The Role of Onium Salts in the Pro-Oxidant Effect of Gold Nanoparticles in Lipophilic Environments

Metal nanoparticles are reported to be toxic due to the generation of free radicals at their surface. Relatively inert thiol-capped gold nanoparticles (AuNPs) have been reported to induce radical formation in the presence of hydroperoxides, which would conflict with their potential use as inert scaffolds for the design of novel nano-antioxidants. With the aim of clarifying this aspect, we investigated the pro-oxidant activity of dodecanethiol-capped AuNPs (approximate to 5nm diameter), prepared through the Brust-Schiffrin synthesis, by oxygen-uptake kinetic studies. The pro-oxidant activity was found to be proportional to the impurities of the transfer agent tetraoctylammonium bromide (TOAB) left from the synthesis and decreased on repeated washing of the nanoparticles. Under identical settings similar batches of AuNP (approximate to 9nm diameter) prepared through the Ulman method without onium salts showed no pro-oxidant behavior. The alternative onium phase-transfer agents Oct(4)NBF(4) (Oct=octyl), Hex(4)NBF(4) (Hex=hexyl), and Hex(4)NPF(6) were comparatively investigated and showed lower pro-oxidant activity depending on the counterion (Br->PF6->BF4-).