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.