Interaction between stabilized droplets of different phases in the same continuous phase of an aqueous three-phase system.

Water-in-water (W/W) emulsions, also called aqueous two-phase systems, are formed by mixing two incompatible polymers in water that phase separate into two distinct phases. They can be stabilized by addition of colloidal particles. Droplets of the dispersed phase can be used to compartmentalize ingredients and induce localized reactions. By mixing more types of incompatible polymers, emulsions containing droplets of different phases can be formed that can potentially capture different ingredients. Here the interaction between dispersed droplets of different types was studied by gently mixing a W/W emulsion containing droplets rich in dextran (DEX) dispersed in a continuous phase rich in polyethylene oxide with an emulsion containing droplets rich in fish gelatin (GEL) dispersed in the same continuous medium. Bis-hydrophilic microgels (MG) consisting of DEX grafted with poly(N-isopropylacrylamide) were added and their effect on the stability of each binary emulsion was investigated. Interestingly, when two very stable emulsions were gently mixed, droplets of different types were observed with confocal scanning laser microscopy to coalesce immediately upon contact. In this manner, Janus-type droplets were formed containing a DEX and a GEL compartment with no MG at the GEL/DEX interface that further associated into strings of alternating droplets. Contact angles between the different phases in emulsions with and without MG were compared and used to determine the effect of the microgels on the interfacial tension between the phases.

Dynamic Covalent Chemistry Enables Reconfigurable All-Polysaccharide Nanogels.

Dynamic covalent bonds are established upon molecular recognition of sugar derivatives by boronic acid molecules. These reversible links can be used in a cross-linking method to fabricate polymer-based responsive nanosystems. Herein, the design of the first dynamic nanogels made entirely of polysaccharides (PS) is reported. Based on PS chains alternately modified with phenyl boronic acid groups and sugar moieties, these colloids self-assemble in physiological conditions and combine the biocompatible nature of their PS backbone with the reconfiguration capacities of their cross-linking chemistry. These dynamic nanogels are easily prepared, stable for a long time, pH responsive, and efficiently internalized by cancer cells.

Modulation of Wetting Gradients by Tuning the Interplay between Surface Structuration and Anisotropic Molecular Layers with Bipolar Electrochemistry.

A new simple and versatile method for the preparation of surface-wetting gradients is proposed. It is based on the combination of electrode surface structuration introduced by a sacrificial template approach and the formation of a tunable molecular gradient by bipolar electrochemistry. The gradient involves the formation of a self-assembled monolayer on a gold surface by selecting an appropriate thiol molecule and subsequent reductive desorption by means of bipolar electrochemistry. Under these conditions, completion of the reductive desorption process evolves along the bipolar surface with a maximum strength localized at the cathodic edge and a decreasing driving force towards the middle of the surface. The remaining quantity of surface-immobilized thiol, therefore, varies as a function of the axial position, resulting in the formation of a molecular gradient. The surface of the bipolar electrode is characterized at each step of the modification by recording heterogeneous electron transfer. Also, the evolution of static contact angles measured with a water droplet deposited on the surface directly reveals the presence of the wetting gradient, which can be modulated by changing the properties of the thiol. This is exemplified with a long, hydrophobic alkane-thiol and a short, hydrophilic mercaptan.

Two-Dimensional Electrochemiluminescence: Light Emission Confined at the Oil-Water Interface in Emulsions Stabilized by Luminophore-Grafted Microgels.

We describe a method to confine electrochemiluminescence (ECL) at the oil-water interface of emulsion droplets that are stabilized by luminophore-grafted microgels. These hydrogel nanoparticles incorporating covalently bound Ru(bpy)32+ as the luminophore are irreversibly adsorbed at the interface of micrometric oil droplets dispersed in a continuous aqueous phase. We study the electrochemical and ECL properties of this multiscale system, composed of a collection of droplets in close contact in the presence of two types of model coreactants. ECL emission is observed upon oxidation of the coreactant and of the luminophore. ECL imaging confirms that light is emitted at the surface of oil droplets. Interestingly, light emission is observed more than 100 µm far from the electrode. It is possibly due to the interconnection between redox-active microgels, making an entangled two-dimensional network at the dodecane-water interface and/or to some optical effects related to the light propagation and refraction at different interfaces in this multiphasic system. Confining ECL in such an inhomogeneous medium should find promising applications in the study of compartmentalized systems, interfacial phenomena, sensors, and analysis of single oil droplets.

Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density.

Poly(N-isopropylacrylamide) (pNIPAM) microgels are soft and deformable particles, which can adsorb at liquid interfaces. In the present paper, we study the two-dimensional organization of charged and quasi-neutral microgels with different cross-linking densities, under compression at the air-water interface and the transfer of the microgel monolayer onto a solid substrate at different surface pressures. At low cross-linking densities, the microgels form highly ordered hexagonal lattices on the solid substrate over large areas, with a unique lattice parameter that decreases continuously as the surface pressure increases. We thus prove that the microgel conformation evolves at the air-water interface. The microgels undergo a continuous transition from a highly flattened state at low surface coverage, where the maximal polymer segments are adsorbed at the interface, to entangled flattened microgels, and finally the thickening of the layer up to a dense hydrogel layer of compacted microgels. Moreover, two batches of microgels, with and without charges, are compared. The contribution of electrostatic interactions is assessed via changing the charge density of the microgels or modulating the Debye length. In both cases, electrostatics does not change the lattice parameter, meaning that, despite the microgel different swelling ratio, charges do not affect neither interactions between particles at the interface nor microgels adsorption. Conversely, the cross-linking density has a strong impact on microgel packing at the interface: increasing the cross-linking density strongly decreases the extent of microgel flattening and promotes the occurrence of coexisting hexagonally ordered domains with different lattice parameters.

Wireless Synthesis and Activation of Electrochemiluminescent Thermoresponsive Janus Objects Using Bipolar Electrochemistry.

In this work, bipolar electrochemistry (BPE) is used as a dual wireless tool to generate and to activate a thermoresponsive electrochemiluminescent (ECL) Janus object. For the first time, BPE allows regioselective growth of a poly(N-isopropylacrylamide) (pNIPAM) hydrogel film on one side of a carbon fiber. It is achieved thanks to the local reduction of persulfate ions, which initiate radical polymerization of NIPAM. By controlling the electric field and the time of the bipolar electrochemical reactions, we are able to control the length and the thickness of the deposit. The resulting pNIPAM film is found to be swollen in water at room temperature and collapsed when heated above 32 °C. We further incorporated a covalently attached ruthenium complex luminophore, Ru(bpy)32+, in the hydrogel film. In the second time, BPE is used to activate remotely the electrogenerated chemiluminescence (ECL) of the Ru(bpy)32+ moieties in the film. We take advantage of the film responsiveness to amplify the ECL signal. Upon collapse of the film, the ECL signal, which is sensitive to the distance between adjacent Ru(bpy)32+ centers, is strongly amplified. It is therefore shown that BPE is a versatile tool to generate highly sophisticated materials based on responsive polymers, which could lead to sensitive sensors.

Antagonistic effects leading to turn-on electrochemiluminescence in thermoresponsive hydrogel films.

We report the effects of the swell-to-collapse transition on the electrochemical and electrochemiluminescence (ECL) properties of thermoresponsive pNIPAM films incorporating covalently-attached Ru(bpy)32+ luminophores. Upon the collapse of the film, the number of electrochemically-active Ru(bpy)32+ centers increases, due to the reduced distance between adjacent redox centers. To generate ECL, cationic and anionic coreactants are employed, which are free to diffuse in the medium. In both cases, the ECL intensity exhibits a remarkable amplification of up to 58-fold at the swell-to-collapse transition, whereas the oxidation current undergoes a steep decrease in the case of the cationic coreactant. Indeed, the diffusion of the coreactant is hindered by the collapse of the film. The fact that ECL emission is enhanced whereas less coreactant radicals are generated is an intriguing observation because it is classically believed that the higher the oxidation rate of the coreactant the stronger the ECL emission. Thanks to the study of such films where antagonistic effects occur, we demonstrate that ECL enhancement is neither correlated with the oxidation efficiency of the coreactant nor with the hydrophilic-hydrophobic transition, but solely due to film shrinking. The decrease of the distance between adjacent redox centers, which favors better electron-transfer processes, is found to be the main parameter governing the ECL enhancement. Finally, it is noteworthy that the present turn-on ECL signal with increasing temperature is unique compared to many thermoresponsive luminescent systems.

