Physical adsorption of anisotropic titania nanoparticles onto poly(2-vinylpyridine) latex and characterisation of the resulting nanocomposite particles.

Four poly(2-vinylpyridine) latexes with intensity-average mean diameters ranging between 246 and 955nm were prepared by aqueous emulsion polymerisation. These latexes were characterised by transmission electron microscopy, field emission scanning electron microscopy, dynamic light scattering, aqueous electrophoresis, disc centrifuge photosedimentometry and thermogravimetry. The adsorption of rice grain-shaped nano-sized titania particles onto the surface of these latex particles from aqueous solution was investigated. It was found that the titania particles adsorb strongly at pH 10 and the optimal loading and packing density of titania was investigated for each latex. The resulting core-shell P2VP-titania nanocomposite particles were characterised in terms of their titania contents, surface coverages and colloidal stabilities. UV-Vis spectra were recorded for the titania nanoparticles, the original P2VP latexes and the poly(2-vinylpyridine)-titania nanocomposite particles. It was found that, for the larger nanocomposite particles, UV-Vis absorption was dominated by the latex core, whereas the smaller P2VP-titania nanocomposite particles exhibited UV attenuation to longer wavelengths compared to both the bare latex and the titania particles. The poly(2-vinylpyridine) cores were selectively removed by calcination of the nanocomposite particles and the resulting hollow titania structures were investigated by transmission electron microscopy.

Measuring sunscreen protection against solar-simulated radiation-induced structural radical damage to skin using ESR/spin trapping: development of an ex vivo test method.

The in vitro star system used for sunscreen UVA-testing is not an absolute measure of skin protection being a ratio of the total integrated UVA/UVB absorption. The in vivo persistent-pigment-darkening method requires human volunteers. We investigated the use of the ESR-detectable DMPO protein radical-adduct in solar-simulator-irradiated skin substitutes for sunscreen testing. Sunscreens SPF rated 20+ with UVA protection, reduced this adduct by 40-65% when applied at 2 mg/cm(2). SPF 15 Organic UVA-UVB (BMDBM-OMC) and TiO(2)-UVB filters and a novel UVA-TiO(2) filter reduced it by 21, 31 and 70% respectively. Conventional broad-spectrum sunscreens do not fully protect against protein radical-damage in skin due to possible visible-light contributions to damage or UVA-filter degradation. Anisotropic spectra of DMPO-trapped oxygen-centred radicals, proposed intermediates of lipid-oxidation, were detected in irradiated sunscreen and DMPO. Sunscreen protection might be improved by the consideration of visible-light protection and the design of filters to minimise radical leakage and lipid-oxidation.

Modified data analysis for thermal conductivity measurements of polycrystalline silicon microbridges using a steady state Joule heating technique.

Accurate knowledge of thermophysical properties is needed to predict and optimize the thermal performance of microsystems. Thermal conductivity is experimentally determined by measuring quantities such as voltage or temperature and then inferring a thermal conductivity from a thermal model. Thermal models used for data analysis contain inherent assumptions, and the resultant thermal conductivity value is sensitive to how well the actual experimental conditions match the model assumptions. In this paper, a modified data analysis procedure for the steady state Joule heating technique is presented that accounts for bond pad effects including thermal resistance, electrical resistance, and Joule heating. This new data analysis method is used to determine the thermal conductivity of polycrystalline silicon (polysilicon) microbridges fabricated using the Sandia National Laboratories SUMMiT V™ micromachining process over the temperature range of 77-350 K, with the value at 300 K being 71.7 ± 1.5 W/(m K). It is shown that making measurements on beams of multiple lengths is useful, if not essential, for inferring the correct thermal conductivity from steady state Joule heating measurements.

An investigation of the factors influencing the detection sensitivity of cavity enhanced Raman scattering for probing aqueous binary aerosol droplets.

Stimulated Raman scattering (SRS) from single aerosol droplets can be observed at extremely low laser threshold intensities at wavelengths commensurate with whispering gallery modes. Although droplet size can routinely be determined from the ensuing cavity enhanced Raman scattering (CERS) fingerprint, determining droplet composition is a considerably more challenging measurement. We present here an examination of the factors that influence and limit the detection sensitivity of CERS in quantifying the concentrations of sulfate and nitrate in water droplets, 20-50 microm in radius. In particular, we consider the variation in nitrate and sulfate SRS signal with variation in species concentration, probe laser intensity and droplet size. We illustrate that the band contour of the OH stretching band can be used as a relative measure of the internal light intensity circulating within the droplet and experimentally investigate how the threshold condition for SRS is achieved.

Heterogeneous atmospheric aerosol chemistry: laboratory studies of chemistry on water droplets.

Heterogeneous chemical reactions on aerosol particles play a pivotal role in atmospheric chemistry. In this review, the fundamental concepts underlying the chemical dynamics of liquid aerosol droplets are discussed, with particular emphasis on the properties of the aqueous-air interface and the reaction mechanisms of key chemical processes. Recent laboratory studies of heterogeneous chemistry on aqueous aerosol particles are reviewed, with techniques that probe the gas phase, liquid phase and the interface directly, discussed in turn.

Chemistry in the clouds: the role of aerosols in atmospheric chemistry.

Ever since the discovery of the ozone hole over the Antarctic and the recognition of the damaging effects of acid rain, the role of atmospheric aerosol particles in determining the chemical balance of the atmosphere has received much attention. Aerosol particles produced in combustion can also have a deleterious effect on human health. In this article we review the chemistry that can occur on aerosol particles, particularly on aqueous based aerosols in the troposphere. The sources, transformation and loss mechanisms of atmospheric aerosol will be discussed. In particular, we will focus on the role of chemical transformation on aerosol particles in promoting reactions that would otherwise be too slow in the homogeneous atmospheric gas phase. Heterogeneous reaction mechanisms of some key chemical reactions will be described. Recent observations of a high organic content of tropospheric aerosol particles will be described and a model of organic coated aerosols will be reviewed.