Considerable research has been directed toward identifying the mechanisms involved in biosilicification to understand and possibly mimic the process for the production of superior silica-based materials while simultaneously minimizing pollution and energy costs. Molecules isolated from diatoms and, most recently sponges, thought to be key to this process contain polyamines with a propylamine backbone and variable levels of methylation. In a chemical approach to understanding the role of amine (especially propylamine) structures in silicification we have explored three key structural features: (i) the degree of polymerization, (ii) the level of amine methylation, and (iii) the size of the amine chain spacers. In this article, we show that there are two factors critical to their function: the ability of the amines to produce microemulsions and the presence of charged and uncharged amine groups within a molecule, with the latter feature helping to catalyze silicic acid condensation by a proton donor/acceptor mechanism. The understanding of amine-silicate interactions obtained from this study has enabled the controlled preparation of hollow and nonporous siliceous materials under mild conditions (circumneutral pH, room temperature, and in all aqueous systems) possibly compatible with the conditions used by biosystems. The ''rules'' identified from our study were further used predictively to modulate the activity of a given amine. We believe that the outcomes of the present contribution will form the basis for an approach to controlling the growth of inorganic materials by using tailor-made organic molecules.bioinspired materials ͉ biosilica ͉ hollow particles T he commercialization of siliceous materials has generated a multibillion pound industry with the value of precipitated silica products alone estimated to have reached 1.4 billion pounds by the year 2006 (1). This value largely depends on the ability to produce silica for specific applications through control of surface chemistry and morphology during the condensation and aggregation process. Industrial processes involved in the production of high-value siliceous materials typically involve harsh conditions of high temperature and acidity or basicity, environmentally damaging waste streams, and often toxic silicate precursors. These processes also generally exert poor chemical and morphological control over the materials produced. In comparison, natural silica production in organisms such as sponges, diatoms, and radiolaria occurs under biologically benign conditions and produces silica of exquisite form and function, all from a monosilicic acid source of only a few parts per million in concentration (2). Considerable research has been directed toward identifying the mechanisms involved in the biosilicification process to understand and possibly mimic the process to allow us to produce superior materials without the attendant pollution and high-energy usage (3, 4).Studies of the processes associated with the formation of siliceous diatom shells (thecae) have i...
The role of polymer (poly(vinylamine)) size (238-11000 units) on silicic acid condensation to yield soluble nanoparticles or composite precipitates has been explored by a combination of light scattering (static and dynamic), laser ablation combined with aerosol spectrometry, IR spectroscopy, and electron microscopy. Soluble nanoparticles or composite precipitates are formed according to the degree of polymerization of the organic polymer and pH. Nanoparticles prepared in the presence of the highest molecular weight polymers have core-shell like structures with dense silica cores. Composite particles formed in the presence of polymers with extent of polymerization below 1000 consist of associates of several polymer-silica nanoparticles. The mechanism of stabilization of the "soluble" silica particles in the tens of nanometer size range involves cooperative interactions with the polymer chains which varies according to chain length and pH. An example of the use of such polymer-poly(silicic acid) nanoparticles in the generation of composite polymeric materials is presented. The results obtained have relevance to the biomimetic design of new composite materials based on silica and polymers and to increasing our understanding of how silica may be manipulated (stored) in the biological environment prior to the formation of stable mineralized structures. We suspect that a similar method of storing silicic acid in an active state is used in silicifying organisms, at least in diatom algae.
A new method for the stepwise synthesis of propylamines containing fragments of N-methyl propylamine as found in diatom bioextracts is presented and their activity in silicic acid condensation is described.
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