"We read [Oberdorster's] paper last year about buckyballs being taken up by bass [fish] and found in the brain tissue, which suggested to us that buckyballs could get inside living tissue they didn't just pass through the gills of the fish. So we asked the question: What would happen if they could get all the way into the nucleus of the cell, how would they impact the DNA?" said Cummings.
Cummings' team found that buckyballs would shut down the human immune system and prevent cells from self-repairing. However, to penetrate to the DNA, the nanoparticles would first have to have an affinity for living tissue which usually means they need to be organic like buckyballs, which are simply a form of carbon, a basic building block of life.
"To contact the DNA, buckyballs would have to get past several natural barriers. First into the tissues which we know they can in fish then through the cell wall, and finally into the nucleus. What we hope will happen as a result of our research is that there will be some further experimental work and theoretical work to investigate whether these natural barriers will hold. We know they can get into the tissues from [Oberdorster's] fish study, but whether they can get past the other two barriers, we just don't know yet."
The team's molecular model showed that buckyballs fit precisely into two spots on the spiraled helix of DNA molecules. Buckyballs could lodge at both the end of DNA strands or in minor grooves along the outside of the DNA. In either case, the binding will cause the DNA molecule to bend over to one side. The damage was most severe when the cell was reproducing by splitting into two separate helices, as it does when it divides or when it manufactures new proteins. The presence of buckyballs prevents both actions from happening.
"Our expectation was that buckyballs were so hydrophobic they don't like water that they would simply clump together and fall out of solution, but in fact they can stay in solution to a sufficient degree to be ingested and then make their way to the tissues," said Cummings.
The strongest evidence showing buckyballs are a biohazard was the finding that they have a stronger affinity for human DNA than they do for each other.
"The worst thing is the strong binding energy between buckyballs and DNA energies comparable to those binding drugs to receptors," said Striolo. "If buckyballs get into the nucleus of a cell, our simulations show they can bind to the DNA and damage it. The cell could even stop repairing itself." He added, "The best we can hope for is that we are wrong."
The team then tested cells under various common circumstances to see how DNA infected with nanoparticles would react. The worst outcome for double strands was complete penetration of the end of the DNA helix, permanently breaking the hydrogen bonds between the pairs of nucleotides. The worst case for single strands of DNA, which are exposed during cell division and self-repairing, was buckyballs moving in to fill vacant sites, thereby preventing the normal reproduction and repair operations from taking place.
Cummings added this caveat: "Our simulation doesn't predict how buckyballs could get into the nucleus just that they could cause real problems if they get in. Now we need more experimental and theoretical studies to find out just how hazardous they are."
Cummings predicts that only organic nanoparticles (composed of carbon or hydrogen) will be a potential problem, thus giving inorganic nanoparticles made of materials like silicon and titanium a clean bill of health. However, only testing will resolve the question conclusively, he said. The high aspect ratio of some nanoparticles such as nanotubes will probably also let them off the biohazard hook, he said.
Even some organic nanoparticles "will probably be safe," said Cummings, "such as carbon nanotubes, which are much larger because they are usually quite long. We expect that the possibility of nanotubes getting through the skin, into the tissues, the cell and nucleus is much more remote than [with] buckyballs."
Cummings' team is already working on qualifying other nanoparticles as potential biohazards by performing the same DNA affinity test as they did for buckyballs. After that, they will institute a new suite of simulations to perform standard environmental protection agency biohazard tests, such as the Octanol-water Partition Coefficient Determination test. The EPA test "gives you some indication whether a given material will go from a water phase into living tissue or not," said Cummings.