Adeline Perro

Design and synthesis of Janus micro- and nanoparticles

Because the Roman god Janus was usually represented with two heads placed back to back, the term Janus is used for the description of particles whose surfaces of both hemispheres are different from a chemical point of view. So, they could be used as building blocks for supraparticular assemblies, as dual-functionalized devices, as particular surfactants if one hemisphere is hydrophilic and the other hydrophobic, etc. If they could allow the segregation of negative charges on one hemisphere and positive charges on the other one, they would display a giant dipole moment allowing their remote positioning by rotation in an electric field as a function of field polarity. This review deals with the great and imaginative efforts which were devoted to the synthesis of Janus particles in the last fifteen years. A special emphasis is made on scalable techniques and on those which apply to the preparation of Janus particles in the nanometer range. Specific properties and applications of Janus particles are discussed.

A Chemical Synthetic Route towards “Colloidal Molecules”

A non-exhaustive list of fields in which extensive research has been dedicated to colloidal particles during the past century includes condensed matter physics, biology, optics, materials science, and chemistry. Both our current understanding of various physical phenomena and our capability to fabricate new functional materials have been considerably enriched by the development of synthetic strategies that are capable of generating copious quantities of colloidal entities of good size uniformity. Nevertheless, most of the available monodisperse colloidal materials are spherical, as the minimization of the interfacial free energy strongly drives a particle to adopt such a shape. [1] This strongly limits the number of new structures which can be engineered by using these colloids as building blocks. For instance, the crystallization of spherical colloids into three-dimensional periodic lattices has recently allowed the emergence of a very active field of research—photonic colloidal crystals, known as artificial opals. Nevertheless, the light diffraction properties of these crystals are rather limited because of their face-centered cubic lattice, which results from the packing of spheres. It has been predicted that crystals with a lower degree of symmetry, such as the diamond lattice, can exhibit a full photonic bandgap. To build such photonic crystals, well-defined colloids with nonspherical shapes are required. Van Blaaderen recently introduced the elegant term of “colloidal molecules”, [2] which takes into account that spherical colloids can be treated as if they were atoms and that molecules can form more complex materials than can atoms. Therefore, the reproducible fabrication of large amounts of colloids that have a good uniformity in

Mastering a Double Emulsion in a Simple Co-Flow Microfluidic to Generate Complex Polymersomes

We show that the production and the geometrical shape of complex polymersomes can be predicted by varying the flow rates of a simple microdevice using an empirical law which predicts the droplet size. This device is constituted of fused silica capillaries associated with adjusted tubing sleeves and T-junctions. Studying the effect of several experimental parameters, double emulsions containing a controlled number of droplets were fabricated. First, this study examines the stability of a jet in a simple confined microfluidic system, probing the conditions required for droplets production. Then, multicompartmental polymersomes were formed, controlling flow velocities. In this work, poly(dimethylsiloxane)-graft-poly(ethylene oxide) (PDMS-g-PEO) and poly(butadiene)-block-poly(ethyleneoxide) (PBut-b-PEO) amphiphilic copolymers were used and dissolved in chloroform/cyclohexane mixture. The ratio of these two solvents was adjusted in order to stabilize the double emulsion formation. The aqueous suspension contained poly(vinyl alcohol) (PVA), limiting the coalescence of the droplets. This work constitutes major progress in the control of double emulsion formation in microfluidic devices and shows that complex structures can be obtained using such a process.

New insights into the nucleation and growth of PS nodules on silica nanoparticles by 3D cryo-electron tomography

The nucleation and growth of polystyrene (PS) nodules on 170 nm silica seeds under emulsion-polymerization conditions have been investigated for the first time by cryo-electron tomography. 3D arrangements were reconstructed from samples collected at several polymerization times (from 5 to 120 min). Early samples display the presence of small PS nodules bound to silica particles in a random distribution. For longer polymerization times, the number of PS nodules per silica seed decreases leading to octopod-like morphologies. The tomographic method allowed us to measure the contact angle that the growing PS nodules form with the silica bead surface. The average value of 142.4° remains constant over all of the observed period of the polymerization reaction. This contact angle appeared to be one of the key parameters for controlling the morphology of PS-silica biphasic particles.

