Preparation of monodispersed carboxylate-functionalized gold nanoparticles using pamoic acid as a reducing and capping reagent

A simple preparation method of gold nanoparticles (AuNPs) using pamoic acid (PA; 4,4'-methylene-bis(3-hydroxy-2-naphthalene carboxylic acid)) with NaOH is described. Although PA is insoluble in water, it can be dissolved in the presence of NaOH and function as a capping and reducing reagent to form the AuNPs. The thus-formed AuNPs have a good monodispersity with diameters of 10.8 +/- 1.2 nm and carboxylate functions that come from the PA. Because PA is a methylene-bridged dimer of 3-hydroxy-2-naphthalene carboxylic acid, the formation of the AuNPs was examined also using the analogous monomer molecules, i.e., 3-hydroxy-2-naphthalene carboxylic acid, 2-hydroxy-1-naphtalene carboxylic acid, or 2-naphthol. Consequently, it was found that the case of PA was specific to forming the spherical monodispersed AuNPs while that differently shaped Au nanostructures were formed in the other cases. The present preparation using PA would be an interesting example that stable, monodispersed, carboxylate-functionalized AuNPs could be prepared without using thiols. Also, the present results may provide some insight into molecular designs of capping reagents for preparing functionalized AuNPs without using thiol derivatives.