Control of physical properties in BiFeO3nanoparticles via Sm3+and Co2+ion doping.

Highly crystalline BiFeO3(BFO), Bi0.97Sm0.03FeO3(Sm-BFO) and BiFe0.97Co0.03O3(Co-BFO) nanoparticles (NPs) were utilized as potential magnetic hyperthermia agents at two different frequencies in the radiofrequency (RF) range, and the effect of Sm3+and Co2+ion doping on the physical properties of the material was examined. The thermal behaviour of the as-prepared powders disclosed that the crystallization temperature of the powders is affected by the incorporation of the dopants into the BFO lattice and the Curie transition temperature is decreased upon doping. Vibrational analysis confirmed the formation of the R3c phase in all compounds through the characteristic FT-IR absorbance bands assigned to O-Fe-O bending vibration and Fe-O stretching of the octahedral FeO6group in the perovskite, as well as through Raman spectroscopy. The shift of the Raman-active phonon modes in Sm-BFO and Co-BFO NPs indicated structural distortion of the BFO lattice, which resulted in increased local polarization and enhanced visible light absorption. The aqueous dispersion of Co-BFO NPs showed the highest magnetic hyperthermia performance at 30 mT/765 kHz, entering the therapeutic temperature window for cancer treatment, whereas the heating efficiency of all samples was increased with increasing frequency from 375 to 765 kHz, making our doped nanoparticles to be suitable candidates for potential biomedical applications.

Toward the Separation of Different Heating Mechanisms in Magnetic Particle Hyperthermia.

Magnetic particle hyperthermia (MPH) is a promising method for cancer treatment using magnetic nanoparticles (MNPs), which are subjected to an alternating magnetic field for local heating to the therapeutic range of 41-45 °C. In this window, the malignant regions (i.e., cancer cells) undergo a severe thermal shock while healthy tissues sustain this thermal regime with significantly milder side effects. Since the heating efficiency is directly associated with nanoparticle size, MNPs should acquire the appropriate size to maximize heating together with minimum toxicity. Herein, we report on facile synthetic controls to synthesize MNPs by an aqueous precipitation method, whereby tuning the pH values of the solution (9.0-13.5) results in a wide range of average MNP diameters from 16 to 76 nm. With respect to their size, the structural and magnetic properties of the MNPs are evaluated by adjusting the most important parameters, i.e. the MNP surrounding medium (water/agarose), the MNP concentration (1-4 mg mL-1), and the field amplitude (20-50 mT) and frequency (103, 375, 765 kHz). Consequently, the maximum heating efficiency is determined for each MNP size and set of parameters, outlining the optimum MNPs for MPH treatment. In this way, we can address the different heat generation mechanisms (Brownian, Néel, and hysteresis losses) to different sizes and separate Brownian and hysteresis losses for optimized sizes by studying the heat generation as a function of the medium viscosity. Finally, MNPs immobilized into agarose solution are studied under low-field MPH treatment to find the optimum conditions for clinical applications.

From MAX Phase Carbides to Nitrides: Synthesis of V2GaC, V2GaN, and the Carbonitride V2GaC1-xNx.

The research in MAX phases is mainly concentrated on the investigation of carbides rather than nitrides (currently >150 carbides and only <15 nitrides) that are predominantly synthesized by conventional solid-state techniques. This is not surprising since the preparation of nitrides and carbonitrides is more demanding due to the high stability and low diffusion rate of nitrogen-containing compounds. This leads to several drawbacks concerning potential variations in the chemical composition of the MAX phases as well as control of morphology, the two aspects that directly affect the resulting materials properties. Here, we report how alternative solid-state hybrid techniques solve these limitations by combining conventional techniques with nonconventional precursor synthesis methods, such as the "urea-glass" sol-gel or liquid ammonia method. We demonstrate the synthesis and morphology control within the V-Ga-C-N system by preparing the MAX phase carbide and nitride─the latter in the form of bulkier and more defined smaller particle structures─as well as a hitherto unknown carbonitride V2GaC1-xNx MAX phase. This shows the versatility of hybrid methods starting, for example, from wet chemically obtained precursors that already contain all of the ingredients needed for carbonitride formation. All products are characterized in detail by X-ray powder diffraction, electron microscopy, and electron and X-ray photoelectron spectroscopies to confirm their structure and morphology and to detect subtle differences between the different chemical compositions.

Magnetic phase diagram of (Mo2/3RE1/3)2AlC, RE=Tb and Dy, studied by magnetization, specific heat, and neutron diffraction analysis.

