Synthesis and characterization of functionalized core-shell γFe2O3-SiO2 nanoparticles

Amino and/or polyethyleneglycol (PEG) functionalized core-shell γFe₂O₃-SiO₂ magnetic nanoparticles were synthesized and characterized. Amino-PEG-functionalized core-shell nanoparticles have calibrated sizes and a good colloidal stability. These bi-functionalized core-shell nanoparticles are potentially useful as biocompatible particles for magnetically targeted chemotherapy.     

Structural Insight into Stabilization of Pickering Emulsions with Fe3O4@SiO2 Nanoparticles for Enzyme Catalysis in Organic Media

Encapsulation of enzymes with enhanced activity and recyclability in water‐in‐oil Pickering emulsions is a simple and efficient method for their immobilization; however, the effect produced by the structure of colloidal particles on the stabilization of the Pickering emulsion for enzyme catalysis has not been investigated in detail. In this study, four types of hydrophobic Fe₃O₄@SiO₂ nanoparticles (NPs) with similar chemical compositions, particle diameters, but different surface characteristics have been prepared and utilized for enzyme encapsulation in various water‐in‐oil magnetic Pickering emulsions, after which the relationship between NPs structure, size of emulsions droplets, and enzyme activity is examined. The obtained results indicate that (i) the more hydrophobic Fe₃O₄@SiO₂ NPs cause the higher enzyme activity; (ii) the higher hydrophobicity of oil phase also increases the enzyme activity, especially for Fe₃O₄@w‐SiO₂ NPs which form in the solvent of water. The results are mainly attributed to the higher specific surface area of emulsion droplets and interfacial mass transfer of substrates through the interfaces of droplets. The reported data provide new insights into the mechanism of stabilization of Pickering emulsions for enhancing enzyme activity and demonstrate efficient theoretical references for enzyme immobilization and synthesis of stable and active biocatalysts with high recyclability.     

Fabrication and characterization of thermo-sensitive magnetic polymer composite nanoparticles

Novel dual-functional nanospheres composed of Fe₃O₄ nanoparticles embedded in a thermo-sensitive polymer were synthesized by emulsifier-free emulsion polymerization. The Fe₃O₄ nanoparticles were prepared by chemical precipitation. The surface of these particles was modified by oleic acid to achieve stability against agglomeration. These stable particles were then polymerized using N-isopropylacrylamide as the main monomer, divinylbenzene as the crosslinker and potassium persulfate as the initiator. The nanospheres were characterized by Fourier-transform infrared spectrum, transmission electron microscopy, thermogravimetric analysis, vibrating sample magnetometer and dynamic light scattering. The results show that the lower critical solution temperature of thermo-sensitive magnetic immunomicrospheres was between 40 and 45 °C.     

Suppression of the Verwey Transition by Charge Trapping

The Verwey transition in Fe₃O₄ nanoparticles with a mean diameter of 6.3 nm is suppressed after capping the particles with a 3.5 nm thick shell of SiO₂. By X‐ray absorption spectroscopy and its associated X‐ray magnetic circular dichroism this suppression can be correlated to localized Fe²⁺ states and a reduced double exchange visible in different site‐specific magnetization behavior in high magnetic fields. The results are discussed in terms of charge trapping at defects in the Fe₃O₄/ SiO₂ interface and the consequent difficulties in the formation of the common phases of Fe₃O₄. By comparison to X‐ray absorption spectra of bare Fe₃O₄ nanoparticles in course of the Verwey transition, particular changes in the spectral shape could be correlated to changes in the number of unoccupied d states for Fe ions at different lattice sites. These findings are supported by density functional theory calculations.     

New evidences of in situ laser irradiation effects on γ‐Fe2O3 nanoparticles: a Raman spectroscopic study

The nature of the physical mechanisms responsible for the structural modification of the γ‐Fe₂O₃ nanoparticles under laser irradiation has been investigated by Raman spectroscopy. In situ micro‐Raman measurements were carried out on as‐prepared γ‐Fe₂O₃ nanoparticles about 4 nm in size as a function of laser power and on annealed γ‐Fe₂O₃ particles. A baseline profile analysis clearly evidenced that the phase transition from maghemite into hematite is caused by local heating due to laser irradiation with an increase of grain size of nanoparticles. This increasing was clearly determined by X‐ray diffraction from 4 nm in nanoparticles up to more than 177 nm beyond 900 °C in a polycrystalline state. Copyright © 2010 John Wiley & Sons, Ltd.     

