Preparation and characterization of Pickering emulsion with directionally embedded antimicrobial peptide MOp2 and its preservation effect on grass carp

The peptide MOp2 obtained from Moringa oleifera seeds showed good antimicrobial activity. However, the stability of its activity has not yet been studied. In the present study, MOp2-loaded thiolated chitosan-stabilized (CMOp2) Pickering emulsion was prepared and applied to prolong the shelf life of grass carp. The encapsulation rate of MOp2 was 57.7% in CMOp2. In addition, the effects of different concentrations of CMOp2 solid particles and pH on droplet size, zeta optional and storage stability of Pickering emulsions were evaluated; the best condition for preparing Pickering emulsion through experiment was 1.75% CMOp2 solid particles at pH 9.5. Moreover, morphological observations and rheological analysis indicated that Pickering emulsions were considered a water-in-oil emulsion with typical non-Newtonian fluid characteristics. Furthermore, the prepared Pickering emulsion could significantly inhibit the growth of Escherichia coli and Staphylococcus aureus. Besides, Pickering emulsion effectively prevented spoilage of grass carp, and the Pickering emulsion-treated group reduced its pH, TVB-N and color values, inhibited microbial growth, and extended shelf life to 9 day at the storage of 4 °C. Overall, the present findings provide a reference for the application of MOp2-loaded Pickering emulsions in food preservation.

Light-responsive Pickering emulsions based on azobenzene-modified particles

Light-responsive particle-stabilised (Pickering) emulsions can in principle be selectively emulsified/demulsified on-demand through the remote application of light. However, despite their wide-ranging potential in applications such as drug delivery and biphasic catalysis, their rational design is extremely challenging and there are very few examples to date. Herein, we investigate a model system based on silica particles functionalised with azobenzene photoswitches to understand the key factors that determine the characteristics of light-responsive Pickering emulsions. The particle hydrophobicity is tuned through judicious variation of the spacer length used to graft the chromophores to the surface, the grafting density, and irradiation to induce trans-cis photoisomerisation. For select emulsions, and for the first time, a reversible transition between emulsified water-in-oil droplets and demulsified water and oil phases is observed with the application of either UV or blue light, which can be repeatedly cycled. A combination of surface energy analysis and optical microscopy is shown to be useful in predicting the stability, and expected light-response, of a given emulsion. Using the observed trends, a set of design rules are presented which will help facilitate the rational design, and therefore, more widespread application of light-responsive Pickering emulsions.

Pickering Emulsion Catalysis: Interfacial Chemistry, Catalyst Design, Challenges, and Perspectives

Pickering emulsions are particle-stabilized surfactant-free dispersions composed of two immiscible liquid phases, and emerge as attractive catalysis platform to surpass traditional technique barrier in some cases. In this review, we have comprehensively summarized the development and the catalysis applications of Pickering emulsions since the pioneering work in 2010. The explicit mechanism for Pickering emulsions will be initially discussed and clarified. Then, summarization is given to the design strategy of amphiphilic emulsion catalysts in two categories of intrinsic and extrinsic amphiphilicity. The progress of the unconventional catalytic reactions in Pickering emulsion is further described, especially for the polarity/solubility difference-driven phase segregation, "smart" emulsion reaction system, continuous flow catalysis, and Pickering interfacial biocatalysis. Challenges and future trends for the development of Pickering emulsion catalysis are finally outlined.

Preparation of Poly(ethylene glycol)@Polyurea Microcapsules Using Oil/Oil Emulsions and Their Application as Microreactors

