Sergei V. Zaitsev

InGaAs/GaAs Quantum Dot Lasers with Ultrahigh Characteristic Temperature (T 0= 385 K) Grown by Metal Organic Chemical Vapour Deposition

Low threshold current density (AlInGa)As/GaAs lasers based on InGaAs quantum dots (QDs) are grown by metal organic chemical vapour deposition (MOCVD). Quantum dots deposited at 490° C and covered with GaAs are directly revealed in the active region. On a transmission electron microscopy (TEM) image of the laser structure no large clusters or dislocations are found over a macroscopic distance. We show that the properties of QD lasers can be strongly improved if the QDs are confined by Al0.3Ga0.7As barriers and the cladding layers are grown at high temperature. Optimisation of the laser structure geometry allows extension of the range of ultrahigh temperature stability (T0=385 K) of the threshold current to 50° C.

Calcium ions as efficient cofactor of polycation-mediated gene transfer.

We investigated the effect of calcium on the transfection of non-viral DNA transfer systems. Cationic proteins such as the nuclear protein H1, the polycation polylysine and a number of commercial transfection agents exhibited high transfection rates in the presence of Ca2+. Without Ca2+ H1 and HMG1 were inactive in transfection of the human permanent endothelial cell line ECV 304 while cationic liposomes such as Lipofectin and Lipofectamine did not show any Ca2+ dependence. More detailed experiments showed that Ca2+ was replaceable by the lysosomotropic agent chloroquine. Furthermore, it was possible to separate the transfection-enhancing role of Ca2+ from the actual transfection process by adding Ca2+ to the cells after the transfection period and still to obtain a significant transgene expression. This makes it possible to distinguish between cellular uptake of H1 (or mediator)-DNA complexes and endocytotic release. We also replaced soluble Ca2+ by Ca-phosphate precipitates not containing DNA and obtained similar transfection results. This allowed us to suggest that the addition of free Ca2+ to the transfection medium resulted in nascent Ca-phosphate microprecipitates. The known fusogenic and membranolytic activity of such microprecipitates could facilitate the transport through and the release of the transfecting complexes from the endosomal/lysosomal compartment.

Acid nuclear extracts as mediators of gene transfer and expression

In an attempt to demonstrate transfection-active DNA packaging proteins in the cell nucleus, we prepared acid nuclear extracts with perchloric acid and subsequent protein fractions by stepwise acetone precipitation. The original extract and these fractions containing different compositions of nuclear proteins were used as DNA packaging agents. After the formation of complexes between these protein fractions and reporter genes, the addition of these complexes to the cells resulted in high transfection rates. Gel electrophoresis shows that the most active fractions contain histone H1 and HMG17. HMG1 exhibits a smaller activity. This result was confirmed by positive transfection experiments with commercial histone H1. Our results show that the transfection activity of acid nuclear protein fractions and histone H1 is dependent on the presence of calcium.

Preparation and characterization of nanocrystalline doped TiO2

Abstract Nanocrystalline TiO 2 powders, doped in range of 0.05-5 mol% Nb and Ta, have been prepared using a chemical method. Synthesized powders have an anatase crystal structure and a fine particle size of 10-20 nm. Energy-dispersive X-ray microanalysis shows uniform dopant distribution in the powder. Temperature dependence of resistivity for both the Nb- and Ta-doped TiO 2 sintered at 1300°C exhibits n-type conductivity in the range 100-500°C with an activation energy of about 0.2 eV. It was found by transmission electron microscope analysis that the dopants in the doped TiO 2 sintered samples are segregated on grain boundaries after repeated high temperature treatment regardless of doping ranges. Addition of Zr in small amount (about 1 wt%) sufficiently suppresses such phase segregation, improving the high temperature stability.