Insight on Thermal Stability of Magnetite Magnetosomes: Implications for the Fossil Record and Biotechnology

Magnetosomes are intracellular magnetic nanocrystals composed of magnetite (Fe3O4) or greigite (Fe3S4), enveloped by a lipid bilayer membrane, produced by magnetotactic bacteria. Because of the stability of these structures in certain environments after cell death and lysis, magnetosome magnetite crystals contribute to the magnetization of sediments as well as providing a fossil record of ancient microbial ecosystems. The persistence or changes of the chemical and magnetic features of magnetosomes under certain conditions in different environments are important factors in biotechnology and paleomagnetism. Here we evaluated the thermal stability of magnetosomes in a temperature range between 150 and 500 °C subjected to oxidizing conditions by using in situ scanning transmission electron microscopy. Results showed that magnetosomes are stable and structurally and chemically unaffected at temperatures up to 300 °C. Interestingly, the membrane of magnetosomes was still observable after heating the samples to 300 °C. When heated between 300 °C and 500 °C cavity formation in the crystals was observed most probably associated to the partial transformation of magnetite into maghemite due to the Kirkendall effect at the nanoscale. This study provides some insight into the stability of magnetosomes in specific environments over geological periods and offers novel tools to investigate biogenic nanomaterials

Tomographie électronique analytique (développement de la cartographie chimique 3D à l'échelle du nanomètre et applications aux catalyseurs hétérogènes)

Cette thèse a été consacrée au développement de la tomographie analytique pour l analyse des nanomatériaux, une technique qui combine la tomographie électronique et l imagerie filtrée en énergie. En enregistrant pour différentes orientations de l objet une série d images filtrées en énergie sur les seuils d ionisation des éléments d intérêt, plusieurs volumes chimiquement sélectifs peuvent être calculés à travers un algorithme de reconstruction. Grâce à sa double sélectivité, au caractère 3D de l objet et à sa composition chimique, cette technique présente un fort potentiel pour la caractérisation des nanomatériaux chimiquement inhomogènes. Dans ce contexte, notre but a été de définir une méthodologie de travail pour permettre son application à l échelle du nanomètre. Pour ceci, une maîtrise parfaite de toutes les étapes à suivre et un ajustement pointu de différents paramètres ont été nécessaires. Validée tout d abord sur des échantillons modèles, la méthodologie a été ensuite appliquée à l étude de deux familles de catalyseurs mixtes avec une résolution de quelques nanomètres. L analyse combinée des volumes chimiques obtenus sur le même échantillon nous a permis de déduire des paramètres crucials pour les applications en catalyse (comme par exemple la proportion relative des composants à la surface du grain). La visualisation en 3D de l'agencement des éléments chimiques constituant le grain analysé a déjà eu un premier impact bénéfique sur le contrôle et l amélioration de la synthèse de ces catalyseurs.This work is devoted to the development of analytical electron tomography for the 3D chemical analysis of nanomaterials, a new technique combining the electron tomography and the energy filtered imaging within an electron microscope. By acquiring series of energy filtered images at different angular orientations of the object, their 3D chemical spatial distribution can be achieved by employing a specific reconstruction algorithm. The energy positions used for the filtering correspond to the ionization edges of the elements of interest. Owing to its double selectivity, this technique is particularly powerful when applied to heterogeneous nanomaterials. In this context, the emphasis has been put on defining a precise methodology that would be implemented at the nanometric scale. To do so, an accurate assessment of all steps of analysis as well as a fine adjustment of experimental conditions and parameters of data treatment are imperatively required. Therefore, model objects have been analyzed in a first step in order to validate our protocol. The procedure as defined was then used for the study of two classes of heterogeneous catalysts, with a spatial resolution of several nanometers. The analysis of all elemental volumes within the very same sample allowed us to quantify key-parameters governing the catalytic activity of these specimens, such as the relative proportion of the components at the surface of a grain. The 3D visualization of the chemical composition of heterogeneous nanomaterials has been already implemented for improving the catalysts synthesis methods.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Étude des couches nano-structurées obtenues par ablation laser pour les implants biomimétiques avancés

