A multicolor cationic conjugated polymer for naked-eye detection and quantification of heparin

A new cationic polyfluorene derivative with 20 mol % 2,1,3-benzothiadiazole (BT) content was synthesized via Suzuki cross-coupling polymerization. The high charge density and cationic oligo(ethylene oxide) side chains endue the polymer with a good water solubility (∼12 mg/mL), leading to a low inherent BT emission background in buffer. Addition of negatively charged heparin into the polymer solution induces polymer aggregation, giving rise to enhanced energy transfer from the fluorene segments to the BT units. With increasing heparin concentrations, the orange BT emission intensity progressively increases at the expense of the blue fluorene emission. In contrast, addition of hyaluronic acid, an analogue of heparin, results in an insignificant enhancement in BT emission. This selective optical signature not only allows distinguishing heparin from hyaluronic acid but also highlights the importance of electrostatic attraction between the polymer and the analyte in an energy transfer process. Heparin quantification is demonstrated by the linear intensity increase in the BT emission as a function of heparin concentration, providing a practical calibration scope ranging from 30 nM to 48 μM. Moreover, the distinguishable solution fluorescent color at different heparin concentrations makes naked-eye heparin detection and quantification feasible. This study hence reports a general approach to construct convenient multicolor biosensors using cationic conjugated polymers with energy donor-acceptor architecture.