We describe a simple one-pot thermal decomposition method for the production of a stable colloidal suspension of narrowly dispersed superparamagnetic Fe₃O₄-Ag core-shell nanostructures. These biocompatible nanostructures are highly toxic to microorganisms. Antimicrobial activity studies were carried out on both Gram negative (Escherichia coli and Proteus vulgaris) and Gram positive (Bacillus megaterium and Staphylococcus aureus) bacterial strains. Efforts have been made to understand the underlying molecular mechanism of such antibacterial actions. The effect of the core-shell nanostructures on Gram negative strains was found to be better than that observed for silver nanoparticles. The minimum inhibitory concentration (MIC) values of these nanostructures were found to be considerably lower than those of commercially available antibiotics. We attribute this enhanced antibacterial effect of the nanostructures to their stability as a colloid in the medium, which modulates the phosphotyrosine profile of the bacterial proteins and arrests bacterial growth. We also demonstrate that these core-shell nanostructures can be removed from the medium by means of an external magnetic field which provides a mechanism to prevent uncontrolled waste disposal of these potentially hazardous nanostructures.

A temperature-sensitive magnetic fluid with a low Curie temperature is synthesized. The fluid is characterized by x-ray diffraction, magnetization, and small angle neutron scattering (SANS). Low Curie temperature of the fluid enables us to separate out the nuclear and magnetic scattering contributions in SANS by recording the SANS pattern at two temperatures.