Brittle bacteria: A biomimetic approach to the formation of fibrous composite materials

Organized bacterial superstructures have been used as 3-D templates for the fabrication of ordered inorganic-organic fibrous composites. Preformed magnetic (Fe3O4) and semiconducting (CdS) inorganic nanoparticles were incorporated into macroscopic threads of Bacillus subtilis by reversible swelling of the superstructure in colloidal sols. The air-dried mineralized fibers consisted of a closely packed array of 0.5 mu m diameter multicellular bacterial filaments, each of which was coated with a 30-70 nm thick layer of aggregated colloidal particles. Inorganic patterning of the interfilament spaces was influenced by the surface charge of the nanoparticles used. Whereas negatively charged magnetite colloids gave good infiltration and replication of the bacterial superstructure, the neutral-ligand-capped CdS colloid, although internalized to some extent, was preferentially localized at the surface of the thread. Positively charged sols of TiO2, in contrast, did not penetrate the swollen fiber but produced coherent surface coatings of uniform thickness. Attempts to pattern the deposition of CdS using molecular precursors by exposing a Cd(II)-containing bacterial fiber to H2S gas produced an uneven surface coat of CdS particles. Removal of the bacterial component from the magnetic composite by heating at elevated temperatures resulted in structural collapse.