Maghemite Nanocrystals Research Articles

Crystalline Magnetic Gels and Aerogels Combining Large Surface Areas and Magnetic Moments.

The production of materials that simultaneously combine large surface areas and high crystallinities is a major challenge. Conventional sol-gel chemistry strategies to produce high-surface-area gels and aerogels generally result in amorphous or poorly crystalline materials. To attain proper crystallinities, materials are exposed to relatively high annealing temperatures that result in significant surface losses. This is a particularly limiting issue in the production of high-surface-area magnetic aerogels owing to the strong relationship between crystallinity and magnetic moment. To overcome this limitation, we demonstrate here the gelation of preformed magnetic crystalline nanodomains to produce magnetic aerogels with high surface area, crystallinity, and magnetic moment. To exemplify this strategy, we use colloidal maghemite nanocrystals as gel building blocks and an epoxide group as the gelation agent. After drying from supercritical CO2, aerogels show surface areas close to 200 m2 g-1 and a well-defined maghemite crystal structure that provides saturation magnetizations close to 60 emu g-1. For comparison, the gelation of hydrated iron chloride with propylene oxide provides amorphous iron oxide gels with slightly larger surface areas, 225 m2 g-1, but very low magnetization, below 2 emu g-1. Thermal treatment at 400 °C is necessary to crystallize the material, which results in a surface area loss down to 87 m2 g-1, well below the values obtained from the nanocrystal building blocks.

Amino-grafted water-soluble maghemite nanocrystals synthesized by deep eutectic solvents electrolysis: Investigation of the formation mechanism

Benefiting from the hydrophilicity and modifiability, aminated iron oxide nanoparticles play significant roles in the construction of magnetically responsive functional materials, and are then widely used in the field of biomedicine and water purification. Using the deep eutectic solvent (DES) composed of urea and choline chloride as an electrolyte, amino-grafted water-soluble maghemite (γ-Fe2O3) nanocrystals have been synthesized by organic electrolysis (DES-electrolysis). In the present work, the DES containing ferrous ions is used as the electrolyte to verify whether magnetite (Fe3O4) can be prepared by that organic electrolysis strategy. The results show that, regardless of the presence or absence of ferrous ions in the DES-based electrolyte, the resulting nanocrystals are maghemite rather than magnetite. The synthesized amino-grafted γ-Fe2O3 nanoparticles can also be highly and stably dispersed in water. The present work deepens the understanding of the formation mechanism of amino-grafted water-soluble maghemite nanocrystals synthesized by DES-electrolysis.

Magnetic Mesoporous Silica Nanorods Loaded with Ceria and Functionalized with Fluorophores for Multimodal Imaging.

Multifunctional magnetic nanocomposites based on mesoporous silica have a wide range of potential applications in catalysis, biomedicine, or sensing. Such particles combine responsiveness to external magnetic fields with other functionalities endowed by the agents loaded inside the pores or conjugated to the particle surface. Different applications might benefit from specific particle morphologies. In the case of biomedical applications, mesoporous silica nanospheres have been extensively studied while nanorods, with a more challenging preparation, have attracted much less attention despite the positive impact on the therapeutic performance shown by seminal studies. Here, we report on a sol–gel synthesis of mesoporous rodlike silica particles of two distinct lengths (1.4 and 0.9 μm) and aspect ratios (4.7 and 2.2) using Pluronic P123 as a structure-directing template and rendering ∼1 g of rods per batch. Iron oxide nanoparticles have been synthesized within the pores yielding maghemite (γ-Fe2O3) nanocrystals of elongated shape (∼7 nm × 5 nm) with a [110] preferential orientation along the rod axis and a superparamagnetic character. The performance of the rods as T2-weighted MRI contrast agents has also been confirmed. In a subsequent step, the mesoporous silica rods were loaded with a cerium compound and their surface was functionalized with fluorophores (fluorescamine and Cyanine5) emitting at λ = 525 and 730 nm, respectively, thus highlighting the possibility of multiple imaging modalities. The biocompatibility of the rods was evaluated in vitro in a zebrafish (Danio rerio) liver cell line (ZFL), with results showing that neither long nor short rods with magnetic particles caused cytotoxicity in ZFL cells for concentrations up to 50 μg/ml. We advocate that such nanocomposites can find applications in medical imaging and therapy, where the influence of shape on performance can be also assessed.

Local Structure and Magnetism of Fe2O3 Maghemite Nanocrystals: The Role of Crystal Dimension.

