Cécile Roux

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.

A sol–gel matrix to preserve the viability of encapsulated bacteria

E. coli bacteria were encapsulated within silica gels and aged at room temperature in the absence of nutrients. Their viability was studied as a function of time using different viability tests. The plate count technique gives the number of culturable bacteria that remain able to form colonies in the presence of a culture medium. Their metabolic activity toward glycolysis was followed by 14C titration and 13C NMR spectroscopy. Several sol–gel matrices were tested in order to improve the viability of the trapped bacteria. The best results were obtained when encapsulation is performed in the presence of glycerol showing that almost 50% of the bacteria were still able to form metabolites after one month of ageing. Moreover, this study demonstrates that a wide range of viability tests can be adapted for use with cells encapsulated in mineral matrices.

Sol–gel encapsulation of bacteria: a comparison between alkoxide and aqueous routes

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 β-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.

One-pot synthesis of phenyl- and amine-functionalized silica fibers through the use of anthracenic and phenazinic organogelators

In a typical one-pot synthesis procedure, hybrid organosilica fibers were grown by templating the co-condensation between TEOS and selected organotrialkoxysilanes (20 mol% of phenyltriethoxysilane, PTES, or aminopropyltriethoxysilane, APTES) through the use of DDOA (2,3-bis(n-decyloxy)anthracene) and DUOP (2,3-bis(n-undecyloxy)phenazine) organogelators in a compatible solvent (ethanol or acetonitrile, respectively). Interestingly, the successful replication of organogelator fibrous assemblies was accomplished working at low pH values (ca. 2–3) in acid catalysed conditions. By SEM and TEM characterizations the hybrid organosilicas were seen to consist of submicronic fibers (100–200 nm) aggregated into highly anisotropic fibrous bundles (1–15 µm thick). In addition, molecular and/or supramolecular interactions between the organogelator and the growing organosiloxane network proved to have an important effect on fiber thickness and on fiber aggregation (intertwined/co-aligned, lamellar- or ribbon-like fibrous bundles). After removal of the organogelator (by calcination or by Soxhlet washing), the fibrous morphologies were perfectly conserved, and the presence of phenyl and amine moieties was confirmed by FTIR spectroscopy and also by elemental chemical analysis. The formation of a well-condensed and interconnected organosiloxane network was proven by 29Si MAS NMR spectroscopy. Moreover, the accessibility and reactivity of the grafted amine groups were also corroborated by performing a simple post-functionalization heterogeneous reaction with diluted benzaldehyde.

Sol–gel synthesis of oxide materials

The sol–gel process is based on the hydrolysis and condensation of molecular precursors. The chemical design of these precursors provide an interesting tool to control condensation reactions and tailor the nanostructure of the oxide materials. The condensation of vanadic acid in aqueous solutions gives lyotropic nematic sols or gels that lead to anisotropic vanadium oxide layers when deposited onto a flat substrate. These oriented layers exhibit improved electrochemical properties as cathode materials. Amorphous oxopolymers are obtained via the controlled hydrolysis of vanadium alkoxides. They can be easily reduced into VO2 thin films that exhibit highly reversible thermochromic behavior. The chemically controlled condensation of zirconium alkoxides leads to stable colloidal solutions of monodispersed zirconia nanoparticles. The mild conditions associated with sol–gel chemistry allow the encapsulation of biomolecules within a silica glass. Even whole cell organisms such as protozoa can be encapsulated. Their cellular organization and antigenic properties are preserved and they can be used for immunoassays.

Bacteria quorum sensing in silica matrices

Serratia marcescens bacteria were encapsulated in silica gels containing glycerol. In agreement with previous studies on Escherichia coli, entrapped cells showed a ca. 50% viability rate after one month. Nutrients were provided to the bacteria, allowing the production of prodigiosin, a red pigment exhibiting some promising therapeutic properties. Addition of “quorum sensing” molecules involved in intercellular communication leads to an enhanced prodigiosin production after four subsequent recyclings of the bacteria-containing gels over one month. Moreover, at the end of this period, nearly 100% of the initial cell population remain viable within the gels. These results suggest that, in the presence of “quorum sensing” molecules, S. marcescens bacteria can enter a stationary state where their metabolism is modified, enhancing their resistance to the stresses induced by encapsulation.

Immunoassays in sol-gel matrices

Parasites have been encapsulated within sol-gel silica matrices. Thermoporometry measurements show that the pore size depends strongly on experimental conditions. Pores as large as 100 Å in diameter can be obtained, allowing the diffusion of large biomolecules such as immunoglobulins. TEM observations, performed on procaryote (bacteria) and eucaryote (protozoa) specimen show that the cellular organization and the integrity of the plasma membrane of entrapped parasites are preserved. Moreover they retain their antigenic activity and can react specifically with the corresponding antibodies. Sol-gel matrices have then been used for the realization of enzyme-linked immunosorbent assays (ELISA) directly with the sera of infected patients. Two examples are described, one with parasitic protozoa, Leishmania, and the other one with the cystic hydatid stage of tapeworm parasites, Echinococcus granulosus.

How to design cell-based biosensors using the sol–gel process

AbstractInorganic 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. FigureOptical fluorescence image of living bacteria in silica thin films

Viability of Bacteria in Hybrid Aqueous Silica Gels

Whole E. coli bacteria have been trapped within silica gels obtained via the acidification of sodium silicate and silica nanoparticles solutions. Their β-galactosidase enzymatic activity increases with time, suggesting that their membrane is partially lysed during the encapsulation process. Such a lysis can be greatly reduced when encapsulation is performed in the presence of gelatin. The biocatalytic activity of trapped bacteria remains almost constant for more than a week. Moreover bacteria trapped in such gels remain able to incorporate glucose, showing that their viability has been preserved.

Pyridine and phenol removal using natural and synthetic apatites as low cost sorbents: Influence of porosity and surface interactions

A natural phosphate rock and two synthetic mesoporous hydroxyapatites were evaluated for the removal of pyridine and phenol from aqueous solutions. Experiments performed by the batch method showed that the sorption process occurs by a first order reaction for both pyridine and phenol. In contrast, the Freundlich model was able to describe sorption isotherms for phenol but not for pyridine. In parallel, the three apatites exhibit similar pyridine sorption capacities whereas phenol loading was in agreement with their respective specific surface area. This was attributed to the strong interaction arising between pyridine and apatite surface that hinders further inter-particular diffusion. This study suggests that, despite its low specific surface area, natural phosphate rock may be used as an efficient sorbent material for specific organic pollutants, with comparable efficiency and lower processing costs than some activated carbons.