Controlled flame synthesis of αFe<Subscript>2</Subscript>O<Subscript>3</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles: effect of flame configuration,flame temperature,and additive loading

Superparamagnetic iron oxide nanoparticles are used in diverse applications, including optical magnetic recording, catalysts, gas sensors, targeted drug delivery, magnetic resonance imaging, and hyperthermic malignant cell therapy. Combustion synthesis of nanoparticles has significant advantages, including improved nanoparticle property control and commercial production rate capability with minimal post-processing. In the current study, superparamagnetic iron oxide nanoparticles were produced by flame synthesis using a coflow flame. The effect of flame configuration (diffusion and inverse diffusion), flame temperature, and additive loading on the final iron oxide nanoparticle morphology, elemental composition, and particle size were analyzed by transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy. The synthesized nanoparticles were primarily composed of two well known forms of iron oxide, namely hematite αFe₂O₃ and magnetite Fe₃O₄. We found that the synthesized nanoparticles were smaller (6–12 nm) for an inverse diffusion flame as compared to a diffusion flame configuration (50–60 nm) when CH₄, O₂, Ar, and N₂ gas flow rates were kept constant. In order to investigate the effect of flame temperature, CH₄, O₂, Ar gas flow rates were kept constant, and N₂ gas was added as a coolant to the system. TEM analysis of iron oxide nanoparticles synthesized using an inverse diffusion flame configuration with N₂ cooling demonstrated that particles no larger than 50–60 nm in diameter can be grown, indicating that nanoparticles did not coalesce in the cooler flame. Raman spectroscopy showed that these nanoparticles were primarily magnetite, as opposed to the primarily hematite nanoparticles produced in the hot flame configuration. In order to understand the effect of additive loading on iron oxide nanoparticle morphology, an Ar stream carrying titanium-tetra-isopropoxide (TTIP) was flowed through the outer annulus along with the CH₄ in the inverse diffusion flame configuration. When particles were synthesized in the presence of the TTIP additive, larger monodispersed individual particles (50–90 nm) were synthesized as observed by TEM. In this article, we show that iron oxide nanoparticles of varied morphology, composition, and size can be synthesized and controlled by varying flame configuration, flame temperature, and additive loading.     

Full Protection for Graphene‐Incorporated Micro‐/Nanocomposites Containing Ultra‐small Active Nanoparticles: the Best Li‐Storage Properties

In recent years, graphene‐incorporated micro‐/nanocomposites represent one of the hottest developing directions for the composite materials. However, a large number of active nanoparticles (NPs) are still in the unprotected state in most constructed graphene‐containing designs, which will seriously impair the effects of the graphene additives. Here, a fully protected Fe₃O₄‐based micro‐/nanocomposite (G/Fe₃O₄@C) is rationally developed by carbon‐boxing the common graphene/Fe₃O₄ microparticulates (G/Fe₃O₄). The processes and results of full protection are tracked in detail and characterized by X‐ray diffraction, X‐ray photoelectron spectroscopy, and nitrogen absorption–desorption isotherms, as well as scanning and transition electron microscopy. When used as the anode for lithium‐ion batteries, the fully protected G/Fe₃O₄@C exhibits the best lithium‐storage properties in terms of the highest rate capabilities and the longest cycle life compared to the common G/Fe₃O₄ composites and commercial Fe₃O₄ products. These much improved properties are mainly attributed to its novel structural features including complete protection of active Fe₃O₄ nanoparticles by the surface carbon box, a robust conductive network composed of nitrogen‐doped graphene nanosheets, ultra‐small Fe₃O₄ NPs of 4–5 nm, abundant mesopores to accommodate the volume variation during cycling, and micrometer‐sized secondary particles.     

