Shining a light on novel polymers

Plastic, after all, is a well-known insulator. But with certain modifications in its chemical structure, a type of plastic called a conjugated polymer becomes conductive, a property that has been used in recent years to make lighted displays, solar cells and some television screens.

At Los Alamos National Laboratory, researchers have discovered an unusual characteristic of conjugated polymers that makes them highly valuable in sensors to detect biological and chemical agents. "This unique property greatly increases a sensor's sensitivity," said Liaohai Chen of Los Alamos' Bioscience Division. "We can detect bioagents or chemical agents rapidly at extremely low levels."

Conjugated polymers are long chains of molecules. They are joined by alternating single and double bonds that can enable the electrons to move along the backbone of the chain. Normally they become conducting when they are "doped" by certain doping molecules. This discovery in the late 1970s earned the aforementioned Nobel Prize in chemistry.

Some conjugated polymers that fluoresce can be treated as a string of glowing "pearls" linked together. The fluorescence of the polymer can be turned off by the presence of molecular "quenchers" attached to the polymer and turned back on when the quenchers are removed.

Chen and Los Alamos colleagues Hsing-Lin Wang, Duncan McBranch and David Whitten discovered in 1999 that one quencher attached to a polymer chain would quench every "pearl" of the entire chain, and that when this single quencher was removed, the whole chain would fluoresce.

"This was a very surprising result, not only to us, but to the whole polymer community," said Chen. "What's more, the property is reversible, so the polymer can be turned on and off, depending on whether the single quencher is attached or not. This was against all expectations. Even though no current theory or models can explain this observation, the tremendous signal amplification process definitely opens a new avenue for sensor development."

The ability for action on a single molecule to affect an entire polymeric chain makes a biological or chemical sensor extremely sensitive. All it takes is for one element in a quenched chain to recognize the target and pull the quencher away for the entire chain to light up, sending a luminescent or electronic signal that can be easily read. In principle, such a sensor can be hundreds or thousands of times more sensitive than sensors using conventional fluorescence mechanisms.

Thus far, conjugated polymer-based biosensors have successfully detected very low concentrations of toxins and viruses. Los Alamos researchers are trying to optimize a sensing system by further increasing the sensitivity and verifying which quenchers and other elements work best with specific bioagents or toxins. Along with researchers elsewhere, they also are trying to understand why the polymeric chain behaves the way it does. The mechanism for the multiplier effect remains mysterious.

The Los Alamos work is supported by the Department of Energy's Office of Biological and Environmental Research.