NANOTECHNOLOGY—TECHNOLOGY ON A MOLECULAR SCALE - Chemistry: Notable Research and Discoveries - page 62

dielectric material—which does not conduct electricity but affects
electric fields—surrounded by a shell of metal. By varying the struc-
ture and composition of the shell, the researchers can “tune” the
nanoparticle, making it resonate at certain frequencies, which makes
the nanoparticle more responsive to these frequencies. Some fre-
quencies, such as those that lie in the infrared portion of the spec-
trum—an invisible band of frequencies next to red—pass through the
body because they are not readily absorbed by cells and tissues. When
nanoshells are resonant at these frequencies, imaging them with vi-
sual equipment becomes more effective. The increase in effectiveness
also occurs when researchers zap the particles with a laser, a process
called photothermal because of the destruction from the heat (ther-
mal energy) that was induced by the laser light (photons).
Ji-Xin Cheng, a scientist at Purdue University in Lafayette, Indiana,
is also interested in this technology. He has used tiny rods of gold, called
nanorods, to tag and destroy cancerous cells. Yet Cheng has discovered
that heat, although important, only initiates the process that eventu-
ally leads to the cell's death. In a 2007 paper, “Gold Nanorods Mediate
Tumor Cell Death by Compromising Membrane Integrity,” published
in Advanced Materials, Ling Tong, Cheng, and their colleagues report
an experiment in which they show that the nanorods get hot when illu-
minated, as expected, but this heat causes the formation of tiny bubbles
of hot gas. This process, known as cavitation, results in little explosions
that puncture the cell's membrane. The integrity of a cell's membrane
is critical to its survival. When holes or gaps appear, the contents of the
cell mingles with the extracellular fluid, a substance that has a much dif-
ferent character than the fluid inside the cell. Nutrients escape, damag-
ing molecules pour in, and the cell dies.
Nanotechnology's small scale is vital to its medical applications, as
it is in all other applications. Defenses of the body's immune system
recognize and attack foreign material. This system is important in fight-
ing off infection, but the immune system did not evolve to distinguish
the efforts of physicians. If a treatment requires the introduction of
large objects that must stay in the body for a while, such as transplant-
ing an organ from a donor into the patient's body, the treatment may
fail because of an immune system attack. But nanotechnology may be
successful where other techniques fail, because tiny particles in the na-
noscale range slip by the immune system. This is yet another reason
why the miniaturization of technology has tangible advantages.