Anodic stripping voltammetry with a symmetric double-step waveform

The high sensitivity of differential pulse anodic stripping voltammetry (d.p.a.s.v.) is based on discrimination against the charging current by using short current-sampling periods near the end of the applied pulses. In this paper, the capacitive current is eliminated by using a symmetric double-step waveform superimposed on the voltage ramp. The areas under the adjacent current peaks are subtracted from each other which effectively cancels the contribution of the double-layer charging current but preserves the changes in the faradaic current. By using a mercury thin-film electrode the base line is reasonably flat down to nanomolar levels of Cd, Cu and Pb with a deposition time of 3–5 min. Compared with d.p.a.s.v., the proposed method has the advantages of a higher scan rate and simple electronic circuitry.     

FREE RADICALS FROM PHOTODECOMPOSITION OF BISPHENOL-A

Abstract— Irradiation of 2,2-bis-(4-hydroxyphenyl)-propane (bisphenol-A) with UV light causes photo-decomposition of the compound. Spin-trapping with α-phenyl-N-tert-butyl-nitrone showed that free radicals were cleaved from the molecule. No direct ESR-spectroscopic information concerning the structure of the remaining radical skeleton could be obtained. Gas chromatographic mass spectrometric analysis of the decomposition products indicated the existence of a semiquinone structure following cleavage of a methyl radical from 2,2-bis-4–(hydroxyphenyl)-propane. The capacity of 2,2-bis-(4- hydroxyphenyl)-propane to give rise to radicals might explain its photoallergenic properties.     

New 2,2′-Bipyridine Derivatives and Their Luminescence Properties with Europium(III) and Terbium(III) Ions

Twenty differently substituted 2,2′,2″,2? -[(2,2′-bipyridine-6,6′-diyl)bis(methylenenitrilo)]tetrakis(acetc acids) 75–94 were synthesized with the purpose of developing new markers to be used in bioaffinity assays based on the unique luminescence properties of Eu^III and Tb^III ions. The relative luminescence yields, excitation maxima, and emission decay constants were determined for the corresponding Eu^III and Tb^III chelates. The substituents at the bipyridine moiety had a significant effect on the luminescence properties: the best relative luminescence yields R were obtained for ligands with electron-donating substituents (e.g. Me, Ph), electron-withdrawing substituents (e.g. NO₂, COOH) had a reverse effect. However, no clear correlation between the relative luminescence yields and the substituent parameters was found.     

Influence of Chelating Groups on the Luminescence Properties of Europium(III) and Terbium(III) chelates in the 2,2′-bipyridine series

Eight different 2,2′-bipyridine derivatives, i.e. 2, 5, 8, 10, 12, 13, 15 , and 19 (Schemes 1 and 2), were prepared to study the influence of the chelating groups on the luminescence properties of their Eu^III and Tb^III chelates. According to our luminescence results, 2,2′-(methylenenitrilo)bis(acetic acid) as well as (methylenenitrilo)bis-(methylphosphonic acid) in 6- and 6′-position of 2,2′-bipyridine is a suitable group when developing luminescent markers for bioaffinity assays based on time-resolved luminescence measurement.     

Deformation of current algebras in 3+1 dimensions

It was shown in an earlier paper that there is an Abelian extension of the general linear algebra gl ₂, that contains the current algebra with anomaly in 3+1 dimensions. We construct a three-parameter family of deformations of . For certain choices of the deformation parameters, we can construct unitary representations. We also construct highest-weight nonunitary representations for all choices of the parameters.This work was supported in part by U.S. Department of Energy Contract No. DE-AC02-76ER13065.     

Photoinduced ultrafast dye-to-semiconductor electron injection from nonthermalized and thermalized donor states.

Electron injection from the transition metal complex Ru(dcbpy)(2)(NCS)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) into a titanium dioxide nanocrystalline film occurs on the femto- and picosecond time scales. Here we show that the dominating part of the electron transfer proceeds extremely rapidly from the initially populated, vibronically nonthermalized, singlet excited state, prior to electronic and nuclear relaxation of the molecule. The results are especially relevant to the understanding and design of molecular-based photovoltaic devices and artificial photosynthetic assemblies.     

