Synthesis of Iron Oxide Nanoparticles Used as MRI Contrast Agents: A Parametric Study

Colloidal iron oxides play an important role as magnetic resonance imaging (MRI) contrast agents. The superparamagnetic particles actually used are constituted by solid cores (diameter of 5-15 nm), generally coated by a thick polysaccharidic layer (hydrodynamic radii of 30-100 nm), and formulated by direct coprecipitation of iron salts in the presence of polymeric material. To better control the synthesis, we attempted to formulate new stable uncoated superparamagnetic nanoparticles. Colloids were generated by coprecipitation of an aqueous solution of iron salts and tetramethylammonium hydroxide (TMAOH) solution. The influence of parameters such as media composition, iron media, injection fluxes, Fe and TMAOH concentrations, temperature, and oxygen on size, magnetic and magnetic resonance relaxometric properties, and colloidal stability of particles were evaluated. We have determined the relative importance of these parameters as well as the optimal conditions for obtaining uncoated stable particles with an average size of 5 nm and interesting relaxivities. The interpretation of the observed limits takes into account diffusibilities of reactants and product, feeding rates of reactants, and surface properties of nanoparticles. A model of synthesis, related to spontaneous emulsification of suspensions, is proposed. Copyright 1999 Academic Press.     

Penetration depth of tin in float glass

Experimental results on float glass by recent analytical techniques — ESCA, photon emission induced by ion bombardment and ellipsometry — support the conclusion that tin from a metal bath penetrates with relatively high concentration only in the first few nanometers from the surface.     

Acid catalysed methanolysis of 2,5-diazabicyclo[2.2.2]octane-3,6-diones: scope and limitations

The selective methanolysis of 2,5-diazabicyclo[2.2.2]octane-3,6-dione systems is a key step in a synthetic procedure leading to 4-amino-6-oxo-2-piperidinecarboxylate systems. This reaction seems to be primarily governed by steric hindrance caused by the substituents at the 1- and 4-bridgehead positions of the dione. In absence of bulky substituents the methanolysis is directed by the secondary or tertiary nature of the two lactam moieties.     

Oxygen reduction in an acid medium: electrocatalysis by CoNPc(1,2) impregnated on a carbon black support; effect of loading and heat treatment

O₂ reduction in an acid medium has been investigated on a transition metal macrocycle, CoNPc(1,2), impregnated on a carbon black support with a high dibutylphthalate adsorption value, using a rotating disk electrode and voltammetry techniques described previously, combined with X-ray photoelectron spectroscopy measurements. Optimal activity was found for a bilayer coverage (n = 2) at 17%–18% w/w loading. Heat treatment seems to be beneficial for n 3: it increases the overall number N of exchanged electrons and improves the electrode wetting. For the most active samples, mixed Co(II)/Co(III) valencies were displayed.