Number 807, December 28, 2006
by Phil Schewe, Ben Stein, and Davide Castelvecchi
Direct Force Sensing at the Piconewton Level
The measurement of mass can be carried out at the 10-21 (zeptogram) level
(see PNU 725) and of force
to the 10-18 newton (attonewton) level (Arlett et al., in
Nano Letters, 2006). But for many measurements in the cell biology world, this is too
much sensitivity. Forces in this realm are typically at the piconewton
(10-12 newton) level. Examples include the force applied by the kinesin
molecular motor protein to transport vesicles (6 piconewton), the force needed to
unzip a DNA molecule at room temperature (9-20 piconewton), or the force needed to
pull a DNA apart by pulling on opposite ends (65 piconewton).
Biophysicists need a cost-effective force sensor that works reliably in water at the
piconewton level. Steven Koch and his colleagues at Sandia National Laboratories
in Albuquerque, N.M., are well along on delivering the needed sensor. The core of
the device is a spring one millimeter long but only a micron thick and is fabricated
using a standard polysilicon micromachining process. This spring operates according
to the classic experiment conducted by Robert Hooke in the 17th century: the force
exerted on the spring equals the amount of the spring’s compression or extension
multiplied by a spring constant, which in this case is about 1 piconewton per nanometer.
The spring, mounted on a substrate, can be used in a number of ways: it can be
entrained to move with the push or pull of a biological sample or it can be made
sensitive to magnetic fields and so function as a field sensor. The displacement
of the spring is currently viewed by a video camera with precision of 2 nanometer,
but faster and more precise methods are possible.
Koch (now at the University of New Mexico, skoch@chtm.unm.edu) says that the most
likely applications of the new sensor will be in measuring forces on the kind of
magnetic microspheres used in single-biomolecule experiments and to calibrate the
electromagnets used in deploying microspheres in doing things such as stretch,
twist, or unzip DNA. He also envisions direct mechanical force measurements, combined
with other MEMS (microelectromechanical systems) implements, in biophysical experiments
where optical tweezers (using laser beams to manipulate the microspheres attached
to molecules) cannot be used.
The Sandia sensor could be adapted to apply an adjustable tension to single
DNA molecules in order to study protein binding or enzymatic processes.
Koch et al.,
Applied Physics Letters, 23 October 2006
Contact Steven Koch
New Cranked-Up Nuclear States
Some nuclear physicists seek to make new elements by fusing two nuclei and hope the
amalgamated body will hold together at least for a while. Other researchers explore the
nuclear world by creating new spin states. A highly spinning nucleus is not "excited"
in the usual sense of possessing a lot of internal energy, but allows nevertheless the
nucleus constituent protons and neutrons to deploy themselves in new ways.
This high-spin universe is reached in off-center smashups of two nuclei. In new
experimental work at the Lawrence Berkeley National Laboratory in Berkeley,
Calif., erbium-158 nuclei were spun up to very high rates and then closely observed as
they slowed down by offloading high energy photons. These gamma rays, each carrying
off two units of angular momentum (each unit equals 2 times pi times Planck's
constant), are observed in the Gammasphere detector surrounding the collision site;
the number of the gamma rays provides information about nuclear spin.
So, for example, a nucleus spun up to a level of 40 units would, by relaxing back
to normal, throw off about 20 gammas; other forms of nuclear radioactive relaxation
-- throwing out electrons or alpha particles -- take too long to come about.
Theorists believe that above a spin value of about 46, the entire erbium-158 nucleus
cannot be spun up any further without a drastic rearrangement of the entire state
of the nucleus. Instead a spherical core of nuclear particles (constituting a
gadolinium-146 nucleus) rotates no more while a fleet of 12 "valence" particles
(neutrons and protons) orbits the core at ever higher spin values (see this
progression of spin states at Physics News Graphics).
Eddie Paul of the University of Liverpool (esp@ns.ph.liv.ac.uk), in England, and
his colleagues have been able to discover new pathways to a higher spin regime by
observing the pattern of gamma rays thrown out. They find evidence that the core
observed in previous experiments can occasionally break up a bit, allowing
collective rotation of all the nucleons to resume, permitting the total spin of
the nucleus to attain higher values. The highest value observed in this way was a
state with 65 units of spin.
The researchers hope to explore even higher values of spin, maybe so high that
the nucleus approaches the fission limit. At this point the nucleus does not
de-excite by losing gammas, but by actually fissioning -- that is by coming apart
into large nuclear fragments.
Enlarge text
Shrink text
Print
E-mail
Digg this
Del.icio.us
Furl