PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 490 June 22, 2000 by Phillip F. Schewe and Ben Stein
ATOMIC SCALE LOCOMOTIVES. Miniaturization has
produced several examples of nm-sized rotors, ratchets, and gears,
but not yet nano-steam engines. But to move raw materials around
the freight yards of future nano-factories one needs nano-
locomotives. Scientists at Tel Aviv University in Israel (Markus
Porto, 011-972-3-640-7229), have proposed how this can be done.
In their scheme the freight yard consists of a lithographically
prepared corrugated surface, something like the shape of an egg
carton (on the microscopic level, anyway; to the naked eye the
surface looks flat). The engine, in its simplest form, consists of
three tiny clusters of metal atoms connected by two "springs."
Each spring is actually a photochromophore molecule, one whose
length can be expanded or shrunk with light. So to get the engine
to move, laser light is shot in from above, the molecule expands,
and one metal particle moves into depression on the surface. By
careful timing and correlating of the light pulses, the engine can be
made to move along like an inchworm (see movie at
www.aip.org/physnews/graphics). Cargo consisting of, say,
inactive chains of molecules or other atomic material would be
coupled to the locomotive and transported to where it is needed.
(Porto et al., Physical Review Letters 26 June 2000; Select
Article.)
NM-RESOLUTION VISIBLE-LIGHT MICROSCOPY, of a sort,
has been accomplished by scientists at the Max Planck Institute for
Biophysical Chemistry in Gottingen, Germany (Stefan Hell, 011-
49-551-201-1366, shell@gwdg.de). The diffraction of light waves
normally limits spatial resolution of nearby objects to no better
than the wavelength of the light source. So called near-field
microscopy beats this limit by moving the source very close to the
subject to be imaged. But the Gottingen group, whose work is an
example of far-field microscopy, in this case does not so much
beat the diffraction limit as circumvent it. They split a laser pulse
(wavelength of 820 nm) into two parts and illuminate a sample
consisting of beads attached to a
Langmuir-Blodgett layer, the kind of filmlike layer of water-hating
and water-attracting molecules poised back-to-back that forms the
membrane of most cells. The sample, positioned close to the place
where the two laser beams meet at the same focal point of two
lenses, starts to fluoresce. This fluorescence is viewed through
filters at two different colors. This "confocal" microscopy does
not exactly "resolve" the objects apart but does measure the
distance between them with a precision as high as 1.2 nm. For the
process to work, however, the contrasting objects, in this case the
layer and a bead, must fluoresce at different colors. This is just
what one gets when attempting the co-localization of proteins and
organelles or vesicles and membranes, etc. And unlike such
imaging techniques as atomic force microscopy (AFM) or
transmission electron microscopy (TEM), the use of non-bleaching
visible light permits the study of living cells. (Schmidt et al.,
Review of Scientific Instruments, July 2000; Select Article.)
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