We report the results of magnetization, heat capacity, and neutron diffraction measurements on (Mo2/3RE1/3)2AlC with RE = Dy and Tb. Temperature and field-dependent magnetization as well as heat capacity were measured on a powder sample and on a single crystal allowing the construction of the magnetic field-temperature phase diagram. To study the magnetic structure of each magnetic phase, we applied neutron diffraction in a magnetic field up to 6 T. For (Mo2/3Dy1/3)2AlC in zero field, a spin density wave is stabilized at 16 K, with antiferromagnetic ordering at 13 K. Furthermore, we identify the coexistence of ferromagnetic and antiferromagnetic phases induced by magnetic fields for both RE = Tb and Dy. The origin of the field induced phases is resulting from the competing ferromagnetic and antiferromagnetic interactions.

The effect of the composition and pressure on the phase stability and electronic, magnetic, and elastic properties of M2AX (M = Mn, Fe; A = Al, Ga, Si, Ge; X = C, N) phases.

The magnetic properties of M2AX (M = Mn, Fe; A = Al, Ga, Si, Ge; X = C, N) phases were studied within DFT-GGA. The magnetic electronic ground state is determined. The investigation of the phase stability of M2AX phases is performed by comparing the total energy of MAX phases to that of the set of competitive phases for calculation of the phase formation enthalpy. As the result of such an approach, we have found one stable compound (Mn2GaC), and seven metastable ones. It is shown that several metastable MAX phases (Mn2AlC, Fe2GaC, Mn2GeC, and Mn2GeN) become stable at a small applied pressure (1.5-7 GPa). The mechanical, electronic and elastic properties of metastable MAX phases are studied.

2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air.

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo2 CTx flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-µm MXene multilayer for different organic vapors and humidity at 101 -104  ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H2 O down to 10 ppm. Moreover, humidity suppresses the response of Mo2 CTx to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10-2 -106  Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo2 CTx chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo2 C surface interaction with organic analytes and H2 O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.

Magnetic Nanoprobes for Spatio-Mechanical Manipulation in Single Cells.

Magnetic nanoparticles (MNPs) are widely known as valuable agents for biomedical applications. Recently, MNPs were further suggested to be used for a remote and non-invasive manipulation, where their spatial redistribution or force response in a magnetic field provides a fine-tunable stimulus to a cell. Here, we investigated the properties of two different MNPs and assessed their suitability for spatio-mechanical manipulations: semisynthetic magnetoferritin nanoparticles and fully synthetic 'nanoflower'-shaped iron oxide nanoparticles. As well as confirming their monodispersity in terms of structure, surface potential, and magnetic response, we monitored the MNP performance in a living cell environment using fluorescence microscopy and asserted their biocompatibility. We then demonstrated facilitated spatial redistribution of magnetoferritin compared to 'nanoflower'-NPs after microinjection, and a higher magnetic force response of these NPs compared to magnetoferritin inside a cell. Our remote manipulation assays present these tailored magnetic materials as suitable agents for applications in magnetogenetics, biomedicine, or nanomaterial research.

Room temperature synthesized solid solution AuFe nanoparticles and their transformation into Au/Fe Janus nanocrystals.

Solid solution AuFe nanoparticles were synthesized for the first time under ambient conditions by an adapted method previously established for the Fe3O4-Au core-shell morphology. These AuFe particles preserved the fcc structure of Au incorporated with paramagnetic Fe atoms. The metastable AuFe can be segregated by transformation into Janus Au/Fe particles with bcc Fe and fcc Au upon annealing. The ferromagnetic Fe was epitaxially grown on low index fcc Au planes. This preparation route delivers new perspective materials for magnetoplasmonics and biomedical applications and suggests the reconsideration of existing protocols for magnetite-gold core-shell synthesis.

Long-Range Ordering Effects in Magnetic Nanoparticles.

The challenge for synthesizing magnetic nanoparticle chains may be achieved under the application of fixation fields, which are the externally applied fields, enhancing collective magnetic features due to adequate control of dipolar interactions among magnetic nanoparticles. However, relatively little attention has been devoted to how size, concentration of magnetic nanoparticles, and intensity of an external magnetic field affect the evolution of chain structures and collective magnetic features. Here, iron oxide nanoparticles are developed by the coprecipitation method at diameters below (10 and 20 nm) and above (50 and 80 nm) their superparamagnetic limit (at about 25 nm) and then are subjected to a tunable fixation field (40-400 mT). Eventually, the fixation field dictates smaller particles to form chain structures in two steps, first forming clusters and then guiding chain formation via "cluster-cluster" interactions, whereas larger particles readily form chains via "particle-particle" interactions. In both cases, dipolar interactions between the neighboring nanoparticles augment, leading to a substantial increase in their collective magnetic features which in turn results in magnetic particle hyperthermia efficiency enhancement of up to one order of magnitude. This study provides new perspectives for magnetic nanoparticles by arranging them in chain formulations as enhanced performance magnetic actors in magnetically driven magnetic applications.

