Liquid Crystals Paving Way Towards "Smart-Paper" Displays

Liquid crystals are most recognized in the form of liquid crystal displays (LCDs)—found in everything from digital watches to notebook computers and flat-panel desktop monitors. But liquid crystals are far more talented than that. In the August 1 issue of the journal Science, for example, University of Wisconsin chemical engineer Nicholas Abbott reported a big step toward using them in flexible, inexpensive “smart-paper” displays, and in ultra-sensitive detectors for biomolecules or toxic chemicals.

Smart paper and biochemical sensors may seem very different, says Abbott, who did this work at Wisconsin’s Materials Research Science and Engineering Center, one of 27 such centers funded by the National Science Foundation. “But the unifying theme of our work is that a thin layer of liquid crystal can greatly amplify a wide range of activities on the underlying surface.”

In earlier work, for example, he and his colleagues showed that when proteins or other small molecules were captured on a specially prepared surface, they would perturb the liquid crystal immediately above. But the long, thin molecules in the fluid are always trying to line up in the same direction, says Abbott; that’s why they’re called “liquid crystals” in the first place. So the tiny distortions caused by the bound molecule propagate upward through the liquid for a tenth of a millimeter or so—a vast distance on a molecular scale. The result is a large, easily detectable change in the optical properties of the liquid crystal.

Now, Abbott and his colleagues have produced the same kind of effect in a way that can be controlled electronically. They start by layering the liquid crystal on top of a thin gold foil, which has been coated with a compound known as ferrocene. When the researchers then apply a voltage to the foil, the ferrocene molecules respond by changing their electrical charge. Once again, the change in charge produces a detectable distortion in the liquid crystal orientations above.

“You only need a very low potential to do this,” says Abbott, “typically a tenth of a volt, versus tens of volts in a conventional display.” That’s exactly what you would need for a flexible, paper-like display, or a rewritable label, he says. But it’s also the kind of voltage that’s typical of biological systems. So you can easily imagine using this technique in a sensor that would allow diabetics, say, to monitor their blood sugar. “When you ask, ’What could you detect?’” he says, “’glucose’ is the first thing that comes to mind.” More generally, he adds, by choosing the kind of receptors bound to the foil surface, it should be possible to use the liquid crystal to detect a wide variety of compounds.