Novel model of negative secondary ion formation and its use to refine the electronegativity of almost fifty elements.

This study aimed to examine the recently proposed idea that the ionic contribution to atomic bonds is essential in determining the charge state of sputtered atoms. Use was made of negative secondary ion yields reported by Wilson for a large number of elements implanted in silicon and then sputter profiled by Cs bombardment. The derived normalized ion yields (or fractions) P vary by 6 orders of magnitude, but the expected exponential dependence on the electron affinity EA is evident only vaguely. Remarkably, a correlation of similar quality is observed if the data are presented as a function of the ionization potential IP. With IP being the dominant (if not sole) contributor to the electronegativity χ, one is led to assume that P depends on the sum χ + EA. About 72% of the "nonsaturated" ion yields are in accordance with a dependence of the form P ∝ exp[(χ + EA)/ε], with ε ≅ 0.2 eV, provided the appropriate value of χ is selected from the electronegativity tables of Pauling (read in eV), Mulliken or Allen. However, each of the three sources contributes only about one-third to the favorable electronegativity data. This unsatisfactory situation initiated the idea to derive the "true" electronegativity χSIMS from the measured ion yields P(χ + EA), verified for 48 elements. Significant negative deviations of χSIMS from a smooth increase with increasing atomic number are evident for elements with special outer-shell electron configurations such as (n-1)d(g-1)ns(1) or (n-1)d(10)ns(2)np(1). The results strongly support the new model of secondary ion formation and provide means for refining electronegativity data.

Novel dose metric for apparent cytotoxicity effects generated by in vitro cell exposure to silica nanoparticles.

This study aimed at identifying the dose metric applicable to studies on the viability of cells exposed to nanoparticles (NPs) in vitro. A previously reported set of data was evaluated very carefully. The extent of cell death after 24-h exposure of three cell lines to suspended silica NPs (<30 nm) was quantified using four different viability/cytotoxicity assays. Data on NP uptake in cells after 6-h exposure were also reported. Evidence is provided that, in spite of the small size of the NPs, mass transport to the cells cannot be explained solely by diffusion. Gravitational settling must have contributed significantly, presumably as the result of the formation of large agglomerates. Appropriately adjusted response data, with typically 22 combinations of mass concentration and height of the medium for each cell line, could be integrated in universal diagrams, provided the dose was quoted in terms of the areal density of NP mass delivered to the cells. Loss of viability became observable only if cells were exposed to the equivalent of 1 to 5 closely packed layers of NPs; the dose required for complete cell death ranged between 4 and about 20 layers of NPs. The results suggest that the cell-death phenomena observed in the evaluated work and in many similar studies reported in the literature constitute a matter of cell overload with nanostructured matter. This finding also implies that the toxic potential of individual silicate NPs is very low. Strategies for the design of advanced future work are outlined.

In search of the most relevant parameter for quantifying lung inflammatory response to nanoparticle exposure: particle number, surface area, or what?

BACKGROUND: Little is known about the mechanisms involved in lung inflammation caused by the inhalation or instillation of nanoparticles. Current research focuses on identifying the particle parameter that can serve as a proper dose metric. OBJECTIVES: The purpose of this study was to review published dose-response data on acute lung inflammation in rats and mice after instillation of titanium dioxide particles or six types of carbon nanoparticles. I explored four types of dose metrics: the number of particles, the joint length--that is, the product of particle number and mean size--and the surface area defined in two different ways. FINDINGS: With the exception of the particle size-based surface area, all other parameters worked quite well as dose metrics, with the particle number tending to work best. The apparent mystery of three equally useful dose metrics could be explained. Linear dose-response relationships were identified at sufficiently low doses, with no evidence of a dose threshold below which nanoparticle instillation ceased to cause inflammation. In appropriately reduced form, the results for three different sets of response parameters agreed quite well, indicating internal consistency of the data. The reduced data revealed particle-specific differences in surface toxicity of the carbon nanoparticles, by up to a factor of four, with diesel soot being at the low end. CONCLUSIONS: The analysis suggests that the physical characterization of nanoparticles and the methods to determine surface toxicity have to be improved significantly before the appropriate dose metric for lung inflammation can be identified safely. There is also a need for refinements in quantifying response to exposure.

