This review focuses on the synthesis, protection, functionalization, and application of magnetic nanoparticles, as well as the magnetic properties of nanostructured systems. Substantial progress in the size and shape control of magnetic nanoparticles has been made by developing methods such as co-precipitation, thermal decomposition and/or reduction, micelle synthesis, and hydrothermal synthesis. A major challenge still is protection against corrosion, and therefore suitable protection strategies will be emphasized, for example, surfactant/polymer coating, silica coating and carbon coating of magnetic nanoparticles or embedding them in a matrix/support. Properly protected magnetic nanoparticles can be used as building blocks for the fabrication of various functional systems, and their application in catalysis and biotechnology will be briefly reviewed. Finally, some future trends and perspectives in these research areas will be outlined.

Magnetic nanoparticles: Synthesis, protection, functionalization, and application

Common bean (Phaseolus vulgaris) has become a cosmopolitan crop, but was originally domesticated in the Americas and has been grown in Latin America for several thousand years. Consequently an enormous diversity of bean nodulating bacteria have developed and in the centers of origin the predominant species in bean nodules is R. etli. In some areas of Latin America, inoculation, which normally promotes nodulation and nitrogen fixation is hampered by the prevalence of native strains. Many other species in addition to R. etli have been found in bean nodules in regions where bean has been introduced. Some of these species such as R. leguminosarum bv. phaseoli, R. gallicum bv. phaseoli and R. giardinii bv. phaseoli might have arisen by acquiring the phaseoli plasmid from R. etli. Others, like R. tropici, are well adapted to acid soils and high temperatures and are good inoculants for bean under these conditions. The large number of rhizobia species capable of nodulating bean supports that bean is a promiscuous host and a diversity of bean-rhizobia interactions exists. Large ranges of dinitrogen fixing capabilities have been documented among bean cultivars and commercial beans have the lowest values among legume crops. Knowledge on bean symbiosis is still incipient but could help to improve bean biological nitrogen fixation.

Diversity of Rhizobium-Phaseolus vulgaris symbiosis: overview and perspectives

Microrobots have the potential to revolutionize many aspects of medicine. These untethered, wirelessly controlled and powered devices will make existing therapeutic and diagnostic procedures less invasive and will enable new procedures never before possible. The aim of this review is threefold: first, to provide a comprehensive survey of the technological state of the art in medical microrobots; second, to explore the potential impact of medical microrobots and inspire future research in this field; and third, to provide a collection of valuable information and engineering tools for the design of medical microrobots.

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Microrobots for Minimally Invasive Medicine

Biological N2 fixation represents the major source of N input in agricultural soils including those in arid regions. The major N2-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. The Rhizobium-legume symbioses have received most attention and have been examined extensively. The behavior of some N2-fixing systems under severe environmental conditions such as salt stress, drought stress, acidity, alkalinity, nutrient deficiency, fertilizers, heavy metals, and pesticides is reviewed. These major stress factors suppress the growth and symbiotic characteristics of most rhizobia; however, several strains, distributed among various species of rhizobia, are tolerant to stress effects. Some strains of rhizobia form effective (N2-fixing) symbioses with their host legumes under salt, heat, and acid stresses, and can sometimes do so under the effect of heavy metals. Reclamation and improvement of the fertility of arid lands by application of organic (manure and sewage sludge) and inorganic (synthetic) fertilizers are expensive and can be a source of pollution. The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to the improvement of soil fertility and the rehabilitation of arid lands and is an important direction for future research.

Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate

In this review an overview about biological applications of magnetic colloidal nanoparticles will be given, which comprises their synthesis, characterization, and in vitro and in vivo applications. The potential future role of magnetic nanoparticles compared to other functional nanoparticles will be discussed by highlighting the possibility of integration with other nanostructures and with existing biotechnology as well as by pointing out the specific properties of magnetic colloids. Current limitations in the fabrication process and issues related with the outcome of the particles in the body will be also pointed out in order to address the remaining challenges for an extended application of magnetic nanoparticles in medicine.

Biological applications of magnetic nanoparticles

Acid pH limits the persistence of Rhizobium strains in soil, and the nodulation and nitrogen fixation of legumes. To identify acid-tolerant strains, we tested the ability of 45 Rhizobium, Azorhizob...

