Morphology control of TiO2 nanoparticles in biomimetic synthesis using affinity peptide

Peptide aptamers that recognize inorganic materials are attracting increasing attention in the field of nanobiotechnology. This is because these peptides can be used not only to allocate various molecules on patterned substrates but also to form nanocrystals of inorganic materials under ambient conditions.1 TiO2 nanoparticles are widely used for many applications including photo-catalysis, photo-voltaics, energy storage and so on. The peptide-directed room-temperature formation of TiO2 nanoparticles can be an attractive alternative to higher-temperature synthetic methods.2 However, the underpinning mechanism for peptide driven biomimetic TiO2 synthesis remains still unclear and it has a long way to go before producing crystalline TiO2 with controlled nanostructure conducive to practical device applications. In the present study, we used STB1 (-CHKKPSKSC-, a constraint heptapeptide cognitive of TiO2) and its derivatives (i.e. point mutants of STB1 and STB1 fused with the tail peptide sequence of R5 reported elsewhre3) to explore a strategy for controlling the morphology (i.e. size and shape) of TiO2 precipitate in various conditions (e.g. peptide concentration, buffer composition, pH of buffer solution). Furthermore, the effect of peptide local structure (i.e. linear vs constraint form) on the morphology of biomineralized TiO2 precipitate was investigated. All the nanoparticles synthesized were characterized by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). It was found that kinetics of TiO2 biomineralization is a key to determine the size of TiO2 nanoparticles: faster kinetics give rise to smaller nanoparticles. However, the kinetics of TiO2 particle formation exhibited little effect on the shape of particles, hence resulting in spherical nanoparticles with interconnected one another. Interestingly, the local structure of peptide molecules was found to affect the shape of TiO2 precipitate, giving rise to spherical nanoparticles with the use of constrained peptide sequences and planar interconnected precipitates with linear form.