Magnetic Nanoparticles as a Tool for Remote DNA Manipulations at a Single-Molecule Level.

Remote control of cells and single molecules by magnetic nanoparticles in nonheating external magnetic fields is a perspective approach for many applications such as cancer treatment and enzyme activity regulation. However, the possibility and mechanisms of direct effects of small individual magnetic nanoparticles on such processes in magneto-mechanical experiments still remain unclear. In this work, we have shown remote-controlled mechanical dissociation of short DNA duplexes (18-60 bp) under the influence of nonheating low-frequency alternating magnetic fields using individual 11 nm magnetic nanoparticles. The developed technique allows (1) simultaneous manipulation of millions of individual DNA molecules and (2) evaluation of energies of intermolecular interactions in short DNA duplexes or in other molecules. Finally, we have shown that DNA duplexes dissociation is mediated by mechanical stress and produced by the movement of magnetic nanoparticles in magnetic fields, but not by local overheating. The presented technique opens a new avenue for high-precision manipulation of DNA and generation of biosensors for quantification of energies of intermolecular interaction.

Cobalt Ferrite Nanoparticles for Tumor Therapy: Effective Heating versus Possible Toxicity.

Magnetic nanoparticles (MNPs) are widely considered for cancer treatment, in particular for magnetic hyperthermia (MHT). Thereby, MNPs are still being optimized for lowest possible toxicity on organisms while the magnetic properties are matched for best heating capabilities. In this study, the biocompatibility of 12 nm cobalt ferrite MNPs, functionalized with citrate ions, in different dosages on mice and rats of both sexes was investigated for 30 days after intraperitoneal injection. The animals' weight, behavior, and blood cells changes, as well as blood biochemical parameters are correlated to histological examination of organs revealing that cobalt ferrite MNPs do not have toxic effects at concentrations close to those used previously for efficient MHT. Moreover, these MNPs demonstrated high specific loss power (SLP) of about 400 W g-1. Importantly the MNPs retained their magnetic properties inside tumor tissue after intratumoral administration for several MHT cycles within three days. Thus, cobalt ferrite MNPs represent a perspective platform for tumor therapy by MHT due to their ability to provide effective heating without exerting a toxic effect on the organism. This opens up new avenues for smaller MNPs sizes while their heating efficiency is maintained.

Manipulation of the Size and Phase Composition of Yttrium Iron Garnet Nanoparticles by Pulsed Laser Post-Processing in Liquid.

Modification of the size and phase composition of magnetic oxide nanomaterials dispersed in liquids by laser synthesis and processing of colloids has high implications for applications in biomedicine, catalysis and for nanoparticle-polymer composites. Controlling these properties for ternary oxides, however, is challenging with typical additives like salts and ligands and can lead to unwanted byproducts and various phases. In our study, we demonstrate how additive-free pulsed laser post-processing (LPP) of colloidal yttrium iron oxide nanoparticles using high repetition rates and power at 355 nm laser wavelength can be used for phase transformation and phase purification of the garnet structure by variation of the laser fluence as well as the applied energy dose. Furthermore, LPP allows particle size modification between 5 nm (ps laser) and 20 nm (ns laser) and significant increase of the monodispersity. Resulting colloidal nanoparticles are investigated regarding their size, structure and temperature-dependent magnetic properties.

Unravelling the nucleation, growth, and faceting of magnetite-gold nanohybrids.

The chemical synthesis of nanoparticles with a preassigned size and shape is important for an optimized performance in any application. Therefore, systematic monitoring of the synthesis is required for the control and detailed understanding of the nucleation and growth of the nanoparticles. Here, we study Fe3O4-Au hybrid nanoparticles in detail using probes of the reaction mixture during synthesis and their thorough characterization. The proposed approach eliminates the problem of repeatability and reproducibility of the chemical synthesis and was carried out using laboratory equipment (standard transmission electron microscopy, X-ray diffraction, and magnetometry) for typically 10 µL samples instead of, for example, a dedicated synthesis and inspection at a synchrotron radiation facility. From the three independent experimental techniques we extract the nanoparticle size at 12 stages of the synthesis. These diameters show identical trends and good quantitative agreement. Two consecutive processes occur during the synthesis of Fe3O4-Au nanoparticles, the nucleation and the growth of spherical Fe3O4 nanoparticles on the surface of Au seeds during the heating stage and their faceting towards octahedral shape during reflux. The final nanoparticles with sizes of 15 nm Fe3O4 and 4 nm Au exhibit superparamagnetic behavior at ambient temperature. These are high-quality, close to stoichiometric Fe3O4 nanocrystals with nearly volumetric magnetic behavior as confirmed by the presence of the Verwey transition. Understanding the processes occurring during the synthesis allows the nanoparticle size and shape to be adjusted, improving their capabilities in biomedical applications.

