FSF Magazine, May 2007

by Spilogale Authors

  Nanobots aside, we'll talk about what nanotechnology is, why it's interesting, and where its effects can be observed in nature. We'll tell you about some of the products of nanotechnology that are already on the market, and we'll consider the potential impact of this new technology.

  Think Small

  To understand nanotechnology, you have to think small. Nanotechnology and nanoscience involve unimaginably tiny objects, which are measured in a ridiculously small unit of measurement: the nanometer. A nanometer is one-billionth of a meter. That's the American billionth, a thousandth of a millionth. (Here Paul jumps in and inserts scientific notation, explaining that a billionth can be written as 10-9 m.)

  Here are a few facts that may help you imagine a nanometer:

  * If a marble were a nanometer across, then the Earth would measure a meter from pole to pole.

  * A human hair is 50,000 nanometers in diameter.

  * Five carbon atoms in a row fill a distance of one nanometer.

  Are you having trouble imagining a nanometer? If you are, don't feel bad. You aren't alone.

  In the course of her work at the Exploratorium, Pat interviewed Don Eigler, a physicist who specializes in studying the physics of surfaces and a nanotechnology pioneer. Having spent some time being frustrated by analogies like the ones included above, Pat was pleased when Eigler admitted that all these analogies and comparisons really didn't help him imagine the nanoscale world. “I can't imagine what a hundred million of anything in a row is like,” he said. “It's just too big."

  A billion is ten times bigger than a hundred million. So if imagining one hundred million is tough, imagining a billion of anything is even tougher. A billion is approximately three times the population of the United States. It's the approximate number of letters in 6000 books of 500 pages each. A billion seconds is about thirty-two years. A billion is the number of dollars the U. S. government spent on research and development related to nanotechnology in 2005.

  A billion is a very big number. And a nanometer is a meter, divided into a billion parts. Can you imagine that? We can't.

  Many discussions of nanotechnology seem to get stuck at this point. Some educators seem to be obsessed with helping people imagine this unimaginably small fraction of a meter.

  We're not like that. We say, “So you can't imagine it? Get over it. Move on."

  Size is the defining feature of the nanoscale world, but size isn't what makes this world so interesting to researchers. So let's get to the interesting stuff.

  Small is Different

  Nanotechnology generally deals with objects with dimensions of between one and one hundred nanometers. One reason these tiny objects are interesting is they obey a different set of rules than the ones you and I are used to. We live at a size scale where certain physical laws apply—the laws of classical mechanics observed by Isaac Newton in the seventeenth century.

  There are two major reasons that working at the nanoscale is different from working at the macro scale of everyday objects: 1) quantum mechanics and 2) the increase in the ratio of the surface area to the volume of a particle. Because of these two factors, a few atoms together in a nanoparticle just don't behave the same way they do when there are trillions of atoms together.

  Let's start with the issue of quantum mechanics—the modern physical theory that deals with the structure and behavior of subatomic particles. One of the tools used in nanotechnology is the scanning tunneling microscope. The word tunneling here is not about boring holes in mountains. Rather, it is about a quantum mechanical effect in which particles (electrons) disappear from one place and reappear at another without passing through the space in between. That's something you don't see every day. (Well, except with cats—but that's another story.) Quantum mechanical effects become noticeable at very short distances, and they usually vanish when we deal with everyday objects. So quantum mechanics is important at the small distances of the nanoscale.

  The change in surface area with particle size is easy to understand. Take a two-by-four and break it down into sawdust. Measure the surface area of each of those little bits of sawdust and add them all together. The resulting surface area will be much greater than the surface of the original two-by-four. Chemical reactions often occur at surfaces. A cloud of wood dust in air can explode, while a two-by-four usually does not.

  Atoms at the surface of a material behave differently from atoms inside a volume of material. Inside, the atoms are completely surrounded by other atoms; outside, they are not. Water molecules at the surface of a piece of ice, for example, behave more like water molecules in a liquid than they do like water molecules in their solid, frozen form. It's that different behavior of water molecules at the surface that allows ice-skating to happen.

