Publications

We describe a six-step method for making colloidal clusters of 2, 3, or 4 silica particles with a radius of 1.2 um. This method, originally described by Manoharan et al. (Manoharan, V. N.; Elsesser, M. T.; Pine, D. J. Science 2003, 301, 483), is based on the encapsulation of silica spheres in emulsion droplets. The originality of our work lies in the preparation of monodisperse emulsions, which allows us to obtain some high yields of small aggregates over a wide range of conditions. Using optical microscopy and disk centrifugation, we show that the relative fractions of 2, 3, and 4 particle aggregates are controlled by the emulsification conditions, particularly the concentration of silica in the dispersed phase. Our best yields are obtained using low to moderate shear rates, a highly viscous continuous phase, and intermediate amounts of silica. The sedimentation of the colloidal solution into a gradient of concentration leads to aqueous suspensions of identical clusters. Since the overall process can easily be scaled up, large quantities of identical clusters may be prepared, which should allow the thermodynamic properties of these new colloidal objects to be measured for the first time. These nonspherical particles could serve as building blocks for more complex assemblies, such as colloidal crystals which could find applications as photonic materials.

We report on frequency and field-dependent complexsusceptibility, , measurements of a magnetic colloidal system consisting of 200 nm spherical beads, containingmaghemite (γFe₂O₃) nanoparticles. The relaxation properties of both the magnetic colloid and a free suspension of the γFe₂O₃ particles, are investigated over the frequency range 200 Hz–1 MHz. Under a polarizing field H, an absorption peak is detected in the component at frequencies f_max between 1.1 and 1.7 kHz. We show that this absorption peak can be attributed to the Néel relaxation of the inner maghemitenanoparticles. It is also shown that the general trend for the value of f_max and the amplitude of both and is to increase with increasing H. Furthermore, the relation between and and their dependence on frequency, ω/2π, is investigated by means of the magnetic analogue of the Cole–Cole plot and a measure of the Cole–Cole distribution parameter α_s is determined.

We have studied the effect of shear on the stability of suspensions made of non-Brownian solid particles. We demonstrate the existence of an irreversible transition where the solid particles aggregate at remarkably low volume fractions (Phi~0.1). This shear-induced aggregation is dramatic and exhibits a very sudden change in the viscosity, which increases sharply after a shear-dependent induction time. We show that this induction time is related exponentially to the shear rate, reflecting the importance of the hydrodynamic forces in reducing the repulsive energy barrier that prevents the particles from aggregating.

Following a novel realization of low-Reynolds-number swimming (Dreyfus et al., Nature, vol. 436, 2005, p. 862), in which self-assembled filaments of paramagnetic micron-sized beads are tethered to red blood cells and then induced to swim under crossed uniform and oscillating magnetic fields, the dynamics of magnetoelastic filaments is studied. The filament is modelled as a slender elastica driven by a magnetic body torque. The model is applied to experiments of Goubault et al. (Phys.Rev. Lett., vol. 91, 2003, art. 260802) to predict the lifetimes of metastable static filament conformations that are known to form under uniform fields. A second experimental swimming scenario, complementary to that of Dreyfus et al. (2005), is described: filaments are capable of swimming even if not tethered to red blood cells. Yet, if both ends of the filament are left free and the material and magnetic parameters are uniform along its length then application of an oscillating transverse field can only generate homogeneous torques, and net translation is prohibited by symmetry. It is shown that fore–aft symmetry is broken when variation of the bending stiffness along the filament is accounted for by including elastic defects, which produces results consistent with the swimming phenomenology. 

When ligands and receptors are both attached on surfaces, because of the restriction of configurational freedom, their recognition kinetics may be substantially reduced as compared with freely diffusing species. In nature, this reduction may influence the efficiency of the capture and adhesion of circulating cells. Here we show that similar consequences are observed for colloids grafted with biomolecules that are used as probes for diagnostics. We exploit Brownian magnetic colloids that self-assemble into linear chains to show also that the resulting one-dimensional confinement considerably accelerates the recognition rate between grafted receptors and their ligands. We propose that because confinement significantly augments the colliding frequency, it also causes a large increase in the attempt frequency of the recognition. This work gives the basis of a rapid, homogeneous, and highly sensitive bioanalysis method.

A new general methodology based on magnetic colloids to study the recognition kinetics of tethered biomolecules is introduced here. By tuning the architecture of the tethers that link molecules to surfaces, we explain the large diversity of observed association times. We thus studied the association kinetics between a streptavidin and a biotin grafted on two opposite surfaces. We also considered the case of cellular adhesion molecules, the cadherins, for which we measured an association kinetic constant smaller than that of the strepatividin-biotin interaction.

We explored the ability of emulsion droplets to bind specifically solid model substrates and we study the relationship between drop composition and the contact angle. We developed a in-sisu method to graft covalently biotin and streptavidin on the O/W emulsion interface. We characterized the size distribution and the surface density of the streptavidine by flow cytometry. We showed that the contact angle and the adhesion energy grow linearly with the biotin density on the glass coverslip, and we observed a different patterning (disk, ring) of the streptavidin within the adhesion patch, depending on the PEG density on the surface. This result suggest a 2D phase transition of the proteins.

Ce manuscrit présente l'étude de la stabilité et des propriétés d'encapsulation et de relargage des émulsions doubles cristallisables. Notre démarche a été de remplacer l'huile liquide classiquement utilisée pour les émulsions doubles par une huile solide à température ambiante. Une barrière répulsive stérique assure la stabilité de ces objets au repos. Sous écoulement, les objets s'agrègent : l'énergie fournie par le cisaillement réduit la barrière répulsive alors comparable à kT et facilement franchissable. Ceci se traduit par une dépendance exponentielle du temps caractéristique d'agrégation avec le taux de cisaillement. Sous régime iso-osmotique, la nature solide de la membrane ralentit les cinétiques de libération par diffusion des espèces initialement encapsulées. Sous régime hypo-osmotique, le matériau solide libère rapidement l'espèce encapsulée. Les processus mis en jeu et les différents paramètres permettant de contrôler les cinétiques de libération ont été identifiés.

The invention describes a method for the identification of compounds which bind to a target component of a biochemical system or modulate the activity of the target, comprising the steps of: a) compartmentalising the compounds into microcapsules together with the target, such that only a subset of the repertoire is represented in multiple copies in any one microcapsule; and b) identifying the compound which binds to or modulates the activity of the target; wherein at least one step is performed under microfluidic control. The invention enables the screening of large repertoires of molecules which can serve as leads for drug development.