The scanning tunneling microscope is less than 10 years old
but has already given rise to at least nine other kinds of
microscopes, each designed to collect a different kind of
information from a sample. Robert Pool has provided a nice
overview of these devices, which all share the the same general
mechanical and imaging technology: samples are placed on a
surface that can be moved by the expansion and contraction of
piezoelectric crystals; images are made by scanning with a
pointed probe brought very close to the sample.
The devices differ in what they measure or do to the sample. The
STM sends an electric current through the sample and, in effect,
measures the electrical resistance. The atomic force microscope
measures the longitudinal force between the probe tip and the
sample (i.e., the force along the line between probe and
sample), whereas the friction force microscope measures the
transverse force. The magnetic force microscope measures the
magnetic field around the sample; the electric force microscope
measures the electric field. The scanning thermal microscope
measures the temperature variations along a sample. The optical
absorption microscope does spectroscopic measurements. The
scanning ion-conductance microscope maps ion flows passing
through the sample. The scanning near-field optical microscope
uses an optical probe to make images with visible light at
molecular-scale resolution. The scanning acoustic microscope uses
sound waves to image samples, permitting a view beneath the
surface. Some of these devices can be used to alter specimens as
well as to view them.
[Science 247:634-247, 9Feb90]
Rather suddenly, science finds itself able to obtain a wealth
of information at atomic or near-atomic resolution. This should
bring on an avalanche of useful structural information, much of
it immediately applicable to problems in molecular engineering.
The direct impact should be felt first in biology and materials
science; and soon after in fields that depend on these, such as
biotechnology, electronics, medicine, and chemistry.
Although scanning probe devices (like the scanning tunneling
microscope) can image objects of atomic size, they lack reliable
methods for positioning and measuring samples. Researchers at the
University of Tokyo are trying to remedy the situation by using
the lattices of crystals as a calibration standard. A dual-probe
scanning device is being developed in which one probe scans the
sample while the other scans a reference crystal. Rough
positioning of the sample would be performed by an impact drive
mechanism in which a piezoelectric element strikes the block
holding the sample, knocking it forward several nanometers. An
impact rate of 80 impacts/sec is being investigated.
[Paper by Hideki Kawakatsu, et al., IEEE document
Scanning probe devices are thus being transformed from novelty
items into reliable instruments for routine research.
The Diehls-Alder reaction, discovered in the mid-1800s, is a
chemical reaction with broad application for building larger
molecules from smaller ones. In the past few years chemists have
begun using it for molecular construction on the basis of
shape--to make molecules designed to look like gears, for
example. In an intriguing review of the subject, Franz H. Kohnke
at Univ. di Messina suggests that the next few years will see
rapid development of such "structure-directed
synthesis," giving rise to molecules "that look like
ball bearings, beads and threads, belts, cages, chains, chimneys,
clefts, coils, collars, knots, ladders, nets, springs, stacks,
strips, washers, and wires--and concurrently and subsequently for
molecules with function--that work like abacuses, capacitors,
catalysts, circuits, clocks, conductors, dynamos, membranes,
motors, nuts and bolts, resistors, screws, semiconductors,
sensors, shuttles, superconductors, and switches."
Julius Rebek and co-workers at MIT have designed and built a
replicator: a molecule that produces copies of itself, given
appropriate raw materials. The raw materials are quite
specialized: one of them is a variant of adenosine (one of the
four building blocks of DNA); the other is a fluorinated ester
having features complementary to the adenosine. The ester
component includes a catalytic group that promotes bonding
between adenosine and ester.
This primitive replicator contains about two hundred atoms. The
initial copies were made by ordinary chemical synthesis. When a
solution of adenosine and the fluorinated ester was seeded with
replicators, the replicator molecules paired up with adenosine
and ester molecules and catalyzed bond formation between them,
forming more replicators.
[Article by I. Amato in Science News, 3Feb90: 69]
So, structures that foster their own formation need not be very
complicated. Replication appears to derive from the
complementarity of the components along with the inclusion of an
appropriately situated catalytic group.
An interesting question left unanswered by this research is
whether it is easier to satisfy the conditions for replication
with a single molecule or with a set of molecules that catalyze
each others' formation.
The "folding problem" for proteins is to figure out
what spatial configuration will be taken by a given chain of
amino acids. Researchers at MIT started with the repressor
protein from the lambda phage (a virus that attacks bacteria).
They made numerous versions of the repressor, each with different
amino acid substitutions at one or several locations, and tested
the activity of phages containing these. It was found that at
certain locations almost any substitution can be made with little
or no effect on activity. At other locations, the protein is
intolerant to any change at all. Generally speaking, amino acids
located in the core of the protein could often be substituted,
but only with other amino acids of similar (hydrophobic) type.
Surface amino acids were usually tolerant of a wider variety of
substitutions except at a few functionally important sites.
[Paper by James U. Bowie, et al. in Science 247:1306-247,
Designing proteins for given roles may require less
computation than one might think, since many different amino acid
sequences can give rise to the same functionality.
