Centimeter-scale materials exhibiting solid state-like spin-crossover properties and easy handling under ambient conditions are obtained that preserve their thermochromic behavior over months.

In this third-year undergraduate experiment, a coordination complex [Fe(NH(2)trz)(3)]Br-2 center dot H2O (NH(2)trz is 4-amino-1,2,4-triazole, C2H4N4) was synthesized and characterized by IR and UV-visible spectroscopies. This compound exhibits a spectacular reversible purple-white thermochromic transition upon heating-cooling with a large hysteresis centered at room temperature. The thermochromic transition is related to the spin crossover (SCO) from the low-spin state (LS, purple, S = 0, diamagnetic) to the high-spin state (HS, white, S = 2, paramagnetic). During this experiment, students discovered synthetic methods, such as anion. metathesis and Ostwald ripening, and physical properties, such as magnetic properties, thermochromism, and bistability phenomenon, altogether leading to possible applications in optical-data storage and display.

In order to develop novel electroactive hosts for biosensor design, the possibility to use nanostructured vanadate phases as alternatives to well-known V2O5 gels was studied. For this purpose, the formation of M[V3O8] and Mx[V6O16] (x = 1 and 2) oxides by the sol–gel process has been studied over a wide range of cations (M+ = Li+, Na+, K+, Cs+, and NH4+; M2+ = Ca2+, Mg2+ and Ba2+). By a combination of XRD, 51V NMR and SEM studies, it was possible to evidence the influence of the nature and hydration state of cations on the size and morphology of the resulting particles as well as on the kinetics of their formation. On this basis, K2[V6O16] was evaluated for glucose oxidase encapsulation, either via impregnation or co-precipitation methods. When compared to V2O5, these novel bioelectrodes exhibit higher stability under pH conditions of optimum enzymatic activity, as well as better sensitivity, and reproducibility for glucose detection via amperometric titration.

Inorganic gels formed using the sol-gel process are promising hosts for the encapsulation of living organisms and the design of cell-based biosensors. However, the possibility to use the biological activity of entrapped cells as a biological signal requires a good understanding and careful control of the chemical and physical conditions in which the organisms are placed before, during, and after gel formation, and their impact on cell viability. Moreover, it is important to examine the possible transduction methods that are compatible with sol-gel encapsulated cells. Through an updated presentation of the current knowledge in this field and based on selected examples, this review shows how it has been possible to convert a chemical technology initially developed for the glass industry into a biotechnological tool, with current limitations and promising specificities.

The development of sol-gel based processes for the encapsulation of living cells is a challenging task where the host material must be optimized both in terms of intrinsic properties and impact on cell viability. Here, the mechanical stability of silica hosts obtained via a mixed aqueous route based on sodium silicate-silica nanoparticle mixtures could be improved by increasing the relative content of molecular precursors over colloidal silica. The specific surface area, pore volume and diffusion properties of the host were also modified. Moreover, this resulted in an increase in the ionic strength, inducing high osmolarity that is detrimental to encapsulated Escherichia coli bacteria. Glycine betaine, a well-known E. coli osmoprotector, could be successfully used as an additive to the sol-gel formulation to limit this osmotic stress. Compared to the previously studied glycerol additive, glycine betaine was found more efficient to preserve the bacterial viability. Moreover, this efficiency was obtained with much lower amounts of glycine betaine, whose addition has therefore no detrimental impact on the silica host structure. These data provide new information about the different cellular stresses resulting from the sol-gel encapsulation process and demonstrate the importance of combining chemical and biological approaches to design more robust functional "living'' materials.

