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 oen 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...

The problems of multidrug-resistant bacteria and environmental pollution associated with the abuse of antibiotics call for effective antibiotic alternatives. Here, gold@silvergold alloy nanoflowers (Au@AgAu ANFs) with distinct atomic structures are first fabricated and then demonstrated for in vivo inhibition of bacteria. The Au@AgAu ANFs display high antibacterial activity against the model Gram-negative bacterium Escherichia coli, with a minimum inhibitory concentration value of 4.8 μg mL-1, which is 3.1 times lower than that of silver nanoparticles. The alloy structure with a rough surface enables Au@AgAu ANFs to firmly adhere to the bacterial surface and damage the cell membrane, resulting in long-term (48 h) and highly stable (30 days) antibacterial activity. Meanwhile, the Au@AgAu ANFs show remarkable biocompatibility with human cells even at a high concentration of 40 μg mL-1. Application of Au@AgAu ANFs in the treatment of bacterial infections in the mouse intestine significantly reduces the reproduction of bacteria compared to an untreated mouse, giving results similar to those of the current antibiotic treatment, with no cytotoxicity. Our study opens up a new avenue for the rational design of safe and highly efficient antibacterial materials.

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