Confinement effects and tunnelling through quantum dots

Several recent theoretical advances concerning semiconductor quantum dots are reviewed. First of all, the effect of the quantum confinement on the energy gap is revisited on the basis of GW and Bethe-Salpeter calculations, showing that the excitonic gap is practically equal to the ordinary eigenvalue gap of single-particle approximations. The second part demonstrates that it is now possible to calculate the conductance peaks for the tunnelling current through a nanostructure. Finally, we discuss in some detail the concept of a macroscopic dielectric constant for nanostructures, showing that, except for a thin surface layer, the local dielectric constant still keeps its bulk value down to pretty small nanostructures.     

Vibrational spectroscopic analysis of silicones: a Fourier transform-Raman and inelastic neutron scattering investigation

An inelastic neutron scattering spectrum of a poly(dimethylsiloxane) (PDMS) is reported, and a spectrum simulated using a monomer molecular unit as a model for comparison. FT-Raman spectra of a series of PDMS derivatives are reported and structure spectra correlations are shown to exist for the estimation of (a) PDMS average chain length, (b) ratio of the number of monofunctional units to quadrifunctional units in silicone resins, and (c) the percentage weight of PDMS in silicone emulsions.     

Separation and quantitation of the stereoisomers of ephedra alkaloids in natural health products using flow injection-electrospray ionization-high field asymmetric waveform ion mobility spectrometry-mass spectrometry

A method is described for the determination of ephedrine (E) and pseudoephedrine (PE) and their metabolites norephedrine (NE), norpseudoephedrine (NPE), methylephedrine (ME), and methylpseudoephedrine (MPE) alkaloids in natural health products by flow injection-electrospray ionization-high field asymmetric waveform ion mobility spectrometry-mass spectrometry (FI-ESI-FAIMS-MS). The determination of the six alkaloids requires the separation of diastereomic pairs of E-PE, NE-NPE, and ME-MPE. FAIMS was able to resolve/separate these isomeric pairs based on their gas-phase ion mobility differences. The FAIMS-based separation and detection approach has been tested on over-the-counter diet pills. Following the extraction of the tablets, either by pressurized fluid extraction developed in-house or with sonication, the ephedra alkaloids were quantified using a modified isotope dilution approach. Detection limits for the alkaloids ranged from 0.1 to 3 ng/mL, and a linear range of at least 2 orders of magnitude was observed for the six analytes. The throughput of the current configuration of the FI-ESI-FAIMS-MS system is 2 min/sample, which is significantly higher than conventional chromatographic approaches. The developed FI-ESI-FAIMS-MS method has been compared with a conventional LC-UV analysis, and good agreement has been found for the major alkaloids.     

Integration of electrokinetic-based multidimensional separation/concentration platform with electrospray ionization-Fourier transform ion cyclotron resonance-mass spectrometry for proteome analysis of Shewanella oneidensis

This work focuses on the development of a multidimensional electrokinetic-based separation/concentration platform coupled with electrospray ionization-Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) for achieving the high resolution and ultrasensitive analysis of complex protein/peptide mixtures. A microdialysis junction is employed as the interface for on-line combination of capillary isoelectric focusing (CIEF) with transient capillary isotachophoresis/zone electrophoresis (CITP/CZE) in an integrated platform. Besides the excellent resolving power afforded by both CIEF and CZE separations, the electrokinetic focusing/stacking effects of CIEF and CITP greatly enhance the dynamic range and detection sensitivity of MS for protein identification. The constructed multidimensional separation/concentration platform is demonstrated for the analysis of Shewanella oneidensis proteome, which has considerable implications toward the bioremediation of environmental pollutants. The electrokinetic-based platform offers the overall peak capacity comparable to those obtained using multidimensional chromatography systems, but with a much shorter run time and no need for column regeneration. Most importantly, a total of 1174 unique proteins, corresponding to 26.5% proteome coverage, are identified from the cytosolic fraction of S. oneidensis, while requiring <500 ng of proteolytic digest loaded in the CIEF capillary. The ultrasensitive capabilities of electrokinetic-based proteome approach are attributed to the concentration effect in CIEF, the electrokinetic stacking of CITP, the nanoscale peak volume in CZE, the "accurate mass tag" strategy for protein/peptide identification, and the high-sensitivity, high-resolution, and high-mass measurement accuracy of FTICR-MS.     

