Malic acid as an organic linker for attaching Ag NPs to Fe3O4 nanoclusters: Synergistic enhancement of antimicrobial and antioxidant activities

The study presents a simple and user-friendly one-pot method for fabricating Ag@Fe₃O₄ nanoclusters (NCs) and demonstrates their superior antimicrobial and antioxidant activity. In contrast, Ag NPs and Fe₂O₃ NPs synthesized with ascorbic acid as a reducing agent showed no antimicrobial effectiveness. The fabrication process of Ag@Fe₃O₄ NCs involved linking Ag NPs to Fe₃O₄ NCs by hydrolyzing FeCl₃ to Fe(OH)₃ with NaOH, reducing Fe(OH)₃ to Fe(OH)₂ using malic acid, and then oxidizing Fe(OH)₂ with AgNO₃. In the colloidal solution, malate ions from malic acid served as linkers, connecting Ag NPs to Fe₃O₄ NCs through surface interactions. FESEM images showed smaller spherical Ag NPs attached to clusters of square-shaped Fe₃O₄ particles, while FTIR analysis confirmed the presence of malate ions in the colloidal solution. The antimicrobial activity was assessed against Gram-positive bacteria (B. cereus, MRSA), Gram-negative bacteria (E. coli, P. aeruginosa, S. liquefaciens), and yeasts (C. albicans, C. tropicalis), showing that Ag@Fe₃O₄ NCs effectively eliminated both bacteria and fungi. MIC and growth curve investigations showed that even at very low concentrations, the individual components of Ag@Fe₃O₄ NCs (Ag NPs and Fe₃O₄ NCs) effectively synergize to inhibit bacterial growth. Additionally, the DPPH assay revealed that the antioxidant efficacy of Ag@Fe₃O₄ NCs was also enhanced through this synergistic interaction. This combined effect is attributed to the external force exerted by Fe₃O₄ NCs on the membrane, disrupting the cell wall and facilitating the entry of Ag NPs into the cell interior. Therefore, attaching Ag NPs to Fe₃O₄ NCs in a colloidal solution represents a novel approach for optimizing both antimicrobial and antioxidant properties.