Iron/iron oxides core-shell nanoparticles by laser pyrolysis: Structural characterization and enhanced particle dispersion

Well-dispersed nanoparticles with iron/iron carbide core and iron oxide shell structures may constitute an excellent magnetic material for different applications as magnetic nanofluids, contrast agents in magnetic resonance imaging, sensors and catalysts. Based on the ability of the CO₂ laser pyrolysis technique to synthesize nanoparticles of the Fe/Fe₂O₃ core-shell type, we further improve the powder dispersion by first collecting the nanoparticles in a toluene bubbler, positioned downstream and prior to the collection filter. Structural characterisation of the samples by electron microscopy and X-ray diffraction was performed. Conditions in which clusters contain a reduced number of nanoparticles (around 50) are evidenced. Mean core-shell particle sizes of 15 nm were estimated. Finally, preliminary results on the morphology of iron/iron oxide core-shell nanoparticles as hydrocarbon-based magnetic nanofluids are presented.     

Fabrication high-purity Fe nanochains with near theoretical limit value of saturation magnetization of bulk Fe

High-yield purity chain-like one-dimensional nanostructures consisting of single crystal Fe nanoparticles have been produced by using solution dispersion approach. Room temperature magnetic measurement shows that the as-fabricated Fe nanochains are ferromagnetic with a high saturation magnetization (203 emu/g) whereas the nanoparticles are single magnetic domains, which indicate that the as-synthesized products have superparamagnetism behavior with the saturation magnetization of about 28 emu/g. Maybe this results from the directional alignment of the nanoparticles. The excellent characteristic may have led to the potential applications in spin filtering, high density magnetic recording, and nanosensors.     

Ex vivo assessment of polyol coated-iron oxide nanoparticles for MRI diagnosis applications: toxicological and MRI contrast enhancement effects

Polyol synthesis is a promising method to obtain directly pharmaceutical grade colloidal dispersion of superparamagnetic iron oxide nanoparticles (SPIONs). Here, we study the biocompatibility and performance as T2-MRI contrast agents (CAs) of high quality magnetic colloidal dispersions (average hydrodynamic aggregate diameter of 16-27 nm) consisting of polyol-synthesized SPIONs (5 nm in mean particle size) coated with triethylene glycol (TEG) chains (TEG-SPIONs), which were subsequently functionalized to carboxyl-terminated meso-2-3-dimercaptosuccinic acid (DMSA) coated-iron oxide nanoparticles (DMSA-SPIONs). Standard MTT assays on HeLa, U87MG, and HepG2 cells revealed that colloidal dispersions of TEG-coated iron oxide nanoparticles did not induce any loss of cell viability after 3 days incubation with dose concentrations below 50 μg Fe/ml. However, after these nanoparticles were functionalized with DMSA molecules, an increase on their cytotoxicity was observed, so that particles bearing free terminal carboxyl groups on their surface were not cytotoxic only at low concentrations (<10 μg Fe/ml). Moreover, cell uptake assays on HeLa and U87MG and hemolysis tests have demonstrated that TEG-SPIONs and DMSA-SPIONs were well internalized by the cells and did not induce any adverse effect on the red blood cells at the tested concentrations. Finally, in vitro relaxivity measurements and post mortem MRI studies in mice indicated that both types of coated-iron oxide nanoparticles produced higher negative T2-MRI contrast enhancement than that measured for a similar commercial T2-MRI CAs consisting in dextran-coated ultra-small iron oxide nanoparticles (Ferumoxtran-10). In conclusion, the above attributes make both types of as synthesized coated-iron oxide nanoparticles, but especially DMSA-SPIONs, promising candidates as T2-MRI CAs for nanoparticle-enhanced MRI diagnosis applications.     

Mössbauer and TDPAC Studies on Fe/Mo Multilayers

Hyperfine fields at Fe and Mo layers in polyimide/Fe(10 nm)/[Mo(1.1 nm)/Fe(2.0 nm)]₁₂₀ and [Mo(1.3 nm) /Fe(2.0 nm)]₁₂₀ multilayers prepared by the electron-beam evaporation technique were measured at room-temperature by Mössbauer spectroscopy and perturbed-angular-correlation spectroscopy. The hyperfine fields in the Fe layers do not show a clear dependence on the Mo layer thickness. On the other hand, the hyperfine fields in the Mo layers show different magnetic structures in these samples. The difference suggests a variation of electron spin polarization in the Mo layers.     

