M. Arturo López-Quintela

Synthesis of nanomaterials in microemulsions: formation mechanisms and growth control ☆

Abstract The evolution of the microemulsion technique in the last years is revised with special emphasis in the mechanisms of control of particle size, namely the control by the proper microemulsions and the control by the surfactant adsorption (capping). The kinetics of the particle formation; the possibility of the preparation of coatings, core-shell and ‘onion-like’ structures with a very precise size control; the use of microemulsions to produce fine ceramics and finally their use in the preparation of superlattices is addressed.

Delivery of a hydrophilic solute through the skin from novel microemulsion systems

Five microemulsions were prepared with a mean radius of the internal phase droplets varying from 10 to 70 nm. The microemulsions were evaluated for their ability to deliver a model hydrophilic solute (sucrose) across hairless mouse skin in vitro. Maximum sucrose fluxes, following application of the different microemulsions for 9 h, were similar and were about an order of magnitude greater than that from a 20 mM sucrose aqueous solution. The five (unloaded) formulations and three controls (water, propylene glycol and 5% oleic acid in propylene glycol) were applied for 3 h to the ventral forearm of six volunteers. Transepidermal water loss (TEWL) and relative skin blood flow (SBF) were measured immediately after removing the formulations and repeatedly over a further 3 hour period. SBF increased significantly only after application of the oleic acid/propylene glycol positive control; for all other treatments, SBF remained at the pretreatment value. Immediately after removing all the formulations, TEWL was elevated. However, these values quickly recovered to the pretreatment control except in the case of oleic acid/propylene glycol. Overall, this preliminary study indicates that microemulsion formulations can be used to improve the delivery of hydrophilic solutes while eliciting insignificant effects on human skin in vivo.

Exceptional oxidation activity with size-controlled supported gold clusters of low atomicity

The catalytic activity of gold depends on particle size, with the reactivity increasing as the particle diameter decreases. However, investigations into behaviour in the subnanometre regime (where gold exists as small clusters of a few atoms) began only recently with advances in synthesis and characterization techniques. Here we report an easy method to prepare isolated gold atoms supported on functionalized carbon nanotubes and their performance in the oxidation of thiophenol with O2. We show that single gold atoms are not active, but they aggregate under reaction conditions into gold clusters of low atomicity that exhibit a catalytic activity comparable to that of sulfhydryl oxidase enzymes. When clusters grow into larger nanoparticles, catalyst activity drops to zero. Theoretical calculations show that gold clusters are able to activate thiophenol and O2 simultaneously, and larger nanoparticles are passivated by strongly adsorbed thiolates. The combination of both reactants activation and facile product desorption makes gold clusters excellent catalysts.

Microemulsions for topical delivery of 8-methoxsalen

8-Methoxsalen (8-MOP) and related furocumarins have been extensively used for the treatment of hyperproliferative skin diseases in association with long-wavelength UVA light. In order to develop alternative formulations for the topical administration of 8-MOP, microemulsions were evaluated as delivery vehicles. Six microemulsion formulations were prepared using water, isopropyl myristate (IPM) and Tween((R)) 80: Span((R)) 80: 1,2-Octanediol (3:1:1.2 w/w). The microemulsions were characterized using conductimetric and dynamic light scattering analyses. The ability of the systems to deliver 8-MOP into and through the skin was evaluated in vitro using newborn pig-skin. The in vitro permeation data showed that the novel microemulsions increased the 8-MOP total penetration through the skin by order of 1.9-4.5, as compared with IPM. In general, the accumulation of 8-MOP into the skin was increased by a factor of 1.5-4.5 by the microemulsion systems with respect to their total amount of drug delivered across the skin. These results suggest that the studied microemulsion systems may be appropriate vehicles for the topical delivery of 8-MOP.

One Step Synthesis of the Smallest Photoluminescent and Paramagnetic PVP-Protected Gold Atomic Clusters

Gold atomic clusters of only two and three atoms were prepared by a simple electrochemical technique based on the anodic dissolution of a gold electrode in the presence of PVP, and subsequent electroreduction of the Au-PVP complexes. These clusters show stable photoluminescent and magnetic properties, which make them the smallest and most elemental gold (0) building blocks in nature (after atoms) bringing new possibilities to construct novel nano/microstructures with large potential interest in biomedicine, catalysis, and so forth.

