Propriétés de transport de nanoparticules magnétiques de FeCo auto-organisées

Je présente dans ce travail de thèse l’étude des propriétés de transport électronique de nanoparticules de FeCo auto-assemblées. Ces nanoparticules, métalliques et ferromagnétiques, sont obtenues par synthèse chimique et stabilisées par des ligands organiques. Deux types de systèmes ont été étudiés : des solides compacts issus directement de la synthèse (auto-assemblage spontané en réseau 3D) et des réseaux 2D obtenus après dépôt d’une solution de particules. Les mesures de magnéto-transport réalisées sur ces deux types d’échantillons nous ont donc permis d’étudier le rôle de la dimensionnalité et du désordre sur les propriétés de conduction.\ud Les deux types de systèmes présentent à basse température un comportement typique du blocage de Coulomb. Quand un champ magnétique est appliqué, deux régimes de magnétorésistance (MR) sont observés :\ud - Entre 1.8 K et 10 K, des effets magnétorésistifs apparaissent pour des valeurs de champ magnétique supérieures au champ de saturation magnétique des particules. Un modèle phénoménologique basé sur la présence d’impuretés paramagnétiques localisées dans la barrière a été développé afin de reproduire les propriétés de cette MR.\ud - En dessous de 1.8 K, cette MR à fort champ disparaît et une MR classique reliée à l’aimantation des particules est mesurée. \ud Enfin, des effets collectifs ont été observés sur certains échantillons. Ils se manifestent sur le transport électronique par des transitions hystérétiques entre deux modes de conduction distincts. L’application d’un champ magnétique induit la transition entre ces deux régimes. Dans des conditions optimisées, cette transition se révèle réversible, donnant lieu à un nouveau type de MR._________________________________________________________________________________________ I report in this study the transport properties of self-assembled CoFe nanoparticles. These nanoparticles, which are metallic and ferromagnetic, are obtained by chemical synthesis and stabilized by organic ligands. Two types of systems have been studied: as-prepared solids resulting from spontaneous self-organization of the nanoparticles in 3D lattices and 2D lattices obtained by a deposition of a solution of nanoparticles. Thus, the magnetotransport measurements of both systems allow us to study the role of the dimensionality and the disorder on the electronic transport.\ud All systems show at low temperature typical features of Coulomb blockade in arrays of nanoparticles. When an external magnetic field is applied, two regimes of magnetoresistance (MR) are observed :\ud - The first one between 1.8 K and 10 K is characterized by a MR which appears for values of the applied field larger than the saturation field of the nanoparticles. A phenomenological model based on magnetic impurities localized between the nanoparticles has been developed in order to describe the properties of this MR.\ud - Below 1.8 K, this high-field MR collapses and a classical MR related to the magnetization of the particles occurs.\ud Finally, novel properties attributed to the collective behaviour of the nanoparticles have been observed in some samples. It is displayed in the electronic conduction by hysteretic transitions between two regimes of conduction. We show that an external magnetic field can induce the transition between both regimes. In optimum conditions, this transition is reversible and leads to a new mechanism of MR. \ud \u

Voltage and Temperature Dependence of High-Field Magnetoresistance in Arrays of Magnetic Nanoparticles

Huge values of high field magnetoresistance have been recently reported in large arrays of CoFe nanoparticles embedded in an organic insulating lattice in the Coulomb blockade regime. An unusual exponential decrease of magnetoresistance with increasing voltage was observed, as well as a characteristic scaling of the magnetoresistance amplitude versus the field-temperature ratio. We propose a model which takes into account the influence of paramagnetic impurities on the transport properties of the system to describe these features. It is assumed that the non-colinearity between the core spins inside the nanoparticles and the paramagnetic impurities can be modelled by an effective tunnel barrier, the height of which depends on the relative angle between the magnetization of both kind of spins. The influence on the magnetotransport properties of the height and the thickness of the effective tunnel barrier of the magnetic moment of the impurity, as well as the bias voltage are studied. This model allows us to reproduce the large magnetoresistance magnitude observed and its strong voltage dependence, with realistic parameters.Comment: 19 pages, with 6 figures, references and figure caption

