Evolution of the magnetic structure with chemical composition in spinel iron oxide nanoparticles.

Magnetic properties of iron oxide nanoparticles with spinel structure are strictly related to a complex interplay between cationic distribution and the presence of a non-collinear spin structure (spin canting). With the aim to gain better insight into the effect of the magnetic structure on magnetic properties, in this paper we investigated a family of small crystalline ferrite nanoparticles of the formula CoxNi1-xFe2O4 (0 ≤x≤ 1) having equal size (≈4.5 nm) and spherical-like shape. The field dependence of magnetization at low temperatures indicated a clear increase of magnetocrystalline anisotropy and saturation magnetization (higher than the bulk value for CoFe2O4: ∼130 A m(2) kg(-1)) with the increase of cobalt content. The magnetic structure of nanoparticles has been investigated by Mössbauer spectroscopy under an intense magnetic field (8 T) at a low temperature (10 K). The magnetic properties have been explained in terms of an evolution of the magnetic structure with the increase of cobalt content. In addition a direct correlation between cationic distribution and spin canting has been proposed, explaining the presence of a noncollinear spin structure in terms of superexchange interaction energy produced by the average cationic distribution and vacancies in the spinel structure.

Human epithelial cells exposed to functionalized multiwalled carbon nanotubes: interactions and cell surface modifications.

With the expansion of the production and applications of multiwalled carbon nanotubes (MWCNTs) in several industrial and science branches, the potential adverse effects on human health have attracted attention. Numerous studies have been conducted to evaluate how chemical functionalization may affect MWCNT effects; however, controversial data have been reported, showing either increased or reduced toxicity. In particular, the impact of carboxylation on MWCNT cytotoxicity is far from being completely understood. The aim of this work was the evaluation of the modifications induced by carboxylated-MWCNTs (MWCNTs-COOH) on cell surface and the study of cell-MWCNT-COOH interactions by means of field emission scanning electron microscope (FESEM). Human pulmonary epithelial cells (A549) were incubated with MWCNTs-COOH for different exposure times and concentrations (10 µg/mL for 1, 2, 4 h; 5, 10, 20 µg/mL for 24 h). At short incubation time, MWCNTs-COOH were easily observed associated with plasma membrane and in contact with microvilli. After 24 h exposure, FESEM analysis revealed that MWCNTs-COOH induced evident changes in the cellular surface in comparison to control cells: treated cells showed blebs, holes and a depletion of the microvilli density in association with structure modifications, such as widening and/or lengthening. In particular, an increase of cells showing holes and microvilli structure alterations was observed at 20 µg/mL concentration. FESEM analysis showed nanotube agglomerates, of different sizes, entering into the cell with two different mechanisms: inward bending of the membrane followed by nanotube sinking, and nanotube internalization directly through holes. The observed morphological microvilli modifications, induced by MWCNTs-COOH, could affect epithelial functions, such as the control of surfactant production and secretion, leading to pathological conditions, such as alveolar proteinosis. More detailed studies will be, however, necessary to examine in depth the effects induced by MWCNTs-COOH and, in particular, the timing of the MWCNT-COOH-cell interaction.

Influence of Cu nanoparticle size on the photo-electrochemical response from Cu-multiwall carbon nanotube composites.

We show that Cu metal nanoparticle-multiwall carbon nanotube (MWCNT) assemblies can act as a new hybrid photoactive layer in photo-electrochemical devices. The carbon nanotube (CNT) composites were formed by a controlled thermal deposition of copper which produced crystalline metal nanoparticles localized on the carbon tube outer walls. The photoresponse evaluated in terms of IPCE (incident photon-to-charge carrier generation efficiency) varied for different sized-Cu-MWCNT samples across all the visible and near ultraviolet photon energy range with respect to the response of bare MWCNTs. In the case of 0.2 nm Cu nominal thickness, the IPCE increased, reaching 15%, a value 2.5 times higher than that measured for bare MWCNTs. As the Cu nominal coverage thickened, the IPCE started to decrease and become totally ineffective after 1 nm deposited Cu. The IPCE increase found was interpreted as being the result of a remarkable charge transfer between the Cu metal nanoparticles and the CNTs due to the formation of a strong ionic bond at their interface. The results obtained prove that the metal nanoparticle-CNT composites have optical, electrical and structural properties that can be applied in a variety of nanoscale architectures for novel photo-electrochemical devices.

