Number 725, April 1, 2005
by Phil Schewe and Ben Stein
Zeptogram Mass Detection---Weighing Molecules
Michael Roukes and
his Caltech colleagues produce some of the finest nanoscopic
electromechanical systems (NEMS) devices in the world. His latest
achievement is performing mass measurements with nearly zeptogram
(zg) sensitivity, that is, with an uncertainty of only a few times
10-21 grams. At this level you can start to weigh molecules one at
a time. In experiments, the presence of xenon accretions of only
about 30 atoms (7 zg, or about 4 kilodaltons, or the same as for a
small protein) have been detected in real time.
Minuscule masses
are measured through their effect on an oscillating doubly clamped
silicon carbide beam, which serves as the frequency-determining
element in a tuned circuit. So, in practice, the beam would be set
to vibrating at a rate of more than 100 MHz and then would be
exposed to a faint puff of biomolecules. Each molecule would strike
the beam, where its presence (and its mass) would show up as a
changed resonant frequency.
After a short sampling time, the
molecule would be removed and another brought in. Through this kind
of miniaturization and automation, the NEMS approach to mass
spectroscopy could change the way bioengineering approaches its
task, especially in the search for cancer and its causes. The
Roukes (roukes@caltech.edu, 626-395-2916) group reported its
findings at last week’s meeting of the American Physical Society
(APS) in Los Angeles.
Laser Scattering of Mitochondria
Laser scattering of mitochondria, the "power plants" of cells, can
immediately identify early-stage liver cancer cells and potentially
monitor stem cells as they undergo various stages of development. At
the APS March Meeting, Paul Gourley of Sandia (plgourl@sandia.gov) reported
the latest uses of the "biocavity laser," an aluminum-gallium-arsenide
based design that continuously pumps in single human cells into a chamber
for analysis. The laser's beams are altered in their passage through
the cells. The 800-nanometer light in the experiments is not absorbed
by most of the cell, except by its hundreds of mitochondria, which are
responsible for scattering 90-95 percent of the light.
By analyzing
the scattering patterns, the researchers determined the distribution
of mitochondria in the cell, and could instantly determine whether the
cell was healthy (in which case the mitochondria cluster cooperatively
around the cell nucleus) or cancerous (in which case they are apathetically
sprawled across the cell). The process is highly accurate, works much
more quickly than traditional techniques, and does not require the usual
pre-treatment of cells with chemical reagents or fluorescent molecules.
Co-author Bob Naviaux of UC-San Diego added the biocavity laser technique
also has the potential to rapidly identify the in-between states of
stem cells as they transform into their final identities. (Also see
Sandia News release at http://www.sandia.gov/news-center)
No Splash on the Moon
Sidney Nagel’s lab at the University of Chicago has explored the behavior
of liquid drops---how and when they fall from a faucet---granular materials,
crumpling, and other everyday-but-difficult-to-explain phenomena. At
the APS meeting, Nagel’s graduate student, Lei Xu, revealed a surprising
discovery concerning one of the commonest physical effects: the splash
a liquid drop makes when it strikes a flat surface.
Under ordinary atmospheric
conditions a liquid drop will flatten out on impact, splay sideways,
and also raise a tiara-like crown of splash droplets. Remove some of
the ambient atmosphere, and surprisingly the splash becomes less. At
about one-fifth atmosphere the splash disappears altogether, leaving
the outward going splat but no upwards splash (see movie at kauzmann.uchicago.edu
). Apparently it is the presence of the air molecules that give the
impacting liquid something to push off of; remove the surrounding atmosphere,
and the splash disappears.
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