pH-responsive Pickering Emulsions Research Articles

Fabrication and Optimization of Pickering Emulsion Stabilized by Lignin Nanoparticles for Curcumin Encapsulation.

To develop reversible pH-responsive emulsifiers of natural origin, alkali lignin (AL) was used to develop oil-in-water Pickering emulsions. AL was first modified to synthesize quaternized alkali lignin (QAL), which displayed pH-responsive properties and demonstrated solubility in both acidic and alkaline solutions. In contrast, QAL exhibited insolubility and formed particles in neutral solutions, thereby making it a suitable candidate for utilization as an emulsifier in doubly pH-responsive Pickering emulsions. At pH 5-9, the emulsions were stable. Above or below this pH range, the system demulsifies, resulting in a reversible Pickering emulsifier with two pH-controlled transitions. On the basis of this pH-dependent behavior, lignin-based Pickering emulsions (LPE) could be subjected to several cycles of emulsification-demulsification by alternating the pH of the aqueous phase between basic and acidic, while the droplet size and storage stability were maintained. Curcumin was used as a drug model to study the loading/release behavior of LPE, finding that 50.08% of curcumin could be encapsulated in LPE. The in vitro release of curcumin was pH-dependent. In addition, LPE exhibited an outstanding protective effect against the ultraviolet-induced degradation of curcumin.

A pH switchable Pickering emulsion stabilised by controlled non-conventional lanthanum sulfide nanoparticles, in situ hydrophobized with a cationic surfactant

Microemulsion mediated controlled lanthanide metal based nanoparticles are employed in the preparation of a pH responsive Pickering emulsion.

PH-responsive Pickering emulsion containing citrus essential oil stabilized by zwitterionically charged chitin nanofibers: Physicochemical properties and antimicrobial activity

In this study, zwitterionic chitin nanofibers (Z-ChNFs) were used to prepare Pickering emulsions containing citrus essential oils (CEO) and their physicochemical properties and antimicrobial activity were investigated. Results show that as-prepared Pickering emulsions exert pH-reversible properties, pH can adjust the charge of Z-ChNFs to influence the stability of the emulsion. As the concentration of Z-ChNFs increase, the droplet size of the emulsion decreases. The high concentration of Z-ChNFs (1.5 wt%) can enhance the viscosity and promote forming nano-network structures within continuous phases, and their amphiphilic nature can strengthen the capacity for adsorption on the oil/water interface, resulting in enhanced physical stability of the encapsulated CEO emulsion. Additionally, Z-ChNFs have positive effects on the improvement of antimicrobial activity of CEO. This study provides valuable implications for the development and application of essential oils as biopreservation in the food field.

Silver Ions Drive Ordered Self-Assembly Mechanisms and Inherent Properties of Lignin Nanoflowers.

Designing anisotropic lignin-based particles and promoting the high-value utilization of lignin have nowadays drawn much attention from scientists. However, systematic studies addressing the self-assembly mechanisms of anisotropic lignin-based particles are scarce. In this work, an interaction including the electrostatic forces and chelating forces between lignin and Ag+ was regulated via carboxymethylation modification. Subsequently, the aggregation morphology of carboxymethylated lignin in a Ag+ solution was observed via SEM. The result showed that a large number of Ag+ intercalated into the lignin molecules when the grafting degree of the carboxyl groups increased from 0.17 mmol/g to 0.53 mmol/g, which caused the lignin molecules to gradually transform from disordered blocks to ordered layers. Dynamics research indicated that the adsorption process of Ag+ in carboxymethylated lignin conforms to the Pseudo-first-order kinetic model. The saturated adsorption amount of Ag+ in the carboxymethylated lignin reached 1981.7 mg/g when the grafting rate of carboxyl groups increased to 0.53 mmol/g, which then fully intercalated into lignin molecules and formed a layered structure. The thermodynamic parameters showed that the thermal adsorption process conforms to the Langmuir model, which indicates that Ag+ is monolayer-adsorbed and intercalated into lignin molecules. Meanwhile, the ΔH values are more than 0, which suggests that this adsorption process is a endothermic reaction and that a higher temperature is conducive to an adsorption reaction. Therefore, self-assembly of lignin in a Ag+ solution under 70 °C is more conducive to the formation of a nanoflower structure, which is consistent with our experimental result. Finally, pH-responsive Pickering emulsions were successfully prepared using a lignin-based nanoflowers, which demonstrated their potential as a catalytic platform in the interface catalysis field. This work offers a deeper understanding into the formation mechanism of anisotropic lignin-based nanoflowers and hopes to be helpful for designing and preparing anisotropic lignin-based particles.

