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

Abstract

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 (∼5 nm 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 (∼9 nm diameter) prepared through the Ulman method without onium salts showed no pro-oxidant behavior. The alternative onium phase-transfer agents Oct NBF (Oct=octyl), Hex NBF (Hex=hexyl), and Hex NPF were comparatively investigated and showed lower pro-oxidant activity depending on the counterion (Br >PF >BF ).

Antioxidant Activity of Metal Nanoparticles Coated with Tocopherol-Like Residues—The Importance of Studies in Homo- and Heterogeneous Systems

“…On the other side, in both hydrocarbons, no pro-oxidant effect of 5 was noticed. Recently, Baschieri and coworkers [12] reported that AuNPs with even traces of onium salts (including TOAB) exhibit clear pro-oxidant behavior. The contradiction between their observations and our results is apparent, but in our system, the autoxidation was initiated with AIBN, while Baschieri et al used tert-butyl hydroperoxide as an initiator, and they rationalized the observed pro-oxidant effect as a result of catalytic decomposition of tBuOOH induced by onium salts.…”

“…Pro-oxidant activity frequently depends on size [9,10] and shape [5,11] of nanoparticles that can be rationalized by increasing the number of structural defects in the crystal lattice or by exposition of some specifically ordered atoms at the edge of the nanocrystal, thus increasing the reactivity of a nanoparticle compared to a macroscopic object made of the same material. Pro-oxidative activity can be caused by the adsorbed impurities; therefore, synthesis and purification are crucial for the redox activity of NPs [12].…”

Chain-Breaking Antioxidant and Peroxyl Radical Trapping Activity of Phenol-Coated Magnetic Iron Oxide Nanoparticles

“…Our experiments show that “bare” SPIONS are capable of accelerating the AIBN-initiated autoxidation of styrene, as expected from the known ability of materials containing surface Fe 2+ ions to participate in heterogeneous Fenton chemistry [ 43 ]. Considering that styrene autoxidation proceeds via the formation of a polyperoxide (see Scheme 2 ) [ 37 ], this observed acceleration is due to the homolytic dissociation of O-O styryl polyperoxide affording alkoxyl radicals [ 44 ]. We also show that the heterogeneous Fenton reaction between bare SPION and three peroxides is clearly detectable and increases in the order t BuOO t Bu < H 2 O 2 < t BuOOH (inset of Figure 4 a).…”

Methods to Determine Chain-Breaking Antioxidant Activity of Nanomaterials beyond DPPH•. A Review

“…For instance, octanethiolcapped gold nanoparticles were found to trigger the autoxidation of an organic substrate (styrene) because the traces of tetra octyl ammonium bromide used as phase transfer agent in the synthesis promoted the homolytic decomposition of organic hydroperoxides ( t BuOOH). The rate of generation of radicals by the system Au-NP/ t BuOOH could be determined by the standard addition of a reference antioxidant [117].…”