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.