Formulation and characterization of pH-responsive pickering emulsions stabilized by soy protein isolate/nicotinamide mononucleotide complexes for controlled drug release

pH-responsive emulsion stabilizer can be used as an ideal Pickering emulsion type drug embedding and sustained release. However, achieving an optimal balance of particle wettability and size, emulsion stability, and drug encapsulation efficiency remains challenging. This study delves into the potential of soy protein isolate (SPI) and nicotinamide mononucleotide (NMN) complexes as pH-responsive stabilizers for Pickering emulsions, aiming to optimize particle properties, emulsion stability, and drug encapsulation efficiency for biomedical applications. Our findings illustrate that 1.5 wt% NMN concentration yields the optimal SPI/NMN composite particle properties, contributing to enhanced emulsion stability. The stability was further improved at pH 7 with the addition of 0.01 mol/L sodium chloride. SPI/NMN complexes under these conditions formed a unique three-dimensional network within the continuous phase, evidenced by 3D-laser confocal microscopy. Furthermore, at this NMN concentration, the emulsion thermal stability index peaked at 73.2% above the initial value, accompanied by optimal water-holding capacity (82.5%) and surface coverage (89.23%). Exploring drug encapsulation, we observed that 2% ibuprofen dosage in the microspheres results in drug loading and encapsulation efficiencies of 33.5% and 89.6%, respectively. Importantly, the SPI/NMN-embedded ibuprofen exhibited pH-responsive behavior, with increased release in alkaline conditions and reduced release in acidic ones, signifying potential for pH-targeted drug delivery. The SPI/NMN-embedded ibuprofen showed pH-responsive behavior, with enhanced release at alkaline pH values and reduced release under acidic conditions, suggesting potential utility in pH-targeted drug delivery applications.

Gastrointestinal pH-Sensitive Pickering Emulsions Stabilized by Zein Nanoparticles Coated with Bioactive Glycyrrhizic Acid for Improving Oral Bioaccessibility of Curcumin.

Pickering emulsions have received considerable attention for their stability and functionality. Environmentally responsive Pickering emulsions could be used as vehicles for oral administration. However, challenges still exist, such as nonbiocompatibility of emulsifier and mismatched response behavior in the gastrointestinal environment. In this study, a strategy was proposed that bioactive saponin glycyrrhizic acid (GA) was used as a pH-responsive substance to functionalize zein nanoparticles, and tannic acid (TA) was used as a primer for cross-linking GA and zein nanoparticles. The Pickering emulsions fabricated by zein/TA/GA nanoparticles (ZTGs) exhibited excellent stability at acid conditions while slowly demulsifying at neutral conditions, which can be further used as an intestine-targeted delivery system. Curcumin was encapsulated into ZTG-stabilized Pickering emulsions, and the encapsulation efficiency results suggested that the presence of GA coating remarkably facilitated the encapsulation of curcumin. An in vitro digestion study suggested that ZTGs provided protection for emulsions from pepsin hydrolysis and exhibited higher free fatty acid release as well as higher bioaccessibility of curcumin during simulated intestine digestion. This study provides an effective strategy to prepare pH-responsive Pickering emulsions for improving the oral bioaccessibility of hydrophobic nutraceuticals.

