Silica not so inert after all

Powdered silica has long had a reputation for being chemically inert, nontoxic, and biocompatible. But that description may be inaccurate, according to a biomolecular study (Proc. Natl. Acad. Sci. U.S.A. 2023, DOI: 10.1073/pnas.2304735120).

Manufacturers in the food and healthcare industries use microscopic particles of silica (SiO₂) in various formulations. For example, the amorphous form of the material, which is approved for use by the US Food and Drug Administration, serves as an anticaking agent to prevent clumping in powdered foods, cosmetics, and skin care products. And the material is being studied for use in drug delivery and as a carrier of imaging agents.

Despite its widespread use, silica may not be inert after all. Yangjie Li and Richard N. Zare of Stanford University, and Kurt W. Kolasinski of West Chester University found that adding commercial silica particles to aqueous solutions of L-cysteine, glutathione, and D-penicillamine, caused the molecules to undergo substantial oxidation in 24 h when stored at room temperature in the dark. Oxidation increased with incubation time, and more oxidation occurred with particles having larger surface areas. On the basis of quantum calculations, the researchers propose that oxidation is mediated by silyloxy radicals (SiO^•) on silica surfaces.

Low levels of glutathione can cause oxidative stress and damage DNA and cell membranes. As a result of these and other potential health risks, the team encourages further investigation into silica’s reactivity.