Publications

This work focuses on the creation of a new biocompatible reactor allowing the encapsulaion and the study of electroactive bacteria. Made by co-extrusion, this millimeter bioreactor is a liquid core capsule with a conducting hydrogel membrane. To create such an object, we formulate a composite hydrogel of alginate/carbon nanotubes in two steps. The first step is rapid and creates the hydrogel matrix by diffusion of divalent ions inside the alginate/carbon nanotubes mix. The second step is slower and permits the dialysis of the surfactant used to stabilize the nanotubes. During this last step, the carbone nanotube network percolates, creating a conducting network in the hydrogel for sufficient nanotube contents (above 0.5 wt%). This composite material has a macroscopic conductivity around 0.1 S.m^-1. An electrochemical study of this material allows to follow the nanotube connection inside the hydrogel. Bacteria can adhere on this composite hydrogel. Then, we demonstrate that the electroactivity of a biofilm developped on the inner side of the conductive capsule shell can be measured. This new biocompatible and electron-conducting compartment opens the way towards the development of a screening tool for the selection of electroactive bacteria but also brings innovative perspectives in the field of microbial fuel cells fabrication.
 

We investigate, through osmotic pressure measurements, the validity of the single-parameter equation of state (EOS) for solutions of polyethylene glycols in water, by Cohen et al.^1,2 We show that it is physically meaningful and that a reasonable good correspondence between the osmotic pressures for PEG35 in large range of concentrations is obtained. We also take the chain length dependence into account in our analysis, as suggested by Cohen et al. By recalculating the experimental pressures in the paper by Jönsson et al.³ applying the new calibration curve, which is based on the experimental results obtained in this study and the EOS obtained by Cohen et al., there is almost a perfect correspondence between the simulations and the experiments. These results have implications for correctly probing macromolecular interactions in wide range of systems when applying the osmotic stress method.

doi:10.1016/j.polymer.2015.10.056

The dissolution and swelling properties of montmorillonite at different pH have been studied, using small angle X-ray scattering (SAXS), imaging and osmotic stress methods combined with Monte Carlo simulations. The acidity of montmorillonite dispersions has been varied as well as the counterions to the net negatively charged platelets. At low pH, Na montmorillonite dissolves and among other species Al3+is released, hydrated, polymerized and then it replaces the counterions in the clay. This dramaticallychanges the microstructure of Na montmorillonite, which instead of having fully exfoliated platelets, turns into a structure of aggregated platelets, so-called tactoids. Montmorillonite dispersion still has a significant extra-lamellar swelling among the tactoids due to the presence of very small nanoplatelets.

Reinforcing precipitated silica systems have a complex hierarchical structure consisting of a branched
network made of connected clusters composed of small silica beads welded together into larger dense
aggregates. Here, we study the evolution of such structural features during a filtration process. The
typical behaviour is that the cakes formed at constant pressure do not reorganize at local scale during
a filtration experiment. Accordingly, the creep resistance of a precipitated silica network is high.
Overall, there is a percolation threshold, which appears when the branches are pushed into each other.
Once this percolation path is reached, the cake withstands compression over more than a decade of
applied pressure. Beyond, it seemed useful to make predictions of the filtration properties knowing the
typical length scales – small silica beads, dense aggregates, and consolidation behaviour of the cake.
A simple approach introducing the concept of an effective medium approximation into Darcy’s law
was tested. This approach treats the network as a pseudo-continuum of porous medium built at two main
length scales: the size of dense aggregates and a length scale representing the typical distance between
the aggregates. The quality of the fit of experimental filtration rates by this simple model indicates that a
description based on a continuous network made of two material phases is accurate.

http://dx.doi.org/10.1016/j.seppur.2015.10.005

We present the principle of a fast magnetic field enhanced colloidal agglutination assay, which is based on the acceleration of the recognition rate between ligands and receptors induced by magnetic forces. By applying a homogeneous magnetic field of 20 mT for only 7 s, we detect CRP (C-reactive protein) in human serum at a concentration as low as 1 pM for a total cycle time of about 1 min in a prototype analyzer. Such a short measurement time does not impair the performances of the assay when compared to longer experiments. The concentration range dynamic is shown to cover 3 orders of magnitude. An analytical model of agglutination is also successfully fitting our data obtained with a short magnetic pulse.

Montmorillonite is a key ingredient in a number of technical applications. However, little is known regarding the microstructure and the forces between silicate platelets. The size of montmorillonite platelets from different natural sources can vary significantly. This has an influence on their swelling behavior in water as well as in salt solutions, particularly when tactoid formation occurs, that is when divalent counterions are present in the system. A tactoid consists of a limited number of platelets aggregated in a parallel arrangement with a constant separation. The tactoid size increases with platelet size and with very small nanoplatelets, ∼30 nm, no tactoids are observed irrespectively of the platelet origin and concentration of divalent ions. The formation and dissociation of tactoids seem to be reversible processes. A large proportion of small nanoplatelets in a mixed-size system affects the tactoid formation, reduces the aggregation number and increases the extra-lamellar swelling in the system.

DOI: 10.1039/C5NR04615G

We present the principle of a fast magnetic field
enhanced colloidal agglutination assay, which is based on the
acceleration of the recognition rate between ligands and
receptors induced by magnetic forces. By applying a
homogeneous magnetic field of 20 mT for only 7 s, we detect
CRP (C-reactive protein) in human serum at a concentration as
low as 1 pM for a total cycle time of about 1 min in a prototype
analyzer. Such a short measurement time does not impair the
performances of the assay when compared to longer experiments.
The concentration range dynamic is shown to cover 3 orders of
magnitude. An analytical model of agglutination is also successfully fitting our data obtained with a short magnetic pulse.