Size controlled synthesis of semiconductor nanocrystals in a continuous-flow mode microcapillary reactor

A microcapillary reactor with 320 μm inner diameter was utilized for CdSe nanoparticle synthesis. The influence of the reaction temperature and flow rate of precursors on the size and size distribution of prepared CdSe nanoparticles was systematically studied. The as-prepared nanoparticles exhibit sharp excitonic absorption and photoluminescence peak (FWHM 30 nm) with a quantum-yield around 10–40%. The microcapillary reactor was also used for CdSe/ZnS core-shell nanoparticle synthesis in continuous-flow mode. The quantum yield of the core-shell nanoparticles was found to be considerably influenced by the reactor temperature and have a close correlation with the thickness of ZnS shell under growth. An optimized quantum yield up to 70% was obtained for the CdSe/ZnS core-shell nanoparticles.     

Microwave absorption and Mössbauer studies of Fe3O4 nanoparticles

Materials consisting of nanometer-sized magnetic particles are currently the subject of intensive research activities. Especially, much attention has been paid to their promising features for microwave magnetic properties. Well dispersed Fe₃O₄ nanoparticles of 30 nm have been synthesized by oxidization method with NaNO₂, and the microwave magnetic properties of the composites have been studied. The real and imaginary part of relative permittivity remained low and nearly constant in the region of 0.1–18 GHz, respectively. As a result, the resin composites having a thickness of 2.0–3.2 mm, and containing 20 vol% Fe₃O₄ in the form of nanoparticles with an average diameter of 30 nm, exhibited excellent electromagnetic wave absorption properties in the frequency range of 4.5–12.0 GHz.     

Effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles

In this work the effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles obtained by the sol-gel method is analyzed. Two sets of samples were prepared: Fe3O4 nanoparticles and Fe3O4@SiO2 core-shell composites. The samples display the characteristic spinel structure associated with the magnetite Fe3O4 phase, with the majority of grain sizes around 5-10 nm. At room temperature the nanoparticles show the characteristic superparamagnetic behavior with mean blocking temperatures around 160 and 120 K for Fe3O4 and Fe3O4@SiO2, respectively. The main effect of the SiO2 coating is reflected in the temperature dependence of the high field magnetization (μ(0)H = 6 T), i.e. deviations from the Bloch law at low temperatures (T < 20 K). Such deviations, enhanced by the introduction of the SiO2 coating, are associated with the occurrence of surface spin disordered effects. The induction heating effects (magnetic hyperthermia) are analyzed under the application of an AC magnetic field. Maximum specific absorption rate (SAR) values around 1.5 W g(-1) were achieved for the Fe3O4 nanoparticles. A significant decrease (around 26%) is found in the SAR values of the SiO2 coated nanocomposite. The different heating response is analyzed in terms of the decrease of the effective nanoparticle magnetization in the Fe3O4@SiO2 core-shell composites at room temperature.     

Magnetic anisotropy and magnetization dynamics of Fe nanoparticles embedded in Cr and Ag matrices

Static and dynamical magnetic properties of Fe nanoparticles (NPs) embedded in non-magnetic (Ag) and antiferromagnetic (Cr) matrices with a volume filling fraction (VFF) of 10% have been investigated. In both Fe@Ag and Fe@Cr nanocomposites, the Fe NPs have a narrow size distribution, with a mean particle diameter around 2 nm. In both samples, the saturation magnetization reaches that of Fe bulk bcc, suggesting the absence of alloying with the matrices. The coercivity at 5 K is much larger in Fe@Cr than in Fe@Ag as a result of the strong interaction between the Fe NPs and the Cr matrix. Temperature-dependent magnetization and ac-susceptibility measurements point out further evidence of the enhanced interparticle interaction in the Fe@Cr system. While the behaviour of Fe@Ag indicates the presence of weakly interacting magnetic monodomain particles with a wide distribution of blocking temperatures, Fe@Cr behaves like a superspin glass produced by the magnetic interactions between NPs.     

