Antimicrobial mechanism of cuprous oxide (Cu₂O) coatings

Some very effective antimicrobial coatings exploit copper or cuprous oxide (Cu₂O) as the active agent. The aim of this study is to determine which species is the active antimicrobial - dissolved ions, the Cu₂O solid, or reactive oxygen species. Copper ions were leached from Cu₂O into various solutions and the leachate tested for both dissolved copper and the efficacy in killing Pseudomonas aeruginosa. The concentration of copper species leached from Cu₂O into aqueous solution varied greatly with the composition of the aqueous solution. For a range of solution buffers, killing of P. aeruginosa was highly correlated with the concentration of copper in the leachate. Further, 10 µL bacterial suspension droplets were placed on Cu₂O coatings, with or without a polymer barrier layer, and tested for bacterial kill. Killing occurred without contact between bacterium and solid, demonstrating that contact with Cu₂O is not necessary. We therefore conclude that soluble copper species are the antimicrobial agent, and that the most potent species is Cu⁺. The solid quickly raises and sustains the concentration of soluble copper species near the bacterium. Killing via soluble copper ions rather than contact should allow copper coatings to kill bacteria even when fouled, which is an important practical consideration.