Effect of surfactant on water-soluble conjugated polymer used in biosensor.

The effect of nonionic surfactants on the cationic conjugated polymer (CCP), poly{9,9-bis[6-(N,N-trimethylammonium)hexyl]fluorene-co 1,4-phenylene} iodide 1, has been investigated. It is shown that the CCP in various solvents exists in three phases: isolated polymer chains, polymer aggregate, and variable size clusters (partially dissolved polymer). It is shown that nonionic surfactants enhance the photoluminescence (PL) quantum yield of the CCP in water by breakup of polymer aggregates, which eliminates the nonemissive interchain quenching with aggregates and increases surface-to-volume ratio of the CCP. Furthermore, the surfactants reduce quenching by incorporation of the CCP into aggregates or binary micelles. Surfactant also reduces the polar interaction strength between CCP and water and enhances CCP quenching by the counterions (iodine) by ion pairing effect. The dynamics of the interactions are complex and reveal that the surfactant induces rapid increase in the PL which imply that the main force that causes the aggregation is weak and may be due to hydrophobic interaction of the CCP in water rather than a solid, particulate-like state. Time-resolved fluorescence measurements at the exciton energy (420 nm) confirm that the CCP in water and in some organic solvents is a multiphase system in which three exponential decay terms are needed to fit the decay profile of the CCP. The change in the decay lifetime explains clearly the effect of surfactant and solvent polarity on the three CCP phases. The average lifetime of the CCP does not increase with surfactant, but the number of isolated polymer chains increases which leads to higher PL quantum yield. The association between the polymer and a quencher, single-strand deoxyribonucleic acid (ssDNA), was investigated. It indicated that CCP:ssDNA forms a weak electrostatic complex that does not alter the absorption spectra of the CCP but induces a strong CCP fluorescence quenching with association constant KS = 5 x 10(7) M(-1). At low ssDNA concentrations, the surfactant reduces quenching in the complex possibly by preventing charge-transfer processes. This may be due to an increase in the distance between the CCP and ssDNA through incorporation of the CCP into aggregates (micelles). However, at high ssDNA concentration, the quenching increases sharply which may be assigned to the increase in the electrostatic force destroying the micelles' structure around the CCP, leading to contact quenching as well as DNA induced CCP aggregation, which in turn leads to CCP-CCP quenching.