Polycation-sensitive membrane electrode for determination of heparin based on controlled release of protamine

A polycation-selective polymeric membrane electrode using dinonylnaphthalene sulfonate as an ion exchanger has been developed as a protamine controlled-release system for potentiometric detection of heparin. The incorporation of tetradodecylammonium tetrakis(4-chlorophenyl)borate as a lipophilic salt in the membrane dramatically improves the sensor's selectivity towards protamine over sodium ions via influencing the activity coefficient of protamine in the membrane, and a stable potential baseline can be obtained in the presence of an electrolyte background. The electrostatic binding interaction between heparin and protamine decreases the concentration of free protamine released at the sample-membrane interface and facilitates the stripping of protamine out of the membrane surface via the ion-exchange process with sodium ions, thus decreasing the membrane potential. Under optimal conditions, the proposed polymeric membrane electrode exhibits a linear relationship between the initial slope of the potential change and the heparin concentration in the range of 0.025-1.25 U mL(-1) with an improved detection limit of 0.01 U mL(-1).