Tuning interfacial microstructure of alginate-based amphiphile by dynamic bonding for stabilizing Pickering emulsion

Polysaccharide-based soft colloidal particles mediated by the dynamic bonding-engineered interfacial self-assembly can regulate the properties of oil-water interfacial films, availing the stability of emulsions under a wide pH range. The amphiphilic phenylboronic alginate soft colloidal particles (Alg-PBA) were designed to stabilize pH-responsive Pickering emulsions (PEs). Combining stability analysis with quartz crystal microbalance and dissipation monitoring (QCM-D), the microstructure and viscoelasticity of Alg-PBA at the oil-water interface were determined. The results showed that PEs stabilized by Alg-PBA due to a thicker and stronger viscoelastic interface film induced by BO bonds and hydrogen bonds. The structure-function relationship of the Alg-PBA emulsifier driven by dynamic bonds was further elaborated at multiple scales by laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Meanwhile, the microstructure of aerogels templated by emulsion could be tuned by adjusting dynamic bonds, which provides a new idea for polysaccharide soft material engineering.

Fabrication and characterization of octenyl succinate anhydride modified amylose with pH-responsive Pickering emulsion behavior

To make stable Pickering emulsion in stomach acid condition which are suitable for small intestine targeted delivery, the emulsifying ability and pH responsiveness mechanisms of octenyl succinate anhydride modified amylose (OSA-AM) with the formless state and nanoparticles (NP) form in the Pickering emulsion were compared. OSA-AMs and OSA-AM NPs were obtained by successively modification of OSA esterification reaction with amylose and nanoprecipitation process, respectively. OSA-AM NPs showed higher contact angle and lower interfacial tension than OSA-AMs, which suggested OSA-AM NPs have the stronger ability to form stable Pickering emulsion. In addition, to compare the stability of Pickering emulsion in different environment, the emulsion index (EI), photographs and microscopy images during storage time of 180 days in pH 2.0, pH 4.0 and pH 7.0 were monitored. Pickering emulsion formed by OSA-AM NPs exhibited stronger stability in acid environment (pH 2.0) than pH 4.0 and pH 7.0. However, Pickering emulsion stabilized by OSA-AMs presented the opposite pH-responsive behaviors with OSA-AM NPs. To further study the pH responsiveness mechanisms of Pickering emulsion, the morphology, contact angle, particle size and surface charge of OSA-AMs and OSA-AM NPs with pH changing were measured. These results suggested that the protonation/deprotonation process of carboxyl groups in difference pH condition revealed the pH-responsible behaviors of Pickering emulsion.

Diblock copolymer worms stabilized pH-responsive Pickering emulsions: An efficient and recyclable platform for Claisen-Schmidt condensation reaction

Pickering emulsions were promising platforms for the heterogeneous reactions due to both their large interfacial areas and high stability, whereas both the product separation and the emulsifier recovery via simple and efficient strategies for remain a concern. In this study, we constructed an efficient and recyclable platform for interfacial catalytic reaction adopting pH-responsive n-decane-in-water Pickering emulsions. Pickering emulsions were prepared employing core cross-linked PEO45-PGMA74 (CCE45G74) diblock copolymer worms as Pickering emulsifiers. CCE45G74 diblock copolymer worms were prepared via the combination of photo-initiated polymerization induced self-assembly strategy and the core covalent cross-linking with ethylenediamine. The prepared n-decane-in-water Pickering emulsions showed a high stability during the storage at the room temperature, whereas a rapid demulsification took place at a lower pH, such as 2.5. Alkali-catalyzed Claisen-Schmidt condensation reactions between benzaldehyde and acetone were conducted in n-decane-in-water Pickering emulsions at 30 °C. The yield of dibenzylidene acetone could reach 99% after 12 h of reaction. The reaction efficiency was far higher than that at the plain n-decane-water (∼30%). This catalytic platform could be repeatedly used for three cycles.

PH-Responsive Pickering Emulsions Synergistically Stabilized by Maleopimaric Acid and Alumina Nanoparticles

pH-Responsive Pickering Emulsions Synergistically Stabilized by Maleopimaric Acid and Alumina Nanoparticles

Amphiphilicity-adaptable graphene quantum dots to stabilize pH-responsive pickering emulsions at a very low concentration