La thèse est consacrée à la compréhension et l application des techniques de transfert laser de la matière afin d obtenir une nouvelle génération d implants d os présentant une meilleure biointégration. Le recouvrement des implants en Ti avec des couches minces bioactives en hydroxyapatite et macromolécules biologiques permet d améliorer la réponse du corps par rapport à l objet implanté. Ce nouveau type d implant conserve les propriétés mécaniques du Ti, en augmentant sa capacité d intégration dans l environnement biologique. La thèse est structurée en trois parties. La première constitue le cadre théorique: la présentation des matériaux métalliques et céramiques candidats pour la fabrication des implants orthopédiques ou dentaires, les avantages de l utilisation des couches minces d HA, ainsi que le potentiel de l utilisation des protéines de la matrice extracellulaire. La deuxième partie décrit les techniques de transfert laser utilisées dans la thèse. La troisième partie est consacrée aux diverses études expérimentales entreprises : (i) la corrélation des mécanismes d ablation laser et de croissance des couches d hydroxyapatite, (ii) l étude des changements induits par la fluence laser, l amélioration (iii) de la cristallinité des couches en appliquant un traitement thermique post-dépôt en vapeur d eau et (iv) de l adhérence entre la couche et le substrat à l aide d une inter-couche à gradient de propriétés. Dans les dernières études, les techniques (v) MAPLE et (vi) LDW ont été proposées pour le transfert des structures de fibronectine afin d obtenir une amélioration de la fonctionnalité des surfaces. La thèse est finie avec les conclusions et les perspectives.The thesis main goal is the understanding and application of material laser transfer techniques applied in order to obtain a new generation of bone implants, exhibiting a better biointegration. The Ti implant thin film covering allows an improvement of the body response in respect to the implanted object. This new implant type preserves the mechanical properties of Ti but increases its integration capacity in the biological environment. The thesis is divided in three parts. The first one presents the theoretical frame: the metallic and ceramic materials appropriate for orthopedic or dental implants, the advantages of using thin films of HA, as well as the potential given by the extracellular matrix proteins. The second part describes the laser transfer techniques used within this thesis. The third part is devoted to the experimental studies: (i) the correlation between the laser ablation and thin films growth mechanisms, (ii) the study of the changes induced by the laser fluence, the improvement (iii) of the thin films crystallinity by applying a post-deposition heat treatment in water vapor and (iv) of the thin film adherence to the substrate by the interposition of a gradual layer. In the last studies, the (v) MAPLE and (vi) LDW techniques were suggested for fibronectin structures transfer in order to obtain the surfaces functionalization. The thesis is accomplished by the conclusions and perspectives.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Physical–chemical and biological behavior of an amorphous calcium phosphate thin film produced by RF-magnetron sputtering

International audienc

New Phenotype and Mineralization of Biogenic Iron Oxide in Magnetotactic Bacteria

Many magnetotactic bacteria (MTB) biomineralize magnetite crystals that nucleate and grow inside intracellular membranous vesicles originating from invaginations of the cytoplasmic membrane. The crystals together with their surrounding membranes are referred to as magnetosomes. Magnetosome magnetite crystals nucleate and grow using iron transported inside the vesicle by specific proteins. Here, we tackle the question of the organization of magnetosomes, which are always described as constituted by linear chains of nanocrystals. In addition, it is commonly accepted that the iron oxide nanocrystals are in the magnetite-based phase. We show, in the case of a wild species of coccus-type bacterium, that there is a double organization of the magnetosomes, relatively perpendicular to each other, and that the nanocrystals are in fact maghemite. These findings were obtained, respectively, by using electron tomography of whole mounts of cells directly from the environment and high-resolution transmission electron microscopy and diffraction. Structure simulations were performed with the MacTempas software. This study opens new perspectives on the diversity of phenotypes within MTBs and allows to envisage other mechanisms of nucleation and formation of biogenic iron oxide crystals

Direct Observation of Stacking Faults and Pore Connections in Ordered Cage-Type Mesoporous Silica FDU-12 by Electron Tomography.

International audienceThe porous structure and the periodic array of cavities in ordered mesoporous materials with large, three-dimensionally arranged and interconnected pores is thoroughly described by combining electron tomography, small-angle X-ray diffraction, and nitrogen sorption techniques. We used the ability of the electron tomography to provide local three-dimensional information of a nano-object and compared the results to those of the other characterization techniques which furnish global information. We showed thus that the face-centered cubic (fcc) structure usually assigned to the FDU-12 materials is in fact an intergrowth of cubic and hexagonal close-packing structures. This agrees with small-angle X-ray scattering (SAXS) modeling, but for the first time a direct visualization of these stacking faults was achieved. Three-dimensional transmission electron microscopy (3D-TEM) provides also a direct and unique evidence of peculiar stacking defects ("z-shifted [111] areas"), as well as an estimate of their density, which have never been reported elsewhere. In addition, interstitial cavities were also observed, revealing the complex defective structure of this material. A direct observation of the nature of the connecting pores was also achieved for the first time, with a resolution limit of 2 nm. Finally, the characteristics of the porous network evidenced by 3D-TEM are used to explain and validate the results obtained by nitrogen sorption experiments

Localized iron accumulation precedes nucleation and growth of magnetite crystals in magnetotactic bacteria

International audienc

Three-Dimensional Chemistry of Multiphase Nanomaterials by Energy-Filtered Transmission Electron Microscopy Tomography

A three-dimensional (3D) study of multiphase nanostructures by chemically selective electron tomography combining tomographic approach and energy-filtered imaging is reported. The implementation of this technique at the nanometer scale requires careful procedures for data acquisition, computing, and analysis. Based on the performances of modern transmission electron microscopy equipment and on developments in data processing, electron tomography in the energy-filtered imaging mode is shown to be a very appropriate analysis tool to provide 3D chemical maps at the nanoscale. Two examples highlight the usefulness of analytical electron tomography to investigate inhomogeneous 3D nanostructures, such as multiphase specimens or core-shell nanoparticles. The capability of discerning in a silica-alumina porous particle the two different components is illustrated. A quantitative analysis in the whole specimen and toward the pore surface is reported. This tool is shown to open new perspectives in catalysis by providing a way to characterize precisely 3D nanostructures from a chemical point of view