We disclose a colorless composition comprising metal particles (e.g., silver nanoparticles) and water, wherein said particles comprise an interior of elemental metal (e.g., silver) and an exterior of metal oxide (e.g., one or more silver oxide(s)), wherein the metal nanoparticles are present in the water at a level of about 5-40 ppm, and wherein the composition manifests significant antimicrobial properties. Methods of use of the composition are described. The composition can be incorporated into a hydrogel with essentially no loss of antimicrobial properties. Various metal-containing compositions with unexpected biological efficacy are also disclosed.
