Olivier J. Cayre

Induced-charge electrophoresis of metallodielectric particles.

The application of ac electric fields in aqueous suspensions of anisotropic particles leads to unbalanced liquid flows and nonlinear, induced-charge electrophoretic motion. We report experimental observations of the motion of Janus microparticles with one dielectric and one metal-coated hemisphere induced by uniform fields of frequency 100 Hz-10 kHz in NaCl solutions. The motion is perpendicular to the field axis and persists after particles are attracted to a glass wall. This phenomenon may find applications in microactuators, microsensors, and microfluidic devices.

Fabrication of asymmetrically coated colloid particles by microcontact printing techniques

We developed a novel method for preparation of asymmetrically coated colloid particles by using a microcontact printing technique. Films of water-insoluble ionic surfactants deposited on PDMS stamps were printed onto latex particle monolayers of opposite surface charge in order to produce spherical latex particles of dipolar surface charge distribution. We studied the effects of salt on the aggregation of such dipolar particles in aqueous suspensions. Upon addition of salt, dipolar colloid particles were found to give “linear” aggregates. We then extended this microcontact printing technique to the directed assembly of colloidal particles. Microcontact printing of one colloidal monolayer over a latex particle monolayer of opposite charge was used to fabricate particles of complex internal structure. We demonstrated that if the two colloid monolayers consist of particles of comparable sizes, this method allows fabrication of particle doublets. When the particle monolayer was stamped with another colloid monolayer of much smaller particle size complex structures as half-coated “raspberry”-like particles were obtained. Possible applications of these asymmetrically coated colloids include photonic crystals with novel symmetries, colloidal substitutes for liquid crystals and water-based electrorheological fluids.

Stimulus responsive core-shell nanoparticles: synthesis and applications of polymer based aqueous systems

In this article we review the current state of knowledge in the field of responsive nanoparticles for aqueous-based systems. The review focuses on the use of stimuli-responsive copolymers that can either self-assemble into ‘soft’ nanoparticles or that are attached to the surface of solid nanoparticles. We first describe the most common methods for synthesizing the different responsive polymers used in the design of such nanoparticles, highlighting living radical polymerization techniques in particular. Subsequently, we give examples of how copolymers containing such responsive blocks can self-assemble into a wide range of ‘soft’ structures. We also depict the main techniques for attaching responsive polymers to the surface of solid nanoparticles and list advantages and drawbacks of each. Finally, for both soft self-assembled systems and solid core-polymer shell systems, we show specific examples of these systems used in a varied range of applications including drug delivery, smart emulsifiers, transport across membranes, sensors and coatings.

Fabrication of novel colloidosome microcapsules with gelled aqueous cores

Novel colloidosome microcapsules with aqueous gel cores and integral shells of polymeric colloid particles have been prepared and characterized. Our method is based on templating water-in-oil emulsions stabilized by polystyrene (PS) latex particles, followed by gelling of the aqueous phase with a suitable hydrocolloid. The obtained microcapsules were transferred into water after dissolving the oil phase in ethanol and multiple centrifugation–washing with ethanol and water. The presence of an aqueous gel core was found to be crucial for the structural integrity and mechanical stability of the obtained colloidosome microcapsules. The effect of the oil type on the final structure of the colloidosome membrane was also studied. It was demonstrated that by using an appropriate oil, the particles within the colloidosome monolayer can be partially or completely swollen which allows direct control over the membrane pore size and its permeability. Such colloidosome microcapsules may find applications as delivery vehicles for controlled release of drugs and cosmetic or food supplements.

Responsive core-shell latex particles as colloidosome microcapsule membranes.

Responsive core-shell latex particles are used to prepare colloidosome microcapsules using thermal annealing and internal cross linking of the shell, allowing the production of the microcapsules at high concentrations. The core-shell particles are composed of a polystyrene core and a shell of poly[2-(dimethylamino)ethyl methacrylate]-b-poly[methyl methacrylate] (PDMA-b-PMMA) chains adsorbed onto the core surface, providing steric stabilization. The PDMA component of the adsorbed polymer shell confers thermally responsive and pH-responsive characteristics to the latex particle, and it also provides glass transitions at temperatures lower than those of the core and reactive amine groups. These features facilitate the formation of stable Pickering emulsion droplets and the immobilization of the latex particle monolayer on these droplets to form colloidosome microcapsules. The immobilization is achieved through thermal annealing or cross linking of the shell under mild conditions feasible for large-scale economic production. We demonstrate here that it is possible to anneal the particle monolayer on the emulsion drop surface at 75-86 °C by using the lower glass-transition temperature of the shell compared to that of the polystyrene cores (∼108 °C). The colloidosome microcapsules that are formed have a rigid membrane basically composed of a densely packed monolayer of particles. Chemical cross linking has also been successfully achieved by confining a cross linker within the disperse droplet. This approach leads to the formation of single-layered stimulus-responsive soft colloidosome membranes and provides the advantage of working at very high emulsion concentrations because interdroplet cross linking is thus avoided. The porosity and mechanical strength of the microcapsules are also discussed here in terms of the observed structure of the latex particle monolayers forming the capsule membrane.