The development process of catalytic core/shell microreactors, possessing a poly(ethylene glycol) (PEG) core and a polyurea (PU) shell, by implementing an emulsion-templated non-aqueous encapsulation method, is presented. The microreactors' fabrication process begins with an emulsification process utilizing an oil-in-oil (o/o) emulsion of PEG-in-heptane, stabilized by a polymeric surfactant. Next, a reaction between a poly(ethylene imine) (PEI) and a toluene-2,4-diisocyanate (TDI) takes place at the boundary of the emulsion droplets, resulting in the creation of a PU shell through an interfacial polymerization (IFP) process. The microreactors were loaded with palladium nanoparticles (NPs) and were utilized for the hydrogenation of alkenes and alkynes. Importantly, it was found that PEG has a positive effect on the catalytic performance of the developed microreactors. Interestingly, besides being an efficient green reaction medium, PEG plays two crucial roles: first, it reduces the palladium ions to palladium NPs; thus, it avoids the unnecessary use of additional reducing agents. Second, it stabilizes the palladium NPs and prevents their aggregation, allowing the formation of highly reactive palladium NPs. Strikingly, in one sense, the suggested system affords highly reactive semi-homogeneous catalysis, whereas in another sense, it enables the facile, rapid, and inexpensive recovery of the catalytic microreactor by simple centrifugation. The durable microreactors exhibit excellent activity and were recycled nine times without any loss in their reactivity.

Effect of Particle Wettability and Particle Concentration on the Enzymatic Dehydration of n-Octanaloxime in Pickering Emulsions

Pickering emulsion systems have emerged as platforms for the synthesis of organic molecules in biphasic biocatalysis. Herein, the catalytic performance was evaluated for biotransformation using whole cells exemplified for the dehydration of n-octanaloxime to n-octanenitrile catalysed by an aldoxime dehydratase (OxdB) overexpressed in E. coli. This study was carried out in Pickering emulsions stabilised solely with silica particles of different hydrophobicity. We correlate, for the first time, the properties of the emulsions with the conversion of the reaction, thus gaining an insight into the impact of the particle wettability and particle concentration. When comparing two emulsions of different type with similar stability and droplet diameter, the oil-in-water (o/w) system displayed a higher conversion than the water-in-oil (w/o) system, despite the conversion in both cases being higher than that in a "classic" two-phase system. Furthermore, an increase in particle concentration prior to emulsification resulted in an increase of the interfacial area and hence a higher conversion.

Catalysis in Pickering emulsions

Particle-stabilised or Pickering emulsions are versatile systems. In the past 10 years a new application has emerged in the field of catalysis to use them as vehicles to carry out catalytic reactions, allowing a more environmentally friendly process with high conversions and selectivities and important advantages for catalyst recovery. As the area has advanced rapidly, the intention of this review is to summarize the latest innovations being reported. An overview is given regarding the kinds of liquid phases comprising the emulsion system, the different types of solid particle stabilizers (whether they contain catalyst or not) and the catalytic reactions studied. A section describing methods for recovering the catalyst is also included, in which various stimuli are discussed. Finally, the importance of using Pickering emulsions to carry out reactions in flow and in multi-step cascade systems is highlighted with various examples to support the benefits of transferring this technology to industrial processes.

Inverse Pickering Emulsion Stabilized by Binary Particles with Contrasting Characteristics and Functionality for Interfacial Biocatalysis

Water-in-oil (w/o) Pickering emulsions have received considerable attention in biphasic enzymatic catalysis for their advantages of good stability, large interfacial area, and ease of product separation. However, enzymes are commonly encapsulated in the interior of aqueous droplets, which inevitably increases the diffusional resistance to catalysis. Alternatively, enzymes are immobilized or trapped into Pickering stabilizers. Often, however, these approaches suffer from leaching and a decrease of enzyme activity during the chemical treatments. We report here a new Pickering interfacial biocatalysis platform with efficient enzyme encapsulation, binary particle composition, and high catalytic performance. Our approach is based on w/o Pickering emulsions stabilized by binary particles consisting of hard silica and soft, pH-responsive microgel particles. We demonstrate that pH-responsive microgels can simultaneously stabilize a w/o Pickering emulsion, encapsulate enzymes, and catalyze reactions at the water/oil interface. In addition, we show that the coordination with rigid silica nanoparticles as additional stabilizers markedly improves the emulsion structure and will provide a new avenue for the preparation of w/o Pickering emulsion and concept of biphasic catalysis.

Static phase transfer catalysis for Williamson reactions: Pickering interfacial catalysis

A static PIC system with silica particles was prepared for PTC reactions and exhibited enhanced catalytic activity.