Bioengineered Bacterial Flagella-Templated in Situ Green Synthesis of Polycrystalline Co₃O₄ Nanowires for Gram-Negative Antibacterial Applications

In recent years, considerable progress has been made on the synthetic chemistry and antibacterial application of tricobalt tetroxide (Co 3 O 4 ) nanomaterials. However, the current approaches to designing and synthesizing Co 3 O 4 nanomaterials are complicated and hard to manipulate. Herein, we developed a one-pot strategy to synthesize Co 3 O 4 nanowires at room temperature with an antibacterial activity. The synthesis process relied on the use of engineered bacterial flagella as a biotemplate, which were genetically modifiable protein nanofibers naturally attached to bacteria for assisting their swimming. We found that the flagella displaying negatively charged peptides (E10 and E20) effectively induced the nucleation of Co 3 O 4 nanoparticles from a cobalt chloride (CoCl 2 ) precursor solution on their surface to form polycrystalline nanowires, with the E20-flagella being more effective than the E10-flagella. However, the wildtype flagella or those displaying neutral (G10) or positively charged (K5) peptides did not effectively induce the Co 3 O 4 nucleation on the flagella. A mechanism investigation discovered that an amorphous phase of Co 3 O 4 was first formed rapidly on the E20-flagella, followed by a crystallization process with both the good crystallinity and nanowire water-dispersibility reached in 2 h. We also found that the E20-flagella formed the Co 3 O 4 nanowires with the good crystallinity at a precursor solution of 1 mM. The E20-flagella-templated Co 3 O 4 nanowires, synthesized using the optimal mineralization conditions (2 h and 1 mM CoCl 2 ), showed the most effective killing of Gram-negative bacteria such as Escherichia coli . This work suggests that flagella with tunable peptide sequences are biotemplates for forming antibacterial nanowires at environmentally benign conditions.