One-Step Formation of Pickering Double Emulsion Costabilized by Hydrophobic Silica Nanoparticles and Sodium Alginate.

Pickering double emulsions exhibit higher stability and biocompatibility compared with surfactant-stabilized double emulsions. However, tailored synthesis of particle stabilizers with appropriate wettability is time consuming and complicated and usually limits their large-scale adoption. Using binary stabilizers may be a simple and scalable strategy for Pickering double emulsion formation. Herein, commercially available hydrophobic silica nanoparticles (SNPs) and sodium alginate (SA) as binary stabilizers are used to prepare O/W/O Pickering double emulsions in one-step emulsification. The influence of system composition on double emulsion preparation is identified by optical microscopy, confocal laser scanning microscopy, and interfacial tension and water contact angle analyses. The formation of the O/W/O Pickering double emulsion depends critically on the aqueous phase viscosity and occurrence of emulsion inversion. Both hydrophobic SNPs and SA adsorb at the droplet surface to provide a steric barrier, while SA also reduces interfacial tension and increases aqueous phase viscosity, giving double emulsion long-term stability. Their microstructure and stability are controlled by adjusting the SA concentration, water-oil volume ratio, concentration and wettability of the particle stabilizer, and oil type. As a demonstration, the middle layer of the as-prepared O/W/O Pickering double emulsions can be cross-linked in situ with calcium ions to produce calcium alginate porous microspheres. We believe that our strategy for double emulsion formation holds great potential for practical applications in food, cosmetics, or pharmaceuticals.

Porous Waterborne Polyurethane Films Templated from Pickering Foams for Fabrication of Synthetic Leather.

Waterborne polyurethane (WPU) latex nanoparticles with proven interfacial activity were utilized to stabilize air-water interfaces of Pickering foams through interfacial interaction with hydrophobic fumed silica particles (SPs). The rheological properties of the Pickering foam were tailored through adjustment of their SP content, which influenced their formability and stability. A Pickering foam stabilized with WPU and SPs was used as a template to prepare a WPU-SP composite porous film. The as-prepared film had intact open-cell porous structures, which increased its water absorption and water-vapor permeability. The porous film was used as a middle layer in the preparation of synthetic leather via a four-step "drying method". Compared with commercial synthetic leather, the lab-made synthetic leather with a middle layer made of the WPU-SP composite porous film exhibited a richer porous structure, acceptable wetting on a fabric substrate, a thicker porous layer, and higher water-vapor permeability. This work provides a novel and facile approach for preparing WPU-SP Pickering foams. Furthermore, the foams have the potential to function as a sustainable material for creating a porous-structured synthetic leather made from WPU, which may be utilized as an alternative to solvent-based synthetic leather.

Light-Driven Spatiotemporal Pickering Emulsion Droplet Manipulation Enabled by Plasmonic Hybrid Microgels.

The past decades have witnessed the development of various stimuli-responsive materials with tailored functionalities, enabling droplet manipulation through external force fields. Among different strategies, light exhibits excellent flexibility for contactless control of droplets, particularly in three-dimensional space. Here, we present a facile synthesis of plasmonic hybrid microgels based on the electrostatic heterocoagulation between cationic microgels and anionic Au nanoparticles. The hybrid microgels are effective stabilizers of oil-in-water Pickering emulsions. In addition, the laser irradiation on Au nanoparticles creats a "cascade effect" to thermally responsive microgels, which triggers a change in microgel wettability, resulting in microgel desorption and emulsion destabilization. More importantly, the localized heating generated by a focused laser induces the generation of a vapor bubble inside oil droplets, leading to the formation of a novel air-in-oil-in-water (A/O/W) emulsion. These A/O/W droplets are able to mimic natural microswimmers in an aqueous environment by tracking the motion of a laser spot, thus achieving on-demand droplet merging and chemical communication between isolated droplets. Such proposed systems are expected to extend the applications of microgel-stabilized Pickering emulsions for substance transport, programmed release and controlled catalytic reactions.

Phase Inversion of Pickering Emulsions Induced by Interfacial Electrostatic Attraction.

