Takuro Niidome1 Kaung Kyaw1 2 Hiroaki Ichimaru1 Masayuki Tsushida3 Yuta Miyazawa4 Daigou Mizoguchi4

1, Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto, , Japan
2, Department of Chemical Engineering, Yangon Technological University, Yangon, , Myanmar
3, Technical Division, Faculty of Engineering, Kumamoto University, Kumamoto, , Japan
4, Dai Nippon Toryo Co., Ltd., Otawara, , Japan

It has been known that silver ions released by silver nanoparticles have bactericidal effect against a wide spectrum of bacteria. Silver ions can induce redox reactions on the membrane transport proteins of bacteria thereby deactivating them. Silver ions are also capable of impairing the energy transfer mechanism of bacteria during respiration process. However, the nanoparticles are unstable and easily form aggregates, which decreases their antibacterial activity.
To improve the dispersion stability of silver nanoparticles in aqueous media, and to increase their effectiveness as antibacterial agents, we coated triangular plate-like silver nanoparticles (silver nanoplates, Ag NPLs) with one or two layers of gold atoms (Ag@Au1L NPLs and Ag@Au2L NPLs, respectively). These gold coatings improved the dispersion stability in aqueous media with high salt concentrations. Ag@Au1L NPLs showed stronger antibacterial activity on pathogenic bacteria than Ag NPLs and Ag@Au2L NPLs. Furthermore, the Ag@Au1L NPLs decreased the number of bacteria living in RAW 264.7 cells. The Ag@Au1L NPLs displayed no cytotoxicity towards RAW 264.7 cells, and the Ag@Au1L NPLs could be used as an antibacterial agent for intracellular bacterial infections.
Next, we prepared hollow-shaped alloy nanoparticles made of silver and gold atoms (Ag/Au NPs) by treating silver nanoparticles with gold ions, not core-shell type gold-coated silver nanoparticles as described in the previous section. The antibacterial activity of the hollowed Ag/Au alloy nanoparticles was stronger than the original silver nanoparticles. Additionally, the release of silver ions from the hollowed Ag/Au nanoparticles was higher than the original silver nanoparticles.
The gold atoms on the surface of silver nanoparticles or in the alloy nanoparticles made of silver and gold atoms affected the oxidation of the silver atoms in the nanoparticles. Since chloride ions can affect the migration of gold atoms on silver, in turn, the migration of gold atoms might affect the exposure of silver metals to the culture medium and the subsequent release of silver ions. These gold treatments are effective methods to improve the antimicrobial activity of silver nanoparticles.