Core–shell structured silver-ferrite nanoparticles for antibacterial action and magnetic removal of bacteria from aqueous media

In this work, we synthesized antibacterial core-shell structured magnetic nanoparticles with silver particles as the core and nickel-ferrite or cobalt-ferrite crystallites as the shell using solvothermal method. The prepared ferromagnetic amine-functionalized Ag/NiFe₂O₄ and Ag/CoFe₂O₄ particles had a saturation magnetization (Ms) of 40 emu/g and 37 emu/g, respectively, and were therefore easily separated from aqueous media by magnetic field, thus making them excellent magnetic adsorbents. The produced core-shell particles had mean diameters of 276 ± 70 nm (Ag/CoFe₂O₄) and 337 ± 109 nm (Ag/NiFe₂O₄). Due to their silver content, these particles showed outstanding antibacterial activity against Gram-positive (Micrococcus luteus) and Gram-negative (Escherichia coli) bacteria, which was confirmed by bacteriological studies. For E. coli, a nanoparticle concentration of less than 0.5 mg/mL was sufficient to inhibit growth on solid surfaces, whereas for M. luteus a concentration of 3 mg/mL was effective. This difference in antibacterial efficacy can be explained by differences in the cell wall structure of Gram-negative and Gram-positive bacteria. In addition to being effective in killing bacteria, both types of Ag-containing magnetic nanoparticles also exhibited excellent adsorption capacity, removing 100% of microorganisms from the tested solution. In liquid-phase assays, complete antibacterial and adsorptive efficacy was achieved at 1.0 mg/mL for E. coli with Ag/CoFe₂O₄, while Ag/NiFe₂O₄ reached full efficiency at concentrations as low as 0.1 mg/mL; for M. luteus, both 1.0 and 3.0 mg/mL proved fully effective. Time-kill kinetic tests further confirmed the rapid action of the nanoparticles, with total bacterial elimination occurring within 10 min. This rapid and high removal efficiency is likely attributed to the silver content and positive surface charge of the magnetic particles, which facilitates electrostatic interactions with the negatively charged bacterial membranes. Given their strong antibacterial and adsorption performance demonstrated in our experiments, these nanoparticles are highly promising candidates for water treatment applications in the future.