Synthesis and physical characterization of magnetite nanoparticles for biomedical applications

Iron oxide nanoparticles for biomedical applications in the size range of 15–130 nm were prepared by either oxidative hydrolysis of ferrous sulfate with KOH or precipitation from ferrous/ferric chloride solutions. The magnetite particle size is controlled by variation of pH and temperature. The synthesized magnetite nanoparticles are partially oxidized as signaled by ferrous concentrations of below 24 wt% Fe²⁺ and lattice parameters of a₀ ≤ 8.39 Å which are smaller compared to 8.39 Å for stoichiometric magnetite. The extend of oxidation increases with decreasing particle size. Heating at 150–350 °C topotactically transforms the magnetite nanoparticles into stoichiometric tetragonal maghemite (ferrous ion concentration c_Fe²⁺=0 and a₀ = 8.34 Å) without significant particle growth. The magnetite–maghemite transformation is studied with thermal analysis, XRD and IR spectroscopy. The saturation magnetizations of the magnetite and maghemite particles decrease with decreasing particle size. The variation of M_s with particle size is interpreted using a magnetic core–shell particle model. Magnetite particles with d ≤ 16 nm show superparamagnetic behavior at room temperature whereas particles with diameter >16 nm display hysteresis behavior. These particles are candidates for biomedical applications, e.g. controlled drug release or hyperthermia.     

Synthesis and characterizations of Ni-Fe@spinel oxide core-shell nanoparticles

Core-shell Ni-Fe@ferrite nanoparticles with an average diameter of 14 nm and shell thickness of 3 nm were synthesized through a redox-transmetalation process. The alloy core and spinel oxide shell were verified by X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy. The hydrophobic oleylamine molecules on the surface were replaced by hydrophilic meso-2,3-Dimercaptosuccinic acid to make the nanoparticles to be water-soluble. X-ray diffraction study of the as-prepared core-shell nanoparticles indicates that they remained face centered cubic alloy core and spinel shell form in air. Magnetic measurements indicate that the core-shell nanoparticles exhibit superparamagnetic and exchange bias characteristics at 300 K and 5 K, respectively.     

Synthesis of nanocrystalline CeAlO3 by solution-combustion route

CeAlO₃ was synthesised by a modified solution-combustion route using a mixture of urea and glycine as fuel. A trivalent oxidation state of cerium was stabilised and high-quality single phase polycrystalline CeAlO₃ was obtained by optimising the ratio of fuels. The transmission electron micrography and powder X-ray diffraction investigations showed that the particles were nanocrystalline in nature. Rietveld refinement confirmed the space group of the structure to be I4/mcm$I4/mcm$ with lattice parameters a=5.3278(1)$a=5.3278(1)$ Å, c=7.5717(3)$c=7.5717(3)$ Å. Magnetisation measurements indicated that the sample was paramagnetic up to 2 K. The susceptibility data fitted the Curie–Weiss model in the temperature range 100–300 K with _θp=−40${\theta }_p=-40$ K. The value of _μeff=2.2_μB${\mu }_{\text{eff}}=2.2{\mu }_B$ was close to that expected for a Ce³⁺ ion. The magnetic properties were comparable to that reported for single crystals indicating the high quality of CeAlO₃ prepared in the present work. The semiconducting band gap as estimated from UV–visible spectroscopy was 3.26 eV.     

Synthesis of magnetic nanostructures: Shape tuning by the addition of a polymer at low temperature

We report a simple method for shape-controlled synthesis of iron oxide spinels such as magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) nanostructures using a thermoresponsive polymer poly(vinyl methyl ether) (PVME) by the alkaline hydrolysis of iron salt at low temperature (20 °C). Microscopic analysis confirmed the formation of needle- and flower-shaped iron oxide nanostructures depending on reaction conditions. High-resolution transmission electron microscopic analysis of the needle- and flower-shaped nanostructures as well as their corresponding selected area electron diffraction patterns revealed that the formed nanostructures are crystalline in nature. X-ray diffraction study reveals the formation of well-crystalline pure Fe₃O₄ and γ-Fe₂O₃ nanostructures under different reaction conditions. Fourier transform Infra-red spectroscopic analysis confirms the adsorption of PVME on the surface of iron oxide nanostructures. Finally, the magnetic properties of γ-Fe₂O₃ and Fe₃O₄ nanostructures is studied that shows the superparamagnetic behavior of the formed iron oxide nanostructures.     

