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Fluorescent monomers and polymers that undergo temperature-induced phase transitions in water have been combined to produce copolymers that fluoresce strongly as soon as a threshold temperature is exceeded. "These are by far the most sensitive fluorescent molecular thermometers known," says Seiichi Uchiyama, a postdoc at Nara Women's University (NWU), in Japan, and a visiting research fellow at Queen's University, Belfast, Northern Ireland. He carried out the work with Ph.D. student Yuriko Matsumura and associate professor of chemistry Kaoru Iwai at NWU, and Queen's chemistry professor A. Prasanna de Silva [Anal. Chem., 75, 5926 (2003)]. "Our molecular thermometers are examples of fully functional nanosized devices," de Silva says. "They can, in principle, go to nanospaces and report the local temperature to us." The thermometers can potentially be used for monitoring the temperature of a microreactor or for biological applications, Uchiyama points out. "It should be possible to use them, for example, as an immediate test for severe acute respiratory syndrome (SARS) by using a film of the material on passengers at airports," he tells C&EN. The materials are random copolymers consisting of a thermosensitive poly(N-alkylacrylamide) labeled with less than 1% of a benzofurazan fluorophore monomer. In aqueous solution, the copolymer undergoes a phase transition above a certain critical solution temperature. The transition causes the local polarities around the main chains of the polymer to decrease. The polarity drop induces a sharp increase in the fluorescence intensity of the fluorophore. The copolymer is almost nonfluorescent at lower temperatures but becomes strongly fluorescent at higher temperatures, Uchiyama says. "For example, the fluorescence signal of one copolymer at 37 °C was over 13-fold [higher] than seen at 29 °C. The changes of their output signals per 1 °C are at least 10 to 30 times larger than those of conventional fluorescent molecular thermometers." The team also demonstrates that the fluorescence responses from the copolymers to changes in temperature are reversible and repeatable over at least 10 cycles of heating and cooling. The range of temperatures to which the thermometers are sensitive can be tuned by changing the alkyl substituents in the N-alkylacrylamide monomer. "Fluorescent thermometers are a long-awaited tool in physiology and molecular biology," comments Luigi Fabbrizzi, a chemistry professor at the University of Pavia, Italy, who is an expert on fluorescent molecular thermometers. "For instance, they could allow the temperature within the cell to be mapped during a variety of processes," he says. "The systems introduced by Uchiyama and coworkers are cleverly designed" and modular, Fabbrizzi points out. "The fluorophore can be changed at will to meet the needs of the user. On the other hand, the desired temperature interval can be selected by changing the polymer." |
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