  The amount of surface area at the nanoscale becomes huge compared to the surface area of everyday objects. In a nanoparticle (a chunk of matter less than one hundred nanometers across), almost all of the atoms or molecules in a substance will be near the surface. That changes the way those atoms or molecules behave.

  Suppose you have a fist-sized lump of gold. The number of atoms at the surface of the lump is small compared to the total number of atoms in the lump. But if you broke that lump of gold down into gold nanoparticles, the ratio of surface to volume changes. In a cluster of 100 atoms, more than half the atoms are on the surface. If you have a cube with five atoms on a side, then you have a total of 125 atoms—and 98 of them are on the surface! The properties of a nanoparticle are really governed by surface effects.

  How does that change the way that gold behaves? When it's a fist-sized chunk of gold, it looks gold in color and it melts at around 1948 degrees Fahrenheit (1064 degrees Celsius).

  Compare that to particles of gold that are between one and one hundred nanometers across. These particles melt when heated to just a few hundred degrees Fahrenheit, a change that probably relates to the increase in the atoms near the surface.

  These particles no longer look gold in color. They can look red, blue, or a variety of other colors, depending on the particles’ sizes and distance from each other. That, we think, is a quantum mechanical effect related to the behavior of electrons in the gold.

  The electrons that make gold a metallic conductor are free to move from gold atom to gold atom. However when there are clusters of just a few gold atoms, quantum mechanics restricts these electrons to specific energy levels. That means the particles can absorb certain colors and give off light of other specific colors. They can turn white light into red, for example, by absorbing the blue-green light and scattering red light.

  The ancient Romans knew how to color glass by adding gold. Initially the glass is colorless, but it becomes ruby-red when heated in a controlled fashion. The Romans knew how to make the glass turn red, but they didn't know that the color came from nanoparticles of gold.

  Butterfly Wings and Gecko Toes

  A close examination of the natural world reveals other examples of how the nanoscale world affects what we see or experience at the human scale. Take, for example, the blue morpho butterfly. This butterfly's wings are a beautiful, shimmering blue, a color so bright that naturalists have reported seeing the flash of blue wings from a quarter of a mile away. You might think that such a vibrant color comes from blue pigment—but there is no blue pigment in the butterfly's wings. In fact, microscopic studies have shown that the butterfly's wing is covered with tightly packed rows of clear scales. No color at all!

  These clear scales form layers that reflect blue light. Each layer is 62 nanometers thick and the layers are 207 nanometers apart. This spacing is exactly what's needed to reflect that shimmering blue light. Spacing of other distances would reflect light of other colors. The interaction of light with these nanoscale structures creates the brilliant blue color of the butterfly's wings.

  Another natural example of how very small structures have very big effects can be found on the feet of geckos, lizards noted for their ability to run across walls and ceilings, sticking e
ffortlessly to the slickest surface. On the bottom of each gecko foot are half a million microscopic hairs, each about one tenth the diameter of a human hair. The end of each hair splits into hundreds of even tinier hairs, measuring just 200 nanometers across. When a gecko presses its foot down, these tiny hairs unfurl, pressing very closely against the surface.

  When molecules are brought very close together, they are weakly attracted to each other. We don't usually notice this attraction, known as van der Waals forces. (For those who must know, van der Waals forces occur when an unequal distribution of the electrons in one atom creates an area of positive charge and an area of negative charge, known as an electric dipole. The dipole affects a neighboring atom: the positively charged area attracts the neighbor's negatively charged electrons, creating an electric dipole in the neighboring atom. The positive charge attracts the negative charge and the two stick together.)

  Van der Waals forces operate at the nanoscale. These forces, multiplied by the millions of hairs on the gecko's feet, hold the lizards to the ceiling quite securely.

