Nanoscale silver: Thin-film structure and antimicrobial functionality

Abstract

Since antiquity, silver has been used as a material to reduce spoilage. Over the past decades, there has been an increasing scientific and commercial interest in developing silver surfaces due to the increasing number of drug resistant microorganisms. In this study, the effect of nanostructuring silver films as an antimicrobial against the bacteria Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) was examined. Films consisting of 3 nm chromium adhesion layers and nominal 20 nm silver surfaces (assuming flat deposition) were deposited by thermal evaporation and nanostructuring was controlled by varying the incident angle of the silver onto the substrate. Four substrate angles were used including 0 °, 18 °, 40 ° and 70 ° to the horizontal. Examination by atomic force microscope, Rutherford backscattering and ellipsometry showed that as the incident angle of deposition increased, so did the nanostructuring and surface roughness. This was coupled with a decrease in film thickness. Incubation of the nanostructured thin-films in bacterial broths with E. coli and S. aureus showed that as the surface roughness increased the antimicrobial activity was enhanced – both in solution and for bacteria adhered to the thin-films. Inductively coupled plasma mass spectrometry was used to measure silver leaching from the thin-films and showed a negligible loss for all films, with corresponding low-levels of antimicrobial activity. Further indicating the enhancing effect of nanostructuring as an antimicrobial. All thin-films showed biological fouling after prolonged exposure to the bacterial solutions, which reduced antimicrobial activity. Cleaning the films with IPA showed that the films could be regenerated but with some loss of antimicrobial activity. The mechanism of thin-film antimicrobial activity is at this time unknown but it is speculated that nanostructuring is capable of penetrating the cell envelopes of bacteria, which enhances the antimicrobial activity of silver.