Systematic control of size,shape, structure,and magnetic properties of uniform magnetite and maghemite particles

As an application of the gel-sol method especially developed for the synthesis of general monodisperse particles in large quantities, uniform hematite (alpha-Fe2O3), magnetite (Fe3O4), and maghemite (gamma-Fe2O3) particles, precisely controlled in size, aspect ratio, and internal structure, have been prepared. For the synthesis of uniform ellipsoidal single-crystal particles of alpha-Fe2O3, a highly condensed suspension of fine beta-FeOOH particles doped with a prescribed amount of PO4(3-) ion in their interiors was aged at 140 degrees C for 24 h with seed particles of alpha-Fe2O3 in an acidic medium containing optimum concentrations of HCl and NaNO3. Systematic control of the aspect ratio and mean size was achieved by regulating the concentration of PO4(3-) ion incorporated into the beta-FeOOH particles and the number of seeds added. The resulting hematite particles were converted into magnetite by reduction in a H2 stream at 330 degrees C for 6 h; the magnetite was then oxidized to maghemite in an air stream at 240 degrees C for 2 h. Magnetite and maghemite thus prepared retained the original shape of the hematite. On the other hand, polycrystalline hematite particles of different sizes and aspect ratios were also prepared by aging a condensed Fe(OH)3 gel in the presence of different concentrations of SO4(2-) ion and seeds. The polycrystalline hematite particles were similarly converted into magnetite and then maghemite. The magnetic properties of these magnetite and maghemite particles were analyzed as a function of their mean particle volume, aspect ratio, and internal structure.     

Hamaker constants of iron oxide nanoparticles

The Hamaker constants for iron oxide nanoparticles in various media have been calculated using Lifshitz theory. Expressions for the dielectric responses of three iron oxide phases (magnetite, maghemite, and hematite) were derived from recently published optical data. The nonretarded Hamaker constants for the iron oxide nanoparticles interacting across water, A(1w1) = 33 - 39 zJ, correlate relatively well with previous reports, whereas the calculated values in nonpolar solvents (hexane and toluene), A(131) = 9 - 29 zJ, are much lower than the previous estimates, particularly for magnetite. The magnitude of van der Waals interactions varies significantly between the studied phases (magnetite < maghemite < hematite), which highlights the importance of a thorough characterization of the particles. The contribution of magnetic dispersion interactions for particle sizes in the superparamagnetic regime was found to be negligible. Previous conjectures related to colloidal stability and self-assembly have been revisited on the basis of the new Lifshitz values of the Hamaker constants.     

Adsorption of trimethyl phosphate on maghemite, hematite, and goethite nanoparticles

Adsorption of trimethyl phosphate (TMP) on well-characterized hematite, maghemite and goethite nanoparticles was studied by in situ DRIFT spectroscopy as a model system for adsorption of organophosphorous (OP) compounds on iron minerals. The iron minerals were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), specific surface area, and pore size distribution. The minerals were found to consist of stoichimetrically and morphologically well-defined maghemite, hematite, and goethite nanoparticles. Analysis of in situ diffuse reflectance Fourier transform (DRIFT) spectroscopy shows that TMP bonds mainly to Lewis acid Fe sites through the O phosphoryl atom (-P═O-Fe) on hematite and maghemite. On goethite most TMP molecules bond to Br?nstedt acid surface OH groups and form hydrogen bonded surface complexes. The vibrational mode analysis and uptake kinetics suggest two main reasons for the observed trend of reactivity toward TMP (hematite > maghemite > goethite): (i) larger number of accessible Lewis acid adsorption sites on hematite; (ii) stronger interaction between the Lewis acid Fe sites and the phosphoryl O atom on TMP for hematite and maghemite compared to goethite with concomitant formation of surface coordinated TMP and dimethyl phosphate intermediates. As a result, on the oxides a surface oxidation pathway dominates during the initial adsorption, which results in the formation of surface methoxy and formate. In contrast, on goethite a slower hydrolysis pathway is identified, which eventually yields phosphoric acid. The observed trends of the reactivity and analysis of the corresponding surface structure and particle morphology suggest an intimate relation between the surface chemistry of exposed crystal facets on the iron minerals. These results are important to understand OP surface chemistry on iron minerals.     