Photovoltaic response of carbon nanotube-silicon heterojunctions: effect of nanotube film thickness and number of walls.

We report on the multiwall carbon nanotube application as energy conversion material to fabricate thin film solar cells, with nanotubes acting as photogeneration sites as well as charge separators, collectors and carrier transporters. The device consists of a semitransparent thin film of nanotubes coating a n-type crystalline silicon substrate. Under illumination electron-hole (e-h) pairs, generated in the nanotubes and in the silicon substrate underneath, are split and charges are transported through the nanotubes (electrons) and the n-Si (holes). We found that a suitable thickness of the nanotube thin film, high density of Schottky junctions between nanotubes and n-Si and lowest number of nanotube walls are all fundamental parameters to improve the device incident photon to electron conversion efficiency. Multiwall carbon nanotubes have been synthesized by chemical vapour deposition in an ultra high vacuum chamber by evaporating a given amount of iron at room temperature and then exposing the substrate kept at 800 degrees C at acetylene gas. The amount of deposited iron is found to directly affect the nanotube size distribution (inner and outer diameter) and therefore the number of walls of the nanotubes.

Tuning photoresponse through size control of Cu nanoparticles deposited on multi wall carbon nanotubes.

In this paper we illustrate a simple method for the production of multiwall carbon nanotubes thin films decorated with copper metal nanoparticles. The structural information obtained from the transmission electron microscopy study performed on samples differing in the quantity of deposited Copper was linked to the opto-electronic properties evaluated with photo-electrochemical measurements. The photo-response evaluated in terms of incident photon-to-charge carrier generation efficiency varied for different sized-Cu-multiwall carbon nanotubes samples across all the visible and near-ultraviolet photon energy range with respect to the response of bare carbon tubes. The photo-response from the sample covered with of 0.5 nm Cu nominal thickness, reached 10.2%, a value 2 times higher than that measured for bare carbon tubes of 5.9%. While this value decreased to 2.8% when the Cu nominal coverage thickened up to 3 nm. The increase in the photo-response found was interpreted as being the result of a remarkable charge transfer between the Cu metal nanoparticles and the carbon atoms in the tube due to the formation of a strong ionic bond at their interface. The results obtained prove that the metal nanoparticle-carbon nanotube composites have optical, electrical and structural properties that can be applied in a variety of nanoscale architectures for novel photo-electrochemical devices.

[An example of size characterization of synthetic vitreous fibers]. / Un esempio di caratterizzazione dimensionale di fibre vetrose artificiali.

BACKGROUND: The size characterization of synthetic vitreous fibres must be carried out to include or exclude them from classification as a carcinogen and to conduct in vivo and in vitro toxicity studies. The length weighted geometric mean diameter, together with its standard error, determines the toxicity of fibres according to Italian legislation relating to the provisions for classification, packaging and labelling of dangerous substances as formulated in the European Commission Directive 97/69/EC. Up to now there is no national or European guideline, which provides a technical procedure to obtain the size characterization parameters. OBJECTIVES: The aim of this paper was to measure the fibre size of some rock wool and ceramic fibre samples in order to estimate the length weighted geometric mean diameter and its standard error, and to calculate the related toxicity parameter. We also wanted to analyse whether the measured lengths and diameters were log-normally distributed or not. METHODS: We reduced the fibre length by the two most used techniques: cutting the rock wool fibres with a blade and pressing the ceramic fibre material in a die for infrared spectroscopy. Each sample was suspended in water by ultrasonic agitation and a small amount of it was filtered on a polycarbonate membrane. The diameter and length measurements of 300 fibres were carried out by a scanning electron microscope and the experimental data were analysed to calculate the main statistical parameters. The size distributions obtained for each sample were studied using the Kolmogorov-Smirnov test for normality and Q-Q plot. RESULTS: The fibre dispersion on the polycarbonate membrane area was suitable: there was a sufficient number of fibres to be measured without an excessive fibre overlapping. The main statistical parameters were calculated for two different cutting times (t = 20 min and t = 1 h) and two different applied forces (F = 1.2 t, corresponding to a pressure of 90 MPa, and F = 1.6 t, corresponding to a pressure of 120 MPa). For each substance the two sets of measurements were acquired by two different operators. This gave us an estimation of the variability of the calculated statistical parameters in the worse case (different applied forces or cutting times, and different operators). For the ceramic fibres only, one of the two operators carried out the measurements for both the applied forces to obtain more close information about the variability related to the method itself. The normality tests and the Q-Q plots showed that some of the data were far from having a lognormal distribution due to the tails of the distributions. CONCLUSIONS: Both fibre length reduction methods were effective, but the press method was found to be much easier and quicker. The data obtained showed that it is relatively easy to estimate the main statistical parameters related to the size characterization of synthetic vitreous fibres. Nevertheless, further studies must be carried out in order to better evaluate the method reproducibility and the variability of the parameters estimated by one and more operators, and to associate an error with the cancerogenicity parameter.

