Synthesis of magnetosome chain-like structures

Magnetite chains with a number of magnetite particles arranged in a line parallel to the outer amorphous carbon coating have been prepared. The sizes of the nanoparticles range from 40 to 120?nm, with nearly identical gaps between every two adjacent particles. The synthesized chains display ferromagnetic properties with several single-magnetic-domain (SD) nanoparticles assembled in an orderly fashion. Based on the formation process of chains in our reaction system, it is suggested that the localized environments favor the growth of SD nanoparticles, and the strong magnetic dipolar-dipolar interactions lead to the self-assembly of SD nanoparticles. This research could offer some useful information in studying the formation mechanism of magnetosome chains and the origin of the special chain-like nanostructures in magnetotactic bacteria.     

Magnetic properties of Acidithiobacillus ferrooxidans

Understanding the magnetic properties of magnetotactic bacteria (MTBs) is of great interest in fields of life sciences, geosciences, biomineralization, biomagnetism, and planetary sciences. Acidithiobacillus ferrooxidans (At. ferrooxidans), obtaining energy through the oxidation of ferrous iron and various reduced inorganic sulfur compounds, can synthesize intracellular magnetite magnetosomes. However, the magnetic properties of such microorganism remain unknown. Here we used transmission electronmicroscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) assay, vibrating sample magnetometer (VSM), magneto–thermogravimetric analysis (MTGA), and low temperature magnetometry to comprehensively investigate the magnetic characteristics of At. ferrooxidans. Results revealed that each cell contained only 1 to 3 magnetite magnetosomes, which were arranged irregularly. The magnetosomes were generally in a stable single-domain (SD) state, but superparamagnetic (SP) magnetite particles were also found. The calcined bacteria exhibited a ferromagnetic behavior with a Curie Temperature of 454 °C and a coercivity of 16.36 mT. Additionally, the low delta ratio (δ_FC/δ_ZFC = 1.27) indicated that there were no intact magnetosome chains in At. ferrooxidans. Our results provided the new insights on the biomineralization of bacterial magnetosomes and magnetic properties of At. ferrooxidans.     

Structural purity of magnetite nanoparticles in magnetotactic bacteria

Magnetosome biomineralization and chain formation in magnetotactic bacteria are two processes that are highly controlled at the cellular level in order to form cellular magnetic dipoles. However, even if the magnetosome chains are well characterized, controversial results about the microstructure of magnetosomes were obtained and its possible influence in the formation of the magnetic dipole is to be specified. For the first time, the microstructure of intracellular magnetosomes was investigated using high-resolution synchrotron X-ray diffraction. Significant differences in the lattice parameter were found between intracellular magnetosomes from cultured magnetotactic bacteria and isolated ones. Through comparison with abiotic control materials of similar size, we show that this difference can be associated with different oxidation states and that the biogenic nanomagnetite is stoichiometric, i.e. structurally pure whereas isolated magnetosomes are slightly oxidized. The hierarchical structuring of the magnetosome chain thus starts with the formation of structurally pure magnetite nanoparticles that in turn might influence the magnetic property of the magnetosome chains.     

Collective behaviour in two-dimensional cobalt nanoparticle assemblies observed by magnetic force microscopy

The use of magnetic nanoparticles in the development of ultra-high-density recording media is the subject of intense research. Much of the attention of this research is devoted to the stability of magnetic moments, often neglecting the influence of dipolar interactions. Here, we explore the magnetic microstructure of different assemblies of monodisperse cobalt single-domain nanoparticles by magnetic force microscopy and magnetometric measurements. We observe that when the density of particles per unit area is higher than a determined threshold, the two-dimensional self-assemblies behave as a continuous ferromagnetic thin film. Correlated areas (similar to domains) of parallel magnetization roughly ten particles in diameter appear. As this magnetic percolation is mediated by dipolar interactions, the magnetic microstructure, its distribution and stability, is strongly dependent on the topological distribution of the dipoles. Thus, the magnetic structures of three-dimensional assemblies are magnetically soft, and an evolution of the magnetic microstructure is observed with consecutive scans of the microscope tip.     

