Dense aqueous colloidal gold nanoparticles prepared from highly concentrated precursor solution

► Casein protein emerges as a novel protective agent for metal nanoparticles. ► A highly concentrated synthetic route to Au and Ag nanoparticle powder is developed. ► Highly stable dense metal nanoparticle colloids is obtained by dissolving the powders. ► The conformational structures of the proteins change during the growth of the nanoparticles.