A two-step process relying on cell encapsulation in alginate beads followed by inorganic gelation from colloidal metal oxides was successfully applied to the immobilization of Escherichia coli bacteria in the presence of boehmite and zirconium oxyhydroxide particles. In the case of the Al-based gel, the alginate bead obtained at low biopolymer content provides an efficient barrier against the encapsulation stress. In contrast, an increase in the alginate concentration together with the phosphate-induced mineralization of the biopolymer bead is found necessary to maintain the viability of entrapped bacteria in Zr-based gels. Diffusion studies using model molecular and colloidal species put in evidence that positively charged Zr-oligomers and ZrO2 nanoparticles may be involved in the cytotoxicity of the precursor solution. This optimization of the encapsulation process allows the first observation of E. coli growth within such metal oxide-alginate hybrid gels. Results presented in this work give a clear evidence that sol-gel based cell encapsulation can now be envisioned within a wide variety of metal oxide hosts through the optimization of the pre-encapsulation environment.

Two enzymes, lipase and β-galactosidase, have been encapsulated within sol-gel matrices. Enzymatic activity of encapsulated lipase for hydrolysis and trans-esterification reactions is maintained. Encapsulation yields depend not only on the sol-gel porous texture but also on the water amount added for the sol-gel synthesis and the hydratation history of the enzyme. When the water amount is low, the highly active enzyme conformation generated by the phase separation is frozen during gelation. Escherichia Coli have been also encapsulated. The cellular organization appears to be well preserved. Their β-galactosidase activity seems to be better in wet gels but decreases dramatically upon drying.

Zirconia-hydroxyapatite nanocomposites were prepared by sol-gel deposition of zirconium oxide from a zirconium alkoxide in the presence of apatite colloidal suspension under ultrasonication. The material porosity evolves from mainly microporous zirconia to mesoporous hydroxyapatite, with decreasing surface area and increasing pore volume. XRD studies indicate that the apatite phase is well-preserved within the composite materials. The homogeneous dispersion of apatite colloids within the zirconia network was supported by TEM observations and nitrogen sorption measurements. (31)P solid-state NMR studies suggest that partial dissolution of apatite may have occurred during the preparation, leading to the adsorption of phosphate species on zirconia particles. This is confirmed by XRD studies of nanocomposites after thermal treatment that demonstrate the preferred formation of tetragonal over monoclinic ZrO(2) in the presence of hydroxyapatite. In order to investigate the surface properties of these novel materials, the adsorption of Pb(2+), Cr(3+), and Cr(2)O(7)(2-) was evaluated. Metal cations were preferentially adsorbed on apatite-rich composites, whereas Cr(2)O(7)(2-) shows a good affinity for the zirconia-rich phases. Zirconia-apatite materials showed the most promising performance in terms of recyclability. These nanocomposites that combine microporosity, mesoporosity and dual sorption properties for these species appear as interesting materials for metal ion remediation and may also find applications as biomaterials.

Zirconium oxyhydroxide nanoparticles prepared by an aqueous route were evaluated for Escherichia colt bacteria encapsulation. A low viability rate was measured 24 hours after immobilization that could be attributed to nanoparticle cytotoxicity Moreover, the presence of glycerol, a long-term cell-preserving molecule, hindered gel formation, probably due to its adsorption on the nanoparticle surface A comparison with boehmite and ferrihydrite gels previously synthesized following a similar aqueous colloidal route suggests that the generalization of this method will rely on a careful control of the nanoparticle surface reactivity and may require surface chemical modification. To cite this article: M. Amoura et al., C. R. Chimie 13 (2010). (C) 2009 Academic des sciences Published by Elsevier Masson SAS All rights reserved.

AbstractAfter 20 years of research, the inorganic sol–gel process can now be considered as a suitable technological alternative for the encapsulation of living cells. Strategies have been developed and optimized that allow the formation of glasses and ceramics in conditions compatible with the preservation of cell membrane integrity and metabolic activity over several weeks. These biocer(amic)s can be prepared as gels, coatings, films, and capsules, which can host a wide variety of organisms including bacteria, protists, yeasts, and plant and animal cells. It was demonstrated that biocers can be used for the design of bioreactors, biosensors, as well as for bioremediation systems, at least at the laboratory scale. Moreover, hybrid materials associating biopolymers and silica were shown to be suitable for the elaboration of bioartificial organs that have been patented and are currently under clinical evaluation. The biocer technology has now reached its maturity, suggesting that industrial developments can now be foreseen in the near future.

