New Method to Form Colloidal Gold: Spontaneous Transformation from Nanoporous Anodic Gold Oxide Film into Gold Nanoparticles in Pure Water

Gold nanoparticles (Au NPs) exhibit strong localized surface plasmon resonance (LSPR) and have applications in biosensing, optical devices, and conductive materials. The conventional chemical synthesis of Au NPs relies on the dissolution of gold salts such as HAuCl4 and reductants followed by the reduction of gold ions, which pose safety, environmental, and scalability challenges. In this study, we present a new route to obtaining colloidal gold: the formation of a nanoporous gold oxide film by gold anodization in an aqueous citric acid solution and the immersion of the film in pure water. UV–vis spectra show the emergence of LSPR at around 525–530 nm for the anodized film immersed in pure water for ∼3 days at 25 °C. Dynamic light scattering measurements indicate that the mean hydrodynamic diameter does not increase with LSPR intensity for the film immersed for 60 days. The nearly constant hydrodynamic diameter despite increasing LSPR extinction is consistent with the continued formation or release of similarly sized particles. SEM observations revealed that when an anodic gold oxide film is kept immersed in water, Au NPs with a diameter of ∼20 nm emerge in the film simultaneously with the dispersion of colloidal gold in water. The data obtained suggest the formation of colloidal gold through the spontaneous morphological transformation of the nanoporous film to NPs during the reduction of gold oxide in water with the assistance of organic matter from citrate. The proposed method is operationally simple and potentially scalable and avoids the use of HAuCl4 and added strong reductants.