Modulating Fluorescent Responses in Organic-Doped Polyethylenimine Composites: Impact on Pyrophosphate Detection and Biogenic Phosphate Analysis

Organic-doped fluorescent polyethylenimine composites have been prepared by incorporating three commercially available aldehydes─salicylaldehyde (SA-PEI), 4-hydroxybenzaldehyde (PHB–PEI), and 2-hydroxynaphthaldehyde (NP-PEI)─in PBS buffer solution (pH = 7.4). The polymers derived from salicylaldehyde and 4-hydroxybenzaldehyde exhibited opposite responses (turn-on with SA-PEI and turn-off with PHB–PEI) to pyrophosphate (PPi), whereas NP-PEI showed a negligible response. Mechanistic investigations indicated that the electrostatic interaction supplemented by hydrogen bonding is responsible for the phosphate-specific optical response. However, the mode of interaction and fluorescence response depended on the nature of the organic dopant to the PEI polymeric backbone. Mechanistic investigation suggested that the SA-PEI composite showed turn-on response due to the restricted intramolecular motion and enhancement of hydrophobicity surrounding the probe due to PPi addition, and PHB–PEI showed turn-off response due to higher hydrogen-bonding interaction leading to more nonradiative decay by charge transfer. Not only this but also the entanglement of polymer chains, leading to aggregated structures, depends on the degree of anionic charges on phosphate analytes. Further, a similar idea was extended for analysis of biogenic phosphates like Calf Thymus DNA (CT DNA) and adenosine triphosphate (ATP), which was relevant for clinical diagnosis.