Incorporation of block copolymer micelles into multilayer films for use as nanodelivery systems

This work demonstrates the potential application of stimulus responsive block copolymer micelles as triggerable delivery systems for use within multilayer films. Cationic, pH-responsive micelles of poly[2-(dimethylamino)ethyl methacrylate-block-poly(2-(diethylamino)ethyl methacrylate)] (PDMA-PDEA) were deposited on anionic polystyrene latex particles. The charge reversal of the surface and the amount of adsorbed polymer were monitored by zeta potential measurements and colloidal titrations, respectively. Prior to adsorption, the PDMA-PDEA micelles were loaded with a hydrophobic dye, and UV-vis spectroscopy was used to determine the amount of dye encapsulated within a monolayer of micelles. It was found that subtle chemical modification of the PDMA-PDEA diblock copolymer via permanent quaternization of the PDEA block results in micelles with tunable loading capacities. Multilayers of cationic micelles of partially quaternized PDMA-PDEA and anionic polyelectrolyte (poly(sodium 4-styrene sulfonate)) were deposited on the surface of polystyrene latex particles by sequential adsorption. UV-vis analysis of the dye present within the multilayer after the addition of each layer demonstrates that the micelles are sufficiently robust to retain encapsulated dye after multiple adsorption/washing cycles and can thus create a film that can be increasingly loaded with dye as more micelle layers are adsorbed. Multiple washing cycles were performed on micellar monolayers of PDMA-PDEA to demonstrate how such systems can be used to bring about triggerable release of actives. When performing several consecutive washing steps at pH 9.3, the micelle structure of the PDMA-PDEA micelles in the monolayer is retained, resulting in only a small reduction in the amount of encapsulated dye. In contrast, washing at pH 4, the structure of the micelle layers is severely disrupted, resulting in a fast release of the encapsulated dye into the bulk. Finally, if a sufficient number of micelle/homopolyelectrolyte layers are adsorbed, it is possible to selectively dissolve the latex template, resulting in hollow capsules.

Fabrication of dipolar colloid particles by microcontact printing

A novel technique for preparation of dipolar colloid particles has been developed which is based on microcontact printing of films of water-insoluble ionic surfactants onto monolayers of colloid particles of opposite surface charge.

Fabrication of microlens arrays by gel trapping of self-assembled particle monolayers at the decane–water interface

A new technique for producing ordered microlens arrays has been developed which is based entirely on self-assembly of charged latex particles spread at an oil–water interface. It includes using a gel trapping technique and replication of the ordered particle monolayers by casting with PDMS. Microlens arrays were fabricated by taking an inverse replica of the PDMS template with a photopolymer.

Polymeric microcapsules assembled from a cationic/zwitterionic pair of responsive block copolymer micelles

Using a layer-by-layer (LbL) approach, this work presents the preparation of hollow microcapsules with a membrane constructed entirely from a cationic/zwitterionic pair of pH-responsive block copolymer micelles. Our previous work with such systems highlighted that, in order to retain the responsive nature of the individual micelles contained within the multilayer membranes, it is important to optimize the conditions required for the selective dissolution of the sacrificial particulate templates. Consequently, here, calcium carbonate particles have been employed as colloidal templates as they can be easily dissolved in aqueous environments with the addition of chelating agents such as ethylenediaminetetraacetic acid (EDTA). Furthermore, the dissolution can be carried out in solutions buffered to a desirable pH so not to adversely affect the pH sensitive micelles forming the capsule membranes. First, we have deposited alternating layers of anionic poly[2-(dimethylamino)ethyl methacrylate-block-poly(2-(diethylamino)ethyl methacrylate)] (PDMA-PDEA) and cationic poly(2-(diethylamino)ethyl)methacrylate-block-poly(methacrylic acid) (PDEA-PMAA) copolymer micelles onto calcium carbonate colloidal templates. After deposition of five micelle bilayers, addition of dilute EDTA solution resulted in dissolution of the calcium carbonate and formation of hollow polymer capsules. The capsules were imaged using atomic force microscopy (AFM) and scanning electron microscopy (SEM), which shows that the micelle/micelle membrane is sufficiently robust to withstand dissolution of the supporting template. Quartz crystal microbalance studies were conducted and provide good evidence that the micelle multilayer structure is retained after EDTA treatment. In addition, a hydrophobic dye was incorporated into the micelle cores prior to adsorption. After dissolution of the particle template, the resulting hollow capsules retained a high concentration of dye, suggesting that the core/shell structure of the micelles remains intact. Finally, thermogravimetric analysis (TGA) of dried capsules confirmed complete removal of the sacrificial inorganic template. As far as we are aware, this is the first demonstration of LbL assembled capsules composed entirely from responsive block copolymer micelles. The results presented here when combined with our previous findings demonstrate that such systems have potential application in the encapsulation and triggered release of actives.

Directed assembly of yeast cells into living yeastosomes by microbubble templating

We report the fabrication of yeastosomes—novel multicellular assemblies, which consist of a spherical monolayer of living yeast cells held together by colloid interactions. Our method is based on templating of microbubbles with cells coated with cationic polyelectrolyte and the layer-by-layer technique. We show the mechanism of formation of yeastosomes which involves a gradual dissolution of the air-cores and their infusion with water. We demonstrate that the cells remain viable in the yeastosome structures. Yeastosomes and similar structures may find applications in the development of novel symbiotic bio-structures, artificial multicellular organisms and in tissue engineering. The unusual structure of these multicellular assemblies resembles the primitive forms of multicellular species, like Volvox, and other algae and could be regarded as one possible stage of the evolutionary development of multicellularity.