We demonstrate how water-soluble gold nanoparticles prepared by the citrate reduction method can be functionalized with 11-azidoundecane-1-thiol in a single-step procedure, which combines phase transfer and ligand exchange. The advantages of our method include high reproducibility, low cost of gold nanoparticle production, and rapid and clean ligand exchange, with minimal intermediate steps and none of the laborious efforts. The resulting azido-functionalized nanoparticles were characterized by ¹H NMR and UV-visible spectroscopy, and visualized by transmission electron microscopy (TEM).