Here we report on the impact of reducing the crystalline size on the structural and magnetic properties of γ-Fe2O3 maghemite nanoparticles. A set of polycrystalline specimens with crystallite size ranging from ~2 to ~50 nm was obtained combining microwave plasma synthesis and commercial samples. Crystallite size was derived by electron microscopy and synchrotron powder diffraction, which was used also to investigate the crystallographic structure. The local atomic structure was inquired combining pair distribution function (PDF) and X-ray absorption spectroscopy (XAS). PDF revealed that reducing the crystal dimension induces the depletion of the amount of Fe tetrahedral sites. XAS confirmed significant bond distance expansion and a loose Fe-Fe connectivity between octahedral and tetrahedral sites. Molecular dynamics revealed important surface effects, whose implementation in PDF reproduces the first shells of experimental curves. The structural disorder affects the magnetic properties more and more with decreasing the nanoparticle size. In particular, the saturation magnetization reduces, revealing a spin canting effect. Moreover, a large effective magnetic anisotropy is measured at low temperature together with an exchange bias effect, a behavior that we related to the existence of a highly disordered glassy magnetic phase.

Removal of Lead Contaminants with γ-Fe2O3 Nanocrystals

γ-Fe2O3 (maghemite) nanocrystals were synthesized by co-precipitation method at room temperature have been used to remove Pb+2 from aqueous solutions. The prepared of this sample was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and vibrating sample magnetometer (VSM). XRD patterns indicated the maghemite structure of average particle size of ∼12 nm, which was further confirmed by the FESEM images of the γ-Fe2O3 nanosrystals. Magnetic saturation of synthesized maghemite was found to be 68.4 emu g–1. Adsorption experiments were tested for pH of the solutions, contact time and initial concentration of metal ions. The kinetic data of adsorption of heavy metal ions on the surface of the synthesized nano-adsorbent was described by a pseudo-second order equation. The adsorption equilibrium study exhibited that the heavy metal ions adsorption of maghemite nanocrystals followed a Langmuir and Freundlich isotherm model. Pb2+ adsorption equilibrium data were best fitted to Langmuir equation and maximum adsorption capacity of 25 mg g–1 was obtained. This work demonstrates that the synthesized γ-Fe2O3 nanocrystals can be considered as prospective nano-adsorbent.

Generation of maghemite nanocrystals from iron-sulfur centres.

Iron oxide nano-crystals 0.1-1.1 μm in diameter were generated on sulfur-doped amorphous carbon surfaces by electron beam irradiation of the novel 13e- high-spin complex [Fe(4-methyl-1,2-benzenedithiolate)2][NHEt3] encapsulated in a triblock copolymer. Possible relevance to iron nano-mineralization from Fe-S ferredoxin proteins and iron dysregulation in neurological disorders is discussed.

Formation of colloidal crystals of maghemite nanoparticles: Experimental and theoretical investigations

Novel colloidal crystals made of maghemite nanocrystals are fabricated by a co-evaporation method with a mixture of ethanol/hexane. Through a series of comprehensive characterization performed by grazing incidence small-angle X-ray scattering (GISAXS) and field emission gun scanning electron microscope (FEG-SEM), we show the first example of well-defined face-centered cubic (fcc) colloidal crystals. In order to obtain a clear picture of the crystal formation, the amount of ethanol in the solution is monitored using gas chromatography. In parallel, the interactions between the nanocrystals are calculated by statistical mechanics theory using solubility parameters. Theory predicts the formation of colloidal crystals at quite high amounts of ethanol around 15%, in perfect agreement with experimental results. We show that the theory can further be applied to predict the optimal experimental conditions for the formation of colloidal crystals using other solvent mixtures.

Synthesis and characterization of maghemite nanocrystals decorated multi-wall carbon nanotubes for methylene blue dye removal

Maghemite nanocrystals decorated multi-wall carbon nanotubes (MWCNTs) magnetic and electronic properties enhanced nanocomposites are synthesized using infrared (IR) irradiation and diazonium chemistry. The present method is simple and very effective as it overcomes the problem faced in the past decade for the decoration of carbon nanotubes (CNTs) with maghemite nanoparticles. The resulting materials are characterized using different techniques such as XPS, TEM, PXRD, STEM, EDX, HRTEM, and UV–Vis spectrophotometry. Firstly, sodium hydroxide treatment leads to the purification of MWCNTs (p-MWCNTs). The p-MWCNTs functionalization with tricarboxylic aryl diazonium salts generated in situ (p-MWCNTs-D3) followed by its impregnation with iron (II) ethylenediammonium sulfate (p-MWCNTs-D3/IEDS) utilizing IR radiation is the key step in homogeneously impregnating functionalized MWCNTs. Calcination of p-MWCNTs-D3/IEDS at 500 °C under argon atmosphere results in a controlled decoration of p-MWCNTs with maghemite nanocrystals (p-MWCNTs/MC). A homogeneous distribution of maghemite nanocrystals (cubic crystal system) on MWCNTs is observed in the size range of 1–6 nm, with a Gaussian mean diameter of ∼ 1.9 nm. To illustrate the applications of p-MWCNTs/MC for the decontamination of pollutants in water, methylene blue (MB), a model pollutant, is used and the performances are compared with the unmodified MWCNTs, Fe2O3 nanopowder and a mixture of MWCNTs and Fe2O3 nanopowder (1:1). Successful integration of properties of both constituents (MWCNTs and maghemite nanocrystals) in the new nanocomposite with superior characteristics is proved. The present method is valid for large-scale preparations which also opens very interesting perspectives in nanotechnology.

