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ASU
RESEARCHERS DEMONSTRATE A NEW NANOTECHNOLOGY
EFFECT – MOVING WATER MOLECULES BY LIGHT
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The
image shows two water drops illuminated with a fluorescent
dye. The drop on the left (the one that looks round)
is sitting on a nanowire surface with a very hydrophobic
coating. The drop on the right, which is spread out,
is sitting on a flat surface with the same coating.
This picture shows that the nanowires create a lotus
leaf-like surface. Arizona State University researchers
have made that surface photoresponsive and can make
the drop on the left move in response to light.
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A team
of researchers at Arizona State University has demonstrated
the ability to move water molecules by light -- a phenomenon
they believe could have widespread use in analytical chemistry
and possibly pharmaceutical research. The discovery could
have an important effect on the fledgling field of microfluidics,
said Tony Garcia, a professor in the Harrington Department
of Bioengineering.
The use of an ordinary beam of light to move water around
without the need for potentially damaging electric fields,
air bubbles (which can denature proteins), or moving microscopic
mechanical pump parts (which are expensive to make and
difficult to repair) could significantly aid development
of microfluidic devices, which are themselves tiny, sophisticated
devices that can analyze samples.
“This discovery can speed the development of microfluidic
devices,” Garcia said. “These devices could
require only one drop of blood for a battery of 20 to
30 tests, with results provided in the time spent waiting
to consult with the physician,” Garcia explained.
“They also could help pharmaceutical companies screen
for a new drug by allowing for tests to be run on an extremely
small scale and in simultaneous fashion.”
The ASU researchers discovered an amplification effect
of the surface change in water contact angles through
nanotechnology. Details of their work will appear in a
paper titled “Lotus Effect Amplifies Light-Induced
Contact Angle Switching,” in the Journal of Physical
Chemistry. It is now available from the Journal’s
online ASAP service (go to http://pubs.acs.org/journals/jpcbfk/
and click on articles ASAP).
In addition to Garcia, the team includes Devens Gust,
professor of chemistry and biochemistry; Tom Picraux,
professor of chemical and materials engineering; Mark
Hayes, associate professor of chemistry and biochemistry;
Rohit Rosario, a postdoctoral researcher in the Harrington
Department of Bioengineering; Jennifer Taraci, a postdoctoral
researcher in chemical and materials engineering; and
graduate students Teresa Clement and Jeff Dailey working
in Picraux’s group.
In nanotechnology, devices are designed from the molecular
level up. As the overall size of these devices shrink,
the nature of the surface plays an increasingly important
role because a greater percentage of the molecules in
a nanotech device reside on the surface. The ability to
manipulate surface molecules using everyday means, such
as shining a light or connecting to a battery, becomes
very important because ordinary tools like pumps and valves
are hard to make on a nano scale.
The ASU team theorized and then proved that a change in
water wettability – the ability of the water molecules
to easily move across a surface – when induced by
light can be significantly amplified through a combination
of very high nanoscale roughness and chemically coating
the surface with molecules.
“What we found was the ‘sweet spot’
in surface roughness where the amplification effect was
the greatest,” Garcia said. “Our theory showed
where the sweet spot would be, meaning the optimal roughness
of the surface, and then we proved it.”
“We have been working on the problem of using light
to move microscopic amounts of water around for drug delivery
and microanalysis applications,” said Tom Picraux.
“However, we were stymied by the vexing problem
of the combined small effect created and the high degree
of attraction that water retains on even a very waxy,
or hydrophobic, flat surface.
“Our advance came when we realized that if the surface
was roughened at the nanoscale, not only would we obtain
the ‘lotus leaf effect,’ but we could also
magnify the small change in water repelling controlled
by light to a level that can overcome the hysteresis,
or the attraction, that causes water to stick even when
a drop is pushed along, ” Picraux said. “Rohit
Rosario mathematically derived the theory for surface
change amplification and proved it in the laboratory.”
The lotus leaf effect is a fairly well known phenomenon
that combines the microscopically rough surface of the
plant’s leaves with a waxy chemical coating and
leads to high water repellency and self-cleaning of the
surface. It is already employed commercially in stain
repelling pants.
What appears to aid this effect is tiny ‘nanowires’
on the surface of a material, the ASU researchers said.
Nanowires are small, high-aspect-ratio wire-like structures
composed of semiconducting and other materials. Typical
wire dimensions are tens to hundreds of nanometers in
diameter and micrometers in length.
“We have used our expertise in nanowire growth to
influence a new physical property for nanowire surfaces,
namely the behavior and motion of fluids,” Picraux
said. “While nanowires give exquisite control over
the surface for creating extremely rough surfaces, we
point out there are many practical ways to nanostructure
the surface once the basic principles of surface amplification
of switching are understood.”
The ASU team now is working to design a device that can
move drugs dissolved in water, or droplets of water and
samples that need to be tested for environmental or biochemical
analyses.
Another potential application is reducing the amount of
proteins or enzymes needed for testing during drug development.
Usually, making and purifying these candidate drugs is
time-consuming and small amounts are made at a time.
In a microfluidic device, the cells, DNA, or proteins
that are used to test the candidate drug efficacy also
are reduced so that a small amount of candidate drug can
be mixed with its target and the result recorded. This
reduces the time needed to screen all of the drug candidates
and allows as many tests as possible to be run simultaneously.
“The payoff of this scientific collaboration is
the first demonstrated ability to use a beam of light
to move microdroplets of water around on surfaces, in
extremely small channels or place them in predetermined
positions for analysis,” Garcia said. “Other
nanotechnology researchers can follow our lead and look
at ways of magnifying the triggering of surface changes
through electric fields or through solution conditions
such as temperature or acidity.”
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Skip
Derra, with ASU's Office of Public Affairs
skip.derra@asu.edu
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