Because of their light, stiff composition, merely sprinkling carbon nanotubes into epoxy strengthens the glue by more than 30 percent. The tubes have also begun turning up in high-end sporting equipment. They strengthen tennis rackets, mountain-bike handlebars, frames for racing bikes, and golf-club shafts. Carbon nanotubes also show promise for use in transparent conductive films for displays on computers, cell phones, PDAs, and automatic teller machines.
Smalley was also an ardent advocate of nanotubes as a solution to the world’s impending energy crisis. His plan was to replace old copper and aluminum power lines with wires spun from carbon nanotubes. Nanotubes can carry far more current than traditional metal wires—over a billion amps of current per square centimeter—and, unlike metal wires, they lose very little of that energy as heat. In theory, the nanotube power lines would carry electricity over thousands of miles. Rather than relying on local coal-fired power plants, cities could use energy generated by giant solar farms in deserts or by wind farms off coasts. “This is the great getting-up morning of nano,” Smalley said. “If Mother Nature allows it, we could restring the electrical grid of the world.”
Not everyone is so sure. Carbon nanotubes come in three types. They all conduct electricity, but only one does it especially well. And so far no one has come up with a way to make those nanotubes very long. Right now, the longest electricity-conducting nanotube in existence measures a fraction of an inch.
At the root of the problem is the fact that there are two ways to make nanoparticles: “top down,” where a bulk material gets chopped down into nanosize bits, and “bottom up,” where molecules grow under controlled conditions, as in crystals, and then snap together into particular configurations based on their charge and molecular chemistry.
Bottom-up constructions—which long carbon nanotubes would require—are where the real power of nano lies. But they’re also far more complicated, subject to all the laws of bonding that limit the ways atoms and molecules can be arranged. Getting carbon to curl into a perfectly aligned tube rather than a thick, twisted scroll is exceedingly complex.
Scientists are still relatively ham-fisted when it comes to the finer points of bottom-up assembly, particularly compared with a far more prolific nanofactory: the human body.
The human body makes quick and constant work of assembling raw materials like calcium and keratin to create elaborate structures like bones and skin. Compared with the work a blood cell does, scientists are “pretty much inept,” admits Jim Heath, a Caltech chemist who is developing nanoscale sensors capable of detecting and diagnosing cancers. “But we’re learning. We’ve come a long way in the past two years.”
Heath’s goal is to identify cancers early, when they are still just a few thousand cells strong and far easier to treat. Unlike HIV or malaria, which produce unique antibodies identifiable from a simple blood test, cancers are difficult to spot. Nonetheless, they do leave what Heath calls a fingerprint: a change in the number and type of proteins that regularly circulate in the blood. Determining which combination of proteins makes up the unique signature of a particular cancer is an ongoing project. “To diagnose one cancer reliably in the early stages, we probably need to measure 20 or 25 different proteins,” Heath says. “So to develop a test that would identify 20 different cancers, we’d need about 500 measurements. And we would want to be able to do that easily, with just a finger prick of blood.”
Heath has already developed nanosize sensors called nanowires that can electronically detect a few protein molecules along with other biochemical markers that are early signs of cancer. Heath’s strategy is to coat a collection of nanowires with different compounds, each of which binds to one particular marker. When the marker, which can be a protein, an antibody, or DNA, latches on, it changes the conductivity of the nanowire, creating a tiny but measurable alteration in current. Heath has combined tens of thousands of these sensors onto a single chip, which allows him to detect cancer-signifying molecules in blood while their concentration is still low. The chips also allow him to identify what types of cancer are present. Currently, Heath reports, his chip can detect between 20 and 30 relevant biomolecules. He plans to begin using the chip to detect brain cancers this summer.
Richard Smalley was one scientist who followed Heath’s progress carefully. Smalley’s non-Hodgkin’s lymphoma was a relatively slow-moving cancer, but even when he was in remission, between a hundred million and a billion cancer cells circulated in his body (a number that doctors consider relatively low).
One of the advantages of treating cancer in an early stage is that the cells are less likely to have mutated and become resistant. Drug resistance is one of the trickiest things about cancer, which adapts so rapidly that medications can rarely keep up. “You don’t want a killing mechanism to be fancy,” Smalley said. It needs to be fast and thorough.
But targeting a brute-force treatment is difficult, says Jennifer West, a bioengineer who is treating tumors in mice using gold nanoshells. Difficult because things that kill cancer cells typically kill healthy cells as well. “That’s what we’d like to avoid,” West says. Her approach relies on the fact that tumors grow blood vessels so quickly to keep up with the rapidly multiplying tumor cells that they don’t have time to knit tightly and instead leak like rusted pipes. West’s gold nanoshells are about 120 nanometers in diameter—a cancer cell is 170 times bigger. So the nanoshells are minute enough to seep through the cracks in the tumor capillaries and become lodged in the tumor.
