Dynamics of Citrate Coordination on Gold Nanoparticles Under Low Specific Power Laser-Induced Heating.

SERS evolution recorded over a drop-coated coffee-ring pattern of citrate-capped gold colloids was investigated as a function of time under low-specific laser power. Spectral changes caused by plasmon-induced reaction could not be detected, but a long-term transient original spectral profile showing additional lines was observed. We performed deep qualitative and quantitative SERS intensity variation analysis based on the complementary use of extreme deviation and cross-correlation statistics, which provided further insights on the behavior of citrate-capping layers of gold nanoparticles upon laser illumination. More precisely, the cross-correlation analysis made possible to follow the so-called individual events denoting particular resonance structures, in which groups of modes were assigned to an evolution of citrate coordination on gold surface driven by photo-activation. As a consequence, the detection limit was increased and new lines were related to the presence of a very low amount of dicarboxy-acetone (DCA), which was already present in the system.

Combining sorption experiments and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) to study the adsorption of propranolol onto environmental solid matrices - Influence of copper(II).

The bioavailability of pharmaceuticals is governed by their sorption in soils/sediments, as the retention processes determine their concentration in surface- and ground-water. The adsorption of these contaminants can involve various solid components such as organic matter, clays and metallic oxides, and their distribution among these solid components depends on contaminant and solid properties. In this paper we studied the adsorption of the pharmaceutical propranolol - a beta-blocker - on eight different solids (six soils, one sediment and one kaolinite-based sample) by batch experiments. The influence of contact time, propranolol concentration and pH was considered, as well as the presence of copper(II). The investigated solids displayed a wide variability in terms of CEC (cationic exchange capacity) and organic carbon and carbonates contents. The influence of pH was negligible in the pH range from 5.5 to 8.6. The adsorbed amounts were greatly dependent on the solid and two groups of solids were evidenced: three soils of high CEC and organic carbon contents which retained high amounts of propranolol, and three soils, the sediment and the kaolinite-based sample (low CEC and organic carbon content) displaying a low adsorption capacity for the beta-blocker. A linear model enabling the determination of the sorption parameters Kd and Koc was pertinent to describe the adsorption isotherms but the Koc values showed a great variability. It was shown that organic carbon content alone could not explain propranolol adsorption. The CEC value was identified as influent parameter and a simple empirical model was proposed to describe propranolol adsorption. At microscopic and molecular scales, ToF-SIMS experiments indicated (i) a decrease of potassium on the surface upon propranolol adsorption with a distribution of the beta-blocker similarly to alumino-silicates, iron and organic carbon on the surface confirming a cation exchange mechanism and (ii) the absence of degradation products and copper-propranolol complexes.

Dynamics of a plasmon-activated p-mercaptobenzoic acid layer deposited over Au nanoparticles using time-resolved SERS.

Time-dependent SERS intensity recorded over a drop-coated coffee-ring pattern of p-MBA with gold colloids was investigated as a function of the specific laser power applied. Pure electromagnetic enhancement produced stochastic intensity variations of the whole SER spectra, which were mainly correlated with evolutions of the background intensity. Besides long-term, non-reversible spectral changes caused by plasmon-induced decarboxylation of p-MBA, transient original spectral profiles showing additional lines were also observed as the specific power reached 5.5 × 10(4) W cm(-2). An unprecedented qualitative and quantitative study of SERS intensity variations based on the complementary use of both extreme deviation and cross-correlation statistics is provided, which resulted in an improved understanding of SERS mechanisms. More precisely, cross-correlation analysis made it possible to follow the evolution of groups of modes assigned to one species or sharing the same symmetry while so-called individual events denote particular resonance structures, whose occurrence was tentatively related to a photo-thermally activated motion of the gold nanostructures.

On the track to silica-supported tungsten oxo metathesis catalysts: input from 17O solid-state NMR.

The grafting of an oxo chloro trisalkyl tungsten derivative on silica dehydroxylated at 700 °C was studied by several techniques that showed reaction via W-Cl cleavage, to afford a well-defined precatalyst for alkene metathesis. This was further confirmed by DFT calculations on the grafting process. (17)O labeling of the oxo moiety of a series of related molecular and supported tungsten oxo derivatives was achieved, and the corresponding (17)O MAS NMR spectra were recorded. Combined experimental and theoretical NMR studies yielded information on the local structure of the surface species. Assessment of the (17)O NMR parameters also confirmed the nature of the grafting pathway by ruling out other possible grafting schemes, thanks to highly characteristic anisotropic features arising from the quadrupolar and chemical shift interactions.

