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Silica Particles Are Not As Inert As We Thought

A pile of small white silica particles against a black background.
Credit: Kier in Sight Archives / Unsplash.
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Silica, a common food additive and cosmetics ingredient, may not be as chemically inert as once thought. According to a new study, published in the Proceedings of the National Academy of Sciences, silica particles can cause thiol-containing biomolecules to undergo redox reactions, potentially affecting how these biomolecules function in the body.

Silica: Odorless, tasteless and frequently used

Silica, or silicon dioxide (SiO2), is a mineral most commonly found in nature as quartz. A highly versatile material, powdered silica has found use as an anticaking agent in many industries to avoid clumping and improve the consistency of powdered products.

The mineral is odorless, tasteless and does not readily react with other compounds. As a result, silica has been readily adopted by the food industry as a subtle way to improve product shelf life and texture. In cosmetics, silica is frequently added to makeup as a bulking or absorbing agent, or as an abrasive in skin scrubs.  

Silica’s inertness is critical to its widespread usage. Indeed, because of this, the US Food and Drug Administration (FDA) currently allows foods to contain up to 2% by weight of silica particles.

“We’ve seen before that so-called inert materials may not really be inert,” said senior author Richard Zare, a professor of chemistry at Stanford University. “That story may be repeating itself with silica particles.”

Silica can react with thiol-containing compounds

For her doctoral thesis, Yangjie Li, now a postdoctoral scholar in the Stanford University Department of Chemistry, investigated the phenomenon whereby inert glass can catalyze and accelerate certain chemical reactions under the right conditions. Continuing this line of inquiry, Li recently sought to examine whether silica particles might do the same.

Working with colleagues at Stanford University, Li purchased a stock of commercially available pure silica powder. This was then added to watery solutions containing one of three thiol-containing biomolecules: cysteine (a key amino acid), glutathione (an antioxidant) and penicillamine (a “heavy metal antagonist” molecule, used in the treatment of copper accumulation in the body).

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After incubation in the dark for a day, small samples were taken from each solution at half-hour, 2-hour, 4-hour and 24-hour marks to study any reactions that may have occurred within the vessels.

Using mass spectrometry, the researchers found that around 95% of the biomolecules in these solutions were oxidized over time. In contrast, experiments that did not include silica in the incubated solutions showed minimal oxidation by the 24-hour mark.

“Silica particles are thought to be benign and inert, but our study’s results indicate that silica is actually reactive,” said Li. “We encourage further investigation into whether silica particle exposure can deplete glutathione and other critical compounds in the body.”

What does this mean for consumer products?

Based on their results, the researchers theorize that silica forms so-called surface-bound silyloxy radicals (a silicon atom bound to one oxygen atom, with an unpaired electron) when exposed to water. When thiol-containing biomolecules come into contact with the radicals, this can cause redox reactions that affect the hydrogen atoms contained in the biomolecules, turning the thiols into disulfide molecules.

The researchers are now planning further experiments that will look at whether the size of the silica particles plays a role in how they influence chemical reactions, and whether other larger biomolecules are also affected.

While too early to conclude whether these reactions might pose any real health risks, the Stanford team hopes that their findings might prompt other researchers and possibly even regulatory agencies to pursue similar investigations.

“Silica is a material that shows up in a lot of places, in the things we eat, in the products we put on our skin and in medical settings,” said Zare. “In light of this new study, we ought to know more about silica and its interactions with other materials.”

“Our findings sound an alarm for the continued use of silica particles,” Zare continued. “While it’s too soon to say that silica is a health risk, at minimum silica poses the potential problem of introducing unwanted chemistry, particularly in food.”

Reference: Li Y, Kolasinski KW, Zare RN. Silica particles convert thiol-containing molecules to disulfides. Proc Natl Acad Sci USA. 2023;120(34):e2304735120. doi: 10.1073/pnas.2304735120

This article is a rework of a press release issued by Stanford University. Material has been edited for length and content.