Magnetic assembly of nonmagnetic particles into photonic crystal structures

We report the rapid formation of photonic crystal structures by assembly of uniform nonmagnetic colloidal particles in ferrofluids using external magnetic fields. Magnetic manipulation of nonmagnetic particles with size down to a few hundred nanometers, suitable building blocks for producing photonic crystals with band gaps located in the visible regime, has been difficult due to their weak magnetic dipole moment. Increasing the dipole moment of magnetic holes has been limited by the instability of ferrofluids toward aggregation at high concentration or under strong magnetic field. By taking advantage of the superior stability of highly surface-charged magnetite nanocrystal-based ferrofluids, in this paper we have been able to successfully assemble 185 nm nonmagnetic polymer beads into photonic crystal structures, from 1D chains to 3D assemblies as determined by the interplay of magnetic dipole force and packing force. In a strong magnetic field with large field gradient, 3D photonic crystals with high reflectance (83%) in the visible range can be rapidly produced within several minutes, making this general strategy promising for fast creation of large-area photonic crystals using nonmagnetic particles as building blocks.     

Influence of extreme dilutions on the magnetic properties of single-domain particle aggregates

Various magnetic properties of samples of single-domain particles dispersed in a nonmagnetic matrix are examined as functions of the packing fractionp. The range of variability ofpis from 0.0003 to 0.20. The squareness ratio and the rotational and alternating hysteresis integrals change withp, while the coercive field, the initial anhysteretic susceptibility, and the areas between the remanence curves are not dependent onp. These results are interpretated as a consequence of the formation of agglomerates interacting with each other, rather than as a consequence of analogous interactions among the single-domain particles which are in the aggregate.     

Biological colloid engineering: Self-assembly of dipolar ferromagnetic chains in a functionalized biogenic ferrofluid

We have studied the dynamic behavior of nanoparticles in ferrofluids consisting of single-domain, biogenic magnetite (Fe(3)O(4)) isolated from Magnetospirillum magnetotacticum (MS-1). Although dipolar chains form in magnetic colloids in zero applied field, when dried upon substrates, the solvent front disorders nanoparticle aggregation. Using avidin-biotin functionalization of the particles and substrate, we generated self-assembled, linear chain motifs that resist solvent front disruption in zero-field. The engineered self-assembly process we describe here provides an approach for the creation of ordered magnetic structures that could impact fields ranging from micro-electro-mechanical systems development to magnetic imaging of biological structures.     

Magnetisation effects of multicore magnetite nanoparticles crystallised from a silicate glass

Magnetic multicore nanoparticles were produced by crystallisation of a glass with the composition Na₂O·Al₂O₃·B₂O₃·SiO₂·Fe₂O₃. During cooling of the melt, in a first step, phase separation occurred and droplets enriched in boron and iron oxide were formed. These droplets crystallised spontaneously during cooling. The phase-separated droplets had sizes in the range from 200 to 800 nm. Inside the droplets, magnetite crystals with a mean size of 33 nm occurred. Magnetometer measurements showed the occurrence of a small hysteresis which indicates predominantly superparamagnetic behaviour of the magnetite crystals. The magnetic domains of the phase-separated droplets were studied by magnetic force microscopy. In this article, a glass which is exposed to a magnetic field shows droplet-shape phase separations where the single-domain magnetite crystals do not have a preferred orientation of the magnetic dipoles. By contrast, the whole droplet shows one magnetic dipole parallel to the external field if the glass is exposed to a magnetic field during measurement.     

Generation of Multishell Magnetic Hybrid Nanoparticles by Encapsulation of Genetically Engineered and Fluorescent Bacterial Magnetosomes with ZnO and SiO2

Magnetic nanoparticles (MNPs) have great potential in biomedical applications, but the chemical synthesis of size‐controlled and functionalized core–shell MNPs remain challenging. Magnetosomes produced by the magnetotactic bacterium Magnetospirillum gryphiswaldense are naturally uniform and chemically pure magnetite MNPs with superior magnetic characteristics. Here, additional functionalities are made possible by the incorporation of biomolecules on the magnetosome surface; the magnetosome system is then chemically encapsulated with an inorganic coating. The novel multishell nanoparticles consist of the magnetosome core—which includes the magnetite crystal, the magnetosome membrane, and additional moieties, such as the enhanced green fluorescent protein (EGFP) and peptides—and an outer shell, comprising either silica or zinc oxide. Coating the functionalized magnetosomes with silica improves their colloidal stability and preserves the EGFP fluorescence in the presence of proteases and detergents. In addition, the surface charge of magnetosomes can be adjusted by varying the coating. This method will be useful for the versatile generation of new, multifunctional, multishell, and magnetic hybrid nanomaterials with potential applications in various biotechnological fields.     

