Bactericidal activity of high concentration Ag nanoparticles immobilized on surface of an aqueous sol-gel silica thin film was investigated against Escherichia coli and Staphylococcus aureus bacteria. Size of the surface nanoparticles was estimated in the range of 35-80 nm by using atomic force microscopy. Due to accumulation of the silver nanoparticles at near the surface (at depth of 6 nm and about 40 times greater than the silver concentration in the sol), the synthesized Ag-SiO2 thin film (with area of 10 mm(2)) presented strong antibacterial activities against E. coli and S. aureus bacteria with relative rate of reduction of the viable bacteria of 1.05 and 0.73 h(-1) for initial concentration of about 10(5) cfu/ml, respectively. In addition, the dominant mechanism of silver release in long times was determined based on water diffusion in surface pores of the silica film, unlike the Usual diffusion of water on the surface of silver-based bulk materials. Therefore, the Ag nanoparticles embedded near the surface of the SiO2 thin film can be utilized in various antibacterial applications with a strong and long life activity. (c) 2009 Elsevier B.V. All rights reserved.