Dynamically Reconfigurable,Multifunctional Emulsions with Controllable Structure and Movement

Dynamically reconfigurable oil‐in‐water (o/w) Pickering emulsions are developed, wherein the assembly of particles (i.e., platinum‐on‐carbon and iron‐on‐carbon particles) can be actively controlled by adjusting interfacial tensions. A balanced adsorption of particles and surfactants at the o/w interface allows for the creation of inhomogeneity of the particle distribution on the emulsion surface. Complex Pickering emulsions with highly controllable and reconfigurable morphologies are produced in a single step by exploiting the temperature‐sensitive miscibility of hydrocarbon and fluorocarbon liquids. Dynamic adsorption/desorption of (polymer) surfactants afford both shape and configuration transitions of multiple Pickering emulsions and encapsulated core/shell structured can be transformed into a Janus configuration. Finally, to demonstrate the intrinsic catalytic or magnetic properties of the particles provided by carbon bound Pt and Fe nanoparticles, two different systems are investigated. Specifically, the creation of a bimetallic microcapsule with controlled payload release and precise modulation of translational and rotational motions of magnetic emulsions are demonstrated, suggesting potential applications for sensing and smart payload delivery.     

Water-in-Water Emulsions Stabilized by Silica Janus Nanosheets

Water-in-water (w/w) emulsions have been recognized for their broad applications in foods, cosmetics, and biomedical engineering. In this work, silica Janus nanosheets (JNs) with polyacrylic acid (PAA) chains grafted on one surface via crushing functional silica foams, and used silica JNs as Pickering stabilizer to produce stable water-in-water (w/w) emulsions from the aqueous two-phase system (ATPS) containing methacrylic acid (MAA) and NaCl are prepared. The interfacial area of w/w emulsions increases linearly with the concentration of silica JNs, and the interfacial coverage of nanosheets is calculated to be about 98%. After polymerizing w/w emulsions prepared from MAA/NaCl ATPS, it is found that silica JNs are entrapped at the interface of w/w emulsions with the smooth PAA-grafted surface located toward MAA-rich phase due to their specific interaction. These results show that functional silica JNs can be used as a promising amphiphilic Pickering stabilizer to produce well-defined w/w emulsions for numerous application fields.     

Microfluidic Generation of All‐Aqueous Double and Triple Emulsions

Higher order emulsions are used in a variety of different applications in biomedicine, biological studies, cosmetics, and the food industry. Conventional droplet generation platforms for making higher order emulsions use organic solvents as the continuous phase, which is not biocompatible and as a result, further washing steps are required to remove the toxic continuous phase. Recently, droplet generation based on aqueous two‐phase systems (ATPS) has emerged in the field of droplet microfluidics due to their intrinsic biocompatibility. Here, a platform to generate all‐aqueous double and triple emulsions by introducing pressure‐driven flows inside a microfluidic hybrid device is presented. This system uses a conventional microfluidic flow‐focusing geometry coupled with a coaxial microneedle and a glass capillary embedded in flow‐focusing junctions. The configuration of the hybrid device enables the focusing of two coaxial two‐phase streams, which helps to avoid commonly observed channel‐wetting problems. It is shown that this approach achieves the fabrication of higher‐order emulsions in a poly(dimethylsiloxane)‐based microfluidic device, and controls the structure of the all‐aqueous emulsions. This hybrid microfluidic approach allows for facile higher‐order biocompatible emulsion formation, and it is anticipated that this platform will find utility for generating biocompatible materials for various biotechnological applications.     

Current Trends in Pickering Emulsions: Particle Morphology and Applications

In recent years, Pickering emulsions and their applications have attracted a great deal of attention due to their special features, which include easy preparation and enhanced stability. In contrast to classical emulsions, in Pickering emulsions, solid microparticles or nanoparticles that localize at the interface between liquids are used as stabilizers, instead of surfactants, to enhance the droplet lifetime. Furthermore, Pickering emulsions show higher stability, lower toxicity, and stimuli-responsiveness, compared with emulsions that are stabilized by surfactants. Therefore, they can be considered attractive components for various uses, such as photocatalysis and the preparation of new materials. Moreover, the nanoparticle morphology strongly influences Pickering emulsion stability as well as the potential utilization of such emulsions. Here, we review recent findings concerning Pickering emulsions, with a particular focus on how the nanoparticles morphology (i.e., cube, ellipsoid, nanosheet, sphere, cylinder, rod, peanut) influences the type and stability of such emulsions, and their current applications in different fields such as antibacterial activity, protein recognition, catalysis, photocatalysis, and water purification.     

