Zengjiang Wei

Simple, Reversible Emulsion System Switched by pH on the Basis of Chitosan without Any Hydrophobic Modification

Chitosan without hydrophobic modification is not a good emulsifier itself. However, it has a pH-tunable sol-gel transition due to free amino groups along its backbone. In the present work, a simple reversible Pickering emulsion system based on the pH-tunable sol-gel transition of chitosan was developed. At pH > 6.0, as adjusted by NaOH, chitosan was insoluble in water. Chitosan nanoparticles or micrometer-sized floccular precipitates were formed in situ. These chitosan aggregates could adsorb at the interface of oil and water to stabilize the o/w emulsions, so-called Pickering emulsions. At pH < 6.0, as adjusted by HCl, chitosan was soluble in water. Demulsification happened. Four organic solvents (liquid paraffin, n-hexane, toluene, and dichloromethane) were chosen as the oil phase. Reversible emulsions were formed for all four oils. Chitosan-based Pickering emulsions could undergo five cycles of emulsification-demulsification with only a slight increase in the emulsion droplet size. They also had good long-term stability for more than 2 months. Herein, we give an example of chitosan without any hydrophobic modification to act as an effective emulsifier for various oil-water systems. From the results, we have determined that natural polymers with a stimulus-responsive sol-gel transition should be a good particulate emulsifier. The method for in situ formation of pH-responsive Pickering emulsions based on chitosan will open up a new route to the preparation of a wide range of reversible emulsions.

Autonomous self-healing of poly(acrylic acid) hydrogels induced by the migration of ferric ions

A facile and versatile strategy was developed for the preparation of self-healing hydrogels containing double networks of both physically and chemically cross-linked polymers. The autonomous self-healing of the hydrogel was achieved through the dynamic bonding of physical cross-linking and the migration of ferric ions.

Hydrodynamically driven self-assembly of giant vesicles of metal nanoparticles for remote-controlled release.

The hydrodynamics of laminar flow in a microfluidic device has been used to control the continuous self-assembly of gold nanoparticles (NPs) tethered with amphiphilic block copolymers. Spherical micelles, giant vesicles (500 nm-2.0 μm), or disk-like micelles could be formed by varying the flow rates of fluids. Such vesicles can release encapsulated hydrophilic species by using near-IR light.

Alkaline lignin extracted from furfural residues for pH-responsive Pickering emulsions and their recyclable polymerization

Recycling of waste leads to a decrease of environmental pollution and landfill. In this work, we describe an interesting application of furfural residues in fabricating pH-responsive Pickering emulsions and their stable emulsion polymerization. Alkaline lignin extracted from furfural residues is soluble in basic water. However, in acidic conditions, lignin becomes insoluble and particles are formed, which could be used as an effective particular emulsifier for reversible styrene-in-water Pickering emulsions. The emulsions will break when the pH value of the aqueous phase increases. The emulsion system based on a small amount of lignin, 0.05 wt% in water, exhibits good pH-responsivity. Lignin-coated polystyrene (PS) microparticles and pure PS microparticles have been fabricated using the emulsion template. Furthermore, this lignin emulsifier can be recirculated after polymerization. Finally, we propose a cheap and recyclable route for industrial emulsion polymerization. This work opens up new possibilities for agricultural residues as a valuable and recyclable emulsifier in green chemistry.

Versatile fabrication of nanocomposite microcapsules with controlled shell thickness and low permeability.

Novel ethyl phenylacetate (EPA)-loaded nanocomposite microcapsules with polyurea (PU) /poly (melamine formaldehyde) (PMF) shells were facilely and fabricated: by using silica nanoparticle-stabilized oil-in-water (o/w) emulsion template and subsequent interfacial reaction and in situ polymerization. SiO2 nanoparticles absorbed at the interface between oil and water to stabilize the o/w emulsions. The oil droplets containing EPA, isophorone diisocyanate (IPDI) and tolylene 2,4-diisocyanate-terminated poly (propylene glycol) (PPG-TDI) were subsequently reacted with MF prepolymer (pre-MF) dissolved in water phases. The interfacial reaction between pre-MF and IPDI produced interior PU walls. Meanwhile, the in situ polymerization of pre-MF generated exterior PMF walls. It was found that these in/out double walls were compact together. The resulting capsules had spherical shapes and rough exterior surfaces, and could be easily isolated, dried, and redispersed in epoxy resins. The size of the produced microcapsules was dependent on the concentration of SiO2 nanoparticles. The dynamic thermal gravimetric analysis (TGA) demonstrated that the capsules showed excellent thermal stability with little weight loss when exposed at 150 °C for 2 h. Interestingly, with a double PU/PMF shell, these capsules exhibited an extra-low permeability. Moreover, these microcapsules can also demonstrate exceelent magnetic responsiveness after introducing magnetic nanoparticles inside. We believe our microcapsules could be potential candidates in microcapsule engineering, self-healing composites, and drug-carrying systems.