The local functionalities of biocompatible objects can be characterized under conditions similar to their operating ones by scanning electrochemical microscopy (SECM). In the case of alginate beads entrapping carbon nanotubes (CNTs), SECM allows evidencing the local conductivity, the organization, and the communication between CNTs. It shows that the CNTs network is active enough to allow long range charge evacuation, enabling the use of alginate/CNT beads as soft 3D electrodes. Direct connection or local interrogation by microelectrodes allows visualizing their communication as a network and eventually studying them individually at the nanoscale.

Microbiology has continuously pushed efforts towards understanding microbial diversity. Technologies and methods have also evolved, from plating, and use of mi- croscopes and cytometers, towards micro-well handling robots and, finally, fluidic- based devices. The aim of this review was to bring microbiologists attention to the outstanding analytical and handling power of millifluidic droplet technologies for analysing and sorting phenotypic diversity in the microbial world. This new format overcomes many limitations of previous approaches. It provides outstanding repro- ducible growth conditions over droplet reservoirs allowing unprecedented sensitive read-out over thousands of colonies over time. The confinement of the millifluidic train within tubes and the implementation of a three phases format excludes any con- tamination issues. The automation and handling of reservoir droplets is inherently facilitated. We show as a proof of principle the efficiency of capturing phenotypic diversity within a bacterial sample submitted to a sub-minimum inhibitory con- centration of antibiotic. The precision offered by the millifluidic format allows the detection of a variety of resistance strategies that compete and coexist. The review finally explores the potential of this approach to address new challenges such as community-based growth of multiple-strain systems.

To address possible cell-to-cell heterogeneity in growth dynamics of isogenic cell popula- tions of Chlamydomonas reinhardtii, we developed a millifluidic drop-based device that not only allows the analysis of populations grown from single cells over periods of a week, but is also able to sort and collect drops of interest, containing viable and healthy cells, which can be used for further experimentation. In this study, we used isogenic algal cells that were first synchronized in mixotrophic growth conditions. We show that these synchronized cells, when placed in droplets and kept in mixotrophic growth conditions, exhibit mostly homoge- neous growth statistics, but with two distinct subpopulations: a major population with a short doubling-time (fast-growers) and a significant subpopulation of slowly dividing cells (slow- growers). These observations suggest that algal cells from an isogenic population may be present in either of two states, a state of restricted division and a state of active division. When isogenic cells were allowed to propagate for about 1000 generations on solid agar plates, they displayed an increased heterogeneity in their growth dynamics. Although we could still identify the original populations of slow- and fast-growers, drops inoculated with a single progenitor cell now displayed a wider diversity of doubling-times. Moreover, popula- tions dividing with the same growth-rate often reached different cell numbers in stationary phase, suggesting that the progenitor cells differed in the number of cell divisions they could undertake. We discuss possible explanations for these cell-to-cell heterogeneities in growth dynamics, such as mutations, differential aging or stochastic variations in metabolites and macromolecules yielding molecular switches, in the light of single-cell heterogeneities that have been reported among isogenic populations of other eu- and prokaryotes.

Two coarsening mechanisms of emulsions are well established: droplet coalescence (fusion of two droplets) and Ostwald ripening (molecular exchange through the continuous phase). Here a third mechanism is identified, contact ripening, which operates through molecular exchange upon droplets collisions. A contrast manipulated small-angle neutron scattering experiment was performed to isolate contact ripening from coalescence and Ostwald ripening. A kinetic study was conducted, using dynamic light scattering and monodisperse nanoemulsions, to obtain the exchange key parameters. Decreasing the concentration or adding ionic repulsions between droplets hinders contact ripening by decreasing the collision frequency. Using long surfactant chains and well-hydrated heads inhibits contact ripening by hindering fluctuations in the film. Contact ripening can be controlled by these parameters, which is essential for both emulsion formulation and delivery of hydrophobic ingredients.

DOI: 10.1002/anie.201407858

We examine the internal structure of milk casein micelles using the contrast variation method in Small-
Angle Neutron Scattering (SANS). Experiments were performed with casein dispersions of different
origins (i.e., milk powder or fresh milk) and extended to very low q-values (
9  10^-4 °A-1), thus making
it possible to precisely determine the apparent gyration radius Rg at each contrast. From the variation of
I(q / 0) with contrast, we determine the distribution of composition of all the particles in the
dispersions. As expected, most of these particles are micelles, made of casein and calcium phosphate,
with a narrow distribution in compositions. These micelles always coexist with a very small fraction of fat
droplets, with sizes in the range of 20–400 nm. For the dispersions prepared from fresh milk, which
were purified under particularly stringent conditions, the number ratio of fat droplets to casein micelles
is as low as 1 to 106. In that case, we are able to subtract from the total intensity the contribution of the
fat droplets and in this way obtain the contribution of the micelles only. We then analyze the variation of
this contribution with contrast using the approach pioneered by H. B. Stuhrmann. We model the casein
micelle as a core–shell spherical object, in which the local scattering length density is determined by the
ratio of calcium phosphate nanoclusters to proteins. We find that models in which the shell has a lower
concentration of calcium phosphate than the core give a better agreement than models in which the
shell has a higher density than the core.