Preparation of Fe3O4/polystyrene composite particles from monolayer oleic acid modified Fe3O4 nanoparticles via miniemulsion polymerization

Fe₃O₄/polystyrene composite particles were prepared from oleic acid (OA) modified Fe₃O₄ nanoparticles via miniemulsion polymerization. It was concluded that the surface properties of OA modified magnetite nanoparticles have a great effect on preparation of the composite particles. When Fe₃O₄ nanoparticles coated by multilayer of OA was employed, there were large amounts of free polystyrene particles in the product. Fe₃O₄/polystyrene composite particles with defined structure and different magnetite content can be readily prepared from monolayer OA modified Fe₃O₄ nanoparticles. It was concluded that surface of the monolayer OA modified Fe₃O₄ nanoparticles is more hydrophobic than that of the multilayer coated ones, thus improving the dispersibility of the Fe₃O₄ nanoparticles in styrene monomer and allowing preparation of the Fe₃O₄/polystyrene composite particles with defined structure and controllable magnetite content.     

Synthesis of magnetite core–shell nanoparticles by surface-initiated ring-opening polymerization of l-lactide

Fe₃O₄–polylactide (PLA) core–shell nanoparticles were perpared by surface functionalization of Fe₃O₄ nanoparticles and subsequent surface-initiated ring-opening polymerization of l-lactide. PLA was directly connected onto the magnetic nanoparticles surface through a chemical linkage. Fourier transform infrared (FT-IR) spectra directly provided evidence of the PLA on the surface of the magnetic nanoparticles. Transmission electron microscopy images (TEM) showed that the magnetic nanoparticles were coated by PLA with a 3-nm-thick shell. The amount of grafted polymer determined by thermal gravimetric analysis was ∼13.3% by weight. X-ray diffraction (XRD) patterns of as-prepared core–shell nanoparticles showed the same structure (spinel cubic lattice type) to that of the bare core materials with similar intensity of the corresponding peaks, and that the polymer coating was amorphous. The particles could be stably dispersed in chloroform for several weeks. The prepared Fe₃O₄–PLA core–shell nanoparticles were superparamagnetic behavior with a saturation magnetization value nearly identical to that of the bare magnetic nanoparticles, rendering the Fe₃O₄–PLA nanoparticles for potential applications in both the material technology and biomedical fields.     

Preparation and characterization of silicon oil based ferrofluid

Stable silicon oil based ferrofluid was prepared in the present investigation. Silicon oil surfactant ethoxy terminated polydimethylsiloxane was used to modify the Fe₃O₄ nanoparticles. The Fe₃O₄ nanoparticles were firstly coated with a SiO₂ layer by the hydrolysis of tetraethoxysilane. Then using the active hydroxyl groups on the surface of the SiO₂, silicon oil surfactant was covalently grafted onto the Fe₃O₄ nanoparticles surface. The ethoxy terminated polydimethylsiloxane has similar molecular chain structure and good compatibility with that of the carrier liquid, thus ensuring stable dispersion of modified Fe₃O₄ in the carrier silicon oil. The interaction between Fe₃O₄ and the modifier was characterized by IR and XPS. The crystal structure and the magnetic properties of the Fe₃O₄ nanoparticles were determined by XRD and VSM, respectively. The size and morphology of the particles were observed using TEM. The properties of the silicon oil based ferrofluid were characterized by Gouy magnetic balance. The results indicated that the ferrofluid had high magnetism and good stability. The rheological properties and thermostability of the ferrofluid were also investigated.     