Small diamonds (3 to 5 nm in diameter) constitute about 20% by
weight of soot formed during detonations, and are also found in
meteorites and in nucleation experiments. A group of
investigators has shown by thermodynamic arguments that in this
size range, diamond is as stable as graphite and its relatives,
and that extremely high pressure is not necessarily required to
[Paper by P. Badziag at Univ. of South Africa, et al., in Nature
Diamond is being considered as a construction material for
assemblers and other nanotechnological devices. Badziag's work
supports the thesis that small structures can be built with
diamond at low pressure. It is also comforting to know that such
devices would be unlikely to disintegrate or turn spontaneously
Vibrations in chemical bonds occur on timescale of about 10-13
sec, and rotations in about 10-10 sec. Laser
observations with a resolution of a few tens of femtoseconds have
enabled researchers to follow these processes in detail. To study
vibrations, a "pump" pulse at a wavelength of 620 nm
prepared iodine molecules by sending them to higher energy. A
probe pulse at 310 nm then sent these excited molecules into a
still higher energy state from which they decayed by
fluorescence. By varying the delay between the pump and probe
pulses, the fluorescence intensity was seen to vary as the
chemical bond stretched and contracted.
Rotations were followed with pulses of polarized laser light.
Only those molecules with axes aligned with electric field of the
pulse were excited. As the delay between pump and probe pulses
was increased, the observed fluorescence declined since the
molecules' axes were rotating out of alignment with the probe's
electric field. The results closely matched the predictions made
from quantum theory.
[Paper by M. Dantus at Cal Tech, et al., in Nature
Assemblers and other nanomachines will have to deal with
vibrational and rotational effects every time a bond is made or
broken. Polarized femtosecond spectroscopy has now provided a
direct source of information about these processes.
Electrons behave more like particles when free to move in
regions much larger than their wavelength, and more like waves
when confined into regions comparable to their wavelength.
Fabrication techniques now being developed will enable
electronics to exploit the wave properties of electrons.
Today's most advanced commercial transistors have features as
small as .75 micrometers (a hundredth of the diameter of a hair);
quantum effect devices being experimented with today are
substantially smaller, having features about 25 nanometers (about
100 atoms) across--and researchers are aiming for 10 nanometers
in the near future. Devices of this scale can be made by x-ray
lithography, analogous to the photolithographic technique that
the electronics industry has relied on for a quarter century.
To study electron wave effects, researchers have etched electron
waveguides about 30 nanometers across into a block of aluminum
gallium arsenide (AlGaAs). Transistor-like behavior can be
obtained from a block of AlGaAs after etching a rectangular
pattern of holes.
One intriguing way to utilize quantum devices would be to arrange
them as arrays of "quantum dots" on a surface, each dot
storing a small amount of information and interacting with its
neighbors according to prescribed rules. Such arrays, called
"cellular automata," can perform computations without a
network of wires to shunt information around. Arrays of 200
million quantum dots per cm2 have been made, but are
not yet programmable.
[Review article by Henry I. Smith at MIT, et al., in Technology
While this kind of electronics exploits only a few basic
quantum mechanical effects in simple crystals, it nevertheless
promises major advances in speed and miniaturization. Even
greater improvements will come from more sophisticated materials,
in which complex structures process information at the molecular
level. The following two reports provide hints of what is to
sieves are lattice structures containing regular patterns of
"host" cavities--channels, cups, or cages. A variety of
molecular sieves can be made, each with its own characteristic
size, shape, and pattern of host cavities.
The host cavities can be used to trap and hold
"guests"--small molecules or clusters of semiconductor
atoms--forcing them into regular arrays called
"superlattices" and constraining their internal
Molecular sieves are attracting the interest of researchers in a
number of fields because they provide new ways to control and
obtain information about guest molecules. Semiconductor
scientists are interested in them because confinement and regular
spacing of clusters of semiconductor atoms gives rise to quantum
phenomena not seen in ordinary crystals or solutions of the same
Superlattices should find early application in optoelectronics
because of the ease with which their optical properties can be
manipulated. Some superlattices, for example, undergo color
changes as the temperature is varied; others respond to pressure,
humidity, light, pH, or electric fields.
[Review article by Galen D. Stucky at UC Santa Barbara, et al.,
in Science 247:669-678, 9Feb90]
Since superlattices are generalizations of the crystalline
state, they have a wider range of bulk properties than
conventional crystals do, and offer more opportunities for the
control of these properties. They are early examples of the kind
of atomically precise manmade materials we will see more of when
molecular assembly machines become available.
Ordinary photosynthesis takes place within a complex structure
embedded in a membrane of a plant or bacterium. In this process a
photon is captured by "antenna" molecules and its
energy transferred to a pigment molecule where it is absorbed by
an electron. The electron moves quickly to nearby quinone
molecules, leaving behind a positive charge. From the quinone the
electron passes along a chain of other structures to the outer
side of the membrane and is transferred to other molecules.
Meanwhile, the positive charge left behind is passed to the inner
side of the membrane where it is neutralized by an electron
pulled from a suitable electron donor. In plants, the electron
donors are water molecules which are converted to oxygen and
protons; the protons are used in the manufacture of adenosine
triphosphate (ATP), the basic energy coinage of the biological
At Arizona State University researchers have made a vastly
simplified version of a significant part of this photosynthetic
apparatus. They have designed and synthesized an
impressive-looking molecule of approximately two hundred atoms
that can absorb photons, transfer the energy to electrons, and
send the electrons down one arm of the molecule and the
positively charged "hole" down the other arm. The
charge-separated state has a lifetime of about 55 microseconds
and preserves 83% of the original photon energy.
[Paper by Devens Gust, et al., in Science 248:199-201,
The design of this artificial photosynthetic device was no
exercise in trial and error--it proceeded from a detailed
understanding of how energy is transferred between quantum states
of molecules. The methods used by Gust's team provide a glimpse
of the awesome capabilities that will soon be routinely available
to molecular engineers.