Characterization of La0.67Ca0.33MnO3±δ particles prepared by the sol–gel route

We report in this work the characterization of La0.67Ca0.33MnO3±δparticles synthesizedvia sol–gel technology starting from an aqueous solution of the metallic nitrates and using urea as gelificant agent. The gelification is assumed to happen through the formation of polynuclear species by condensation reactions between hydroxo complexes. Gels were decomposed at 250 °C and calcined for 3 h at temperatures ranging from 300 to 1000 °C. Complete crystallization takes place at ca. 600 °C. The powders were structurally characterized by X-ray diffraction and their structural parameters were calculated using the Rietveld method. The MnIV content of the several samples was determined to be higher than the stoichiometric 33%. TEM micrographs show elongated particles of which the polar (long) axis size increases from 40 to 300 nm as the calcination temperature increases. Magnetization and magnetoresistance studies are reported showing that the particles smaller than 80 nm behave as single magnetic domains while the large ones behave as multidomains. A magnetoresistance of 12% at 1 kOe was observed for all the particles synthesized by this sol–gel method.

Synthesis of small atomic copper clusters in microemulsions.

We report evidence of the formation of small atomic copper clusters, Cu(n), by the microemulsion technique, and how their size can be controlled by adjusting the percentage of the reducing agent used. Copper clusters were characterized by UV-visible spectrophotometry and atomic force microscopy. Photoluminescent copper clusters, Cu(n), with n less, similar 13, can be obtained using very low percentages of the reducing agent (<10% of the stochiometric amount). Photoluminescent clusters disappear for larger percentages of reducing agent, giving rise to larger copper clusters (0.8-2.0 nm), showing a red-shift of their UV-visible absorption bands as they grow in size. Finally, by using near stoichiometric amounts, nanoparticles of 2.9 +/- 1.1 nm in size, displaying the characteristic plasmon band, can be obtained.

The influence of colloidal parameters on the specific power absorption of PAA-coated magnetite nanoparticles

The suitability of magnetic nanoparticles (MNPs) to act as heat nano-sources by application of an alternating magnetic field has recently been studied due to their promising applications in biomedicine. The understanding of the magnetic relaxation mechanism in biocompatible nanoparticle systems is crucial in order to optimize the magnetic properties and maximize the specific absorption rate (SAR). With this aim, the SAR of magnetic dispersions containing superparamagnetic magnetite nanoparticles bio-coated with polyacrylic acid of an average particle size of ≈10 nm has been evaluated separately by changing colloidal parameters such as the MNP concentration and the viscosity of the solvent. A remarkable decrease of the SAR values with increasing particle concentration and solvent viscosity was found. These behaviours have been discussed on the basis of the magnetic relaxation mechanisms involved.PACS: 80; 87; 87.85jf

Synthesis of yttrium aluminium garnet by the citrate gel process

We describe the synthesis of polycrystalline yttrium aluminium garnet (YAG) by the citrate gel process. The conditions for obtaining the pure phase have been determined and the YAG powders were characterized by X-ray diffraction, IR spectroscopy and transmission electron microscopy. YAG powders were obtained at significantly lower temperatures than by other conventional techniques. Moreover, the particle size of the powders prepared through this process is in the range 20–70 nm, depending on the thermal treatment. The lattice parameter of the YAG powders was also determined and we observed that the lattice parameter depends on the particle size. An explanation for this behaviour is provided.

Magnetic properties of chromium (III) oxide nanoparticles

Cr2O3 nanoparticles of controlled particle size were prepared by calcination of a precursor, Cr(OH)3, obtained by precipitation with sodium hydroxide. Samples were characterized by transmission electron microscopy and x-ray diffraction. Average particle sizes ranged from 20 to 200 nm. The magnetic properties of Cr2O3 nanoparticles show the presence of a net magnetic moment at the surface due to the large surface/volume ratio. This fact modifies the classical behaviour expected for bulk antiferromagnetic particles. Below the Neel temperature, magnetization curves as a function of the applied magnetic field show the presence of coercive forces in the low-field range.