Valérie Toniazzo

Self-assembly of tetramers of 5,6-dihydroxyindole explains the primary physical properties of eumelanin: experiment, simulation, and design.

Eumelanin is a ubiquitous pigment in nature and has many intriguing physicochemical properties, such as broad-band and monotonous absorption spectrum, antioxidant and free radical scavenging behavior, and strong nonradiative relaxation of photoexcited electronic states. These properties are highly related to its structural and mechanical properties and make eumelanin a fascinating candidate for the design of multifunctional nanomaterials. Here we report joint experimental-computational investigation of the structural and mechanical properties of eumelanin assemblies produced from dopamine, revealing that the mass density of dry eumelanin is 1.55 g/cm³ and its Youngs modulus is ≈5 GPa. We also find that wet eumelanin has a lower mass density and Youngs modulus depending on the water-to-melanin ratio. Most importantly, our data show that eumelanin molecules tend to form secondary structures based on noncovalent π stacking in both dry and wet conditions, with an interlayer distance between eumelanin molecules of 3.3 Å. Corresponding transmission electron microscope images confirm the supramolecular organization predicted in our simulations. Our simulations show that eumelanin is an isotropic material at a larger scale when eumelanin molecules are randomly oriented to form secondary structures. These results are in good agreement with experimental observations, density functional theory calculations, and bridge the gap between earlier experimental and small-scale quantum mechanical studies of eumelanin. We use the knowledge acquired from the simulations to select a partner molecule, a cationic phthalocyanine, allowing us to produce layer-by-layer films containing eumelanin that display an electrical conductivity 5 orders of magnitudes higher than that of pure eumelanin films.

Recent advances in high performance poly(lactide): from “green” plasticization to super-tough materials via (reactive) compounding

Due to its origin from renewable resources, its biodegradability, and recently, its industrial implementation at low costs, poly(lactide) (PLA) is considered as one of the most promising ecological, bio-sourced and biodegradable plastic materials to potentially and increasingly replace traditional petroleum derived polymers in many commodity and engineering applications. Beside its relatively high rigidity [high tensile strength and modulus compared with many common thermoplastics such as poly(ethylene terephthalate) (PET), high impact poly(styrene) (HIPS) and poly(propylene) (PP)], PLA suffers from an inherent brittleness, which can limit its applications especially where mechanical toughness such as plastic deformation at high impact rates or elongation is required. Therefore, the curve plotting stiffness vs. impact resistance and ductility must be shifted to higher values for PLA-based materials, while being preferably fully bio-based and biodegradable upon the application. This review aims to establish a state of the art focused on the recent progresses and preferably economically viable strategies developed in the literature for significantly improve the mechanical performances of PLA. A particular attention is given to plasticization as well as to impact resistance modification of PLA in the case of (reactive) blending PLA-based systems.

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.

Enhanced Adhesion over Aluminum Solid Substrates by Controlled Atmospheric Plasma Deposition of Amine-Rich Primers

Controlled chemical modification of aluminum surface is carried by atmospheric plasma polymerization of allylamine. The amine-rich coatings are characterized and tested for their behavior as adhesion promoter. The adhesion strength of aluminum-epoxy assemblies is shown to increase according to primary amino group content and coating thickness, which in turn can be regulated by plasma power parameters, allowing tailoring the coating chemical properties. The increase in adherence can be correlated to the total and primary amino group contents in the film, indicating covalent bonding of epoxy groups to the primer as the basis of the mechanical improvement.

Deposition Mechanisms in Layer-by-Layer or Step-by-Step Deposition Methods: From Elastic and Impermeable Films to Soft Membranes with Ion Exchange Properties

The modification of solid-liquid interfaces with polyelectrolyte multilayer films appears as a versatile tool to confer new functionalities to surfaces in environmentally friendly conditions. Indeed such films are deposited by alternate dipping of the substrates in aqueous solutions containing the interacting species or spraying these solutions on the surface of the substrate. Spin coating is more and more used to produce similar films. The aim of this short review article is to provide an unifying picture about the deposition mechanisms of polyelectrolyte multilayer films. Often those films are described as growing either in a linear or in a supralinear growth regime with the number of deposited “layer pairs”. The growth regime of PEM films can be controlled by operational parameters like the temperature or the ionic strength of the used solutions. The control over the growth regime of the films as a function of the number of deposition steps allows to control their functional properties: either hard and impermeable films in the case of linear growth or soft and permeable films in the case of supralinear growth. Such different properties can be obtained with a given combination of interacting species by changing the operational parameters during the film deposition.