Phase inversion of Pickering emulsions from water-in-oil (W/O) to oil-in-water (O/W) is achieved by the formation of an interfacial particle bilayer using negatively charged and positively charged particles dispersed in water and oil, respectively, before emulsification. A mechanism based on electrostatic attraction across the toluene-water interface is proposed and verified by systematic investigation of the parameters that affect the surface charge of negatively charged particles such as pH and salt concentration. Cationic silica-FITC particles (600 nm) can be dispersed in toluene and stabilize W/O emulsions alone; phase inversion of this emulsion can be induced by the addition of anionic silica-RB particles in the aqueous phase at a concentration of 1.0 wt % or above. It is revealed that silica-RB particles of a smaller size (100 nm) can induce emulsion phase inversion at a much lower concentration (0.4 wt %) and an interfacial particle bilayer is clearly revealed by CLSM and SEM images. By tuning the surface charge density of silica-RB particles, the electrostatic attraction mechanism leading to the formation of the interfacial particle bilayer is confirmed and emulsion stability can be tuned as demonstrated by osmotic pressure enhancement results obtained from centrifugation.

Total internal reflection microscopy: a powerful tool for exploring interactions and dynamics near interfaces.

The occurrence of many micro/macrophenomena is closely related to interactions and dynamics near interfaces. Hence, developing powerful tools for characterizing near-interface interactions and dynamics has attached great importance among researchers. In this review, we introduce a noninvasive and ultrasensitive technique called total internal reflection microscopy (TIRM). The principles of TIRM are introduced first, demonstrating the characteristics of this technique. Then, typical measurements with TIRM and the recent development of the technique are reviewed in detail. At the end of the review, we highlight the great progress of TIRM during the past several decades and show its potential to be more influential in measuring interactions and dynamics near interfaces in various research fields.

Water-resistance chitosan film through enzymatic treatment and layer-by-layer assembly with bacterial cellulose for food packaging materials.

The pervasive presence of plastic packaging has led to significant environmental contamination due to excessive reliance on petrochemicals and the inherent non-biodegradability of these materials. Both bacterial cellulose (BC) and chitosan (CT) films offer a promising option for food packaging purposes due to their sturdy mechanical strength, biodegradability, environmentally friendly manufacturing process, and non-toxic composition. However, the considerable moisture absorption capacity of these eco-friendly materials has hindered their extensive use, as it leads to a reduction in their strength and ability to serve as a barrier. In the present study, we introduced a composite material of BC reinforced with a lauryl gallate grafted CT coating. After grafting CT with lauryl gallate (CT-LG) through enzymatic modification, it showed excellent hydrophobic properties also in a green route of chemistry synthesis. Based on the results of the study, the duration of the water droplet test of the pure CT-LG film and BC coated with CT-LG (BC/CT-LG) films was more than 15 min, showing that water droplets can be completely blocked by the CT-LG coating without water penetration. For the mechanical properties, the wet flexural strength and wet tensile strength of BC/CT-LG films have improved 400% and 70% compared with the original BC. This method produces a composite material with enhanced hydrophobicity and green properties and shows great potential for use in drinking straws or packaging bags.

Dual-responsive colloidosome-like microgels as the building blocks for phase inversion of Pickering emulsions.

The intelligent regulation of microgel-stabilized Pickering emulsions with multi-responsiveness is presently constrained to the processes of emulsification and destabilization. However, the expansion of multi-control over Pickering emulsions to involve phase inversion and the investigation of the accompanying processes and mechanisms present a great challenge. In this study, a microgel with dual responsiveness to both pH and temperature was synthesized using an emulsion template. The resulting microgel exhibited a robust colloidosome-like structure, distinguished by the presence of monolayer-adsorbed silica nanoparticles. The regulation of the packing of surface-covered silica nanoparticles was easily achieved through the swelling of the microgel matrix. Furthermore, the wettability of the microgel can be adjusted between hydrophilic and hydrophobic intervals, allowing for the effective and dual-responsive phase inversion of Pickering emulsions. Moreover, it has been observed that colloidosome-like microgels can lead to unique interfacial structures during the emulsification process, thereby elucidating the fundamental mechanism governing emulsion phase inversion.

Edible, strong, and low-hygroscopic bacterial cellulose derived from biosynthesis and physical modification for food packaging.