Formation and characterization of high-density Fe cluster-assembled films with soft magnetic behaviors

High-density, magnetically soft Fe cluster-assembled films were obtained at room temperature by an energetic cluster deposition. Size-monodispersed Fe clusters with the mean cluster size d = 9, 13 and 16 nm were produced using a plasma-gas-condensation technique. Ionized clusters in cluster beam were accelerated electrically and deposited onto the substrate together with neutral clusters from the same cluster source. The morphology, microstructure and magnetic properties of the cluster-assembled films have been studied by an atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and superconducting quantum interference device magnetometer. By increasing the impact energy of the ionized clusters up to 0.6 eV/atom, the Fe cluster-assembled film has a packing fraction of 0.86±0.03, and reveals a soft magnetic behavior. In addition, it is found that oxidization of the cluster-assembled films is remarkably suppressed with the increase in the density of the films.     

Gas-phase preparation and size control of Fe nanoparticles

The magnetic, electrical and optical properties of nanoparticle systems often depend on the size and size distribution of nanoparticles. In order to optimize those properties of nanoparticle-assembled materials, only varying the mean size of nanoparticles was not enough, and narrowing the size distribution was also of immense importance. In this study, uniform-sized Fe nanoparticles with different diameters were prepared using a magnetron sputtering combined with inert gas condensation technique. The size and morphology of nanoparticles were observed by transmission electron microscopy (TEM). The statistic results revealed that the size of Fe nanoparticles increased with increasing the flow rate of Ar gas, but a reverse trend was observed when increasing the flow rate of He gas. By adjusting the flow rate of Ar and He gases, uniform-sized Fe nanoparticles with diameter ranging from 6 to 13?nm were obtained. Moreover, the size effects on the electrical and magnetic properties of Fe nanoparticle-assembled films with thickness of about 500?nm were also investigated. The magnetic properties showed that the coercivity increased with increasing the nanoparticle size. The in-situ resistance measurement results of Fe nanoparticle assembled-films also showed a size-dependent behavior.     

Local magnetic fields in the Mo layer of Mo/Fe multilayer

Using time-differential perturbed-angular-correlation technique, hyperfine fields at ⁹⁹Tc (←⁹⁹Mo) in the Mo layers in polyimide/Fe (10 nm)/[Mo (t _Mo)/Fe (2.0 nm)]₁₂₀, where t _Mo is in the range between 0.4 and 1.5 nm, were measured at room temperature. The values of the magnetic hyperfine field at the Mo/Fe interface were extracted. Its dependence on the Mo layer thickness suggests that the oscillatory interlayer exchange coupling is due to conduction electron spin polarization in the Mo layer, which in turn is produced via an RKKY-type mechanism.     

Synthesis and characterization of microporous hollow core-shell silica nanoparticles (HCSNs) of tunable thickness for controlled release of doxorubicin

Hollow core-shell silica nanoparticles (HCSNs) are being considered as one of the most favorable drug carriers to accomplish targeted drug delivery. In the present study, we developed a simple two-step method, employing polystyrene (PS) nanoparticles (150?±?20 nm) as a sacrificial template for the synthesis of microporous HCSNs of size 230?±?30 nm. PS core and the wall structure directing agent cetyl trimethyl ammonium bromide (CTAB) were removed by calcination. Monodispersed spherical HCSNs were synthesized by optimising the parameters like water/ethanol volume ratio, PS/tetraethyl orthosilicate (TEOS) weight ratio, concentration of ammonia, and CTAB. Transmission electron microscopy (TEM) revealed the formation of hollow core-shell structure of silica with tunable thickness from 15 to 30 nm while tailoring the concentration of silica precursor. The results obtained from the cumulative release studies of doxorubicin loaded microporous HCSNs demonstrated the dependence of shell thickness on the controlled drug release behavior. HCSNs with highest shell thickness of 30 nm and lowest surface area of 600 m²/g showed delay in the doxorubicin release, proving their application as a drug carrier in targeted drug delivery systems. The novel concept of application of microporous HCSNs of pore size ~?1.3 nm with large specific surface area in the field of drug delivery is successful.     

