We have resolved single-exponential relaxation dynamics of the 2-, 3-, and 4-electron-hole pair states in nearly monodisperse cadmium selenide quantum dots with radii ranging from 1 to 4 nanometers. Comparison of the discrete relaxation constants measured for different multiple-pair states indicates that the carrier decay rate is cubic in carrier concentration, which is characteristic of an Auger process. We observe that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed.

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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 millionfold 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. With the rising awareness of the public vulnerability to chemical and biological terrorism, there is a heightened need for detection techniques that show both high sensitivity and selectivity. Such techniques also would find wide use in medical diagnostics and biomedical research applications. Methods of identifying biological molecules such as the enzyme-linked immunosorbant assay (ELISA) achieve selectivity by using specific antibody͞antigen interactions to anchor the antigen to a substrate, with a subsequent colorimetric change or fluorescence signal on addition of secondary reagents; these techniques can be time-consuming and require multistep procedures. Other approaches have used molecular recognition ligands to link to specific receptor sites on a biological species, usually as a means also of fixing the biomolecule to a substrate or membrane (1-6) It has remained a challenge to incorporate the selectivity offered by ligand͞receptor interactions into a sensor that can be extremely sensitive, robust, and versatile.We have recently explored the photophysical properties of a fluorescent, water-soluble polyanionic conjugated polymer [poly (2-methoxy-5-propyloxy sulfonate phenylene vinylene (MPS-PPV)] (Fig. 1B), one of a larger class of related molecules [poly phenylene vinylene (PPV)] (Fig. 1 A and derivatives) that has been the subject of almost explosive recent interest (7-13). Although much attention has focused on the well known potential for use of PPV derivatives as electronic materials [e.g., electrochemical sensors (14-16) light-emitting diodes (17, 18), and integrated circuits (19,20)], the highly charged backbone of MPS-PPV (with charge density approximating that of polynucleic acids such as DNA and RNA), also makes it a model polymer for understanding the interactions and self-assembly properties of charged biopolymers. In this paper, we report a striking discovery: the use of this fluorescent anionic polymer leads to a greater than million-fold amplificatio...

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