BACKGROUND: The pervasive presence of plastic packaging has led to significant environmental contamination due to excessive reliance on petrochemicals and the inherent non-biodegradability of these materials. Bacterial cellulose (BC) films present a viable alternative for food packaging applications, owing to their environmentally friendly synthesis process, non-toxic nature, robust mechanical strength, and biodegradability. However, the high hygroscopicity of such bio-based materials has limited their widespread adoption, as it results in diminished strength and barrier properties. In this study, a novel approach for creating edible, transparent, robust, and high-barrier BC-based composite packaging was proposed through biosynthesis with the incorporation of soy protein isolate and the physical interpenetration of calcium alginate-polyethylene glycol as a composite coating. RESULTS: The finding demonstrated that the synthesized bio-based composite material exhibits stability in water, high optical transparency, complete oil resistance, and full degradability within 1 to 2 months. Furthermore, the composite material displayed enhanced mechanical properties in both dry and wet conditions, with a tensile strength of approximately 84 MPa, outperforming commercially available kraft paper and low-density polyethylene. CONCLUSIONS: Soy protein isolate established a rigid, coherent, and homogeneous network with BC fibrils, thereby augmenting mechanical properties. Calcium alginate can be effectively combined with BC, utilizing polyethylene glycol as a binder and plasticizer, to generate a densely packed structure with reduced hygroscopicity. This bio-based composite material demonstrated considerable potential for application in food packaging and other value-added sectors as a substitute for non-degradable plastics. © 2023 Society of Chemical Industry.

Pickering Emulsions Stabilized by Binary Mixtures of Colloidal Particles: Synergies between Contrasting Properties.

Pickering emulsions that are stabilized by colloidal particles have attracted substantial research attention because of their potential applications in various industries. Previously, single colloidal particles have usually been used to fabricate Pickering emulsions and to investigate the stabilization mechanism. However, surface modification of the colloidal stabilizer is normally required to adjust the particle wettability, which often involves chemical modification, the adsorption of a surfactant or polymer, and the addition of an electrolyte. Such a modification is expensive, time-consuming, and thus only partially effective. In this Perspective, we describe an alternative approach that uses binary mixtures of particles as stabilizers and could be an effective solution to the above-described problems with Pickering emulsions. We introduce various types of Pickering emulsions stabilized by binary mixtures of particles with different functional groups, opposite charges, or opposite wettabilities (i.e., they are hydrophilic or hydrophobic). Examples of stabilizing mechanisms are discussed, showing that compared with emulsions stabilized by single colloidal particles, emulsions stabilized by binary mixtures of particles are generated via simpler particle-pretreatment processes and have higher stability and customizable properties and thus can enable the exploration of the next generation of Pickering emulsions.

Multifunctional Silica-Modified Hybrid Microgels Templated from Inverse Pickering Emulsions.

Microgels are regarded as soft colloids with environmental responsiveness. However, the majority of reported microgels are inherently hydrophilic, resulting in aqueous dispersions, and only used in water-based applications. Herein, we reported an efficient method for hybridization of poly(N-isopropylacrylamide) microgel by coating hydrophobic silica nanoparticles on their surface. The resultant hybrid microgel had switchable surface wettability and could be dispersed in both aqueous and oil phases. Meanwhile, the coated hydrophobic silica nanoparticles solved the difficulty in redispersing microgels caused by particle aggregation and film formation during the drying process, providing a significant advantage in dried storage. Furthermore, the introduction of hydrophobic silica nanoparticles endowed the hybrid microgel with a variety of applications, including cargo encapsulation, active release induced by emulsion reversion, and trace water absorption.

Water-in-Oil Pickering Emulsions Stabilized by Hydrophobized Protein Microspheres.

Water-in-oil (w/o) Pickering emulsions have gained considerable attention in colloid science and daily applications. However, for the formation of w/o emulsions, especially those with high internal water content, the particulate stabilizers are required to be sufficiently hydrophobic, and synthetic or chemically modified particles have been mostly reported until now, which are not biocompatible and sustainable. We present a zein protein-based microsphere derived from the Pickering emulsion template, in which protein microspheres are feasibly in situ hydrophobized by silica nanoparticles, enabling the stabilization of w/o Pickering emulsions. The effects of microsphere concentration, water/oil volume ratio, oil types, and pH on the stabilization of prepared w/o emulsions are systematically studied, revealing prominent characteristics of the controllable size, high water fraction, universal adaptation of oils, as well as broad pH stability. As a demonstration, the Pickering emulsion effectively encapsulates vitamin C and shows high stability for long storage duration against ultraviolet radiation/heat. Therefore, this novel proteinaceous particle-stabilized w/o Pickering emulsion has great potential in the delivery and protection of water-soluble bioactive substrates.

Nanocomposite Polymer Colloids Prepared via Emulsion Polymerization and Stabilized Using Polydopamine-Coated Silica Particles.

