In this article, gold-silica composite nanoparticles with a core-shell structure are successfully fabricated using a simple sol-gel method without using primer, surfactant, or surface modification of the core. The reactions solution at room temperature is prepared by tetraethyl orthosilicate (TEOS), isopropanol, ammonia, and water. The growth of gold-silica core-shell (GSCS) nanoparticles involved the base-catalyzed hydrolysis of TEOS and subsequent condensation of silica onto the surface of gold nanoparticles. Silica is observed to be sol phase without adding water in the reaction solution. Thus, these silica sols could not be uniformly coated onto the surface of gold nanoparticles. To investigate the influence of water on the synthesis of GSCS nanoparticles, the amount of water was varied in the range of 1 to 9 mL. It was found that the thickness of the silica shell could be formed in the range of 10 to 120 nm, and the UV-visible optical absorption measurement revealed a pronounced redshift of the surface plasmon resonance band from 528 to 537 nm. Furthermore, the analysis of the reaction solution pH indicated that the silica shell cannot be coated at pH>10.85. The growth of silica could be increased substantially to form the thicker silica shell within the pH of the reaction solution from 10.85 to 10.06. However, the reaction solution in the lightly alkaline condition is suitable for the growth of silica. (C) 2008 The Electrochemical Society.