E.E. van Faassen

Origin of superoxide production by endothelial nitric oxide synthase

Using fluorescence optical and electron spin resonance spectroscopy, we have investigated the production of superoxide by bovine endothelial nitric oxide synthase (NOS). In contrast to neuronal NOS, the heme moiety is identified as the exclusive source of superoxide production by endothelial NOS. Thus, calmodulin‐mediated enzyme regulation affects production of nitric oxide and superoxide simultaneously and inseparably. The balance between the nitric oxide/superoxide reaction pathways may be shifted by addition of exogenous heme‐specific agents, such as tetrahydrobiopterin. Our results have direct relevance for the pathophysiology of atherosclerosis.

Exciton transport in organic dye layers for photovoltaic applications.

Abstract The mean free path length for excitons was determined in flat organic dye films. From simulations of the photocurrent response of solar cells, consisting of n-type perylene tetracarboxy diimide and a p-type organic dye, we found an exciton diffusion length of 30±10 nm for zinc phthalocyanine (ZnPc), and 7±3 nm for zinctetra-(octylphenyl)-porphyrin (ZnTOPP). The value for ZnTOPP was independently confirmed by the luminescence quenching observed in ZnTOPP layers doped with a variable concentration of quenchers. The results show the direct correlation between the photovoltaic yield of a macroscopic device and the process of energy migration on a molecular scale.

Effects of oxygen on exciton transport in zinc phthalocyanine layers

Abstract The effect of oxygen on the photovoltaic properties of organic solar cells consisting of zinc phthalocyanine and a perylene pigment has been investigated. Under solar illumination, it was possible to raise the short-circuit current by a factor of 1.5 when increasing the partial O 2 pressure threefold from atmospheric pressure in a surrounding gas atmosphere consisting of different oxygen:nitrogen ratios with a total pressure of 1 bar. On the other hand, the exciton diffusion length in ZnPc was observed to decrease at higher oxygen pressures, from which we conclude that the range of exciton transport in the ZnPc layer is limited by the presence of ionic impurities such as O 2 − .

Explanation of the low oxygen sensitivity of thin film phthalocyanine gas sensors

The conductivity of phthalocyanine (Pc) is affected upon exposure to an atmosphere containing oxidizing molecules like oxygen, ozone, nitrogen oxides or chlorine. This phenomenon finds application in the use of thin phthalocyanine films as resistive gas sensors. Such sensors have shown that the sensitivity to oxygen is orders of magnitudes lower than that for the other oxidizing species. We explain this fact by quantifying the oxygen uptake and doping density of copper-, zinc-, manganese- and free base phthalocyanine films. The oxygen inside the film is found in molecular form as O2. Only the metal phthalocyanines acquire significant doping levels upon exposure to oxygen. The number of charge carriers is four orders of magnitude lower than the uptake of molecular oxygen in the material. It explains the observation that resistive phthalocyanine gas sensors have low sensitivity for the detection of oxygen.

The Selective Oxidation of n-Butane to Maleic Anhydride : Comparison of Bulk and Supported V-P-O Catalysts

V–P–O catalysts supported on the surface of silica and titania particles were studied and compared with bulk V–P–O. The catalytic performance was tested in the η‐butane oxidation reaction to maleic anhydride, and the structure of the equilibrated catalysts was characterised with X‐ray absorption spectroscopy (EXAFS) and (low‐temperature) ESR spectroscopy. Our results show considerable differences in catalytic performance between VPO/TiO2 on the one hand, and VPO/SiO2 and VPO/bulk on the other hand, the yield to maleic anhydride being comparable for VPO/bulk and VPO/SiO2. The differences in catalytic behaviour are attributed to differences in the local structure around vanadium (EXAFS). Furthermore, different spin exchange interactions between vanadium atoms in the three samples have been observed (ESR). The combination of characterisation methods suggests that the structure of the supported V–P–O phase is amorphous and differs considerably from that of bulk crystalline vanadylpyrophosphate. We therefore propose that the oxidation of η‐butane to maleic anhydride takes place over an amorphous surface V–P–O phase. This finding has high relevance for our understanding of the catalytic activity of bulk crystalline V–P–O catalysts as well.

