Anne Hébraud

From self-assembly of electrospun nanofibers to 3D cm thick hierarchical foams

Electrospinning usually results in the formation of scaffolds that are a few hundred microns in thickness with pore sizes in the micron range. However some applications, such as tissue engineering, necessitate the fabrication of cm-thick nanofibrous scaffolds with large pore sizes that allow for cell infiltration. Here, we demonstrate for the first time the production of bioresorbable poly(e-caprolactone) nanofibrous cm-thick foams using the electrospinning technique. These scaffolds were obtained through the dynamic self-assembly of electrospun nanofibers into honeycomb patterns, which resulted in a unique columnar hierarchical structure with both micropores and mesopores of up to several hundreds of microns in size. This specific morphology leads to mechanical properties of thick scaffolds, suitable for handling and implanting in vivo.

Simultaneous Electrospinning and Electrospraying: A Straightforward Approach for Fabricating Hierarchically Structured Composite Membranes

We present here for the first time a simple method for micropatterning nonwoven composite membranes. The approach is based on the simultaneous electrospraying of microparticles and electrospinning of nanofibers from different polymer solution feeds (polyethylene glycol and poly(D,L-lactide)) on a common support. The mechanism of self-organization between fibers and particles into hierarchical honeycomb-like structures, as well as the evolution of the later as a function of the thickness of the composite, is investigated. We demonstrate that aggregates of particles, leading to a nonuniform distribution of the electrostatic field near the collector, are necessary to form the self-organized composite. Furthermore, it is shown that the specific dimensions of the generated patterns can be controlled by tuning the flow rate of electrospraying. The obtained composite mat exhibits a multilevel porous structure, with pore sizes ranging from few up to several hundreds of micrometers. Finally, it is shown that the microparticles can be selectively leached, allowing the production of a monocomponent membrane and retaining the hierarchical organization of the nanofibers suitable for biomedical and filtration applications.

Differentiation of human adipose-derived stem cells seeded on mineralized electrospun co-axial poly(ε-caprolactone) (PCL)/gelatin nanofibers

Abstract Mineralized poly(ε-caprolactone)/gelatin core–shell nanofibers were prepared via co-axial electrospinning and subsequent incubation in biomimetic simulated body fluid containing ten times the calcium and phosphate ion concentrations found in human blood plasma. The deposition of calcium phosphate on the nanofiber surfaces was investigated through scanning electronic microscopy and X-ray diffraction. Energy dispersive spectroscopy results indicated that calcium-deficient hydroxyapatite had grown on the fibers. Fourier transform infrared spectroscopy analysis suggested the presence of hydroxyl-carbonate-apatite. The results of a viability assay (MTT) and alkaline phosphatase activity analysis suggested that these mineralized matrices promote osteogenic differentiation of human adipose-derived stem cells (hASCs) when cultured in an osteogenic medium and have the potential to be used as a scaffold in bone tissue engineering. hASCs cultured in the presence of nanofibers in endothelial differentiation medium showed lower rates of proliferation than cells cultured without the nanofibers. However, endothelial cell markers were detected in cells cultured in the presence of nanofibers in endothelial differentiation medium.

Design of Colored Multilayered Electrophoretic Particles for Electronic Inks

The preparation of multilayered latex particles with surface functional groups suitable for use as electrophoretic particles in electronic inks has been studied. The particles are formed by dispersion polymerization and have a polystyrene core, slightly cross-linked with divinylbenzene (DVB), and a poly(methyl methacrylate) (PMMA) or a poly(acrylic acid) (PAA) shell. After grafting alkyl chains on their surface, the particles are negatively or positively charged and sterically stabilized against aggregation in nonpolar solvent. The particles were dyed by incorporation of Nigrosin during polymerization or by swelling in supercritical CO(2) in the presence of a dye. Particle size, morphology, incorporated dye content and zeta potential were determined. A dual-particle electronic ink based on a mixture of the colored multilayered particles and white hybrid TiO(2)-polymer particles was prepared and electro-optically tested.

Thiazole-based scaffolding for high performance solar cells

An interesting way of decreasing both HOMO and LUMO energy levels simultaneously while keeping the band-gap constant in soluble electron-donor small molecules for photovoltaic applications is presented. This consists in the replacement of thiophene rings by thiazole units in small molecules based on the alternation of electron-rich and electron-deficient moieties. A new diketopyrrolopyrrole-based dumbbell-shaped electron-donor soluble molecule for organic photovoltaic applications has been synthesized and characterized. It includes thiazole units as linkers between the bis-lactam core and the triazatruxene moieties used as π-stacking platforms. A power conversion efficiency of 6.3% has been attained with this thiazole derivative and in particular with an increase of the open-circuit voltage of 0.15 V with respect to the thiophene-based organic semiconducting counterpart. This open-circuit voltage increase is due to the lowering of the HOMO level of the thiazole derivative while its LUMO level has also been stabilized as highlighted by the similar band-gap measured for the thiazole and thiophene derivatives.

