remains underexplored. Subsequently, Ulijn et al. immobilized enzymes within peptide gel microparticles formed by the co-assembly of aromatic peptide amphiphiles (e.g., Fmoc-FF) and Fmoc-amino acids with various functional groups.¹⁰² Their results indicated that CalB immobilized within the Fmoc-FF/S and Fmoc-FF/D peptide particles exhibited good retention of activity over multiple cycles. This peptide network mimicked the enzyme's native crowded microenvironments, providing a biocompatible environment that helped maintain the structural integrity and activity of the enzyme during storage and reuse. However, some potential drawbacks remain. For instance, hydrophobic substrates must overcome internal diffusion resistance before reaching the enzymes. Additionally, while the Pickering emulsion system facilitates enzyme recovery through centrifugation, concerns about the long-term stability of the emulsion and enzyme activity over multiple cycles persist.

In contrast, PIB involves the direct immobilization of enzymes onto suitable carriers, anchoring them at the droplet interface.¹⁹ This configuration enhances the reaction rate by improving contact between enzymes and substrates while preventing enzyme loss and inactivation.¹⁰³ In this context, selecting appropriate immobilization carriers and methods is crucial for maintaining enzyme catalytic activity, enhancing reusability, and stabilizing the emulsion.¹⁹ Considering the properties of enzymes and the nature of their carriers, several immobilization methods have been extensively explored, including physical adsorption, embedding, and covalent coupling.

Physical adsorption is a relatively economical and straightforward approach for binding enzymes to carriers, primarily achieved through various interactions, such as hydrogen bonds, ionic bonds, salt bridges, and hydrophobic interactions.¹⁰⁴ For instance, particles with nanopores, like mesoporous silica¹⁰⁵ (Fig. 9a) and carbon nanoparticles,¹⁰⁶ provide ample space for immobilization of enzymes via physical absorption. Pickering emulsions stabilized by these composite nanoparticles have a large interfacial area and reduced mass transfer resistance, which significantly improves the efficiency of the enzymatic reaction. In addition, after the enzyme is adsorbed and immobilized on the carrier, it can directly contact the substrate in the external phase from the interface, which further improves the efficiency of the enzyme. However, the inherent weak binding strength of this method can make enzymes susceptible to inactivation and leakage under varying external conditions.

Fig. 9 Directed anchoring of enzyme. (a) Schematic illustration of the strategy for adsorbing enzymes in FDU-12 nanocages and performing Pickering interfacial catalysis. Reproduced with permission from ref. 105. Copyright 2013 Royal Society of Chemistry. (b) Schematic illustration of the strategy for encapsulating enzymes within ZIF-8 and conducting Pickering interfacial catalysis. Reproduced with permission from ref. 107. Copyright 2019 American Chemical Society. (c) Scheme of an interfacial biocatalytic cascade (GOx/CALB) constructed by spatial localization of different enzymes via covalent coupling at the water droplet interface. Reproduced with permission from ref. 108. Copyright 2023 Wiley.

Embedding is a process of confining enzymes within porous carriers such as polymeric grid structures, porous organic or metal–organic frameworks. For example, Lu et al. synthesized a lipase-loaded zeolitic imidazolate framework-8 (ZIF-8) and utilized this enzyme@ZIF-8 composite to create a Pickering emulsion microreactor (Fig. 9b).¹⁰⁷ The protective characteristics of ZIF-8 enhance enzyme stability in organic media, allowing substrates and products to permeate while shielding the enzymes from the harmful effects of organic solvents, thereby prolonging their operational lifespan. However, while embedding provides protection from external influences, it may also lead to enzyme aggregation, which can reduce reaction rates.

Covalent coupling is a widely employed method for enzyme immobilization, enhancing enzyme stability by establishing robust chemical bonds with the support. In a study by Wu et al., benzaldehyde lyase was utilized as a model enzyme and an enzyme–polymer conjugate was synthesized by grafting PNIPAAm via atom transfer radical polymerization.¹⁰⁹ This approach not only preserved the structural integrity and stability of the enzyme but also significantly improved its catalytic performance at the oil–water interface. In another study, Yang et al. used Janus mesoporous SiO₂ nanosheets (JMSNs) with asymmetric surface modifications to selectively immobilize two different enzymes on their hydrophilic and hydrophobic surfaces, respectively (Fig. 9c).¹⁰⁸ When these enzyme-immobilized JMSNs were used to create Pickering emulsions, the two different enzymes were precisely positioned in the inner and outer interfaces of the droplets, allowing for efficient cascade reactions by maintaining the enzymes in their optimal reaction media while ensuring close proximity for effective intermediate transport.