Impact of electrostatics on the adsorption of microgels at the interface of Pickering emulsions.

The importance of electrostatics on microgel adsorption at a liquid interface is studied, as well as its consequence on emulsion stabilization. In this work, poly(N-isopropylacrylamide) (pNIPAM) microgels bearing different numbers of charges and various distribution profiles are studied, both in solution and at the oil-water interface of emulsion drops. Charged microgels are compared to neutral ones, and electrostatic interactions are screened by adding salt to the aqueous solution. In solution, electrostatics has a significant impact on microgel swelling, as induced by the osmotic pressure exerted by mobile counterions in the gel network. At the interface of drops, microgels pack in a hexagonal array, whose lattice parameter is independent of the number of charges and range of electrostatic interactions. Microgel morphology and packing are ruled only by the adsorption of the pNIPAM chain at the interface. Conversely, decreasing the charge density of microgels by the protonation of the carboxylic groups leads to unstable emulsions, possibly as a result of the impact of hydrogen bonding on microgel deformability.

Impact of pNIPAM microgel size on its ability to stabilize Pickering emulsions.

We study the influence of the particle size on the ability of poly(N-isoprolylacrylamide) microgels to stabilize direct oil-in-water Pickering emulsions. The microgel size is varied from 250 to 760 nm, the cross-linking density being kept constant. The emulsion properties strongly depend on the stabilizer size: increasing the particle size induces an evolution from dispersed drops and fluid emulsions toward strongly adhesive drops and flocculated emulsions. In order to get insight into this dependency, we study how particles adsorb at the interface and we determine the extent of their deformation. We propose a correlation between microgel ability to deform and emulsion macroscopic behavior. Indeed, as the microgels size increases, their internal structure becomes more heterogeneous and so does the polymeric interfacial layer they form. The loss of a uniform dense layer favors bridging between neighboring drops, leading to flocculated and therefore less handleable emulsions.

Adsorption of microgels at an oil-water interface: correlation between packing and 2D elasticity.

The aim of this paper is to determine how microgels adsorb at a model oil-water interface and how they adapt their conformation to compression, which gives rise to surface elasticity depending on the microgel packing. The structure of the film is determined by the Langmuir films approach (forced compression) and compared to spontaneous adsorption using the pendant drop method. The behaviour of microgels differs significantly from that of non-deformable particles but resembles that of linear polymers or proteins. We also correlate the properties of microgels spontaneously adsorbed at model interfaces to their forced adsorption during emulsification. Finally we propose a route to easily control a posteriori the microgel packing at the surface of droplets and the flow properties of emulsions stabilised by the microgels.

Readily prepared dynamic hydrogels by combining phenyl boronic acid- and maltose-modified anionic polysaccharides at neutral pH.

Dynamic covalent hydrogels are facilely prepared from biocompatible polysaccharides in physiological conditions by the formation of phenylboronate ester cross-links. This is based on the simple mixing of carboxylate-containing polysaccharides (i.e., hyaluronic acid or carboxymethylcellulose) modified with phenylboronic acid and maltose moieties according to mild coupling reactions performed in aqueous solution. The formation of dynamic networks based on reversible boronic-ester cross-links is demonstrated by analyzing their rheological behavior. This study shows that these gels can adapt their structure in response to chemical stimuli such as variations in pH or addition of glucose and self-heal.

Modulation of circularly polarized luminescence by swelling of microgels functionalized with enantiopure [Ru(bpy)3]2+ luminophores.

Chemoresponsive microgels functionalized with enantiomeric Δ- or Λ-[Ru(bpy)3]2+ showed tunable chiroptical properties upon swelling and shrinking. The tuning is triggered by a modulation of the local mobility of [Ru(bpy)3]2+ upon addition of fructose, controlling interactions and distances between [Ru(bpy)3]2+ and phenylboronic acid.

Enhanced electrogenerated chemiluminescence in thermoresponsive microgels.