[Exposure to artificial mineral fibers in public buildings]. / Esposizione a fibre minerali artificiali in edifici pubblici.

Man-made mineral fibers are widely used as asbestos substitutes in many application fields. Therefore toxicology studies and indoor pollution surveys, related to man-made mineral fibers, are becoming very important. Several surveys were made in office buildings where the main source of indoor pollution was the dispersion of man-made mineral fibers contained in the insulation materials (under-ceiling panels, partitions and material used in air-conditioning installations). Samples of the insulation materials were taken and analyzed by phase contrast optical microscopy and scanning electron microscopy equipped with energy-dispersive X-ray analysis. For some samples the fiber diameter measurements were made and the related statistical parameters were calculated. The measured airborne fiber concentrations showed non relevant man-made mineral fiber dispersion in the environment. The relative exposure limits and regulations in force are discussed.

Electromagnetic radiation from VDT units: study of the effectiveness of an active shielding device.

Measurements of extremely low frequency electromagnetic fields and low frequency magnetic fields emitted by a set of video display terminal (VDT) units are reported. The field values measured at the position normally occupied by the user are below the safety limits. This is because the field amplitudes decrease rapidly (following a 1/R3 law) with the distance from the source, as has been verified in this work. Measurements with a commercial shielding device consisting of small plastic balls filled with a water solution of rare earth elements were also performed. The only physical mechanism that could be hypothesized to produce an active suppression of the VDT field is that rare earth atoms, which probably were chosen due to their large magnetic moment, behave as oscillating magnetic dipoles capable of emitting a secondary magnetic field that, along some particular directions, has a phase that is opposite to that of the exciting field. Unfortunately, if one analyzes this mechanism quantitatively, it is easy to show that the secondary magnetic field is absolutely negligible, as was confirmed by experimental measurements performed in this study.

Measurement of segmental transit through the gut in man. A novel approach by the biomagnetic method.

The techniques commonly used to evaluate the transit of contents through the gut feature some limitations for being either inaccurate, invasive, inconvenient, or potentially dangerous for the subjects. Aim of this study was to establish a safe, noninvasive and accurate technique for the measurement of segmental oroanal transit time. We localized an orally ingested magnetic marker by means of a biomagnetic instrumentation that allows us to identify in a three-dimensional pattern the position of a biomagnetic source inside the body. The biomagnetic localizations were compared with the anatomical data obtained by magnetic resonance imaging investigations. The study was performed in 12 healthy subjects, and scans were taken every hour up to the arrival of the marker into the cecum; thereafter, scans were taken every 4 hr up to the elimination of the marker. In 99% of the isofield maps obtained from each field scan, the marker was localized within the bowel walls. The mean oroanal transit time was 56 +/- 5 hr, the mouth-to-cecum transit time was 13 +/- 1.7 hr, and the total colonic transit time was 43.5 +/- 5 hr (mean +/- SEM). Segmental colon transit did not show major differences among the regions considered, although most of the time was spent in the right colon. In fact, a good correlation was found between transit time through the right colon and oroanal and total colonic transit (r = 0.77, P < 0.02, r = 0.79, P < 0.02 respectively). In conclusion, this method might be a safe alternative to the techniques presently used in the clinical setting for the measurement of intestinal transit.

Calculation of static susceptibility maps using the reciprocity theorem.

In performing measurements of magnetic susceptibility of human tissues, it is useful to calculate the field produced by a magnetisation distribution over a relatively large measurement area. This involves costly computations that can be made easier by the use of the reciprocity theorem.

Measurement of gastrointestinal transit time by means of biomagnetic instrumentation: preliminary results.

The measurement of gastrointestinal transit time, GITT, is of particular interest in the study of gastrointestinal activity and in the diagnosis of some pathologies. In the present work we report the measurements of GITT obtained by applying the biomagnetic method.