Chemical Peels For Molecular Films

A new method peels molecules off surfaces to create patterned arrays, rather than printing them onto surfaces as in conventional soft lithography (Science, DOI: 10.1126/science.1221774).

By removing patches of molecules, the technique can generate molecular arrays with 40-nm features. That resolution is higher than is possible with many soft lithography techniques, which add molecules to surfaces. Furthermore, compounds printed onto bare substrates by conventional methods tend to spread, blurring the edges of array features. Peeling molecules from the surface prevents molecular diffusion, say the researchers who developed the method.

“While soft lithography has already empowered the materials science community to produce micropatterns over large areas, this work adds another attractive and enabling aspect to this technology,” says Gang-yu Liu, a chemist at the University of California, Davis, who was not involved in the study. “This approach certainly will find wide application immediately.”

To make molecular arrays, the research team, led by Anne M. Andrews and Paul S. Weiss of UCLA, first forms a hydroxyl-terminated monolayer of alkanethiols on a gold surface. Then, the researchers use an oxygen plasma treatment to activate a patterned stamp made of polydimethylsiloxane (PDMS). This treatment produces hydrophilic siloxyl groups on the stamp’s surface.

When the researchers bring the stamp in contact with the thiol film, strong bonds form between the siloxyl groups from the pattern and the hydroxyl groups of the monolayer. Peeling the stamp away yanks the thiols, along with a single layer of gold atoms, off the surface.

The idea had been around for years that “if you somehow ripped thiolates off gold, you would break gold-gold bonds before breaking sulfur-gold bonds,” Weiss says. These studies confirm that hypothesis, he adds.

Andrews thinks the team can improve the resolution of its technique. “To date, we’re limited by the stamp features that we have,” she says. The researchers plan to next use the method to pattern small-molecule neurotransmitters onto surfaces, with the goal of capturing and studying receptor proteins that bind those compounds in the brain.