Nanoparticles Assemble By Millions To Encase Oil Drops,
Could Be Used To Clean Up Oil Spills
June 2, 2008
Houston -- In a development that could lead to new technologies for cleaning
up oil spills and polluted groundwater, scientists at Rice University have
shown how tiny, stick-shaped particles of metal and carbon can trap oil
droplets in water by spontaneously assembling into bag-like sacs.
The tiny particles were found to assemble spontaneously by the tens of
millions into spherical sacs as large as BB pellets around droplets of oil
in water. In addition, the scientists found that ultraviolet light and
magnetic fields could be used to flip the nanoparticles, causing the bags to
instantly turn inside out and release their cargo -- a feature that could
ultimately be handy for delivering drugs.
"The core of the nanotechnology revolution lies in designing inorganic
nanoparticles that can self-assemble into larger structures like a 'smart
dust' that performs different functions in the world – for example, cleaning
up pollution," said lead research Pulickel Ajayan, Rice's Benjamin M. and
Mary Greenwood Anderson Professor in Mechanical Engineering and Materials
Science. "Our approach brings the concept of self-assembling, functional
nanomaterials one step closer to reality."
The research was published online today by the American Chemical Society's
journal Nano Letters.
The multisegmented nanowires, akin to "nanoscale batons," were made by
connecting two nanomaterials with different properties, much like an eraser
is attached to the end of a wooden pencil. In the study, the researchers
started with carbon nanotubes -- hollow tubes of pure carbon. Atop the
nanotubes, they added short segments of gold. Ajayan said that by adding
various other segments -- like sections of nickel or other materials -- the
researchers can create truly multifunctional nanostructures.
The tendency of these nanobatons to assemble in water-oil mixtures derives
from basic chemistry. The gold end of the wire is water-loving, or
hydrophilic, while the carbon end is water-averse, or hydrophobic. The thin,
water-tight sacs that surround all living cells are formed by interlocking
arrangements of hydrophilic and hydrophobic chemicals, and the sac-like
structures created in the study are very similar.
Ajayan, graduate student Fung Suong Ou and postdoctoral researcher Shaijumon
Manikoth demonstrated that oil droplets suspended in water became
encapsulated because of the structures' tendency to align their carbon ends
facing the oil. By reversing the conditions -- suspending water droplets in
oil – the team was able to coax the gold ends to face inward and encase the
water.
"For oil droplets suspended in water, the spheres give off a light yellow
color because of the exposed gold ends," Ou said. "With water droplets, we
observe a dark sphere due to the protruding black nanotubes."
The team is next preparing to test whether chemical modifications to the "nanobatons"
could result in spheres that can both capture and break down oily chemicals.
For example, they hope to attach catalysts to the water-hating ends of the
nanowires that will cause compounds like trichloroethene, or TCE, to break
into nontoxic constituents. Another option would be to attach drugs whose
release can be controlled with an external stimulus.
"The idea is to go beyond just capturing the compound and initiate a process
that will make it less toxic," Ajayan said. "We want to build upon the
method of self assembly and start adding functionality so these particles
can carry out tasks in the real world." The research was supported by Rice
University, Applied Materials Inc. and the New York State Foundation for
Science, Technology and Innovation.
SOURCE: Rice University
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