Hypothesis: Stimuli-responsive Pickering emulsions have attracted considerable interest due to their widespread potential applications. Especially pH-responsive behavior could be easily implemented. In this work, we reported a pH-responsive Pickering emulsion based on amphiphilic graphene quantum dots at a low concentration which shows a great potential from the environmental and economic perspective. The stimuli responsive properties would make the smart Pickering emulsifiers recyclable and reusable.Experiments: The amphiphilic-adaptable graphene quantum dots functionalized by alkyl groups (C-GQDs) were synthesized by a facile one-step pyrolysis method. The pH-responsive emulsion performances were investigated, and the mechanism of pH-responsive of C-GQDs was studied by dynamic light scattering.Findings: The amphiphilicity of C-GQDs could be acquired controllably and effectively by this facile one-step pyrolysis method, which are able to stabilize Pickering emulsion at a very low concentration (0.001%). The amphiphilicity of C-GQDs are capable of changing in response to environmental stimuli. When the pH value of aqueous solution adjusts to 2, these C-GQDs aggregate in contrast to their stability in neutral condition due to the alternation of surface charges. The pH-responsive aggregation/ dispersion behavior of C-GQDs allows us to tune the interactions between oil-in-water emulsion droplets without introduction of destabilization agents. This will provide huge economic benefits in industrial applications in the future.

PH-Responsive Pickering emulsions stabilized solely by surface-inactive nanoparticles via an unconventional stabilization mechanism.

Using solely highly hydrophilic particles to stabilize emulsions, especially high internal phase emulsions, has always been an important challenge. Here pH-responsive Pickering emulsions stabilized by a low concentration of bare highly hydrophilic Ludox CL nanoparticles without surface modification or addition of surfactants are developed at neutral pH. The dispersed nanoparticles can be transformed into an aggregate state with a network-like structure near the isoelectric point, which contributes to the stabilization of the emulsions. Moreover, the vdW attraction between particles and droplets also plays a key role in the formation of emulsions, which can make the aggregated nanoparticles adsorb tightly around the droplets rather than penetrate the oil-water interface. The formed protective armor and network-like aggregates separate droplets from each other to prevent coalescence. At a low nanoparticle concentration (0.5 wt%), a high internal phase emulsion can be formed and can last up to half a year. This system can emulsify not only the hydrocarbon oil but also the fluoroalkane oil phase. Finally, organic-inorganic composite particles are fabricated using the template action of the Pickering emulsions. The method of preparing composite particles is more convenient than the traditional Pickering emulsion polymerization which often requires the modification of the surface of the hydrophilic particles or the addition of auxiliary monomers. This study provides a simple green strategy for the preparation of a more stable Pickering emulsion stabilized by surface-inactive nanoparticles and will broaden the scope of applications.

Stability of pH-responsive Pickering emulsion stabilized by carboxymethyl starch/xanthan gum combinations

The objective of this study was to investigate the effect of different pH values (3, 5, 7, 9 and 11) on the stability of Pickering emulsions stabilized by carboxymethyl starch (CMS)/xanthan gum (XG) combinations. The CMS/XG combinations could be used as an effective stabilizer for the preparation of Pickering emulsion. The effects of CMS/XG combinations on oil droplet size, Zeta potential, rheological properties, backscattering profiles, Low-field nuclear magnetic resonance imaging and storage stability of oil-in-water emulsion were evaluated as a function of pH. As pH increased from 3 to 11, the droplet size of the emulsion increased and the absolute value of Zeta potential decreased. Besides, the consistency coefficient of the emulsion was reduced and the pseudoplasticity was weakened. The variation of backscattered light intensity of the emulsion at pH 3 was the smallest. The combination of CMS and XG formed a good network structure and inhibited the phase separation of emulsion. The color distribution of Low-field nuclear magnetic resonance imaging of emulsions at pH 3, 5 and 7 was more uniform and had better storage stability at 25 °C for 9 days. pH affected the aggregation of oil droplets. The Pickering emulsion stabilized by CMS/XG combinations was pH-responsive and could be used as an effective carrier for biologically active substances.

Multistimuli-Responsive Pickering Emulsion Stabilized by Se-Containing Surfactant-Modified Chitosan.