A deep-purple-grey thiophene–benzothiadiazole–thiophene BODIPY dye for solution-processed solar cells

In this work we explore the synthesis of an extended 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dye (BODIPY) engineered from thiophene–benzothiadiazole–thiophene modules linked in the 3,5-substitution positions. We found that this highly soluble dye absorbs up to 800 nm in solution and up to 900 nm in thin films. An effective charge transfer absorption band was found at around 479 nm. The hybrid dye emits at 778 nm with a quantum yield of about 6%. Similar electrochemical and optical gaps were determined about 1.36 eV. When deposited in thin films the dye exhibits an ambipolar nature with well-balanced hole and electron mobilities. Bulk heterojunction solar cells based upon this dye blended with [6,6]phenylC61or71butyricacid methylester (PC61BM or PC71BM) provide a power conversion efficiency of about 1.26% upon a mild thermal annealing.

One‐Pot Synthesis of a Nitrogen‐Doped Carbon Composite by Electrospinning as a Metal‐Free Catalyst for Oxidation of H2S to Sulfur

A macroscopic composite consisting of nitrogen‐doped carbon fibers (N@CFs) was synthesized by electrospinning. The as‐prepared N@CF material was further applied as a metal‐free catalyst in the catalytic oxidation of H2S to sulfur, which is one of the most important purification processes for raw chemical resources (that is, biogas, natural gas, and petrochemical compounds). The catalyst, after a carbonization step at T=800 °C, exhibits a high and stable desulfurization activity for more than 100 h of testing with 57 % H2S conversion and 95 % sulfur selectivity at T=230 °C, which is two times higher than that of the most active metal‐based catalyst (Fe2O3/SiC). The desulfurization performance could also be improved by changing the reactant velocity. Moreover, the macroscopic shaping with an inner hierarchical structure network allows the avoidance of problems linked with the transport and handling of nanoscopic carbon‐based materials and also enhances the mass diffusion during the oxidation reaction.

Rational Engineering of BODIPY-Bridged Trisindole Derivatives for Solar Cell Applications

Boron dipyrromethene (BODIPY) and its derivatives are known to be efficient photon-harvesting chromophores. However, their study as active materials in bulk heterojunction (BHJ) solar cells is still scarce. In this study, the development of new synthetic ways to design original BODIPY-based dumbbell-shape molecules, including a first 2,3,5,6-tetravinyl aromatic BODIPY molecule, is reported. High fill factors can be obtained in BHJ solar cells when blended with a fullerene derivative, leading to a new record BODIPY-based power conversion efficiency of 5.8 %.

Sampling the structure and chemical order in assemblies of ferromagnetic nanoparticles by nuclear magnetic resonance

Assemblies of nanoparticles are studied in many research fields from physics to medicine. However, as it is often difficult to produce mono-dispersed particles, investigating the key parameters enhancing their efficiency is blurred by wide size distributions. Indeed, near-field methods analyse a part of the sample that might not be representative of the full size distribution and macroscopic methods give average information including all particle sizes. Here, we introduce temperature differential ferromagnetic nuclear resonance spectra that allow sampling the crystallographic structure, the chemical composition and the chemical order of non-interacting ferromagnetic nanoparticles for specific size ranges within their size distribution. The method is applied to cobalt nanoparticles for catalysis and allows extracting the size effect from the crystallographic structure effect on their catalytic activity. It also allows sampling of the chemical composition and chemical order within the size distribution of alloyed nanoparticles and can thus be useful in many research fields.

Hierarchical porous carbon fibers/carbon nanofibers monolith from electrospinning/CVD processes as a high effective surface area support platform

Nanocarbons with unique physicochemical properties have been considered typical sustainable materials for use as catalyst supports and directly as catalysts. Unfortunately, the powder form of nanocarbons renders them difficult to use in industrial processes due to the high pressure drop, their difficulty of handling as well as health injuries caused to human beings. Herein, hierarchical carbon fibers/carbon nanofibers (CF/CNF) composites, with high effective surface areas and controlled macroscopic shapes, were successfully synthesized through a combination of electrospinning (ES) and chemical vapour deposition (CVD). A web of poly(acrylonitrile)/poly(vinyl pyrrolidone) (PAN/PVP) composite fibers embedding a nickel salt was firstly produced by electrospinning. After a carbonization step, the polymeric material was converted into porous carbon embedding nickel nanoparticles, available on the fiber surface. Then, the catalytic growth of the CNFs was carried out from the nickel nanoparticles by CVD leading finally to the formation of a hierarchical carbon web of hairy fibers with a high effective surface area. The density, diameters and lengths of the CNFs attached on the surface of the CFs could be finely tuned by adjusting the CVD conditions. The specific surface area of the CF/CNF monolith amounted to more than 200 m2 g−1 along with high accessibility due to the small dimensions. The hierarchical CF/CNF composite has been used as a metal-free catalyst for the steam- and oxygen-free catalytic dehydrogenation of ethylbenzene to styrene. The catalytic results have pointed out that such a monolith can be efficiently used as a material platform for different applications , going from catalysis to wastewater treatment, thanks to the high effective surface area and reactivity of the CNF with prismatic planes.