Shining Light on the Toxic Effects of Nanoparticles

This post is long overdue, both because I haven’t posted in  more than a month, and because I’ve actually had this research highlight written since January – it was one of the writing samples I submitted for my application to the AAAS Mass Media Fellowship this spring.

I’m pleased to say that I’ve received the fellowship this year, and will be spending the summer writing for the Milwaukee Journal Sentinel before heading off to do a postdoc in the fall.  This will make my graduation & moving plans a little complicated, but I’m really excited about this opportunity.

At any rate, I’ll keep you updated as I go, but for the moment – zebrafish!

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Exposure to light and tiny titanium dioxide nanoparticles can be a lethal combination – at least, if you’re a zebrafish.

According to researchers at the University of Wisconsin-Madison, raising zebrafish in water containing titanium dioxide nanoparticles and exposing them to light caused the fish to be badly deformed and die much more quickly than fish raised in normal environments.

Understanding the toxicity of these materials is critical because titanium dioxide nanoparticles are finding their way into a growing range of consumer products.

Titanium dioxide nanoparticles are especially useful in sunscreens, because they effectively absorb harmful UV rays without making the sunscreen pasty and opaque. However, similar nanoparticles also form the foundation of some next-generation solar cells and are used to treat wastewater in sewage plants.

Yet according to the researchers at UW-Madison, nobody really knows what effects these nanoparticles have when they get out into the environment.

The UW team explored this question by raising zebrafish embryos in water containing titanium dioxide nanoparticles. They kept some fish in the dark and exposed others to light to see whether or not light exposure changed the nanoparticles’ effects.

Other researchers have studied nanoparticle toxicity using isolated cells grown in the lab. However, the toxic effects on cells in a dish can sometimes be different than the effects on a whole organism.

By using zebrafish, the researchers at UW-Madison were able to look at the toxicity on a larger scale. Zebrafish are ideal for these sorts of experiments because they grow quickly. They are also clear, so the researchers can easily see any growth defects that appear.

Researcher Ofek Bar-Ilan and coworkers found that fish exposed to the nanoparticles but kept in the dark looked normal. Fish exposed to the nanoparticles and kept under bright light, however, had deformed heads and tails, and their growth was stunted. The fish exposed to both nanoparticles and light also died much more quickly than fish kept in the dark.

For something so small, the nanoparticles seemed to do a lot of damage.

The problem is that these particles aren’t just small, they’re tiny. At 25 nanometers in diameter, they are nearly three hundred times smaller than red blood cells. This small size means that the nanoparticles can get into places that larger particles can’t. When the UW-Madison researchers sliced open the dead zebrafish, they found nanoparticles not just on the skin, but also throughout the muscle tissue.

Ironically, part of what makes titanium dioxide nanoparticles so useful – their ability to absorb light – is also what makes them toxic.

When light hits a titanium dioxide nanoparticle, it excites electrons. At the surface of the particle, these electrons react with oxygen to form peroxides, like the hydrogen peroxide commonly used to disinfect scrapes and cuts.

A similar reaction between water and the positive charge the electron leaves behind creates the highly reactive hydroxyl radical. These so-called “reactive oxygen species” then wreak havoc by chewing up DNA, proteins, fatty acids, and other molecules crucial to healthy cells.

By showing that the nanoparticles weren’t as toxic to fish also exposed to a protective antioxidant, the UW-Madison team demonstrated that these reactive oxygen species caused the nanoparticles’ toxic effects.

Titanium dioxide nanoparticles exposed to light can thus do a lot of damage to developing zebrafish. But it is unclear how these effects translate to commercial products and to other animals.

Nanoparticles in sunscreens and cosmetics probably don’t pose much of a threat, for example, because living skin prevents the particles from penetrating into the body.

Zebrafish, by contrast, have very little to protect them from toxins in their environment once they hatch. Because the zebrafish are transparent, it is also much easier for light to penetrate and interact with nanoparticles deep in their bodies.

The UW-Madison team also used much higher doses of nanoparticles than those in commercial products. Lower doses may or may not have as much of an effect. Studying the long-term effects of low exposures is the subject of ongoing experiments in the UW labs.

As these types of nanoparticles come into greater use, however, being able to predict and assess their toxicity is essential to understanding their effects on health and the environment. By showing that zebrafish are a good model system for these types of studies, the UW-Madison team has taken a big step in the right direction.

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The original article can be found here:

Bar-Ilan O., Louis K.M., Yang S.P., Pedersen J.A., Hamers R.J., Peterson R.E. & Heideman W. (2012). Titanium dioxide nanoparticles produce phototoxicity in the developing zebrafish, Nanotoxicology, 6 (6) 670-679. DOI: 10.3109/17435390.2011.604438