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.
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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
Nanoparticles encaged in cross-linked resorcinarene shells
Alexander Wei, Stephen V. Pusztay, Kevin B. Stavens, and Ronald P. Andres
Monolayers of calix  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.