Scanning near-field optical microscopy measurements and simulations of regularly arranged silver nanoparticles.

Silver nanoparticles on a glass substrate are experimentally investigated by aperture scanning near-field optical microscopy (a-SNOM). To understand the experimental results, finite-element-method simulations are performed building a theoretical model of the a-SNOM geometry. We systematically vary parameters like aperture size, aluminum-coating thickness, tip cone angle, and tip-surface distance and discuss their influence on the near-field enhancement. All these investigations are performed comparatively for constant-height and constant-gap scanning modes. In the end, we establish a reliable and stable optical model for simulating a-SNOM measurements, which is capable of reproducing trends observed in experimental data.

Paramagnetic gold in a highly disordered Au-Ni-O alloy.

Magnetic materials are usually classified into a distinct category such as diamagnets, paramagnets or ferromagnets. The enormous progress in materials science allows one nowadays, however, to change the magnetic nature of an element in a material. Gold, in bulk form, is traditionally a diamagnet. But in a ferromagnetic environment, it can adopt an induced ferromagnetic moment. Moreover, the growth of gold under certain conditions may lead to a spontaneous ferromagnetic or paramagnetic response. Here, we report on paramagnetic gold in a highly disordered Au-Ni-O alloy and focus on the unusual magnetic response. Such materials are mainly considered for plasmonic applications. Thin films containing Au, Ni and NiO are fabricated by co-deposition of Ni and Au in a medium vacuum of 2 × 10-2 mbar. As a result, Au is in a fully disordered state forming in some cases isolated nanocrystallites of up to 4 nm in diameter as revealed by high resolution transmission electron microscopy. The disorder and the environment, which is rich in oxygen, lead to remarkable magnetic properties of Au: an induced ferromagnetic and a paramagnetic state. This can be proven by measuring the x-ray magnetic circular dichroism. Our experiments show a way to establish and monitor Au paramagnetism in alloys.

Local photocurrent mapping and cell performance behaviour on a nanometre scale for monolithically interconnected Cu(In,Ga)Se2 solar cells.

The local efficiency of lamellar shaped Cu(In,Ga)Se2 solar cells has been investigated using scanning near-field optical microscopy (SNOM). Topographic and photocurrent measurements have been performed simultaneously with a 100 nm tip aperture. The lamellar shaped solar cell with monolithic interconnects (P scribe) has been investigated on a nanometre scale for the first time at different regions using SNOM. It was found that, the cell region between P1 and P2 significantly contributes to the solar cells overall photocurrent generation. The photocurrent produced depends locally on the sample topography and it is concluded that it is mainly due to roughness changes of the ZnO:Al/i-ZnO top electrode. Regions lying under large grains of ZnO produce significantly less current than regions under small granules. The observed photocurrent features were allocated primarily to the ZnO:Al/i-ZnO top electrode. They were found to be independent of the wavelength of the light used (532 nm and 633 nm).

Direct evidence for significant spin-polarization of EuS in Co/EuS multilayers at room temperature.

The new era of spintronics promises the development of nanodevices, where the electron spin will be used to store information and charge currents will be replaced by spin currents. For this, ferromagnetic semiconductors at room temperature are needed. We report on significant room-temperature spin polarization of EuS in Co/EuS multilayers recorded by x-ray magnetic circular dichroism (XMCD). The films were found to contain a mixture of divalent and trivalent europium, but only Eu(++) is responsible for the ferromagnetic behavior of EuS. The magnetic XMCD signal of Eu at room temperature could unambiguously be assigned to magnetic ordering of EuS and was found to be only one order of magnitude smaller than that at 2.5 K. The room temperature magnetic moment of EuS is as large as the one of bulk ferromagnetic Ni. Our findings pave the path for fabrication of room-temperature spintronic devices using spin polarized EuS layers.

Magnetic properties of textured CoPd nanocrystalline thin films.