Temperature-controlled magnetic nanoparticles hyperthermia inhibits primary tumor growth and metastases dissemination.

Magnetic hyperthermia (MHT) is a promising approach for cancer therapy. However, a systematic MHT characterization as function of temperature on the therapeutic efficiency is barely analyzed. Here, we first perform comparative temperature-dependent analysis of the cobalt ferrite nanoparticles-mediated MHT effectiveness in two murine tumors models - breast (4T1) and colon (CT26) cancer in vitro and in vivo. The overall MHT killing capacity in vitro increased with the temperature and CT26 cells were more sensitive than 4T1 when heated to 43 °C. Well in line with the in vitro data, such heating cured non-metastatic CT26 tumors in vivo, while only inhibiting metastatic 4T1 tumor growth without improving the overall survival. High-temperature MHT (>47 °C) resulted in complete 4T1 primary tumor clearance, 25-40% long-term survival rates, and, importantly, more effective prevention of metastasis comparing to surgical extraction. Thus, the specific MHT temperature must be defined for each tumor individually to ensure a successful antitumor therapy.

Size-selected Fe3O4-Au hybrid nanoparticles for improved magnetism-based theranostics.

Size-selected Fe3O4-Au hybrid nanoparticles with diameters of 6-44 nm (Fe3O4) and 3-11 nm (Au) were prepared by high temperature, wet chemical synthesis. High-quality Fe3O4 nanocrystals with bulk-like magnetic behavior were obtained as confirmed by the presence of the Verwey transition. The 25 nm diameter Fe3O4-Au hybrid nanomaterial sample (in aqueous and agarose phantom systems) showed the best characteristics for application as contrast agents in magnetic resonance imaging and for local heating using magnetic particle hyperthermia. Due to the octahedral shape and the large saturation magnetization of the magnetite particles, we obtained an extraordinarily high r 2-relaxivity of 495 mM-1·s-1 along with a specific loss power of 617 W·gFe -1 and 327 W·gFe -1 for hyperthermia in aqueous and agarose systems, respectively. The functional in vitro hyperthermia test for the 4T1 mouse breast cancer cell line demonstrated 80% and 100% cell death for immediate exposure and after precultivation of the cells for 6 h with 25 nm Fe3O4-Au hybrid nanomaterials, respectively. This confirms that the improved magnetic properties of the bifunctional particles present a next step in magnetic-particle-based theranostics.

Magnetic Fe@FeOx, Fe@C and α-Fe2O3 Single-Crystal Nanoblends Synthesized by Femtosecond Laser Ablation of Fe in Acetone.

There are few reports on zero-field-cooled (ZFC) magnetization measurements for Fe@FeOx or FeOx particles synthesized by laser ablation in liquids (LAL) of Fe, and the minimum blocking temperature (TB) of 120 K reported so far is still much higher than those of their counterparts synthesized by chemical methods. In this work, the minimum blocking temperature was lowered to 52 K for 4⁻5 nm α-Fe2O3 particles synthesized by femtosecond laser ablation of Fe in acetone. The effective magnetic anisotropy energy density (Keff) is calculated to be 2.7⁻5.4 × 105 J/m³, further extending the Keff values for smaller hematite particles synthesized by different methods. Large amorphous-Fe@α-Fe2O3 and amorphous-Fe@C particles of 10⁻100 nm in diameter display a soft magnetic behavior with saturation magnetization (Ms) and coercivities (Hc) values of 72.5 emu/g and 160 Oe at 5 K and 61.9 emu/g and 70 Oe at 300 K, respectively, which mainly stem from the magnetism of amorphous Fe cores. Generally, the nanoparticles obtained by LAL are either amorphous or polycrystalline, seldom in a single-crystalline state. This work also demonstrates the possibility of synthesizing single-crystalline α-Fe2O3 hematite crystals of several nanometers with (104), (113), (116) or (214) crystallographic orientations, which were produced simultaneously with other products including carbon encapsulated amorphous Fe (a-Fe@C) and Fe@FeOx core-shell particles by LAL in one step. Finally, the formation mechanisms for these nanomaterials are proposed and the key factors in series events of LAL are discussed.

