Cell-cell contact and membrane spreading in an ultrasound trap

An ultrasonic standing wave trap [Langmuir 19 (2003) 3635] in which the morphologies of 2-D latex–microparticle aggregates, forming a pressure node plane, were characterised has been applied here to different cell suspensions with increasing order of specificity of cross-linking molecule, i.e. polylysine with chondrocytes; wheat germ agglutinin (WGA) with erythrocytes and surface receptors on neural cells. The outcome of initial cell–cell contact, i.e. whether the cells stuck at the point of contact (collision efficiency=1) or rolled around each other (collision efficiency=0), was monitored in situ by video-microscopy. The perimeter fractal dimensions (FD) of 2-D hexagonally symmetric, closely packed aggregates of control erythrocytes and chondrocytes were 1.16 and 1.18, respectively while those for the dendrititc aggregates formed initially by erythrocytes in 0.5 μg/ml WGA and chondrocytes in 20 μg/ml polylysine were 1.49 and 1.66. The FDs for control and molecularly cross-linked cells were typical of reaction-limited aggregation (RLA) and transport diffusion-limited aggregation (DLA), respectively. The FDs of the aggregates of cross-linked cells decreased with time to give more closely packed aggregates without clear hexagonal symmetry. Suspensions of neural cells formed dendritic aggregates. Spreading of inter-cellular membrane contact area occurred over 15 min for both erythrocyte and neural cell dendritic aggregates. The potential of the technique to characterise and control the progression of cell adhesion in suspension away from solid substrata is discussed.     

Effect of Light on the Electrokinetic Behavior of TiO(2) Particles in Contact with Cr(VI) Aqueous Solutions

The electrokinetic behavior of titanium dioxide particles (Degussa P25, mainly composed of anatase) put in contact with chromium(VI) aqueous solutions is highly sensitive to light exposure under normal laboratory conditions. In the dark, adsorption of Cr(VI) gives rise to substantial decrements in the mobilities, especially in the acidic branch, and, at higher concentrations, to shifts in the isoelectric point (pH(piep)) to lower values, as expected for anionic chemisorption. A two-mode adsorption model accounts qualitatively for the results. Under light, pH(piep) shifts in the opposite direction as the Cr(VI) concentration increases. A maximum value is attained at [Cr(VI)] approximately 10(-3) mol dm(-3), pH(piep) 8.2, which coincides with values reported for hydrous chromium(III) oxides. At higher concentrations, Cr(VI) adsorption defines a shift of pH(piep) to lower values. It is concluded that light absorption by titanium dioxide promotes the reduction of Cr(VI) and the surface precipitation of the hydrous Cr(III) oxide. The values of pH(piep) for partially covered surfaces are well described by a simple model of surface ionization derived earlier. FTIR/ATR analysis of the surfaces supports this interpretation, and further suggests that one of the modes of Cr(VI) adsorption implies surface dimerization to yield adsorbed dichromate. Copyright 2000 Academic Press.     

Low-temperature activation and deactivation of high-Curie-temperature ferromagnetism in a new diluted magnetic semiconductor: Ni2+-Doped SnO2

We report the synthesis of colloidal Ni(2+)-doped SnO(2) (Ni(2+):SnO(2)) nanocrystals and their characterization by electronic absorption, magnetic circular dichroism, X-ray absorption, magnetic susceptibility, scanning electron microscopy, and X-ray diffraction measurements. The Ni(2+) dopants are found to occupy pseudooctahedral Sn(4+) cation sites of rutile SnO(2) without local charge compensation. The paramagnetic nanocrystals exhibit robust high-Curie-temperature (T(C)) ferromagnetism (M(s)(300 K) = 0.8 mu(B)/Ni(2+), T(C) > 300 K) when spin-coated into films, attributed to the formation of interfacial fusion defects. Facile reversibility of the paramagnetic-ferromagnetic phase transition is also observed. This magnetic phase transition is studied as a function of temperature, time, and atmospheric composition, from which the barrier to ferromagnetic activation (E(a)) is estimated to be 1200 cm(-1). This energy is associated with ligand mobility on the surfaces of the Ni(2+):SnO(2) nanocrystals. The phase transition is reversed under air but not under N(2), from which the microscopic identity of the activating defect is proposed to be interfacial oxygen vacancies.     

Chromatographic separation of conformers of substituted asymmetric nitrosamines.

The syn and anti conformers of N-nitrosoproline, N-nitrososarcosine and N-nitroso-2-(ethylamino)-ethanol, have been separated by liquid chromatography. These conformers result from hindered rotation about the N-N bond. Separation was achieved using adsorption, reversed-phase, and ion-exchange modes. For the nitroso-amino acids, a shift in the equilibrium conformer concentration was observed with changes in pH.     

A Robust, Environmentally Benign Catalyst for Highly Selective Hydroformylation

By a sol-gel process a rhodium complex containing a diphosphane with a large natural P-Rh-P bite angle is covalently anchored in a silica matrix (see picture). The immobilized catalyst is a very selective hydroformylation catalyst that is completely and conveniently separated from the product and can be reused in numerous cycles.