Crystal growth of bullet-shaped magnetite in magnetotactic bacteria of the Nitrospirae phylum

Magnetotactic bacteria (MTB) are known to produce single-domain magnetite or greigite crystals within intracellular membrane organelles and to navigate along the Earth''s magnetic field lines. MTB have been suggested as being one of the most ancient biomineralizing metabolisms on the Earth and they represent a fundamental model of intracellular biomineralization. Moreover, the determination of their specific crystallographic signature (e.g. structure and morphology) is essential for palaeoenvironmental and ancient-life studies. Yet, the mechanisms of MTB biomineralization remain poorly understood, although this process has been extensively studied in several cultured MTB strains in the Proteobacteria phylum. Here, we show a comprehensive transmission electron microscopy (TEM) study of magnetic and structural properties down to atomic scales on bullet-shaped magnetites produced by the uncultured strain MYR-1 belonging to the Nitrospirae phylum, a deeply branching phylogenetic MTB group. We observed a multiple-step crystal growth of MYR-1 magnetite: initial isotropic growth forming cubo-octahedral particles (less than approx. 40 nm), subsequent anisotropic growth and a systematic final elongation along [001] direction. During the crystal growth, one major {111} face is well developed and preserved at the larger basal end of the crystal. The basal {111} face appears to be terminated by a tetrahedral–octahedral-mixed iron surface, suggesting dimensional advantages for binding protein(s), which may template the crystallization of magnetite. This study offers new insights for understanding magnetite biomineralization within the Nitrospirae phylum.     

Microbial synthesis of iron-based nanomaterials—A review

Nanoparticles are the materials having dimensions of the order of 100 nm or less. They exhibit a high surface/volume ratio leading to different properties far different from those of the bulk materials. The development of uniform nanoparticles has been intensively pursued because of their technological and fundamental scientific importance. A number of chemical methods are available and are extensively used, but these are often energy intensive and employ toxic chemicals. An alternative approach for the synthesis of uniform nanoparticles is the biological route that occurs at ambient temperature, pressure and at neutral pH. The main aim of this review is to enlist and compare various methods of synthesis of iron-based nanoparticles with emphasis on the biological method. Biologically induced and controlled mineralization mechanisms are the two modes through which the micro-organisms synthesize iron oxide nanoparticles. In biologically induced mineralization (BIM) mode, the environmental factors like pH, pO₂, pCO₂, redox potential, temperature etc govern the synthesis of iron oxide nanoparticles. In contrast, biologically controlled mineralization (BCM) process initiates the micro-organism itself to control the synthesis. BIM can be observed in the Fe(III) reducing bacterial species of Shewanella, Geobacter, Thermoanaerobacter, and sulphate reducing bacterial species of Archaeoglobus fulgidus, Desulfuromonas acetoxidans, whereas BCM mode can be observed in the magnetotactic bacteria (MTB) like Magnetospirillum magnetotacticum, M. gryphiswaldense and sulphate-reducing magnetic bacteria (Desulfovibrio magneticus). Magnetite crystals formed by Fe(III)-reducing bacteria are epicellular, poorly crystalline, irregular in shapes, having a size range of 10–50 nm super-paramagnetic particles, with a saturation magnetization value ranging from 75–77 emu/g and are not aligned in chains. Magnetite crystals produced by MTB have uniform species-specific morphologies and sizes, which are mostly unknown from inorganic systems. The unusual characteristics of magnetosome particles have attracted a great interdisciplinary interest and inspired numerous ideas for their biotechnological applications. The nanoparticles synthesized through biological method are uniform with size ranging from 5 to 100 nm, which can potentially be used for various applications.     

磁场诱导和晶种联合法制备链状Fe_3O_4

以粒径为700nm的球形Fe3O4为晶种,将晶种加入到含有FeSO4.7H2O、FeCl3.6H2O与尿素的混合溶液中,在磁场诱导下制备链状Fe3O4。研究了磁场强度、晶种添加量、分散剂聚乙烯吡咯烷酮(PVP)质量浓度以及反应时间对产品形貌的影响。研究结果表明,随着磁场强度的增大、晶种量的增加,链状粒子数目显著增多;适量的PVP能使链状粒子分散平行排列。合成链状Fe3O4粒子的最佳工艺条件为磁场强度0.35T,晶种用量10%,PVP质量浓度7.5g/L,反应18h。磁性能测试表明,该链状Fe3O4粒子具有亚铁磁性,磁饱和强度为72.3emu/g,矫顽力为381Oe。     