CoPd is an important nanomaterial for magnetic and magneto-optic storage of information. In this work, CoPd alloyed thin films are grown via radio frequency magnetron sputtering on silicon, glass and polyimide substrates in a vacuum chamber with base pressure of 5 x 10(-8) mbar. The films are nanocrystalline with grain size between 4 and 80 nm. The magnetic properties of thoroughly textured CoPd alloyed thin films are compared to random polycrystalline ones. Magnetization hysteresis loops recorded under fields up to 12 kOe via a home-made magneto-optic Kerr-effect magnetometer reveal strong tendency for perpendicular magnetic anisotropy for the textured film. This anisotropy leads to the formation of well-defined stripe or labyrinthine ferromagnetic domains with the local spins oriented perpendicular to the film plane. The domain patterns and the hysteresis loops are simulated with micromagnetic calculations. Finally, an induced magnetic moment of 0.44 microB/atom is measured for Pd via X-ray magnetic circular dichroism and it is separated into spin and orbital magnetic moment contributions.

Optical and magnetic properties of hexagonal arrays of subwavelength holes in optically thin cobalt films.

In this study, we present our experimental results on the optical, magnetic, as well as magneto-optic properties of hexagonal arrays of subwavelength holes in optically thin cobalt films. Different meshes were used with hole diameters ranging between 220 and 330 nm while the interhole distance has been kept constant at 470 nm. The hole pattern modifies completely the magnetic behavior of the cobalt films; it gives rise to an increase of the coercive field of the in-plane magnetization with increasing hole diameter and to the appearance of out-of-plane magnetization components. Magneto-optic measurements show a spectacular magneto-optic response at wavelengths where surface plasmon-polaritons are supported by the structure as deduced in optical measurements. The experiments demonstrate the ability to artificially control the magnetic and thus the magneto-optic properties in hole array structures.

Magnetic, magneto-optic and magnetotransport properties of nanocrystalline Co/Au multilayers with ultrathin Au interlayers.

A series of nanocrystalline Co/Au multilayers with ultrathin Au interlayers was grown at room temperature by electron beam evaporation on Si(111), glass and polyimide substrates. X-ray diffraction measurements reveal a face centered cubic multilayered structure with very small nanograins within 7-10 nm in diameter. Magneto-optic polar Kerr effect experiments show an enhancement of the Kerr rotation around 3 eV as the Au interlayer thickness increases. The experimental data are interpreted with the help of simulated Kerr spectra. The magnetization curves and magnetic force microscopy images indicate the existence of perpendicularly magnetized stripe-domain structures at remanence. The magnitude of the magnetoresistance ratio reaches values of 0.4%. The investigation of the interplay between magnetic and magnetotransport properties demonstrates the contribution of the domain-wall spin-dependent scattering to the magnetoresistance.

Structural, magnetic, and magneto-optical properties of nanocrystalline face centered cubic Co70Cr30/Pt multilayers with perpendicular magnetic anisotropy.

Co70Cr30 alloyed layers are combined with extremely thin Pt layers in order to produce novel face-centered-cubic multilayered films to be considered as a potential perpendicular magnetic recording medium. The films were grown on Si, glass and polyimide substrates by e-beam evaporation at a temperature slightly higher than room temperature. The multilayered structure of the films was verified by X-ray diffraction experiments. Plane-view transmission electron microscopy images have revealed the formation of very small grains in the range of 7-9 nm. Hysteresis loops as a function of temperature were recorded via the magneto-optic Kerr effect in the polar geometry configuration. The system exhibits perpendicular magnetic anisotropy, which enhances with decreasing temperature. Hysteresis loops with a squareness of 1 and a coercivity of 1.45 kOe were obtained at 10 K. Furthermore, complete magneto-optic spectra of the films are recorded, showing a strong magneto-optic enhancement in the ultraviolet region at around 4.5 eV.

Catheter-related bacteremia due to Lactobacillus rhamnosus in a single-lung transplant recipient.

There is still a major debate about the pathogenicity of Lactobacillus spp. and some reports emphasize that these microorganisms are never isolated from endovascular devices. In this report we present a case of catheter-related bacteremia due to L. rhamnosus in a patient who underwent a single-lung transplant.

[Methicillin-resistant Staphylococcus aureus (MRSA) in an intensive care unit: a one-year survey]. / Colonizzazioni e infezioni da Staphylococcus aureus meticillino-resistente (SAMR) in una terapia intensiva polivalente: analisi su un anno di osservazione.