In vitro and in vivo evaluation of a <Superscript>64</Superscript>Cu-labeled propylene amine oxime complex as a potential hypoxia imaging agent bearing two 3-nitrotriazole groups

The effect of coating with nine different carboxylic acids (glycolic, propionic, lactic, malic, tartaric, citric, mandelic, caproic and caprylic) on nanostructured magnetite (D ~ 10 nm) was studied by Raman and photoacoustic, magnetic and ⁵⁷Fe Mössbauer measurements. Mössbauer spectra of frozen suspensions showed dominantly magnetically split envelopes at lower temperatures, which were evaluated by hyperfine field distribution method. Mössbauer and Raman spectroscopy indicated similar variation of relative occurrence of magnetite and maghemite phases. These results are discussed on the basis of the hypothesis that different carboxylic acids can promote either the oxidation or reduction of iron oxide nanoparticles.     

Aging effects in the electrokinetics of colloidal iron oxides

We analyze in this contribution the effect of aging on the electrokinetic properties of magnetite (Fe(3)O(4)) and hematite (alpha-Fe(2)O(3)). In both cases, high-purity commercial samples and monodisperse synthetic particles were studied. Commercial magnetite showed a rather erratic dependence of its electrophoretic mobility u(e) with the concentration of NaCl. Furthermore, sufficient concentrations of the latter were able to change the sign of the mobility. When KNO(3) solutions were used, although no such change was observed, no clear effect of [KNO(3)] on the mobility was found, and, in addition, an intense aging effect was detected, as the mobility became increasingly positive in suspensions that were stored over 1 day. The picture was radically different with synthetic magnetite spheres, as the expected overall decrease of u(e) with either NaCl or KNO(3) concentration was measured. However, also in this case the aging effect was clearly observed: u(e) tended in this case to more negative values upon suspension storage, and a steady value of the mobility was reached only after 5 days in NaCl (and even longer in KNO(3) solutions). Because of the crystal structure similarities between magnetite and maghemite (gamma-Fe(2)O(3)), it has been shown that the final step of magnetite oxidation is maghemite. This is confirmed in the present study, as the mobility-pH trends of magnetite progressively approach those of maghemite after about 7 days of storage. Since hematite is chemically more stable than magnetite, our study focused in this case on the comparison between commercial and synthetic particles. The former showed a negative mobility at pH 5.5 under all conditions, suggesting an isoelectric point well below the value accepted for hematite (>/=7). The effect of aging on commercial samples was again very significant, as u(e) decreased in absolute value, apparently without limit as the time since preparation was longer. In contrast, synthetic hematite showed a more predictable dependence on ionic strength, and more limited aging effects, as u(e) reached equilibrium values after around 5 days in NaCl; longer times were required in KNO(3) solutions.     

Infrared- and Raman-active phonons of magnetite, maghemite, and hematite: a computer simulation and spectroscopic study

Magnetite, maghemite, and hematite have been the subject of numerous studies using vibration spectroscopy to determine their infrared- and Raman-active phonons. However, no complete and unambiguous set of experimentally observed optically active phonons has yet been reported for these iron oxides. The use of atomistic simulation methods with a transferable Buckingham potential provides new data for the phonon densities of states of magnetite and the two associated phases, hematite and maghemite.     