Particle-stabilized emulsions that can respond to external stimuli have attracted significant concerns due to their intelligent-controlled stability, whereas particle-stabilized Pickering emulsions responding to multistimuli but based on biomass have been rarely reported. Here, a multistimuli-responsive Pickering emulsion was developed using the modified chitosan as stabilizer. Due to electrostatic attraction, Se-containing anionic surfactant, sodium 11-(butylselenyl)undecylsulfate (C4SeC11S), can bind with CS at an acidic pH and form CS-C4SeC11S complexes which can further self-associate to form micrometer-sized particles with the character of partially hydrophobicity. Therefore, at pH < pKa, an oil-in-water Pickering emulsion can be formed using CS-C4SeC11S particles as stabilizers and can spontaneously respond to redox, ion, and pH. First, with the addition of oxidation, the hydrophilicity of C4SeC11S was enhanced, and thus, hydrophobic association of CS-C4SeC11S decreased, leading to the disruption of CS-C4SeC11S particles. Hence, the emulsion destabilized. The demulsification process is closely related with the dosage of oxidant and the oxidation time. Second, introduction of a competitive ion (e.g., CTAB) could break the binding between C4SeC11S and CS, leading to the disruption of particle emulsifier. Thereby, demulsification occurred. Third, with sequentially increasing/decreasing pH, the emulsion can be switched from stable to unstable and then to stable again accordingly. Such a unique pH-responsive behavior has never been discovered in other pH-responsive Pickering emulsions. All of the stimuli-responsive behaviors were reversible. Upon alternately adding oxidant/reductant, CTAB/C4SeC11S, or base/acid, the current emulsion can be reversibly switched off (destabilization) and on (stabilization). Such a Pickering emulsion may be a good candidate as a vehicle of functional ingredient.

PH-Responsive Pickering emulsion stabilized by polymer-coated silica nanoaggregates and applied to recyclable interfacial catalysis.

We first synthesized a diblock copolymer poly[tert-butyl methacrylate]-b-poly[3-(trimethoxysilyl)propyl methacrylate] (PtBMA-b-PTMSPMA) through reversible addition–fragmentation chain transfer (RAFT) living radical polymerization and grafted it onto fumed silica by converting the PTMSPMA segment to silanol and the PtBMA segment to polymethylacrylic acid (PMAA) in the presence of trifluoroacetic acid in order to obtain PMAA brush-coated silica nanoaggregates P-Si. TEM, DLS, FTIR, and TGA results confirmed the successful modification of the starting materials. The nanoaggregates flocculated and stabilized a toluene-in-water Pickering emulsion at low pH, while the nanoaggregates were well dispersed in water and broke the emulsion under both neutral and basic conditions. Alternatively, the addition of acid/base induced emulsification/demulsification cycles that were sustained for several cycles. Moreover, when the P-Si was mixed with Rh-loaded silica, Rh-Si, the mixture had the same pH-responsive Pickering emulsion behavior as the single P-Si. This Pickering emulsion system can be used in the biphasic interfacial catalytic hydrogenation of olefins and had excellent yields under a hydrogen atmosphere. The yield of Pickering emulsion catalysis rapidly reached more than 99% in 3 h, while that of the demulsified mixture failed to reach 20% in 4 h, which verified the promotion of catalysis by the Pickering emulsion. Base-induced demulsification can be used to separate the products and recycle the catalyst. This pH-responsive Pickering emulsion catalytic system was capable of several cycles of reuse, and there was no significant decrease in catalytic efficiency even after eight cycles.

Preparation and characterization of pH-responsive Pickering emulsion stabilized by grafted carboxymethyl starch nanoparticles

Copolymer nanoparticles with pH-responsive properties were prepared by grafting different amounts of 2-(dimethylamine) ethyl methacrylate (DMAEMA) on carboxymethyl maize starch (CMS). The formation of CMS-g-DMAEMA was verified by different techniques and was then used for stabilization of Pickering emulsion at different pH values. The CMS-g-DMAEMA nanoparticles showed pH-responsive properties in Pickering emulsion and the pH-responsivity increased as the amount of DMAEMA increased in the CMS-g-DMAEMA copolymer. The prepared Pickering emulsion underwent emulsion/demulsification transitions at different pH values. The microscopy analysis verified the attachment of CMS-g-DMAEMA on oil-water interface of Pickering emulsion. Pickering emulsion showed different droplets size, contact angle, and Zeta-potential depend on pH value and CMS/DMAEMA ratio in the CMS-g-DMAEMA copolymer. The obtained results revealed that CMS-g-DMAEMA copolymer can be successfully used for preparing and stabilizing of Pickering emulsion at pH 10.