Methicillin-resistant Staphylococcus aureus (MRSA) is frequently isolated in nosocomial outbreaks. In our study, we analysed the occurrence of colonisation and infection in an Intensive Care Unit of our hospital during a 12-month period. We also evaluated the possibility of using automated ribotyping as a molecular method in order to type the isolates. Twice a week a nasal swab and a rectal swab were performed on all patients; from ventilator-assisted patients, a sputum culture was also taken. All the MRSA isolated were identified by using commonly phenotypic procedures and on all isolates susceptibility tests were performed. An automated ribotyping using EcoRI was also done. Out of 292 patients enrolled in the study, 205 were never colonised (group N); among the other 87 who were colonised by MRSA (29.8%), 40 patients (group A) were MRSA carriers at the time of admission, while 47 (group B) were colonised in the ICU. Twenty-seven patients (11 from group A, 15 from group B and 1 from group N) developed 31 infections due to MRSA. Patients from group A exhibited, as a rule, worse clinical conditions than those from the other two groups. For the former group, MRSA infection was frequently systemic (sepsis), while in group B pneumonia was the predominant infection. The prevalence of colonisations in our study was 30%, which is a value comparable to those presented by other authors in similar cases. MRSA colonisation is a necessary condition for subsequent infections in almost all cases, with an average lag of 7 days. Susceptibility tests were non-discriminating among the isolates: all the strains were susceptible to glycopeptides; nearly all of them were resistant to erythromycin, clindamycin, ciprofloxacin and gentamicin. Automated ribotyping allowed us to distinguish 12 different ribogroups, the most frequent of which was composed of 146 isolates. In our study, this molecular method was able to define a possible endemic clone that should be better investigated by using methods with a higher discriminatory power, such as RAPD or PFGE. The method that we employed is highly reliable, easy to perform and not time-consuming. In our opinion, it could be the method of choice in the first screening of high numbers of isolates.

The gene encoding DRAP (BACE2), a glycosylated transmembrane protein of the aspartic protease family, maps to the down critical region.

We applied cDNA selection methods to a genomic clone (YAC 761B5) from chromosome 21 located in the so-called 'Down critical region' in 21q22.3. Starting from human fetal heart and brain mRNAs we obtained and sequenced several cDNA clones. One of these clones (Down region aspartic protease (DRAP), named also BACE2 according to the gene nomenclature) revealed a striking nucleotide and amino acid sequence identity with several motifs present in members of the aspartic protease family. In particular the amino acid sequences comprising the two catalytic sites found in all mammalian aspartic proteases are perfectly conserved. Interestingly, the predicted protein shows a typical membrane spanning region; this is at variance with most other known aspartic proteases, which are soluble molecules. We present preliminary evidence, on the basis of in vitro translation studies and cell transfection, that this gene encodes a glycosylated protein which localizes mainly intracellularly but to some extent also to the plasma membrane. Furthermore DRAP/BACE2 shares a high homology with a newly described beta-secretase enzyme (BACE-1) which is a transmembrane aspartic protease. The implications of this finding for Down syndrome are discussed.

Human NRD convertase: a highly conserved metalloendopeptidase expressed at specific sites during development and in adult tissues.

We report the cloning of the human homologue of the rat metalloprotease N-arginine dibasic convertase (NRD convertase). This endopeptidase is responsible for the processing, at the Arg-Lys dibasic site on the N-terminal side of the arginine residue, of propeptides and proproteins. Comparisons of the human and rat full-length cDNAs show similarity and identity of 94 and 91%, respectively. In humans NRD convertase is predominantly expressed in heart, skeletal muscle, and testis. We have also studied the expression of this gene in mouse at various developmental stages and found that the neural tissue is the almost exclusive site of expression in early development (between E 10.5 and E 16.5). To gain information about the possibility that defects in this gene are linked to inherited neuromuscular disorders, we determined the chromosomal location of the human NRD convertase gene by FISH analysis, showing that the gene resides at 1p32.2.

Cloning a new human gene from chromosome 21q22.3 encoding a glutamic acid-rich protein expressed in heart and skeletal muscle.