Control of Particle Adsorption for Stability of Pickering Emulsions in Microfluidics

Studying the stability of Pickering emulsion is of great interest for applications including catalysis, oil recovery, and cosmetics. Conventional methods emphasize the overall behavior of bulk emulsions and neglect the influence of particle adsorbing dynamics, leading to discrepancies in predicting the shelf‐life of Pickering emulsion–based products. By employing a microfluidic method, the particle adsorption is controlled and the stability of the Pickering emulsions is consequently examined. This approach enables us to elucidate the relationship between the particle adsorption dynamics and the stability of Pickering emulsions on droplet‐level quantitatively. Using oil/water emulsions stabilized by polystyrene nanoparticles as an example, the diffusion‐limited particle adsorption is demonstrated and investigated the stability criteria with respect to particle size, particle concentration, surface chemistry, and ionic strength. This approach offers important insights for application involving Pickering emulsions and provides guidelines to formulate and quantify the Pickering emulsion–based products.     

Nanoparticle cages for enzyme catalysis in organic media

Encapsulation of enzymes in Pickering emulsions results in a large interfacial area of the enzyme-containing aqueous phase for biocatalysis in organic media. This immobilization technique minimizes enzyme inactivation through stabilizing immiscible liquids by particles, facilitates separation processes, and significantly increases catalytic performance of both stable and vulnerable enzymes. Thus, a broad technical applicability can be envisioned.     

Nanocellulose-stabilized Pickering emulsions and their applications

Abstract

Pickering emulsion, which is an emulsion stabilized by solid particles, offers a wide range of potential applications because it generally provides a more stable system than surfactant-stabilized emulsion. Among various solid stabilizers, nanocellulose may open up new opportunities for future Pickering emulsions owing to its unique nanosizes, amphiphilicity, and other favorable properties (e.g. chemical stability, biodegradability, biocompatibility, and renewability). In this review, the preparation and properties of nanocellulose-stabilized Pickering emulsions are summarized. We also provide future perspectives on their applications, such as drug delivery, food, and composite materials.     

Confined Synthesis of Carbon Nitride in a Layered Host Matrix with Unprecedented Solid‐State Quantum Yield and Stability

Fluorescent carbon nanomaterials have drawn tremendous attention for their intriguing optical performances, but their employment in solid‐state luminescent devices is rather limited as a result of aggregation‐induced photoluminescence quenching. Herein, ultrathin carbon nitride (CN) is synthesized within the 2D confined region of layered double hydroxide (LDH) via triggering the interlayer condensation reaction of citric acid and urea. The resulting CN/LDH phosphor emits strong cyan light under UV‐light irradiation with an absolute solid‐state quantum yield (SSQY) of 95.9 ± 2.2%, which is, to the best of our knowledge, the highest value of carbon‐based fluorescent materials ever reported. Furthermore, it exhibits a strong luminescence stability toward temperature, environmental pH, and photocorrosion. Both experimental studies and theoretical calculations reveal that the host–guest interactions between the rigid LDH matrix and interlayer carbon nitride give the predominant contribution to the unprecedented SSQY and stability. In addition, prospective applications of the CN/LDH material are demonstrated in both white light‐emitting diodes and upconversion fluorescence imaging of cancer cells.     

