A. I. Shames

The second international intercomparison on EPR tooth dosimetry

Eighteen international EPR laboratories participated in the second intercomparison programme. Each participant had to prepare enamel samples and evaluate the absorbed dose from molars that were irradiated in vitro in the range 0-1000 mGy. The objective of the programme was to bring together all methods which are currently applied by different laboratories for EPR dose reconstruction and to demonstrate the present state of dosimetry. An overview of the essential features of the different methods is presented. The current accuracy of EPR tooth enamel dosimetry under defined conditions of irradiation is evaluated

Dual fluorophore–nitroxide probes for analysis of vitamin C in biological liquids

A new method for quantitative analysis of vitamin C in biological and chemical liquids was proposed. The method is based on the use of dual molecule consisting of a fluorescent chromophore and a nitroxide radical. In the dual molecule, the nitroxide acts as a quencher of the fluorescence of the chromophore fragment. Reduction of the nitroxide fragment by ascorbic acid results in decay of ESR signal and enhancement of the fluorescence. By performing the series of pseudo-first-order reactions between the dual molecule and ascorbic acid and consequent plotting rate constants versus ascorbic acid concentrations the calibration curves for the vitamin C analysis were obtained. Variations of chemical structure of fluorophore and nitroxide fragments allow to regulate fluorescent properties and redox potentials of the dual molecules. The proposed fluorophore-nitroxide hybrids retain all features of the spin labels and fluorescence probes gaining new advantages for monitoring redox reactions and radical processes by two independent techniques: ESR and steady-state fluorescent spectroscopy. The method was applied to the vitamin C analysis in commercial fruit juices.

On paramagnetism in fluorinated graphite: EPR and solid state NMR study

Abstract The study of the origin of paramagnetism in fluorinated graphics by means of EPR and 19 F NMR spin–lattice relaxation measurements is presented. We show that paramagnetism in fluorinated graphite is caused by local magnetic moments of dangling bonds. Localized spins in the high fluorine concentration region behave like isolated spins. In the partially fluorinated samples, strong exchange interaction between localized moments is observed.

Do π-dimers of tetrathiafulvalene cation radicals really exist at room temperature?

The longest wave absorption band of the tetramethylthio- tetrathiafulvalene cation radical, which is usually interpreted as a π-dimer band, is shown to be the intrinsic cation radical absorption, all studied cation radicals in solution at room temperature exist as paramagnetic monomers and only tetrathiafulvelene and tetramethyltetrathiafulvalene cation radicals undergo π-dimerization at low temperatures.

Lipid binding and membrane penetration of polymyxin B derivatives studied in a biomimetic vesicle system

Understanding membrane interactions and cell-wall permeation of Gram-negative bacteria is of great importance, owing to increasing bacterial resistance to existing drugs and therapeutic treatments. Here we use biomimetic lipid vesicles to analyse membrane association and penetration by synthetic derivatives of polymyxin B (PMB), a potent naturally occurring antibacterial cyclic peptide. The PMB analogues studied were PMB nonapeptide (PMBN), in which the hydrophobic alkyl residue was cleaved, PMBN diastereomer containing D-instead of L-amino acids within the cyclic ring (dPMBN) and PMBN where the hydrophobic alkyl chain was replaced with an Ala6 repeat (Ala6-PMBN). Peptide binding measurements, colorimetric transitions induced within the vesicles, fluorescence quenching experiments and ESR spectroscopy were applied to investigate the structural parameters underlying the different membrane-permeation profiles and biological activities of the analogues. The experiments point to the role of negatively charged lipids in membrane binding and confirm the prominence of lipopolisaccharide (LPS) in promoting membrane association and penetration by the peptides. Examination of the lipid interactions of the PMB derivatives shows that the cyclic moiety of PMB is not only implicated in lipid attachment and LPS binding, but also affects penetration into the inner bilayer core. The addition of the Ala6 peptide moiety, however, does not significantly promote peptide insertion into the hydrophobic lipid environment. The data also indicate that the extent of penetration into the lipid bilayer is not related to the overall affinity of the peptides to the membrane.