Directed anchoring of biocatalytic units can protect them from harmful factors in the reaction environment. Furthermore, in multi-enzyme catalytic systems, this approach facilitates the orderly arrangement and synergy of different enzymes, thereby improving the efficiency of multi-step reactions. It is important to conduct a thorough evaluation of enzyme characteristics, substrate properties, and the types of carriers used to select suitable enzyme immobilization methods for more efficient biocatalytic processes.

3.4. Holistic emulsion design

Unlike conventional catalytic platforms, Pickering emulsions exhibit exceptional adaptability in accommodating reactants with diverse solubilities (aqueous, lipophilic, or gaseous) by tailoring their phase-segregated architectures.²⁵ This includes the formation of oil-in-water droplets for reactions involving water- and oil-soluble substrates, as well as gas-stabilized interfaces for reactions that engage gas and water-soluble substrates. The effectiveness of multiphase catalytic reactions facilitated by Pickering emulsions, such as oil–water, oil-ionic liquids (ILs), water–gas, or triphasic systems, is heavily reliant on the interfacial engineering of colloidal particle stabilizers, the strategic spatial arrangement of catalytic sites across different phases, and the selection of appropriate solvents.

In oil–water systems, solid particles must exhibit amphiphilic characteristics to effectively adsorb at the oil–water interface. The surface chemistry of these particles, including the arrangement of hydrophilic and hydrophobic groups, is crucial for their ability to stabilize emulsions and modulate catalytic performance. Particles can be engineered to incorporate catalytic centres either within their matrix or on their surface, allowing them to function as both stabilizers and catalysts. For instance, Kim et al. demonstrated the use of amphiphilic nanoplatelets with hydrophobic and hydrophilic brushes for stabilizing oil–water interfaces, with metal nanoparticles (e.g., Ag, Pd, Au) positioned on the hydrophilic face to catalyze reactions.¹¹⁰ This configuration facilitates efficient interactions between hydrophobic reactants in the oil phase and catalysts at the interface, exemplified by the rapid reduction of 4-nitrophenol in a Pickering emulsion system.

In water–gas systems, the primary challenge is stabilizing gas bubbles within a liquid phase. Here, particles must effectively adsorb at the gas–liquid interface to create stable foam-like structures, thereby enabling efficient triphasic (gas–liquid–solid) catalysis. The design of such particles often entails optimizing surface properties to ensure strong adsorption at the gas–liquid interface while maintaining resistance to coalescence and disproportionation. For example, Huang and Yang reported the surface modification of SiO₂ with hydrophobic octyl silanes and hydrophilic triamine silanes, which allows particles to assemble at the gas–liquid interface and stabilize micrometer-sized gas bubbles.¹¹¹ The catalytic activity in these systems can be enhanced by loading catalytic nanoparticles (e.g., Pd, Au) onto these particles, allowing reactions such as hydrogenation and oxidation to occur at the gas–liquid interface.

ILs are salts that exist in a liquid state at or near room temperature, composed of cations and anions.¹¹² They exhibit unique properties such as low volatility, high thermal stability, and the ability to dissolve a wide range of organic compounds,^113,114 making them attractive solvents for building Pickering emulsion-based microreactors with high stability and long-term activity. For instance, Yang and Zhang have reported a series of dimethyldichlorosilane-modified SiO₂ nanoparticle stabilized IL-in-octane Pickering emulsions using various ionic liquids.⁸⁴ The favourable solvation properties of ILs for organic substrates and their compatibility with enzymes boosted the conversion efficiency. Furthermore, the resulting IL droplets provided excellent operational stability, enabling 4000-hour continuous operation, with minimized batch-to-batch variability. Their work underscores the importance of selecting appropriate solvents to improve the catalytic efficiency, reduce product inhibition, and enhance scalability for Pickering emulsions.

The design of Pickering emulsions for multiphase catalytic systems requires a holistic approach that considers the interfacial properties of the particles, the nature of the immiscible phases, and the specific catalytic roles of each phase. By meticulously optimizing these parameters, it is possible to create highly efficient and stable Pickering emulsion catalytic systems applicable to a wide range of complex reactions.

