INTERFACE
INTERFACE
Research
Controlled cobalt doping in the spinel structure of magnetosome magnetite: new evidences from element- and site-specific X-ray magnetic circular dichroism analyses
The biomineralization of magnetite nanocrystals (called magnetosomes) by magnetotactic bacteria has attracted intense interest in biology, geology, and materials science due to the precise morphology of the particles, the chain-like assembly and their unique magnetic properties. Great efforts have been recently made in producing transition metal-doped magnetosomes with modified magnetic properties for a range of applications. Despite some successful outcomes, the coordination chemistry and magnetism of such metal-doped magnetosomes still remain largely unknown. Here, we present new evidences from X-ray magnetic circular dichroism for element- and site-specific magnetic analyses that cobalt is incorporated in the spinel structure of the magnetosomes within Magnetospirillum magneticum AMB-One through the replacement of iron two ions by cobalt two ions in octahedral sites of magnetite. Both X-ray magnetic circular dichroism at iron and cobalt L two, three edges, and energy-dispersive X-ray spectroscopy on transmission electron microscopy analyses reveal a heterogeneous distribution of cobalt occurring either in different particles or inside individual particles. Compared with non-doped one, cobalt-doped magnetosome sample has lower Verwey transition temperature and larger magnetic coercivity, related to the amount of doped cobalt. This study also demonstrates that the addition of trace cobalt in the growth medium can significantly improve both the cell growth and the magnetosome formation within M. magneticum AMB-One. Together with the cobalt occupancy within the spinel structure of magnetosomes, this study indicates that magnetotactic bacteria may provide a promising biomimetic system for producing chains of metal-doped single-domain magnetite with an appropriate tuning of the magnetic properties for technological and biomedical applications.
One. Introduction
One. Introduction
Magnetic nanoparticles have a broad range of applications from data storage to medical imaging and to hyperthermia treatment of tumors. Tailored synthesis of magnetic nanoparticles by which the physical and magnetic properties are well controlled is essential for determining the technological and biomedical applications that they are suitable for. For instance, magnetic nanoparticles with single-domain sizes and one-dimensional arrays have advantages in magnetic memory devices and magnetic hyperthermia due to their stronger magnetic anisotropy and hysteresis than their random assemblages.
A number of methods including co-precipitation, hydrothermal approaches and mechanical ball milling have been developed to produce nanocrystals of magnetite or maghemite, the most popular magnetic nanoparticles in biomedical applications due to their low toxicity and high magnetization, for different commercial uses. However, these methods often involve high temperature with toxic solvents resulting in high environmental and energy costs. Recently, synthesis of magnetic nanoparticles by biotic processes such as microbial biomineralization or biocatalysis by biomolecules has attracted a great interest in producing novel magnetic nanoparticles with high quality and biocompatibility under environment friendly conditions (e.g. at room temperature and ambient pressure and in toxic-free solutions).
Magnetotactic bacteria have an incredible ability to form nanocrystals of single-domain magnetite within intracellular lipid vesicles (named as magnetosomes) that have high chemical purity, narrow size and shape distributions, species- or strain-specific crystal morphologies, and are usually organized into chain(s). By using the magnetosome chain(s), magnetotactic bacteria cells orient themselves and navigate along the Earth's magnetic field lines when they are swimming in aquatic environments. As an intriguing model system, magnetotactic bacteria have, therefore, been extensively studied for better understanding the magnetite biomineralization and geomagnetic sensitivity in organisms, but also have recently been modified for producing physically and biologically tailored magnetic nanoparticles. For instance, to enhance the magnetic hardness of magnetosome magnetite, Staniland et al. pioneered a cobalt-doping study of magnetosomes within three cultured strains of the bacterium Magnetospirillum. They have shown that the presence of cobalt increases the coercivity of the magnetosomes by thirty-six to forty-five percent, depending on the bacterial strains and the cobalt contents. Recently, Tanaka et al. further increased the levels of cobalt doping approximately up to three point zero percent by increasing the concentration of cobalt ions in the initial culture medium. It has been evidenced that the presence of cobalt in the magnetosomes significantly increases the magnetocrystalline anisotropy constant and the magnetic hysteresis, and therefore increases the heating efficiency for applications in alternating magnetic field cancer therapy. These previous studies are extremely important for the development of functionalized magnetosomes for biotechnological and biomedical applications. However, one fundamental question, i.e. the coordination chemistry and magnetism of cobalt within the magnetosomes, still remains largely unknown, and information on the structure and the magnetic properties of cobalt-doped magnetosomes is very limited.
In this study, we investigate the magnetosome formation in Magnetospirillum magneticum AMB-One anaerobically grown under three cobalt two plus over iron three plus ratio conditions: zero over twenty-two point one micromolar, two point one over twenty micromolar and twelve point one over ten micromolar, hereafter referred to as cobalt zero, cobalt two point one and cobalt twelve point one, respectively. Scanning transmission electron microscopy observations show that the presence of trace cobalt in the initial growth medium (e.g. two point one picomolar) significantly improved the cell growth and magnetosome formation. Rock magnetic measurements on the whole-cell samples reveal that, compared with the cobalt zero magnetosomes,
the C O left two point one right and Co twelve point one magnetosomes have reduced Verwey transition temperature T sub v right and increased coercivity B sub c right values, e.g. T sub v equals one hundred eight Kelvin, B sub c left three hundred Kelvin right equals fourteen point four millitesla and B sub c left five Kelvin right equals seventy-four point zero millitesla for the C O left zero right versus T sub v equals one hundred Kelvin, B sub c left three hundred Kelvin right equals twenty-seven point three millitesla and B sub c left five Kelvin right equals two hundred seventy-five point four millitesla for the Co twelve point one. The site occupancy, valence and distribution of cobalt within the magnetosomes have further been studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism at both Fe and C O L sub two point three edges. Experimental and theoretical X-ray magnetic circular dichroism analyses reveal a heterogeneous distribution of cobalt occurring either in different particles or inside individual particles. Within cobalt-doped magnetosomes, only C O superscript two plus was incorporated in octahedral O sub h sites of magnetite through the substitution of F e superscript two plus. These findings are supported by energy-dispersive X-ray spectroscopy performed by transmission electron microscopy on individual particles. Finally, the potential process of cobalt doping in the magnetosomes and its significance in the tailored synthesis of magnetically usable nanoparticles are discussed.