Human Endeavors: Microscopes: From Animalcules to Atoms - From Leeuwenhoek's first report of microscopic animals (animalcules) in 1676 to the viewing of atoms in 1983, humans have endeavored to probe the invisible through the development of better microscopes and other technology.
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In 1676 Antonie van Leeuwenhoek reported to the Royal Society in London that he had observed an invisible world made visible by the microscope, a world in which small animals, living creatures that he called animalcules, were alive in a drop of rainwater. Thus began the first step in a human endeavor that would lead to the cell theory, to germ theory, and eventually to our imaging of atoms and even to detecting the changes that occur in atoms as they undergo chemical reactions. Imagine, only a century ago scientists debated the existence of atoms, and until January 1983 no one had ever seen the image of an atom. Today, using modern scanning-probe microscopes, scientists are not only seeing atoms but can move them about. All this is due to our new microscopic technology.
Since the early seventeenth century, lens makers were viewing the microscopic world through their very primitive light microscopes, and the light optical microscope was the primary research tool of biologists until the 1930's. But visible light has limits as to how small an object you can see. This limit is due to a phenomenon known as resolving power. Resolving power is the ability to distinguish between two points and is due to the wavelength of light energy used. Light travels in waves, and the distance between the crests of the waves is known as its wavelength. The resolving power is equal to one-half of the wavelength of light used. For example, because visible light ranges from 380 to 720 nanometers, or billionths of a meter (a nanometer is equal to 1 billionth of a meter), visible light can only distinguish objects about 160 nanometers in diameter. Obviously, if you used a smaller wavelength, for example the wavelength of electrons (which is about 1 nanometer in length)you can see objects 0.5 nanometers in size. In the 1930s, a new type of microscope was developed, the electron microscope, which could be used to see very small objects. Electron microscopes use electrons rather than light and magnetic lenses rather than the glass lens used in the optical microscope to view specimens, but the electron microscope also has its limitations. One limitation is that the electrons have to be beamed in a vacuum to avoid scattering. Because of this, the specimen has to be specially prepared and sectioned for viewing. For the most part, except when using the high-voltage electron microscope, no living specimens can be viewed. If you try to see atoms, because the electron beam is so concentrated, the energy of the beam would destroy the fragile atoms in organic molecules or pass right through them, showing nothing. Scientists also developed modifications of the electron microscopes, like the scanning electron microscope, which has the advantage of scanning the surface of the specimen with a narrow beam of electrons. This allows for the production of a three-dimensional image that can be viewed on a visual monitor (like a computer screen). Also, computer programs can enhance the image and even add color. But still, the limits of the wavelength prevent the viewing of atoms. But what if we imaged by feel (touch), not sight?
In 1981, Gerd Binning and Heinrich Rohrer in Zurich, Switzerland, developed the scanning-probe microscope. The scanning-probe microscope is based on the principle that if you place a probe near the surface of a sample and measure the distance between the probe and the sample, you can produce an accurate image of the sample. It is sort of like being in the dark and running (or scanning) your hands over the surface of eggs sticking out of an egg carton to detect the nature of the surface of the eggs, rather than using light to see them. The probes are like your hands, and they detect the peaks and valleys of the image; as you move your hands, you can scan the sample, but in order to have an accurate image of atoms, you have to be able to move the probe in controlled steps, each step less than the diameter of an atom. The technological breakthrough that allowed for that exact scanning was the development of piezoelectric crystalsmaterial that can change shape as electric current is applied, which is translated into minute movement of the probe. Three sets of crystals are used to allow for a three-dimensional image. Today, using a finewire platinum probe and piezoelectric crystals, scientists are literally tunneling into atoms. The scanning-tunneling microscope charts the shape of the surface being scanned, just like you touching the eggs to detect their size, shape, number, and so on. And the computer message, the electrical current between the probe and the specimen, is used to produce an image of the atom or molecule. But what if the specimen does not conduct electrons? Today that limitation of the scanning-tunneling microscope has been overcome by the atomic-force microscope, developed in 1985. The atomic-force microscope uses a diamond point glued to a piece of silicon about the size of a pinpoint. This forms a silicon cantilever, sort of like a diving board. The repulsive forces produced by the electron cloud of the atom bend the silicon cantilever. A laser beam can then measure the amount of bending and produce the image of the specimen. Using this microscope, scientists can image very delicate molecules like DNA and other organic materials and even film the molecular events that lead to a biological process, like the formation of protein fibers in a blood clot.
As you can see, we literally can image and see atoms. But this is just the beginning. We will discuss how this technology at the nanometer level is rapidly evolving into nanotechnology, where we can actually micromanipulate atoms and image the changes in atoms as chemical reactions occur.
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