Maghemite nanoparticles protectively coated with poly(ethylene imine) and poly(ethylene oxide)-block-poly(glutamic acid)

Superparamagnetic iron oxide particles (SPIO) of maghemite were prepared in aqueous solution and subsequently stabilized with polymers in two layer-by-layer deposition steps. The first layer around the maghemite core is formed by poly(ethylene imine) (PEI), and the second one is formed by poly(ethylene oxide)-block-poly(glutamic acid) (PEO-PGA). The hydrodynamic diameter of the particles increases stepwise from D(h) = 25 nm (parent) via 35 nm (PEI) to 46 nm (PEI plus PEO-PGA) due to stabilization. This is accompanied by a switching of their zeta-potentials from moderately positive (+28 mV) to highly positive (+50 mV) and finally slightly negative (-3 mV). By contrast, the polydispersity indexes of the particles remain constant (ca. 0.15). M?ssbauer spectroscopy revealed that the iron oxide, which forms the core of the particles, is only present as Fe(III) in the form of superparamagnetic maghemite nanocrystals. The magnetic domains and the maghemite crystallites were found to be identical with a size of 12.0 +/- 0.5 nm. The coated maghemite nanoparticles were tested to be stable in water and in physiological salt solution for longer than 6 months. In contrast to novel methods for magnetic nanoparticle production, where organic solvents are necessary, the procedure proposed here can dispense with organic solvents. Magnetic resonance imaging (MRI) experiments on living rats indicate that the nanoparticles are useful as an MRI contrast agent.     

Solvothermally Driven Mn Doping and Clustering of Iron Oxide Nanoparticles for Heat Delivery Applications

Direct interactions between nanoparticles of Mn‐doped magnetite or maghemite (clearly differentiated by Raman spectroscopy) grouped in spherical clusters minimize the effect related to their characteristic magnetic dead layer at the surface. Hence, the clustering process jointly with the manganese doping renders these ferrite nanostructures very attractive as displaying increased saturation magnetization, offering, consequently, outstanding values of the specific absorption rate (SAR) for heat delivery. The whole picture for bio‐related applications has been considered, with issues related to magnetic manipulation, colloidal stability, and biocompatibility.     

The magnetocaloric effect and heat capacity of ferrimagnetic nanosystems: High-dispersity magnetite

The magnetocaloric effects of aqueous and ethanolic high-dispersity magnetite suspensions and the magnetite magnetic liquid were determined calorimetrically over the temperature range 15–80°C. The temperature dependence of the magnetocaloric effect of suspensions was evidence of the thermal oxidation of magnetite to maghemite. The temperature dependences of the magnetocaloric effect of the magnetic liquid passed extrema related to the second-order magnetic phase transition.     

Particle size effects on the thermal behaviour of maghemite synthesised by flame pyrolysis

Thermal treatment of pure nanoscale maghemite (γ-Fe₂O₃) powders under argon yields different products depending on particle size. Particles with a mean diameter above 15 nm transform to hematite (-Fe₂O₃). Smaller particles, which were synthesised under identical conditions but from solutions with lower Fe³⁺ concentrations, transform into magnetite (Fe₃O₄) during thermal treatment. This phenomenon can be explained by adsorbed or incorporated reducing carboxylate groups. The origin of the reducing species and possible reaction mechanisms with the maghemite particles are discussed.     

Size-tunable silicon/iron oxide hybrid nanoparticles with fluorescence, superparamagnetism, and biocompatibility

Magnetic/fluorescent composite materials have become one of the most important tools in the imaging modality in vivo using magnetic resonance imaging (MRI) monitoring and fluorescence optical imaging. We report herein on a simplified procedure to synthesize hybrid nanoparticles (HNPs) that combine silicon and magnetic iron oxides consisting of magnetite (Fe(3)O(4)) and maghemite (γ-Fe(2)O(3)). Intriguingly, our unique synthetic approach can control magnetic and optical behaviors by reducing the particle size, demonstrating that the HNPs with the mean diameter of 3.0 nm exhibit superparamagnetic behavior and green fluorescence in an aqueous solution, ambient air, and a cellular environment, whereas the HNPs with the mean diameter more than 5.0 nm indicate ferromagnetic behavior without fluorescence. Additionally, both HNPs with different diameters possess excellent magnetic responsivity for external applied magnetic field and good biocompatibility due to the low cytotoxicity. Our biocompatible HNPs with the superparamagnetism can provide an attractive approach for diagnostic imaging system in vivo.     

Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition

Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (alpha-Fe(2)O(3)) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (gamma-Fe(2)O(3))-like defects in the near surface regions, to which a vibrational mode at 690 cm(-1), active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced alpha-Fe(2)O(3)<-->gamma-Fe(2)O(3) phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates-concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the alpha-->gamma phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the E(u) band at about 460 cm(-1) and the spinel band at 690 cm(-1), instead of the A(2u)/E(u) band at about 550 cm(-1) used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.     

Gold-doped ceria–lanthana solid solution: surfactant assisted preparation,nanostructural and catalytic properties

The effect of preparation methods on the formation of gold doped ceria–lanthana solid solution (Au-CLSS) nanoparticles (NPs) was studied. The nanoparticles were prepared by surfactant assisted precipitation methods viz., co-precipitation (CP) and deposition precipitation (DP) in the presence of cetyltrimethylammonium bromide (CTAB). The as prepared nanoparticles were characterized using BET, FESEM, EDS, Raman, TEM, XRD and UV–Vis. spectroscopic techniques to investigate the preferred method for the preparation of nanoparticles for catalytic applications. The cubic fluorite phase formation of nanoparticles was confirmed by XRD analysis and the average crystallite size was calculated to be around 7–10 nm. Surface area studies revealed that the NPs formed by CTAB assisted method have higher surface area. The morphology of NPs formed by both methods was flaky. Raman studies confirmed that the samples prepared by DP method generated increased oxygen vacancies than those prepared by CP method. In the present work, catalytic oxidation efficiency of the catalysts studied with toluene vapours showed maximum efficiency for Au-CLSS at low temperatures (450 K) as compared to undoped CLSS.     

Mössbauer and X-ray study of natural magnetite sands

Black sands originating from north, west and south seaside strips of the Bay of Burgas, Black Sea, were investigated. It was found that these sands are martitized magnetites. Their composition involves mixtures of nonstoichiometric magnetite and maghemite or stoichiometric magnetite and hematite.     

Iron oxide nanopropellers prepared by a low-temperature solution approach

The alpha-Fe(2)O(3) (hematite) nanopropellers were synthesized via a low-temperature solution-based method using FeCl(2) as a precursor in the presence of urea and glycine hydrochloride. The formation of alpha-Fe(2)O(3) nanopropellers is strongly depended on the addition of glycine hydrochloride, which serves as a pH modulator and affects the oxidation rate of Fe(2+). The structural evolution of the propeller-structured hematite was found to follow dissolution and recrystallization processes. For the structural conformation, each nanopropeller presents a hexagonal central column closed by six equivalent surfaces of {(-)1100} and the six arrays of the nanopropeller structure are a result of growth along +/- [(-)1100], +/- [(-)1010], and +/-[0(-)110]. Preliminary results show that the magnetic maghemite (gamma-Fe(2)O(3)) nanopropellers could also be prepared by a reduction and reoxidation process from the alpha-Fe(2)O(3) (hematite) nanopropeller precursors.     

Kinetics of Thermal Transformations of Precipitated Magnetite and Goethite

The aim of the present study was to determine the kinetic equations for the thermal transformations of precipitated iron oxides and hydroxides, namely for the process of thermal dehydroxylation of goethite and consecutive of hematite crystal structure growth as well as for the oxidation of magnetite to maghemite and its thermal transformation into crystalline hematite. The investigations have been carried out using thermogravimetry (TG/DTG/DTA), X-ray powder diffractometry (XRD) and high temperature powder diffractometry (HT-XRD). This presentation contains the continuation of our earlier works. This revised version was published online in July 2006 with corrections to the Cover Date.     

