Extracellular Bacterial Synthesis of Protein-Functionalized Ferromagnetic Co₃O₄ Nanocrystals and Imaging of Self-Organization of Bacterial Cells under Stress after Exposure to Metal Ions

Co 3 O 4, which crystallizes in the spinel phase at temperatures much higher than ambient temperatures, orders antiferromagnetically below its Neel temperature (∼40 K). However, in nanosize, it shows ferromagnetic ordering due to surface canting, disorder, imperfect oxygen atom coordination, etc. Here, for the first time, we report the synthesis of single-crystalline, ferromagnetic Co 3 O 4 nanoparticles functionalized with proteins (size 5–7 nm) using cobalt acetate as precursor at room temperature in aqueous medium by a metal-tolerant marine bacterium isolated from the coast of the Arabian Sea. Energetically unfavorable change in the oxidation state of Co atoms from (+2 to +3) during synthesis by the bacterial strain was evidenced by X-ray photoelectron spectroscopy. The effect on the morphology of bacterial cells after exposure to the cobalt acetate was imaged by scanning electron microscopy showing cooperative, self-organized, structured colony formation, possibly due to the interbacterial communication under external stress.