New insights into the antimicrobial treatment of water on Ag-supported solids

Abstract

BACKGROUND: Silver (Ag) has been long known to be a strong antimicrobial agent and has been used as such either as AgNO3 or in the form of nanoparticles. The antimicrobial activity of nanosilver is believed to be due to free metal ion toxicity, the consequent generation of excess reactive oxygen species and inhibition of gene expression in several cells. RESULTS: The antimicrobial activity of Ag/Al2O3 spheres was studied after suppression of free Ag ions by using a suitable complexing agent (Ag+ scavenger). It was found that Ag/Al2O3 retained its antimicrobial activity even after the addition of the Ag+ complexing agent, which is in contrast to the behaviour of an AgNO3 solution which became completely inactive. Initial/preliminary transmission electron microscopy and Fourier transform infrared studies indicate possible phospholipid residues on the Ag-supported solid surface. •OH radicals were confirmed to be formed during the antimicrobial process. CONCLUSIONS: The present work provides strong evidence that the antimicrobial property of Ag-supported solids is not exclusively due to the dissolution of surface silver (free Ag+). A possible simplified mechanism is proposed in which the initiation of the antimicrobial reaction is proposed to be a heterogeneous intersurface process, which might include the interaction between the partially positively charged, surface silver atoms and the negatively charged outer membrane (OM) of microbes, and the subsequent activation of a free radical mechanism. Further study and confirmation of the above findings might be decisive for the development of novel Ag-supported solids with limited metal surface dissolution but strong antimicrobial activity useful for the confrontation of particular environmental challenges.