High-field and low field magnetoresistance of CoFe nanoparticles elaborated by organometallic chemistry

We report on magnetotransport measurements on CoFe nanoparticles surrounded by an insulating organic layer. Samples were obtained by evaporating a solution of nanoparticles on a patterned substrate. Typical behaviour of Coulomb blockade in array of nanoparticles is observed. High and low field magnetoresistance have been evidenced. Below 10 K, a large high-field magnetoresistance is measured, reaching up to 500 %. Its amplitude decreases strongly with increasing voltage. At 1.6 K, this high-field magnetoresistance vanishes and an inverse low field tunnelling magnetoresistance is observed.Comment: 12 pages, with 3 figures, references and figure captions. Proceeding of the 52nd MMM conferenc

Transport in Magnetic Nanoparticles Super-Lattices : Coulomb Blockade, Hysteresis and Magnetic Field Induced Switching

We report on magnetotransport measurements on millimetric super-lattices of Co-Fe nanoparticles surrounded by an organic layer. At low temperature, the transition between the Coulomb blockade and the conductive regime becomes abrupt and hysteretic. The transition between both regime can be induced by a magnetic field, leading to a novel mechanism of magnetoresistance. Between 1.8 and 10 K, high-field magnetoresistance due to magnetic disorder at the surface of the particles is also observed. Below 1.8 K, this magnetoresistance abruptly collapses and a low-field magnetoresistance is observed.Comment: 9 pages (text, figures, figures legends and references). 3 figures reference 33 added : "arXiv:0710.1750v1

3000 % high-field magnetoresistance in super-lattices of CoFe nanoparticles

We report on magnetotransport measurements on millimetre-large super-lattices of CoFe nanoparticles surrounded by an organic layer. Electrical properties are typical of Coulomb blockade in three dimensional arrays of nanoparticles. A large high-field magnetoresistance, reaching up to 3000 %, is measured between 1.8 and 10 K. This exceeds by two orders of magnitude magnetoresistance values generally measured in arrays of 3d metals ferromagnetic nanoparticles. The magnetoresistance amplitude scales with the magnetic field / temperature ratio and displays an unusual exponential dependency with the applied voltage. The magnetoresistance abruptly disappears below 1.8 K. We propose that the magnetoresistance is due to some individual paramagnetic moments localized between the metallic core of the nanoparticles, the origin of which is discussed.Comment: 9 pages (text, references, figures and legends

Targeting a G-Protein-Coupled Receptor Overexpressed in Endocrine Tumors by Magnetic Nanoparticles To Induce Cell Death

Nanotherapy using targeted magnetic nanoparticles grafted with peptidic ligands of receptors overexpressed in cancers is a promising therapeutic strategy. However, nanoconjugation of peptides can dramatically affect their properties with respect to receptor recognition, mechanism of internalization, intracellular trafficking, and fate. Furthermore, investigations are needed to better understand the mechanism whereby application of an alternating magnetic field to cells containing targeted nanoparticles induces cell death. Here, we designed a nanoplatform (termed MG-IONP-DY647) composed of an iron oxide nanocrystal decorated with a ligand of a G-protein coupled receptor, the cholecystokinin-2 receptor (CCK2R) that is overexpressed in several malignant cancers. MG-IONP-DY647 did not stimulate inflammasome of Raw 264.7 macrophages. They recognized cells expressing CCK2R with a high specificity, subsequently internalized via a mechanism involving recruitment of β-arrestins, clathrin-coated pits, and dynamin and were directed to lysosomes. Binding and internalization of MG-IONP-DY647 were dependent on the density of the ligand at the nanoparticle surface and were slowed down relative to free ligand. Trafficking of CCK2R internalized with the nanoparticles was slightly modified relative to CCK2R internalized in response to free ligand. Application of an alternating magnetic field to cells containing MG-IONP-DY647 induced apoptosis and cell death through a lysosomal death pathway, demonstrating that cell death is triggered even though nanoparticles of low thermal power are internalized in minute amounts by the cells. Together with pioneer findings using iron oxide nanoparticles targeting tumoral cells expressing epidermal growth factor receptor, these data represent a solid basis for future studies aiming at establishing the proof-of-concept of nanotherapy of cancers using ligand-grafted magnetic nanoparticles specifically internalized via cell surface receptors