The encapsulation of Escherichia coli bacteria within ferrihydrite gels favours the long-term viability of the entrapped cells while preserving the magnetic properties of the host material.

The control of silica growth by living organisms such as diatoms is known to involve the templating effect of several biomolecules working concomitantly. However, until now, biomimetic studies involving model molecules have mainly been performed with single templates. We show here that the addition of two biopolymers, gelatin and alginic acid, to silicate solutions allows the formation of complex structures resulting from the combined templating effect of both components at different scales. Gelatin is able to activate silica formation resulting in hybrid aggregates at the nanoscale. Alginic acid does not interfere with silica condensation but is able to control silica morphology through the assembly of these gelatin-silica aggregates at the microscale. For all materials, calcination up to 700 degrees C degrades the polymer component of the hybrid material and opens macroporosity in the silica network. In parallel, the high thermal stability of gelatin allows a good preservation of initial silica nanoparticle size upon heating whereas a coarsening process is observed in the sole presence of alginate. These results correlate well with previous models of biosilicification and suggest that the use of multiple templates is a suitable approach to elaborate more complex silica architectures.

A colloidal route to aqueous alumina gels is described, allowing the long-term viability of encapsulated bacteria.

The self-assembly of the polyoxomolybdate [MnMo6O18[(OCH2)3CNHCO(4-C5H4N)]2]3- and [PdCl2(PhCN)2] yields a transparent and birefringent gel.

Organically functionalised hybrid silica fibres with accessible and modifiable functionalities have been reproducibly obtained for the first time through the use of organogelators as structure-directing agents.

The encapsulation of enzymes within silica gels has been extensively studied during the past decade for the design of biosensors and bioreactors1,2,3,4,5,6,7,8,9,10,11. Yeast spores and bacteria have also been recently immobilized within silica gels12,13,14,15,16,17 where they retain their enzymatic activity, but the problem of the long-term viability of whole cells in an inorganic matrix has never been fully addressed. It is a real challenge for the development of sol–gel processes18. Generic tests have been performed to check the viability of Escherichia coli bacteria in silica gels. Surprisingly, more bacteria remain culturable in the gel than in an aqueous suspension. The metabolic activity of the bacteria towards glycolysis decreases slowly, but half of the bacteria are still viable after one month. When confined within a mineral environment, bacteria do not form colonies. The exchange of chemical signals between isolated bacteria rather than aggregates can then be studied, a point that could be very important for 'quorum sensing'19.

Silica polymerisation activation and structure templating using arginine-based surfactants led to bio-inspired multi-scale porous materials.

Low cost materials based on hybrid organic-inorganic sol-gel systems have been developed for use in the trapping and detection of monocyclic aromatic hydrocarbon contaminants emitted into the atmosphere. To this end, two strategies are followed; the first one aims at tailoring the pore size in order to selectively trap and discriminate benzene and toluene. The second is to decrease the pore polarity in order to eliminate the main interfering gas of the atmosphere, water vapor. The results of these strategies are reported in this paper. After describing the synthesis of silicon hybrid xerogels and thin films and the characterisation of their structural properties, we report the study of the local polarity of the pores, of the porosity and optical properties of the materials. These characterisation data will enable us to define the materials suited for hosting benzene and toluene.

The viability of bacteria in the presence of sol-gel reagents has been studied in order to define the best experimental conditions for the sol-gel encapsulation of E. coli. The beta -galactosidase activity of these bacteria, trapped in sol-gel silica matrices, was then analyzed. Two routes, using alkoxide and aqueous precursors, have been used and compared. It appears that the aqueous route is less damaging than the alkoxide one. Moreover the aqueous silica matrix appears to slow down the lysis of cell membranes when bacteria are aged without nutrient.