Compositions of nanoparticles functionalized with at least one zwitterionic moiety, methods for making a plurality of nanoparticles, and methods of their use as diagnostic agents are provided. The nanoparticles have characteristics that result in minimal retention of the particles in the body compared to other nanoparticles. The nanoparticle comprises a core, having a core surface essentially free of silica, and a shell attached to the core surface. The shell comprises at least one silane-functionalized zwitterionic moiety.