NANOTECHNOLOGY—TECHNOLOGY ON A MOLECULAR SCALE - Chemistry: Notable Research and Discoveries - page 45

rate series of steps to encourage the union. Self-assembly occurs when
the components are more stable together than they are apart. In other
words, the assembly reduces the energy so that it happens spontane-
ously, similar to a spontaneous chemical reaction such as a pellet of
sodium reacting violently when immersed in water. Spontaneous pro-
cesses result in more disorder—this is a law of thermodynamics—and
self-assembly would, at first glance, seem to violate this principle. But
the principle is upheld because these processes raise the disorder of the
overall environment. This is also what happens in living organisms,
which is why life fails to violate the laws of thermodynamics even as it
grows, evolves, and sustains itself spontaneously.
The job of the chemist is to set up the necessary conditions. Hydro-
phobic forces can be involved, and the researcher Eugene Zubarev at Rice
University in Houston, Texas, and his colleagues have made nanopar-
ticles of gold with the aid of these forces. (The size of a nanoparticle is in
the nanotechnology range—0.00000004-0.000004 inches [1-100 nm].)
Zubarev attaches a tiny gold particle to an amphiphilic molecule—this
molecule has both hydrophobic and hydrophilic ends. When placed in
water, the amphiphilic molecules spontaneously form cylinders or spheres
as their hydrophobic portions bunch together, avoiding water, while the
hydrophilic portions are exposed. The same thing occurs as proteins fold,
as well as in cellular membranes—these membranes are composed of am-
phiphilic molecules that form a two-layered structure that enclose cells.
Zubarev's assembly is so biologically adept that it travels widely in
the body, and by attaching other molecules such as medications to the
gold particle, the assembly can transport substances to specific points
in a patient. Pinpoint delivery of drugs would vastly improve treatment
regimens because it would tend to avoid side effects, which commonly
occur when drugs act on tissues other than the intended target.
Other researchers are employing DNA to promote nanoparticle as-
semblies. As mentioned in the sidebar on pages 42-44, hydrogen bonds
knit the strands of the DNA's double helix. DNA consists of strings of
four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The
structures of the bases and the helix permit only the binding of A with T,
and C with G. This is called complementarity, for if the base at one loca-
tion is an A, the base on the opposite strand will be its complement, T, not
a C or a G. If it is a G, then the other base will be a C.
Mathew Maye, Oleg Gang, and their colleagues at Brookhaven Na-
tional Laboratory in New York use DNA to help assemble nanoparticles.