Nanostructures of gold coated iron core-shell nanoparticles and the nanobands assembled under magnetic field

Pure metal iron nanoparticles are unstable in the air. By a coating iron on nanoparticle surface with a stable noble metal, these air-stable nanoparticles are protected from the oxidation and retain most of the favorable magnetic properties, which possess the potential application in high density memory device by forming self-assembling nanoarrays. Gold-coated iron core-shell structure nanoparticles (Fe/Au) synthesized using reverse micelles were characterized by transmission electron microscopy (TEM). The average nanoparticle size of the core-shell structure is about 8 nm, with about 6 nm diameter core and 1∼2 nm shell. Since the gold shell is not epitaxial growth related to the iron core, the morié pattern can be seen from the overlapping of iron core and gold shell. However, the gold shell lattice can be seen by changing the defocus of TEM. An energy dispersive X-ray spectrum (EDS) also shows the nanoparticles are air-stable. The magnetic measurement of the nanoparticles also proved successful synthesis of gold coated iron core-shell structure. The nanoparticles were then assembled under 0.5 T magnetic field and formed parallel nanobands with about 10 μm long. Assembling two dimensional ordered nanoarrays are still under going. Received 29 November 2000     

磁性聚苯胺纳米微球的合成与表征

报道了具有核壳结构的Fe3 O4 聚苯胺磁性纳米微球的合成方法和表征结果 .微球同时具有导电性和磁性能 .在优化的实验条件下 ,可得到饱和磁化强度Ms 为 5 5 .4emu/g ,矫顽力Hc 为 6 2Oe的磁性微球 .微球的导电性随着微球中Fe含量的增加而下降 .微球的磁性能则随着Fe含量的增加而增大 .Fe3 O4 磁流体的粒径和磁性聚苯胺微球的粒径均在纳米量级 .纳米Fe3 O4 粒子能够提高复合物的热性能 .实验表明 ,磁流体和聚苯胺之间可能存在着一定的相互作用 ,但这种相互作用较为复杂 ,难于研究 .     

Magnetic Properties of Iron/Graphite Core-shell Structured Nanoparticles Prepared by Annealing of Fe-C-N Nanocomposite

We are reporting the core-shell structured iron/graphite nanoparticles formed during annealing of a nanopowder prepared by laser pyrolysis of gas phase reactants. The originally synthesized Fe-C-N nanocomposite powder has been characterized by TEM, XRD and magnetic measurements. Nanopowder was heated up to 800 °C at ～ 1 Pa vacuum. Presence of iron nanoparticles with mean diameter 40 nm in the annealed state of nanopowder was proved by XRD and TEM analyses. Mössbauer spectroscopy was used for characterization of synthesized/annealed nanopowder to confirm the qualitative change in phase composition.     

Iron/iron oxide core-shell nanoclusters for biomedical applications

Biocompatible magnetic nanoparticles have been found promising in several biomedical applications for tagging, imaging, sensing and separation in recent years. Most magnetic particles or beads currently used in biomedical applications are based on ferromagnetic iron oxides with very low specific magnetic moments of about 20–30 emu/g. Here we report a new approach to synthesize monodispersed core-shell nanostructured clusters with high specific magnetic moments above 200 emu/g. Iron nanoclusters with monodispersive size of diameters from 2 nm to 100 nm are produced by our newly developed nanocluster source and go to a deposition chamber, where a chemical reaction starts, and the nanoclusters are coated with iron oxides. HRTEM Images show the coatings are very uniform and stable. The core-shell nanoclusters are superparamagnetic at room temperature for sizes less than 15 nm, and then become ferromagnetic when the cluster size increases. The specific magnetic moment of core-shell nanoclusters is size dependent, and increases rapidly from about 80 emu/g at the cluster size of around 3 nm to over 200 emu/g up to the size of 100 nm. The use of high magnetic moment nanoclusters for biomedical applications could dramatically enhance the contrast for MRI, reduce the concentration of magnetic particle needs for cell separation, or make drug delivery possible with much lower magnetic field gradients     

Synthesis and characterization of L10 FePt nanoparticles from Pt–Fe3O4 core-shell nanoparticles

Pt/Fe₃O₄ core-shell nanoparticles have been prepared by a modified polyol method. Pt nanoparticles were first prepared via the reduction of Pt(acac)₂ by polyethylene glycol-200 (PEG-200), and layers of iron oxide were subsequently deposited on the surface of Pt nanoparticles by the thermal decomposition of Fe(acac)₃. The nanoparticles were characterized by XRD and HR-TEM. The as-prepared Pt/Fe₃O₄ nanoparticles have a chemically disordered FCC structure and transformed into chemically ordered fct structure after annealing in reducing atmosphere (4% H₂, 96% Ar) at 700 °C. The ordered fct FePt phase has high magnetic anisotropy with coercivity reaching 7.5 kOe at room temperature and 9.3 kOe at 10 K.     

