Antibacterial activity of silver-doped silica glass microspheres prepared by a sol-gel method

Various kinds of inorganic substances doped with silver ions have been developed as antibacterial materials, and some have already been commercialized. Previously, we successfully prepared colorless silica glass microspheres doped with silver ions in combination with aluminum ions by a sol-gel method. However, the antibacterial activity of the microspheres was not maintained for long periods in an aqueous environment, since the silver ions were located only in a thin layer near the surface of the microspheres. In this study, silica glass microspheres in which silver ions are uniformly distributed were attempted to be prepared. A tetraethoxysilane ethanol solution was mixed with aqueous silver nitrate and aluminum nitrate solutions to be subjected to almost simultaneous hydrolysis and polycondensation. An ammonia solution was then added, to form microspheres. Monodispersed microspheres about 0.1 μm in diameter were obtained, which did not show coloring even after heat treatment at 600-1000°C, indicating that the silver in the microspheres took the form of Ag⁺ ions and not colloid, even after the heat treatments. Microspheres heat-treated at temperatures ranging from 700 to 800°C showed much higher antibacterial activity than commercial antibacterial zeolites and maintained their high antibacterial activities for long periods in an aqueous environment. Polypropylene plates and films mixed with the microspheres heat-treated at 800°C showed excellent antibacterial properties.