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A biocompatible, reliable, and particularly versatile synthesis of magnetic iron oxide nanoparticles (IONPs) is presented that uses iron(III) acetylacetonate Fe(acac)3 as an iron precursor and sodium borohydride as a reducing agent. Both the reaction temperature and the concentration of the reducing agent have considerable effects on the IONP size. These dependencies can be used to prepare IONPs ranging in size from 5 to 8 nm, as determined by transmission electron microscopy (TEM). Synthesis at room temperature or with higher sodium borohydride concentrations always resulted in smaller particle sizes. Powder X-ray diffraction patterns show the presence of an iron oxide phase with a cubic unit cell and allow for the determination of the lattice parameters and average crystallite sizes for all synthesized IONPs. Transmission Mössbauer spectroscopy shows that the as-synthesized IONPs are pure magnetite (Fe3O4) and is further used to elucidate the reaction pathway by analyzing iron intermediates formed prior to nanoparticle formation and precipitation. TEM and high-resolution TEM reveal quasi-spherical shapes and lattice fringes for most IONPs. With only minor modifications of the synthesis procedure, this versatile, one-pot synthesis is proven to be suitable for the production of bare (uncoated) IONPs, IONPs with hydrophilic poly(ethylene glycol), l-arginine, and l-glutamic acid coatings, as well as IONPs with hydrophobic coatings such as oleic acid. All coated IONPs were characterized by FT-IR spectroscopy. In addition, the bare IONPs could easily be modified post-synthesis with a suitable capping agent using ultrasonication. To verify the biocompatibility of the IONPs, in vitro cytotoxicity studies were carried out on bare IONPs with intestinal (Caco2) and liver epithelial (HepG2) cell cultures using an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Phase contrast microscopy after hematoxylin-eosin staining showed the intact morphology of the Caco2 and HepG2 cells treated with IONPs.

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Fe/SiO2 catalysts have been prepared by two different sol–gel methods, cogelation and dissolution. The cogelation and dissolution preparative methods lead to xerogels with fundamentally different pore width distributions. The nature of the iron species obtained has been examined in detail by UV–visible and Mössbauer spectroscopy, and magnetic, transmission electron microscopy, and X-ray diffraction studies. There is no evidence for the presence of any ordered iron(III) oxides in the samples but all three contain two types of iron species, specifically paramagnetic high-spin iron(III) ions isolated in silica and iron(III) containing nanoparticles with a broad width distribution centered on 1.5 nm, nanoparticles that contain antiferromagnetically coupled clusters of a few ligated bridged iron(III) ions.

#sol-gel #Mossbauer #nanoparticles

The Mössbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11 nm have been measured between 4.2 and 315 K and fitted within the formalism for stochastic fluctuations of the hyperfine Hamiltonian. In this model, the hyperfine field is assumed to relax between the six ±x, ±y, and ±z directions in space with a distribution of relaxation rates that is temperature dependent.Muon spin relaxation measurements have been carried out on the 9 nm particles between 4.2 and 295 K.

Both techniques reveal three regimes in the magnetic dynamics of these nanoparticles.

In the low-temperature regime, between 4.2 and ∼30 K, the nanoparticle magnetic moments are blocked and a spin-glass-like state is observed with nearly static hyperfine fields, as is indicated by the well resolved magnetic Mössbauer spectra and the slow exponential decay of the muon asymmetry functions. In the high-temperature regime, above ∼125 K, the nanoparticle magnetic moments and, hence, the hyperfine fields, relax rapidly and a typical thermally activated superparamagnetic behavior is observed, as is indicated by the Mössbauer doublet line shape and the muon asymmetry functions that are unquestionably characteristic of monodispersed nanoparticles.

In the intermediate regime between ∼30 and 125 K, the Mössbauer spectra are the superposition of broad sextets and doublets and the muon asymmetry functions have been fitted with a sum of two terms, one relaxing term similar to that observed at and above 125 K and one term characteristic of static local fields. Hence, in this intermediate regime, the sample is magnetically inhomogeneous and composed of nanoparticles rapidly and slowly relaxing as a result of interparticle interactions. The magnetic anisotropy constants determined from both the Mössbauer spectral and magnetic susceptibility results decrease by a factor ∼4 with increasing diameter from 4 to 22 nm and increase linearly with the percentage of iron(III) ions present at the surface of the nanoparticles.

The interparticle interaction energy is estimated to be between 89 and 212 K from the temperature dependence of the magnetic hyperfine field measured on the 9 nm nanoparticles.

#Mossbauer #nanoparticles #muon