Synthesis and Characterization of La‐Doped Fe3O4‐Polyaniline Magnetic Response Nanocomposites

Lanathum (La)‐doped Fe₃O₄ magnetic nanoparticles were prepared in aqueous solution at room temperature, then La‐doped Fe₃O₄‐polyaniline (PANI) nanocomposites containing a dispersion of La‐doped Fe₃O₄ nanoparticles were synthesized via in‐situ polymerization of aniline monomer. The structure and properties of the synthesized samples were characterized with X‐ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectrometry (FTIR), thermogravimetric analysis (TGA), inductively coupled plasma atomic emission spectrometry (ICPAES), and a vibrating sample magnetometer (VSM). The resulting particles of La‐doped Fe₃O₄ and La‐doped Fe₃O₄‐PANI were almost spherical with diameters ranging from 15 to 25 nm and 25 to 85 nm, respectively. The La‐doped Fe₃O₄‐PANI composite presented core‐shell structures; polyaniline covered the La‐doped Fe₃O₄ completely. The specific saturation magnetization of La‐doped Fe₃O₄‐PANI depended on the starting material of La‐doped Fe₃O_4. It increased with increasing amounts of La and Fe₃O₄ content.     

Glutathione-capped core-shell structured magnetite nanoparticles: Fabrication and their nonlinear optical characteristics

Core-shell structured magnetic Fe₃O₄@glutathione composite nanoparticles were synthesized and examined using diverse methods including Fourier-transform infrared spectroscopy, X-ray diffraction, energy-dispersive X-ray analysis, transmission electron microscope, thermogravimetric analysis, and vibrating sample magnetometer analysis. In addition, the nonlinear optical measurements were performed by both open and closed aperture z-scan methods using an aqueous colloidal solution of the fabricated nanoparticles. The colloidal system exhibited a positive nonlinear refractive index because of the self-focusing effect arising from optical re-orientation. Although optical re-orientation is a rare phenomenon in nanocolloids, high polarizability of the enveloping organic ligands caused optical re-orientation of the composite nanoparticles in the electrical field direction of the incident beam. Finally, the effect of external voltage on the nonlinear optical index was further investigated.     

Synthesis of magnetic core–shell Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>–Au nanoparticle for biomolecule immobilization and detection

The production of monodispersed magnetic nanoparticles with appropriate surface modification has attracted increasing attention in biomedical applications including drug delivery, separation, and purification of biomolecules from the matrices. In the present study, we report rapid and room temperature reaction synthesis of gold-coated iron nanoparticles in aqueous solution using the borohydride reduction of HAuCl₄ under sonication for the first time. The resulting nanoparticles were characterized with transmission electron microscopy (TEM), electron spectroscopy for chemical analysis (ESCA), ultraviolet visible spectroscopy (UV–Vis), and X-ray diffraction (XRD). Surface charges and magnetic properties of the nanoparticles were also examined. The pattern of Fe₃O₄ nanoparticles is face centered cubic with an average diameter of 9.5 nm and the initial reduction of gold on the surface of Fe₃O₄ particles exhibits uniform Fe₃O₄–Au nanoparticles with an average diameter of 12.5 nm. The saturation magnetization values for the uncoated and gold-coated Fe₃O₄ nanoparticles were found to be 30 and 4.5 emu/g, respectively, at 300 K. The progression of binding events between boronic acid terminated ligand shell and fructose based on the covalent bonding interaction was measured by absorbance spectral changes. Immunomagnetic separation was also performed at different E. coli concentration to evaluate capturing efficiency of resulting nanoparticles. Immunomagnetic separation percentages were varied in a range of 52.1 and 21.9% depend on the initial bacteria counts.     

Superparamagnetic silica nanoparticles with immobilized metal affinity ligands for protein adsorption

Superparamagnetic silica-coated magnetite (Fe₃O₄) nanoparticles with immobilized metal affinity ligands were prepared for protein adsorption. First, magnetite nanoparticles were synthesized by co-precipitating Fe²⁺ and Fe³⁺ in an ammonia solution. Then silica was coated on the Fe₃O₄ nanoparticles using a sol–gel method to obtain magnetic silica nanoparticles. The condensation product of 3-Glycidoxypropyltrimethoxysilane (GLYMO) and iminodiacetic acid (IDA) was immobilized on them and after charged with Cu²⁺, the magnetic silica nanoparticles with immobilized Cu²⁺ were applied for the adsorption of bovine serum albumin (BSA). Scanning electron micrograph showed that the magnetic silica nanoparticles with an average size of 190 nm were well dispersed without aggregation. X-ray diffraction showed the spinel structure for the magnetite particles coated with silica. Magnetic measurement revealed the magnetic silica nanoparticles were superparamagnetic and the saturation magnetization was about 15.0 emu/g. Protein adsorption results showed that the nanoparticles had high adsorption capacity for BSA (73 mg/g) and low nonspecific adsorption. The regeneration of these nanoparticles was also studied.     