Polymer/inorganic colloidal nanocomposites can be prepared via Pickering emulsion polymerization (PEP); however, this process usually requires the use of surfactants, auxiliary comonomers, and volatile organic compounds. Herein, we report a versatile and efficient method for synthesizing stable monodisperse polymer/silica colloidal nanocomposite particles via PEP. First, silica nanoparticles were modified by depositing a multifunctional polydopamine (PDA) film. The outermost PDA film could enhance the precipitation of oligomeric polymer radicals on the silica surface, which is crucial for the preparation of stable polymer/inorganic colloidal nanocomposites via PEP. Notably, this PDA modification approach can employ different initiator systems, such as cationic initiators and redox initiator couples, and can be applied to various monomers and monomer pairs (St, St/nBA, MMA, MMA/nBA, Vac, Vac/nBA). The influence of the concentration and size of polydopamine-coated silica (SiO2@PDA) on the colloidal nanocomposite was investigated. Increasing the diameter of SiO2@PDA and decreasing the concentration of SiO2@PDA both lead to the formation of larger nanocomposite particles. Considering its wide applicability, the proposed PDA modification approach can be applied to other functional inorganic particles to prepare multifunctional polymer/inorganic nanocomposite particles.

Investigation of the Contact Angle and Packing Density of Silica Nanoparticles at a Pickering Emulsion Interface Fixed by UV Polymerization.

The contact angle of colloidal particles at an oil-water interface plays a crucial role in determining Pickering emulsion stability and emulsion type, but the contact angle cannot be directly determined using conventional methods. In this work, a Pickering emulsion was prepared with photocurable resin as the internal phase containing silica nanoparticle stabilizers. Particles adsorbed at the oil-water interface were then fixed through UV curing, allowing for the investigation of various parameters that influence the contact angle of colloidal particles at the interface. After curing, the contact angle can then be observed using scanning electron microscopy and subsequently measured. The contact angle of interfacial adsorbed silica nanoparticles gradually decreases as the size increases due to the line tension at the three-phase contact line, but, more importantly, we found that the surface chemistry of the silica nanoparticles plays the most important role in determining the contact angle. The fast fixation of solid nanoparticles at emulsion interfaces facilitates accurate measurements of the partition of particles between oil and water, providing a new method for studying the factors that affect Pickering emulsion stability.

Fortification of edible films with bioactive agents: a review of their formation, properties, and application in food preservation.

Biodegradable films constructed from food ingredients are being developed for food coating and packaging applications to create more sustainable and environmentally friendly alternatives to plastics and other synthetic film-forming materials. In particular, there is a focus on the creation of active packaging materials from natural ingredients, especially plant-based ones. The film matrix is typically constructed from film-forming food components, such as proteins, polysaccharides and lipids. These matrices can be fortified with active ingredients, such as antioxidants and antimicrobials, so as to enhance their functional properties. Edible active films must be carefully designed to have the required optical, mechanical, barrier, and preservative properties needed for commercial applications. This review focuses on the fabrication, properties, and functional performance of edible films constructed from natural active ingredients. It provides an overview of the type of active ingredients that can be used, how they interact with the film matrix, how they migrate through the films, and how they are released. It also discusses the potential application of these active films for food preservation. Finally, future trends are highlighted and areas where further research are required are discussed.

One-Step Formation of Double Emulsions Stabilized by PNIPAM-based Microgels: The Role of Co-monomer.

Microgels have been widely used as particulate emulsifiers to stabilize emulsions due to their multiresponsiveness and deformability. Generally, microgels stabilize oil-in-water (o/w) emulsions, whereas occasionally water-in-oil (w/o) emulsions are reported using oils like n-octanol in which microgels can swell. However, the use of microgels to stabilize double emulsions (DEs) remains scarce. In this work, we report a special poly(N-isopropylacrylamide)- (PNIPAM-) based microgel to obtain water-in-oil-in-water (w/o/w) DEs in one step with the introduction of 1-vinylimidazole (VIM) as comonomer and hydroxy silicone oil as the oily phase. By comparison, when methacrylic acid (MAA) is used, an o/w emulsion will be obtained. The same holds true even when we freeze-dry and redisperse the microgels in the oil. Compared with PNIPAM-co-MAA microgel, PNIPAM-co-VIM microgel achieves a lower interfacial tension (IFT) when dispersed in the aqueous phase. This interfacial affinity of PNIPAM-co-VIM is believed to result from acid-base interaction between VIM and hydroxyl groups of the silicone oil, the same interaction used for preparing silica-vinyl polymer composite particles. Increasing the particle concentrations from 0.05% to 0.9% (w/v), we observe the inversion from w/o to o/w/o and w/o/w emulsions. When the oil fraction is changed from 0.1 to 0.9, the emulsion morphology evolves from o/w and w/o/w to w/o emulsions. At last, we examine the emulsifying ability of PNIPAM-co-VIM microgel with other oils and find that w/o/w emulsions are obtained with edible oils as well. Considering the similarity between microgels and biopolymers, the discovery in this work will help in designing food-grade emulsifiers to form edible DEs.

pH-Sensitive W/O Pickering High Internal Phase Emulsions and W/O/W High Internal Water-Phase Double Emulsions with Tailored Microstructures Costabilized by Lecithin and Silica Inorganic Particles.

