Naked-Eye Detection and Quantification of Heparin in Serum with a Cationic Polythiophene

A strategy for naked-eye detection and quantification of heparin in biological media, such as fetal bovine serum (FBS) is demonstrated by monitoring the absorbance change of a water-soluble cationic polythiophene. The negatively charged heparin interacts with positively charged polythiophene through electrostatic interaction, which leads to polymer conformation and color change from yellow to orange in solution. Under optimized conditions, addition of heparin derivatives, such as hyaluronic acid or chondroitin 4-sulfate to the same polymer solution leads to less change in polymer conformation and solution color due to their lower charge density as compared to that of heparin. Increasing the detection temperature or simply adding some organic solvent to the aqueous media reduces the polymer-polymer interchain pi stacking, and the polymer color change can be used to clearly differentiate heparin from its analogues in homogeneous solutions. Quantification of heparin is also demonstrated by correlating the changes in polymer absorbance to the heparin concentration. A linear calibration curve is observed in the 0-6.7 U/mL and 0-2.2 U/mL ranges for heparin quantification in pure water and in FBS, respectively.