Synthèse et activité catalytique des cristaux ZIF-8 et ZIF-8 dopés : évaluation de leur stabilité et leur toxicité

Metal organic frameworks (MOFs) have gained considerable attention as heterogeneous catalytic systems and also have been studied in the area of separation, gas storage, controlled release or as drug delivery systems. According to their complex structure formed by metal centers coordinated with polydentate linkers, MOFs expose abundance of Lewis and/or Brönsted acid-base sites that are crucial for the materials catalytic activity and selectivity towards specific reactions. Moreover, these materials have many other attractive properties, including a large surface area, a low density and a high porosity. In this work, we focused on the zeolithic imidazolate framework (ZIF-8) material – a MOF exhibiting high porosity and stability and which can also be used as a template for further functionalization and modification. Firstly, we focused on the preparation of ZIF-8 crystals and ZIF-8 heterostructures with properties adapted to the desired application, and then shaping of the catalyst to obtain the best form of material for industrial scale-up utilization. By varying Zn2+ precursors used for the synthesis, we demonstrated that the properties (size, porosity,…) of ZIF-8 crystals can be controlled and tuned depending on the applications. These ZIF-8 crystals were successfully applied as heterogeneous catalysts in Knoevenagel and Friedländer reactions. Next, we developed protocols for the synthesis of Cu2+-doped ZIF-8 crystals. The use of these crystals could be extended to Cu-mediated reactions, like the Combes condensation and the Huisgen cycloaddition. We evaluated recyclability and we showed that the nanomaterials could be reused up to ten times without any loss of catalytic activity. Moreover, we functionalized ZIF-8 crystals with magnetic Fe3O4 nanoparticles. The hybrid Fe3O4@ZIF-8 heterostructures could be easily recovered by magnetic separation after catalytic experiments. To show multiple benefits originating from the ZIF-8 structure and properties, we also used this material for the conversion of CO2 into cyclic carbonates using a Parr reactor. As the reaction could be scale-up at the industrial level, we shaped the powder in the form of pellets and use it under the same conditions. In the last chapter, we evaluated the toxicity and the stability in biological media of ZIF-8, Cu- and Fe-doped ZIF-8 particles using A549 alveolar cells, IHK skin cells as models and in vitro ingestion under fed conditions. These models were chosen according to the most probable first contact entering gates for nanoparticles inside human body, skin, lungs and digestive tract. Outcomes from these preliminary studies motivated us to conduct extended stability tests of the particles in different media. We showed that the particles are altered by pH changes and medium complexityDepuis plusieurs années, les MOFs (Metal Organic Frameworks) suscitent une grande attention pour leurs applications potentielles en catalyse hétérogène. Ces matériaux sont également étudiés dans les domaines de la séparation, du stockage de gaz, de la libération contrôlée ou comme systèmes de vectorisation de médicaments. Leur structure complexe étant formée par des centres de coordination métalliques unis par des ligands polydentates, les MOFs disposent de nombreux sites acido-basiques de Lewis ou de Brönsted qui sont cruciaux pour l'activité des matériaux et la sélectivité des produits lors des réactions catalysées par les MOFs. Dans ce travail, nous nous sommes tout particulièrement intéressés à une classe de MOFs appelée « zéolithic imidazolate frameworks » (ZIF-8). Les ZIF-8 présentent de nombreuses propriétés intéressantes, notamment une grande surface spécifique, une faible densité, une forte porosité ainsi qu’une excellente stabilité chimique et thermique. Dans une première partie, la préparation de matériaux ZIF-8 et d’hétérostructures à base ZIF-8 a été développée afin de conférer à ces matériaux des propriétés adaptées à l'application souhaitée. La mise en forme de ces catalyseurs a également été étudiée afin d’obtenir la forme optimale pour une utilisation industrielle de ces matériaux. En variant le précurseur d’ions Zn2+ utilisé pour la synthèse, nous avons démontré que les propriétés (taille, porosité, ...) de ZIF-8 cristaux pouvaient être contrôlées en fonction de l’application catalytique recherchée. Ces ZIF-8 cristaux ont été utilisés avec succès en tant que catalyseurs hétérogènes dans les réactions de Knoevenagel et Friedländer. Nous avons développé de nouveaux matériaux ZIF-8 dopés par des ions Cu2+. Les particules Cu/ZIF-8 se sont montrés être des catalyseurs efficaces dans la réaction de Combes et la cycloaddition de Huisgen. La recyclabilité du matériau a été évaluée et il a notamment été montré que les particules ZIF-8 pouvaient être réutilisées jusqu'à dix fois sans perte d'activité catalytique. Nous avons également fonctionnalisé les cristaux ZIF-8 avec des particules magnétiques Fe3O4. L’hétérostructure hybride Fe3O4@ZIF-8 peut facilement être récupérée par séparation magnétique après les expériences de catalyse. Afin d’étendre le champ d’application des catalyseurs ZIF-8, le matériau a également utilisé pour la conversion du dioxyde de carbone en carbonates cycliques en utilisant un réacteur du Parr. Comme la réaction est d’un grand intérêt industriel, le catalyseur a été mis en forme par compression. Dans la dernière partie de ce mémoire, la toxicité des particules ZIF-8 et ZIF-8 dopé par Cu ou Fe a été évaluée en utilisant des cellules alvéolaires A549 et de la peau IHK comme modèles. La stabilité des particules a été déterminée à l’aide de milieux mimant la digestion des particules in vivo. Les résultats obtenus montrent que les particules sont très sensibles aux variations de pH ainsi qu’aux sels présents dans les différents milieu

