Silver distribution and binding mode as key determinants of the antimicrobial performance of iron oxide/silver nanocomposites

Abstract

The emerging threat of bacterial resistance to antibiotics prompts the urgent search for biomaterials for the treatment of infectious disease. Here we report on the synthesis and characterization of a multiphasic nanocomposite comprising magnetic iron oxide and silver nanoparticles. The method of synthesis involved the combustion of a metalorganic complex and reduction of the silver ions that were exchanged and/or adsorbed on the surface of iron oxide. Different physical and chemical treatments coupled to the reduction process, including ultrasound and Lugol's iodine solution, respectively, homogenized the distribution of the silver nanoparticles on the iron oxide phase. Remarkably, using ascorbic acid as a reductant enhanced the magnetic properties of the material as a result of the reduction of the magnetic matrix alongside the silver cations. The treatment with ultrasound detached large amounts of silver from the iron oxide phase and resulted in the lowest amount of silver incorporated in the nanocomposite. Despite that, this treatment led to the highest antibacterial activity against both Gram-positive and Gram-negative strains, indicating that the homogeneity of the distribution of silver on the iron oxide matrix is a more important determinant of the antibacterial performance than the amount of silver incorporated in the material. At the same time, the treatment with Lugol's iodine equally increased the distribution homogeneity, but induced excessive ion exchange and crystal lattice substitutions, thereby adversely affecting the antibacterial performance. This has indicated that the mode of binding silver to iron oxide can compensate for the positive effects of homogeneous distribution with respect to the antibacterial performance.