Stéphane Reculusa

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.

Raman enhancement of azobenzene monolayers on substrates prepared by Langmuir-Blodgett deposition and electron-beam lithography techniques.

Nanostructured metallic platforms for Raman enhancement were fabricated using Langmuir-Blodgett and electron beam (e-beam) lithography techniques. The gold platforms were inscribed on thin glass slides with the purpose of using them in a transmission geometry experimental setup under a confocal microscope. The plasmon frequency of the gold nanostructures was determined in the visible-near-infrared range for various pattern sizes prepared by Langmuir-Blodgett transfer and e-beam lithography. The surface Raman enhancement factors were determined for a monolayer of azobenzene molecules adsorbed on gold through thiol bonding and compared for both LB transfer and e-beam samples for nanostructures of comparable geometries.

Syntheses and characterization of highly mesoporous crystalline TiO2 macrocellular foams

Titanium dioxide open-cell macro-cellular foams have been generated with emphasis toward controlling macro-, meso- and microstructures thus reaching hierarchically organized inorganic architectures. At the macroscopic length scale a non-static air–liquid foam strategy allows strong control over the open-cell morphologies. At the meso- and/or nanoscopic length scales various mesogenic templates or latex colloids have been used to promote mesoporosity. Among the strategies in use, a Pluronic copolymer P-123 combined with tetradecyltrimethylammonium bromide induce vermicular-like mesoporosity associated with a specific surface area around 400 m2 g−1. At the microscopic length scale, upon the use of specific thermal treatment, either monophasic anatase, biphasic anatase–rutile or monophasic rutile allotropic forms are obtained.

Combined macro-/mesoporous microelectrode arrays for low-noise extracellular recording of neural networks

Microelectrode arrays (MEAs) are appealing tools to probe large neural ensembles and build neural prostheses. Microelectronics microfabrication technologies now allow building high-density MEAs containing several hundreds of microelectrodes. However, several major problems become limiting factors when the size of the microelectrodes decreases. In particular, regarding recording of neural activity, the intrinsic noise level of a microelectrode dramatically increases when the size becomes small (typically below 20-μm diameter). Here, we propose to overcome this limitation using a template-based, single-scale meso- or two-scale macro-/mesoporous modification of the microelectrodes, combining the advantages of an overall small geometric surface and an active surface increased by several orders of magnitude. For this purpose, standard platinum MEAs were covered with a highly porous platinum overlayer obtained by lyotropic liquid crystal templating possibly in combination with a microsphere templating approach. These porous coatings were mechanically more robust than Pt-black coating and avoid potential toxicity issues. They had a highly increased active surface, resulting in a noise level ∼3 times smaller than that of conventional flat electrodes. This approach can thus be used to build highly dense arrays of small-size microelectrodes for sensitive neural signal detection.

Ring-opening metathesis polymerization on well defined silica nanoparticles leading to hybrid core–shell particles

The grafting and catalytic activity of a metathesis catalyst on well defined silica nanoparticles is described. The aim of this was to test the catalysis properties of a ligand-linked catalyst in ring-opening metathesis polymerization and also the possibility to obtain controlled hybrid materials. This was carried out by reaction of a synthesized hydroxy functionalized phosphine Cy2P(CH2)10OH with Cl2Ru(PPh3)2(CH–Ph), leading to a metathesis catalyst bearing a hydroxy group at the end of a ligand. This group was reacted with well defined silica nanoparticles with a diameter of 200 nm, that had acyl chloride functions at their surface. The grafting density was calculated from thermogravimetry experiments and found to be around 7 µmol m−2. The activity of the catalyst was tested for the ring-opening polymerization of norbornene. This showed that a fraction as high as 30% of the catalyst could initiate the polymerization and that the polymerization proceeded until completion. TEM characterization revealed that in diluted solutions, core–shell morphologies could be obtained. The efficiency of the polymerization may therefore lead to control of the thickness of the polymer coating.

