Synthesis of anionic water-soluble polyfluorenes with iridium (III) as energy acceptor and applications in bio-sensing

A series of water-soluble carboxylated polyfluorenes containing iridium complex as the energy acceptor was synthesized via Suzuki coupling reaction. These anionic polymers (P4,P5 and P6) have 0,2 and 5 molar percentage of iridium (III) complex units in the polymer backbone, respectively. The polymer (P6) with 5 mol% iridium (III) complex shows efficient intrinsic fluorescence resonance energy transfer (FRET) in aqueous solution. The photoluminescence (PL) spectra studies show that the polymers (P4 and P6) have higher fluorescence intensity at high pH and quenched in water at low pH. As pH decreases the carboxylate groups become more protonation, which leads to more aggregation of the polymer chains. The polymer (P6) fluorescence changes were studied with lysozyme, hemoglobin, histone and heparin in PBS buffer solutions. Addition of lysozyme to the P6 solution led to quenching of the fluorene segment and gradual increase in the iridium (III) complex emission band. Addition of hemoglobin to the polymer solution led to weak quenching due to the metalloprotein but neutral charged. Addition of histone and heparin to the P6 solution induced increase in emission for fluorene segments, which was due to the hydrophobic nature and the repulsion interaction between negatively charged heparin and P6, respectively.