Bruno Alonso

Chemical bonding differences evidenced from J-coupling in solid state NMR experiments involving quadrupolar nuclei

Small scalar J-coupling between quadrupolar nuclei and spin 1/2 can be measured in inorganic solids using J-Resolved experiments and further used to acquire 2D J-HQMC heteronuclear correlation, giving detailed insight into the chemical bonding scheme.

Chitin-silica nanocomposites by self-assembly.

Self-assembly of chemical entities is at the basis of many biological processes and increasingly in materials syntheses. Self-assembled surfactants and block copolymers are widely and successfully used to prepare ordered hybrid and mesoporous materials with outstanding properties. 7–9] A strong interest also rises for energy-saving chemical processes and reactants from renewable resources. 11] Self-assembled biomacromolecules and other biological objects have been used previously as liquid-crystal templates for the formation of mesoporous silica. Herein we present a novel and versatile colloid-based approach for the large-scale synthesis of a new family of hybrid bioorganic–inorganic nanocomposites with an unprecedented control in texture and morphology. This approach combines the self-assembly properties of polysaccharide chitin nanorods, with the flexibility of sol– gel processes involving siloxane oligomers. The resulting optical and mechanical properties of the chitin–silica nanocomposites can be tuned by varying the chitin volume fraction fCHI. Nanorod alignment inside these materials was achieved under moderate magnetic fields (9 T), generating highly oriented textures and providing an alternative method to that developed for surfactant-templated materials. Furthermore, sol–gel chemistry is amenable to a variety of processing techniques such as spray-drying, which allowed us to prepare micrometer-size chitin–silica particles with variable porosity in the calcined replicas. Our approach consists in the formation and processing of a stable suspension containing two colloids, a-chitin nanorods and siloxane precursors, both in a dispersed state. This is a challenging issue owing to differences in stability and reactivity of the colloids. Chitin nanorods purified from shrimp shells (L = 260 80 nm, D = 23 3 nm) are bundles of monocrystals (D = 3.2 0.6 nm) with amino groups at their surface (Figure 1a). They are stably dispersed in water by electrostatic repulsions in slightly acidic conditions. The simple addition of silica precursors would lead to uncontrolled chitin/silica precipitation owing to electrostatic interactions and/or rapid siloxane condensation. To avoid this, we prepared mixed alcoholic suspensions of the chitin nanorods with siloxane oligomers through repeated solvent exchange cycles. The siloxane oligomers were formed by controlled acid-catalyzed hydrolysis with a hydrodynamic diameter of about 4 nm and a degree of siloxane condensation c of 0.75 (Figure 1a). The resulting clear to translucent suspensions (depending on chitin concentration) are stable over a long period of time (several months). Upon evaporation, the colloidal suspension shows a marked increase in viscosity and turbidity, which is accompanied by a gradual rise in optical birefringence. In a standard procedure, the resulting pasty mixture was cast and further dried overnight (348 K), yielding hard bulk materials with condensation degrees c in the expected range of 0.8–0.9. Preliminary compression tests show that the elastic mechanFigure 1. a) Representation of the siloxane oligomers (upper panel), and of the chitin nanorods (core: poly[b-(1!4)-2-acetamido-2-deoxy-dglucopyranose]) as bundles of monocrystals with amino groups at their surface (lower panel). The colloid sizes were estimated by DLS and TEM. b–d) TEM analysis: b) bulk nanocomposite (fCHI = 0.28); spray-dried microparticles (fCHI = 0.28) before (c) and after (d) calcination (porous replicas).

Multi-scale NMR characterisation of mesostructured materials using 1H→13C through-bond polarisation transfer, fast MAS, and 1H spin diffusion

A NMR method for the characterisation of materials at different length scales, robust and simple to implement, is presented. It combines selection of 1H-13C pairs by a through-bond polarisation transfer (INEPT here) and exploration of larger distances by the introduction of 1H spin diffusion. This characterisation method is well adapted to the highest MAS rates and takes benefits of it. The effect of 1H dephasing on the efficiency of the 1H-->13C through-bond polarisation transfer is determined. This allows consecutively the quantification of signals. Mesostructured spherical silica-based particles containing CTA+ cations were studied by this multi-scale characterisation method. Contrasted spin diffusion curves were found and qualitatively explained by differences in terms of mobility and spatial distributions.

Sol-gel reactions of 3-glycidoxypropyltrimethoxysilane in a highly basic aqueous solution

The reactions of 3-glycidoxypropyltrimethoxysilane in a highly basic aqueous solution have been studied by multinuclear magnetic resonance and light scattering techniques. The study has shown that in this peculiar chemical environment the alkoxy groups of 3-glycidoxypropyltrimethoxysilane undergo a fast hydrolysis and condensation which favor the formation of open hybrid silica cages. The silica condensation reaches 90% at a short aging time but does not go to completion even after 9 days. The highly basic conditions also slow down the opening of the epoxies which fully react only after several days of aging. The epoxy opening generates different chemical species and several reaction pathways have been observed; in particular, the formation of polyethylene oxide chains, diols, termination of the organic chain by methyl ether groups and formation of dioxane species. These reactions are slow and proceed gradually with aging; light scattering analysis has shown that clusters of dimensions lower than 20 nm are formed after two days of reactions, but their further growth is hindered by the highly basic conditions which limit full silica condensation and formation of organic chains.