From Raspberry-like to Dumbbell-like Hybrid Colloids through Surface-assisted Nucleation and Growth of Polystyrene Nodules onto Macromonomer-modified Silica Nanoparticles

Colloidal particles with a controlled morphology combining both organic and inorganic parts were synthesized through a seeded emulsion polymerization process. Silica seed particles (from 50 to 150 nm in diameter) were first surface-modified by the adsorption of an oxyethylene-based macromonomer. Then, emulsion polymerization of styrene was carried out in presence of these particles, the formation of polystyrene nodules being highly favored at the silica surface in such conditions. The ratio between the number of silica seeds and the number of growing polystyrene nodules appeared to be one of the key parameters to control the morphology of the final hybrid nanoparticles. When this ratio is close to 1, original hybrid dumbbell-like nanoparticles were mainly obtained.

Multilayer Langmuir-Blodgett films as diffractive external 3D photonic crystal in blue OLEDs

Three-dimensional Langmuir-Blodgett films made of silica beads are theoretically and experimentally investigated in order to improve the relatively small efficiency of blue OLEDs. Using films made of 5 layers of beads, we fabricated OLEDs emitting at 476 nm, and measured a gain of around 40% on their external quantum efficiency. An optical model has been developed to accurately handle the fact that the organic emitting layer and the photonic extraction layer are separated by a distance greater than 1000 wavelength. The latter also permits to describe rapidly this three-dimensional optical OLED cavity, without redoing all the numerical simulations when the optical properties of the organic layers are changed (material index, thicknesses).

Highly Ordered Macroporous Poly-3,4-ortho-xylendioxythiophene Electrodes as a Sensitive Analytical Tool for Heavy Metal Quantification

Highly ordered macroporous electrodes of the conducting polymer poly-3,4- ortho-xylendioxythiophene (PXDOT) are presented as a sensitive analytical tool for heavy metal ion quantification due to a controlled gain in electroactive area. They were designed by using colloidal crystal templates. A direct correlation between the final number of porous layers and the deposition charge ( Qd) employed for electropolymerization is observed. All the electrodes exhibit a surface-templated structure due to an interaction between the radical cation, formed during the electropolymerization, and the surface groups of the silica beads. The voltamperometric response of the macroporous PXDOT electrodes shows a rather fast electron transfer with Δ Ep values between 70 mV and 110 mV. Square wave anodic stripping voltammetric (SWASV) analysis of Cu2+ as a representative heavy metal ion shows a linear response in the subppm range. As a model application, the efficient quantification of Cu2+ in a commercial mezcal sample is validated by the standard addition method and the results correlate adequately with the values obtained by atomic absorption spectroscopy.

Modulation of Wetting Gradients by Tuning the Interplay between Surface Structuration and Anisotropic Molecular Layers with Bipolar Electrochemistry

A new simple and versatile method for the preparation of surface-wetting gradients is proposed. It is based on the combination of electrode surface structuration introduced by a sacrificial template approach and the formation of a tunable molecular gradient by bipolar electrochemistry. The gradient involves the formation of a self-assembled monolayer on a gold surface by selecting an appropriate thiol molecule and subsequent reductive desorption by means of bipolar electrochemistry. Under these conditions, completion of the reductive desorption process evolves along the bipolar surface with a maximum strength localized at the cathodic edge and a decreasing driving force towards the middle of the surface. The remaining quantity of surface-immobilized thiol, therefore, varies as a function of the axial position, resulting in the formation of a molecular gradient. The surface of the bipolar electrode is characterized at each step of the modification by recording heterogeneous electron transfer. Also, the evolution of static contact angles measured with a water droplet deposited on the surface directly reveals the presence of the wetting gradient, which can be modulated by changing the properties of the thiol. This is exemplified with a long, hydrophobic alkane-thiol and a short, hydrophilic mercaptan.