Improved stability of gold nanoparticles (AuNPs) in water is achieved by capping the particles. Such stability improvements are significant because of the potential applications of AuNPs in various biologically relevant areas. The highly stabilized water-soluble AuNPs described in this study have precisely tunable sizes ranging from 2 to 5 nm (see figure; inset: fast Fourier transform) and a narrow monodispersity, and are prepared by using nucleotides as capping ligands.