A novel material, described in the 17 January issue of Science, has a unique love-hate relationship with water. With a flip of a switch, it alternates between attraction and repulsion. The secret to the switchable material, which scientists say could have a variety of uses, is electronically twisting molecules arrayed on the surface.
Teams have been able to switch surface properties before, but only by laboriously adjusting their chemistry. Physical chemist Joerg Lahann, a postdoc in lab of chemical engineer Robert Langer at the Massachusetts Institute of Technology, wanted something easier. Lahann and colleagues turned to chainlike polymers called alkanethiols, which naturally assemble into what looks like rows of tightly packed cornstalks. If they could synthesize alkanethiols with different chemical properties on their tops and sides and then attach them to a plate, the researchers thought, they could alter the surface properties of the plate simply by making the molecules stand straight or bend over.
Getting the alkanethiols to stand up was easy. Sulfur atoms at one end of the molecules naturally bind to gold surfaces, and the molecular stalks stick straight up if packed in tight. To bend over, however, the alkanethiols needed breathing room. Lahann supplied it by synthesizing novel alkanethiol stalks with bulky mushroomlike heads. As the molecules, dubbed MHAEs, latched onto the gold surface, the bulky heads prevented them from packing tightly together. Lahann and his colleagues then used a standard chemical reaction to lop off the tops of the mushrooms, leaving each molecular cornstalk tipped with a negatively charged, water-loving carboxylic acid group.
To persuade the MHAEs to bend over, Lahann and his colleagues needed only to wire up the gold surface to a power supply. When the researchers applied a positive electric potential, the plate yanked down on the carboxylic acids, exposing their water-repelling hydrocarbon chains.
"The idea that you can change the surface is extremely exciting, because many technologies are based on surface structures and most surfaces are static," says Edith Mathiowitz, a chemist at Brown University in Providence, Rhode Island. Mathiowitz and others hope the technique will make possible novel schemes for releasing drug compounds on cue, trapping and releasing proteins for large-scale proteomics studies, and manipulating liquids in microfluidic chips.