Biologically Benign Multi-functional Mesoporous Silica Encapsulated Gold/Silver Nanorods for Anti-bacterial Applications by On-demand Release of Silver Ions

Although silver (Ag)-based nanoparticles (NPs) are frequently used for bactericidal purposes, they have critical issues including excessive release of Ag⁺, severe oxidation, and cytotoxicity. In this study, we designed a multifunctional, on-demand antibacterial agent by successively encapsulating bimetallic gold/silver nanorods (Ag/AgNRs) with mesoporous silica (mSiO₂) shells. Au/AuNRs were synthesized by coating Ag on AuNRs in a controlled manner, so that they exhibited a localized surface plasmon resonance peak in the near-infrared (NIR) region. When Au/AgNR@mSiO₂ NPs were irradiated with an NIR laser under optimal conditions (0.4 W/cm²), they generated a small amount of heat (40–45 ^oC), which successively triggered the release of Ag⁺ and induced bacterial cell death. Here, mSiO₂ shells play critical roles because they not only protect Ag from oxidation but also prevent the burst release of Ag⁺ and improve biocompatibility of the antibacterial agent against normal cells. We found that this multifunctional bacterial agent effectively kills gram-negative Escherichia coli and gram-positive Staphylococcus aureus without significantly increasing the temperature of the medium. Au/AgNR@mSiO₂ NPs were also biologically benign with high biocompatibility against mammalian cells.