Increased hydrogen peroxide concentration in human tumor cells due to a nitroxide free radical

Evidence is presented that the nitroxide free radical, TEMPO, at concentrations commonly used to prevent oxidative damage, increases the intracellular hydrogen peroxide concentration. To investigate the origin of this increased hydrogen peroxide concentration, we have incubated various human tumor cell lines with compounds interfering with the generation of active oxygen metabolites. Sodium azide, inhibitor of the respiratory chain, the iron-chelating agent desferrioxamine, superoxide dismutase and catalase had no effect on the hydrogen peroxide concentration. Metyrapone, inhibitor of the cytochrome P450 system, was demonstrated to decrease, but not completely prevent, the hydrogen peroxide production. N-ethylmaleimide, a sulphydryl-bond alkylating agent, was able to completely prevent the increased hydrogen peroxide production. We conclude that, by increasing the cellular hydrogen peroxide concentration, TEMPO exerts a pro-oxidant effect. This increase in hydrogen peroxide production seems to be mediated by the induction of oxidase activity in the cytochrome P450 system, but other cellular systems involved in electron transport may also play a role.

Time-domain simulation of ESR spectra of nitroxide spin probes

Abstract A time-domain method of simulating the ESR spectrum of a nitroxide spin probe undergoing reorientational motion is described. The method reqiures as input a time trajectory of the stochastic orientation of the nitroxide radical. This trajectory may be generated from any arbitrary prescription for the mode of molecular motion. The potential of the method is demonstrated by the simulation of the ESR spectra of steroid nitroxide spin probes in oriented lipid bilayers over a large range of motional correlation times.

Recombination of Photogenerated Charge Carriers in Nanoporous Gallium Phosphide

Using time resolved microwave conductivity experiments, the dynamics of charge carrier recombination in nanoporous GaP was studied. The photoinduced conductivity initially decays very rapidly on a microsecond timescale but slows down by eight orders of magnitude in its final stages. The experimental results can be explained by a model in which charge carrier recombination takes place at the semiconductor surface, the rate being determined by the band bending across the depletion layer. The conductivity response is characteristic for a situation where the typical length scale for the nanoporous semiconductor network (10−7 m) is comparable to the width of the depletion layer.

Structure of sputtered silicon suboxide single- and multi-layers

The microscopic structure of silicon-rich and oxygen-rich SiOx (0<x<2) layers are very different. Generally, the Random Mixing Model (RMM) is used to describe the oxygen-rich SiOx layer structure in terms of microdomains of high- and low-oxygen content, respectively. We have studied the dimensions of spatial inhomogeneities in a-SiO2/a-Si multilayer stacks obtained by sputter deposition of Si in an Ar–O2 mixture. By using stacks of very thin layers, we have fabricated spatially inhomogenous structures as a model for the RMM. All stacks have the same total thickness (256 nm) and the thickness/layer is from 128 nm down to 2 nm. The composition and spatial inhomogeneities in the stacks were investigated by ion beam analyses techniques (Rutherford backscattering spectrometry (RBS) and high resolution RBS) and electron paramagnetic resonance (EPR). Infrared spectroscopy (IR) was used to study the local atomic structure of the samples. The EPR measurements, using different values of the microwave power, revealed two types of uncharged dangling bond defects. Their density amounts to approximately 1020 cm−3. We are able to detect spatial inhomogeneities down to 2 nm. This value is a firm upper limit for the spatial extension of domains in an RMM material.

EFFECTS OF THE DEFECT STRUCTURE ON HYDROGEN TRANSPORT IN AMORPHOUS SILICON

Hydrogen evolution transients were measured for hydrogenated amorphous silicon prepared by Si implantation of crystalline silicon and subsequent hydrogen implantation. The evolution curves are found to be similar for different H concentrations but with entirely different atomic and nanoscale structures, as was evidenced by small-angle x-ray scattering and infrared absorption investigations [Phys. Rev. B 53, 4415 (1996)]. This behavior is explained by a hydrogen-diffusion controlled effusion with a limited density of sites in the amorphous material that can be occupied by hydrogen. The experimental effusion curves are modeled by using diffusion coefficients in the implanted layers that were determined by secondary-ion mass spectrometry. Diffusion through a highly disordered material of low H content is found to have an activation energy of 2.26 eV.