One-pot synthesis of maghemite nanocrystals across aqueous and organic solvents for magnetic hyperthermia

Superparamagnetic iron oxide nanocrystals (SPIONs) continue to receive intensive interest due to their significant prospects in biomedical applications. However, a critical requirement for such applications is the ability to synthesise high quality nanocrystals that are not only readily dispersible in aqueous solvents, but also retain good magnetic properties in an aqueous medium. This is not easy to achieve because, although organic solvent-based syntheses provide better control over SPIONs properties, their magnetic properties in particular, tend to deteriorate during aqueous phase transfer. In the current work, we propose a new route for the synthesis of high quality SPIONs using triethylamine (TEA) as a dynamic amphiphilic molecule. We demonstrate that in a single step, TEA can directly synthesise SPIONs that are readily dispersible in both aqueous and organic media. We further demonstrate the versatility of TEA in facilitating one-step phase transfer of SPIONs from organic to aqueous solvents. We then extend the applicability of this approach to demonstrate for the first time that organic molecules-capped SPIONs can also be directly dispersed in aqueous solvents. These SPIONs show remarkable potential for hyperthermia, as demonstrated through their outstanding magnetic field induced heating performance, which improves after their phase transfer to the aqueous phase. The current study establishes the importance of a suitable nanoparticle phase transfer protocol to ensure that the final product retains all the desirable application-focussed properties.

Wet Chemical Approach for Vanadium Doped Maghemite (Fe2-xVxO3) Nanocrystals

Background: Nanomaterials are widely synthesized by wet chemical routes. Magnetic nanoparticles with size ranging from 2 to 20 nm are of significant importance, resulting in a unique magnetic property of the nanoparticles. Maghemite nanostructures with partially disordered (Fe3+)[Fe5/3⁏1/3]O4 structures, undergo cation-insertion reactions without resorting to a drastic change in the material composition. Tailoring the superparamagnetic properties of nanoparticles by doping with transition metals, such as vanadium could be advantageous, as it would provide control over properties that are important for biological applications, such as magnetization dependent changes in temperature, relaxivity, and hysteresis. Method: We describe the obtention of vanadium doped maghemite (Fe2-xVxO3) from colloidal suspensions of vanadium and iron salts, using a precipitation–annealing method. A single step simpler synthesis scheme has been proposed in this work, with annealing temperature (350 °C), for a short period of time (~ 15 minutes), in order to achieve smaller in size but developed nanocrystals. Results: Superparamagnetic vanadium doped maghemite with particles (~8 nm) size has been synthesized. For the first time, vanadium up to 5 mol% has been doped on maghemite nanocrystals and characterized successfully for the presence of stable V3+. These results open a new era of vanadium applications in industrial Nanosciences; possessing superparamagnetic properties. Conclusion: A wet chemical route to direct the synthesis of vanadium doped γ- Fe2O3 nanocrystals, has been studied. The doping controls the crystallite size by occupying the vacant octahedral spaces available in the cubic system (space group P4332) maghemite crystals. High surface area magnetic particles, with an average size of ~8 nm were obtained. Keywords: Characterization, synthesis, vanadium doped maghemite.

Structural behavior of laser-irradiated γ-Fe2O3 nanocrystals dispersed in porous silica matrix : γ-Fe2O3 to α-Fe2O3 phase transition and formation of ε-Fe2O3

The effects of laser irradiation on γ-Fe2O3 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-Fe2O3 phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-Fe2O3 nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-Fe2O3 is an intermediate phase between γ-Fe2O3 and α-Fe2O3 and a series of distinct Raman vibrational bands were identified with the ε-Fe2O3 phase. The structural transformation of γ-Fe2O3 into α-Fe2O3 occurs either directly or via ε-Fe2O3, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.

