“Quantification of Water Solubilized CdSe/ZnS Quantum Dots in Daphnia magna” N. A. Lewinski, H Zhu, H J Jo, D. Pham, R R Kamath, C R Ouyang, C D Vulpe, V L Colvin, and R A Drezek. Environ. Sci. Technol. 2010, 1841-1846. 10.1021/es902728a
“Quantum Dot Weathering Results in Microbial Toxicity” S Mahendra, H Zhu, V L Colvin, P J Alvarez. Environ. Sci. Technol. 2008, 9424-9430. 10.1021/es8023385
The goal of green chemistry is to design chemicals and chemical processes which are inherently less toxic. For the emerging discipline of nanoscience, the potential toxicological properties remain largely unknown. That said I would like to discuss two related articles concerning the fate and toxicity of nanoparticles (NPs) in the aquatic environment. I like this set of articles because they are both simple, and yet give a hint of the complexities involved with understanding NP fate and toxicity. The more recent, examines the interaction between CdSe/ZnS NPs with Daphnia (a model species for aquatic toxicity). And the original article that drew my attention to this work concerned the same NP and studies the effect that chemical degradation has on NP toxicity.
The CdSe/Zs NPs used in this study are relatively common and have been commercialized for their use as biological dyes. Additionally, CdSe and related metal chalcogenide particles are being used in solar cell modules. (While the scale of commercialization is currently small, energy applications demand many more material resources and hence would represent a larger threat to human health and the environment.)
Jumping right into the tasty results of the daphnia study, the authors demonstrated that CdSe/ZnS NPs accumulate in the gut of the Daphnia. These NPs persisted (ie stuck around) after 48 hours of exposure to clean water and food. So what, they didn’t die, right? Well the issue is that if you expose Daphnia to Cd2+ at similar concentrations they are able to clear the cadmium from their system in 6-8 hours. This doesn’t mean the nanoparticles are inherently worse. But it does mean that they hang around longer in the Daphnia bodies. I don’t know about you, but if I am exposed to a known toxin, like cadmium, I sure want to get it out of my body ASAP. The quantitative results are shown below. Look at how long the CdSe/ZnS NP persist in the Daphnia guts under different experimental conditions.
The authors show that over 50% of the NPs remain in the Daphnia after exposure to clean water . Even after feeding on clean algae for 48 hours, about 30% of the NPs remain in the Daphnia. Additionally, the authors looked at the effect of the NP surface coating on the residence time of NPs in Daphnia. They looked at four surface coatings: octylamine modified polyacrylic acid, polyethylene glycol(PEG) modified polyacrylic acid (PAA), poly(maleic anhydride-alt-1-octadecene) (PMAO), and PEG modified poly(maleic anhydride-alt-1-octadecene). In both cases the PEG conjugated polymers were less accumlative than the parent polymer. The most accumulative by a factor of seven over the PEG modified PAA, was the PMAO. Although all of the NP’s accumulate in the Daphnia gut, we see that surface coating does play a large role.
Alright, so we have some Daphnia that look cool under a black light, they are still swimming, right?
This is where the second study comes into play. This study looks at CdSe nanoparticles again, but this time we actually get to quantify how many of these nanoparticles it takes to kill something. Unfortunately for my narrative flow, this second study evaluated the toxicity of the same CdSe/ZnS NPs against 3 strains of bacteria, instead of Daphnia. So the pictures are not as cute, and the skeptics out there will cry foul when I compare the two studies. Regardless, if you have stuck with me this far, I think it will be worth your while to humor me for a few more paragraphs while I explain why I think it is a bad thing to have CdSe/ZnS particles hanging around in your gut.
In their 2008 report, the authors looked at the effect of weathering on NPs coated with PAA, PMAO, and polyethylene imine (PEI). They also subjected these particles to acidic (pH=2) or basic (pH=12) solutions to simulate weathering. It turned out that weathered NPs were more toxic than expected based on what would be expected from the toxicity of the constituent ions Cd2+ and Se2-. On the other hand, the intact coated particles were less toxic.
Alright we are in the clear, let’s coat our particles and then coat the earth with them! (Based on the first study I hope we are coating them with PEG of some sort…)
The problem is, although I don’t know what the pH of a Daphnia stomach is, I do know that the pH of a human stomach is between 1-3. We have seen that NPs hang out in the GI track of Daphnia, and that when you expose particles to low pH environments the damage to their surface coatings makes them more toxic (to bacteria). So there may be a reason for concern if you start connecting the dots. Based on these studies, particles persist longer than their known toxic constituents, and under certain environmental exposures, their toxicity increases even when they have been coated.
We cannot conclude from these studies what the risk of CdSe/ZnS exposure is for humans, or even Daphnia directly. But I for one, will no longer be putting those tasty NPs in my morning coffee! (For more information on the pH of coffee, check this out.)