Preparation of cobalt oxide nanoparticles and cobalt powders by solvothermal process and their characterization

Co₃O₄ nanoparticles and cobalt (fcc-Co) powders were successfully synthesized by solvothermal process from a single precursor. The reaction of Co(Ac)₂ with sodium dodecylbenzenesulfonate (SDBS) shows evident-dependent temperature effect. At 180 °C, Co(Ac)₂ reacts with SDBS to produce precursor CoCO₃ plate structures, which are assembled by small nanoparticles. At the temperature of 250 °C, the precursor CoCO₃ can be gradually decomposed to form Co₃O₄ nanoparticles with diameter of ca. 70 nm. While, at 250 °C, the reaction of Co(Ac)₂ with SDBS also produce precursor CoCO₃ nanoparticles/plates, but the CoCO₃ nanoparticles/plates would only decompose to give metal Co. In this process, SDBS acts as not only a surfactant but also a reagent. Magnetic measurements reveal that the as-prepared Co₃O₄ nanoparticles exhibit weak ferromagnetic properties and Co powders show ferromagnetic properties. In addition, a possible formation mechanism was elaborately discussed.     

Studies on initial permeability and loss factor in Ni-Zn ferrites synthesized by oxalate precursors

Ni_XZn_1−XFe₂O₄ ferrites with (X = 0.28-0.40 in step of 0.2) have been synthesized by oxalate precursor method and investigated for their, initial permeability and loss factor measurements. Initial permeability has been observed to increase with the increase in Ni²⁺ up to X = 0.32, beyond which it decreases. The variation of initial permeability has been explained by considering the factors such as grain size, saturation magnetization and anisotropy constant. Thermal variation of initial permeability reveals a peak height in μ_i-T curves which tends to increase with increase in Ni²⁺ content. μ_i-T curves also exhibit thermal hysteresis, which reveals the inverse relationship between the difference in heating and cooling curves at which hysteresis falls between Hopkinson peak and T_c with value of initial permeability. Loss factor values are small which is attributed to high density of the samples and processing techniques.     

Synthesis of polypyrrole coated manganese nanowires and their application in hydrogen peroxide detection

This study focuses on the synthesis and application of polypyrrole coated manganese nanowires (Mn/PPy NWs) as an enzyme-less sensor for the detection of hydrogen peroxide (H₂O₂). The X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) results confirm a core–shell structure with the Mn nanowires encapsulated by the PPy. An electrochemical sensor based on the Mn/PPy NWs for amperometric determination of H₂O₂ is prepared. The electrochemical behaviour of H₂O₂ is investigated by cyclic voltammetry with the use of modified glassy carbon electrode (GCE) with Mn/PPy NWs film. The modified glassy carbon electrode (GCE) with Mn/PPy NWs shows enhanced amperometric response for the detection of H₂O₂. This is due to the high available surface area of Mn/PPy NWs which can provide a suitable area for the reaction of H₂O₂. The detection limit and limit of quantification (S/N = 3) for two linear segments (low and high concentration of H₂O₂) are estimated to be 2.12 μmol L⁻¹, 7.07 μmol L⁻¹ and 22.3 μmol L⁻¹, 74.5 μmol L⁻¹, respectively. In addition, the sensitivity for these two linear segments is 0.4762 μA mM⁻¹ and 0.0452 μA mM⁻¹ respectively.     

