Herein, we report a method that uses free-base amino acids to mediate the controlled hydrothermal growth of amorphous zinc oxide (a-ZnO) or nanocrystalline zinc sulfide (c-ZnS) shells on gold nanoparticles. By screening through a set of 13 candidate amino acids, we have identified four as being capable of mediating inorganic shell growth using an aqueous, low-temperature, one-pot process. In particular, unaggregated and monodisperse sols of exceptional quality are produced using L-histidine, which preserves colloidal stability and mediates the growth of continuous and remarkably uniform a-ZnO shells with a tunable thickness between 2 and 25 nm while avoiding the nucleation of free particles. By coupling spectral extinction measurements with generalized Mie theory calculations, we estimated the complex refractive index of the a-ZnO shell to be 1.47 + i0.09. It is expected not only that our Au@a-ZnO core−shell particles are suitable for both energy and biological applications but also that our process for growing inorganic shells could be extended to other nanocomposite systems comprised of different materials and geometries.