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Chemosensor shows enhanced sensitivity Chemists at the University of Pennsyl vania have devised a new way to en hance the sensitivity of a fluorescencebased chemosensor [/. Am. Clwm. Soc, 117, 7017 (1995)]. They believe the meth od, which makes use of the special elec tronic properties of conjugated poly mers, will find wide use in a variety of sensing applications. A fluorescent chemosensor is a nonbiological molecule that binds to an analyte and signals that binding by a change in the light emitted by the che mosensor when it is excited by pho tons. Ideally, the chemosensor's recep tor (host) binds with the analyte (guest) selectively and reversibly. Fluorescence, by its nature, is a very sensitive method of detection. But ana lysts are always striving for greater and greater sensitivity so they can detect smaller and smaller amounts of analytes. The most straightforward way to gain sensitivity, says Timothy M. Swager, an assistant professor of chemistry at Pennsylvania, is to design receptors that bind an analyte more strongly. But if the binding is too strong, it will be ir reversible, and the sensor will work only once. Swager's approach focuses not on achieving optimal binding but on ampli fying the signal: He uses a conjugated polymer with a receptor attached to ev ery repeating unit. When the polymer is stimulated with light, it transports ener gy along the chain, and the excited state can sample every receptor on the chain, in effect amplifying the response pro duced by any one receptor. In their initial studies, Swager and graduate student Qin Zhou have used poly(phenylene-ethynylene)s in which every other phenylene unit is part of a 34-atom macrocycle known as a cyclo phane. This particular cyclophane is known to be an exceptional receptor for the dicationic molecule paraquat (l,l / -dimethyl-4,4 / -bipyridinium),
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Sensor system involves host-guest binding
which serves as the analyte in this sys tem. Binding occurs when paraquat slips into the "doughnut hole" of the cyclophane receptor. Swager views the conjugated poly mer as a molecular wire. "In effect, we wire all the cyclophane receptors in se ries," he explains. This creates "a poly mer whose binding constant for para quat is [theoretically] η times greater than that of a similar single receptor, where η is the number of repeating units in the polymer. The molecular wire functions as an antenna for optical
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Poly(phenylene-ethynylene) with cyclophane receptor (host)
photons much like the antenna on your radio." Absorption of a photon creates an excitation—an electron-hole pair— that migrates throughout the polymer. In the absence of paraquat, the polymer
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Paraquat (guest)
fluoresces strongly. But if a single para quat molecule binds to the polymer, it will quench the fluorescence by an electron-transfer mechanism, signaling the analyte's presence. In effect, Swager continues, "the para quat acts like a short in the wire," cap turing the excitation and deactivating it. By comparing the polymer's response to that of a single-receptor model com pound, he and Zhou have shown that the energy migration produces a 65-fold enhancement in quenching efficiency. That is, the polymer senses paraquat with 65 times the sensitivity of a single receptor. And each monomer unit in the polymer binds paraquat as strongly and reversibly as a molecule with a sin gle receptor. In more recent work, the Penn re searchers have demonstrated enhance ments greater than 200-fold, which may open some new applications. But Swager is hoping to go further and achieve a 1,000-fold enhancement, which he says would really get peo ple's attention. For convenience, the chemosensor's performance has been studied thus far only in solution. Real devices, though, will require attaching a thin film of the polymer to an optical fiber, for example. Swager notes that applications for fluo rescent chemosensors are expanding in medical instrumentation, groundwater testing, and in monitoring of industrial waste streams. He believes that his method of enhancing the sensitivity of such sensors is a general one that could be used to detect a variety of analytes, such as heavy-metal ions. Anthony W. Czarnik, a research di rector at Parke-Davis Pharmaceutical Research in Ann Arbor, Mich., and an expert in this field, says Swager's work is "creative," "clever," and "is going to open up some new possibilities" for chemosensors. "This is just a beginning," Swager tells C&EN about these new chemosen sors. "I see no end to what we can do with this." Ron Dagani