Synthesis and electrochemical properties of nanorod-shaped LiMn1.5Ni0.5O4 cathode materials for lithium-ion batteries

Nanorod-shaped LiMn_1.5Ni_0.5O₄ cathode powders were synthesized by a co-precipitation method with oxalic acid. Their structures and electrochemical properties were characterized by SEM, XRD and galvanostatic charge-discharge tests. The resulting nanorod-shaped LiMn_1.5Ni_0.5O₄ cathode active materials delivered a specific discharge capacity of 126 mAh g⁻¹ at 0.1 C rate. These active materials exhibited better capacity retention and higher rate performance than those of LiMn_1.5Ni_0.5O₄ cathode powders with irregular morphology.     

Magnetic and microwave absorption properties of Ni microcrystals with hierarchical branch-like and flowers-like shapes

The Ni microcrystals with hierarchical branch-like and flowers-like shapes have been fabricated by hydrothermal method. The flowers-like microspheres with an average diameter of 500 nm are built of intercrossed flakes. Magnetic study of the Ni microcrystals indicates that the flowers-like microspheres sample has a little higher coercive force (276 Oe) than that of the branch-like sample (161 Oe). An important application of these hierarchical Ni microcrystals is used as high-performance microwave absorbers in 2–8 GHz range. Compared to the branch-like sample, flowers-like microspheres sample exhibits more excellent microwave absorbing performance and the reflection loss reaches a minimum of −17 dB at 13 GHz with −6 dB bandwidth covering the 6.5–14 GHz range. The further analysis of electromagnetic parameters indicates the branch-like sample has a significant eddy current effect which leads to the μ^′${\mu }^{\prime }$ value below 1.0, the μ^″${\mu }^{″}$ value below zero and the poor microwave absorption in 11.5–14 GHz range. However, flowers-like microspheres sample has relatively low eddy current effects coming from the particle shape effects, which results in relatively high permeability and more suitable impedance matching between permittivity and permeability in 2–18 GHz range.     

Synthesis and characterization of WO3 thin films by surfactant assisted spray pyrolysis for electrochromic applications

Thin films of WO₃ were prepared by surfactant assisted spray pyrolysis on F-doped SnO₂ (FTO) conductive glass by using hexadecyltrimethylammonium bromide (HTAB) and polyethylene glycol (PEG400):HTAB as growth controlling agents. The surface tension of the spraying solutions was experimentally evaluated and was correlated with the deposition processes (nucleation and growth) of very smooth and homogenous films. The effect of the surfactant, alone and associated with PEG, on the structure (XRD), morphology (AFM), surface composition (XPS), FTIR and hydrophilicity (contact angle) were investigated and their influence on the electrochromic activity was discussed. Using surfactants and PEG, the coloration efficiency, transmission modulation and cycling stability of the WO₃ thin films can be enhanced.     

Shape-controlled synthesis of manganese oxide nanoplates by a polyol-based precursor route

Manganese oxide nanoplates with different shapes have been prepared based on an ethylene glycol-mediated route. The first step consists of precipitating manganese alkoxide precursor in a polyol process from the reaction of manganese acetate with ethylene glycol. During this process, the morphologies of the prepared precursor could be tuned from disc-shaped to hexagonal nanoplates by introducing different organic additives. The second step involves the treatment of the precursor under different conditions. Crystalline Mn₂O₃ with the same morphology was readily obtained by calcination of the manganese alkoxide precursor. Furthermore, Mn₃O₄ nanoplates could be obtained by immersing the precursor into the deionized water.     

Synthesis and anomalous magnetic properties of hexagonal CoO nanoparticles

CoO nanoparticles in the 38-93 nm range have been prepared by thermal decomposition. The particles were characterized to be pyramid shape with a hexagonal close-packed structure. Their anomalous magnetic behavior includes: (i) vanishing of antiferromagnetic transition around 300 K; (ii) creation of hysteresis below a blocking temperature of 6-11 K; (iii) presence of relatively large moments and coercivities accompany with specific loop shifts at 5 K; and (iv) appearance of an additional small peak located in low field in the electron spin resonance spectrum. Further, the present results provide evidence for the existence of uncompensated surface spins. The coercivity and exchange bias decrease with increasing particle size, indicating a distinct size effect. These observations can be explained by the multisublattice model, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the total CoO particle.     

