Véronique Lapeyre

Glucose-responsive microgels with a core–shell structure

New multiresponsive core-shell microgels have been synthesized, with a thermoresponsive core and a glucose-responsive shell, made respectively of poly(N-isopropylacrylamide) (pNIPAM) and pNIPAM-co-acrylamidophenylboronic acid (pNIPAM-co-APBA). The structure of the particles was elucidated by means of dynamic light scattering. Their thermal properties were investigated and compared to those of the core alone. Without glucose, the hydrophobic shell prevented the core from swelling in a certain temperature range where the shell was shown to be collapsed. This core compression vanished upon glucose addition, when the shell became hydrophilic and swelled. Therefore, the extent of core swelling was regulated by two processes: its own internal stimulus, i.e. temperature, and shell compression, which is proportional to glucose concentration, even at physiological salinity. The concept was applied to a selected chemical composition. Core-shell microgels with a response to glucose at physiological pH were obtained and used to encapsulate insulin. Insulin release was shown to be regulated by the presence of glucose.

Dissymmetric carbon nanotubes by bipolar electrochemistry.

Short carbon nanotubes have been modified selectively on one end with metal using a bulk technique based on bipolar electrochemistry. A stabilized suspension of nanotubes is introduced in a capillary containing an aqueous metal salt solution, and a high electric field is applied to orientate and polarize the individual tubes. During their transport through the capillary under sufficient polarization (30 kV), each nanotube is the site of water oxidation on one end and the site of metal ion reduction on the other end with the size of the formed metal cluster being proportional to the potential drop along the nanotube.

Multiresponsive hybrid microgels and hollow capsules with a layered structure.

Various stimuli-responsive composite particles with a high control of their internal structure and their corresponding hollow capsules are synthesized and characterized by photon correlation spectroscopy, TEM, and AFM. Core-shell particles with a silica core and a thermoresponsive shell are obtained by polymerization of N-isopropylacrylamide (NIPAM) in the presence of silica seeds grafted with a high density of gamma-methacryloxypropyltrimethoxysilane (MPS). The influence of the synthesis conditions is studied. The shell thickness increases when the monomer concentration increases in a limited range where uniform composite particles with a single core are obtained. At constant monomer concentration, the shell thickness does not depend on the size of the silica seeds, but the presence of free unbound microgels is observed when the silica surface area decreases. A range of particle diameters and shell thicknesses is thus obtained, which can lead to the corresponding hollow capsules by exposure to hydrofluoric acid solution. The volume phase transition temperature of these materials can be easily tuned by replacing the NIPAM monomer by another N-alkylacrylamide derivative. However, the incorporation of comonomers such as acrylic acid (AA) and a phenylboronic acid (PBA) derivative inhibits the formation of core-shell structures. In order to get pH or glucose responsiveness, these functional groups can be incorporated in the outer shell of a core-double shell structure, with pNIPAM as intermediate shell. pH-responsive and glucose-responsive composite particles are obtained by this method with a high control of their internal structure.

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.

Water-in-oil emulsions stabilized by water-dispersible poly(N-isopropylacrylamide) microgels: understanding anti-Finkle behavior.

Emulsions were prepared using poly(N-isopropylacrylamide) microgels as thermoresponsive stabilizers. The latter are well-known for their sensitivity to temperature: they are swollen by water below the so-called volume phase transition temperature (VPTT = 33 °C) and shrink when heated above it. Most of the studies reported in the literature reveal that the corresponding emulsions are of the oil-in-water type (O/W) and undergo fast destabilization upon warming above the VPTT. In the present study, whereas O/W emulsions were obtained with a wide panel of oils of variable polarity and were all thermoresponsive, water-in-oil (W/O) emulsions were found only in the presence of fatty alcohols and did not exhibit any thermal sensitivity. To understand the peculiar behavior of emulsions based on fatty alcohols, we investigated the organization of microgels at the oil-water interface and we studied the interactions of pNIPAM microgels with octanol. By combining several microscopy methods and by exploiting the limited coalescence process, we provided evidence that W/O emulsions are stabilized by multilayers of nondeformed microgels located inside the aqueous drops. Such behavior is in contradiction with the empirical Finkle rule stating that the continuous phase of the preferred emulsion is the one in which the stabilizer is preferentially dispersed. The study of microgels in nonemulsified binary water/octanol systems revealed that octanol diffused through the aqueous phase and was incorporated in the microgels. Thus, W/O emulsions were stabilized by microgels whose properties were substantially different from the native ones. In particular, after octanol uptake, they were no longer thermoresponsive, which explained the loss of responsiveness of the corresponding W/O emulsions. Finally, we showed that the incorporation of octanol modified the interfacial properties of the microgels: the higher the octanol uptake before emulsification, the lower the amount of particles in direct contact with the interface. The multilayer arrangement was thus necessary to ensure efficient stabilization against coalescence, as it increased interface cohesiveness. We discussed the origin of this counterexample of the Finkles rule.

