Silica particles convert thiol-containing molecules to disulfides

Synthetic amorphous silica is a common food additive and a popular cosmetic ingredient. Mesoporous silica nanoparticles are also widely studied for their potential use in drug delivery and imaging applications because of their unique properties, such as tunable pore sizes, large surfaces areas, and assumed biocompatibility. Such a nanomaterial, when consisting of pure silicon dioxide, is generally considered to be inert, but in this study, we showed that oxidation yields for different compounds were facilitated by simply incubating aqueous solutions with pure silica particles in the dark. Three thiol-containing molecules, L-cysteine, glutathione, and D-penicillamine, were studied separately, and it was found that more than 95% of oxidation happened after incubating any of these compounds with mesoporous silica nanoparticles in the dark for a day at room temperature. Oxidation increased over incubation time and more oxidation was found for particles having larger surface areas. For nonporous silica nanoparticles, yields of oxidation were different based on structures of molecules, correlating with steric hindrance while accessing surfaces. We propose that the silyloxy radical on silica surfaces is what facilitates oxidation. Density functional theory calculations were conducted for total energy changes for reactions between different aqueous species and silicon dioxide surfaces. These calculations identified two most plausible pathways of lowest energy to generate SiO• radicals from water radical cations and •OH radicals, previously known to exist at water interfaces.