The big ban on bituminous coal sales revisited: serious epidemics and pronounced trends feign excess mortality previously attributed to heavy black-smoke exposure.

The effect of banning bituminous coal sales on the black-smoke concentration and the mortality rates in Dublin, Ireland, has been analyzed recently. Based on the application of standard epidemiological procedures, the authors concluded that, as a result of the ban, the total nontrauma death rate was reduced strongly (-8.0% unadjusted, -5.7% adjusted). The purpose of this study was to reanalyze the original data with the aim of clarifying the three most important aspects of the study, (a) the effect of epidemics, (b) the trends in mortality rates due to advances in public health care, and (c) the correlation between mortality rates and black-smoke concentrations. Particular attention has been devoted to a detailed evaluation of the time dependence of mortality rates, stratified by season. Death rates were found to be strongly enhanced during three severe pre-ban winter-spring epidemics. The cardiovascular mortality rates exhibited a continuous decrease over the whole study period, in general accordance with trends in the rest of Ireland. These two effects can fully account for the previously identified apparent correlation between reduced mortality and the very pronounced ban-related lowering of the black-smoke concentration. The third important finding was that in nonepidemic pre-ban seasons even large changes in the concentration of black smoke had no detectable effect on mortality rates. The reanalysis suggests that epidemiological studies exploring the effect of ambient particulate matter on mortality require improved tools allowing proper adjustment for epidemics and trends. Aspects of harvesting and more recent results derived from a distributed lag model covering the effects of black smoke and temperature are also discussed.

Characterization of carbon nanoparticles in ambient aerosols by scanning electron microscopy and model calculations.

Size-selected aerosol samples were analyzed by scanning electron microscopy (SEM) to explore (1) the relative concentration of individual and aggregated carbon nanoparticles (C-NPs) and (2) the combustion behavior of C-NP agglomerates. SEM analysis of low-coverage aerosol deposits showed that most of the C-NP matter is present in the form of chain-type agglomerates. The individual C-NPs in the agglomerates are remarkably similar in diameter (40+/-5 nm) and appear to be very tightly bound to the neighboring NPs. Comparison with literature data suggests that the agglomerates originated from diesel exhaust. After gently removing the water-soluble compounds from relatively thick layers of aerosol matter, the residues were exposed to increasing temperatures, for 1 hr at each step, followed by SEM analysis of the same sample area. C-NP agglomerates were found to disappear rapidly at temperatures exceeding approximately 470 degrees C. This observation constitutes the first direct visualization of the combustion of what appears to be the most important fraction of elemental carbon in ambient aerosol matter. The experimental studies were complemented by simple model calculations that aimed at assessing the size-dependent mass fraction of individual C-NPs in ambient aerosol matter. The results suggest that the mass fraction decreases from 20+/-10% at particle diameters of approximately 30-40 nm to less than 0.1% above 300 nm.

Excessive delivery of nanostructured matter to submersed cells caused by rapid gravitational settling.

With very few exceptions, previous nanotoxicity studies implicitly involved the assumption that the techniques developed for risk assessment of hazardous chemical substances can be applied in unchanged form to explore cell response in NP laden media. This misleading approach has the consequence that the actual dose of exposure is ill defined or, more often, completely unknown. Here the effect of gravitational settling on the dose of exposure was explored for commercially available engineered nanostructured matter (nanopowder). Micrometer sized aggregates abundantly present in all nanopowders were fractured as much as possible by probe-type sonication in water or cell culture media. The morphology of cracked aggregates was studied by scanning electron microscopy. Size distributions were determined by dynamic light scattering (DLS). Possible pitfalls encountered in using DLS were documented. Absorbance measurements and optical microscopy served to monitor the rate of gravitational settling on time sales ranging from minutes up to several days. The sonicated particles settled rapidly in all liquid media. At the well bottom, they exhibited intense lateral (two-dimensional) Brownian-like motion, which allowed them to travel large distances. Taken together, the probability for particle-cell contact may be enhanced by a factor of more than 1000 compared to the commonly advocated picture. The very high levels of exposure can give rise to overload effects which are often misinterpreted as evidence of cytotoxicity. To identify the true toxic potential of NPs, future studies must account for these phenomena. It is also argued that stable dispersions of NPs are not required in nanotoxicity studies.

