Claire Arnoult

The reduction of Ag+ in metallic silver on pseudomelanin films allows for antibacterial activity but does not imply unpaired electrons

Dopamine-melanin films produced through the oxidation of dopamine in the presence of oxygen as an oxidant allow to reduce silver ions onto silver particles as already described in the paper by Lee et al. (H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Science 318 (2007) 426.). This reduction process has to occur through the oxidation of moieties present in the melanin film. This investigation shows that the free radicals present in the pseudomelanin film, quantified by means of electron spin resonance spectroscopy (ESR) for the first time, are not used in the transformation of Ag(+) cations to deposit silver. The ESR signal is hardly affected by the deposition of silver particles. On the other hand, X-ray photoelectron spectroscopy shows a small increase in the density of quinone groups and a small decrease of catechol groups on the surface of the film during the deposition of silver. This suggests that the deposited pseudomelanin films contain a significant fraction of catechol groups able to trigger reduction processes of metallic cations. These silver nanoparticles remain adherent to the melanin films and allow for a quantitative killing of Escherichia coli over a broad range of bacterial dilutions. However, the presence of the bacteria induces a release of the nanoparticles. The pseudomelanin films cannot be reused again for a silver ion reduction step. Nevertheless, the easy preparation of the pseudomelanin-silver composite and its effective one shot bacterial killing activity renders the strategy presented in this paper attractive. Some fundamental questions about redox process allowed by the pseudomelanin films will also be asked.

Pressure and scattering regime influence on the EDS profile resolution at a composite interface in environmental SEM

Gas impact on the EDS profile resolution at the interface of composite interface resin/Al was investigated with two gaseous environments: helium and water vapor. Two main components of the global profile at the interface were investigated: the contrast of the profile and the spatial resolution. A complementary approach was developed by comparing gas nature impact versus the pressure and versus the scattering regime. The results show that the unscattered electron beam mainly governs EDS profile spatial resolution as long as the scattering regime is single or oligo scattering. Then for plural scattering, spatial resolution is dramatically degraded. In addition, the contrast is degraded since a gas is introduced, whatever the gas, the pressure and so the scattering regime. This approach would enable to better understand the respective contributions of the unscattered beam and the skirt and the influence of the gases nature on them.

A new approach to reach the best resolution of X-ray microanalysis in the variable pressure SEM

A validation of our recent new approach is presented here in order to better interpret the EDS analysis results in low vacuum SEM. This approach is based on correlation between two concepts: the electron beam skirt radius in the gas characterized by RS and the X-ray emission volume radius in the material characterized by RX. If RS≤RX; then the skirt impact on the analysis is null and the best possible X-ray lateral resolution within the limitations imposed by gas scattering is obtained. In order to follow the relationship between RS and RX, two aluminum foils with different thickness (2 μm and 20 μm) embedded separately in epoxy resin were used. The results showed the existence of the optimal experimental conditions depending on the pressure and the energy that verify the condition of RS≤RX. The experimental and simulated results show the great consistency of this approach.

Reactive Layer-by-Layer Films from Solutions Containing Silicic Acid and a Ti(IV) Complex: Preferential Incorporation of Silica and Interactions of the Obtained Films with Hexacyanoferrate Anions

The concept of reactive layer-by-layer (LBL) deposition allows the build-up of films containing polycations and oxide particles, namely, silica and poorly crystalline anatase. Because polyelectrolyte multilayer films have been produced from blended polyanions or polycations solutions and since preferential incorporation of one of the partners of the blend has been found in most cases, one should wonder if a preferential polycondensation of either silica or titania should occur when the reactive deposition is performed from a solution containing a precursor of both inorganic species. X-ray photoelectron (XPS) and UV-visible spectroscopies show that the reactive LBL films made from the blend and poly(diallyldimethylammonium chloride) (PDADMAC) incorporate predominantly silica over TiO(2) over the whole molar fraction range of the silicic acic/hydrosoluble Ti(IV) complex. The transparency of the films below 365 nm, corresponding to the band edge of TiO(2), can easily be modulated. The silica/TiO(2) films are all able to bind hexacyanoferrate owing to the presence of the polycation allowing the binding of the oxide particles to the substrate. However, the binding capacity of the film does not scale proportionally to its thickness. The films made from eight dipping cycles showed a sudden decrease in their binding capacity for hexacyanoferrate when the molar fraction of the titanium complex was higher than ∼0.6 in the blend. For the same films, electrochemical impedance spectra (EIS) showed marked differences with a change in film composition: the more TiO(2) in the film, the higher the resistance to electron and to mass transfer. Therefore, EIS helps to explain the reduced surface concentration measured by means of cyclic voltammetry for films rich in TiO(2).

Prediction and limitation of polymer degradation in Environmental SEM

Polymer materials degradation is a well-known limitation to their characterization in SEM. In this paper authors present an additional possibility for polymer imaging offered by the Environmental SEM applied to the PMMA with micrometric relief. As shown by the so-called double-scan procedure, increasing the pressure enables to delay the degradation of the polymer surface. To evaluate quantitatively this observation, the classical formula to calculate the electron dose is adapted to ESEM characteristic. It is shown first that pressure enables to decrease the electron dose and all that takes place as if the skirt electrons do not participate in the irradiation. Secondly this procedure allows to obtain the critical dose of topographic degradation, applied to the PMMA with micrometric relief D(C) was found to be 1.31 ± 0.28 Cm(-2). Finally it becomes then possible, knowing the D(C) for a given sample, to evaluate the limit of investigation without topographic surface degradation: for example, the maximal field of view (linked to the magnification) depending on the other parameters.