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.     

Inducing symmetry breaking in nanostructures: anisotropic stretch-tuning photonic crystals

We use elastically induced phase transitions to break the structural symmetry of self-assembled nanostructures, producing significantly modified functional properties. Stretching ordered polymer opals in different directions transforms the fcc photonic crystal into correspondingly distorted monoclinic lattices. This breaks the conventional selection rules for scattering from the crystal planes, yielding extra multiply scattered colors when the phase-breaking stretch is in specific directions. Scattering is spectroscopically tracked in real time as the samples distort, revealing a new phase transition that appears for <121>-oriented deformations.     

Self refractive non-linearities in chalcogenide based glasses

We report third order non-linear absorption and refraction measurements at 1.20 and 1.52 μm on selected gallium-Lanthanum sulfide-based glasses (Ga:La:S) showing negligible non-linear absorption and a refractive non-linearity close to one hundred times that of SiO₂. Their potential use in telecommunication as base materials for all-optical switching practical devices is evaluated resulting in large figures of merit. The addition of a glass modifier to the Ga:La:S matrix has improved thermal and optical properties, resulting in ease of fibre drawing. The non-linear optical response of this new variant of the Ga:La:S family is studied.     

Quenching of CdSe quantum dot emission, a new approach for biosensing

The emission of CdSe quantum dots linked to the 5'-end of a DNA sequence is efficiently quenched by hybridisation with a complementary DNA strand with a gold nanoparticle attached at the 3'-end; contact of the quantum dot and gold nanoparticle occurs.