Biferroic LuCrO3: Structural characterization,magnetic and dielectric properties

Multiferroic LuCrO₃ perovskite-type structure (Pbnm) obtained via auto-ignition synthesis was characterized by a combination of X-ray diffraction (XRD) and thermogravimetric (TG) techniques, and through magnetization and permittivity measurements. Results showed that amorphous combustion powders were fully transformed to orthorhombic LuCrO₃ structure at 1200 K after the first LuCrO₄ crystallization at 700 K. The magnetic response displays thermal irreversibility between zero-field-cooling and field-cooling condition which is due to spin canted AF switching at 116 K. Accordingly, a hysteresis loop in the M(H) data confirms weak ferromagnetism in LuCrO₃. On the other hand, the permittivity measurement shows a broad peak transition typical of relaxor-type ferroelectrics transition at ∼450 K. Electrical conductivity increases as temperature increases showing an anomaly around the diffuse phase transition. The diffuse phase transition and the formation of the charge carriers are discussed in terms of a local distortion around the Lu Site.     

Structural,optical and magnetic characterizations of Mn-doped MgO nanoparticles

Structural, optical and room temperature magnetic properties of Mn-doped MgO nanoparticles with Mn fractions (5–50 at.%), were investigated. The as-prepared pure MgO, with grain size of about 15 nm, exhibits two magnetization components, one is diamagnetic and another is superparamagnetic. After removing the diamagnetic contribution, the magnetization curve exhibits superparamagnetic behavior which may be attributed to vacancy defects. As the Mn content increases, the lattice parameter decreases, the ferromagnetism appears and the emission bands were considerably blue shifted. First principle electronic structure calculations reveal the decrease of both the gap and the Curie temperature with increasing Mn concentration. The obtained results suggest that both Mn doping and oxygen vacancies play an important role in the development of room temperature ferromagnetism.     

Hydrothermal preparation of high saturation magnetization and coercivity cobalt ferrite nanocrystals without subsequent calcination

In this work, CoFe₂O₄ nanocrystals with high saturation magnetization (M_s) and high coercivity (H_c) have been fabricated via a simple hydrothermal method and without subsequent calcination. The resulting CoFe₂O₄ nanocrystals are characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, differential scanning calorimetry and vibrating sample magnetometry. The results indicate that CoFe₂O₄ nanocrystals are single crystal and the average crystallite size is increasing with the hydrothermal temperature. The electron micrographs show that the nanocrystals are well-dispersed and possess uniform size. The shape of CoFe₂O₄ nanocrystals is transformed from spherical into rod by increasing the hydrothermal temperature. The nanocrystals show relatively high M_s of 74.8 emu g⁻¹ and H_c of 2216 Oe, as compared to previous reported results. The obtained results reveal the applicability of this method for efficiently producing well crystallized and relatively high magnetic properties CoFe₂O₄ nanocrystals as compared to other methods. More importantly, it does not require further calcination processes.     

Preparation of porous SnO2 helical nanotubes and SnO2 sheets

We report a surfactant-free chemical solution route for synthesizing one-dimensional porous SnO₂ helical nanotubes templated by helical carbon nanotubes and two-dimensional SnO₂ sheets templated by graphite sheets. Transmission electron microscopy, X-ray diffraction, cyclic voltammetry, and galvanostatic discharge–charge analysis are used to characterize the SnO₂ samples. The unique nanostructure and morphology make them promising anode materials for lithium-ion batteries. Both the SnO₂ with the tubular structure and the sheet structure shows small initial irreversible capacity loss of 3.2% and 2.2%, respectively. The SnO₂ helical nanotubes show a specific discharge capacity of above 800 mAh g⁻¹ after 10 charge and discharge cycles, exceeding the theoretical capacity of 781 mAh g⁻¹ for SnO₂. The nanotubes remain a specific discharge capacity of 439 mAh g⁻¹ after 30 cycles, which is better than that of SnO₂ sheets (323 mAh g⁻¹).     