Novel amphiphilic cellulose nanocrystals for pH-responsive Pickering emulsions

Development of a green, recyclable emulsifier for pH-responsive Pickering emulsion would be of great importance to many industries. To this end, a novel emulsifier, benzyl-polyethyleneimine modified cellulose nanocrystals (Ben-PEI-CNCs), was developed via the periodate oxidation of cellulose nanocrystals and reductive amination. Ben-PEI-CNCs possess pH-responsive amphiphilicity due to the existence of hydrophilic amino and hydrophobic benzyl groups. The Pickering emulsions stabilized by Ben-PEI-CNC2 and Ben-PEI-CNC18 are very responsive to pH changes, and adjusting the pH from 3 to 7 effectively triggers oil-water separation and emulsification. Additionally, cyclic testing establishes the robustness of this process. Overall, this study demonstrates that Ben-PEI-CNCs can promote the transition from a stable emulsion to an unstable emulsion by adjusting the pH, allowing the recovery of oil and the recycling of the emulsifier.

Doubly pH Responsive Emulsions by Exploiting Aggregation of Oppositely Charged Nanoparticles and Polyelectrolytes.

We present a simple and modular approach to realize highly stable pH responsive Pickering emulsion from mixtures of commercially available oppositely charged nanoparticle and polyelectrolyte. While highly charged nanoparticles and polyelectrolytes when used solely do not stabilize emulsions, we show that the electrostatic attraction between oppositely charged nanoparticles and polyelectrolytes can be exploited to formulate emulsions with long-term stability of up to 8 months. The Ludox CL nanoparticles and poly(4-styrenesulfonate) sodium salt (PSS) when dispersed in aqueous solution at pH 2-11 form particle polyelectrolyte complexes (PPCs) due to heteroaggregation. These complexes are effective in stabilizing oil-in-water Pickering emulsions. We demonstrate that this is due to the formation of weakly charged complexes that are surface active and hence readily adsorbed to the oil-water interface created during emulsification. We show that the composition of nanoparticles and polyelectrolytes in the mixture as well as the pH can be tuned to control the average diameter of the emulsions droplets. Immediate destabilization and doubled responsiveness of the emulsions stabilized by particle polyelectrolyte complexes are illustrated by changing the pH of the stable emulsions formed at intermediate pH to either 1 or 13. The aggregation behavior of nanoparticle-polyelectrolyte mixtures and the effect of various parameters such as mixing fraction, pH, and energy input on the formation of Pickering emulsions is discussed. Furthermore, we show that the formation of near charge neutral aggregates that exhibit optimal wetting conditions is a requirement to accomplish emulsion formation. The visualization of particle polyelectrolyte complexes around the emulsion droplets, their morphology prior to emulsification, and their wetting properties are also investigated to elucidate the mechanism of emulsification.

PH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant.

pH-responsive oil-in-water Pickering emulsions were prepared simply by using negatively charged silica nanoparticles in combination with a trace amount of a zwitterionic carboxyl betaine surfactant as stabilizer. Emulsions are stable to coalescence at pH ≤ 5 but phase separate completely at pH > 8.5. In acidic solution, the carboxyl betaine molecules become cationic, allowing them to adsorb on silica nanoparticles via electrostatic interactions, thus hydrophobizing and flocculating them and enhancing their surface activity. Upon increasing the pH, surfactant molecules are converted to zwitterionic form and significantly desorb from particles' surfaces, triggering dehydrophobization and coalescence of oil droplets within the emulsion. The pH-responsive emulsion can be cycled between stable and unstable many times upon alternating the pH of the aqueous phase. The average droplet size in restabilized emulsions at low pH, however, increases gradually after four cycles due to the accumulation of NaCl. Experimental evidence including adsorption isotherms, zeta potentials, microscopy, and three-phase contact angles is given to support the postulated mechanisms.

High-yield preparation of a zwitterionically charged chitin nanofiber and its application in a doubly pH-responsive Pickering emulsion

A rapid and high-yield (up to 85%) preparation of a zwitterionic chitin nanofiber (DE-CO-ChNF) has been demonstrated.