To kill the tumor, West activates the shells with infrared rays that pass harmlessly through the skin but heat the gold, killing the adjacent tumor cells. Because the cancer cells die, they don’t develop the resistance that can plague drug-based cures.
Moreover, because the nanoshells lodge only in the tumor and are nontoxic unless activated by infrared light, West expects her treatment to be nearly side-effect free—particularly compared with treatments like chemotherapy and radiation. As part of the FDA approval process, West has injected mice with increasingly large doses of nanoshells. Not a single mouse has died. “We haven’t even been able to induce any adverse effects,” she says with a shrug. “If we had injected these mice with the same amount of table salt, they would have keeled over long ago.”
Unfortunately, the very thing that makes nanoshells such a promising therapy—their ability to move easily through the body and to interact with different cells— is a downside when it comes to the problem of nanoparticle pollution.
In 2004 Eva Oberdörster, a toxicologist at Southern Methodist University in Dallas, reported that largemouth bass exposed to water containing buckyballs at a concentration of 500 parts per billion suffered brain damage. And people are similarly vulnerable. After exposing lab-grown human skin and liver cells to an even weaker solution—a mere 20 parts per billion—Rice University chemist Vicki Colvin found that fully half the exposed cells died.
Results like these are troubling, in part because of the rapidly growing number of products already on the market that contain nanoparticles. “With nanomaterials, it’s not enough to look at the properties of the bulk material,” Colvin warns. “Whether you’re working with gold or lead, the toxicity will be hard to predict.” There is some evidence, for example, that the nanoscale particles of titanium dioxide used in sunscreen, depending on the way they are nanosized, can produce high amounts of free radicals when exposed to sunlight. Free radicals can damage cells, making some more likely to turn cancerous.
Colvin’s concern is that companies are currently optimizing their particles for processability rather than for human health. A recent study found that buckyballs could be made less toxic fairly easily—by attaching inert molecules known as hydroxyl groups. The more hydroxyl groups attached, the less dangerous the buckyballs became. For the most thoroughly coated, the safe exposure level went up by a factor of ten thousand.
But it’s hard to get funding for this kind of research, Colvin says. “Funding managers want a sexy story at the end of the day. They want to be able to say that they’re helping to cure cancer. It doesn’t sound as glorious when your finding is that a certain particle you were hoping to use ought to have hydroxyl groups put on it in order to be safe.”
Still, researchers are making important advances. They are finding new ways to use nanosize sensors in water purification systems that will filter everything from bacteria to industrial pollutants like arsenic. The key feature of the new filters is the fact that nanoparticles have a vast amount of surface area for their weight: One ounce of nanobeads, for instance, contains a staggering 300,000 square feet of surface area. Because the chemical reactions that neutralize pollutants take place on the surface of the beads, the greater the available area, the more effective the filter.
The potential impact of nanofilters is substantial. Many regions in China have drinking water that contains dangerously high levels of arsenic and other industrial pollutants. Because of this, Colvin predicts that Asia will be a test bed for point-of-use water treatment systems that utilize nanoparticles to eliminate toxic chemicals. “Right now, nanoscale iron is a bit too expensive to be used to treat wastewater,” she says. “But it’s the best way to clean up concentrated arsenic, and I expect the cost will come down soon.”
Because nanotech applications are so potentially useful, Colvin doesn’t think research should be stopped, or even slowed. But she does think that a larger proportion of government money should be directed toward safety and related questions—like whether nanoparticles could accumulate undetected in the water and food chains.
Such safety issues are key, given the speed with which the nanotech tsunami is moving. Corporations will invest more than four billion dollars in nanotech this year alone, and a recent nanotech conference in Japan drew a crowd of 30,000.
Meanwhile, commercial applications continue to spread. Homeowners now have the option of installing windows manufactured by PPG Industries, a company that uses nanoscale particles of titanium dioxide to make glass that doesn’t streak and never needs washing. Food companies have begun experimenting with nanopackaging that changes color when food spoils or contains bacteria like E. coli. The prefix has even trickled over into popular culture, where it’s the advertising hit du jour, with GM hawking a “nano” Hummer, and Apple its iPod Nano digital music player.
“What’s amazing is how quickly this is evolving,” Colvin says. “Even ten years ago, a lot of these applications would have seemed pretty unrealistic.”
The boom left Richard Smalley downright nostalgic. “Nano is a baby that’s all growed up,” he mused shortly before his death. Perhaps, but we’ve still got some interesting years ahead.