Energy-transfer enhanced quenching in fluorescent conjugated polymer chemosensors.

摘要

This dissertation explores the fluorescence sensitivity and energy-transfer properties of conjugated fluorescent polymer chemosensors in the presence of analytes. In this work we develop a polymer system involving the systematic design of pendant-functionalized polymers for the detection by fluorescence quenching of transition metal ions in aqueous media designated as priority contaminants by the U.S. EPA. A modular system was designed to allow tuning of the receptor pendant and the polymer backbone for optimal sensitivity and specificity to target analytes. Lewis-bases such as tolylterpyridine and bipyridine were linked in conjugation to polymer backbones such as polyphenylene ethynylene thienylene ethynylene (PPETE) or polyphenylene ethynylene (PPE).; Fluorescence quenching was observed in the presence of transition metal ions and treated quantitatively by Stern-Volmer analysis. Stern-Volmer analysis typically results in linear relationships between either the relative intensity or fluorescence lifetimes versus quencher concentration. This method distinguishes between complexational (static) or collisional (dynamic) quenching mechanisms. We observed a static quenching mechanism for our polymer system. Also, these materials demonstrate significant deviation from linear Stern-Volmer analysis, with curvature that describes increasing static quenching Stern-Volmer constants. Aggregation or conformational changes were investigated by viscosimetry, light-scattering, and time resolved anisotropy but were ruled out for these polymers.; Conjugated polymers are predicted to exhibit enhanced sensitivity due to energy-transfer along the backbone. Models exist in the literature to describe the energy-transfer enhancement to dynamically quenched polymer systems. We developed a new model that describes energy-transfer enhanced quenching for statically quenched polymer systems. This new model can distinguish between through-bond (Dexter) and through-space (Forster) energy-transfer mechanisms. It also can account for differences in Stern-Volmer curvature observed for different analytes as aggregation or conformational changes could not. Further work has investigated the model empirically through the synthesis and characterization of a series of chemosensor polymers with increased spacing between receptor units along the polymer backbone. Analysis of the fluorescence and quenching data yield an empirical approximation of the distance of energy-transfer enhancement to quenching in this polymer system.