Binary Superlattices from {Mo132} Polyoxometalates and Maghemite Nanocrystals: Long-Range Ordering and Fine-Tuning of Dipole Interactions.

In the present article, the successful coassembly of spherical 6.2 nm maghemite (γ-Fe2O3) nanocrystals and giant polyoxometalates (POMs) such as 2.9 nm {Mo132} is demonstrated. To do so, colloidal solutions of oleic acid-capped γ-Fe2O3 and long-chain alkylammonium-encapsulated {Mo132 } dispersed in chloroform are mixed together and supported self-organized binary superlattices are obtained upon the solvent evaporation on immersed substrates. Both electronic microscopy and small angles X-ray scattering data reveal an AB-type structure and an enhanced structuration of the magnetic nanocrystals (MNCs) assembly with POMs in octahedral interstices. Therefore, {Mo132} acts as an efficient binder constituent for improving the nanocrystals ordering in 3D films. Interestingly, in the case of didodecyldimethylammonium (C12)-encapsulated POMs, the long-range ordered binary assemblies are obtained while preserving the nanocrystals magnetic properties due to weak POMs-MNCs interactions. On the other hand, POMs of larger effective diameter can be employed as spacer blocks for MNCs as shown by using {Mo132} capped with dioctadecyldimethylammonium (C18) displaying longer chains. In that case, it is shown that POMs can also be used for fine-tuning the dipolar interactions in γ-Fe2O3 nanocrystal assemblies.

Magnetic field induced controllable self-assembly of maghemite nanocrystals: From 3D arrays to 1D nanochains

A hydrothermal process has been used to synthesize walnut-like maghemite superstructures which can be further self-assembled in a controllable manner into ordered three-dimensional (3D) architectures and one-dimensional (1D) nanochains in the presence of different external magnetic field. The assembly behavior of the maghemite nanoparticles isclosely related to the van der Waals interactions and external-field-induced magnetic dipole interactions. The magnetic properties of these nanostructures are also investigated.

Effect of Iron Oxide Nanocrystal Content on the Morphology and Magnetic Properties of Polystyrene-block-poly(methyl methacrylate) Diblock Copolymer Based Nanocomposites

Iron oxide nanocrystals, synthesized by surfactant-assisted thermal decomposition of Fe(CO)5, were selectively incorporated into the microseparated PS block phase of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer to design novel nanostructured inorganic/organic materials with magnetic properties. The colloidal synthesis led to oleic acid- and oleylamine- capped magnetic γ-Fe2O3 maghemite nanocrystals, which resulted well-dispersible in the block copolymer up to 60 wt % due to the affinity between PS block and organic surfactants. Atomic force and scanning electron microscopy confirmed that nanocomposites with nanocrystal loading lower than 40 wt % maintained cylindrical morphology, even without additional treatment applied to enhance their nanostructuration. Nanocrystals appeared well-dispersed in the nanocomposites at low contents, while nanocrystal clusters, with size depending on their loading, were observed at higher contents. At the highest nanocrystal content, the nanostru...

Superparamagnetic Liposomes for MRI Monitoring and External Magnetic Field‐Induced Selective Targeting of Malignant Brain Tumors

Magnetic‐fluid‐loadedliposomes (MFLs) of optimized magnetic responsiveness are newly worked out from the entrapment of superparamagnetic maghemite nanocrystals in submicronic PEG‐ylated rhodamine‐labelled phospholipid vesicles. This nanoplatform provides an efficient tool for the selective magnetic targeting of malignant tumors localized in brain and non‐invasive traceability by MRI through intravascular administration. As assessed by in vivo 7‐T MRI and ex vivo electron spin resonance, 4‐h exposure to 190‐T m–1 magnetic field gradient efficiently concentrates MFLs into human U87 glioblastoma implanted in the striatum of mice. The magnetoliposomes are then longer retained therein as checked by MRI monitoring over a 24‐h period. Histological analysis by confocal fluorescence microscopy confirms the significantly boosted accumulation of MFLs in the malignant tissue up to the intracellular level. Electron transmission microscopy reveals effective internalization by endothelial and glioblastoma cells of the magnetically conveyed MFLs as preserved vesicle structures. The magnetic field gradient emphasizes MFL distribution solely in the tumors according to the enhanced permeability and retention (EPR) effect while comparatively very low amounts are recovered in the other cerebral areas. Such a selective targeting precisely traceable by MRI is promising for therapeutic applications since the healthy brain tissue can be expected to be spared during treatments by deleterious anticancer drugs carried by magnetically guided MFLs.

