Synthesis of Gold Nanoparticles in Cyclodextrin-Tetrachloroaurate System

Tetrachloroaurate reduction with α-, β-, and γ-cyclodextrin at pH 10.56 was studied in this study by dual-angle dynamic light scattering and spectrophotometry. The nanoparticles were also characterized by scanning electron microscopy. In contrast with our previous study, the nanoparticles were purified by centrifugation prior to characterization. The reaction is considered to be a promising one-step method for preparation of gold nanoparticles with immobilized cyclodextrins without the need for seeding. Unlike in our previous study, the reaction mixture was boiled under reflux conditions instead of an open vial. This change sped up the reaction, and extensive aggregation was avoided. For the first time, this study demonstrated that all three cyclodextrins were able to reduce tetrachloroaurate at room temperature as well, but long incubation periods were necessary. This is the first publication discussing the mechanism of tetrachloroaurate reduction by cyclodextrins. The first stage of the reaction involved tetrachloroaurate hydrolysis despite presence of hydrochloric acid in the commercial preparation of tetrachloroauric acid (HAuCl4). Tetrachloroaurate was therefore the precursor but not the actual oxidant. The previous hypothesis on autocatalytic decarboxylation of cyclodextrins was therefore proven wrong. Particle growth was accompanied by nucleation, leading to coexistence of nanoparticles and nanoclusters. The resulting nanoparticles contained a small fraction of aggregates, probably because of sodium chloride generated from hydrochloric acid. However, besides playing this detrimental role, chloride ions facilitated nucleation by stabilizing gold intermediates. The nanoparticles solutions could be stored at least for three months, which contrasted with our previous work, where the nanoparticles were stable for only three days. For the first time, freeze-drying and reconstitution of the resulting nanoparticles were investigated, and they were shown to be free from aggregation. The synthesized gold nanoparticles are recommended for use as drug carriers.