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Better Molecular Pens


Someday, nanotechnologists fancy, they’ll be able to build materials atom by atom from the bottom up, LEGO-style. Right now they’re still working on the two-dimensional equivalent: writing ultrafine lines and dots of selected molecules on ultrasmooth surfaces. Unfortunately, all the molecular “pens” developed so far have been either too blunt-tipped or too costly for broad use. Now a new technique might enable nanotechnologists to quickly and cheaply write molecular features across a large area. Down the road, the approach could help scientists rapidly prototype novel nanostructures for use in everything from studying stem cells to the molecular triggers involved in cancer.

The tried-and-true way to put fine patterns on surfaces is optical lithography, used for decades to carve circuits onto computer chips. But lithography is very expensive and doesn’t work well with many materials, such as fragile biomolecules. To solve those problems, researchers have tried stamping molecular inks onto surfaces with rubber molds, or using arrays of tiny pyramids with ultrasharp tips as quills. The stamps are cheap, but they have trouble printing features less than 50 nanometers wide. The arrays have much better resolution. But because they depend on springlike cantilevers to keep the moving tips in contact with the surface, they are complicated to operate—and, again, expensive.

Three years ago, researchers led by Chad Mirkin of Northwestern University in Evanston, Illinois, reported a possible fix. In an article in Science, they unveiled a new design that did away with cantilevers by making the tips out of a springy plastic that flexed to maintain contact with the surface. The downside was that the softer plastic couldn’t take as sharp a point as the hard silicon pyramids could.

Now, Mirkin and colleagues are back with a hybrid design: hard tips mounted on a springy polymer layer instead of cantilevers. It’s the best of both worlds, Mirkin says: The new tips can write molecular patterns with a resolution less than 50 nanometers, but an array of thousands of them costs less than $1. In a paper published online this week in Nature, the researchers describe using an array of 4750 tips to write 19,000 copies of the pyramid portrayed on the United States $1 bill, each consisting of 6982 42-nanometer-wide dots.

“This is excellent work,” says Stephen Chou, a nanopatterning expert at Princeton University. Joseph DeSimone, a nanopatterning and nanomedicine expert at the University of North Carolina, Chapel Hill, agrees. “Placing the spring in the polymer is pretty clever,” he says. “It gets rid of the cantilevers and reduces the complexity and cost of the system.” Mirkin says the new arrays could create cheap arrays of DNA and other biomolecules for diagnosing diseases or studying how different combinations of biomolecules affect things such as the development of stem cells or the progression of cancer cells. That may not be molecular LEGO, but it ain’t hay.