Investigation of conjugated polymers for metal ion sensing

Two carboxylate-containing fluorene-based anoinic water-soluble conjugated polymers, which are specially designed to link fluorene with alternating moieties such as 5% benzo[2,1,3]thiadiazole (BT) and amine groups have been synthesized via the Pd-catalyzed Suzuki-coupling reaction, respectively. The optical properties of the neutral precursory polymers in CHCl₃ and final carboxylic-anionic conjugated polyelectrolytes in aqueous solution were investigated. These polymers had good solubility in water and showed different responses for metal ions in aqueous environments. The fluorescence of carboxylic-anionic polyfluorene (PFA) can be quenched by most of metal ions selected (except K⁺, Li⁺) in H₂O/CH₃OH solution (9/1). The carboxylic acid-functionalized polyfluorene containing benzo[2,1,3]thiadiazole (PFBTA) showed good quenching with the addition of Cu²⁺ ions and had a little response for K⁺, Li⁺. While addition of other metal ions selected results in a change in the emission color of PFBTA from blue to orange. These spectral changes can be ascribed to increased interchain contacts in aggregated structures of PFBTA and increased levels of fluorescence resonance energy transfer (FRET) from the fluorene segments to BT units. The carboxylic acid-functionalized polyfluorene containing benzo[2,1,3]thiadiazole and amine group (PFBTNA) had similar responses for all metal ions selected with PFBTA in H₂O/CH₃OH solution (9/1). Moreover, PFBTNA showed improved sensitivity to most of metal ions selected, which may be contributed to the interaction between amine group and metal ions. After investigation of the UV-vis absorption spectra and PL emission spectra, we found that the energy or electron-transfer reactions were the main reason for ionochromic fluorescence quenching of these polymers. On the other hand, interchain aggregation caused by metal ions also lead to fluorescence quenching for these polymers. Moreover, the results also indicated that introduction of recognition group can obviously increase the sensitivity of the polymers and suggested the potential application for polymer PFBTA and PFBTNA as novel colorimetric and universal chemosensor with higher sensing sensitivity for many metal ions in aqueous environments.