4. Biomimetic catalysis of Pickering emulsion-based biomimetic microreactors

Cellular functions are governed by a series of regulated and confined biochemical reactions and metabolic pathways that operate in concert. Learning from nature, scientists across diverse disciplines, such as biology, chemistry, and materials science, have made significant strides in understanding intracellular processes and simulating these systems with synthetic materials to recreate innovative biomimetic designs.^26,115–118 Recent advancements include the development of biomimetic catalytic platforms utilizing Pickering emulsions to replicate cellular compartmentalization.^119,120 These platforms can be classified into three primary strategies: (1) biomimetic metabolism strategy for the simulation of metabolic reactions and metabolic regulatory mechanisms; (2) biomimetic cascade catalysis strategy for the simulation of multi-enzyme cascades; (3) biomimetic strategies for extending to industrial applications. Each section includes a brief introduction of the biological origin, design of the reaction system as well as development of the strategy.

4.1. Biomimetic metabolism

Cell metabolism involves a series of complex and interrelated biochemical reactions that maintain normal physiological functions, including growth, reproduction, and homeostasis.¹¹⁶ At the core of these cellular processes is the conversion of enzyme-specific substrates or other energy-rich molecules into smaller substances and energy (catabolism).¹²¹ A common approach to mimic this process is through the encapsulation of enzymes or biocatalysts within the interior of Pickering emulsion-based compartments, allowing for selective control over substrate penetration. Various enzymes have been utilized in this context, including lipases,^101,105,122 chymotrypsin,¹²² alkaline phosphatase (ALP),^61,65,92 glucose amylase (GA),⁶⁶ glucose oxidase (GOx),⁹³ horseradish peroxidase (HRP),¹²³ and benzaldehyde lyase (BAL).^101,109

Mann et al. reported the pioneering construction of a primitive bio-inorganic protocell model utilizing Pickering emulsions stabilized by SiO₂ nanoparticles.¹²² This model demonstrated the capability to support a range of functionally bioactive molecules involved in metabolism and informational processing. They initially focused on cell-free in vitro gene expression of enhanced green fluorescent protein and found that the expression rate within the Pickering emulsion interior was comparable to that observed in bulk aqueous solutions. Subsequently, they encapsulated lipoprotein lipases, chymotrypsin, and ALP within the inner aqueous phase of the Pickering emulsions to facilitate hydrolysis and enzymatic dephosphorylation reactions. Their findings revealed a significant enhancement in the specific activity of these enzymes. Designed as biomimetic microreactors, Pickering emulsions not only provided a stable environment for the enzymes but also enabled material exchange with the external environment, which is essential for sustaining metabolic processes. Despite these contributions, the study lacks information on regulatory mechanisms—commonly found in natural biological systems—that would allow for adjustments in the reaction processes in response to changes in both internal and external environments.

Natural cells are remarkably complex bioreactors capable of performing intricate metabolic reactions in response to environmental stimuli, such as light, temperature, and pH.^66,123–125 Drawing inspiration from this, Huang et al. presented a sophisticated approach that integrated a photo-switchable spiropyran unit and thermos-responsive PNIPAAm into the membranes of proteinosomes (Fig. 10a and b).¹²⁶ These proteinosomes could modulate the release of hydrophilic FITC-dextran and hydrophobic Nile red, facilitating interface reactions between enzymes and substrates, including lipase with fluorescein diacetate and alkaline phosphatase with fluorescein diphosphate. The regulation of these processes occurred through exposure to various conditions, including temperature changes, ultraviolet light, and variations in environmental redox balances, effectively mimicking the dynamic nature of biological membranes in regulating the transport of molecules in and out of cells. Additionally, Chen et al. reported on using light to drive proton pumping and ATP synthesis by integrating bacteriorhodopsin with plasmonic colloidal capsules (Fig. 10c and d).¹²⁷ This setup converted solar energy into electrochemical gradients, enabling the enzymatic synthesis of energy-storage molecules like ATP. Utilizing light as a stimulus to control biochemical reactions offers several advantages, including high selectivity, spatiotemporal resolution, remote control, and the ability to trigger photosynthesis-mimicking reactions.¹²⁸

