Credit: © 2009 Wiley

Silver nanoparticles are known for their ability to kill a variety of bacteria and are used in wound dressings, textiles and various biomedical devices. Researchers at the University of New South Wales now report that it is possible to 'tune' the antimicrobial activity of nanosilver by simply irradiating it with light.

Rose Amal and co-workers1 dispersed nanosilver on a semiconducting titanium-dioxide support using conventional techniques, and tested its ability to kill E. coli bacteria. Concentrations greater than 50 mg L−1 of the nanosilver/titanium-dioxide material completely suppressed the growth of the bacteria over 6 h. Materials that were pre-irradiated with ultraviolet A (UVA) radiation allowed the bacteria to grow, and with an 80 h pre-irradiation, up to 75% of the antimicrobial activity could be switched off. However, when the material was subsequently subjected to visible light at wavelengths greater than 450 nm for 8 h, the bacterial growth was again suppressed. Irradiation with UVA or visible light moves electrons between the silver deposits and the titanium support causing the nanosilver to undergo oxidation or reduction reactions, which determines the amount of silver ions (one of the causes of bacterial death) that are present.

This new finding may lead to other interesting applications, but it also highlights the importance of the oxidation states of silver towards its antimicrobial action.