Core–shell–shell nanorods for controlled release of silver that can serve as a nanoheater for photothermal treatment on bacteria

A novel bactericidal material comprising rod-shaped core–shell–shell Au–Ag–Au nanorods is constructed as a nanoheater in the near-infrared (NIR) region. The outer Au shell melts under laser irradiation and results in exposure of the inner Ag shell, facilitating the controlled release of the antibacterial Ag shell/layer or Ag+. This results in the Au–Ag–Au nanorods having a favorable bactericidal ability as it combines the features of physical photothermal ablation sterilization of the outer Au shell and the antibacterial effect of the inner Ag shell or Ag+ to the surrounding bacteria. The sterilizing ability of Au–Ag–Au nanorods is investigated with Escherichia coli O157:H7 as a model bacterial strain. Under low-power NIR laser irradiation (785 nm, 50 mW cm−2), the Au–Ag–Au nanoheater exhibits a higher photothermal conversion efficiency (with a solution temperature of 44 °C) with respect to that for the Au–Ag nanorods (39 °C). Meanwhile, a much improved stability with respect to Au–Ag nanorods is observed, i.e., 16 successive days of monitoring reveal virtually no change in the ultraviolet–visible spectrum of Au–Ag–Au nanorods, while a significant drop in absorption along with a 92 nm red shift of Localized Surface Plasmon Resonance is recorded for the Au–Ag nanorods. This brings an increasing bactericidal efficiency and long-term stability for the Au–Ag–Au nanorods. At a dosage of 10 μg ml−1, a killing rate of 100% is reached for the E. coli O157:H7 cells under 20 min of irradiation. The use of Au–Ag–Au nanorods avoids the abuse of broad-spectrum antibiotics and reduces the damage of tissues by alleviating the toxicity of silver under controlled release and by the use of low-power laser irradiation. These features could make the bimetallic core–shell–shell nanorods a favorable nanoheater for in vivo biomedical applications.