Highly sensitive biological and chemical sensors based on reversible fluorescence quenching in a conjugated polymer

The fluorescence of a polyanionic conjugated polymer can be quenched by ext remely low concentrations of cationic electron accepters in aqueous solutio ns. We report a greater than millionfold amplification of the sensitivity t o fluorescence quenching compared with corresponding "molecular excited sta tes." Using a combination of steady-state and ultrafast spectroscopy, we ha ve established that the dramatic quenching results from weak complex format ion [polymer((-))/quencher((+))], followed by ultrafast electron transfer f rom excitations on the entire polymer chain to the quencher, with a time co nstant of 650 fs. Because of the weak complex formation, the quenching can be selectively reversed by using a quencher-recognition diad. We have const ructed such a diad and demonstrate that the fluorescence is fully recovered on binding between the recognition site and a specific analyte protein. In both solutions and thin films, this reversible fluorescence quenching prov ides the basis for a new class of highly sensitive biological and chemical sensors.