X-Message-Number: 14459
From: "George Smith" <>
Subject: One small step for nanotech...
Date: Tue, 12 Sep 2000 16:30:33 -0700

Press release from Purdue University

 September 1, 2000

Tiny coated particles smooth way for nanoscale technologies
WEST LAFAYETTE, Ind.   Purdue University chemists have devised a way to
remove a major obstacle in designing new materials for use in the atom-size
realm of nanotechnology.

Nanoparticles   tailor made of selected metals or other materials and
measuring just billionths of a meter in diameter   are the building blocks
for this new generation of materials. Scientists are trying to use these to
build new, stronger materials one molecule at a time for applications
ranging from medicine to aerospace.

But this bottoms-up approach has had a downside: Nanoparticles can be so
fragile and unstable that if their surfaces touch, they will fuse together,
losing their special shape and properties.

Now, researchers at Purdue University have found a way to stabilize
nanoparticles made of metal by wrapping the tiny structures in a "plastic
coat" of molecular thickness. The coating prevents the nanoparticles from
fusing together upon contact and allows them to be easily manipulated.

The new coating process can be used to stabilize nanoparticles with magnetic
properties, allowing scientists to develop new materials for use in
microelectronic devices and magnetic sensors, says Alexander Wei, assistant
professor of chemistry who developed the new stabilization method.

"Though many of the applications are yet to come, our new method opens the
doors to a variety of new nano-structured materials," he says. "For example,
this coating process may be useful in developing materials for use in
biomedicine, such as new drug-delivery systems or probes and sensors
designed to target specific cells or tissues."

The research also has been used to process and manipulate nanoparticles that
are slightly larger in size, presenting opportunities that have yet to be
explored in nanoscale science and technology, Wei says.

Nanoparticles are developed in the laboratory using inorganic or metallic
particles one to 100 nanometers in diameter. Their name comes from
nanometer, which is one-billionth of a meter, about 100,000 times smaller
than the width of a human hair. These building blocks are part of a large
scientific effort, called nanotechnology, in progress in laboratories
throughout the world aimed at developing new technologies at the molecular
level.

Scientists are especially interested in developing nanoparticles made of
metals, semiconductors and magnetic materials. These substances have special
properties that make them useful for specific tasks. Because nanoparticles'
properties depend on their size, scientists can create materials with
distinct characteristics, such as electronic function, by fine-tuning the
size of the particles.

"Being able to control structures at the nanoscale level will allow
scientists to custom design materials to perform very specific functions,"
Wei says. "Ultra-small devices with unique electronic or magnetic functions,
and materials with superior strength and hardness are just two of the many
possible benefits of this technology."

Though scientists have been working for the past decade to develop various
types of nano-sized particles to use as building blocks for the next
generation of materials, stabilizing the tiny structures has remained a
challenge, Wei says.

"There are several issues to address in stabilizing nanoparticles," he says.
"One is keeping them dispersed, which means keeping them apart from each
other when working with them. Another is to stabilize them against
degradation, because you don't want them to change shape or get destroyed by
chemical interactions."

As the nanoparticles increase in size, they become even more difficult to
control.

"Metal particles larger than 10 nanometers in diameter are often challenging
to work with because of their strong tendency to stick to each other," Wei
says.

His group discovered a novel approach that addresses all these issues.
Working with nanoclusters of gold 10 to 20 nanometers in diameter, the
researchers first encapsulated the tiny structures in a shell of molecules
called resorcinarenes, which have bowl-shaped "heads" with several "tails"
fastened at one end.

"The resorcinarenes work well because they have a curvature which is
complementary to the surface of the nanoparticles, so they stick to the
metal," Wei explains.

Next, the researchers created a polymer cage around the surface of particles
by chemically "stitching" the resorcinarene tails together. The porous
coating permits the particle inside to interact with substances outside, but
keeps the nanoparticles from interacting with each other.

"The result is a very stable, permanent coat that keeps the particles
dispersed in solution," Wei says. "And the coating can be customized by
adding different chemicals, to make the nanoparticles function in a specific
manner."

Wei says the stabilization process also works well with larger size
nanoparticles. For example, his group has used the process to stabilize
nanoparticles of cobalt   a magnetic material   in sizes up to 40 nanometers
in diameter.

"Scientists working with nanoparticles have often been restricted to working
with structures one to ten nanometers in diameter," Wei says. "We think that
this is going to extend our ability to manipulate and process particles in
the 10 to 50 nanometer range."

The Purdue group also has shown that the encaged cobalt particles can be
used to create structures in the shapes of rings or chains, suggesting that
the magnetic properties of the nanoparticles can be precisely controlled to
create new structures.

"The way the magnetic particles behave in an external field is what will
allow us to create a lot of exotic structures that haven't been seen yet,"
Wei says. "Magnetic materials are inherently functional because they respond
to magnetic fields, so I think there are new applications just waiting to
happen for these particles."

Wei's studies at Purdue are supported by the National Science Foundation. He
presented details of his findings in August at the American Chemical
Society's national meeting in Washington, D.C.

Source: Alexander Wei, (765) 494-5257, 

Writer: Susan Gaidos, (765) 494-2081; 

Purdue News Service: (765) 494-2096; 

PHOTO CAPTION:


Stephen Pusztay, a fourth-year graduate student working at Purdue
University, shows how tiny magnetic nanoparticles can respond when exposed
to a magnetic field. Pusztay is working with Purdue researcher Alexander Wei
to develop ways to stabilize and manipulate nanoparticles for use in
nanotechnology. (Purdue News Service Photo by David Umberger)

A publication-quality photograph is available at the News Service Web site
and at the ftp site. Photo ID: Wei.nanoparticles

Download Photo Here

ABSTRACT

Nanoparticles encaged in cross-linked resorcinarene shells

Alexander Wei, Stephen V. Pusztay, Kevin B. Stavens, and Ronald P. Andres

Monolayers of calix [4] resorcinarene 1 (R=CH3) on the surfaces of neutral
metal nanocrystals can be cross-linked into shells of molecular thickness by
olefin metathesis. This paper describes the synthesis and characterization
of these hybrid materials, and discusses the implications of nanoparticle
encagement for colloidal stabilization and the fabrication of
nanoparticle-based devices.

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