Maria Bacia

Mg alloy for hydrogen storage processed by SPD

Abstract Mg-based nanocomposites are promising candidates for hydrogen storage applications exhibiting fast H-sorption kinetics at reasonably low temperatures when processed by high-energy ball milling techniques. However, since compaction of the highly reactive nanometric powder is desirable before application, the search for other effective processing routes for the preparation of Mg-based nanocomposites is relevant. In this work, we have used a combination of equal channel angular pressing, cold rolling and high-energy ball milling in the processing of the commercial AZ31 extruded alloy to evaluate its use as a hydrogen storage material. Severe plastic deformation carried out at different temperatures, combined with further mechanical processing resulted in a controlled texture and signifiant grain refinement, which are desirable microstructural characteristics for hydrogen storage applications.

Confined nucleation and growth of organic nanocrystals in sol–gel matrices

We have engineered new hybrid organic–inorganic materials through a simple and generic preparation of stable organic nanocrystals grown in gel–glass matrices. This process was based on confined nucleation and growth of dyes in the pores of dense gels. For bulk samples, narrow size distributions of particles were obtained between 10 and 20 nm in diameter. We extended this method to the preparation of organic nanocrystals embedded in sol–gel thin films by spin-coating. For all these nanocomposite samples, we significantly increased the dye stability and obtained promising optical properties (luminescence, NLO properties, SHG). Moreover we also demonstrated the basic working principles of a new type of fluorescent nanosensors through the preparation of organic luminescent nanocrystals grown in silicate films.

Fluorescent molecular nanocrystals anchored in sol–gel thin films: a label-free signalization function for biosensing applications

A DNA sensor based on highly fluorescent and photostable molecular nanocrystals has been investigated. These nanocrystals are confined in a sol–gel thin film by a simple one-step process. After controlled dissolution of the sol–gel thin film layer-by-layer, nanocrystals emerge from the silicate matrix. Hairpin-shaped DNA fragments, functionalized by a probe molecule, were chemically bound to these nanocrystals. This grafting brings molecular probes close to the nanocrystal surface, resulting in quenching of the nanocrystal fluorescence by Forster Resonance Energy Transfer. After hybridization of the hairpin with their complementary DNA, they become unfolded, moving the probes away from the nanocrystals. This “turns on” the fluorescence of the nanocrystals and allows the hybridization detection of non-fluorescent DNA to be achieved. These first results on fluorescent nanocomposite coatings open up promising possibilities for making label-free generic sensors.

Structural and electrochemical properties of new nanospherical manganese oxides for lithium batteries

The products of decomposition of manganese carbonate with and without doping by copper or cobalt, at temperatures <500 °C in air were studied. Doped systems with 20 atom% Cu or Co give new nanometric manganese oxides agglomerated in submicronic spheres at 370 °C. Transmission electron microscopy (TEM) shows that these X-ray amorphous compounds are nanocrystalline with grain size in the 10 nm range and a spinel substructure. The electrochemical behaviour of these materials in lithium cells was studied. Whereas non-doped manganese oxide exhibits poor intercalation capabilities, the freshly co-precipitated Cu or Co doped materials can be cycled successfully around 3 V vs. Li/Li+. Step-potential electrochemical spectroscopy shows that the initial discharge gives rise to a two-phase transition and is followed by stable, reversible, single-phase cycling. Best results are obtained on a cobalt-doped manganese oxide (Co : Mn = 1 : 5), which can sustain more than 100 charge–discharge cycles with a 175 mA h g−1 capacity in the 1.8–4.2 V range. XAS spectra were measured on pristine and electrochemically discharged materials, showing that (1) in the cobalt-doped material, cobalt is divalent and manganese is the only redox-active species, (2) the variations in local structure around Mn on discharge are much smaller than in long-range ordered compounds such as Li–Mn–O spinels.

Assembly principles of a unique cage formed by hexameric and decameric E. coli proteins

A 3.3 MDa macromolecular cage between two Escherichia coli proteins with seemingly incompatible symmetries–the hexameric AAA+ ATPase RavA and the decameric inducible lysine decarboxylase LdcI–is reconstructed by cryo-electron microscopy to 11 Å resolution. Combined with a 7.5 Å resolution reconstruction of the minimal complex between LdcI and the LdcI-binding domain of RavA, and the previously solved crystal structures of the individual components, this work enables to build a reliable pseudoatomic model of this unusual architecture and to identify conformational rearrangements and specific elements essential for complex formation. The design of the cage created via lateral interactions between five RavA rings is unique for the diverse AAA+ ATPase superfamily. DOI: http://dx.doi.org/10.7554/eLife.03653.001

Structural insights into the Escherichia coli lysine decarboxylases and molecular determinants of interaction with the AAA+ ATPase RavA

The inducible lysine decarboxylase LdcI is an important enterobacterial acid stress response enzyme whereas LdcC is its close paralogue thought to play mainly a metabolic role. A unique macromolecular cage formed by two decamers of the Escherichia coli LdcI and five hexamers of the AAA+ ATPase RavA was shown to counteract acid stress under starvation. Previously, we proposed a pseudoatomic model of the LdcI-RavA cage based on its cryo-electron microscopy map and crystal structures of an inactive LdcI decamer and a RavA monomer. We now present cryo-electron microscopy 3D reconstructions of the E. coli LdcI and LdcC, and an improved map of the LdcI bound to the LARA domain of RavA, at pH optimal for their enzymatic activity. Comparison with each other and with available structures uncovers differences between LdcI and LdcC explaining why only the acid stress response enzyme is capable of binding RavA. We identify interdomain movements associated with the pH-dependent enzyme activation and with the RavA binding. Multiple sequence alignment coupled to a phylogenetic analysis reveals that certain enterobacteria exert evolutionary pressure on the lysine decarboxylase towards the cage-like assembly with RavA, implying that this complex may have an important function under particular stress conditions.

Molecular nanocrystals grown in sol-gel thin films for ultrabright chemical sensor applications

Polyaromatic dyes were used to synthesize molecular nanocrystals in sol-gel thin films for sensor applications. Fluorescence Confocal Microscopy (FCM) and Transmission Electron Microscopy (TEM) experiments showed the advantages of our nanocrystallization process compared with microcrystallization in free solutions. Indeed, we obtained well-dispersed and spherical nanocrystals with a narrow size distribution, exhibiting a good crystallinity. Time-resolved fluorescence spectroscopy allowed us to measure fluorescence lifetimes of nanocrystals in presence of molecular probes. Then, chemical sensoring properties of these molecular nanocrystals were demonstrated.