Design and Synthesis of Model Transparent Aqueous Colloids with Optimal Scattering Properties

We demonstrate the synthesis and self-assembly of colloidal particles with independently controlled diameter and scattering cross section. We show that it is possible to prepare bulk colloidal suspensions that are nearly transparent in water, while the particles themselves can be individually resolved using optical microscopy. These particles may be ideal model colloids for real-space studies of self-assembly in aqueous media. Moreover, they illustrate the degree to which the optical properties of colloids can be engineered through straightforward chemistry.

Clogging by sieving in microchannels: Application to the detection of contaminants in colloidal suspensions

We report on a microfluidic method that allows measurement of a small concentration of large contaminants in suspensions of solid micrometer-scale particles. To perform the measurement, we flow the colloidal suspension through a series of constrictions, i.e., a microchannel of varying cross-section. We show and quantify the role of large contaminants in the formation of clogs at a constriction and the growth of the resulting filter cake. By measuring the time interval between two clogging events in an array of parallel microchannels, we are able to estimate the concentration of contaminants whose size is selected by the geometry of the microfluidic device. This technique for characterizing colloidal suspensions offers a versatile and rapid tool to explore the role of contaminants on the properties of the suspensions.

Synthesis of multivalent silica nanoparticles combining both enthalpic and entropic patchiness

Silica particles with a controlled number of entropic patches, i.e. dimples, are synthesized through the growth of the silica core of binary multipods that have been produced by a seeded-growth emulsion polymerization reaction. Transmission electron microscopy studies indicate that the silica surface conforms to the shape of the polystyrene (PS) nodules of the multipods while growing, allowing good control of the final shape of the dimpled silica particles. The PS nodules are also used as protecting masks to regioselectively graft amino groups, as revealed by the adsorption of gold markers. After dissolution of the PS nodules, some polymer chains remain grafted onto the silica surface, forming organic bumps. These residues are also selectively functionalized, leading to silica particles with both entropic and enthalpic patches.

Planar submicronic silica–polystyrene particles obtained by substrate-directed shaping

Original planar biphasic colloids are obtained by (i) using a growth-seeded emulsion polymerization process where a controlled number of polystyrene nodules are able to nucleate and grow on submicronic silica seeds to form multipods and (ii) shaping these multipods on a planar substrate by taking advantage of the softness of the polystyrene nodules. Hence, triangles, squares, diamonds, pentagons, hexagons, octagons or decagons are observed. The opportunity to control these morphologies thanks to experimental parameters such as the silica size and the styrene conversion is presented and discussed.

Bulk synthesis of polymer-inorganic colloidal clusters.

We describe a procedure to synthesize colloidal clusters with polyhedral morphologies in high yield (liter quantities at up to 70% purity) using a combination of emulsion polymerization and inorganic surface chemistry. We show that the synthesis initially used for silica-polystyrene hybrid clusters can be generalized to create clusters from other inorganic and polymer particles. We also show that high yields of particular morphologies can be obtained by precise control of the inorganic seed particle size, a finding that can be explained using a hard-sphere packing model. These clusters can be further chemically modified for a variety of applications. Introducing a cross-linker leads to colloidal clusters that can be index matched in an appropriate solvent, allowing them to be used for particle tracking or optical studies of colloidal self-assembly. Also, depositing a thin silica layer on these colloids allows the surface properties to be controlled using silane chemistry.

Oil-in-microgel strategy for enzymatic-triggered release of hydrophobic drugs

Polymer microgels have received considerable attention due to their great potential in the biomedical field as drug delivery systems. Hyaluronic acid (HA) is a naturally occurring glycosaminoglycan composed of N-acetyl-d-glucosamine and d-glucuronic acid. This polymer is biodegradable, nontoxic, and can be chemically modified. In this work, a co-flow microfluidic strategy for the preparation of biodegradable HA microgels encapsulating hydrophobic drugs is presented. The approach relies on: (i) generation of a primary oil-in-water (O/W) nanoemulsion by the ultrasonication method, (ii) formation of a double oil-in-water-in-oil emulsion (O/W/O) using microfluidics, and (iii) cross-linking of microgels by photopolymerization of HA precursors modified with methacrylate groups (HA-MA) present in the aqueous phase of the droplets. The procedure is used for the encapsulation and controlled release of progesterone. Degradability and encapsulation/release studies in PBS buffer at 37°C in presence of different concentrations of hyaluronidase are performed. It is demonstrated that enzymatic degradation can be used to trigger the release of progesterone from microgels. This method provides precise control of the release system and can be applied for the encapsulation and controlled release of different types of hydrophobic drugs.