Acid pH tolerance in strains of Rhizobium and Bradyrhizobium, and initial studies on the basis for acid tolerance of Rhizobium tropici UMR1899

Magnetic nanoparticles can be used for a variety of biomedical applications. They can be used in the targeted delivery of therapeutic agents in vivo, in the hyperthermic treatment of cancers, in magnetic resonance (MR) imaging as contrast agents and in the biomagnetic separations of biomolecules. In this study, a characterization of the movement and heating of three different types of magnetic nanoparticles in physiological systems in vitro is made in a known external magnetic field and alternating field respectively. Infra-red (IR) imaging and MR imaging were used to visualize these nanoparticles in vitro. A strong dependence on the size and the suspending medium is observed on the movement and heating of these nanoparticles. First, two of the particles (mean diameter d = 10 nm, uncoated Fe3O4 and d = 2.8 µm, polystyrene coated Fe3O4+γ-Fe2O3) did not move while only a dextran coated nanoparticle (d = 50 nm, γ-Fe2O3) moved in type 1 collagen used as an in vitro model system. It is also observed that the time taken by a collection of these nanoparticles to move even a smaller distance (5 mm) in collagen (~100 min) is almost ten times higher when compared to the time taken to move twice the distance (10 mm) in glycerol (~10 min) under the same external field. Second, the amount of temperature rise increases with the concentration of nanoparticles regardless of the microenvironments in the heating studies. However, the amount of heating in collagen (maximum change in temperature ΔTmax~9 °C at 1.9 mg Fe ml−1 and 19 °C at 3.7 mg Fe ml−1) is significantly less than that in water (ΔTmax~15 °C at 1.9 mg Fe ml−1 and 33 °C at 3.7 mg Fe ml−1) and glycerol (ΔTmax~13.5 °C at 1.9 mg Fe ml−1 and 30 °C at 3.7 mg Fe ml−1). Further, IR imaging provides at least a ten times improvement in the range of imaging magnetic nanoparticles, whereby a concentration of (0–4 mg Fe ml−1) could bevisualized as compared to (0–0.4 mg Fe ml−1) by MR imaging. Based on these in vitro studies, important issues and parameters that require further understanding and characterization of these nanoparticles in vivo are discussed.

https://iopscience.iop.org/article/10.1088/0957-4484/16/8/041/pdf

In vitro characterization of movement, heating and visualization of magnetic nanoparticles for biomedical applications

Spatially resolved velocity profiles and spatially nonresolved velocity distributions of steady flow in a tube and bead packs were measured. Two different NMR experiments were used to measure velocity distributions. In one, the velocity histogram was calculated from spatially resolved velocity phase encoded images acquired in a 6 mm bead pack. In the other, a Fourier flow method was used to measure the velocity distribution directly in a 0.25 mm bead pack. Axial velocity profiles in the pore space of the 6 mm bead pack at Reynolds numbers of 14.9, 29.9, and 44.8 proved to be roughly parabolic, with maxima near the pore centers. Both NMR methods yielded the same dimensionless velocity distributions that contain negative as well as positive velocity components. The velocity distribution function derived from a bundle‐of‐tubes‐model accounts for the positive part of the velocity distribution.

NMR imaging of velocity profiles and velocity distributions in bead packs

Detector geometry, spatial sampling, and more fundamentally, positron range and noncollinearity of annihilation photon emission define Positron Emission Tomography(PET)spatial resolution. In this paper, a strong magnetic field is used to constrain positron travel transverse to the field. Measurement of the spread function from a 500 μm diameter 68Ga impregnated resin bead shows a squeezing of the full width at half maximum (FWHM) by a factor of 1.0, 1.22, 1.42, and 2.05, at 0, 4.0, 5.0, and 9.4 Tesla, respectively. The full width at tenth maximum (FWTM) decreases by a factor of 1.0, 1.73, 2.09, and 3.20, at 0, 4.0, 5.0, and 9.0 Tesla, respectively. Acquiring a PET image in a magnetic field should significantly reduce resolution loss due to positron range.

Use of a magnetic field to increase the spatial resolution of positron emission tomography.

We have developed a protocol to evaluate the magnetic resonance (MR) compatibility of implantable medical devices. The testing protocol consists of the evaluation of magnetic field-induced movement, electric current, heating, image distortion, and device operation. In addition, current induction is evaluated with a finite element analysis simulation technique that models the effect of radiofrequency fields on each device. The protocol has been applied to several implantable infusion pumps and neurostimulators with associated attachments. Experiments were performed using a 1.5-T whole-body MR system with parameters selected to approximate the intended clinical and worst case configuration. The devices exhibited moderate magnetic field-induced deflection and torque but had significant image artifacts. No heating was detected for any of the devices. Pump operation was halted in the magnetic field, but resumed after removed. Exposure to the magnetic field activated some of the neurostimulators.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/%28SICI%291522-2586%28199904%299%3A4%3C596%3A%3AAID-JMRI14%3E3.0.CO%3B2-T

Bruce E. Hammer

Papers

Acid pH tolerance in strains of Rhizobium and Bradyrhizobium, and initial studies on the basis for acid tolerance of Rhizobium tropici UMR1899

In vitro characterization of movement, heating and visualization of magnetic nanoparticles for biomedical applications

NMR imaging of velocity profiles and velocity distributions in bead packs

Use of a magnetic field to increase the spatial resolution of positron emission tomography.

MRI compatibility and visibility assessment of implantable medical devices.