(131a) Silica Particle Formation Using a Biomimetic Catalyst within Confined Environments

In nature, marine organisms such as diatoms produce biosilica that is essential for the formation of their architecturally diverse and complex shells. The remarkable control that diatoms exhibit during the creation of their morphologically ornate structures at nanoscales has attracted a great deal of interest. Consequently, researchers are inspired to study the mechanisms that allow these organisms to form structural features that far exceed the capabilities of present day nanotechnology. These marine organisms have drawn closer attention because they can create these three-dimensional nanostructures at ambient temperatures and near neutral pH; as opposed to the present-day processes which require harsher conditions: extreme pressures, elevated temperatures, and strong acid or alkaline chemicals to produce simple silicates [1]. Diatoms are a group of unicellular brown algae that uptake silicic acid from their environments for processing into ornate silica shells, known as frustules. The shell creation is regulated by templating biomolecules the first set of these macromolecules are polycationic polypeptides named silaffins [2, 3] and the second are long-chain polyamines [4, 5].

We present a biomimetic approach, inspired by diatoms, to create synthetically derived silica-based particles by confining the reaction environment using reverse micelles. Before investigating the role of confinement, initial bulk silica studies showed that (1mg/ml) of a synthetically derived peptide (R5H), similar to that of the native silaffins found in diatoms, reacted with a 25mM monosilicic acid (Si(OH)4) catalyzed a silica reaction within 30 seconds as determined using a molybdate assay; whereas the 25mM monosilicic acid without the peptide took on the order or 30 minutes to begin to autopolymerize. Similarly, scanning electron microscope (SEM) images taken on samples allowed to react for 5 minutes showed silica material on the samples that were reacted with the peptide and no silica material for the samples where no peptide was used. The material observed on the samples was confirmed to be silica using an energy dispersive x-ray spectrometer (EDS).

To investigate the role of confinement, we examined the incorporation of a (1mg/ml) R5H peptide and 25mM Si(OH)4 monosilicic acid, which was pH adjusted to 7.4, into spherical reverse micelles formed by 100mM bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) in isooctane. We determined that the high salt concentrations found inside the anionic micelle provided an environment where the monosilicic acid polymerized without the need of the peptide. Hence a high salt environment would cause silica precipitation of the silicic acid solution. To study the dependence of salt concentration on silica precipitation we demonstrated, in bulk studies, that 2M and 4M sodium chloride reacted with 25mM silicic acid to form silica particles which we characterized using a molybdate assay, SEM and EDS. Since AOT micelles did not favor our studies we decided to use the nonionic surfactant polyoxyethylene (10) tert-octylphenyl ether (Triton X-100) in cyclohexane/1-hexanol as a means for bioinspired, controlled silica growth in a confined environment. By varying the size of the water pool from Wo = 5, 10, and 15, where (Wo = [H2O]/[TritonX-100]), micelles with hydrodynamic radius of (1.7 ± 0.017)nm up to (7.4 ± 0.015)nm were measured using dynamic light scattering and the morphologies of the resultant silica were studies using SEM. We also demonstrated in the Triton X-100 micelles that both the R5H and sodium chloride would catalyze the precipitation of silica when reacted with monosilicic acid.

Using diatoms as inspiration, we demonstrated the role of confinement by reacting the peptide, or sodium chloride with monosilicic acid to form silica particles, by combing template-directed synthesis and biomimetic chemistry. This technique opens the possibility of tailoring materials specific to their intended applications such as bioseparation filters, high surface area catalytic supports, size-exclusion chromatography, and potentially three-dimensional metamaterials for photonics applications.

[1]. Barrer, R.M., Hydrothermal Chemistry of Zeolites. London: Academic Press, 1982

[2]. Kroger, N., et al., ?Polycationic Peptides from Diatom Biosilica That Direct Silica Nanosphere Formation?. Science. 286(5442), p. 1129-1132, 1999.

[3]. Kroger, N., et al., ?Silica-precipitating Peptides from Diatoms. THE CHEMICAL STRUCTURE OF SILAFFIN-1A FROM CYLINDROTHECA FUSIFORMIS?. J. Biol. Chem. 276(28), p. 26066-26070, 2001

[4]. Sumper, M., ?A Phase Separation Model for the Nanopatterning of Diatom Biosilica?. Science. 295(5564), p. 2430-2433, 2002.

[5]. Sumper, M., ?Biomimetic Patterning of Silica by Long-Chain Polyamines?. Angew. Chem. Int. Ed. Engl. 43(17), p. 2251-2254, 2004.