Thibaud Coradin

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.

Biogenic Silica Patterning: Simple Chemistry or Subtle Biology?

Among biogenic minerals, silica appears rather singular.Whereaswidespreadcarbonateandphosphatesaltsarecrystal-line iono-covalent solids whose precipitation is dictated bysolubilityequilibria,silicaisanamorphousmetaloxide formedbymorecomplexinorganicpolymerizationprocesses.Biogenicsilicahasmainlybeenstudiedwithregardtothediversityofthespecies that achieve this biomineralization process, and at thelevelofdiversityinthemorphologyofsilicastructures.

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.

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.

Silica–alginate composites for microencapsulation

Optimisation of membrane properties of alginate microcapsules is a key factor for the application of microencapsulation techniques to bioartificial organ elaboration. Coacervation and layer-by-layer processes involving additional biopolymers have been extensively studied. Recently, the use of silica as a membrane-forming agent was investigated. This approach was rendered possible by the development of biocompatible routes to silica formation. The composites exhibit enhanced mechanical and thermal stability as well as suitable diffusion properties. Moreover, encapsulated enzymes and cells retain their biological activities. Similarly, silica can be associated to many other biopolymers, opening a promising route for new biocomposites design and biotechnology applications.

Mimicking Biogenic Silica Nanostructures Formation

Biomineralization processes are now fully recognized as inspiring systems for the design of new materials. In the case of silica, the formation of diatom shell or sponge spicule has attracted much attention in the last decade since it could provide key information to elaborate new hierarchically structured materials and nanodevices. In these two examples, the mineral phase is thought to be formed by the controlled assembly of nanoparticles generated in vivo from diluted precursor solutions, in the presence of biomolecular templates. The elucidation of biosilicifica tion processes therefore relies on the understanding of biomolecules capacity to form and structure colloidal silica. Two different approaches have been developed. The first one starts with the extraction and identification of biomolecules present in silicifying organisms and then addresses the in vitro specific activity of these molecules towards silicon species. Alternatively, model macromolecules are used to understand the role of functionality and of structure on silica formation. This review aims at providing a critical overview of the most recent advances in these domains. Relevance for both the understanding of biosilicification process and the design of new bio-inspired nanomaterials are also discussed.

Spectroscopic characterization of biogenic silica

Amorphous biogenic silicas were studied by FTIR and MAS-NMR spectroscopy. Fossil diatom frustules and sponge spicules exhibit a highly condensed and well-organised silica whereas the silica frustule of living diatoms is much less condensed, suggesting that some condensation process still happens upon fossilisation during the diagenetic evolution of silica. Moreover, the silica network of living diatoms appears to be linked to the bio-organic components of the cell, in agreement with the biosilicification mechanisms suggesting that some proteins or polysaccharides favour the formation of silica.

Type I collagen, a versatile liquid crystal biological template for silica structuration from nano- to microscopic scales

Type I collagen is a suitable and versatile template for the structuration of silica at different length scales.

Sol-Gel Biopolymer/Silica Nanocomposites in Biotechnology

Bioencapsulation in silica gels has become a very popular field of research, leading to the design of biosensors and bioreactors. If pure silica gels appear suitable to maintain the biological activity of entrapped enzymes, there are many cases where hybrid materials are necessary to reach the long-term preservation of biomolecular or cellular species and/or to provide new functionalities. This review focuses on the design of such nanocomposite materials combining silica with biopolymers. In the first part, the synthesis and characterization of these bio-hybrid materials are described, emphasizing the importance of the polymer influence on the reactivity of silica precursors. In the second part, the benefits of biopolymer incorporation in silica gels are illustrated in the context of biotechnologi cal devices. As a conclusion, a parallel is drawn between biohybrids and biominerals, opening new perspectives for the design of multi-compone nt biologically-active materials.