The identification and functional characterization of genes on chromosome 21 is a necessary step to understand the pathogenesis of the various phenotypic anomalies that affect Down syndrome patients. Using direct cDNA selection we have identified a new gene, SH3BGR, that maps to 21q22.3, proximal to HMG14, and is differentially expressed in heart and skeletal muscle. SH3BGR encodes a novel protein that is characterized by the presence of a proline-rich region containing the consensus sequence for a SH3-binding domain and by an acidic carboxyl-terminal region containing a glutamic acid-rich domain predicted to assume a coiled coil. The presence of two functional domains involved in protein-protein interactions suggests that SH3BGR could be part of a multimeric complex. Its overexpression might alter specific functions of muscular tissue and therefore take part in the pathophysiology of muscular hypotonia in Down syndrome.

Vulnerability of mitochondrial complex I in PC12 cells exposed to manganese.

The present findings provide experimental evidence for the hypothesis that an impairment of mitochondrial function may be involved in manganese neurotoxicity. Specifically, the treatment of dopaminergic neuronal-derived cell line (PC12) with MnCl2 produced a significant inhibition of some mitochondrial complexes of the respiratory chain, while in the glial-derived cell line (C6) this effect was not observed. In PC12 the decrease in complex I activity was more pronounce than in other mitochondrial complexes. However treatment of cells with ZnSO4 exerted no significant variations in enzymatic activities. A direct exposure of mitochondrial fraction to MnCl2 reduced enzymatic activities of mitochondria in both cell lines adding further support to the proposed theory that the different sensitivity of the cells to manganese may be explained by a difference in uptake or intracellular storage. These data indicate that manganese neurotoxicity could be the result of a direct effect just on complex I activity or due to a secondary effect of oxidative stress induced by an excess of this transition metal.

Retrograde cannulation of the jugular vein: erroneous positioning of the catheter in the subarachnoid space.

This report describes one complication related to retrograde positioning of a catheter in the jugular vein in a patient in a coma resulting from subarachnoid haemorrhage. The catheter was found in the cervical subarachnoid space, as confirmed by radiography with contrast medium. Attention is focused on the fact that this technique, usually performed easily and safely, may occasionally present potentially severe complications.

Interdomain loops and conformational changes of glutamate synthase as detected by limited proteolysis.

Azospirillum brasilense glutamate synthase, a complex iron-sulfur flavoprotein, was subjected to limited proteolysis using trypsin and chymotrypsin, in the absence or presence of its substrates or their analogs. Time-dependent degradation of glutamate synthase alpha and beta subunits, to yield several fragments of different stability, was observed, the alpha subunit being more sensitive than the beta to proteolytic attack. The main sites of proteolytic cleavage were determined by densitometric analysis of the electrophoretic patterns obtained under denaturing conditions and by N-terminal sequencing of the major proteolytic products. These analyses showed that most of the peptide bonds sensitive to the proteases are clustered in two regions of the alpha subunit, outside the proposed substrate and cofactor binding regions of glutamate synthase [Pelanda, R., Vanoni, M. A., Perego, M., Piubelli, L., Galizzi, A., Curti, B. & Zanetti, G. (1993) J. Biol. Chem. 268, 3099-3106]. Therefore, these protease-sensitive sites can be identified as flexible loops, exposed to solvent, connecting adjacent domains of the protein. The presence of the enzyme substrates or their analogs caused significant changes in the proteolytic patterns. NADP+ protected the C-terminal region of glutamate synthase beta subunit from tryptic cleavage, supporting the proposal that it contains the pyridine-nucleotide-binding site. Furthermore, NADP+, and to a lesser extent the glutamine analog L-methionine sulfone, which binds presumably to the N-terminal region of the alpha subunit, altered the sensitivity to proteolysis of the sites of the alpha subunit proposed to be part of links between domains of glutamate synthase. These results show that long-range conformational changes of glutamate synthase occur on binding of its substrates. The study of several NADPH-dependent diaphorase activities of glutamate synthase was also undertaken in order to test if proteolytic fragments of the enzyme retained their ability to transfer electrons from NADPH to synthetic electron acceptors. Although proteolysis yielded partial loss of all enzyme NADPH-dependent reactions, the kinetic analysis showed that the rates of reduction of iodonitrotetrazolium, ferricyanide and dichlorophenolindophenol were at least twofold faster than the rate of the physiological glutamate synthase reaction. These results indicate that enzyme reduction and intramolecular electron transfer are not rate limiting during catalysis of the physiological glutamate synthase reaction.