Vesicular self-assembly of colloidal amphiphiles in microfluidics.

Hydrodynamic flow in a microfluidic (MF) device offers a high-throughput platform for the continuous and controllable self-assembly of amphiphiles. However, the role of hydrodynamics on the assembly of colloidal amphiphiles (CAMs) is still not well understood. This Article reports a systematic study of the assembly of CAMs, which consist of Au nanoparticles (AuNPs) grafted with amphiphilic block copolymers, into vesicles with a monolayer of CAMs in the membranes using laminar flows in MF flow-focusing devices. Our experimental and simulation studies indicate that the transverse diffusion of solvents and colloids across the boundary of neighboring lamellar flows plays a critical role in the assembly of CAMs into vesicles. The dimension of the vesicles can be controlled in the range of 100-600 nm by tuning the hydrodynamic conditions of the flows. In addition, the diffusion coefficient of CAMs was also critical for their assembly. Under the same flow conditions, larger CAMs generated larger assemblies as a result of the reduced diffusion rate of large amphiphiles. This work could provide fundamental guidance for the preparation of nanoparticle vesicles with applications in bioimaging, drug delivery, and nano- and microreactors.

Macroporous antibacterial hydrogels with tunable pore structures fabricated by using Pickering high internal phase emulsions as templates

Artemisia argyi oil (AAO)-loaded macroporous antibacterial hydrogels were prepared by polymerization of oil-in-water Pickering high internal phase emulsions (HIPEs). The HIPEs were stabilized by the synergy of hydrophilic silica nanoparticles (N20) and surfactant Tween 80. The void interconnectivity and pore size of the hydrogels could be tailored readily by varying the concentrations of N20 nanoparticles and Tween 80. The mechanical properties of the porous hydrogels were related to the pore structure of the materials. There was an optimal condition for the N20 particle and Tween 80 contents where the hydrogel exhibited high compressive stress and strain. The in vitro release of the AAO-loaded hydrogels with different inner morphologies was evaluated and showed controlled release activity. The antibacterial activity of the AAO-loaded hydrogel was evaluated against Staphylococcus aureus and Escherichia coli. This kind of hydrogel exhibited excellent and long-term antibacterial activity indicating its potential use in biomedical and infection prevention applications.

Fabrication of degradable polymer microspheres via pH-responsive chitosan-based Pickering emulsion photopolymerization

Pickering emulsion stabilized by pH-reversible chitosan is developed to prepare degradable polymer microspheres by emulsion photopolymerization, where chitosan acts as a green and recyclable particulate emulsifier. The thiol–ene photopolymerization of trimethylolpropane tris(3-mercaptopropionate) and trimethylolpropane triacrylate is initiated by UV irradiation. Chitosan adsorbed at the surface of the microspheres can be recycled for polymerization at least three times. Moreover, ibuprofen (IBU) is loaded into the microspheres. The higher the release temperature or pH value, the faster release rate and higher release extent of IBU from the microspheres are found. Meanwhile, the resulting microspheres exhibit a good degradability in 1 M NaOH aqueous solution, with a weight loss of about 90 wt% after 35 days. This study demonstrates a potential green and recyclable application of chitosan in fabrication of degradable polymer microspheres.

Synergistic Stabilization of High Internal Phase Pickering Emulsions by a Mixture of Nanoparticle and Polymer

In this paper, we have prepared high internal phase Pickering emulsions (Pickering HIPEs) with internal phase volume fraction 90% which are stabilized by a mixture of silica nanoparticles (H30) and poly(L-lactide-co-glycolide) (PLGA). HIPEs with an internal phase volume not more than 75% could be prepared solely by H30 nanoparticles. Using PLGA as stabilizer, phase separation takes place and stable emulsions are not formed. Synergistic interaction between H30 particles and PLGA molecules plays an im- portant part in preparation of HIPEs. Using inorganic particles and polymer as co-stabilizer will be a new and effective method to prepare HIPEs.