Fig. 10 Examples of Pickering emulsion-based biomimetic microreactors with stimuli response for bio-inspired catalytic transformation. (a) and (b) Schematic representation of a multifunctional microcapsule based on proteinosomes and regulated co-release of hydrophilic ALP-FITC and hydrophobic Nile red under UV light. Reproduced with permission from ref. 126. Copyright 2018 American Chemical Society. (c) Scheme of the formation process of synthetic protocells based on plasma colloidal capsules coated with a purple membrane. These synthetic protocells are able to pump protons upon light irradiation resulting in photoinduced pH change (inset). (d) The strategy for pumping protons to trigger ATP biosynthesis. Different reaction conditions: (i) AuAgNR SPCs + proteoliposomes + light, (ii) AuAgNR SPCs + proteoliposomes in the dark, (iii) proteoliposomes + light, (iv) AuNP SPCs + proteoliposomes + light, and (v) SiO₂NP SPCs + proteoliposomes + light. Reproduced with permission from ref. 127. Copyright 2019 Wiley.

In addition to exogenous stimuli, Gobbo et al. introduced a novel approach utilizing endogenous pH regulation.¹²⁹ They first synthesized a polyethylene glycol-based NHS ester-activated ketal crosslinker that remained stable at pH values greater than 7.5 but rapidly hydrolyzed at pH values below 6.0. The controlled degradation of carboxymethyl-chitosan hydrogels and proteinosome membranes was mediated by encapsulated GOx. Once exposed to glucose, GOx catalyzed the generation of gluconic acid, which lowered the pH within the hydrogel and activated the acid-degrading ketone crosslinkers. The hydrolysis of the crosslinker resulted in the disassembly of the hydrogel, demonstrating a controlled and programmable degradation process triggered by enzyme activity.

These studies have successfully developed Pickering emulsion-based artificial systems that can simulate and modulate metabolic processes with responsiveness and adaptability akin to those in living cells. However, cellular metabolic pathways typically involve multiple enzymes working in concert to progressively convert a substrate into an end product, a complexity that was not explored in depth in these works. For more comprehensive surveys on dynamically controlled catalysis in Pickering emulsion systems under external stimulus (e.g., light, pH, thermal, magnetism and CO₂/N₂), interested readers can refer to reviews by Xu, Wu, Qiu, and others.^15,130–132

4.2. Biomimetic cascade catalysis

Biological cells enable various biocatalytic transformations to occur efficiently through multi-enzyme cascade networks, providing a valuable model for the development of cascade catalytic microreactors. Huang et al. first prepared proteinosomes consisting of three enzyme-PNIPAAm building blocks and investigated the impact of enzyme positioning within the membrane on the cascade by co-localizing and spatially separating three enzymes (GA, GOx, and HRP).¹³³ The efficiency of the cascade reaction was highest when all three enzymes were situated in the membrane, indicating that colocalization reduced diffusion limitations for the intermediates. Moreover, the temperature responsiveness of PNIPAAm mimics how temperature regulates enzyme activity and membrane permeability in biological systems, while the co-localization and spatial separation of enzymes in the colloidosome shell mimic the localization and function of intracellular enzymes on the membrane.

Later on, building upon the concept of compartmentalization, He et al. focused on the cascade cycling of nicotinamide cofactor (NAD⁺/NADH) in a dual enzyme microsystem.¹³⁴ The researchers encapsulated alcohol dehydrogenase (ADH) and GOx within colloidosomes constructed by peroxidase-like tourmaline microparticle. The ADH/GOx@TM microsystem was capable of reducing NAD⁺ to NADH in the presence of ethanol and oxidizing NADH back to NAD⁺ with the assistance of H₂O₂ generated by GOx. This study highlighted the importance of spatial confinement in regulating enzyme activities and cascades.

To advance the concept of enzyme and cofactor co-compartmentalization for catalytic applications, Yang and colleagues utilized the unique interfacial adsorption and confinement effects of Pickering emulsion droplets to co-compartmentalize enzymes and cofactors, enabling the regeneration of cofactors within the droplets (Fig. 11).⁸⁵ The system was demonstrated through enzyme-catalyzed ketone enantioselective reduction and enantioselective transamination, showing long-term stability, outstanding total turnover numbers, and enhanced catalytic efficiency compared to conventional biphasic reactions. This work represents a significant step towards the practical applications of biomimetic catalysis, offering a more efficient and sustainable approach to chemical synthesis. Recently, they further developed a biomimetic microreactor where different catalytically active sub-compartments were co-localized within the interior of Pickering emulsion droplets, creating cell-like microreactors (Fig. 12).¹⁸ These biomimetic microreactors were packed into a column reactor for continuous-flow cascade catalysis. The study demonstrated significant enhancement in catalysis efficiency and enantioselectivity for the synthesis of chiral cyanohydrins and chiral esters, highlighting the importance of compartmentalization and reactant enrichment in boosting catalysis efficiency.