Creation of Nonspherical Microparticles through Osmosis‐Driven Arrested Coalescence of Microfluidic Emulsions

Droplet‐based microfluidics enable the production of emulsions and microparticles with spherical shapes, but the high‐throughput fabrication of nonspherical emulsions and microparticles still remains challenging because interfacial tension plays a dominant role during preparation. Herein, ionic liquids (ILs) containing salts, which possess sufficient osmotic pressure to realize water transport and phase separation, are introduced as inner cores of oil‐in‐oil‐in‐water double emulsions and it is shown that nonspherical emulsions can be constructed by osmosis‐driven arrested coalescence of inner cores. Subsequently, ultraviolet polymerization of the nonspherical emulsions leads to nonspherical microparticles. By tailoring the number, composition, and size of inner cores as well as coalescence time, a variety of nonspherical shapes such as dumbbell, rod, spindle, snowman, tumbler, three‐pointed star, triangle, and scalene triangle are created. Importantly, benefitting from excellent solvency of ILs, this system can serve as a general platform to produce nonspherical microparticles made from different materials. Moreover, by controlling the osmotic pressure, programmed coalescence of inner cores in double emulsions is realizable, which indicates the potential to build microreactors. Thus, a simple and high‐throughput strategy to create nonspherical microparticles with arrested coalescence shapes is developed for the first time and can be further used to construct novel materials and microreactors.     

Drying oil-in-water Pickering emulsions to make redispersible powders

We report on the formation of powder by drying oil-in-water emulsions stabilized by silanised silica nanoparticles. Drying was achieved by spraying fine droplets (up to a hundred micrometres in diameter) of the emulsions into a chamber of hot, flowing air. We show that the surfaces of droplets of the emulsions become enriched with nanoparticles as the water evaporates in the drying chamber. Controlling the relative amounts of oil and particles in the droplets being dried is the key to encapsulating the oil drops within the powder. Dried Pickering emulsions containing up to 40 wt% encapsulated oil that could be dispersed in water as drops of the same size as in the original emulsion were produced.     

纳微米颗粒的表面改性及其在Pickering乳液中的应用进展

以固体粒子替代表面活性剂稳定的Pickering乳液,不仅可以赋予乳液许多特殊的性能(电磁和温度感应性能等),还可消除由添加表面活性剂所带来的副作用(过敏性和毒性等),这使得Pickering乳液可以应用到对表面活性物质具有很大限制的生物材料领域。系统综述了常见Pickering乳液用固体粒子,以及为了获得不同类型的乳液而对固体粒子进行表面改性的主要方法,着重归纳了表面改性前亲水性羟基磷灰石(HAp)和改性后疏水性HAp作为颗粒乳化剂稳定的Pickering乳液的类型及其进一步合成的产物。     

Selectively Permeable Double Emulsions

Natural organisms are made of different types of microcompartments, many of which are enclosed by cell membranes. For these organisms to display a proper function, the microcompartments must be selectively permeable. For example, cell membranes are typically permeable toward small, uncharged molecules such as water, selected nutrients, and cell signaling molecules, but impermeable toward many larger biomolecules. Here, it is reported for the first time dynamic compartments, namely surfactant‐stabilized double emulsions, that display selective and tunable permeability. Selective permeability is imparted to double emulsions by stabilizing them with catechol‐functionalized surfactants that transport molecules across the oil shell of double emulsions only if they electrostatically or hydrophobically attract encapsulants. These double emulsions are employed as semipermeable picoliter‐sized vessels to controllably perform complexation reactions inside picoliter‐sized aqueous cores. This thus far unmet level of control over the transport of reagents across oil phases opens up new possibilities to use double emulsion drops as dynamic and selectively permeable microcompartments to initiate and maintain chemical and biochemical reactions in picoliter‐sized cell‐mimetic compartments.     

Self‐Sensitized Carbon Nitride Microspheres for Long‐Lasting Visible‐Light‐Driven Hydrogen Generation

A new type of metal‐free photocatalyst is reported having a microsphere core of oxygen‐containing carbon nitride and self‐sensitized surfaces by covalently linked polymeric triazine dyes. These self‐sensitized carbon nitride microspheres exhibit high visible‐light activities in photocatalytic H₂ generation with excellent stability for more than 100 h reaction. Comparing to the traditional g‐C₃N₄ with activities terminated at 450 nm, the polymeric triazine dyes on the carbon nitride microsphere surface allow for effective wide‐range visible‐light harvesting and extend the H₂ generation activities up to 600 nm. It is believed that this new type of highly stable self‐sensitized metal‐free structure opens a new direction of future development of low‐cost photocatalysts for efficient and long‐term solar fuels production.     

