Controlled Magnetic Anisotropy in Single Domain Mn-doped Biosynthesized Nanoparticles

Magnetotactic bacteria Magnetospirillum gryphiswaldense synthesize cubo-octahedral shaped magnetite nanoparticles, called magnetosomes, with a mean diameter of 40 nm. The high quality of the biosynthesized nanoparticles makes them suitable for numerous applications in fields like cancer therapy, among others. The magnetic properties of magnetite magnetosomes can be tailored by doping them with transition metal elements, increasing their potential applications. In this work, we address the effect of Mn doping on the main properties of magnetosomes by the combination of structural and magnetic characterization techniques. Energy-dispersive X-ray spectroscopy, X-ray absorption nearedge structure, and X-ray magnetic circular dichroism results reveal a Mn dopant percentage of utmost 2.3%, where Mn cations are incorporated as a combination of Mn2+ and Mn3+, preferably occupying tetrahedral and octahedral sites, respectively. Fe substitution by Mn notably alters the magnetic behavior of the doped magnetosomes. Theoretical modeling of the experimental hysteresis loops taken between 5 and 300 K with a modified Stoner-Wohlfarth approach highlights the different anisotropy contributions of the doped magnetosomes as a function of temperature. In comparison with the undoped magnetosomes, Mn incorporation alters the magnetocrystalline anisotropy introducing a negative and larger cubic anisotropy down to the Verwey transition, which appears shifted to lower temperature values as a consequence of Mn doping. On the other hand, Mn-doped magnetosomes show a decrease in the uniaxial anisotropy in the whole temperature range, most likely associated with a morphological modification of the Mn-doped magnetosomes.The Spanish and Basque Governments are acknowledged for funding under project numbers MAT2017- 83631-C3-R and IT-1245-19, respectively

Intracellular transformation and disposal mechanisms of magnetosomes in macrophages and cancer cells

International audienceMagnetosomes are magnetite nanoparticles biosynthesized by magnetotactic bacteria. Given their potential clinical applications for the diagnosis and treatment of cancer, it is essential to understand what becomes of them once they are within the body. With this aim, here we have followed the intracellular long‐term fate of magnetosomes in two cell types: cancer cells (A549 cell line), because they are the actual target for the therapeutic activity of the magnetosomes, and macrophages (RAW 264.7 cell line), because of their role at capturing foreign agents. It is shown that cells dispose of magnetosomes using three mechanisms: splitting them into daughter cells, excreting them to the surrounding environment, and degrading them yielding less or non‐magnetic iron products. A deeper insight into the degradation mechanisms by means of time‐resolved X‐ray absorption near‐edge structure (XANES) spectroscopy has allowed us to follow the intracellular biotransformation of magnetosomes by identifying and quantifying the iron species occurring during the process. In both cell types there is a first oxidation of magnetite to maghemite and then, earlier in macrophages than in cancer cells, ferrihydrite starts to appear. Given that ferrihydrite is the iron mineral phase stored in the cores of ferritin proteins, this suggests that cells use the iron released from the degradation of magnetosomes to load ferritin. Comparison of both cellular types evidences that macrophages are more efficient at disposing of magnetosomes than cancer cells, attributed to their role in degrading external debris and in iron homeostasis

Magnetite Biomineralization in <i>Magnetospirillum gryphiswaldense</i>: Time-Resolved Magnetic and Structural Studies

Magnetotactic bacteria biosynthesize magnetite nanoparticles of high structural and chemical purity that allow them to orientate in the geomagnetic field. In this work we have followed the process of biomineralization of these magnetite nanoparticles. We have performed a time-resolved study on magnetotactic bacteria <i>Magnetospirillum gryphiswaldense</i> strain MSR-1. From the combination of magnetic and structural studies by means of Fe K-edge X-ray absorption near edge structure (XANES) and high-resolution transmission electron microscopy we have identified and quantified two phases of Fe (ferrihydrite and magnetite) involved in the biomineralization process, confirming the role of ferrihydrite as the source of Fe ions for magnetite biomineralization in <i>M. gryphiswaldense</i>. We have distinguished two steps in the biomineralization process: the first, in which Fe is accumulated in the form of ferrihydrite, and the second, in which the magnetite is rapidly biomineralized from ferrihydrite. Finally, the XANES analysis suggests that the origin of the ferrihydrite could be at bacterial ferritin cores, characterized by a poorly crystalline structure and high phosphorus content

Sodium Distribution and Reaction Mechanisms of a Na₃V₂O₂(PO₄)₂F Electrode during Use in a Sodium-Ion Battery