Magnetite-Gold nanohybrids as ideal all-in-one platforms for theranostics.

High-quality, 25 nm octahedral-shaped Fe3O4 magnetite nanocrystals are epitaxially grown on 9 nm Au seed nanoparticles using a modified wet-chemical synthesis. These Fe3O4-Au Janus nanoparticles exhibit bulk-like magnetic properties. Due to their high magnetization and octahedral shape, the hybrids show superior in vitro and in vivo T2 relaxivity for magnetic resonance imaging as compared to other types of Fe3O4-Au hybrids and commercial contrast agents. The nanoparticles provide two functional surfaces for theranostic applications. For the first time, Fe3O4-Au hybrids are conjugated with two fluorescent dyes or the combination of drug and dye allowing the simultaneous tracking of the nanoparticle vehicle and the drug cargo in vitro and in vivo. The delivery to tumors and payload release are demonstrated in real time by intravital microscopy. Replacing the dyes by cell-specific molecules and drugs makes the Fe3O4-Au hybrids a unique all-in-one platform for theranostics.

Large uniaxial magnetostriction with sign inversion at the first order phase transition in the nanolaminated Mn2GaC MAX phase.

In 2013, a new class of inherently nanolaminated magnetic materials, the so called magnetic MAX phases, was discovered. Following predictive material stability calculations, the hexagonal Mn2GaC compound was synthesized as hetero-epitaxial films containing Mn as the exclusive M-element. Recent theoretical and experimental studies suggested a high magnetic ordering temperature and non-collinear antiferromagnetic (AFM) spin states as a result of competitive ferromagnetic and antiferromagnetic exchange interactions. In order to assess the potential for practical applications of Mn2GaC, we have studied the temperature-dependent magnetization, and the magnetoresistive, magnetostrictive as well as magnetocaloric properties of the compound. The material exhibits two magnetic phase transitions. The Néel temperature is T N ~ 507 K, at which the system changes from a collinear AFM state to the paramagnetic state. At T t = 214 K the material undergoes a first order magnetic phase transition from AFM at higher temperature to a non-collinear AFM spin structure. Both states show large uniaxial c-axis magnetostriction of 450 ppm. Remarkably, the magnetostriction changes sign, being compressive (negative) above T t and tensile (positive) below the T t . The sign change of the magnetostriction is accompanied by a sign change in the magnetoresistance indicating a coupling among the spin, lattice and electrical transport properties.

Ultrasmall Yttrium Iron Garnet Nanoparticles with High Coercivity at Low Temperature Synthesized by Laser Ablation and Fragmentation of Pressed Powders.

Pulsed laser ablation of pressed yttrium iron garnet powders in water is studied and compared to the ablation of a single-crystal target. We find that target porosity is a crucial factor, which has far-reaching implications on nanoparticle productivity. Although nanoparticle size distributions obtained by analytical disc centrifugation and transmission electron microscopy (TEM) are in agreement, X-ray diffraction and energy dispersive X-ray analysis show that only nanoparticles obtained from targets with densities close to that of a bulk target lead to comparable properties. Our findings also show why the gravimetrical measurement of nanoparticle productivity is often flawed and needs to be complemented by colloidal productivity measurements. The synthesized YIG nanoparticles are further reduced in size by laser fragmentation to obtain sizes smaller than 3 nm. Since the particle diameters are close to the YIG lattice constant, these ultrasmall nanoparticles reveal an immense change of the magnetic properties, exhibiting huge coercivity (0.11 T) and irreversibility fields (8 T) at low temperatures.

Enhanced magnetocrystalline anisotropy of Fe30Co70 nanowires by Cu additives and annealing.

The use of 3d transition metal-based magnetic nanowires (NWs) for permanent magnet applications requires large magnetocrystalline anisotropy energy (MAE), which in combination with the NWs' magnetic shape anisotropy yields magnetic hardening and an enhancement of the magnetic energy product. Here, we report on the significant increase in MAE by 125 kJ m(-3) in Fe30Co70 NWs with diameters of 20-150 nm embedded in anodic aluminum oxide templates by adding 5 at.% Cu and subsequent annealing at 900 K. Ferromagnetic resonance (FMR) reveals that this enhancement of MAE is twice as large as the enhancement of MAE in annealed, but undoped NWs. X-ray diffraction (XRD) analysis suggests that upon annealing the immiscible Cu in FeCo NWs causes a crystal reorientation with respect to the NW axis with a considerable distortion of the bcc FeCo lattice. This strain is most likely the origin of the strongly enhanced MAE.