Self-indexing of multiview fringe systems

The algorithm proposed in this paper enables the registration of two three-dimensional surface of an object when recording with a fringe-projection technique from two different viewpoints. Based on the redundancy and a distance minimization between the views, it leads iteratively to the absolute positioning of the surfaces without fringe coding. The robustness of the algorithm is demonstrated on natural shapes reconstructed from a four-head system.     

Tuning the bipolar conductance of an alkali-doped C(60) layer sandwiched between two tunneling barriers

Resonant tunneling through a C(60) monolayer doped with single Na, K, Rb, and Cs atoms was measured between the tip of a scanning tunneling microscope and a NiAl(110) substrate. By supporting the monolayer on a thin aluminum oxide film grown on the substrate, a double barrier tunnel junction is formed, consisting of the vacuum and oxide. This geometry enables conductance through an electronic state of the alkali-C(60) complex at both positive and negative sample bias. The positions of the conductance peaks can be varied by tuning the vacuum barrier. An opposite variation is found for Na and K as compared to Rb and Cs, suggesting the influence of bonding on nanoscale transport.     

Single-walled carbon nanotubes under the influence of dynamic coordination and supramolecular chemistry

A dynamic coordinative-directed solubilization of single-walled carbon nanotubes (SWNTs) in aqueous solutions has been achieved through a combination of a Zn(II) metalloporphyrin complex and a cis-protected Pd(II) complex, which are believed to form charged acyclic and/or cyclic adducts on or around the side walls of SWNTs. The solubilization of SWNTs in aqueous solution only occurs when these acyclic and/or cyclic complexes are allowed to enter simultaneously into a self-assembly process with SWNTs under mild conditions. The aqueous solubility properties that these dynamic complexes confer upon SWNTs are believed to involve noncovalent bonding interactions between the two entities. They have been probed in solution using ultraviolet and visible absorption spectroscopy and in thin films using high-resolution transmission electron microscopy. The supramolecular electronic effects that the individual components of their acyclic and/or cyclic complexes impart upon a single semiconducting SWNT have been probed within a nanotube field-effect transistor device.     

Evaluation of toxic agent effects on lung cells by fiber evanescent wave spectroscopy

Biochemical changes in living cells are detected using a fiber probe system composed of a single chalcogenide fiber acting as both the sensor and transmission line for infrared optical signals. The signal is collected via evanescent wave absorption along the tapered sensing zone of the fiber. We spectroscopically monitored the effects of the surfactant Triton X-100, which serves as a toxic agent simulant on a transformed human lung carcinoma type II epithelial cell line (A549). We observe spectral changes between 2800-3000 cm(-1) in four absorptions bands, which are assigned to hydrocarbon vibrations of methylene and methyl groups in membrane lipids. Comparison of fiber and transmission spectra shows that the present technique allows one to locally probe the cell plasma membrane in the lipid spectral region. These optical responses are correlated with cellular metabolic activity measurements and LDH (lactate dehydrogenase) release assays that indicate a loss of cellular function and membrane integrity as would be expected in response to the membrane solubilizing Triton. The spectroscopic technique shows a significantly greater detection resolution in time and concentration.     

Nanoscale borromean rings

The molecular expression of topologically interesting structures represents a formidable challenge for synthetic chemists. The nontrivial link known as the Borromean rings has long been regarded as one of the most ambitious targets in this field. Of ancient provenance, this symbol comprises three interlocked rings in an inseparable union, but cut any one of the rings and the whole assembly unravels into three separate pieces. This Account delineates different strategies that can be applied to the formation of molecules possessing this distinctive topology, culminating with two successful syntheses of such compounds, thus cutting the Gordian knot of topological chemistry.     

Rheological characterization of interfacial reaction in reactive polymer blends

Summary An original method has been developped which allows to study the influence of chain diffusion on the reaction kinetics in reactive polymer blends. Blends with controlled multilayer morphologies were directly prepared in a rotational rheometer and time-dependent dynamic mechanical measurements allowed to follow the progress of the reaction. Model miscible copolymers with different acrylic monomers were prepared for this study by controlling the composition, molecular weight and average number of reactive groups per chain. Reduced variables for the modulus and the reaction time were introduced to compare the results obtained for different concentrations of reactive groups. In comparison to homogeneous blends, the normalized results for the multilayered structures show that the extent of the reaction depends directly on the chain length and is thus controlled by chain diffusion.