In the past, chemists, physicists, and biologists had to rely on
indirect methods to provide information on the structures of
the smallest molecules. But technological advances have created
a new generation of microscopes that “see” atomic structure
by actually feeling it. Scanning probe microscopes operate with a
minute needle tapered to a tip that can be as narrow as a single
atom! This probe scans over the exposed surface of a material
on the end of an arm and records an image of its outer texture.
(Think of an old-fashioned record player. . . .) These revolutionary
microscopes have such profound resolution that they have the
potential to image single atoms (but not subatomic structure yet)
and to magnify 100 million times. There are two types of scanning
probe microscopes, the atomic force microscope (AFM) and the
scanning tunneling microscope (STM). The STM uses a tungsten
probe that hovers near the surface of an object and follows its
topography while simultaneously giving off an electrical signal
of its pathway, which is then imaged on a screen. The STM was
used initially for detecting defects on the surfaces of electrical
The Evolution in Resolution: Probing Microscopes
not all microbes exhibit capsules is a useful feature for identifying
pathogens. One example is Cryptococcus, which causes
a serious fungal meningitis in AIDS patients (see chapter 19).
Flagellar staining is a method of revealing fl agella,
the tiny, slender fi laments used by bacteria for locomotion.
Because the width of bacterial fl agella lies beyond the resolving
power of the light microscope, in order to be seen, they
must be enlarged by depositing a coating on the outside of
the fi lament and then staining it. Their presence, number, and
arrangement on a cell are taxonomically useful.
3.2 Learning Outcomes—Can You . . .
4. . . . convert among different lengths within the metric system?
5. . . . describe the earliest microscopes?
6. . . . list and describe the three elements of good microscopy?
7. . . . differentiate between the principles of light and electron
microscopy?
8. . . . name the two main categories of stains?
9. . . . give examples of a simple, differential, and special stain?
Scanning tunneling microscopy. The figure
on the left was created when scientists dragged
iron atoms over a copper matrix to spell (in kanji,
a Japanese written alphabet) “atom” (literally:
“original child”). On the right you see a chemical
reaction performed by an STM microscope. At the
top (a), two iodobenzene molecules appear as two
bumps on a copper surface. The STM tip emits a
burst of electrons and causes the iodine groups to
dissociate from each of the benzene groups (b). The
tip then drags away the iodine groups (c), and the
two carbon groups bind to one another (d and e).
Source: http://www.almaden.ibm.com/vis/stm/
atomo.html, page 80.
conductors and computer chips composed of silicon, but it has
also provided the first incredible close-up views of DNA.
The atomic force microscope (AFM) gently forces a diamond
and metal probe down onto the surface of a specimen like a needle
on a record. As it moves along the surface, any deflection of the
metal probe is detected by a sensitive device that relays the information
to an imager. The AFM is very useful in viewing the
detailed structures of biological molecules such as antibodies and
enzymes.
These powerful new microscopes can also move and position
atoms, spawning a field called nanotechnology—the science
of the “small.” When this ability to move atoms was first discovered,
scientists had some fun (see illustration on the left).
But it has opened up an entirely new way to manipulate atoms
in chemical reactions (illustration on the right) and to create
nanoscale devices for computers and other electronics. In the
future, it may be possible to use microstructures to deliver drugs
and treat disease.