A synthetic, free-floating nanosheet just two molecules thick may provide the perfect substrate for creating future electronic devices.
The biologically inspired sheet is made of polymers, or long molecules with repeating units, that mimic the precision and order seen in proteins and crystal structures. But these synthetic sheets are made of molecular building blocks that are more durable than their natural counterparts.
"We're making molecular plywood -- a flat piece of building material that you can build nanoscale structures with," said chemist Ronald Zuckermann of Lawrence Berkeley National Laboratory, coauthor of a study April 11 in Nature Materials. "This study will open people's eyes and make them talk about proteins and plastics in the same sentence."
Zuckermann's team made the discovery by stumbling upon a particular sequence of repeating units that formed perfectly aligned two-dimensional crystals. "Ours is the largest and thinnest two-dimensional self-assembled organic crystal known," he said.
Proteins are made of a chain of amino acids that fold up into three-dimensional structures, such as alpha-helices and beta-sheets. Zuckermann had previously developed polymers that mimic alpha-helices, and here for the first time he has developed a material that mimics beta-sheets.
"This study is a great advancement," said materials scientist Yi Cui of Stanford University. "The fact that they can produce a really large sheet on a nanometer-scale is really surprising."
By using only two types of molecular building blocks, the team dramatically reduced the number of possible sequences and simplified the self-assembly of the polymers into larger structures, such as sheets. They created 3-nanometer-thick sheets with hydrophobic, or water-fearing, chemical groups facing the inside and hydrophilic, or water-loving, molecular units on the surface.
The team systematically adjusted the hydrophilic and hydrophobic groups until they discovered a pattern of molecular sequences that self-assemble into layered sheets. The sheets resemble a plasma membrane, the bilayered structure made of lipids and proteins that surrounds cells.
When Zuckermann looked at the polymer chains directly under the most powerful electron microscope in the world, he observed them wiggling around like little worms as they slid against each other. The idea of using high-resolution electron microscopy to visualize the shape of individual polymer chains was previously unheard of, he said
"It completely blew us away that these crystallizine sheets are so well-ordered and have very straight edges, even though their component polymer chains are flexible and spaghetti-like," Zuckermann said. "It was a real thrill to figure out how to really order material in a precise way at the atomic level." His team knows exactly where each atom is located in the structure, so it's possible to chemically engineer the material to serve specific functions.
A smooth, layered surface may be ideal for building flat electrical components, such as photovoltaic devices, batteries and fuel cells, Zuckermann said. Decorating the hydrophilic surface of the sheet with molecules that specifically bind to proteins may be useful for biosensing applications, such as developing catalysts and recognizing molecules, he added.
What's more, the sheets form layers that can separate and selectively transport different materials. He foresees developing more complicated three-dimensional structures using the same technology. Scientists may also one day use the technology for biological applications, such as drug delivery or tissue engineering.
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