Naturally derived and synthetic polymers as biomimetic enhancers of silicic acid solubility in (bio)silicification processes

Abstract

Abstract Numerous publications report the existence of intracellular “Si” storage pools in diatoms representing intracellular concentrations of ca. 19–340 mM depending on the species. “Si” storage pools in diatom cells, if present, are supposed to accumulate “Si” for the production of new valves. The accumulated “Si” is then transported into the silicon deposition vesicle (SDV) where the new cell wall is synthesized. Interestingly, the reported concentrations of intracellular “Si” within the storage pool sometimes strongly exceed the solubility of monosilicic acid (ca. 2 mM pH <9). Various types of “Si” storage pools are discussed in the literature. It is usually assumed that “Si” species are stabilized by the association with some kind of organic material such as special proteins, thus forming a soluble silicic acid pools inside the cells. In an effort to mimic the above phenomenon, we have used a variety of neutral or cationic polymers that stabilize two soluble forms of “Si,” silicic and disilicic acids. These polymers include amine-terminated dendrimers, amine-containing linear polymers (with primary, secondary or tertiary amines), organic ammonium polymers, polyethylene glycol (PEG) neutral polymers, co-polymers (containing neutral and cationic parts) and phosphonium end-grafted PEG polymers. All the aforementioned polymeric entities affect the rate of silicic acid polycondensation and also the silica particle growth. Synergistic combinations of cationic and anionic polymers create in situ supramolecular assemblies that can also affect the condensation of silicic acid. Possible mechanisms for their effect on the condensation reaction are presented, with an eye towards their relevance to the “Si pools,” from a bioinspired/biomimetic point of view.