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 extremely low concentrations of cationic electron acceptors in aqueous solutions. We report a greater than million-fold amplification of the sensitivity to fluorescence quenching compared with corresponding “molecular excited states.” Using a combination of steady-state and ultrafast spectroscopy, we have established that the dramatic quenching results from weak complex formation [polymer(−)/quencher(+)], followed by ultrafast electron transfer from excitations on the entire polymer chain to the quencher, with a time constant of 650 fs. Because of the weak complex formation, the quenching can be selectively reversed by using a quencher-recognition diad. We have constructed 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 provides the basis for a new class of highly sensitive biological and chemical sensors.

Brownian flocculation of polymer colloids in the presence of a secondary minimum

Most analyses of Brownian flocculation apply to conditions where London–van der Waals attractive forces cause particles to be strongly bound in a deep interparticle potential well. In this paper, results are reported that show the interaction between primary- and secondary-minimum flocculation when the interparticle potential curve reflects both attractive and electrostatic repulsive forces. The process is highly time-dependent because of transfer of particles from secondary- to primary-minimum flocculation. Essential features of the analysis are corroborated by experiments with 0.80-μm polystyrene spheres suspended in aqueous solutions of NaCl over a range of ionic strengths. In all cases, experiments were restricted to the initial stage of coagulation, where singlets and doublets predominate.