Origin of precious opal revisited: Possible quick formation of precious opal

It has been well known that a sedimentary precious opal is composed of closely packed uni-size silica spheres with voids filled by an air or water, and that an interference among reflections from the boundaries of those spheres and filler yields play-of-colour (iridescence). So, occurrence of a play-of-colour means occurrence of natural selection in size of spheres, or suppression of further nucleation after initial outburst of spheres, during its formation process. We had been exploring the possibility if we can regard a Stöber process as an analogue of the formation process of precious opal. The key is the reason why variation in size is rarely found on both precious opal and Stöber colloid. To give a clue, we examined the internal structure of Stöber particles and how those particles were formed at very initial stage of the process. The answers for evenness in shape and size are a quick supersaturation of reactive silica species, consecutive formation of large and loose polymers by fast dehydration, and their quick aggregation as the initial burst of silica spheres in highly diffusive medium. These can be achieved in nature by quick but continuous decrease in temperature on “basic” (high pH) geothermal hot water moving upward through cracks in rocks. Sedimentary precious opal can thus be formed when such naturally occurring colloid is filtered by a permeable bed.

Effect of Calcium Precursor on the Bioactivity and Biocompatibility of Sol-Gel-Derived Glasses

This study investigated the impact of different calcium reagents on the morphology, composition, bioactivity and biocompatibility of two-component (CaO-SiO2) glasses produced by the Stöber process with respect to their potential application in guided tissue regeneration (GTR) membranes for periodontal repair. The properties of the binary glasses were compared with those of pure silica Stöber particles. The direct addition of calcium chloride (CC), calcium nitrate (CN), calcium methoxide (CM) or calcium ethoxide (CE) at 5 mol % with respect to tetraethyl orthosilicate in the reagent mixture gave rise to textured, micron-sized aggregates rather than monodispersed ~500 nm spheres obtained from the pure silica Stöber synthesis. The broadening of the Si-O-Si band at ~1100 cm−1 in the infrared spectra of the calcium-doped glasses indicated that the silicate network was depolymerised by the incorporation of Ca2+ ions and energy dispersive X-ray analysis revealed that, in all cases, the Ca:Si ratios were significantly lower than the nominal value of 0.05. The distribution of Ca2+ ions was also found to be highly inhomogeneous in the methoxide-derived glass. All samples released soluble silica species on exposure to simulated body fluid, although only calcium-doped glasses exhibited in vitro bioactivity via the formation of hydroxyapatite. The biocompatibilities of model chitosan-glass GTR membranes were assessed using human MG63 osteosarcoma cells and were found to be of the order: CN < pure silica ≈ CC << CM ≈ CE. Calcium nitrate is the most commonly reported precursor for the sol-gel synthesis of bioactive glasses; however, the incomplete removal of nitrate ions during washing compromised the cytocompatibility of the resulting glass. The superior bioactivity and biocompatibility of the alkoxide-derived glasses is attributed to their ease of dissolution and lack of residual toxic anions. Overall, calcium ethoxide was found to be the preferred precursor with respect to extent of calcium-incorporation, homogeneity, bioactivity and biocompatibility.

Imprinting of Stöber particles for chirally-resolved adsorption of target monosaccharides and disaccharides

Soft imprinting of silica particles using sugar surfactants targets chirally resolved binding of saccharides.

Porosity of silica Stöber particles determined by spin-echo small angle neutron scattering

Open and closed volume fractions determined for Stöber silica in deuterated and hydrogenous solvents.