Jacques Livage

Chemical modification of alkoxide precursors

Abstract The chemical reactivity of metal alkoxides offers a broad range of possibilities for chemical modification of these molecular precursors. The whole hydrolysis-condensation process may then be completely different leading to new products. An analysis is presented concerning some of the most common chemical additives used in the sol-gel process. Their role is explained in terms of chemical reactivity. The most important parameters appear to be the reactivity of the new ligand towards hydrolysis, the charge distribution in the new molecular precursor and the coordination numbers of the metal atom.

Recent bio-applications of sol-gel materials

This review is devoted to the most recent developments (2000–2005) of sol–gel materials at the interface with biology. In the context of bioencapsulation in mineral hosts, novel synthetic approaches have been designed, allowing the immobilization of numerous proteins, enzymes and immune molecules as well as poly-saccharides, phospholipids and nucleic acids. These efforts have led to the development of new biosensors and bioreactors. A similar trend was also observed for whole cell encapsulation, survival periods over several weeks now being achieved. This has opened the possibility of designing hybrid hosts for cell-based biosensing and bioproduction, ultimately allowing the development of artificial organs. Indeed, applications of sol–gel processes are not restricted to bioencapsulation, as demonstrated by recent progress in drug release systems and bioactive materials. Finally, the considerable efforts devoted to the biomimetic elaboration of mineral structures suggest that they might be the key for future development of improved sol–gel materials for bio-applications.

Hydrolysis of titanium alkoxides: modification of the molecular precursor by acetic acid

Monolithic TiO 2 gels can be reproducibly obtained when the hydrolysis of titanium alkoxides is performed in the presence of acetic acid. This carboxilic acid does not act only as an acid catalyst, but also as a ligand and changes the alkoxide precursor at a molecular level therefore modifying the whole hydrolysis condensation process. Infra-red experiments show that bidentate acetates replace OR groups and are directly bounded to the titanium. Both, chelating and bridging acetates, are observed, leading to Ti(OR) x (Ac) y . oligomers. Hydrolysis of this new molecular precursor removes first (OR) groups and bridging acetates. Chelating acetates are still observed in the gel. They can only be removed upon heating above 200 °C.

Sol-gel chemistry

Sol-gel chemistry involves nucleophilic reactions. The chemical reactivity of metal alkoxides toward hydrolysis, condensation and complexation mainly depends on the electronegativity of the metal atom, its coordination number and the steric hindrance of alkoxide groups. Silicon alkoxides are poorly reactive. Hydrolysis and condensation rates have to be enhanced by acid and base catalysis or nucleophilic activation. Transition metal alkoxides are usually too reactive. They have to be stabilized by complexation in order to avoid fast condensation. The molecular design of alkoxide precursors opens the way to tailor-made materials.

Encapsulation of biomolecules in silica gels

A wide variety of biomolecules, ranging over proteins, enzymes, antibodies and even whole cells, have been embedded within sol-gel glasses. They retain their bioactivity and remain accessible to external reagents by diffusion through the porous silica. Sol-gel glasses can be cast into desired shapes and are optically transparent, so it is possible to couple optics and bioactivity to make photonic devices and biosensors. The high specificity and sensitivity of enzymes and antibodies allows the detection of traces of chemicals. Entrapped living cells can be used for the production of metabolites, the realization of immunoassays and even for cell transplantation.

Layered structure of vanadium pentoxide gels

Abstract This paper gives evidence for a layered structure of vanadium pentoxide gels, analogous to that of montmorillonites. In such a gel, V 2 O 5 layers are formed by cross-linked fibers and separated by water molecules. The interlayer spacing depends on the water content. It discontinuously decreases by steps of 2.8 A when water is removed. Interlayer spacings of 11.55 A and 8.75 A have been measured for V 2 O 5 , 1.6 H 2 O and V 2 O 5 , 0.5 H 2 O respectively. The structure of the fibers themselves is not modified during the swelling process and can be presumably related to the lamellar structure of orthorhombic vanadium pentoxide.

Synthesis of polyoxovanadates via “chimie douce”

Abstract A large variety of new polyoxovanadates have been synthesized during the past few years by sol–gel chemistry or hydrothermal methods. These wet chemistry methods offer many advantages compared to the usual solid state syntheses. New open structures have been obtained from aqueous precursors. They result from the self-assembling of ionic species in the solution. Vanadium oxide gels and sols, V 2 O 5 · n H 2 O, are formed around the point of zero charge (pH≈2). They have a ribbon-like structure and exhibit a liquid crystal behavior. These mesophases are similar to those currently observed with nematic polymers. Xerogel layers deposited from V 2 O 5 · n H 2 O gels exhibit some preferred orientation and behave as versatile host structures for intercalation giving new hybrid organic–inorganic nanocomposites. Layered structures are formed around pH≈7 in the presence of large organic cations. They are built of mixed valence polyoxovanadate planes made of [VO 5 ] pyramids and [VO 4 ] tetrahedra. Organic cations lie between the oxide layers where they interact with the negative oxygen of the VO double bonds. Anions can behave as templating agents. Hollow cluster shells are formed around anions that remain encapsulated within the negatively charged polyvanadate cage. Large cations only behave as counter ions for the formation of a neutral crystalline network. It appears that the molecular structure of V V precursors depends mainly on pH, but the way they self-assemble may be governed by other ionic species in the solution.

Vanadium pentoxide gels:: I. Structural study by electron diffraction

Abstract Vanadium pentoxide gels consist of entangled polymeric fibers. Electron diffraction experiments have shown that these fibers are actually arranged as flat ribbons about 103 A long, 102 A wide, and 10 A thick. A two-dimensional order is observed along these ribbons which does not vary upon swelling. The two-dimensional cell parameters a = 27.0 A and b = 3.6A suggest that the organization within the fibers is closely related to the lamellar structure of orthorhombic V2O5. The fiber is made of basic blocks containing 10 vanadium atoms along the a direction. These blocks seem to be linked together by water molecules which are strongly bonded to the structure and cannot be removed without crystallization of the xerogel into orthorhombic V2O5.

Sol–gel electrochromic coatings and devices: A review

A brief updated review is made on sol-gel-derived electrochromic films (some of which used as ion storage films) of different chemical systems. Performances of selected films measured in electrochemical cells or in devices are discussed and the degradation problems experienced by different authors enumerated.

Effect of some amino acids and peptides on silicic acid polymerization

The polymerization of silicic acid in aqueous solutions at different pH was followed by the colorimetric molybdosilicate method. The role of four amino acids (serine, lysine, proline and aspartic acid) and the corresponding homopeptides was studied. All four amino acids behave the same way and favor the condensation of silicic acid. Peptides exhibit a stronger catalytic effect than amino acids but they appear to behave in very different ways depending on the nature of side-groups and pH. Poly-lysine and poly-proline for instance lead to the precipitation of solid phases containing both silica and peptides. The role of these biomolecules on the polymerization of silicic acid is discussed in terms of electrostatic interactions, hydrogen bonds and solubility.