Fig. 11 Schematic illustration of the co-compartmentalization design principle of enzymes and cofactors inside Pickering emulsion droplets. (a) Representation of the co-localization strategy for enzymes and cofactors encapsulated within Pickering emulsion droplets. 2.5D confocal fluorescence microscopy images showing the location of Rhodamine B-labelled AKR (b) and NADPH (c) at the droplet interface and within the emulsion droplet, respectively. Reproduced with permission from ref. 85. Copyright 2022 Wiley.

Fig. 12 Schematic illustration of the construction of Pickering emulsion-based biomimetic microreactors for cascade reactions. (a) and (b) Biological prototype and the construction process of a biomimetic microreactor based on Pickering emulsion droplets. (c) Pickering emulsion-based biomimetic microreactors for continuous flow cascade reactions. Reproduced with permission from ref. 18. Copyright 2022 Springer Nature.

Very recently, inspired by the architectures and cascade enzyme-involved detoxification pathways of bacterial microcompartments, Xu et al. advanced cascade enzyme catalysis by immobilizing sugar-related enzymes and chloroperoxidase in hydrogen-bonded organic frameworks (HOFs).¹³⁵ They utilized these HOFs as building blocks to construct a series of Pickering emulsion droplets for the detoxification of a mustard gas simulant (2-chloroethyl ethyl sulfide, CEES). In this cascade enzyme system, sugar was converted by sugar-related enzymes into hydrogen peroxide, which was then used by chloroperoxidase to catalyze the oxidation of CEES into a non-toxic sulfoxide. The advantage of this catalytic system was its mild, multi-enzyme-based approach, which avoided harsh chemicals and toxic byproducts associated with traditional decontamination methods. Additionally, the favorable mass transfer of substrates to the enzymes at the droplet interface, combined with facilitated substrate channeling between the proximally immobilized enzymes, significantly enhanced the decontamination efficiency. Furthermore, the modularity and adaptability of this strategy open new possibilities for developing customized detoxification solutions tailored to various chemical threats and operational conditions.

4.3. Scalability in industrial applications

Current laboratory studies have demonstrated the high efficiency and stability of Pickering emulsion-based biomimetic microreactors in a variety of reactions. However, scaling these biomimetic microreactors from laboratory settings to industrial applications presents several challenges, including high energy consumption for large-scale production, maintaining emulsion stability under continuous operations, and ensuring consistent performance over time.^15,136 Researchers are actively exploring solutions such as optimizing the emulsification process, developing new materials, and implementing crosslinking technologies to address these issues.

As a pivotal step toward industrial catalytic applications, continuous flow catalysis has emerged as a promising prototype.¹³⁷ Its core advantages include enhanced interfacial mass transfer, continuous operation, environmental friendliness, and adaptability to diverse scenarios, all of which contribute to efficient, stable, and green industrial catalysis. For instance, Yang's group developed a Pickering emulsion droplet-integrated electrode system for continuous-flow electrosynthesis of oximes, achieving stable operation for over 100 hours with high Faraday efficiency (83.8%) and producing 10 grams of cyclohexanone oxime, showcasing the potential for industrial scalability.¹³⁸

Concurrently, emulsion stability challenges are being addressed through enhancing emulsification processes and selecting suitable stabilizers, which can improve flow uniformity and reduce clogging. High-pressure homogenization parameterization (pressure, shear rate tuning) has proven effective in maintaining droplet uniformity, as evidenced by Chevalier and Bolzinger's work showing >95% stability retention in flow regimes.¹³⁹

In addition, material innovations, like the use of composite nanoparticles, have demonstrated the potential to stabilize emulsions. For example, Thesium chinense Turcz. crude polysaccharide (TTP) particles have been reported by Wu et al. to effectively stabilize O/W Pickering emulsions.¹⁴⁰ The resulting emulsions remained stable over a broad pH range of 1–11, ionic strengths from 0 to 500 mM, and temperatures from 4 to 55 °C. This stability was attributed to the reduced interfacial tension and increased viscosity resulting from the network formed by the soluble components of TTP, along with the anti-coalescence bridging structure created by the insoluble components between droplets. This dual-action mechanism not only enhanced the robustness of the Pickering emulsions but also rendered them suitable for industrial applications by effectively addressing key instability factors, such as droplet coalescence and phase separation.