Determination of the glass transition temperature of latex films: Comparison of various methods

The glass transition temperature (T_g) for two latices with different styrene/butadiene compositions was determined by the thermal SPM probe resonance frequency method. The results were compared with the T_g values obtained by differential scanning calorimetry (DSC), dynamical mechanical analysis (DMA), process rheometer (PR) and thermo-mechanical analyzer (TMA) measurements. The T_g values detected by the thermal SPM method agreed well with the T_g values obtained by DSC and calculated by the Fox–Flory equation. DMA, on the other hand, showed a significantly higher T_g value for both latices than those obtained from theoretical calculations, the thermal SPM method and DSC. The T_g obtained from the PR curve was slightly higher for the latex with a low styrene content, whereas good agreement was obtained with the thermal SPM data for the latex with a high styrene content. The glass transition temperature determined by TMA agreed fairly well with the thermal SPM data for the latex with the low styrene content, whilst the value of T_g for the second latex was much less than those obtained by the other methods. The thermal SPM method detects changes in thermal behavior (thermal diffusivity, heat capacity) during heating of the latex films rather than changes in the mechanical properties. Information about the sample history could be seen in the thermal SPM curves, which was further associated with the degree of latex film formation, especially when the roughness of the films was taken into consideration.     

Effect of temperature on the buildup of polyelectrolyte multilayers

The effect of temperature on the buildup of polyelectrolyte multilayers consisting of poly(styrenesulfonate) (PSS), poly(diallyldimethylammonium) (PDADMA), and poly(allylamine) (PAH) was studied by using a quartz crystal microbalance. The increase of temperature in the deposition process was shown to have a considerable effect on the rate of the layer-by-layer buildup. The effect of temperature on the PDADMA/PSS deposition was found to be stronger than on the PAH/PSS deposition. The increasing temperature was found to extend the exponential buildup regime in all of the studied systems. A buildup model was created to simulate the buildup and to explain the effect of temperature. The model is based on the assumption that each deposition step leads to a quasi-equilibrium between the concentration of the polymer repeating unit in solution and the composition of the layer. According to the model, the layer-by-layer buildup is inherently exponential, becoming linear whenever diffusion is not fast enough to carry the polymer within the entire thickness of the film. This buildup model is discussed jointly with the earlier published three-zone model of the polyelectrolyte multilayers. The rate of the buildup is characterized by growth exponent beta. The temperature dependence of the growth exponent is discussed in connection with the thermodynamic parameters of the deposition.     

Thermal and topographical characterization of polyester- and styrene/acrylate-based composite powders by scanning probe microscopy

The thermal properties of two conventional polyester-based toners and a chemically prepared styrene/acrylate toner with different thermal histories were studied by scanning probe microscopy (SPM) and differential scanning calorimetry (DSC). The thermal transition temperatures detected by SPM agreed with the results of the DSC measurements. The validity of SPM for detecting thermal transitions was further confirmed by studying two amorphous reference polymers with different glass transition points (T_g) and three crystalline reference polymers with different melting points (T_m). When the toner sample was heated by the SPM probe above the glass transition temperature of the toner powder (T_probe > T_g), changes occurred in the surface topography and roughness causing different levels of local sintering of the particles. A set of roughness parameters calculated from the SPM image data were used to quantify the most essential features of toner surfaces. Environmental scanning electron microscopy (ESEM) was used to study the penetration depth of heat dissipated by the SPM probe. The probe-annealing was compared with oven-annealing in order to establish the effect of thermal history on the thermal properties of the materials.     

An efficient strategy for the synthesis of 1-chloroethyl phosphates and phosphoramidates

[reaction: see text] A versatile, efficient, and simple method for the preparation of various 1-chloroethyl phosphates and phosphoramidates is described. The protected chlorophosphates or phosphoramidates are synthesized to the vinyl derivative under mild conditions, followed by conversion to the chloroethylidene phosphate or phosphoramidate by dry HCl gas, resulting in good to excellent yields. 1-Chloroethyl phosphates and phosphoramidates are excellent building blocks for the synthesis of novel ethylidene-linked phosphate prodrugs.