Synthesis of ZIF-8 crystals and application to carbon dioxide conversion

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Photo-induced design of reflective metallized gold@polymer coatings with tuned architecture

International audience. This paper presents an efficient, eco-friendly, low cost and totally photo-induced approach to obtain new gold metallized coatings with highly reflective feature. Depending on gold precursor used and experimental conditions , the metal nanoparticles are distributed full depth or concentrated at the surface of the polymer matrix. The spatial organization and distribution of gold nanoparticles (AuNPs) is obtained through a perfect tuning of two concurrent processes, i.e. photo-polymerization of the binder and photo-reduction of the gold precursor to Au(0) nanoparticles, the balance of which is influenced depthwise, by internal filter effects associated to the absorption of gold particles or precursor. Two distinct classes of architectures were obtained. One of them was a bi-layered coating, i.e. a polymer rich sub-layer with a metal cover layer with a sharp delimitation between them; as for itself, the second type of metal@polymer coating exhibited a clear depthwise gradient of Au NPs concentration with a quasi-compact metal layer at the air surface. The mechanisms underlying these two types of architectures are discussed. The metal@polymer coatings developed with this innovative approach are full of promise in many fields of application such as large scale functionalized surfaces, adaptive optical devices (plasmonic and mirror), flexible conductive surfaces and aesthetics

From visible to white-light emission by siloxane-capped ZnO quantum dots upon interaction with thiols

International audienceThe interaction of thiols (glutathione, cysteine, and cysteamine) with yellow-emitting siloxane-capped ZnO QDs was studied. A gradual enlargement of the PL emission band resulting in white-light emission was observed upon reaction with thiols, while the diameter (ca. 4 nm) and the crystallinity of the dots were not affected. The appearance of broad white-emission was accompanied by a decrease of the photoluminescence quantum yield from 16% to 5-6%. Generation of surface defect states through interaction of the thiols with Zn surface atoms of the dots provoking shrunk of the siloxane capping may be responsible of that broadband emission throughout most of the light spectrum

Fe3O4@ZIF-8: magnetically recoverable catalysts by loading Fe3O4 nanoparticles inside a zinc imidazolate framework

International audienceA simple methodology for encapsulating ca. 10 nm-sized superparamagnetic Fe3O4 nanoparticles in zeolitic imidazolate frameworks (ZIF-8) crystals was developed. The corresponding Fe3O4@ZIF-8 heterostructured material exhibits bifunctional properties with both high magnetization (Fe3O4) and high thermal stability, large specific surface, and catalytic properties (ZIF-8). The Fe3O4@ZIF-8 catalyst exhibits fair separation ability and reusability, which can be repeatedly applied for Knoevenagel condensations and Huisgen cycloadditions for at least ten successive cycles