Practical Works on Nanotechnology: Middle School to Undergraduate Students

Since its emergence a few decades ago, nanotechnology has been shown to be a perfect example of a crossroad between different fundamentals sciences. In the last 10 years, the continuous progress of classical top-down lithography and the use of alternative bottom-up elaboration methods has allowed new and smaller components to be created. Their combination has led to very complex and innovative architectures. At the same time, flexible, low-cost, and low-ecological-footprint devices have emerged. Thus, the diversity and multidisciplinary features present challenges in addressing these issues in educational programs

Prospects for Food Fermentation in South-East Asia, Topics From the Tropical Fermentation and Biotechnology Network at the End of the AsiFood Erasmus+Project

Fermentation has been used for centuries to produce food in South-East Asia and some foods of this region are famous in the whole world. However, in the twenty first century, issues like food safety and quality must be addressed in a world changing from local business to globalization. In Western countries, the answer to these questions has been made through hygienisation, generalization of the use of starters, specialization of agriculture and use of long-distance transportation. This may have resulted in a loss in the taste and typicity of the products, in an extensive use of antibiotics and other chemicals and eventually, in a loss in the confidence of consumers to the products. The challenges awaiting fermentation in South-East Asia are thus to improve safety and quality in a sustainable system producing tasty and typical fermented products and valorising by-products. At the end of the “AsiFood Erasmus+ project” (www.asifood.org), the goal of this paper is to present and discuss these challenges as addressed by the Tropical Fermentation Network, a group of researchers from universities, research centers and companies in Asia and Europe. This paper presents current actions and prospects on hygienic, environmental, sensorial and nutritional qualities of traditional fermented food including screening of functional bacteria and starters, food safety strategies, research for new antimicrobial compounds, development of more sustainable fermentations and valorisation of by-products. A specificity of this network is also the multidisciplinary approach dealing with microbiology, food, chemical, sensorial, and genetic analyses, biotechnology, food supply chain, consumers and ethnology

Bioremediation study of pentachlorophenol (PCP) in soil matrix using PCP degrader isolated from PCP acclimated sludge

Pentachlorophenol (PCP) is listed as a priority toxic pollutant because of its high toxicity and low biodegradability in soil matrix. PCP biodegradation in soil is slow due to insufficient number of PCP degrader and inhibition by toxic concentration of PCP. Inoculation with PCP degraders (i.e. bioaugmentation) may, in some cases, be the only way for microbial clean-up of soil contaminated sites (Miethling and Karlson, 1996). The first phase of this study involved the acclimation of PCP as this process allowed microbes to adapt with the new substrate PCP by increasing the PCP concentration stepwise. PCP degrader was then isolated and identified. Moreover, physiological characteristics of this PCP degrader were also investigated. The second phase concerned with bioremediation of PCP in soil with different treatments including bioaugmentation, natural attenuation and control. The soil pH, conductivity, temperature, moisture content, microbial growth, CO2 evolution, PCP and chloride concentration were then investigated. Results showed that the isolated PCP degrader derived from PCP acclimated sludge was Pseudomonas putida. It was characterized as round gramnegative bacteria, non-sporulating rod, 1.25-1.5μm length and 0.4-0.5μm width with growth rate and specific growth rate calculated to be 0.0204 min-1 and 0.1416 h-1, respectively. The ability of P. putida was tested in a batch culture with PCP as sole carbon source and energy and test results showed that P. putida can degrade different PCP concentrations. Bioaugmentation had a faster degradation time of 10 days compared to that of 15 days using natural attenuation method. The chloride concentrations were observed to be 11.36ppm and 10.1ppm, respectively. This study concluded that it is possible to isolate PCP degrader from PCP acclimated sludge. The inoculation of PCP degrader has the potential for microbial clean-up of soil contaminated sites