An easily accessible Re-based catalyst for the selective conversion of methanol: evidence for an unprecedented active site structure through combined operando techniques.

Heterogeneous Re/SiO(2) catalysts prepared using a one pot sol-gel synthesis were found to display high activity in the direct, selective methanol conversion to methylal, which is correlated to an unprecedented rhenium oxide structure.

Development of the first well-defined tungsten oxo alkyl derivatives supported on silica by SOMC: towards a model of WO3/SiO2 olefin metathesis catalyst.

A well-defined, silica-supported tungsten oxo alkyl species prepared by the surface organometallic chemistry approach displays high and sustained activity in propene metathesis. Remarkably, its catalytic performances outpace those of the parent imido derivative, underlining the importance of the oxo ligand in the design of robust catalysts.

Correlation between gelation time, structure and texture of low-doped silica gels.

It is shown how the properties of a porous silica xerogel prepared using a classical sol-gel synthesis can be fine-tuned by a minor modification of the composition. The addition of a doping cation (Cu(2+), Ca(2+), Na(+)) in trace quantities in the silica sol was found to exert a dramatic effect at all stages of material preparation. An investigation of both liquid and solid phases is presented, making it possible to highlight strong correlations. The time-resolved speciation of Si-containing moieties in the sol was found to be an indication of the structuration of the gel, which was reflected by the porosity and by the molecular structure of the resulting porous material. Based on a careful comparison of several slightly doped silica gels, a model is proposed which makes it possible to predict the structure and the texture of a silica gel from data recorded early in the liquid phase.

A well-defined silica-supported dinuclear tungsten(III) amido species: synthesis, characterization and reactivity.

Grafting of [W(2)(NMe(2))(6)] onto dehydroxylated silica affords the well-defined surface species [([triple bond, length as m-dash]Si-O)W(2)(NMe(2))(5)], characterized by elemental analysis, and infrared, Raman and NMR spectroscopies, and the catalytic reactivity of this supported tungsten(III) d(3)-d(3) dimer and of its alkoxide derivatives towards alkynes has been probed.

Alkaline treatment of iron-containing MFI zeolites. Influence on mesoporosity development and iron speciation.

The effects of alkaline treatment on the mesoporosity development and iron speciation in Fe-MFI zeolites have been investigated. To this end, a variety of samples derived from different synthetic routes and having distinct Si/Al ratios and Fe content were treated in NaOH solutions and characterized by N2 adsorption, SEM, TEM, UV/vis spectroscopy, and EPR. The alkaline treatment induces a significant intracrystalline mesoporosity development by framework silicon extraction and promotes disintegration of oligomeric iron species. Iron in framework positions has shown to provoke mesopore formation, whereas nonframework iron species suppresses silicon leaching and lowers the extent of extra porosity.

Microstructures and structural properties of sol-gel silica foams.

Silica xerogels were synthesized and annealed at 1000 degrees C for different durations to yield stable silica materials. The samples were prepared through base-catalyzed hydrolysis and condensation of tetramethyl orthosilicate in methanol. After aging and drying steps, clear and solid xerogels exhibiting a narrow pore size distribution were achieved. The annealing treatment of these xerogels was performed at 1000 degrees C and proved in the present study to lead to a monolithic glass when a progressive heat-treatment procedure was employed to attain 1000 degrees C. In addition to the expected glass, silica foams and ordered phases were observed when the samples were instantaneously heat-treated at 1000 degrees C. Raman spectra of the foamed materials exhibit the classical features of amorphous silica, whereas transmission electronic microscopy pictures reveal the presence of crystallized domains within the vitreous matrix. These crystallites are prone to nucleation and growth processes, which jeopardize the believed stability of the silica foam. The assessment of the hydroxyl content by IR spectroscopy reveals the role played by the latter polycondensation of silanols. The occurrence of foaming process was thus found to result from two competitive phenomena occurring at 1000 degrees C: evacuation of water-related species and viscous sintering.