The growth of CaCO3 crystals on free-standing carboxylic-acid-functionalized gold nanoparticle membranes is described. The gold nanoparticle membrane is synthesized by the spontaneous reduction of aqueous chloroaurate ions by a diamine molecule at a liquid-liquid interface. This membrane is robust and malleable, and most importantly, the gold nanoparticles in the membrane may be functionalized with suitable ligands. In this study, the amino acids aspartic acid and cysteine together with an aromatic bifunctional molecule, anthranilic acid, were used to modify the surface of the gold nanoparticles in the membrane. The free carboxylic acid groups on the gold nanoparticles that further functionalize these molecules were then used to bind Ca2+ ions, and they reacted with CO 2 to yield CaCO3 crystals. The nature of the nano-gold surface modifier directed the formation of either spherical vaterite crystals or rhombohedral calcite. The nano-gold membrane thus suggests potential biomedical application as biocompatible implants, etc.