Dependence of amine-accelerated silicate condensation on amine structure

Abstract

Diatoms are known to grow elaborate nano- and microstructured silica shells by depositing material from precursor-containing vesicles at mild temperature and pH. Oligo(1-methylazetane) and related moieties, in some cases attached to proteins, are believed to facilitate this process. To complement prior studies of more complex amines, we aim to understand why such a unique structure has evolved through a systematic study of a set of simple model compounds. The degree to which a series of diamines in solution enhances condensation of phosphate-buffered silicic acid at neutral pH increases with increasing alkylation, a factor more important than amine pK_a. This suggests why diatoms often use methylated oligomers. Bis(quaternary ammonium) salts result in even greater reactivity enhancement, constituting a new class of compounds that promote condensation under mild conditions. Methods are presented for incorporation of these new moieties into artificial peptides or other template-forming molecules, which should allow for more effective production of tailored silica nanostructures.