Yolk–Shell‐Structured Cu/Fe@γ‐Fe2O3 Nanoparticles Loaded Graphitic Porous Carbon for the Oxygen Reduction Reaction

Core–shell Cu/γ‐Fe₂O₃@C and yolk–shell‐structured Cu/Fe@γ‐Fe₂O₃@C particles are prepared by a facile synthesis method using copper oxide as template particles, resorcinol‐formaldehyde as the carbon precursor, and iron nitrate solution as the iron source via pyrolysis. With increasing carbonization temperature and time, solid γ‐Fe₂O₃ cores are formed and then transformed into Fe@γ‐Fe₂O₃ yolk–shell‐structured particles via Ostwald ripening under nitrogen gas flow. The composition variations are studied, and the formation mechanism is proposed for the generation of the hollow and yolk–shell‐structured metal and metal oxides. Moreover, highly graphitic carbons can be obtained by etching the metal and metal oxide nanoparticles through an acid treatment. The electrocatalytic activity for oxygen reduction reaction is investigated on Cu/γ‐Fe₂O₃@C, Cu/Fe@γ‐Fe₂O₃@C, and graphitic carbons, indicating comparable or even superior performance to other Fe‐based nanocatalysts.     

Designing Zwitterionic SiO2NH2‐Au Particles with Tunable Patchiness using Wrinkles

A facile template‐based approach toward zwitterionic SiO₂NH₂‐Au patchy particles is presented. Therefore, wrinkle templates prepared by stress release in a bilayer system comprised of an elastic PDMS fundament and a thin SiO_x top layer are used. After aligning positively charged, amine‐functionalized silica particles in wrinkle grooves, their surfaces are partially modified with negatively charged gold nanoparticles in an electrostatic adsorption step. Patchiness is precisely controlled by the degree of immersion of the initial particles into wrinkles of varying dimensions. By ultrasonication or wetting with a water droplet, patchy particles are easily released from the substrate‐yielding particles with two oppositely charged hemispheres. Interfacial tension measurements prove the surface activity of the SiO₂NH₂‐Au particles in an oil/water system and are explained in the view of the Janus‐type surface charges of the particles and the charge of the oil/water interface.     

Nanostructural and magnetic studies of virtually monodispersed NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocrystals synthesized by a liquid–solid-solution assisted hydrothermal route

This study presents a comprehensively and systematically structural, chemical and magnetic characterization of ~9.5 nm virtually monodispersed nickel ferrite (NiFe₂O₄) nanoparticles prepared using a modified liquid–solid-solution (LSS) assisted hydrothermal method. Lattice-resolution scanning transmission electron microscope (STEM) and converged beam electron diffraction pattern (CBED) techniques are adapted to characterize the detailed spatial morphology and crystal structure of individual NiFe₂O₄ particles at nano scale for the first time. It is found that each NiFe₂O₄ nanoparticle is single crystal with an fcc structure. The morphology investigation reveals that the prepared NiFe₂O₄ nanoparticles of which the surfaces are decorated by oleic acid are dispersed individually in hexane. The chemical composition of nickel ferrite nanoparticles is measured to be 1:2 atomic ratio of Ni:Fe, indicating a pure NiFe₂O₄ composition. Magnetic measurements reveal that the as-synthesized nanocrystals displayed superparamagnetic behavior at room temperature and were ferromagnetic at 10 K. The nanoscale characterization and magnetic investigation of monodispersed NiFe₂O₄ nanoparticles should be significant for its potential applications in the field of biomedicine and magnetic fluid using them as magnetic materials.     