Synergistic stabilization of Pickering emulsions by a mixture of surfactants and colloidal particles has received increasing interest in recent years but only a few of them can produce high internal phase double emulsions (HIPDEs) with good stability. In this study, we present a feasible and common method of preparing Pickering high internal phase emulsions (HIPEs) with tunable inner morphology costabilized by a biosurfactant lecithin and silica nanoparticles. We investigate the influence of the pH value on the interfacial behavior of lecithin and elucidate the synergistic mechanism between lecithin and silica nanoparticles in different conditions in the stability of as-prepared emulsions. Specifically, water-in-oil (W/O) Pickering HIPEs can be successfully stabilized by lecithin and hydrophobic silica nanoparticles in a wide pH range (pH 1-10), while catastrophic phase inversion occurred at high pH values (pH ≥ 11). Interestingly, stable water-in-oil-in-water (W/O/W) high internal phase double emulsions (HIPDEs) can also be prepared via a two-step method by the cooperation of lecithin and silica nanoparticles. Moreover, functional interconnected porous monoliths and microspheres are facilely fabricated by emulsion templates and their potential applications are explored.

Pickering Emulsions Simultaneously Stabilized by Starch Nanocrystals and Zein Nanoparticles: Fabrication, Characterization, and Application.

Using two types of colloidal particles having natural origins to synergistically stabilize Pickering emulsions is essential for food, cosmetics, and pharmaceutics, especially when neither particle can stabilize the Pickering emulsions alone. The use of two natural stabilizers avoids the complicated surface treatments of particles and the introduction of poisonous or harmful chemicals. In this work, we report an all-natural Pickering emulsion stabilized synergistically by starch nanocrystals and zein protein nanoparticles. Our result shows that the electrostatic interaction between the two types of particles greatly affects their assembled structure at the oil/water interface, which is closely related to the emulsion stability. Specifically, particle bilayers could form with oppositely charged particles at the interface to endow the emulsion with improved stability. As a demonstration, the resultant Pickering emulsions effectively carry ß-carotene and have high stability against high temperatures and ultraviolet radiation. This type of all-natural Pickering emulsion is a promising tool to protect and deliver liposoluble bioactive components.

Measurements of interactions between fluorescent molecules and polyethylene glycol self-assembled monolayers.

Blocking the non-specific binding of fluorescent biomolecules to substrates is one of the most important approaches to minimize the background noise in single-molecule fluorescence detection. Polyethylene glycol (PEG) and its derivatives are the most frequently used self-assembled monolayers (SAMs) for surface passivation because they are particularly effective to reduce the adsorption of a majority of biomolecules. Most studies related to PEG SAMs focus only on the interactions between biomolecules and substrates, while few reports exist in which the interactions between fluorophores and organosilane SAMs are directly examined. The objective of this study is to try to clarify the interactions between fluorescein isothiocyanate (FITC) and PEG SAMs at different ionic strengths. Total internal reflection microscopy (TIRM) was utilized for quantitative analysis of the interactions. At low ionic strength, long-range attractions between FITC-modified polystyrene-silica particles and PEG SAM grafting substrates were observed, even though both of them had an ensemble-averaged negative charge. The origin of this attraction could be correlated to their nonuniformly charged surfaces. At high ionic strength, van der Waals attraction at short distances was measured as the electrostatic interactions were completely screened. Due to the polarizability of the FITC molecule, the van der Waals attractions increased with the thickness of the PEG SAMs. This phenomenon is explained by the hydration shell of the PEG SAMs.

A facile evanescent-field imaging approach for monitoring colloidal gel evolution near a surface.

A facile evanescent-field imaging approach is developed to probe the aggregation behavior of near-wall colloids/clusters during colloidal gel evolution. Total internal reflection microscope (TIRM) images are directly utilized to access the structural relaxation time via density-fluctuation theory. The behaviors of cluster-cluster aggregation and physical aging of the colloidal gel networks are resolved in both time and space under fractal scaling criteria.

Pickering emulsions: Versatility of colloidal particles and recent applications.

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