Cu2+-doped zeolitic imidazolate frameworks (ZIF-8): efficient and stable catalysts for cycloadditions and condensation reactions

International audienceCu2+-doped zeolitic imidazolate framework (ZIF) crystals were efficiently prepared by reaction of Cu(NO3)2, Zn(NO3)2, and 2-methylimidazole in methanol at room temperature. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction showed that the Cu/ZIF-8 particles were nanosized (between ca. 120 and 170 nm) and that the body-centered cubic crystal lattice of the parent ZIF-8 framework is continuously maintained, regardless of the doping percentage. Moreover, thermogravimetric analyses and specific BET surface area measurements demonstrated that the doping does not alter the high stability of ZIF-8 crystals and that the porosity only decreases at a high doping percentage (25% in Cu2+). The Cu/ZIF-8 material showed excellent catalytic activity in the [3 + 2] cycloaddition of organic azides with alkynes and in Friedländer and Combes condensations due to the high catalyst surface area and the high dispersion of Cu/ZIF-8 particles. Notably, the Cu/ZIF-8 particles not only exhibit excellent performance but also show great stability in the reaction, allowing their reuse up to ten times in condensation reactions. Our findings explored a simple and powerful way to incorporate metal ions into the backbones of open framework materials without losing their properties

Thermoswitchable fluorescent nanoparticles: preparation, properties and applications

International audienceIn this work, we developed a new process to covalently graft a thermo-responsive polymer on the surface of fluorescent nanocrystals in order to synthesize hybrid materials that combine both responsive and fluorescent properties. Nontoxic ZnO quantum dots (QDs) formed the core because usually QDs shows excellent fluorescence properties. These dots were synthesized by a sol-gel method followed by stabilization with organosilane. For the first time, poly(N-isopropylacrylamide) (PNIPAM) and poly(ether oxide) derivatives copolymers were grown by Activator ReGenerated by Electron Transfer-Atom Transfer Radical Polymerization (ARGET-ATRP) from ZnO quantum dots by surface-initiated polymerization. This process allowed the formation of fluorescent and responsive ZnO/PNIPAM core/shell QDs and ZnO/(co-)polymer while only requiring the use of ppm amount of copper for the synthesis. Each step of the synthesis process was monitored by the combination of various techniques such as UV measurements, fluorescence spectroscopy, dynamic light scattering and transmission electron microscopy. More particularly, thermo-responsive polymers are generally characterized by either lower critical solution temperature (LCST) or upper critical solution temperature (UCST). The LCST represents the temperature above which the polymer phase is separated from water, and below the LCST this polymer is miscible in water. We demonstrated that aqueous dispersible core/shell ZnO@PNIPAM QDs could be prepared by growing PNIPAM chains by ARGET-ATRP from silanized ZnO surfaces. The influence of the nature of the silanized layer and the PNIPAM polymerization time on the properties of the final nanomaterials were investigated. Results clearly evidence that the nanomaterial displays temperature-dependent optical properties and sizes. Both the PNIPAM layer thickness and the temperature affected the luminescence properties of the core/shell nanoparticles. In addition, the PNIPAM layer, when it is thick enough, could stabilize the QDs optical properties. Copolymer with different length chain and proportions of 2-(2-methoxyethoxy) ethyl methacrylate (MEO2MA) and hydroxyl-terminated oligo (ethylene glycol) methacrylate (HOEGMA) were also used as thermoswitchable copolymers. The nanomaterials exhibit temperature-dependent optical and sizes properties which depend mainly on the content of each monomer. By tuning the amount of each monomer, LCST ranging from 20 to 50°C can be obtained. Then, drug loading and release capacity of ZnO/(co-)polymer as well as their cytotoxicity towards cancer cells were evaluated. Results demonstrate the impact of the co-polymer structure on quantum dots properties in view of the application in cancer therapy