Antibacterial activity of nanoporous gold against Escherichia coli and Staphylococcus epidermidis
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Conventional metallic antibacterial materials release metal ions and reactive oxygen species (ROS) for killing bacteria. Herein, we found that nanoporous gold (NPG) exhibits antibacterial activity (AA) at an intermediate relative humidity (RH) of 60% against Escherichia coli and Staphylococcus epidermidis in contrast to the inert behavior of bulk gold. The dependence of AA on RH, morphological observations of bacteria on NPG, and transcriptomic analyses of NPGtreated Escherichia coli were investigated. These observations collectively suggest that biological processes in cell walls containing peptidoglycan and cell membranes are significantly disrupted by direct contact with NPG. Metal ions and ROS were not detected, and therefore are not responsible for the present antibacterial properties of NPG. The catalytic nature of NPG may be responsible for its AA, probably because of lattice distortion at the surface of nanosized ligaments with large curvature.
I. INTRODUCTION
Several metals and alloys such as silver and copper exhibit antibacterial properties because they release metal ions1,2 and reactive oxygen species (ROS)3 in aqueous environments. Certain metallic nanoparticles that release no harmful diffusive species (e.g., metal ions and ROS) also exhibit antibacterial activity (AA), which is due to their incorporation into the cytoplasm of bacteria, and subsequent deterioration of cytoplasmic proteins such as ribosomes.4–7 Nanoporous metals are nanostructured materials with open porous structures containing nanometer-scale pores and ligaments.8 The first electron microscopy observations of nanoporous metals were reported in 1979–1980 by Forty and Durkin,9,10 and nanoporous metals have since received much interest from researchers in various fields. Nanoporous gold (NPG) has been intensively investigated because it is easily fabricated by the dealloying of gold– silver alloys. Lattice distortion occurs at the surface of the nanosized ligaments because of their large curvature. This results in an interesting catalysis behavior of NPG in organic reaction that is not observed in bulk gold.11–15 NPG reportedly exhibits catalytic activity in the oxidation of carbon monoxide11–13 and methanol14 in contrast to bulk gold. NPG also catalytically decomposes methyl Contributing Editor: Jinju Chen a) Address all correspondence to this author. e-mail: [email protected] b) Present Address: Sapporo Breweries Ltd., 2 Takasecho, Funahashi, Chiba 273-0014, Japan. DOI: 10.1557/jmr.2017.157
orange (MO, an organic azo dye), whereas bulk gold does not.15 The decomposition kinetics of MO was not simply proportional to the surface area of NPG, which suggests that active sites due to lattice distortion are important in the catalysis by NPG. Thus, NPG and its catalytic properties strongly affect the stability of the surrounding organic matter. On the other hand, NPG has been also investigated as a substrate for mammalian cells.16–18 These studies demonstrated that NPG generally does not cause severe deactivation of cells, a
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