NANOTECHNOLOGY—TECHNOLOGY ON A MOLECULAR SCALE - Chemistry: Notable Research and Discoveries - page 54

interact with the material. A small, thin object exposes more surface
area than the same amount of matter compressed into a thick block,
and the greater exposure speeds up interactions with the environment
and other objects. For example, crushed ice melts faster than the same
mass of ice cubes—the smaller, thinner particles of the crushed ice have
more surface area with which to absorb heat, so they require less time
to melt.
Nanoparticles have an exceptionally high ratio of surface area to
volume—unlike bulky forms of matter, most of the material in nanopar-
ticles is exposed rather than hidden inside the interior. This feature of
nanoparticles can drastically alter the material's properties, which is an-
other example of the changes that occur at the small end of the scale.
Gold, for instance, is usually not very chemically active in its bulk form,
yet gold nanoparticles are quite reactive. An example of this phenom-
enon will be described in the following section.
Manufacturers have already begun to take advantage of some of
these nanoparticle properties. Sunscreens, which protect users from
burns by absorbing or deflecting harmful rays, are often made from
chemicals such as titanium dioxide or zinc oxide that are particularly
effective. These sunscreens often leave a whitish residue—which used to
be common on the nose of a pool or beach lifeguard—but when compa-
nies embedded nanoparticles of titanium dioxide or zinc oxide instead
of bulkier particles, the creams become transparent yet maintained or
even increased their effectiveness. With no embarrassing residue, these
sunscreens have become popular.
But some health and environmental groups are concerned over such
uses of nanoparticles. New technologies may introduce new risks as well
as new benefits. The Food and Drug Administration (FDA), the U.S.
government agency that oversees drugs and health-care products such
as sunscreens, had already approved the use of titanium dioxide and zinc
oxide, and the administration decided in 1996 that the use of tiny particles
of these chemicals in sunscreens and other health or beauty products did
not pose any additional risks. But in 2007, Friends of the Earth, a group
based in Washington, D.C., urged a ban on this use of nanoparticles un-
til more research has been done. Proponents of such a ban argue that
the novel properties of nanotechnology materials make them behave like
new substances and therefore require a new round of tests even though
the chemicals themselves have already been approved.