Linear assemblies of magnetic nanoparticles as MRI contrast agents

Using a one-step procedure we have prepared magnetic fluids comprising of polyelectrolyte stabilized magnetite nanoparticles. These nanocomposites are comprised of linear, chain-like assemblies of magnetic nanoparticles, which can be aligned in parallel arrays by an external magnetic field. We have shown the potential use of these materials as contrast agents by measuring their MR response in live rats. The new magnetic fluids have demonstrated good biocompatibility and potential for in vivo MRI diagnostics.     

Microwave-assisted one-step hydrothermal synthesis of pure iron oxide nanoparticles: magnetite, maghemite and hematite

A simple, rapid, one-step synthesis way of pure iron oxide nanoparticles: magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃) was investigated. Nanoparticles were prepared by microwave synthesis, from ethanol/water solutions of chloride salts of iron (FeCl₂ and FeCl₃) in the presence of sodium hydroxide NaOH. X-ray powder diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize these nanoparticles.     

Synthesis of EDTA-modified magnetic activated carbon nanocomposite for removal of permanganate from aqueous solutions

In this study, activated carbon particles were magnetized by different amounts of maghemite in different temperatures using co-precipitation method and the resultant nanocomposite were modified with ethylenediaminetetraacetic acid (EDTA) to increase the permanganate contaminants adsorption capacity and to prevent degradation and oxidation of maghemite nanoparticles. Various properties of nanocomposite were investigated using different techniques including, vibrating sample magnetometer, scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectrometer. Different kinetics, isotherms and thermodynamic models of adsorption process were investigated. Comparing data with kinetic models showed that the adsorption process complies with the pseudo-second-order kinetic model. The study of equilibrium isotherms data at different temperatures indicated that the adsorption process is more compatible with Langmuir model. Negative values of ΔG and positive values of ΔH revealed that adsorption process is spontaneous and endothermic. Response surface methodology was used to determine optimal parameters of an adsorbent dose of 1 g L⁻¹, pH = 2 and initial permanganate concentration of 50 mg L⁻¹, according to which, the maximum capacity of permanganate adsorption obtained under optimal conditions was 93.86 mg g⁻¹.     

Synthesis and real-time magnetic manipulation of a biaxial superparamagnetic colloid

Superparamagnetic colloidal plates were synthesized from tetrabutylammonium stabilized Ca(2)Nb(3)O(10) nanosheets and oleic acid-stabilized Fe(3)O(4) nanoparticles. Modification with 3-aminopropyltrimethoxysilane produces amine-terminated Ca(2)Nb(3)O(10) with an amine concentration of 0.43 +/- 0.06 groups per Ca(2)Nb(3)O(10) unit as follows from spectroscopic quantification with trinitrobenzenesulfonic acid as a dye. Treatment of the modified sheets in THF/ethanol with 5.3 nm oleic acid-stabilized magnetite nanoparticles yields pseudo-2D assemblies that consist of 2 nm thick nanosheets decorated on both sides with a dense collection (9.3 +/- 0.5 x 10(3) particles per square micrometer per side) of magnetite particles. In noncoordinating or weakly coordinating solvents, these composite particles further aggregate into stacked aggregates with a mean edge length of 1.6 +/- 0.7 microm and a thickness of 79 +/- 30 nm. The colloidal plates were characterized by elemental analysis, X-ray powder diffraction, and infrared and UV/vis spectroscopy. SQUID measurements show that films of the aligned particles are superparamagnetic at room temperature. The magnetic hysteresis that is observed at 5 K reveals that the plates have a magnetic anisotropy with the easy axis in the plane of the plates and the hard axis perpendicular to it. Calculations show that the magnetic anisotropy is a direct consequence of the two-dimensional distribution of the magnetic nanoparticles on the sheets. Optical microscopy reveals that when suspended in ethanol or THF, the colloidal plates can be rotated in real time with a variable external magnetic field (200 Oe). Magnetic alignment of the particles in suspensions also produces asymmetric light scattering patterns and magnetic birefringence. These effects and the observed magneto-orientational properties make the biaxial colloids interesting as components in displays and as magnetic actuators.