Strengthening liberation and separation of magnetite ore via magnetic pulse pretreatment: An industrial test study

Performing ore pretreatment before grinding is particularly crucial for improving mineral liberation and conserving energy. This study proposed an innovative magnetic pulse pretreatment (MPP) technology that primarily used the magnetostrictive effect in an alternating magnetic field to enhance mineral liberation. The effect of MPP on the liberation degree and magnetite separation for magnetite ore were systematically investigated. The results show that the size and volume of the magnetic mineral particles in the ore due to the stretching effect of the alternating magnetic field resulted in microcracks at the interface between different minerals. Compared with the unpretreated process in an industrial test, increment in the −0.043 mm content of the ball mill discharge increased by 3.33 percentage points, increment in the liberation degree of magnetite across the entire particle size range increased by 5.82 percentage points, and increment in the iron concentrate grade increased by 1.22 percentage points, with an error range of 0.1%, due to MPP. The industrial application of MPP brings an enormous economic benefit potential to iron ore utilization.     

Control of nanoparticle size, reactivity and magnetic properties during the bioproduction of magnetite by Geobacter sulfurreducens

The bioproduction of nanoscale magnetite by Fe(III)-reducing bacteria offers a potentially tunable, environmentally benign route to magnetic nanoparticle synthesis. Here, we demonstrate that it is possible to control the size of magnetite nanoparticles produced by Geobacter sulfurreducens by adjusting the total biomass introduced at the start of the process. The particles have a narrow size distribution and can be controlled within the range of 10-50?nm. X-ray diffraction analysis indicates that controlled production of a number of different biominerals is possible via this method including goethite, magnetite and siderite, but their formation is strongly dependent upon the rate of Fe(III) reduction and total concentration and rate of Fe(II) produced by the bacteria during the reduction process. Relative cation distributions within the structure of the nanoparticles have been investigated by x-ray magnetic circular dichroism and indicate the presence of a highly reduced surface layer which is not observed when magnetite is produced through abiotic methods. The enhanced Fe(II)-rich surface, combined with small particle size, has important environmental applications such as in the reductive bioremediation of organics, radionuclides and metals. In the case of Cr(VI), as a model high-valence toxic metal, optimized biogenic magnetite is able to reduce and sequester the toxic hexavalent chromium very efficiently to the less harmful trivalent form.     

Magnetic properties of multicore magnetite nanoparticles prepared by glass crystallisation

A glass with the composition 13K₂O*13Al₂O₃*16B₂O₃*43SiO₂*15Fe₂O_3?x was melted and rapidly quenched in water. This leads to the formation of phase-separated droplets with diameters from 100 to 150 nm. Magnetite crystals with a size of 10–20 nm precipitate within these droplets. The magnetite containing phase-separated regions can be separated from the glass by dissolving the SiO₂-rich amorphous glass matrix through boiling the pulverized glass in a concentrated aqueous sodium hydroxide solution. The residual, magnetite containing phase-separated droplets match multicore magnetite nanoparticles (McNP). The magnetite nanoparticles show superparamagnetic behaviour and as McNP, lead to a higher effective magnetic radius than single crystals. Magnetisation measurements of the McNP indicate that the particles show a narrow hysteresis, but the ratio of remanent to saturation magnetisation is not high enough for uniaxial anisotropy. The additionally performed temperature-dependent magnetorelaxometry (TMRX) measurements show peaks at 13 and 39 K in the distribution of the magnetic moment relaxation. The obtained inter-particle distance of the magnetite within the McNP is smaller than 5 d _C (core diameter), leading to strong magnetic interactions.     

Hierarchical magnetite/silica nanoassemblies as magnetically recoverable catalyst-supports

We report the synthesis of magnetically responsive hierarchical assemblies of silica colloids that can be used as recoverable supports for nanocatalysts. Each assembly is composed of a central magnetite/silica composite core and many small satellite silica spheres. The two regions are held together as a stable unit by a polymer network of poly(N-isopropylacrylamide). The central magnetite particles are superparamagnetic at room temperature with strong magnetic response to external fields, thus providing a convenient means for separating the entire assembly from the solution. The satellite silica particles provide large surface areas for loading nanocatalysts through the well-developed silane chemistry. As an example, we demonstrate the use of such magnetically responsive hierarchical assemblies as recoverable supports for Au nanocatalysts for the reduction of 4-nitrophenol with NaBH4.     