Efficient solar cells are more stable: the impact of polymer molecular weight on performance of organic photovoltaics

The principle remaining challenge in the research area of organic photovoltaic (OPV) materials is to develop solar cells that combine high efficiency, stability and reproducibility. Here, we demonstrate an experimental strategy which has successfully addressed this challenge. We produced a number of samples of the highly efficient PTB7 polymer with various molecular weights (Mn ∼ 40–220k). OPV cells fabricated with this polymer demonstrated significant improvement of the cell efficiency (by ∼90% relative) and lifetime (by ∼300% relative) with the Mn increase. We attribute these effects to the lower density of recombination centers (persistent radical defects revealed by EPR spectroscopy) and better photoactive layer morphology in the samples with higher Mn. Relevance of the observed correlation between the OPV efficiency and stability is discussed.

Structure and optical properties of C60 thin films

Abstract By varying the deposition conditions of C 60 thin films and using X-ray diffraction techniques to study the results, we obtain a continuum of structure types ranging from crystalline to amorphous. By measuring the absorption spectra of these films we find that as the film crystallinity increases the fundamental absorption edge and nearby spectral features shift progressively to longer wavelengths. Using a Tauc-type approximation, we find that the optical bandgap E g = 1.3–1.6 eV for crystalline films and E g = 2.4–2.6 eV for amorphous films. Intermediate values of E g are obtained depending upon the degree of crystallinity of the film.

Proton magnetic resonance study of diamond nanoparticles decorated by transition metal ions

We report on a 1H NMR study of diamond nanoparticles decorated by copper and cobalt. Increase in the 1H relaxation rate under decoration results from the interactions of hydrogen nuclear spins of the surface hydrocarbon and hydroxyl groups with paramagnetic copper and cobalt ions. This finding reveals the appearance of paramagnetic Cu2+ or Co2+ ions on the detonation nanodiamond (DND) surface rather than as a separate phase, which is consistent with the 13C NMR data of the same samples. Our results shed light on the mechanism of ion incorporation. A topological model for relative position of paramagnetic Cu2+ or Co2+ ions and hydrogen atoms on the DND surface is suggested. An application of the studied nanomaterials in the field of biomedicine is discussed.

Structure and magnetic properties of detonation nanodiamond chemically modified by copper

We report on detailed study of detonation nanodiamonds (DNDs) whose surface has been chemically modified by copper with the aid of ion exchange in water DND suspension. High resolution transmission electron microscopy, Raman, IR, electron magnetic resonance (EMR), nuclear magnetic resonance (NMR), and superconducting quantum interference device techniques were used for the characterization of DND. Carboxyl groups, appearing on the surface of a nanodiamond particle during its synthesis and purification processes, provide an effective binding of divalent copper ions to the surface. The binding results from the ion exchange between metal cations and protons of surface carboxyl groups in water solutions. IR data evidence the presence of multiple COC groups in the dried copper-modified DND product. Both EMR and C13 NMR provide direct evidences of the appearance of isolated Cu2+ ions on the surface of the 5 nm nanodiamond particles. EMR spectra reveal well-pronounced hyperfine structure due to C63,65u nuclear...

Nanodiamond graphitization: a magnetic resonance study.

We report on the first nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) study of the high-temperature nanodiamond-to-onion transformation. (1)H, (13)C NMR and EPR spectra of the initial nanodiamond samples and those annealed at 600, 700, 800 and 1800 ° C were measured. For the samples annealed at 600 to 800 ° C, our NMR data reveal the early stages of the surface modification, as well as a progressive increase in sp(2) carbon content with increased annealing temperature. Such quantitative experimental data were recorded for the first time. These findings correlate with EPR data on the sensitivity of the dangling bond EPR line width to air content, progressing with rising annealing temperature, that evidences consequent graphitization of the external layers of the diamond core. The sample annealed at 1800 ° C shows complete conversion of nanodiamond particles into carbon onions.