Antimicrobial activity of silver is highly effective and broad-spectrum; however, poor long-term antibacterial efficiency and cytotoxicity toward mammalian cells have restricted their applications. Here, we fabricated Au@Ag NPs with tailored shell thickness, and investigated their antibacterial activities against both Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus) and the cytotoxicity toward SH-SY5Y human cells, for the first time. Our results demonstrate that Au@Ag NPs with a thickness of 5 nm or Au : Ag ratio 1 : 1 (Au@Ag-2 NPs) have the highest antibacterial activity and excellent biocompatibility. The minimum inhibitory concentration (MIC) values of Au@Ag-2 NPs in terms of effective silver concentration are 5 mg mL À1 for E. coli and 7.5 mg mL À1 for S. aureus,
IntroductionTo cope with the overwhelming challenges of infectious diseases resulting from pathogenic bacteria, nano-sized materials, emerging as new types of safe and cost-effective bactericidal materials, have been widely investigated. Among those numerous nanomaterials, silver nanoparticles (Ag NPs) are the most promising antibacterial agents because of their inherent properties of high thermal stability, limited microbial resistance, and broad-spectrum antimicrobial activity against bacteria, viruses and other eukaryotic microorganisms. 1-4 Sondi and co-workers 5 demonstrated that Ag NPs can easily adsorb to the membrane surface of bacteria through electrostatic interaction, form permeable pits, and cause an osmotic collapse in the cells. The other possible mechanism is the release of silver ions (Ag + ) from the oxidized Ag NPs. 6-8 Ag NPs are easy to oxidize and produce a high concentration of Ag + , the released Ag + then destroys the respiratory chain and leads to the formation of reactive oxygen species (ROS), such as hydroxyl radicals, H 2 O 2 and hydroperoxyl radicals, which nally trigger cell damage of the cellular components and leads to the death of the bacteria. 9-11 In spite of this, the easy oxidation of pure Ag NPs and the toxicity in mammalian cells remain challenges in Ag NP applications.
12-14To address this problem, Ag NPs have been decorated with other nanomaterials by the formation of nanostructures. Song et al.
15synthesized the silver/polyrhodanine compositedecorated silica nanoparticles, which exhibited excellent antimicrobial activity toward Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) because of the antibacterial effects of both the silver nanoparticles and the polyrhodanine. Tan et al.
16synthesized antibacterial Ag@dsDNA@GO, which can wrap around the Xanthomonas perforans cell and results in rapid cell apoptosis, showing the synergistic antibacterial effect. While these nanostructures can effectively enhance the antibacterial activity of Ag NPs, they oen require complex and tedious preparation methods, and suffering from low yields, poor longterm antibacterial activity, and biocompatibility problems.
17,18Bimetallic core-shell nanoparticles are graduall...