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Citric acid changes environmental impact of buckyballs

By David Robson , 16-Apr-2008

Buckyballs have the annoying tendency of sticking together in floating clumps in water - a property that prevents them from settling to be removed from waste water from industrial production.

Now, researchers have found that the presence of citrates - common chemical and biological products - modify this behaviour, creating tighter clusters that may need to be treated differently. "It could have interesting consequences in terms of the environmental treatment of the materials - it's possible that it could be a problem," says Peter Vikesland from Virginia Tech, who conducted the study. The findings could also have an impact on the potential toxicity of buckyballs for therapeutic purposes; scientists have been investigating the use of the particles in drug delivery, but the research suggests that citrates present in living cells could have an impact on how the particles move through our body. Buckyballs (a type of fullerene) stick together in water because they are highly hydrophobic. The molecules contain 60 carbon atoms, making the attraction to another buckyball greater than the attraction to the hydrogen and oxygen atoms in water. This causes them to swarm together in large aggregates. The clusters are very stable, with the smaller clumps remaining suspended in the water without sinking or floating to the surface, making it more difficult to remove the particles from industrial waste. Vikesland has shown that the presence of citric acid and its salts may make the situation even worse, creating smaller, more spherical clusters that are even more stable in water. Vikesland presented the work at the American Chemical Society's annual meeting last week. "Instead of having irregular shaped aggregates, you get spherical objects with a reduced surface charge that makes them more stable," he said. "It could have interesting consequences in terms of the environmental impact of the materials." So far, the researchers do not know exactly what causes this effect, but they do have a theory. "We have no answers but we have a hypothesis, still unproven, that there are weak interactions between citrate and individual carbon molecules that cause the spherical shape," Vikesland said. He also found that the smaller, spherical clusters look very similar to previous images from studies of buckyballs within living cells, suggesting that the presence of citrates may change the toxicity of the particles within the body. Vikesland has not investigated the full effect this finding will have on the environmental impact of buckyballs, but this is something he plans to follow up. One experiment will involve passing the acidified clusters through a sand column, to simulate how they would pass through the soil in ground water supplies. Overall, he believes that companies must be cautious about using buckyballs on a large scale until we better understand the way they interact with the environment. "We don't fully understand what's going on," he said. "If you have a face cream with fullerenes as an antioxidant -- we don't know how they will react. There are many organic acids in the environment."

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