Self-Assembly of Metal Nanoparticles and Nanotubes on Bioengineered Flagella Scaffolds

The previously described FliTrx E. coli flagellin protein was genetically engineered to display rationally designed histidine, arginine-lysine, and aspartic acid-glutamic acid peptide loops on the solvent-accessible outer domain region. The resulting flagellin monomers were self-assembled to obtain the corresponding oligomeric flagella bionanotubes in which the peptide loops were 5 nm apart. These flagella nanotubes were equilibrated with solutions of various metal ions (Co(II), Cu(II), Cd(II), Ag(I), Au(I), and Pd(II)). Controlled reduction of these metal ions yielded ordered arrays of nanoparticles or nanotubes, and in some cases, extensive aggregation resulted in formation of metal nanotube bundles. Both metal nanoparticles and nanotubes were generated with Cu(II) and Au(I), depending on the initial concentration of Cu(II) ions, while Ag(I) consistently formed metal nanowires, even under relatively mild conditions of reduction. The covalent attachment of separately synthesized Au nanoparticles to the flagella scaffold was also demonstrated. Controlled reduction of Co(II), Cd(II), and Pd(II) complexed with histidine and aspartic acid-glutamic acid peptide loops yielded ordered arrays of the respective metal nanoparticles on individual flagella nanotubes with minimal aggregation. The peptide loop modified flagella nanotubes have been demonstrated to be useful scaffolds for the generation of ordered arrays of metal nanoparticles or uniform nanotubes.