Nitroxide-coated silver nanoparticles: synthesis, surface physicochemistry and antibacterial activity

In this paper, we describe the facile synthesis and physicochemical characteristics of nitroxide-coated silver nanoparticles. The proposed procedure allows us to obtain isolatable, devoid of Ag+ impurities, long-term stable, spherical nanoparticles with an average diameter ca. 7 nm, which exhibit high antibacterial activity towards both Gram-negative and Gram-positive strains. The determined Minimum Bactericidal Concentrations (MBCs) are significantly lower than the values reported for other thiolate-capped silver nanoparticles and range from 4 mu g ml(-1) (against Pseudomonas aeruginosa) to 12 mu g ml(-1) (against Staphylococcus aureus). Our studies proved that the nitroxide coverage favours the antibacterial activity of silver nanoparticles, probably due to the ability of nitroxides to be oxidized by reactive oxygen species (ROS) to positively charged oxoammonium ions which can interact strongly with the bacterial membrane. Furthermore, the mechanism of chemisorption of disulphide bisnitroxide on a silver surface has been discussed on the basis of XPS, FTIR and ESR results.