Photophysics and phosphate fluorescence sensing by poly(phenylene ethynylene) conjugated polyelectrolytes with branched ammonium side groups

Abstract

Conjugated polyelectrolytes (CPEs) have been applied as sensors for various small molecules and ions including biologically important ions. Here we report the synthesis of two different polyphenylene ethynylene (PPE) type cationic CPEs and studied their biologically important phosphate ion sensing properties. These CPEs are substituted with branched polyammonium units; polymer P-O-3 features an –O–CH₂– linker between phenylene units and the cationic side chains, whereas P-C-3 has an –CH₂– linker. The different linkers lead to a difference in intrinsic tendency of the CPEs to aggregate in water. We report amplified fluorescence quenching, Stern–Volmer constants (K_sv), fluorescence correlation spectroscopy, and different anion sensor response of the polymers. The investigation of these two CPEs for anion sensing reveals that they are considerably more sensitive to di-phosphonates than to di-carboxylates or di-sulfonates. Fluorescence quenching is a result of polymer aggregate formation due to electrostatic interactions and hydrogen-bonding between dianions and the cationic CPEs. Polymer P-C-3, which has lower tendency to aggregate in water, was found to be a more efficient sensor than P-O-3 for PPi and ATP. Stern–Volmer quenching constants (K_sv) of P-C-3 by PPi and ATP are greater than 10⁵ M⁻¹. Furthermore, a real-time fluorescence assay for ALP-catalyzed hydrolysis of pyrophosphate (PPi) is demonstrated.