Silaffin primary structure and its effects on the precipitation morphology of titanium dioxide
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Inorganic oxides exhibit numerous applications influenced by particle size and morphology. While industrial methods for forming oxides involve harsh conditions, nature has the ability to form intricate structures of silicon dioxide (silica) using small peptides and polyamines under environmentally friendly conditions. Recent research has demonstrated that these biomaterials will precipitate other inorganic oxides, such as titanium dioxide (titania). Using the diatom-derived R5 peptide, new peptides with systematic changes (e.g., truncation and substitution) in the R5 primary structure were surveyed for reactivities and the impact on the morphology of the titania. The results demonstrated that (i) basic residues are vital to initiating the reaction, and a minimum local concentration is necessary to sustain the precipitation, (ii) residues containing hydroxyl side chains are important to imparting morphological control on the precipitate, and (iii) buffer conditions can dramatically alter both precipitation and morphology.
I. INTRODUCTION
Recent advances in the field of inorganic oxides (e.g., titanium oxide) have led to consideration of these materials for use in diverse applications such as medicine, energy conversion, electronics, and decontamination. The size and morphology of the oxide structures affect the ability of these oxides to perform these varied functions.1–11 Current industrial methods for forming inorganic oxides involve the use of organic solvents, corrosive pH, and/or high temperature and pressure. Though effective at producing the oxide, they provide limited control over morphology.12 In contrast, nature has the ability to form intricate and unique structures of silicon dioxide (silica) in an aqueous environment, at neutral pH and low temperature. For example, the diatom, a single-celled organism, produces intricately designed exoskeletons largely made out of silica, with each subspecies possessing its own unique structure. Mechanistic studies have revealed that the diatom uses small peptides, modified with a polyamine, to form these silica structures. The peptide (silaffin) and the polyamine act as polycationic agents, forming Lewis acid/base complexes with silicic acid. The silicic acid is rapidly hydrolyzed within the complexes, followed by condensing to the silica. Exoskeleton morphology can be
Contributing Editor: Colin Freeman a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.165
controlled in part by varying the composition of the peptide sequence and the chain length of the polyamine.13,14 In laboratory experiments, both the silaffin peptide and polyamines alone have been shown to produce silica.14–16 Using the mechanistic knowledge obtained from studying the diatom, several compounds have been identified as potential bioinspired agents17 for silica precipitation. Block co-polymers, small biomolecules, and polyamines have all demonstrated the ability to form silica in vitro using a mechanism similar to nature, though with greater morphologic
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