Synthesis and characterization of Bi2Fe4O9 powders

Bi₂Fe₄O₉ have been successfully prepared using ethylenediaminetetraacetic (EDTA) acid as a chelating agent and ethylene glycol as an esterification agent. Heating of a mixed solution of EDTA, ethylene glycol, and nitrates of iron and bismuth at 140 °C produced a transparent polymeric resin without any precipitation, which after pyrolysis at 250 °C was converted to a powder precursor for Bi₂Fe₄O₉. The precursors were heated at 400–800 °C in air to obtain Bi₂Fe₄O₉ powder and differential scanning calorimetry (DSC), thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) techniques were used to characterize the precursors and the derived oxide powders. XRD analysis showed that well-crystallized and single-phase Bi₂Fe₄O₉ with orthorhombic symmetry was obtained at 700 °C for 2 h and BiFeO₃ and Fe₂O₃/FeCO₃ were intermediate phases before the formation of Bi₂Fe₄O₉. Bi₂Fe₄O₉ powders show weak ferromagnetism at room temperature.     

Synthesis, characterization and magnetic properties of nanocrystalline Ni-Zn-Co ferrites

Nanocrystalline magnetic particles of Ni_0.7−xZn_0.3Co_xFe₂O₄ with x lying between 0.0 and 0.3 were synthesized by combustion method using metal nitrates, sucrose and polyvinyl alcohol (PVA). The synthesized powders where characterized by X-ray diffraction and Transmission electron microscopy (TEM). The average crystallite size determined from XRD data using Scherrer formula lie in the range of 20-30 nm. TEM micrographs show a well defined nano-crystallite state with an average particle size of around ~ 10 nm. The electron diffraction patterns confirm the spinel crystal structure of the ferrite. Magnetic properties measured at room temperature by vibrating sample magnetometer (VSM) reveal an increase in saturation magnetization with increase in cobalt concentration. Non-linear increase in saturation magnetization is related to surface effects and method of preparation.     

The synthesis and characterization of electrical and magnetic nanocomposite: PEDOT/PSS–Fe3O4

The poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate)–Fe₃O₄ (PEDOT/PSS–Fe₃O₄) nanoparticles have been prepared by using polystyrene sulfonic sodium (NaPSS) as a dispersant and dopant. The characterization of nanocomposites was investigated by transmission electron microscope, X-ray diffraction, UV spectroscopy, electrochemical study, four-probe, thermogravimetric analysis and magnetic property measurement system. XRD revealed the presence of spinel phase of Fe₃O₄ and the average size was calculated to be about 12 nm. The conductivity of nanocomposites at room temperature is excellent and it depends on the Fe₃O₄ content. The thermal stability of composites is outstanding. Higher saturation magnetization of 6.47 emu g⁻¹ (20 wt.% Fe₃O₄) was observed at 300 K.     

Low temperature synthesis of Fe3O4 micro-spheres and its microwave absorption properties

Fe₃O₄ micro-spheres were synthesized by a simple chemical method at 90 °C. It was demonstrated that the final size and morphology of Fe₃O₄ was significantly affected by the additive NaF. The as-synthesized products were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and vector network analysis. The complex permittivity and permeability of paraffin wax and Fe₃O₄ with different Fe₃O₄ volume fraction were measured to increase linearly with increasing volume fraction of Fe₃O₄. When the matching thickness is 4 mm, the calculated reflection loss reaches a maximum value of −45.2 dB at 4.67 GHz with 40% volume fraction of Fe₃O₄.