Characterization, purification, and stability of gold nanoparticles.

Impurities in the synthesized gold nanoparticle (AuNP) solution are systematically identified followed by determining an optimal purification process and evaluating the stability as well as oxidation state of the purified 20-nm AuNPs. Quantified non-AuNP components and a newly speciated byproduct (acetate) complete the stoichiometric equation of AuNP synthesis through the citrate reduction method. Among the five tested centrifugation forces (3000-11,000g) and durations (10-60 min), optimal purification of AuNPs was achieved by centrifugation operating at 7000 g for 20 min which satisfactorily recovers ∼80% of AuNPs without detectable impurities. Storage in the dark at 4 °C prolongs the stability of the purified AuNP suspensions up to 20 days. AuNPs employed in this study persist in their atomic status without being oxidized, even after they were aerosolized in air or heated at 500 °C. This work demonstrates how impurities are identified and removed, and the purified AuNPs can be a reference material to evaluate toxicity or reactivity of other engineered nanomaterials.