Ambient temperature sodium-ion batteries are emerging as an exciting alternative to commercially dominant lithium-ion batteries for larger scale stationary applications. In order to realize such a sodium-ion battery, electrodes need to be developed, understood, and improved. Here, Na₃V₂O₂(PO₄)₂F is investigated from the perspective of sodium. Reaction mechanisms for this cathode during battery function include the following: a region comprising at least three phases with subtly varying sodium compositions that transform via two two-phase reaction mechanisms, which appears at the lower potential plateau-like region during both charge and discharge; an extended solid solution region for majority of the cycling process, including most of the higher potential plateau; and a second two-phase region near the highest charge state during charge and between the first and second plateau-like regions during discharge. Notably, the distinct asymmetry in the reaction mechanism, lattice, and volume evolution on charge relative to discharge manifests an interesting question: Is such an asymmetry beneficial for this cathode? These reaction mechanisms are inherently related to sodium evolution, which shows complex behavior between the two sodium crystallographic sites in this compound that in turn mediate the lattice and reaction evolution. Thus, this work relates atomic-level sodium perturbations directly with electrochemical cycling

Incorporation of Tb and Gd improves the diagnostic functionality of magnetotactic bacteria

Magnetotactic bacteria are envisaged as potential theranostic agents. Their internal magnetic compass, chemical environment specificity and natural motility enable these microorganisms to behave as nanorobots, as they can be tracked and guided towards specific regions in the body and activated to generate a therapeutic response. Here we provide additional diagnostic functionalities to magnetotactic bacteria Magnetospirillum gryphiswaldense MSR-1 while retaining their intrinsic capabilities. These additional functionalities are achieved by incorporating Tb or Gd in the bacteria by culturing them in Tb/Gd supplemented media. The incorporation of Tb provides luminescence properties, enabling potential applications of bacteria as biomarkers. The incorporation of Gd turns bacteria into dual contrast agents for magnetic resonance imaging, since Gd adds T1 contrast to the existing T2 contrast of unmodified bacteria. Given their potential clinical applications, the diagnostic ability of the modified MSR-1 has been successfully tested in vitro in two cell models, confirming their suitability as fluorescent markers (Tb-MSR-1) and dual contrast agents for MRI (Gd-MSR-1)

Optimal Parameters for Hyperthermia Treatment Using Biomineralized Magnetite Nanoparticles: Theoretical and Experimental Approach

We hereby present experimental and theoretical insights on the use of biomineralized magnetite nanoparticles, called magnetosomes, as heat nanoinductors in the magnetic hyperthermia technique. The heating efficiency or specific absorption rate of magnetosomes extracted from Magnetospirillum gryphiswaldense bacteria and immersed in water and agarose gel, was directly determined from the hysteresis loops obtained at different frequencies and magnetic field amplitudes. We demonstrate that heat production of magnetosomes can be predicted in the framework of the Stoner−Wohlfarth theory of uniaxial magnetic anisotropy subjected to significant dipolar interactions, which can be described in terms of an interaction anisotropy superimposed to that of each particle. Based on these findings, we propose optimal magnetic field amplitude and frequency values in order to maximize the heat production while keeping the undesired eddy current effects below safe and tolerable limits. The efficiency of magnetosomes as heat generators and their impact on cell viability has been checked in macrophage cells. Our results clearly indicate that the hyperthermia treatment causes both cell death and inhibition of cell proliferation. Specifically, only 36% of the treated macrophages remained alive 2 h after alternating magnetic field exposure, and 24 h later the percentage fell to 22%

Electrochemical Na Extraction/Insertion of Na 3

A mixed-valence V3+/V4+ composite material belonging to the Na3V2O2x(PO4)2F3-2x/C family is synthesized and the electrochemical Na extraction/insertion mechanism is determined using a combination of high-resolution synchrotron X-ray diffraction (XRD) data, X-ray absorption spectroscopy (XAS), 23Na and 19F solid state nuclear magnetic resonance (NMR), double titration (for the elucidation of the vanadium oxidation state), and electrochemical measurements. The vanadium oxidation state is found to be +3.8 for the as-prepared sample. Detailed analysis of the cathode structural evolution illustrated that the V4+/V5+ couple is active in this compound during electrochemical cycling between 2.8 V and 4.3 V. This study demonstrates how the sodium-ion extraction and insertion pathways in cathode materials can be followed (and verified) using several experimental techniques, especially when multiple potential oxidation states are present in the parent compound.This work was financially supported by the Ministerio de Educación y Ciencia (No. MAT2010-19442) and the Gobierno Vasco/Eusko Jaurlaritza (Etortek CICEnergigune-10, SAIOTEK-12-ENERGIBA and IT570-13). University of Basque Country is acknowledged for funding under Project No. UFI11/53. Access to synchrotron X-ray radiation facilities of ELETTRA, Trieste, Italy beamline XAFS (Project No. 20125173) is acknowledged. We thank Giuliana Aquilanti and Luca Olivi, from the XAFS beamline at ELETTRA, for their support, help, and expert advice on beamline operation. N.S. would like to thank AINSE, Ltd., for providing support through the research fellowship scheme. Part of this research was undertaken on the Powder Diffraction beamline at the Australian Synchrotron, Victoria, Australia