Spray-dried mesoporous silica microspheres with adjustable textures and pore surfaces homogenously covered by accessible thiol functions

Micrometric mesoporous spheres with textures possessing a relative high degree of ordering and incorporating up to 0.1 thiol function per siloxane unit have been synthesised through sol–gel, self-assembly and—for the first time—spray-drying processes. The sol preparation was optimised as a function of the final properties: morphology, texture, and proportion of thiol functions. A two step synthesis allows the positioning of these functions at the pore surface as proven in particular by 1H solid state NMR. From Ag+ sorption experiments, we have shown that the thiol functions are also homogenously distributed in the spheres volume and fully accessible to external chemical species. So these materials could be used for environmental remediation or metallic nano-particle syntheses. In order to increase the stability and applicability of the materials, different post-synthesis treatments have been studied. A thermal treatment under mild conditions is enough to preserve most of the properties. From the hydrothermal treatments tested, the use of ammonia is shown to be quite interesting for the modulation of the properties. In particular, hierarchical porosities and high specific surface areas have been created. The high degree of ordering observed for the smaller pores is accompanied by a re-ordering of the surfactant polar head groups as deduced from 14N NMR. Lastly, the synthesis can be extended under some conditions to other functions or to higher proportions of thiol functions (up to 0.2).

Organosilicas based on purine–pyrimidinebase pair assemblies: a solid state NMR point of view

Organosilicas based on adenine (A) and thymine (T) assemblies have been synthesized. A surfactant-free route, based on specific molecular recognition between A and T entities, has been developed. The characterization of the H-bond networks, in both homo- and hetero-assemblies, has been emphasized by using 1H solid state NMR (nuclear magnetic resonance). The latest experimental developments were implemented (i.e. very fast MAS (magic angle spinning) experiments at 750 MHz and 33 kHz), in order to enhance drastically the spectral resolution. Moreover, 1H experiments at 67 kHz in 1.3 mm rotors were performed. Spatial connectivities between protons were established by using 1H–1H DQ (double quantum) MAS experiments, allowing the precise characterization of A/A, T/T and A/T associations.

Tunable hierarchical porosity from self-assembled chitin–silica nano-composites

We studied new mesoporous materials with original properties and obtained from self-assembled chitin–silica nano-composites. Our novel synthesis allows the controlled colloidal assembly of α-chitin nanorods (bundles of elongated chitin monocrystals) and siloxane oligomers. Calcination of nano-composites results in mesoporous silica materials. Their pore volume fraction ϕPOR (0–0.52) is strongly correlated to the initial chitin content. Using N2 sorption and TEM data, we identify and characterize primary and secondary textural units related to the imprints of chitin monocrystals (2.5 nm wide) and nanorods (20–30 nm wide) respectively. Primary textural units are preserved over a wide ϕPOR range (linear relationship between pore volume and specific surface area). The coating of monocrystals by siloxane oligomers leads to a siloxane network of fractal nature as deduced from complementary SAXS data. Beyond a critical value ϕPOR′ estimated near 0.2, the coating is partial, and the porosity becomes more open and connected. At larger scales, the arrangements of secondary textural units result in complex textures and long-range ordering, showing similarities with textural features found in natural materials. We discuss the competition between entropy-driven transitions typical of anisotropic particles and kinetic arrest due to colloidal gelation and inorganic condensation. Finally, a schematic model for texture formation is given.

14N and 81Br quadrupolar nuclei as sensitive NMR Probes of n-alkyltrimethylammonium bromide crystal structures. An experimental and theoretical study.

This is the first time a comprehensive study has been carried out on n-alkyltrimethylammonium bromide salts using (14)N and (81)Br solid state NMR, X-ray diffraction, and theoretical calculations. The investigation represents a necessary step toward further (14)N and (81)Br NMR characterization of the environment of cationic and anionic groups in materials, accounting for the amphiphilic properties of cationic surfactants. The NMR spectra of five C(x)H(2x+1)(CH(3))(3)N(+)Br(-) polycrystalline samples with different n-alkyl chain lengths (x = 1, 12, 14, 16, 18) were recorded and modeled. The (14)N and (81)Br quadrupolar coupling interaction parameters (C(Q), eta(Q)) were also estimated from spectrum modeling and from computer simulation. The obtained results were discussed in depth making use of the experimental and reoptimized crystal structures. In the study, both (14)N and (81)Br nuclei were found to be sensitive probes for small structural variations. The parameters which influence the NMR properties the most are mobility, deviation of C-N-C bond angles from T(d) angles, and variations in r(N-Br) distances.

Efficient mesoporous silica–titania catalysts from colloidal self-assembly

Mesoporous silica-titania materials of tunable composition and texture, which present a high catalytic activity in the mild oxidation of sulfur compounds, have been obtained by combining the spray-drying process with the colloidal self-assembly of α-chitin nanorods (biopolymer acting as a template) and organometallic oligomers.

Structural investigation of polydimethylsiloxane–vanadate hybrid materials

The structure of polydimethylsiloxane–vanadate (PDMSV) hybrid materials has been fully investigated. In particular, the use and interpretation of thermogravimetric analysis, IR, Raman and multinuclear NMR (1H, 13C, 17O, 29Si, 51V) spectroscopies allowed an accurate description of their structure on a nanometric scale. The networks are composed of dimethylsiloxane segments with an average size of five to six units linked to monomeric vanadate units VO(OSiMe2..)3. Both units have been identified in cross-linked areas and in end groups. The dissolution and the degradation behaviour of the PDMSV materials are also described.