The electrochemistry, photoluminescence and electrogenerated chemiluminescence of thermoresponsive redox microgels were investigated. For the first time, reversible ECL enhancement is demonstrated in stimuli-responsive 100-nm microgel particles. Such an unexpected amplification reached 2 orders of magnitude, and it is intrinsically correlated with the collapse of the microgel particles. The swell-collapse transition decreases the average distance between adjacent redox sites and favors the electron-transfer processes in the microgels resulting in the enhanced ECL emission.

Pickering emulsions stabilized by soft microgels: influence of the emulsification process on particle interfacial organization and emulsion properties.

This work reports a new evidence of the versatility of soft responsive microgels as stabilizers for Pickering emulsions. The organization of microgels at the oil-water interface is a function of the preparation pathway. The present results show that emulsification energy can be used as a trigger to modify microgel deformation at the oil-water interface and their packing density: high shear rates bring strong flattening of the microgels, whereas low shear rates lead to dense monolayers, where the microgels are laterally compressed. As a consequence, the resulting emulsions have opposite behavior in terms of flocculation, which arises from bridging between neighboring drops and is strongly dependent on their surface coverage. This strategy can be applied to any microgel which can sufficiently adsorb at low shear rates, i.e. small microgels or lightly cross-linked ones. The control of the organization of microgels at the interface does not only modify emulsion end-use properties but also constitutes a new tool for the development of Janus-type microgels, obtained by chemical modification of the adsorbed microgels.

Interfacial rheology of model water-air microgels laden interfaces: Effect of cross-linking.

HYPOTHESIS: The mechanical properties of model air/water interfaces covered by poly(N-isopropylacrylamide) microgels depend on the microgels deformability or in other words on the amount of cross-linker added during synthesis. EXPERIMENTS: The study is carried out by measuring the apparent dilational, the compression and the shear moduli using three complementary methods: (1) the pendant drop method with perturbative areas, (2) the Langmuir trough compression, and (3) shear rheology using a double wall ring cell mounted onto a Langmuir through. FINDINGS: In the range of surface coverages studied, the interfaces exhibit a solid-like behavior and elasticity goes through a maximum as a function of the surface pressure. This is observable whatever the investigation method. This maximum elasticity depends on the microgel deformability: the softer the microgels the higher the value of the moduli. The mechanical behavior of model interfaces is discussed, taking into account the core-shell structure of the particles and their packing at the interface.

Effect of charge on the stabilization of water-in-water emulsions by thermosensitive bis-hydrophilic microgels.

HYPOTHESIS: Molecular surfactants are not able to stabilize water-in-water (W/W) emulsions, unlike nano or micro-particles, which can achieve this in some cases. However, the effect of electrostatic interactions between particles on the emulsion stability has rarely been investigated. We hypothesize that introducing charges modifies the stabilization capacity of particles and renders it both pH- and ionic strength-dependent. EXPERIMENTS: Charge was introduced into bis-hydrophilic and thermoresponsive dextran/polyN-isopropylacrylamide microgels by replacing a small fraction of polyN-isopropylacrylamide with acrylic acid groups. The size of the microgels was obtained by dynamic light scattering. The stability and microstructure of dextran/poly(ethyleneoxide)-based W/W emulsions, was studied as a function of pH, NaCl concentration and temperature using confocal microscopy and by analytical centrifugation. FINDINGS: The swelling degree of charged microgels depends on the pH, ionic strength and the temperature. In the absence of salt, charged microgels do not adsorb at the interface and have little stabilizing effect even after neutralization. However, the interfacial coverage and the stability increase with rising concentration of NaCl. Saltinduced stabilization of these emulsions was also observed at 50 °C. Increasing the temperature strongly influences the emulsion stability at low pH.

Enzymatic cascade reaction in simple-coacervates.