  The nanoscale characteristics of butterfly wings and gecko toes have inspired researchers to contemplate commercial products that make use of the same principles. Researchers at Manchester University's Centre for Mesoscience and Nanotechnology in the United Kingdom have developed what they call “gecko tape,” a super-sticky reattachable dry adhesive that uses synthetic hairs that mimic those on the gecko's feet. Researchers at cosmetic manufacturer L'Oral are working to produce cosmetics that reflect brilliantly colored light like the blue morpho butterfly's wings.

  It's Happening Now

  Gecko tape and blue morpho makeup aren't on the market just yet. Many reports on nanotechnology focus on future possibilities, describing how nanotechnology could change the world. Those are certainly interesting to contemplate. But we've decided it's more interesting to contemplate the ways that nanotechnology has already affected our lives.

  Take, for example, the gas in your car. That gas was extracted from crude or unprocessed oil, the stuff that comes out of the ground—and nanotechnology has made a big difference to how much gasoline is extracted from every barrel of oil.

  Crude oil is a mixture of hundreds of different hydrocarbons, compounds made of hydrogen and carbon. When crude oil is refined, large hydrocarbon molecules are broken into smaller ones in a process called cracking. How much gasoline can be extracted from a barrel of crude oil depends on the efficiency of the cracking process.

  Back in 1962, researchers at Mobil dramatically increased the efficiency of the cracking process, upping the quantity of gasoline extracted from a barrel of oil by a whopping forty percent. They accomplished this revolutionary change in petroleum refining with a porous crystal called zeolite. Riddled with pore openings small enough to distinguish between molecules of different sizes and shapes, zeolite acts as a catalyst, an additive that accelerates and increases the efficiency of a chemical reaction. (Those zeolite crystals qualify as nanotechnology because the holes that riddle them are tiny—less than a nanometer across in some cases.) According to a 1992 National Academy of Sciences estimate, the shift to a zeolite catalyst saves the United States more than 400 million barrels per year of oil.

  That change in 1962 didn't make the newspaper headlines. It was a change in an industrial process—not something to get worked up about. It wasn't called “nanotechnology” back then, but that's what it's called now.

  Mobil's use of zeolite can be said to be one of the first broad-scale applications of nanotechnology. It's an example of what Mark and Daniel Ratner, authors of Nanotechnology—A Gentle Introduction to the Next Big Idea, call “stealth nanotechnology.” That's nanotechnology that's hidden in other products, nanotechnology that we, as consumers, don't even notice, though it stealthily makes a difference in our lives.

  Carbon Nanotubes and Hockey Sticks

  The products of nanotechnology seem to inspire superlatives: super small, super sticky like gecko tape, or, in the case of carbon nanotubes, super strong.

  Carbon has different forms, the best known of which are graphite, a soft substance made of layers of carbon, and diamond, a super-hard substance made by carbon atoms joined in a rigid crystal. In 1985, three scientists discovered the buckminsterfullerene molecule, also known as a buckyball or fullerene. This previously unknown form of carbon is an arrangement of sixty carbon atoms in a spherical structure that looks a lot like a geodesic dome.

  Following the discovery of the fullerene, researchers worldwide were inspired to look for other forms of carbon. In 1991, Japanese scientist Sumio Iijima discovered the carbon nanotube.

  Carbon nanotubes are ridiculously strong (much stronger than steel), light, and flexible. NASA is very interested in using them to create lighter and stronger spacecraft. Nanotubes have already been put to work in aircraft, lightweight bicycle frames, super-strong hockey sticks, and other sporting equipment. They may also be used to make car bodies stronger and lighter, contributing to fuel economy.

  That's what's happening now. But the unique characteristics of carbon nanotubes have inspired much speculation about what could happen in the future. Maybe you've heard of the “space elevator.” This theoretical device would replace the rockets that propel material into space. Instead, a permanent structure would lift material into orbit. Descriptions of this device usually involve a tremendously long and tremendously strong tether or cable that connects the Earth's surface with a point beyond geosynchronous orbit. Carbon nanotubes are the first thing that's come along that could be strong and light enough to serve as the cable in a space elevator.

