How homogeneous are the trehalose, maltose, and sucrose water solutions? An insight from molecular dynamics simulations

The structural properties resulting from the reciprocal influence between water and three well-known homologous disaccharides, namely, trehalose, maltose, and sucrose, in aqueous solutions have been investigated in the 4-66 wt % concentration range by means of molecular dynamics computer simulations. Hydration numbers clearly show that trehalose binds to a larger number of water molecules than do maltose or sucrose, thus affecting the water structure to a deeper extent. Two-dimensional radial distribution functions of trehalose solutions definitely reveal that water is preferentially localized at the hydration sites found in the trehalose dihydrate crystal, this tendency being enhanced when increasing trehalose concentration. Over a rather wide concentration range (4-49 wt %), the fluctuations of the radius of gyration and of the glycosidic dihedral angles of trehalose indicate a higher flexibility with respect to maltose and sucrose. At sugar concentrations between 33 and 66 wt %, the mean sugar cluster size and the number of sugar-sugar hydrogen bonds formed within sugar clusters reveal that trehalose is able to form larger clusters than sucrose but smaller than maltose. These features suggest that trehalose-water mixtures would be more homogeneous than the two others, thus reducing both desiccation stresses and ice formation.     

Series expansion calculation of the elastic neutron diffuse scattering: Ice Ih

A theoretical systematic method for the evaluation of the neutron elastic, coherent, diffuse scattering intensity of ice Ih is presented. For each Q vector, the latter is expressed directly as a series expansion of graphs. This technique is analogous to the Ising susceptibility calculation and avoids the explicit use of individual correlation functions. Therefore the series can be easily and exactly expanded to rather high orders. For the computation of the scattering intensity, the correlation model used is based on the ice rules and the normalized probability is written according to Nagle's formulation. The resulting series is convergent and theoretical isodiffusion patterns have been drawn by extending the series up to the 8th order; the accordance with experiment is good.     

Dynamics of the rotational degrees of freedom in an undercooled crystal of cyanoadamantane

Results of molecular dynamics are presented for a simple model of cyanoadamantane crystal. Rotator cubic phase was simulated over a wide range of temperatures. In this system, glass formation is not induced by quenched diluted disorder, but it occurs similarly to conventional glasses. Simulations have shown that the system evolves from free small-step rotational diffusion to jump like motion. The results obtained with this model are used to discuss the validity of the Mode Coupling Theory (MCT) predictions.     

Bipolar Electrochemiluminescence Imaging: A Way to Investigate the Passivation of Silicon Surfaces

This work depicts the original combination of electrochemiluminescence (ECL) and bipolar electrochemistry (BPE) to map in real-time the oxidation of silicon in microchannels. We fabricated model silicon-PDMS microfluidic chips, optionally containing a restriction, and monitored the evolution of the surface reactivity using ECL. BPE was used to remotely promote ECL at the silicon surface inside microfluidic channels. The effects of the fluidic design, the applied potential and the resistance of the channel (controlled by the fluidic configuration) on the silicon polarization and oxide formation were investigated. A potential difference down to 6 V was sufficient to induce ECL, which is two orders of magnitude less than in classical BPE configurations. Increasing the resistance of the channel led to an increase in the current passing through the silicon and boosted the intensity of ECL signals. Finally, the possibility of achieving electrochemical reactions at predetermined locations on the microfluidic chip was investigated using a patterning of the silicon oxide surface by etched micrometric squares. This ECL imaging approach opens exciting perspectives for the precise understanding and implementation of electrochemical functionalization on passivating materials. In addition, it may help the development and the design of fully integrated microfluidic biochips paving the way for development of original bioanalytical applications.     

Applications of high-order harmonics

We review applications of high-order harmonic generation in different fields of physics, from spectroscopic studies of atoms and molecules, to interferometry and plasma diagnostics and nonlinear optics.Received: 11 December 2002, Published online: 24 April 2003PACS: 32.80.Fb Photoionization of atoms and ions - 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generationD. Descamps: Present address: CEA/DRECAM/SPAM, CEN Saclay, 91105 Gif-sur-Yvette, France.     

High-gain amplification in Yb:CaF2 crystals pumped by a high-brightness Yb-doped 976 nm fiber laser

We report on high single-pass gain in Yb:CaF₂ crystal longitudinally pumped with a 40 W high-brightness fiber laser source based on an ytterbium-doped ultra-large core photonic crystal rod-type fiber operating at 976 nm. A single-pass small-signal gain of 3.2 has been achieved in a 6 % Yb-doped 10-mm-long CaF₂ crystal at room temperature, outperforming any CW-diode-pumped scheme and paving the way towards very promising innovative lasers and amplifiers schemes merging the Yb-doped solid-state and fiber technologies.     

Phase characterization of the reflection on an extreme UV multilayer: comparison between attosecond metrology and standing wave measurements

We characterize the phase shift induced by reflection on a multilayer mirror in the extreme UV range (80-93?eV) using two techniques: one based on high order harmonic generation and attosecond metrology (reconstruction of attosecond beating by interference of two-photon transitions), and a second based on synchrotron radiation and measurements of standing waves (total electron yield). We find an excellent agreement between the results from the two measurements and a flat group delay shift (±40?as) over the main reflectivity peak of the mirror.     

Fast intramolecular dynamics of triphenyl phosphite investigated by <Emphasis Type="Bold">2</Emphasis>H NMR

The first analysis of rapid intramolecular motions of triphenyl phosphite by ²H NMR is presented. The fragile slowing down of the primary relaxation is followed by a solid-echo method. The occurrence of a fast reorientation of the phenyl side groups is demonstrated in the supercooled liquid state, identified as a two-fold flip on the basis of simple lineshape simulations. Coexistence of both static and motionally averaged components in “two phase” spectra indicate a broad distribution of correlation times for this relaxation. This dynamical behavior is shown to persist in the glacial phase. Received 28 May 2002 / Received in final form 1st October 2002 Published online 31 December 2002     

Influence of gas injection on phase inversion in an oil–water flow through a vertical tube

An experimental study has been made of the influence of gas injection on the phase inversion between oil and water flowing through a vertical tube. Particular attention was paid to the influence on the critical concentration of oil and water where phase inversion occurs and on the pressure drop increase over the tube during phase inversion. By using different types of gas injectors also the influence of the bubble size of the injected gas on the phase inversion was studied. It was found that gas injection does not significantly change the critical concentration, but the influence on the pressure drop is considerable. For mixture velocities larger than 1 m/s, the pressure drop over the tube increases with decreasing bubble size and at inversion can become even larger than the pressure drop during the flow of oil and water without gas injection.     

Opto-electrochemical nanosensor array for remote DNA detection

A high-density array of opto-electrochemical nanosensors is presented for remote DNA detection. It was fabricated by chemical etching of a coherent optical fibre bundle to produce a nanotip array. The surface of the etched bundle was sputter-coated with a thin ITO layer which was eventually insulated by an electrophoretic paint. The fabrication steps produced a high-density array of electrochemical nanosensors which retains the optical fibre bundle architecture and its imaging properties. A DNA probe was then immobilized on the nanosensor array surface in a polypyrrole film by electropolymerisation. After hybridisation with the complementary sequence, detection of the strepavidin-R-phycoerythrin label is performed by fluorescence imaging through the optical fibre bundle itself. Control experiments and regeneration steps have also been successfully demonstrated on this nanostructured opto-electrochemical platform.