Detection of non-Gaussian fluctuations in a quantum point contact

An experimental study of current fluctuations through a tunable transmission barrier, a quantum point contact, is reported. We measure the probability distribution function of transmitted charge with precision sufficient to extract the first three cumulants. To obtain the intrinsic quantities, corresponding to voltage-biased barrier, we employ a procedure that accounts for the response of the external circuit and the amplifier. The third cumulant, obtained with a high precision, is found to agree with the prediction for the statistics of transport in the non-Poissonian regime.     

Reversibly permeable nanomembranes of polymeric microcapsules

Polymeric nanometer-thick membranes or nanomembranes possessing photocontrollable permeability are presented. Microcapsules are used as membrane model systems, while gold nanoparticle aggregates are used as active absorption centers. Upon laser light illumination the membranes change permeability reversibly because nanoparticles transiently affect the nearby polymeric network. Nanomembranes reseal to their impermeable state when the laser is switched off. This presents a novel and simple way of reversible permeability control of interest to intracellular signaling and membranes.     

Magnetoreflection of light in CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> magnetostrictive spinel

The reflection and magnetoreflection of natural light within the infrared spectral range is studied in single crystals of CoFe₂O₄ ferrimagnetic ferrite spinel. Correlation between the reflection of light and magnetoelastic characteristics of this spinel is found. It is shown that the most significant magnetic-field-induced changes in the magnetoreflection of the spinel occur near the fundamental absorption edge and within the range of the phonon spectrum.     

Synthesis of 1,4,6,10-tetraazaadamantane quaternary derivatives

Methods for the preparation of stable 1,4,6,10-tetraazaadamantane quaternary derivatives were developed based on quaternization of a tertiary nitrogen atom in tris(β-oximinoalkyl)amines or isomeric 4,6,10-trihydroxy-1,4,6,10-tetraazaadamantanes. This process constitutes a convenient approach to the introduction of a hydrophilic 1,4,6,10-tetraazaadamantane moiety into lipophilic molecules in order to increase their solubility in water. 4,6,10-Trihydroxy-1,4,6,10-tetraazaadamantane N-oxide was synthesized by the oxidation of the tertiary nitrogen atom in the tris-oxime and subsequent intramolecular cyclotrimerization of the oximino groups. Quantum chemical calculations showed that the quaternization of the annular nitrogen atom led to a considerable stabilization of tetraazaadamantane framework as compared to the open-chain form of tris-oxime.     

Reversible pH‐Dependent Properties of Multilayer Microcapsules Made of Weak Polyelectrolytes

Summary: We investigated microcapsules composed of the weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(methacrylic acid) (PMA) assembled on calcium carbonate cores. These capsules are stable in the pH range from 2.5 to 11.5, undergoing reversible swelling in the pH interval from 2.7 to 2.6. Capsule swelling occurs at a protonation degree above 90%. The pH‐dependent size variation of PAH/PMA capsules is blocked after crosslinking of the polyelectrolyte layers.

Schematic of the swelling and de‐swelling of the capsules with changing pH.     

Contribution of carriers to optical conductivity spectra of lanthanum manganites

To study the band structure and carriers in lanthanum manganites, measurements have been made of the reflectance spectra of single crystals and polycrystals in the 0.04–1.6-eV range and of the optical conductivity σ _opt calculated by the Kramers-Kronig method as functions of the concentration and species of divalent ions in the paramagnetic (PM) and ferromagnetic (FM) regions. The optical gap for single-crystal La_0.9Sr_0.1MnO₃ is ∼0.17 eV, and the polaronband energy is 0.12 eV. In the PM region, σ _opt spectra do not indicate a band-carrier contribution, and conduction is dominated by polaron hopping and activation to the mobility edge. In the FM region, the variation in the σ _opt and absorption spectra of La_0.7Sr_0.3MnO₃ epitaxial films indicate the appearance of band carriers and a red shift of the absorption edge. The two band-carrier contributions, with weak and strong dependences on photon energy, are related to conduction in the antiferromagnetic matrix and the ferromagnetic regions. Fiz. Tverd. Tela (St. Petersburg) 41, 475–482 (March 1999)     

Electromanipulation of mammalian cells: fundamentals andapplication

Electroinjection of membrane-impermeable xenomolecules into freely suspended mammalian cells (so-called electroporation) and cell-to-cell electrofusion are powerful tools for manipulation of the genom and the cytosol of cells. Both field pulse techniques are based on the temporary increase of the membrane permeability due to reversible electrical breakdown of the plasma membrane upon application of external high-intensity field pulses of very short duration. Membrane charging and permabilization caused by high-intensity field pulses are preceded and accompanied by transient electrodeformation forces, which lead to an elongation of the cells in low-conductivity media, thus affecting the membrane area of electropermabilization in response to a breakdown pulse. Transient stretching force assumes a maximum value in low-conductivities pulse media. This facilitates incorporation of membrane-impermeable xenomolecules and field-mediated hybridization as well. Therefore, high and reproducible fields of (genetically) manipulated cells can be expected provided that: 1) the duration of the high-intensity field pulses does not exceed shout 100 μs and 2) that the (pulse or fusion) media are hypo-osmolar and exhibit a relatively low conductivities. Such media are also beneficial because field-inducted apoptosis does not occur under these conditions (in contrast to highly conductive media). Indeed, electroporation and electrofusion protocols that fulfill these requirements lead: 1) to high incorporation rates of plasmids (DNA) or artificial chromesomes into living cells without deterioration and 2) to the production of hybridoma cells (by fusion of tumor-infiltrating lymphocytes with heteromyeloma cells), which secrete functional human monoclonal antibodies. Human monoclonal antibodies that bind to and induce apoptosis in autologous tumor cells are promising gents for cancer treatment, as shown by first clinical trials     

Entrapment of α‐Chymotrypsin into Hollow Polyelectrolyte Microcapsules

Stable hollow polyelectrolyte capsules were produced by the layer‐by‐layer assembling of non‐biodegradable polyelectrolytes – poly(allylamine) and poly(styrenesulfonate) on melamine formaldehyde microcores followed by the core decomposition at low pH. A proteolytic enzyme, α‐chymotrypsin, was encapsulated into these microcapsules with high yields of up to 100%. The encapsulation procedure was based on the protein adsorption onto the capsule shells and on the pH‐dependent opening and closing of capsule wall pores. The protein in the capsules retained a high activity, and thermo‐ and storage stability. The nanostructured polyelectrolyte shell protected the proteinase from a high molecular weight inhibitor. Such enzyme‐loaded capsules can be used as microreactors for biocatalysis.     

Spatially Confined Tungstate Ion Polymerization in Microcapsules

A new approach for performing the WO₄²⁻ polymerization reaction exclusively inside polyelectrolyte capsules of a micron scale size is demonstrated. This approach is based upon a pH gradient across the capsule shell (2.5 inside the capsule volume and 6.5 outside the capsule) caused by encapsulated poly(styrene sulfonate) molecules. During the first stage of the reaction, different polytungstate anions were synthesized. Crystalline WO₃ nanoparticles were formed inside the capsule as the final polymerization product.

The tungstate ion polymerization confined within a micron‐sized polyelectrolyte capsule described herein.