The allure of nanoscience can be understood by considering what happens if a macroscopic gold nugget is divided into ever smaller pieces.1 At first, the piece of gold will retain all of its typical characteristics—it simply gets smaller. When the pieces reach a few micrometers in size, we are no longer able to distinguish individual particles with our eyes. However, given enough of them, we still see gold dust. However, when the particles reach a size of -100 nm, something very dramatic happens: The particles change color. For particle diameters between approximately 100 and 30 nm (i.e., for particles containing between approximately 30 million and 1 million gold atoms) the particles change from red or yellow, to green or blue. The particle’s color is determined by its size. Quite amazingly, these colored gold particles have been known since the Middle Ages, when they were used to make beautiful colors in stained glass windows. Of course, the medieval artisans did not know that they were using nanotechnology, or even why the gold produced the colors it did: They just knew that a particular process produced a beautiful effect (see Figure 6–1).

It is only in the last few years that we have begun to understand the size-dependent changes that occur in gold and other metallic nanoparticles. The size of a nanoparticle determines the character of its surface plasmons, a type of collective motion of the electrons within the particle that gives rise to its color. The strong dependence of the particle’s characteristics (in this case its color) on the size of the particle is one of the key features of nanoscience. With our understanding of the nature of the color changes comes the opportunity to tune the particles to achieve the behavior we desire.

OPPORTUNITIES IN SIZE-DEPENDENT DESIGN

The nanoworld is intriguing because it is not just the color of nanoparticles that depends on size. At the nanoscale, a wide range of physical, chemical, and even biological properties can be strongly dependent on the particle size. We expect that by working at this scale, we will soon be able to design and engineer structures with a tremendous variety of desirable features. This is exactly what nature does in the inner workings of every cell. Proteins are nanoscale molecules that are assembled by nanomachines called ribosomes, which connect a series of amino acid molecules together according to a pattern furnished by RNA, another nanoscale molecule. Each protein is essentially a nanoengineered molecule that has been optimized to perform specific cellular functions. A typical cell is filled with numerous nanomachines that serve a multitude of functions, including regulation

1

For more information, see Mark Ratner, Introduction to Nanoscale Materials BehaviorWhy All the Fuss? Available at <http://www.blueskybroadcast.com/Client/ARVO/trans/arvo_ratner_RF_OK.pdf>.



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