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.     

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.     

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.     

Formation and stabilization of Pickering emulsions using salt-sensitive core–shell cationic nanoparticles

Pickering emulsions known for their solid emulsifiers and brilliant stability characters have attracted many researchers’ attention. The controlled stability and demulsification of emulsion are necessary in some cases such as crude oil extraction and drug release. Stimuli-responsive Pickering emulsion could provide suitable controllability and emerged in the last decade. Among various controllable factors, salt ion is known as a critical parameter, but it is rarely investigated. Here, core–shell cationic nanoparticles with a poly-(2-aminoethyl methacrylate hydrochloride) shell and a polystyrene core were used in the preparation of Pickering emulsion. The size and morphology of nanoparticles were monitored by transmission electron microscopy and dynamic light scattering. The microstructure and stability of the formed Pickering emulsion were studied via dynamic light scattering and a polarizing optical microscope under various salt ion types and concentrations. The effect of salt types (Cl^?, ClO₄^?, and PO₄^3?) and salt concentrations on the Pickering emulsion was investigated. Cl^?, ClO₄^?, and PO₄^3? are in situ generated from NaCl, NaClO₄, and (NaPO₃)₆, respectively. It showed that PO₄^3? (100–1000 mM) was unable to form stable Pickering emulsion, while Cl^? and ClO₄^? could induce stable Pickering emulsions under optimized conditions. Furthermore, after increasing the salt concentration over a critical salt concentration, the Pickering emulsion underwent rapid demulsification. This work revealed the effects of salt on size, conformation, charge, wettability, interaction, and adsorption state of nanoparticles and proposed the stability mechanisms of the Pickering emulsion. This opened up more potential applications in the field of controlled demulsification, petroleum recovery, catalyst recovery, and so on triggered by salt ions.

Graphical abstract

Salt could affect the size, conformation, and interaction of core–shell cationic nanoparticles, which then affect the formation mechanism and stability properties of Pickering emulsions from them.

     

纳米粒子对表面活性剂乳液稳定性的影响

制备了由三种不同类型的表面活性剂稳定的乳状液,考察了纳米粒子对表面活性剂乳液稳定性的影响作用,结果表明,ZnO纳米粒子SDBS、Oπ-10和CTAB稳定的乳液均具有破乳作用;MgO纳米粒子对SDBS稳定的乳液具有破乳作用;Fe2O3纳米粒子对三种表面活性剂乳液的稳定性没有显著影响。     

Formation of Janus TiO2 nanoparticles by a pickering emulsion approach applying phosphonate coupling agents

Anisotropic surface modification of TiO2 nanoparticles was achieved applying a Pickering emulsion approach. TiO2 nanoparticles were prepared by sol-gel routes which allowed an excellent control over their size and morphology. The obtained colloids were further used as stabilizers in the formation of oil-in-water Pickering emulsion. For reasons of comparison, also commercially available titanium dioxide nanoparticles (Evonik AEROXIDE TiO2 P25) were used in the functionalization experiments. An organophosphorus coupling agent present in the oil phase coordinated to the surface of the anatase nanoparticles. In such a way an anisotropic surface modification of the particles was achieved which increased the stability of the Pickering emulsion. Spectroscopic studies revealed the presence of organophosphorus coupling agents which exhibited a covalent bonding to the surface of the particles. Thermogravimetric analyses confirmed a lower surface coverage of the particles modified in emulsion compared to those modified in suspension. Reactions of organophosphorus coupling agents containing an additional methacrylate group applying an organic monomer (methyl methacrylate) as the oil phase of the Pickering emulsion resulted in hybrid TiO2@polymer spheres. Spectroscopic characterization of the resulting particles revealed that the phosphonates were coordinated to the TiO2 surface and at the same time copolymerized with the MMA within the oil droplet. Morphological investigations of the isolated final product showed that the material was composed of polymer spheres with the stabilizing TiO2 nanoparticles on their surface.     

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.     

铁酸锌作为光催化剂的研究进展

铁酸锌是具有较高光催化活性及对可见光敏感的n型半导体,其独特的磁性能和化学稳定性使其在光催化领域有着广泛的应用。针对不同形貌的铁酸锌,采用不同的改性方法,可将其与其他材料结合制备出更为高效、实用的光催化剂。近年来,利用铁酸锌的特性,制备磁性可回收且性能稳定的复合光催化剂引起了科研人员的广泛关注。概述了铁酸锌作为光催化剂的基本性质,对不同形貌铁酸锌的制备方法进行了总结,并重点讨论了铁酸锌在光催化方面的改性技术及改性机理,最后对目前存在的问题和未来的研究方向做了简要的总结和预测。     

Facile Fabrication of Monodisperse Micron-Sized Dual Janus Silica Particles with Asymmetric Morphology and Chemical Environment

Janus particles are a kind of materials with asymmetric morphology or surface chemical environment. But so far, the preparation of particles with dual asymmetry is still a challenging problem. Hence the cation surfactant hexadecyl trimethyl ammonium bromide and co-surfactant octadecylamine are applied to improve the Pickering emulsion stability, and the micron-sized silica particles are arranged in a single layer at the toluene–water interface through electrostatic interaction. Furthermore, organosilane reagents are added in the preparation process, resulting in the construction of asymmetric hydrophilic or hydrophobic mesoporous precisely onto the micron-sized silica particles surface. The cation surfactant-assisted Pickering emulsion method is simple, effective, and convenience, which can be applied in the synthesis of various dual Janus silica particles for specific applications.     