Novel approach to identifying supersaturated metastable ambient aerosol particles.

Atomic force microscopy (AFM) was used to determine the shape of fine and ultrafine ambient aerosol particles with sizes between 25 and 700 nm after soft landing on a solid substrate. The particles were collected in summer during daytime at a relative humidity around 50%. To avoid kinetically induced deformation, as previously observed using high-velocity sampling in impactors, the particles were collected on pore filters at very low face velocities (on the order of 10 cm/s). The shape of the collected particles was quantified in terms of their height and apparent diameter. The amount of broadening introduced by the pyramidal shape of the nonideally sharp AFM tips was calibrated using Latex reference spheres with a range of diameters. The height-to-diameter ratios, H/D, of the collected aerosol particles could be extracted from the measured data. Specified in terms of volume-equivalent (dry) diameters, Dv, the size selected frequency distributions of the H/Dv-ratios were found to be bimodal. A small mode centered at H/Dv = 1.0 +/- 0.1 is attributed to nonhygroscopic particles that retained their shape after deposition on the substrate. The large mode, with a peak at H/Dv = 0.65 +/- 0.05, reflects soft particles which were strongly deformed due to vertical collapse after deposition. The pronounced deformation suggests that these particles had previously experienced deliquescence and, when collected at a comparatively low humidity, were in a metastable, supersaturated aqueous state. After landing and indoor sample storage the water evaporated, resulting in minimum H/Dv-ratios as low as 0.45. The dried metastable fraction amounted to 81 +/- 12% in the size range 150 < Dv < 700 nm, and 79 +/- 10% for 50 < or = Dv < or = 150 nm, but only 26 +/- 10% for Dv < 50 nm. Comparison with recently reported data suggests that the observed metastable fraction is the same as the hygroscopic fraction identified by other means. The interpretation is further substantiated by a comparison of the size distributions of collected and airborne particles.

Effect of water treatment on analyte and matrix ion yields in matrix-assisted time-of-flight secondary ion mass spectrometry: the case of insulin in and on hydroxycinnamic acid.

A systematic study was performed to identify the origin of surprisingly high analyte-to-matrix yield ratios recently observed in time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis of oligo- and polypeptides mixed in matrices of alpha-cyano-4-hydroxycinnamic acid (4HCCA). Several sets of samples of porcine insulin in 4HCCA (1:3100 molar) were prepared from liquid solutions by a nebuliser technique, with more than one order of magnitude variation in sprayed material (substrate silicon). Following different periods of storage in air and/or vacuum as well as exposure to high-purity water, TOF-SIMS analysis was performed under oblique impact of 22 keV SF5+. Treatment with water involved either deposition of a droplet covering the whole sample for times between 1 and 20 min or spraying with water in droplet equivalent quantities. The analyte and matrix molecules were detected as protonated molecules (insulin also in doubly protonated form). Even the as-prepared samples usually showed insulin-to-4HCCA yield ratios exceeding the molar ratio of the mixed material. Upon ageing in vacuum the matrix ion yields remained constant but the analyte yields decreased, partly due to break-up of intrachain disulfide bonds. Water treatment resulted in a pronounced decrease in the 4HCCA yield, typically by a factor of five, in parallel with an increase of the insulin yield, by up to a factor of four. Evidence is provided that these changes occur concurrently with a partial dissolution of 4HCCA at the sample surface. The enhanced insulin yield was not correlated with the Na+ yield. The typically 20-fold increase in the insulin-to-4HCCA yield ratio, generated by water exposure of the samples, provides the explanation for the high yield ratios observed previously with water-treated samples. Spraying with water or repeated exposure to water droplets caused a pronounced degradation of the insulin parent yields in combination with an increasing appearance of signals due to the B- and A-chains of insulin. To clarify the issue of surface segregation, a few samples were prepared by spraying acetone-diluted solutions of insulin on previously deposited layers of 4HCCA. Whereas the insulin yields from as-prepared samples were rather low, the yields observed after water treatment were comparable with those observed with samples of insulin in 4HCCA. The results suggest that a large amount of insulin is present at the surface of samples prepared from liquid mixtures of insulin in 4HCCA. With both methods of sample preparation, however, high secondary ion yields of insulin were only obtained after exposure of the samples to water. The chemical changes responsible for this beneficial effect still need to be identified.