NASA scientists have invented a breakthrough biological method to make ultra-small structures that may well be used to produce electronics 10 to 100 times smaller than today's components.
As part of their new method, scientists use modified proteins from 'extremophile' microbes that live in near-boiling, acidic hot springs to grow mesh-like structures so small that an electron microscope is needed to see them. A research article describing the new technique appeared in the Nov. 24 on-line version of the journal Nature Materials and is scheduled to be published in its December issue.
"Our innovation takes advantage of the innate ability of proteins to form into ordered structures and for us to use genetic engineering to change nature's plans, transforming these structures into something useful," said Jonathan Trent, principal investigator of a research project to produce 'nano-electronics' at NASA Ames Research Center in California's Silicon Valley. A nanometer is roughly 100,000 times smaller than the width of a human hair. "Building structures on the nano scale is an incredible engineering challenge," he said.
Proteins are the building blocks in all living things. Scientists can use genetic engineering to modify proteins in a wide variety of ways by altering the deoxyribonucleic acid (DNA) of the genes that contain nature's recipes for making proteins.
"We took a gene from a single-celled organism, Sulfolobus shibatae, which lives in near-boiling acid mud, and changed the gene to add instructions that describe how to make a protein that sticks to gold or semiconductors," said Andrew McMillan, a co-investigator at NASA Ames and primary author of the paper. "What is novel in our work is that we designed this protein so that when it self-assembles into a two-dimensional lattice or template, it also is able to capture metal and semiconductor particles at specific locations on the template surface."
"We cloned, or added, this modified gene segment into a harmless form of E. coli bacteria that rapidly multiplies, producing vast quantities of the new protein," said Chad Paavola, a project co-investigator, also from NASA Ames. Scientists can grow the E.coli bacteria in a watery broth in vats. The new protein starts out just a few nanometers wide. It self-assembles into an organized lattice, or template.
One reason scientists decided to modify a protein from an organism living at high temperatures is that this protein is robust. Because the genetically engineered protein is more heat-stable than the proteins E. coli naturally makes, scientists can easily purify the new protein by heating the broth containing the bacteria to destroy unwanted natural E. coli proteins. The engineered protein remains intact.
Then scientists crystallize the new protein to form tiny, flat, lattice-like structures that act as nano-templates. These crystalline structures, made of rings about 20 nanometers across, are about 5,000 times smaller than the width of a human hair. A nanometer is a billionth of a meter.
"We apply the crystals to a substrate such as a silicon wafer, and we add a gold or semiconductor slurry," said McMillan. "The tiny particles of gold or semiconductor (cadmium selenide/zinc sulfide) stick to the lattices." According to McMillan, the minute pieces that adhere to the protein lattice are 'quantum dots' that are about one to 10 nanometers across. Today's standard computer chips have features that are roughly 130 nanometers apart.
"After further development, an array of nanoparticles could serve as computer memory, a sensor or as a logic device that could calculate," said McMillan.
"Much of the success of today's electronics industry comes from knowing how to arrange materials in an organized fashion on a silicon substrate, and the prospects of using proteins to improve that process on a nanometer scale is encouraging," Trent added.
"There are several other interesting applications for these protein nano structures," said Meyya Meyyappan, director of the Center for Nanotechnology at NASA Ames. "For example, you can use them for biomedical applications."
"We have demonstrated the feasibility of using genetically engineered proteins to manipulate and arrange materials on a nanometer scale," Trent said. "Our ultimate goal is to prove that we can use proteins to build devices that will be of value to NASA in the search for life beyond Earth."
Technical information about the new process can be found on the World Wide Web at:
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