For long-term catalytic durability, interface rigidification strategies like colloidosome formation via chemical crosslinking are gaining traction.¹⁶ This approach solidifies the emulsion droplet interface, preventing coalescence and breakage under high shear, elevated temperatures, or prolonged operational conditions.¹⁴¹ The rigid shell not only strengthens the mechanical stability of the emulsion but also preserves the dispersion and activity of interfacial catalysts, ensuring consistent reaction efficiency.

These strategies enhance the durability and broaden the applicability of Pickering emulsions in demanding industrial processes. However, further technical advancements are required to fully realize the potential of Pickering emulsion-based microreactors in industrial settings.

In summary, these studies highlight the promising potential of using Pickering emulsion-based microreactors for biomimetic catalysis. Advances in materials science have facilitated the development of more efficient, stable, and versatile Pickering emulsion-based biomimetic microreactors. Furthermore, the evolution of these biomimetic strategies toward more complex and stable systems, such as the packing of Pickering emulsion droplets into continuous-flow apparatuses, addresses the challenges of scaling up from laboratory to industrial settings.^97,99,142 Importantly, the shift from merely replicating to actively applying biomimetic strategies in catalysis and synthesis enables researchers to harness the inherent advantages of cellular processes. This approach helps overcome traditional limitations in chemical engineering, leading to more efficient, selective, stable, and scalable methods for complex chemical synthesis.

5. Conclusions and perspectives

In this review, we have summarized the current advances in the design and construction of Pickering emulsion-based biomimetic microreactors. Our focus has been particularly on the classification of colloidal particle stabilizers, including inorganic nanoparticles, polymer-based nanoparticles, and biological nanoparticles, as well as the design principles underlying these microreactors and their applications in biomimetic catalysis. Given the diversity of nanoparticles as emulsifiers and catalysts, along with the flexibility offered by nanotechnology, Pickering emulsion-based biomimetic microreactors are poised to adapt to a variety of environments and demands.

Despite significant progress, several challenges remain to be addressed in order to achieve precise regulation of the emulsion interface and to construct biomimetic microreactors with integrated, tailor-made catalytic functionalities. First, the stability of Pickering emulsions over extended periods, particularly under extreme or unconventional conditions, is a concern. While these emulsions are generally stable against coalescence, they can be sensitive to fluctuations in pH and high salt concentrations, which may disrupt the electrostatic interactions among particle-stabilized droplets, leading to aggregation or desorption of solid nanoparticles. Second, when solid nanoparticles function as both emulsifiers and catalysts, the exposure and activity of active sites may be limited, potentially hindering catalytic efficiency. Moreover, designing multifunctional catalysts capable of performing multiple catalytic steps within a single compartment of biomimetic microreactors presents a considerable challenge, as it necessitates a high degree of activity, selectivity, and stability. Lastly, the spatial organization and compartmentalization of different catalytic components within the Pickering emulsion biomimetic microreactor are crucial for achieving efficient cascade reactions. It is essential to ensure that each catalyst is optimally positioned and that reactants flow through the microreactor in a controlled manner to maximize overall reaction efficiency.

In summary, the field of Pickering emulsion microreactors holds immense potential for advancing sophisticated synthesis and biomimetic catalysis. By addressing existing challenges and exploring innovative solutions, researchers can enhance the performance, stability, and applicability of these systems, paving the way for their successful implementation in both laboratory research and industrial applications.

Author contributions

X. X.: conceptualization, writing – original draft. M. Z. and T.W.: writing – review & editing. Z. C. and H. Y.: conceptualization, funding acquisition, supervision, validation, writing – review & editing.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (22277011, Z.C.; 22421002, H. Y.; 22027805, H.Y.; 22334004, H.Y.), the National Key Research and Development Program of China (2020YFA0210800, H. Y.), and the Major Project of Science and Technology of Fujian Province (2020HZ06006, H.Y.).

Notes and references

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