The design of enzymatic droplet-sized reactors constitutes an important challenge with many potential applications such as medical diagnostics, water purification, bioengineering, or food industry. Coacervates, which are all-aqueous droplets, afford a simple model for the investigation of enzymatic cascade reaction since the reactions occur in all-aqueous media, which preserve the enzymes integrity. However, the question relative to how the sequestration and the proximity of enzymes within the coacervates might affect their activity remains open. Herein, we report the construction of enzymatic reactors exploiting the simple coacervation of ampholyte polymer chains, stabilized with agar. We demonstrate that these coacervates have the ability to sequester enzymes such as glucose oxidase and catalase and preserve their catalytic activity. The study is carried out by analyzing the color variation induced by the reduction of resazurin. Usually, phenoxazine molecules acting as electron acceptors are used to characterize glucose oxidase activity. Resazurin (pink) undergoes a first reduction to resorufin (salmon) and then to dihydroresorufin (transparent) in presence of glucose oxidase and glucose. We have observed that resorufin is partially regenerated in the presence of catalase, which demonstrates the enzymatic cascade reaction. Studying this enzymatic cascade reaction within coacervates as reactors provide new insights into the role of the proximity, confinement towards enzymatic activity.

Responsive microgels-based colloidosomes constructed from all-aqueous pH-switchable coacervate droplets.

HYPOTHESIS: Colloidosomes made of stimuli-responsive microgels offer the opportunity to design polymeric capsules with a hierarchical and tunable pore distribution. Coacervates stabilized by a microgel monolayer represent a unique strategy to build colloidosomes from all-aqueous emulsion drops, while exploiting the sequestration and dissolution properties of the coacervates. EXPERIMENTS: Methacrylated poly(N-isopropylacrylamide) (pNIPAM) microgels are used to stabilize coacervates made of an ampholyte polymer at a pH close to its isoelectric point. They are further cross-linked under UV-irradiation. The resulting assemblies are studied by means of confocal microscopy. Their permeability towards dextrans and nanoparticles is studied before and after dissolution of the coacervate. FINDINGS: PNIPAM microgels are found to stabilize the coacervates by adsorbing at their surface. Inter cross-linking the microgels results in the formation of an elastic colloidosome that persists after the coacervate dissolution and withstands surface deformations up to about 200%. The coacervate is exploited as a sequestrating core to entrap a water-soluble payload, which can be further released upon coacervate dissolution, while the membrane exhibits a size-selecting permeability. The membrane properties can also be switched by the volume phase transition of the microgels. Coacervate-embedded colloidosomes open new perspectives in the area of encapsulation/extraction and controlled transport of water-soluble/dispersed species.

Origin and control of adhesion between emulsion drops stabilized by thermally sensitive soft colloidal particles.

We used soft microgels made of poly(N-isopropylacrylamide) (pNIPAM) of variable cross-linking degrees and the same colloidal size to stabilize oil-in-water Pickering emulsions. The extent of droplet flocculation increased and the resistance of the emulsions to mechanical stresses decreased as the cross-linking density was augmented. Large flat films were separating the droplets, and we could measure the adhesion angle at the junction with the free interfaces through several microscopy methods. The size of the flat films and the values of the angles were reflecting strong adhesive interactions between the interfaces as a result of microgel bridging. In parallel, cryo-SEM imaging of the thin films allowed a precise determination of their structure. The evolution of the adhesion angle and of the film structure as a function of microgels cross-linking density provided interesting insights into the impact of particle softness on film adhesiveness and emulsion stability. We exploited our main findings to propose a novel route for controlling the emulsions end-use properties (flocculation and stability). Owing to particle softness and thermal sensitivity, the interfacial coverage was a path function (it depended on the sample "history"). As a consequence, by adapting the emulsification conditions, the interfacial monolayer could be trapped in a very dense and rigid configuration, providing improved resistance to bridging flocculation and to flow-induced coalescence.

Enhanced electrochemiluminescence at microgel-functionalized beads.

Bead-based assays are successfully combined with electrochemiluminescence (ECL) technology for detection of a wide range of biomarkers. Herein, we demonstrate a novel approach to enhance the ECL signal by decorating micrometric beads with [Ru(bpy)3]2+-grafted microgels (diameter ∼100 nm). Rapid and stable light emission was spatially resolved at the level of single functionalized beads. An enhancement of the ECL signal of microgel-labeled beads by 9-fold was observed in comparison to molecularly linked [Ru(bpy)3]2+ beads prepared by a sandwich immunoassay or an amide bond. Imaging the ECL signal at the single bead level shows that the size of the ECL-emitting layer is extended using the microgels. The reported method offers a great promise for the optimization of bead-based ECL detection and subsequent development of ECL microscopy.