Characterization of ethanolysis products of poly(dimethylsiloxane) species by electrospray ionization tandem mass spectrometry

RATIONALE The partial and controlled degradation of insoluble cross-linked silicon-based polymers is a promising approach to enable their characterization by mass spectrometry. Providing that the chemolysis reaction specifically proceeds at cross-linking sites, the size of linear poly(dimethylsiloxane)s (PDMS) produced during the treatment should reflect the length of linear segments between branching points in the original network. In this context, the specificity of ethanol to act as a nucleophilic agent towards tri-functional silicon atoms in a D3TD(n)TD3 model was evaluated. METHODS Tandem mass spectrometry (MS/MS) combined with accurate mass measurements, MS(3) experiments and collision-induced dissociation of authentic compounds was used for structural characterization of D3TD(n)TD3 ethanolysis products. All MS/MS data were obtained from electrosprayed ammonium adducts, previously reported to provide the most informative data for silicon-based polymers. RESULTS Since the expected ethanolysis products were hydroxy- and ethoxy-terminated PDMS, the dissociation behavior of such polymeric species was established, using electrosprayed ammonium adducts as the precursor ions. Diagnostic product ions were identified, allowing four main D3TD(n)TD3 ethanolysis products to be structurally characterized. End-group analysis of these polymeric distributions clearly indicated that ethanolysis was mostly occurring on tri-functional silicon atoms but also, to a lesser extent, on those D atoms close to T silicons. CONCLUSIONS The size of the linear skeleton located between two tri-functional silicon atoms could be accurately determined by mass spectrometric analyses of a polymeric model submitted to ethanolysis. This soft and rapid pre-treatment is thus a promising approach for determining the length of linear segments between branching points in the original network of cross-linked silicon-based polymers.

Collision-induced dissociation of synthetic polymers containing hydride groups: the case of poly(methylhydrosiloxane) homopolymers and poly(methylhydrosiloxane)-co-(dimethylsiloxane) copolymers.

RATIONALE When substituting one methyl moiety by a hydrogen atom in each end-group of a trimethylsilyl-terminated poly(dimethylsiloxane) (PDMS), dissociation reactions of oligomers adducted with ammonium were observed to proceed at a much higher rate, evidencing the high reactivity of hydride groups. Polymeric molecules containing methylhydrosiloxane (MHS) units could thus be expected to exhibit a different tandem mass spectrometric (MS/MS) behavior from PDMS. METHODS Trimethylsilyl-terminated PMHS and trimethylsilyl-terminated poly(MHS)-co-(DMS) were electrosprayed in the gas phase either as ammonium adducts or lithium adducts. Product ions generated upon collision-induced dissociation (CID) were accurately mass measured in an orthogonal acceleration time-of-flight mass analyzer. RESULTS In contrast to PDMS adducted with lithium, useful structural features could be obtained from product ions generated upon CID of lithium adducts of PMHS. The presence of multiple hydride groups in PMHS induced numerous rearrangements when activating ammonium adducts of these oligomers. MS/MS reactions observed for cationic adducts of MHS-DMS co-oligomers were clearly a combination of major dissociation routes established for the corresponding homopolymers. However, the concerted loss of H(2) and ammonia typically observed from ammonium adducts of PMHS was always shown to generate a quite abundant product ion even from co-oligomers enriched with DMS units. CONCLUSIONS The high reactivity of hydride moieties, previously evidenced when these groups were at the end of PDMS chains, is also at work in PMHS, where each monomer contains a Si-H function. The presence of these hydride groups would increase the nucleophilic character of the oxygen atoms, favoring a tight bonding of lithium, and hence allowing in-chain cleavages to occur. In PMHS ammonium adducts, the particular reactivity of hydride moieties was illustrated by multiple hydride transfers but also by a dehydrogenation reaction systematically observed to proceed, together with the loss of ammonia, from all precursor ions. This latter reaction remained a very competitive process even from MHS/DMS co-oligomers with a low relative number of MHS units.

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.

Nano-ordered thin films achieved by soft atmospheric plasma polymerization

Plasma polymer thin films are of great interest in surface engineering in a wide range of applications. Herein, by using soft atmospheric plasma deposition parameters and by adapting these conditions to the used perfluorodecyl and dodecyl acrylates precursors, it is possible to get a high retention of monomer functionalities and a polymerization close to conventional methods. Molecular investigation revealed the presence of polymeric moieties and the mechanism of plasma polymerization has been mainly based on the polymerization by activation of the ethylenic groups. X-ray diffraction analyses have shown the presence of a smectic lamellar where the polyacrylate backbone was the amorphous phase and fluorinated and alkyl side chains were the hexagonal crystalline section. Wetting properties have been evaluated and finally showed hydrophobic surfaces

Use of doubly charged precursors to validate dissociation mechanisms of singly charged poly(dimethylsiloxane) oligomers.

AbstractCollision-induced dissociation of doubly charged poly(dimethylsiloxane) (PDMS) molecules was investigated to provide experimental evidence for fragmentation reactions proposed to occur upon activation of singly charged oligomers. This study focuses on two PDMS species holding trimethylsilyl or methoxy end-groups and cationized with ammonium. In both cases, introduction of the additional charge did not induce significant differences in dissociation behavior, and the use of doubly charged precursors enabled the occurrence of charge-separation reactions, allowing molecules always eliminated as neutrals upon activation of singly charged oligomers to be detected as cationized species. In the case of trimethylsilyl-terminated oligomers, random location of the adducted charge combined with rapid consecutive reactions proposed to occur from singly charged precursors could be validated based on MS/MS data of doubly charged oligomers. In the case of methoxy-terminated PDMS, favored interaction of the adducted ammonium with both end-groups, proposed to rationalize the dissociation behavior of singly charged molecules, was also supported by MS/MS data obtained for molecules adducted with two ammonium cations. Figureᇵ