Modulation of charge density of cationic conjugated polyelectrolytes for improving the FRET-induced sensory signal with enhanced on/off ratio

Three types of cationic polyfluorenes with a different number of ionic groups are synthesized to modulate their solubility in water and the interactions with anionic oligonucleotide probes (DNA-C). Increasing the charge density improves the water-solubility of conjugated polyelectrolytes (CPEs) but decreases its optical antenna effect in the electrostatic sensory platform (CPE/DNA-C), resulting in low fluorescence resonance energy transfer (FRET) signal. With increasing charge density in CPE, the concentration of conjugated units in CPE/DNA-C decreases at [+]:[-] = 1:1, after which little driving force is expected to bring positive CPEs and negative DNA-C into close proximity for efficient FRET. CPF2 with 2 ionic groups (per repeat unit) and hydrophilic polyether side-chains exhibits good water-solubility and a higher FRET signal with an enhanced signal on/off ratio. In addition, the charge density in CPEs modulates the range of sensory responses by forming a tighter or weaker electrostatic CPE/DNA-C complex, which perturbs the probe-target binding interactions. Three types of cationic polyfluorenes with a different number of ionic groups are synthesized. In the conjugated polyelectrolyte (CPE) and DNA probe-based sensory system, ionic density of CPE (per repeat unit) needs to be controlled carefully to modulate the water-solubility, D-A intermolecular separation, and the resulting fluorescence resonance energy transfer sensory signal with high signal on/off ratio.