Multiwalled carbon nanotubes anchored with maghemite nanocrystals for high-performance lithium storage

In this paper, we have anchored maghemite (γ-Fe2O3) nanocrystals compactly and uniformly on multiwalled carbon nanotubes (MWCNT) via a polyelectrolyte-assisted layer-by-layer assembly approach based on electrostatic attraction. When evaluated as an anode for lithium-ion batteries (LIBs), the as-synthesized MWCNT-γ-Fe2O3 nanohybrid displays high reversible capacities, remarkable cycling stability, and magnificent high rate capability, facilitating its application as an advanced anode for high-energy, long-life, and high-power LIBs.

Size- and composition-dependent radio frequency magnetic permeability of iron oxide nanocrystals.

We investigate the size- and composition-dependent ac magnetic permeability of superparamagnetic iron oxide nanocrystals for radio frequency (RF) applications. The nanocrystals are obtained through high-temperature decomposition synthesis, and their stoichiometry is determined by Mössbauer spectroscopy. Two sets of oxides are studied: (a) as-synthesized magnetite-rich and (b) aged maghemite nanocrystals. All nanocrystalline samples are confirmed to be in the superparamagnetic state at room temperature by SQUID magnetometry. Through the one-turn inductor method, the ac magnetic properties of the nanocrystalline oxides are characterized. In magnetite-rich iron oxide nanocrystals, size-dependent magnetic permeability is not observed, while maghemite iron oxide nanocrystals show clear size dependence. The inductance, resistance, and quality factor of hand-wound inductors with a superparamagnetic composite core are measured. The superparamagnetic nanocrystals are successfully embedded into hand-wound inductors to function as inductor cores.

Size and Size Distribution Control of γ-Fe2O3 Nanocrystallites: An in Situ Study

The formation and growth of maghemite (γ-Fe2O3) nanocrystals during the hydrothermal synthesis from aqueous solutions of ammonium iron(III) citrate (C6H8O7·xFe(III)·yNH3) have been studied by in situ powder X-ray diffraction (PXRD). Data analysis by Rietveld refinement and whole powder pattern modeling (WPPM) reveals that the crystallite size and size distribution can be precisely tuned through simple adjustments of the reaction temperature and time. Increasing the reaction temperature causes faster growth and results in larger crystallites while the size distribution broaden as reaction times increase, regardless of temperature. The crystallization kinetics were investigated by fitting the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetic model to the growth curves. The activation energy was found to be 67(15) kJ/mol, and the limiting mechanisms of crystallite formation were determined. The growth was studied with various precursor compositions of ammonium iron(III) citrate and Fe(NO3)3·9H2O or FeCl3·6H2O. Increasing the fraction of Fe(NO3)3·9H2O in the precursor results in larger maghemite nanocrystallites. The addition of Fe(NO3)3·9H2O also results in the formation of the thermodynamically more stable hematite (α-Fe2O3) phase as a byproduct.

Colloidal assemblies of oriented maghemite nanocrystals and their NMR relaxometric properties.

An elevated-temperature polyol-based colloidal-chemistry approach allows for the development of size-tunable (50 and 86 nm) assemblies of maghemite iso-oriented nanocrystals, with enhanced magnetization. (1)H-nuclear magnetic resonance (NMR) relaxometric experiments show that the ferrimagnetic cluster-like colloidal entities exhibit a remarkable enhancement (4-5 times) in transverse relaxivity when compared to that of the superparamagnetic contrast agent Endorem®, over an extended frequency range (1-60 MHz). The marked increase in the transverse relaxivity r2 at a clinical magnetic field strength (∼1.41 T), which is 405.1 and 508.3 mM(-1) s(-1) for small and large assemblies, respectively, makes it possible to relate the observed response to the raised intra-aggregate magnetic material volume fraction. Furthermore, cell tests with a murine fibroblast culture medium confirmed cell viability in the presence of the clusters. We discuss the NMR dispersion profiles on the basis of relaxivity models to highlight the magneto-structural characteristics of the materials for improved T2-weighted magnetic resonance images.

6-Aminohexanoic Acid를 이용하여 물에 분산되는 Maghemite 나노입자의 합성

This paper describes a simple route to synthesis of water-dispersible monodisperse maghemite () nanocrystals using 6-aminohexanoic acid (AHA) as a stabilizer. The water-dispersible nanocrystals with an average size of 5 nm were obtained simply by addition of into an octyl ether solution containing AHA at under argon condition. As-prepared AHA coated nanocrystals exhibited highly crystallinity and magnetic property while keeping a good dispersity in an aqueous phase. We also obtained water-dispersible AHA coated nanocrystals using ligand-exchange method, demonstrating that AHA can be a good candidate for preparing water-dispersible uniform metal oxide nanocrystals.