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.

Photochemical crosslinking of hyaluronic acid confined in nanoemulsions: towards nanogels with a controlled structure

We present the preparation of nanogels made of hyaluronic acid (HA) with a well-controlled structure. To this end, HA precursors with polymerizable methacrylate groups (HA-MA) were confined within water-in-oil nanoemulsion droplets as nanoreactors and further photopolymerized under UV. Particular attention was paid to the preparation of a stable nanoemulsion template with a homogeneous droplet size. Upon UV irradiation of the emulsion containing HA-MA, crosslinked HA-MA particles with a well-defined size were obtained. Moreover, by varying the photopolymerization conditions, i.e. the number of received photons, we could control the conversion rate of the polymerization, as proved by 1H-NMR. Nanogels with controlled cross-linking densities were thus obtained. Not only could their crosslinking densities be controlled by the number of incident photons, but also by the degree of methacrylation (DM) of HA-MA derivatives. In addition, the swelling properties of the nanogels depended on external factors, showing their pH and ionic strength responsiveness. We show that these structures were highly biocompatible, stable under storage and enzymatically biodegradable, which opens the route for their application as drug delivery systems. Finally, insulin was loaded in the nanogels and its pH-dependent release was demonstrated. This versatile method of nanogel preparation, which can be applied to every type of hydrophilic precursor, offers a potential synthetic route to design other types of fully biocompatible drug delivery systems.

Asymmetric synthesis using chiral-encoded metal

The synthesis of chiral compounds is of crucial importance in many areas of society and science, including medicine, biology, chemistry, biotechnology and agriculture. Thus, there is a fundamental interest in developing new approaches for the selective production of enantiomers. Here we report the use of mesoporous metal structures with encoded geometric chiral information for inducing asymmetry in the electrochemical synthesis of mandelic acid as a model molecule. The chiral-encoded mesoporous metal, obtained by the electrochemical reduction of platinum salts in the presence of a liquid crystal phase and the chiral template molecule, perfectly retains the chiral information after removal of the template. Starting from a prochiral compound we demonstrate enantiomeric excess of the (R)-enantiomer when using (R)-imprinted electrodes and vice versa for the (S)-imprinted ones. Moreover, changing the amount of chiral cavities in the material allows tuning the enantioselectivity.

Direct Visualization of Symmetry Breaking During Janus Nanoparticle Formation

The straight-forward synthesis of Janus nanoparticles composed of Ag and AgBr is reported. For their formation, cucurbit[n]uril (CB)-stabilized AgBr nanoparticles are first generated in water by precipitation. Subsequent irradiation with an electron beam transforms a fraction of each AgBr nanoparticle into Ag(0) , leading to well-defined Janus particles, stabilized by the binding of CB to the surface of both AgBr and Ag(0) . With the silver ion reduction being triggered by the electron beam, the progress of the transformation can be directly monitored with a transmission electron microscope.

Capillary electrophoresis as a production tool for asymmetric microhybrids.

The site selective electrodeposition of silver metal onto a conducting object such as carbon microtubes (CMTs) in an electrolytic solution could be achieved by means of bipolar electrochemistry. Two half reactions are simultaneously carried out at both extremities of the CMT, which act as a bipolar electrode. The thermodynamic threshold value of the process, which consists in metal electroreduction and concomitant water oxidation is directly related to the length of CMT. That is the reason why, when scaling down the methodology to microscale objects, electric fields in the range of tenths of kilovolts per meter are necessary. In that context, a CE apparatus provides a convenient experimental platform to achieve in a straightforward manner such experimental conditions. We exemplify this methodology with the efficient and quick electroreduction of Ag+ on CMTs from a low‐concentration aqueous electrolytic solution during the migration across a fused capillary. CE allows applying safely a large enough electric field (typically ∼30 kV/m) for the successful modification of 15 to 20 μm‐long substrates. The corresponding hybrid materials have been characterized by optical microscopy as well as SEM and energy‐dispersive X‐ray spectroscopy.