Crab Chitin‐Based 2D Soft Nanomaterials for Fully Biobased Electric Devices

2D nanomaterials have various size/morphology‐dependent properties applicable in electronics, optics, sensing, and actuating. However, intensively studied inorganic 2D nanomaterials are frequently hindered to apply in some particular and industrial fields, owing to harsh synthesis, high‐cost, cytotoxicity, and nondegradability. Endeavor has been made to search for biobased 2D nanomaterials with biocompatibility, sustainability, and biodegradability. A method of hydrophobization‐induced interfacial‐assembly is reported to produce an unprecedented type of nanosheets from marine chitin. During this process, two layers of chitin aggregations assemble into nanosheets with high aspect ratio. With super stability and amphiphilicity, these nanosheets have super ability in creating highly stable Pickering emulsions with internal phase up to 83.4% and droplet size up to 140 μm, in analogue to graphene oxide. Combining emulsifying and carbonization can further convert these 2D precursors to carbon nanosheets with thickness as low as ≈3.8 nm. Having biologic origin, conductivity, and dispersibility in various solvents, resultant carbon nanosheets start a new scenario of exploiting marine resources for fully biobased electric devices with sustainability and biodegradability, e.g., supercapacitor, flexible circuits, and electronic sensors. Hybrid films of chitin and carbon nanosheets also offer low‐cost and environment‐friendly alternative of conductive components desirable in green electronics, wearable electronics, biodegradable circuits, and biologic devices.     

One-pot fabrication of magnetic nanocomposite microcapsules

Fe₂O₃ nanoparticles can self-assembly at liquid-liquid interfaces to form stable water-in-oil Pickering emulsions. Novel magnetic and thermo-sensitive microcapsules were one-pot fabricated by radical polymerization of N-isopropylacrylamide (NIPAm) at the aqueous phases of Pickering emulsions at 60 °C. The obtained PNIPAm was deposited from the water phases onto the interfaces of water-in-oil Pickering emulsions to form Fe₂O₃/PNIPAm nanocomposite shells because of its hydrophobicity at this reaction temperature. Pickering emulsion polymerization opens up a new route to fabricate a variety of hollow and hybrid microcapsules.     

Facile synthesis of porous carbon nitride spheres with hierarchical three-dimensional mesostructures for CO<Subscript>2</Subscript> capture

Porous carbon nitride (CN) spheres with partially crystalline frameworks have been successfully synthesized via a nanocasting approach by using spherical mesoporous cellular silica foams (MCFs) as a hard template, and ethylenediamine and carbon tetrachloride as precursors. The resulting spherical CN materials have uniform diameters of ca. 4 μm, hierarchical three-dimensional (3-D) mesostructures with small and large mesopores with pore diameters centered at ca. 4.0 and 43 nm, respectively, a relatively high BET surface area of ∼550 m²/g, and a pore volume of 0.90 cm³/g. High-resolution transmission electron microscope (HRTEM) images, wide-angle X-ray diffraction (XRD) patterns, and Raman spectra demonstrate that the porous CN material has a partly graphitized structure. In addition, elemental analyses, X-ray photoelectron spectra (XPS), Fourier transform infrared spectra (FT-IR), and CO₂ temperature-programmed desorption (CO₂-TPD) show that the material has a high nitrogen content (17.8 wt%) with nitrogen-containing groups and abundant basic sites. The hierarchical porous CN spheres have excellent CO₂ capture properties with a capacity of 2.90 mmol/g at 25 °C and 0.97 mmol/g at 75 °C, superior to those of the pure carbon materials with analogous mesostructures. This can be mainly attributed to the abundant nitrogen-containing basic groups, hierarchical mesostructure, relatively high BET surface area and stable framework. Furthermore, the presence of a large number of micropores and small mesopores also enhance the CO₂ capture performance, owing to the capillary condensation effect.     

