Chemists are masters of materials, and they often work in the world of the very small. Using tools made from the building blocks of life, chemists can spy on the movements of single molecules and make miniature devices that pick up trace levels of contaminants in food and the environment.
The space between two carbon atoms within a molecule is about one-tenth of a nanometer. The DNA double helix has a diameter of about two nanometers. The smallest bacteria, on the other hand, are much bigger: a few hundred nanometers in length.
A nanometer is one-billionth the length of a meter—or about the circumference of a marble in comparison to that of the Earth!
And nanotechnology is the study of the control of matter on an atomic and molecular scale. Some say it is chemistry by a different name.
Fittingly, some entire modern chemistry "labs" are extremely small—cramming all the necessary tools and molecules onto a rectangular wafer smaller than a business card. Such mini-machines contain an expansive network of miniature tubes and columns, each only as big as a fraction of a drop of water.
Chemists want to use tiny devices to deliver drugs to specific sites in the body, allowing for highly targeted treatments with minimal side effects. Other devices could measure cholesterol, sugar and electrolytes in blood, saliva, urine or tears.
Seeing the Body in a New Light
Small tools also allow scientists to watch biology happen in real time. Bright, rainbow-colored dyes and a green fluorescent protein (GFP) that comes from jellyfish let scientists track how molecules move around in living organisms. Often, these experiments are done in simple organisms like bacteria and yeast, which consist of only a single cell but have inner workings with a striking degree of similarity to human biology.
In studies with human cells, researchers have tagged cancer cells with GFP to watch how they spread to other parts of the body. They mark insulin-producing cells in the pancreas to see how they're made and gain insights into new diabetes treatments.
Another technology, called quantum dots, uses microscopic semiconductor crystals to label proteins and genes. Quantum dots enable scientists to study molecules in a cell as a group, rather than in isolation.
Dots of slightly different sizes glow in different fluorescent colors—larger dots shine red, while slightly smaller dots shine blue, with a whole spectrum in between. Researchers can create up to 40,000 labels by mixing quantum dots of various colors and intensities, much like an artist mixes paint.
Another group of modern chemistry tools is sensors, devices that measures a physical quantity and convert it into a signal that can be read by an observer or instrument.
We use sensors all the time in our daily lives. A simple example is a thermometer, which transforms a measured temperature into the expansion and contraction of a liquid that can be read on a calibrated glass tube. Another is a touch-sensitive elevator button.
Scientists and doctors use sensors all the time, too. Biosensors can scan a wide range of biological materials, from microbes, enzymes and antibodies to pollutants. The output, or signal, varies widely as well, ranging from color to light to electricity.
One of the most common examples of a health-related biosensor is a blood glucose monitor.
This miniature device uses the enzyme glucose oxidase to break down blood glucose and produce a readable signal, which indicates how much sugar is present in a person's blood. It is a vital tool for people with diabetes who must check their blood sugar several times a day.
Some biosensors rely on modified microorganisms that detect toxic substances at very low levels. They can give us early warning of environmental contaminants, poisonous gases and even bioterror agents, such as ricin or anthrax.
Chemistry also plays a central role in making biomaterials such as artificial joints, implants, heart valves and skin patches filled with hormones or other medicines.
Chemistry is critical to efforts to engineer artificial organs—like a liver, pancreas or bladder. Researchers have succeeded in making test versions of many artificial organs. And some, like artificial skin for the treatment of severe burns and traumatic injuries, are already in wide use.
The Many Faces of DNA
DNA—it's not just for heredity anymore! Although its primary function is to pass on genes from parents to offspring, chemists are capitalizing on some of the unique features of this versatile substance.
Imagine electronic devices wired with DNA that can work faster and fit into a tiny fraction of the space that today's larger machines require. DNA-based mini-machines will be extremely efficient, consuming less power and producing less heat than the equipment currently in routine use.
Although DNA computers aren't mainstream yet, they offer researchers a powerful tool for solving bewilderingly complicated problems.
Computers made of DNA make sense: DNA evolved as the carrier of the information of life for good reason: It is stable, predictable and correctible. Intact strands of DNA have been unearthed in specimens thousands of years old. A set of mathematical rules defines how DNA is transmitted across generations. And to top it off, DNA is self-replicating—it can copy itself.
Take a famously hard-to-solve math puzzle called the Hamiltonian Path Problem, in which for several points—cities, for instance—the goal is to find the shortest trip from the start city to the end city. But the rules say that you can travel through each intervening city only once.
Seem easy? Conventional computers have a miserable time finding the answer, because the only way machines can solve the problem is to try all the possibilities, one by one. In fact, with 100 or so cities, a supercomputer is needed, and with 1,000 cities, no existing computer can tackle it.
Scientists made a DNA-based computer that could test all the possibilities at once, in parallel. The "cities" were made out of synthetic strands of DNA that they made in the lab (much like a gene, each DNA-city had a different combination of the four different nucleotides). Connector strands, which linked the end of each DNA-city with the start of another (several cities could be hooked together in this way), were sized according to the distance between the cities. The scientists then mixed everything together, and all of the matching DNA-cities came together in all possible combinations.
The answer to the problem is the shortest string of DNA that emerges from all combinations.
Some of the factories of the future will be far smaller than those of today. That's because such manufacturing facilities will employ tiny robots, not humans, to perform routine and repetitive tasks. Scientists have used lab-prepared strands of DNA to construct miniature robots.
Like with DNA computers, DNA robots are made from synthetic DNA molecules of varying rigidity (based on the molecular sequence of chemical letters). Bacterial enzymes link together the different parts of such a device, creating hinge-like connections. The junctions of the mini-machine's parts are twists and turns that naturally occur in DNA.
The versatility of DNA as a building material allows researchers to make DNA tweezers that open and close and "nanowalkers" that can move along a footpath, or track, made itself of DNA.
Why on earth would anyone want or need walking DNA? These nanodevices, which are much too small to see, could carry loads like molecules of medicine or other biomaterials within the body.
Yet another cool use for DNA is to transmit electrical currents, like a wire. Scientists have known for half a century that the DNA in our bodies-and in microbes, plants and animals-has a special structure (called a double helix) that looks a lot like an upward spiraling staircase. The two halves of the staircase are complementary: they stick to each other much like the opposite strands of VELCRO®.
Each railing of the staircase molecule consists of ringed sugar molecules held together by chemical units called phosphates. The steps in between are stacks of aligned ringed molecules called bases.
Because of the geometric arrangement of the whole thing, strands of DNA have defined electrical properties. The nucleotide stairs exhibit orderly displays of electrons and form what scientists call "pi-ways." In chemistry, pi bonds (p bonds) are covalent chemical bonds formed by the overlap of electron clouds, or orbitals, that swirl around atoms. Electrons can literally hop along these routes, carrying current like any other electrical circuit.