  Just as interesting as the strength of carbon nanotubes is their electrical conductivity. They may be the perfect material for making tiny electrical circuits, since electricity passes through them with very little resistance. In 2002, researchers succeeded in making nanotube transistors. At IBM, researchers used a single nanotube to create a working computer circuit. The nanotube became a voltage inverter, or NOT gate. The NOT gate, which flips an incoming bit of binary code from a zero to a one (or from a one to a zero), is one of the three fundamental types of logic gates on which all computers rely. Since each nanotube is basically one large molecule, the researchers essentially created a circuit using a single molecule. Paul reports seeing a wonderful picture of what looked like a thin clothesline draped across a city block of buildings. Except the picture had been made using an electron microscope; the buildings were the smallest transistors made so far on an integrated circuit chip, and the clothesline was a carbon nanotube!

  Why does this matter? Well, maybe you've heard of Moore's Law. In 1965, Intel co-founder Gordon Moore predicted that computer processing power, or the number of transistors on an integrated chip, would double every eighteen months. This prediction became known as “Moore's Law,” and so far, it's held true. In 1965, a single chip held thirty transistors. Six years later, Intel introduced its first chip, which held 2,000 transistors. Today's chips have over one hundred million transistors.

  As chipmakers pack more chips into less space, we get faster computing and greater data storage. But there's a limit to how many transistors can be packed onto a silicon chip. Researchers are looking to carbon nanotubes to come in when the features on silicon chips just can't be made any smaller.

  A Trojan Horse for Cancer Cells

  Nanomedicine is another promising area of research. That makes sense, since life operates at the nanoscale. In all the cells of your body, tiny molecular machines are constantly working to keep you alive.

  One promising area of research is the use of nanoparticles to destroy cancerous cells selectively, rather than killing healthy cells along with the cancerous ones. Many researchers are working on this problem, but our favorite example is the work of Dr. James R. Baker and a team of researchers at the University of Michigan.

  Methotrexate, a powerful chemotherapeutic drug, can be toxic to both cancer cells and healthy cells. Baker and his team knew that all cells need the vitamin kn
own as folate, or folic acid. They also knew cancer cells had a particularly voracious appetite for folate. Using a synthetic molecule called a dendrimer, researchers connected a molecule of folate to a molecule of methotrexate. The cancer cells grabbed the folate and yanked it inside. Along with the folate came the methotrexate, which poisoned the cell. The dendrimer, acting as a molecular trojan horse, fools the cell into admitting the source of its own destruction.

  When tested with laboratory mice, this Trojan horse therapy was ten times more effective than the drug alone. This study, according to the University of Michigan, is the first in which a nanoparticle-targeted drug leaves the bloodstream, is concentrated in cancer cells, and has a biological effect on an animal's tumor.

  Research is underway to create other ways to combat cancer, to monitor blood chemistry and release drugs only when they are needed, and to develop artificial skin, artificial bone, and artificial cartilage that the body won't reject.

  Now and the Future

  It's tough to write an article about nanotechnology that doesn't come across as a laundry list of seemingly unrelated possibilities. We have super-strong tennis rackets and targeted cancer drugs; catalysts for petroleum refining and shimmery eye shadow. Products of nanotechnology that are currently on the market include stain-resistant pants made of nanotechnology-enhanced cotton, sunscreen containing nanoparticles of zinc oxide that block the most dangerous form of ultraviolet radiation, socks with embedded silver nanoparticles that kill bacteria and keep the socks from stinking, and water filters equipped with nanopores that filter out viruses. (To see a more complete list of products that make use of nanotechnologies, see the Project on Emerging Nanotechnologies’ nanotechnology consumer products inventory at www.nanotechproject.org/44.) The varied products on this list have one thing in common: they have all emerged from a new way of examining and manipulating the world.