Inorganic Magnetite Precipitation at 25?��C: A Low-Cost Inorganic Coprecipitation Method

An easy, low-cost coprecipitation method to inorganically produce magnetite nanoparticles from solutions, in free-drift experiments, under anoxic conditions, at 25?°C and 1 atm pressure is here presented. By using this method, pure magnetite is obtained as the final solid, which shows the typical magnetic properties and thermal stability behavior of magnetite produced by other methods. The size of the magnetite crystals produced by the present method varies from relatively big sizes (200?C300 nm), to sizes within the single magnetic domain range, just depending on the incubation time. The solution from which magnetite precipitates may be representative of certain natural environments where bacteria that produce magnetite may live and, thus, our magnetite may be used as an inorganic reference to compare to biologically produced magnetites.     

The yield stress of jammed magnetofluidized beds

By means of a magnetic field externally imposed, fluidized beds of magnetizable particles may experience a transition from a fluidlike unstable to a solidlike stable state. In our work, measurements have been taken of the gas velocity and particle volume fraction at the jamming transition as well as of the tensile yield stress of the stabilized bed subjected to a small consolidation. The influence of diverse physical parameters such as initialization mode, magnetic field orientation, average particle size and size polydispersion, are analyzed. Noninvasive visualization of the bed structure has revealed that magnetic stabilization is determined by the formation of particle chains. Due to the enhancement of the interparticle attractive force with field strength and particle size, the transition to stability takes place at higher gas velocities as the magnitude of these parameters is increased. The magnetic yield stress of magnetofluidized beds of naturally aggregated particles because of a large presence of fines is significantly larger than that corresponding to naturally nonaggregated particles. Moreover, the jamming transition occurs at larger gas velocities (or smaller field strengths) in the former case since the agglomerates behave magnetically as large effective particles. The effect of the magnetic field on the yield stress ia only relevant when it is applied during initialization of the bed in the bubbling regime and particles are free to move and restructure in chains. Measurements of the yield stress are presented when the applied magnetic field is oriented either in the vertical or horizontal direction (B co-flow and B cross-flow modes, respectively). The variation of the magnetic yield stress with particle size was found to be dependent on the field direction. This can be justified by the dependence of the interparticle magnetic force on the chain average angle with the field, which is affected by particle size as the stabilized bed is subjected to small consolidations.     

Biocompatibility evaluation of magnetosomes formed by Acidithiobacillus ferrooxidans

Magnetite nanocrystal has been extensively used in biomedical field. Currently, an interesting alternative to synthetic magnetic Fe₃O₄ nanoparticles, called magnetosome, has been found in magnetotactic bacteria. It has been reported that Acidithiobacillus ferrooxidans (At. ferrooxidans) has a potential to synthesize magnetosome. In this study, transmission electron microscope (TEM) was used to analyze the magnetite particles in At. ferrooxidans BY-3. The magnetosomes formed by this bacterium were isolated by a method combining ultracentrifugation and magnetic separation. Crystalline phase and surface functional group of the magnetosomes were investigated by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), respectively. Biocompatibility of the magnetosomes was systematically evaluated at various concentrations (0.5, 1.0, 2.0 and 4.0 mg/ml). MTT test, hemolysis assay and Micronucleus Test were carried out to evaluate in vitro cytotoxicity, blood toxicity and genotoxicity of magnetosomes, respectively. Under these conditions, magnetosomes showed no cytotoxic, genotoxic and hemolytic effects up to 4.0 mg/ml indicating good biocompatibility of these biological nanoparticles. These revealed that the magnetosomes might have a potential for biotechnological and biomedical applications in the future.     

Production, modification and bio-applications of magnetic nanoparticles gestated by magnetotactic bacteria

Magnetotactic bacteria (MTB) were first discovered by Richard P. Blakemore in 1975, and this led to the discovery of a wide collection of microorganisms with similar features i.e., the ability to internalize Fe and convert it into magnetic nanoparticles, in the form of either magnetite (Fe₃O₄) or greigite (Fe₃S₄). Studies showed that these particles are highly crystalline, monodisperse, bioengineerable and have high magnetism that is comparable to those made by advanced synthetic methods, making them candidate materials for a broad range of bio-applications. In this review article, the history of the discovery of MTB and subsequent efforts to elucidate the mechanisms behind the magnetosome formation are briefly covered. The focus is on how to utilize the knowledge gained from fundamental studies to fabricate functional MTB nanoparticles (MTB-NPs) that are capable of tackling real biomedical problems.