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.     

Recent progress in mesoporous titania materials: adjusting morphology for innovative applications

Abstract

This review article summarizes recent developments in mesoporous titania materials, particularly in the fields of morphology control and applications. We first briefly introduce the history of mesoporous titania materials and then review several synthesis approaches. Currently, mesoporous titania nanoparticles (MTNs) have attracted much attention in various fields, such as medicine, catalysis, separation and optics. Compared with bulk mesoporous titania materials, which are above a micrometer in size, nanometer-sized MTNs have additional properties, such as fast mass transport, strong adhesion to substrates and good dispersion in solution. However, it has generally been known that the successful synthesis of MTNs is very difficult owing to the rapid hydrolysis of titanium-containing precursors and the crystallization of titania upon thermal treatment. Finally, we review four emerging fields including photocatalysis, photovoltaic devices, sensing and biomedical applications of mesoporous titania materials. Because of its high surface area, controlled porous structure, suitable morphology and semiconducting behavior, mesoporous titania is expected to be used in innovative applications.     

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.     

不同表面活性剂对铟锡氧化物纳米悬浮液稳定性的影响

在纳米铟锡氧化物纳米粉体应用中悬浮液分散稳定是一个十分重要的问题。本文采用化学改性的方法,分别用阴离子表面活性剂、阳离子表面活性剂、非离子表面活性剂和硅表面活性剂对铟锡氧化物进行表面修饰以改进其悬浮液的稳定性。结果表明,除阳离子表面活性剂外,其他表面活性剂均可以使纳米铟锡氧化物在特定的分散体系中得到很好的分散,纳米铟锡氧化物悬浮液稳定性良好。     

Charged nanoparticles as supramolecular surfactants for controlling the growth and stability of microcrystals

Microcrystals of desired sizes are important in a range of processes and materials, including controlled drug release, production of pharmaceutics and food, bio- and photocatalysis, thin-film solar cells and antibacterial fabrics. The growth of microcrystals can be controlled by a variety of agents, such as multivalent ions, charged small molecules, mixed cationic-anionic surfactants, polyelectrolytes and other polymers, micropatterned self-assembled monolayers, proteins and also biological organisms during biomineralization. However, the chief limitation of current approaches is that the growth-modifying agents are typically specific to the crystalizing material. Here, we show that oppositely charged nanoparticles can function as universal surfactants that control the growth and stability of microcrystals of monovalent or multivalent inorganic salts, and of charged organic molecules. We also show that the solubility of the microcrystals can be further tuned by varying the thickness of the nanoparticle surfactant layers and by reinforcing these layers with dithiol crosslinks.     

Recent Progress of Three-dimensionally Ordered Macroporous (3DOM) Materials in Photocatalytic Applications: A Review

In recent years, three-dimensionally ordered macroporous (3DOM) materials have attracted tremendous interest in the field of photocatalysis due to the periodic spatial structure and unique physicochemical properties of 3DOM catalysts. In this review, the fundamentals and principles of 3DOM photocatalysts are briefly introduced, including the overview of 3DOM materials, the photocatalytic principles based on 3DOM materials, and the advantages of 3DOM materials in photocatalysis. The preparation methods of 3DOM materials are also presented. The structure and properties of 3DOM materials and their effects on photocatalytic performance are briefly summarized. More importantly, 3DOM materials, as a supported catalyst, are extensively employed to combine with various common materials, including metal nanoparticles, metal oxides, metal sulfides, and carbon materials, to enhance photocatalytic performance. Finally, the prospects and challenges for the development of 3DOM materials in the field of photocatalysis are presented.     

Calcium phosphate-based nanosystems for advanced targeted nanomedicine

Synthetic calcium phosphates (CaPs) are the most widely accepted bioceramics for the repair and reconstruction of bone tissue defects. The recent advancements in materials science have prompted a rapid progress in the preparation of CaPs with nanometric dimensions, tailored surface characteristics, and colloidal stability opening new perspectives in their use for applications not strictly related to bone. In particular, the employment of CaPs nanoparticles as carriers of therapeutic and imaging agents has recently raised great interest in nanomedicine. CaPs nanoparticles, as well as other kinds of nanoparticles, can be engineered to specifically target the site of the disease (cells or organs), thus minimizing their dispersion in the body and undesired organism-nanoparticles interactions. The most promising and efficient approach to improve their specificity is the ‘active targeting’, where nanoparticles are conjugated with a targeting moiety able to recognize and bind with high efficacy and selectivity to receptors that are highly expressed only in the therapeutic site. The aim of this review is to give an overview on advanced targeted nanomedicine with a focus on the most recent reports on CaP nanoparticles-based systems, specifically designed for the active targeting. The distinctive characteristics of CaP nanoparticles with respect to the other kinds of nanomaterials used in nanomedicine are also discussed.     

