Design Online - December 2009
Nano-level DNA research underway in Israel
Dec 14, 2009 3:49 PM, by Jim McMahon, technology
Scanning probe microscopy and negative-stiffness vibration
isolation are enabling nano-level DNA research at Israel's
Weizmann Institute of Science. DNA is able to recognize other
molecules, other strands of DNA, and because it binds together
with similar DNA strands in a unique way, scientists are considering
the possibility of using DNA as an electronic circuit without
having to build in any other circuitry. The DNA would bind
with other similar DNA strands and use the connecting properties
of the DNA to create a self-assembled biological wire for
electrical conduction. Recent research on the capacity of
single molecules of DNA to transport current along individual
strands and conducted by Sidney R. Cohen in collaboration
with Ron Naaman and Claude Nogues of the Weizmann Institute
of Science, Scanned Probe Microscopy Unit, in Rehovot, Israel,
has shed new light on the electrical transport properties
of DNA, focusing on the capacity of single molecules of DNA
to transport current along individual strands.
DNA (deoxyribonucleic acid) is a nucleic acid that contains
the genetic instructions used in the development and functioning
of all known living organisms and some viruses. The main role
of DNA molecules is long-term storage of information. DNA
nanotechnology uses the unique molecular-recognition properties
of DNA to create self-assembling branched DNA complexes with
To measure the electronic properties of DNA, Cohen and his
staff connected a volt electrode to the end of a DNA molecule
(a few nanometers in length), using an AFM (atomic force microscope).
To facilitate this bio-molecular connection, the lab attached
a bio-link, a gold electrode, to a single strand of DNA, and
then attached a gold ball just 10 to 20 nanometers in size
to a complementary DNA strand. Then these two strands were
hybridized (linked, aided by genetic similarity between corresponding
DNA sequences). When strands are complementary, a double strand
is formed. While single strands of DNA do not conduct electricity,
double strands conduct for certain configurations. Using an
AFM, with the DNA double strand displayed on a flat surface,
researchers could locate the gold ball, put the AFM tip on
top of the ball, flow a current through the double strand,
and view the current voltage characteristics.
"There are two possibilities when we talk about electrons
flowing through a DNA molecule," explains Cohen. "One
is a 'tunneling process,' when the electron effectively shoots
through the molecule without caring too much about the internal
structure of the molecule. The other is a 'hopping process,'
where the electron actually resides for small periods of time
in certain positions along the molecule. In this case the
electron will be affected by temperature."
Researchers also found that variations in both the sequence
and the composition of a strand's base pairs can affect the
progress of electron transport through the strand. Similarly,
bases that are electron rich have better electron conductivity
than those with fewer available electrons.
The characteristics of electron conductivity in DNA also
have implications in molecular electronics, which may lead
to devices that, instead of working on the standard silicon
circuitry, function through innocuous molecules. Because of
DNA's facility to bind with similar types of DNA molecules,
it is not necessary to physically place each molecule in a
set location. DNA put into solution can be expected to organize
itself in the right way and become a predictable medium for
The Weizmann Institute is one of the few research groups
in the world that has actually managed to measure the electrical
transport properties of a single molecule of DNA. A critical
factor in the Weizmann Institute's ability to consistently
measure DNA electron structures at such extreme nano-level
resolutions is the lab's use of negative-stiffness vibration
isolation systems by Minus K Technology.
Negative-stiffness mechanism (NSM) isolators have the flexibility
of custom tailoring resonant frequencies vertically and horizontally.
They employ a mechanical concept in low-frequency vibration
isolation. Vertical-motion isolation is provided by a stiff
spring that supports a weight load, combined with an NSM.
The net vertical stiffness is made low without affecting static
load-supporting capability of the spring.
Beam-columns connected in series with the vertical-motion
isolator provide horizontal-motion isolation. The horizontal
stiffness of the beam-columns is reduced by the "beam-column"
effect. A beam-column behaves as a spring combined with an
NSM. The result is a compact passive isolator capable of low
vertical and horizontal natural frequencies and high internal
Transmissibility with negative-stiffness is substantially
improved over air systems, which can make vibration isolation
problems worse since they have a resonant frequency that can
match floor vibrations. Transmissibility is a measure of the
vibrations that transmit through the isolator relative to
the input vibrations. The NSM isolators, when adjusted to
0.5Hz, achieve 93 percent isolation efficiency at 2Hz; 99
percent at 5Hz; and 99.7 percent at 10Hz. NSM transmissibility
is also improved over active systems.
Since they run on electricity, active systems can be negatively
influenced by electronic dysfunction and power modulations,
which can interrupt scanning. They also have a limited dynamic
range, which is easy to exceed - causing the isolator to go
into positive feedback and generate noise underneath the equipment.
Although active isolation systems have fundamentally no resonance,
their transmissibility does not roll off as fast as negative-stiffness
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