C&EN: TODAY'S HEADLINES - SWITCHABLE SURFACE

CELIA HENRY

The properties of a surface can be controlled by changing the conformation of molecules in a self-assembled monolayer (SAM) on that surface. Chemical engineering professor Robert S. Langer and postdoctoral fellow Joerg Lahann at MIT, along with their coworkers, use electrical potential to reversibly switch an alkanethiolate monolayer between hydrophilic and hydrophobic states [Science, 299, 371 (2003)].

Langer likens the monolayer to what he calls “the fingertip model,” in which the fingertips and the knuckles represent opposing properties that are switched by bending the molecules.

Such surfaces could have a variety of uses, Langer believes, depending on the properties that are switchable. He envisions such diverse applications as microfluidics, drug delivery, electro-optics, and offset printing.

The researchers give the molecules enough room to bend by forming a monolayer with large globular end groups—similar to mushroom caps on thin stalks. Once the SAM forms, the “caps” are lopped off, leaving a free carboxylate end group. Applying an electrical potential to the underlying gold substrate attracts the carboxylate to the surface, bending the SAM’s alkane chains and exposing them to the surroundings.

“This work is interesting both because it represents a new approach to an old problem—the molecular-level design of surfaces having electrochemically switchable properties—and because it uses intentionally and rationally disordered SAMs,” says Harvard chemistry professor George M. Whitesides.