Experiment uses biomolecules to write on a gold substrate
Duke University engineers have demonstrated that enzymes can be used to create nanoscale patterns on a gold surface. Since many enzymes are already commercially available and well characterized, the potential for writing with enzyme "ink" represents an important advance in nanomanufacturing. This research was funded by the National Science Foundation through a Nanotechnology Interdisciplinary Research Initiative (NIRT) grant.
Enzymes are natures catalysts -- proteins that stimulate chemical reactions in the body and are used in a wide range of industrial processes, from wastewater treatment to cheese making to dissolving blood clots after heart attacks.
In their experiments, the engineers used an enzyme called DNase I as an "ink" in a process called dip-pen nanolithography -- a technique for etching or writing at the nanoscale level. The dip pen allowed them to inscribe precise stripes of DNase I ink on a gold plate, which they had previously coated with a thick forest of short DNA strands. The stripes of the enzyme were 100 nanometers wide -- about one-millionth the diameter of a human hair.
Once the researchers had created the stripes, they then activated the enzyme with a magnesium-containing solution. This changed the DNase I into a form that efficiently breaks down any DNA in its path. As a result, the team reports in the May 2004, issue of the Journal of the American Chemical Society, available online as of March 27, 2004, the stripes of activated enzyme carved out 400 nm wide "troughs" in the DNA coating.
"We were surprised that the enzyme ink worked so well, because it was simply deposited on the surface and could have washed away during the processing steps," says biomedical engineer Ashutosh Chilkoti of Dukes Pratt School of Engineering, who leads the project.
Chilkoti credits much of the experiments success to the laboratory skills of Jinho Hyun, who was a post-doctoral fellow in his group, and who is now an assistant professor at Seoul National University. But this experiment was also an important proof of principle, says Chilkoti: until now, few researchers have explored biological substances for nanoscale manufacturing, and even fewer have taken the approach of putting down chemically active biomolecules on a surface.
"We wanted to see if we could steal functionality from biology to make the complex structures we need," says Chilkoti. The outcome, he says, was everything he and his colleagues could have hoped for: "In an afternoon, we inexpensively created a nanostructure that would have taken weeks to develop using expensive, traditional methods of etching circuits into chips."
Now that the team has demonstrated that enzymes can "subtract" from the substrate to make precise troughs, they envision many other possibilities. Instead of using enzymes that degrade DNA, for example, they could use other enzymes that link DNA strands together. That would allow them to make "additions" to the substrate, causing the DNA layer to grow thicker in certain places. Alternatively, they could use still other enzymes that make chemical changes in the DNA substrate itself, allowing them to build complex structures with "different colored bricks," as Chilkoti puts it.
The team could even do away with the DNA entirely, and use a different substrate, Chilkoti says. "We used DNA because it is pretty robust, because you can buy synthetic DNA strands off the shelf, and because there are lots of enzymes that work on it. But there is nothing unique about it for this kind of application."
"Enzymes have evolved to carry out an incredible variety of processes," comments team member Stephen Craig, a Duke chemist. "By harnessing the diverse power available in nature, it may be possible to selectively erase structures at one point, add structures at a second location, transform them from one state to another at a third location, and so on. The potential exists to create very small and very complex architectures."
"A lot more work is needed to optimize the process, but we feel this enzyme-inking technique has tremendous promise for wide applicability," says Chilkoti.
Enzyme-based nanomanufacturing is of interest because it could an incredibly versatile tool. This is critical because nanomanufacturing is at the heart of efforts to see if we can make new devices that are far smaller, cheaper, faster and better than existing devices," says Chilkoti
To date, dip-pen nanolithography has been primarily a bench top laboratory technique.
Scaling up the technique to truly make it a viable manufacturing technique will require new instrumental technology such as dippen lithography machines with multiple, articulated tips that can move independently to deposit several different types of enzymes. Chilkoti envisions machines that can work on a sheet of chips using different enzymes, so that the chips can be snapped apart after the enzyme inking and processing. Chilkoti notes that such machines are already being commercially developed, so the day might not be too far off when enzyme-based nanomanufacturing might be possible on an industrial scale.
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