Size-controlled synthesis of superparamagnetic iron oxide nanoparticles and their surface coating by gold for biomedical applications

The size mono-dispersity, saturation magnetization, and surface chemistry of magnetic nanoparticles (NPs) are recognized as critical factors for efficient biomedical applications. Here, we performed modified water-in-oil inverse nano-emulsion procedure for preparation of stable colloidal superparamagnetic iron oxide NPs (SPIONs) with high saturation magnetization. To achieve mono-dispersed SPIONs, optimization process was probed on several important factors including molar ratio of iron salts [Fe³⁺ and Fe²⁺], the concentration of ammonium hydroxide as reducing agent, and molar ratio of water to surfactant. The biocompatibility of the obtained NPs, at various concentrations, was evaluated via MTT (3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide) assay and the results showed that the NPs were non-toxic at concentrations <0.1 mg/mL. Surface functionalization was performed by conformal coating of the NPs with a thin shell of gold (∼4 nm) through chemical reduction of attached gold salts at the surface of the SPIONs. The Fe₃O₄ core/Au shell particles demonstrate strong plasmon resonance absorption and can be separated from solution using an external magnetic field. Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, phase components, core–shell surface composition, and magnetic properties have confirmed the formation of the mono-dispersed core–shell nanostructure.     

Fabrication of magnetic gold nanorod particles for immunomagnetic separation and SERS application

The preparation and application of rod-shaped core–shell structured Fe₃O₄–Au nanoparticles for immunomagnetic separation and sensing were described for the first time with this study. To synthesize magnetic gold nanorod particles, the seed-mediated synthetic method was carried out and the resulting nanoparticles were characterized with transmission electron microscopy (TEM), ultraviolet visible spectroscopy (UV–Vis), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD). Magnetic properties of the nanoparticles were also examined. Characterization of the magnetic gold nanorod particles has proven that the resulting nanoparticles were composed of Fe₃O₄ core and the gold shell. The rod-shaped gold-coated iron nanoparticles have an average diameter of 16 ± 2 nm and an average length of about 50 ± 5 nm (corresponding aspect ratio of 3). The saturation magnetization value for the magnetic gold nanorod particles was found to be 37 emu/g at 300 K. Rapid and room temperature reaction synthesis of magnetic gold nanorod particles and subsequent surface modification with E. coli antibodies provide immunomagnetic separation and SERS application. The analytical performance of the SERS-based homogenous sandwich immunoassay system with respect to linear range, detection limit, and response time is also presented.     

Facile Synthesis of Gd(OH)3‐Doped Fe3O4 Nanoparticles for Dual‐Mode T1‐ and T2‐Weighted Magnetic Resonance Imaging Applications

The facile hydrothermal synthesis of polyethyleneimine (PEI)‐coated iron oxide (Fe₃O₄) nanoparticles (NPs) doped with Gd(OH)₃ (Fe₃O₄‐Gd(OH)₃‐PEI NPs) for dual mode T₁‐ and T₂‐weighted magnetic resonance (MR) imaging applications is reported. In this approach, Fe₃O₄‐Gd(OH)₃‐PEI NPs are synthesized via a hydrothermal method in the presence of branched PEI and Gd(III) ions. The PEI coating onto the particle surfaces enables further modification of poly(ethylene glycol) (PEG) in order to render the particles with good water dispersibility and improved biocompatibility. The formed Fe₃O₄‐Gd(OH)₃‐PEI‐PEG NPs have a Gd/Fe molar ratio of 0.25:1 and a mean particle size of 14.4 nm and display a relatively high r₂ (151.37 × 10^?3m ^?1 s^?1) and r₁ (5.63 × 10^?3m ^?1 s^?1) relaxivity, affording their uses as a unique contrast agent for T₁‐ and T₂‐weighted MR imaging of rat livers after mesenteric vein injection of the particles and the mouse liver after intravenous injection of the particles, respectively. The developed Fe₃O₄‐Gd(OH)₃‐PEI‐PEG NPs may hold great promise to be used as a contrast agent for dual mode T₁‐ and T₂‐weighted self‐confirmation MR imaging of different biological systems.