Surfactant‐Free Shape Control of Gold Nanoparticles Enabled by Unified Theoretical Framework of Nanocrystal Synthesis

Gold nanoparticles have unique properties that are highly dependent on their shape and size. Synthetic methods that enable precise control over nanoparticle morphology currently require shape‐directing agents such as surfactants or polymers that force growth in a particular direction by adsorbing to specific crystal facets. These auxiliary reagents passivate the nanoparticles' surface, and thus decrease their performance in applications like catalysis and surface‐enhanced Raman scattering. Here, a surfactant‐ and polymer‐free approach to achieving high‐performance gold nanoparticles is reported. A theoretical framework to elucidate the growth mechanism of nanoparticles in surfactant‐free media is developed and it is applied to identify strategies for shape‐controlled syntheses. Using the results of the analyses, a simple, green‐chemistry synthesis of the four most commonly used morphologies: nanostars, nanospheres, nanorods, and nanoplates is designed. The nanoparticles synthesized by this method outperform analogous particles with surfactant and polymer coatings in both catalysis and surface‐enhanced Raman scattering.     

镍 -纳米氧化铝复合电镀液的制备及影响因素研究

性质均匀稳定的镍/纳米颗粒复合镀液是制备镍/纳米复合镀层的物质和工艺基础.在瓦特镀镍溶液中加入纳米Al2O3粉末,混合液静置10 h后,因颗粒沉淀而产生不同程度的分层,通过比色法研究了分散剂、分散形式、镀液pH值对纳米Al2O3粉末在镀液中均匀稳定分散的影响.结果表明,在镀液中加入适量的聚羧酸铵、柠檬酸三铵或十六烷基三甲基溴化铵分散剂,并通过超声分散,可得到稳定分散10 h以上的复合电镀液.原子力显微镜分析表明,复合镀液中纳米颗粒的平均尺寸为63 nm,略大于其原料颗粒的尺寸(40 nm),大部分的纳米颗粒在复合镀液中能实现高度分散.     

Stability of orange oil/water nanoemulsions prepared by the PIT method

This article reports the preparation and characterization of orange oil/water nanoemulsions stabilized by commercial nonionic surfactants based on ethoxylated lauryl ether (Ultrol line), by the phase inversion temperature (PIT) method. The orange oil/surfactant/water dispersions were prepared at different HLB values, by varying the concentrations of the surfactants as well as the concentration of the oil phase. The stability of the o/w nanoemulsions and the size distribution of the dispersed particles in these systems in general depended on the concentration of the oil phase used: the emulsions prepared with an oil phase of 14 wt% had smaller droplet size in the dispersed phase than the emulsions prepared in the presence of oil phases of 20 and 30 wt%. The nanoemulsions prepared with pure surfactants were more stable in the presence of Ultrol L60, but the surfactants' cloud point had a strong influence on the stability of the emulsions formed when this was very near room temperature. Because of this, we prepared systems containing mixtures of surfactants. Among these systems, the most stable nanoemulsions were those prepared with a Ultrol L100/Ultrol L20 mixture with HLB of 12.40. This behavior can be attributed to the complete solubilization in mixed micelles of the more hydrophobic surfactant.     

Encapsulation of Single Plasmonic Nanoparticles within ZIF‐8 and SERS Analysis of the MOF Flexibility

Hybrid nanostructures composed of metal nanoparticles and metal‐organic frameworks (MOFs) have recently received increasing attention toward various applications due to the combination of optical and catalytic properties of nanometals with the large internal surface area, tunable crystal porosity and unique chemical properties of MOFs. Encapsulation of metal nanoparticles of well‐defined shapes into porous MOFs in a core–shell type configuration can thus lead to enhanced stability and selectivity in applications such as sensing or catalysis. In this study, the encapsulation of single noble metal nanoparticles with arbitrary shapes within zeolitic imidazolate‐based metal organic frameworks (ZIF‐8) is demonstrated. The synthetic strategy is based on the enhanced interaction between ZIF‐8 nanocrystals and metal nanoparticle surfaces covered by quaternary ammonium surfactants. High resolution electron microscopy and tomography confirm a complete core–shell morphology. Such a well‐defined morphology allowed us to study the transport of guest molecules through the ZIF‐8 porous shell by means of surface‐enhanced Raman scattering by the metal cores. The results demonstrate that even molecules larger than the ZIF‐8 aperture and pore size may be able to diffuse through the framework and reach the metal core.