Synthesis and characterization of a water-soluble poly(Fluorene-co- thiophene) and its fluorescence quenching by proteins

An amomc water-soluble polymer was synthesized by Suzuki-coupling between 2, 7-[ bis(4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)-9, 9-bis(3-(tert-butyl propanoate)) ] fluorene, 2, 7-dibromo-9, 9-bis (3 '-(tert-butylpropanoate)) fluorene, and 2, 5-dibromothiophene, which was followed by treatment in trifluoroacetic acid and aqueous Na₂CO ₃ to afford the functional carboxylic acid groups, with number-averaged molecular weight about 9000 and polydispersity of 1. 37. The polymer emits green fluorescence with a main peak at around 446 nm and a shoulder at about 464 nm. The absorption and fluorescence spectra of the polymer are strongly dependent on pH value owing to the partial protonation of carboxylate groups and the aggregation of the polymer chains. The change in polymer fluorescence upon interaction with different proteins in phosphate buffer solution (PBS) was also studied. The difference between the proteins is that bovine serum albumin (BSA) is negatively charged, histone is positively charged and hemoglobin is neutral. Strong fluorescence quenching of the polymer was observed in the presence of histone, the lowest quenching concentration was observed to be 5 × 10^-8 mol/L. Under similar experimental conditions, whereas weak quenching was observed in the other proteins hemoglobin and BSA. The variation in the fluorescence of the polymer in the presence of different proteins is due to the different electrostatic interactions between the polymer and the proteins.