Phosphonate-Functionalized Polyfluorene as a Highly Water-Soluble Iron(III) Chemosensor

An anionic, phosphonate-functionalized polyfluorene, i.e., poly(9,9-bis(3'-phosphatepropyl)fluorene-alt-1,4-phenylene) sodium salt (PFPNa), has been synthesized by copolymerization of phosphonic acid-substituted 2,7-dibromofluorene and phenyldiboronic ester via direct Suzuki polycondensation reaction in DMF/water. Polymer PFPNa is highly soluble and emissive in water with a solubility of 60 mg/mL and a photoluminescence quantum yield of 75%. The absorption and fluorescence spectra of PFPNa are strongly dependent on pH value owing to the partial protonation of phosphate groups and the aggregation of the polymer chains. In aqueous media polymer PFPNa shows high sensitivity and selectivity for Fe3+. A 400-fold fluorescence quenching was achieved upon adding Fe3+ into its aqueous solution. Most importantly, ultrathin multilayer films constructed by alternatively depositing polyanionic PFPNa and polycationic poly(diallyldimethylamine) via the layer-by-layer (LbL) self-assembling method are stable and highly sensitive to Fe3+ with a sensitivity up to 10(-7) M.