The study, published in Angewandte Chemie, was performed by Prof. Israel Rubinstein, Dr. Alexander Vaskevich, postdoctoral associate Dr. Michal Lahav and doctoral student Tali Sehayek, all of the Institute's Department of Materials and Interfaces.
Nanotubes are tiny cylinder-shaped structures (a nanometer is one millionth of a millimeter). Discovered in 1991, the first nanotubes were made of carbon and captured the attention of scientists worldwide when they proved to be the strongest material ever made (100 times stronger than steel), as well as being excellent conductors of electricity and heat.
The new nanotube created at the WIS lacks the mechanical strength of carbon nanotubes. Its advantages lie instead in its use of nanoparticles as building blocks, which makes it possible to tailor the tube's properties for diverse applications. The properties can be altered by choosing different types of nanoparticles or even a mixture, thus creating composite tubes. Moreover, the nanoparticle building blocks can serve as a scaffold for various add-ons, such as metallic, semiconducting or polymeric materials - thus further expanding the available properties.
The tubes are produced at room temperature - a first-time achievement - in a three-step process. The scientists start out with a nanoporous aluminum oxide template that they modify chemically to make it bind readily to gold or silver nanoparticles. When a solution containing the nanoparticles (each only 14 nanometers in diameter) is poured through, they bind both to the aluminum oxide membrane and to themselves, creating multi-layered nanotubes in the membrane pores. In step three, the aluminum oxide membrane is dissolved, leaving an assembly of free-standing, solid nanotubes.
"We were amazed when we discovered the beautifully formed tubes," says Rubinstein. "The construction of nanotubes out of nanoparticles is unprecedented. We expected the nanoparticles to bind to the aluminum oxide template - that had been done before; but we did not expect them to bind to each other, creating the tubes."
The discovery process held other surprises for the Institute team. They had set out to accomplish something else entirely - to create a nanoporous template for studying the passage of biological molecules through different membranes. Likewise, having employed annealing - a process that uses heat to bind structures - they found that annealing actually prevented tube formation. "Everything interesting, in fact, happened at room temperature," says Rubinstein. "This exceptional process, of spontaneous room-temperature binding of nanoparticles to form tubes, is not yet fully understood and is currently being studied."
The resulting tube is porous and has a high surface area, distinct optical properties and electrical conductivity. Collectively, the tube's unusual properties may enable the design of future sensors and catalysts (both requiring high surface area), as well as microfluidic, chemistry-on- a-chip systems applied in biotechnology, such as DNA chips (used to detect genetic mutations and evaluate drug performance).
Applying their approach, the team has succeeded in creating various metal and composite nanotubes, including gold, silver, gold/palladium and copper-coated gold tubes. Yeda, the Institute's technology transfer arm, has filed a patent application for the new tubes.
Prof. Israel Rubinstein's research is supported by the Clore Center for Biological Physics, The Philip M. Klutznick Fund for Research, the Fritz Haber Center for Physical Chemistry, the Minerva Stiftung Gesellschaft fuer die Forschung m.b.H., and the Angel Faivovich Foundation for Ecological Studies.
The Weizmann Institute of Science in Rehovot, Israel is one of the world's top-ranking multidisciplinary research institutions. Noted for its wide-ranging exploration of the natural and exact sciences, the Institute is home to 2,500 scientists, students, technicians and supporting staff. Institute research efforts include the search for new ways of fighting disease and hunger, examining leading questions in mathematics and computer science, probing the physics of matter and the universe, creating novel materials and developing new strategies for protecting the environment.