Magnetic nanoparticles for in vivo use: a critical assessment of their composition.

Three different magnetic samples with particle sizes ranging from 10 to 30 nm were prepared by wet chemical methods. The powders were heated at 100, 150, 200, and 250 °C during 30 min under air. Ferrous and total iron contents were determined immediately after the synthesis and after the thermal treatments. All samples were characterized by X-ray diffraction, transmission and integral low-energy electron Mössbauer spectroscopy (ILEEMS) at 298 K. These samples are composed of a mixture of individual particles of maghemite and magnetite, which implies that once oxidation starts in this kind of material, it occurs throughout the entire particle volume. The existence of a maghemite/magnetite core-shell model was ruled out. A linear correlation between the average isomer shift and the magnetite content was found, allowing the estimation of the amounts of magnetite and maghemite in an unknown sample without the need of performing chemical analysis.

Low-temperature Mössbauer study of heterosite, (Fe, Mn)PO4.

The heterosite phase occurring in a pegmatitic rock sample was characterized by X-ray diffraction, by energy-dispersive X-ray spectroscopy and by Mössbauer spectroscopy. The orthorhombic unit-cell parameters, expressed in Å, were found as a=9.733 (1), b=5.837 (1) and c=4.776 (1). The composition was determined to be (Fe(0.54)Mn(0.43)Mg(0.04))PO(4). Mössbauer spectra recorded at temperatures T of 65 K and higher consist of two broadened quadrupole doublets. Their isomer shifts δ are both diagnostic for the ferric state. The dominant doublet (~60% of total area) exhibits an average quadrupole splitting ΔE(Q,av) of 1.62 mm/s at room temperature, while the weaker broader doublet has ΔE(Q,av)=0.68 mm/s. For temperatures T≤60 K the spectra are composed of a broad sextet and a central quadrupole doublet. The doublet persists down to the lowest applied temperature of 17 K. It is concluded that this doublet is due to an Fe-bearing phase other than heterosite and which gives rise to the inner doublet appearing in the spectra recorded at T≥65 K. The broad sextets, attributable to the heterosite phase, were fitted with model-independent hyperfine-field distributions. However, it was consistently experienced that using the common Lorentzian-shaped elementary sextets composing the distribution, could not adequately reproduce the observed line shapes. Instead, the calculations had to be based on the diagonalization of the complete hyperfine-interaction Hamiltonian. This is due to the unusually strong quadrupole interaction. The as-such calculated hyperfine parameters of the heterosite phase at 17 K may be summarized as follows: maximum-probability hyperfine field B(hf,m)=473 kOe, isomer shift δ(Fe)=0.54 mm/s, average quadrupole coupling constant ½e(2)qQ=1.50 mm/s, asymmetry parameter of the EFG η=0.80, and polar angles of the hyperfine field with respect to the EFGs principal axes frame Ω=40° and Ψ=90°. The temperature variation of the hyperfine field was interpreted in terms of the Bean-Rodbell (BR) model. The BR parameter, η(BR), was found to be 0.90, indicating a first-order magnetic transition at T(N)=59.7 K. The temperature variation of the isomer shift is explained by the second-order Doppler shift δ(SOD). Using the Debye model for the lattice vibrational spectrum for calculating δ(SOD), the characteristic Mössbauer temperature Θ(M) was found to be 400 K, which is unusually low for a ferric compound.

Fe/Co alloys for the catalytic chemical vapor deposition synthesis of single- and double-walled carbon nanotubes (CNTs). 1. The CNT-Fe/Co-MgO system.

Mg(0.90)Fe(x)Co(y)O (x + y = 0.1) solid solutions were synthesized by the ureic combustion route. Upon reduction at 1000 degrees C in H2-CH4 of these powders, Fe/Co alloy nanoparticles are formed, which are involved in the formation of carbon nanotubes, which are mostly single and double walled, with an average diameter close to 2.5 nm. Characterizations of the materials are performed using 57Fe Mössbauer spectroscopy and electron microscopy, and a well-established macroscopic method, based on specific-surface-area measurements, was applied to quantify the carbon quality and the nanotubes quantity. A detailed investigation of the Fe/Co alloys' formation and composition is reported. An increasing fraction of Co2+ ions hinders the dissolution of iron in the MgO lattice and favors the formation of MgFe2O4-like particles in the oxide powders. Upon reduction, these particles form alpha-Fe/Co particles with a size and composition (close to Fe(0.50)Co(0.50)) adequate for the increased production of carbon nanotubes. However, larger particles are also produced resulting in the formation of undesirable carbon species. The highest CNT quantity and carbon quality are eventually obtained upon reduction of the iron-free Mg(0.90)Co(0.10)O solid solution, in the absence of clusters of metal ions in the starting material.

Fe/Co alloys for the catalytic chemical vapor deposition synthesis of single- and double-walled carbon nanotubes (CNTs). 2. The CNT-Fe/Co-MgAl2O4 system.

A detailed 57Fe Mössbauer study of the Mg(0.8)Fe(0.2-y)Co(y)Al2O4 (y = 0, 0.05, 0.1, 0.15, 0.2) solid solutions and of the CNT-Fe/Co-MgAl2O4 nanocomposite powders prepared by reduction in H2-CH4 has allowed characterization of the different iron phases involved in the catalytic process of carbon nanotube (CNT) formation and to correlate these results with the carbon and CNT contents. The oxide precursors consist of defective spinels of general formulas (Mg(1-x-y)(2+)Fe(x-3alpha)(2+)Fe(2alpha)(3+)[symbol: see text](alpha)Co(y)(2+)Al2(3+))O4(2-) . The metallic phase in the CNT-Fe/Co-MgAl2O4 nanocomposite powders is mostly in the form of the ferromagnetic alpha-Fe/Co alloy with the desired composition. For high iron initial proportions, the additional formation of Fe3C and gamma-Fe-C is observed while for high cobalt initial proportions, the additional formation of a gamma-Fe/Co-C phase is favored. The higher yield of CNTs is observed for postreaction alpha-Fe(0.50)Co(0.50) catalytic particles, which form no carbide and have a narrow size distribution. Alloying is beneficial for this system with respect to the formation of CNTs.