纳米纤维素凝胶微粒及其在Pickering泡沫中的应用

目的研究氧化纳米纤维素/乳酸链球菌素(TONCC/nisin)凝胶粒子的性质及其在环保抗菌泡沫中的应用。方法利用TONCC的表面羧基基团与nisin的表面阳离子的吸附耦合作用,制备TONCC/nisin水凝胶和微凝胶,以微凝胶作为稳定粒子,环氧大豆油丙烯酸酯(AESO)为油相,制备TONCC/nisin/AESO Pickering乳液,对水凝胶、微凝胶、乳液的稳定性进行研究;通过热固化乳液得到环保抗菌的泡沫材料,并对泡沫材料的结构和抗菌效果进行表征。结果水凝胶的结构随着在水中浸泡时长的增加而发生变化,宏观表现为坍塌变形,nisin逐渐析出,微凝胶随着静置时间的延长其粒径变化不大;以微凝胶作为界面稳定剂的AESO乳液的热稳定性较好,在90℃下加热30 min乳液液滴并未发生聚并现象,该乳液固化后形成的多孔泡沫材料对李斯特菌的抑制作用明显,当泡沫中nisin含量为2μg/g时,其抑菌率为43%。结论TONCC和nisin形成的微凝胶粒子在水中稳定性较好,可以用于乳化AESO制备Pickering泡沫,同时赋予泡沫多孔性和抗菌性,在制备环保抗菌泡沫方面有很大的应用潜力。     

Polymersomes Containing a Hydrogel Network for High Stability and Controlled Release

Capillary microfluidic devices are used to prepare monodisperse polymersomes consisting of a hydrogel core and a bilayer membrane of amphiphilic diblock‐copolymers. To make polymersomes, water‐in‐oil‐in‐water double‐emulsion drops are prepared as templates through single‐step emulsification in a capillary microfluidic device. The amphiphile‐laden middle oil phase of the double‐emulsion drop dewets from the surface of the innermost water drop, which contains hydrogel prepolymers; this dewetting leads to the formation of a bilayer membrane. Subsequently, the oil phase completely separates from the innermost water drop, leaving a polymersome. Upon UV illumination of the polymersome, the prepolymers encapsulated within the interior are crosslinked, forming a hydrogel core. The hydrogel network within the polymersomes facilitates sustained release of the encapsulated materials and increases the stability of the polymersomes through the formation of a scaffold to support the bilayer. In addition, this approach provides a facile method to make monodisperse hydrogel particles directly dispersed in water.     

New 2D Carbon Nitride Organic Materials Synthesis with Huge‐Application Prospects in CN Photocatalyst

In recent years, 2D materials are attracting increased attention because of their excellent properties. In this paper, new 2D carbon nitride (CN) organic materials are successfully prepared on the basis of the organic synthesis theory, and the thickness is about 1.5 nm. This new 2D CN organic material further strengthens the 2D materials family. Meanwhile, their synthetic mechanism is theoretically speculated. Then CN photocatalysts of several structures are obtained by roasting 2D CN organic materials. Through the photocatalytic hydrogen production experiments, the results exhibit that these kinds of photocatalysts have good photocatalytic effects compared to common g‐C₃N₄.     

Sequential Coalescence Enabled Two‐Step Microreactions in Triple‐Core Double‐Emulsion Droplets Triggered by an Electric Field

Advances in microfluidic emulsification have enabled the generation of exquisite multiple‐core droplets, which are promising structures to accommodate microreactions. An essential requirement for conducting reactions is the sequential coalescence of the multiple cores encapsulated within these droplets, therefore, mixing the reagents together in a controlled sequence. Here, a microfluidic approach is reported for the conduction of two‐step microreactions by electrically fusing three cores inside double‐emulsion droplets. Using a microcapillary glass device, monodisperse water‐in‐oil‐in‐water droplets are fabricated with three compartmented reagents encapsulated inside. An AC electric field is then applied through a polydimethylsiloxane chip to trigger the sequential mixing of the reagents, where the precise sequence is guaranteed by the discrepancy of the volume or conductivity of the inner cores. A two‐step reaction in each droplet is ensured by two times of core coalescence, which totally takes 20–40 s depending on varying conditions. The optimal parameters of the AC signal for the sequential fusion of the inner droplets are identified. Moreover, the capability of this technique is demonstrated by conducting an enzyme‐catalyzed reaction used for glucose detection with the double‐emulsion droplets. This technique should benefit a wide range of applications that require multistep reactions in micrometer scale.