Stability of tetraoctylammonium bromide-protected gold nanoparticles: Effects of anion treatments

Tetraoctylammonium bromide-capped gold nanoparticles (Oct4NBr-NPs) are known to have a poor long-term stability. This article shows a simple way of improving the stability of Oct4NBr-NPs, so that these nanoparticles can be used in different applications and fundamental studies in the future. When a toluene solution containing Oct4NBr-NPs was mixed with an aqueous solution containing an excess of different anions (Na2S2O3, Na2S, NaNO3, and NaH2PO4), the stability of gold nanoparticles was found to be altered. The effects of the post anion treatment on the chemical and thermal stability of Oct4NBr-NPs were studied using UV-vis spectroscopy. When treated with an aqueous Na2S2O3 solution, the chemical and thermal stability of Oct4NBr-NPs was greatly improved and the long-term stability in the solution was enhanced. However, the other anions studied in this article were found to be less effective in stabilizing the Oct4NBr-NPs. The anion treatment with NaNO3 and NaH2PO4 led the nanoparticles to completely precipitate out of the solution in less than 2 weeks. Transmission electron microscopy results showed that the anion treatments of Oct4NBr-NPs with Na2S2O3 and Na2S resulted in a slight decrease in the average size of the nanoparticles.