Size effects in parameters of both interatomic exchange interactions in Fe–Pt alloy nanoparticles and their superparamagnetism

The objective of this study is to calculate the parameters of strong exchange interactions within magnetic nanoparticles and weak (dipolar + anisotropic) interactions for the patterns of nanoparticles injected into non-magnetic substrate. The Fe–Pt magnetic system was chosen as the best applicable for this purpose. However, computations done in this work may be extended to the other f.c.c. magnetic systems. In this paper, we estimated the parameters of exchange interactions within the magnetic Fe–Pt nanoparticles close to equiatomic composition with 4–10 nm in diameter. Size effect for exchange interaction parameters was found. Temperature dependences of spontaneous magnetizations for Fe and Pt subsystems of nanoparticles with different sizes at fixed equiatomic composition were obtained. Total magnetic energies of weak interactions between Fe and Pt nanoparticles injected different matrixes were also estimated. Magnetic moment ordering temperature was evaluated within the simple model for ordered and disordered Fe–Pt nanoparticles of various sizes (4–10 nm) separated by different interparticle distances (30–50 nm).     

Magnetic annealing of the ion-beam sputtered IrMn/CoFeB bilayers – positive exchange bias and coercivity behaviour

The effect of optimum dilution of antiferromagnetic (AF)/ferromagnetic (FM) interface necessary for observance of positive exchange bias in ion-beam sputtered Si/Ir₂₂Mn₇₈ (t _AF = 12, 18, 24 nm)/Co₂₀Fe₆₀B₂₀(t _FM = 6,9,15 nm) exchange coupled bilayers is investigated by magnetic annealing at 380, 420 and 460 °C for 1 h at 5 × 10^-6 Torr in presence of 500 Oe magnetic field. While the coercivity of the exchange coupled FM layer decreases with the increase in annealing temperature irrespective of the value of t _AF or t _FM, the hysteresis loops however shift by ≈+ 10 Oe whenever the coercivity drops in the 10–15 Oe range. This is consistent with the phase diagram of exchange bias field and coercivity derived from Meiklejohn and Bean model. The X-ray diffraction and X-ray reflectivity measurements confirmed that the texture, grain size and interface roughness of IrMn/CoFeB bilayers are thickness dependent and are correlated to the observed magnetic response of the bilayers. The results establish that optimum dilution of the IrMn/CoFeB interface by thermally diffused Mn-spins is necessary in inducing the effective coupling between the IrMn domains and diluted CoFeB layer. It is further shown that the annealing temperature required for the optimum dilution of the CoFeB interface critically depends on the thickness of the layers.     

Small-angle neutron scattering study of the nano-sized features in an oxide dispersion-strengthened Fe12Cr alloy

Small-angle neutron scattering experiments, along with positron lifetime measurements and transmission electron microscopy observations, were performed on samples of an oxide dispersion-strengthened (ODS) Fe12Cr alloy and its non-ODS counterpart in order to characterize their nano-sized features. The nuclear and magnetic scattering data were analysed using the maximum entropy approach for obtaining the size distribution of the scattering centres in these materials. The positron annihilation results and the TEM information have made possible an interpretation of the volume distribution of the scattering centres having sizes below ~16 nm and their proper quantitative analyses. The smaller scattering centres in the ODS alloy exhibit distributions with modal values at ~6–7 and 12–14 nm. The peak at ~6–7 nm appears to be due to the overlapping of more than one type of scattering centres, while the one at ~12–14 nm can be exclusively attributed to the Y-rich centres. The quantitative analysis of the magnetic scattering data yields a volume fraction and number density of the Y-rich particles estimated in 0.70?±?0.03% and 0.77 × 10²² m^?3, respectively.