Abstract
Chronopotentiometry and electrochemical impedance spectroscopy were used to study the transient behavior and the potentiometric response mechanism of the polymer membrane-based sensor for heparin. Membrane with a composition of 66 wt % poly(vinyl chloride), 33 wt % o-nitrophenyl octyl ether (plasticizer), and 0.05 M tridodecylmethylammonium chloride (ion exchanger) was deposited on the surface of a silver or a glassy carbon (GC) electrode.
In the latter case, the membrane contained also 0.1 M 1,1?-dimethylferrocene/1,1?-dimethylferricenium+ couple ensuring the electronic contact between the membrane and GC. The sensor was dipped in an aqueous solution of 0.1 M LiCl, which was stirred with a magnetic stirrer (2MINUS SIGN 18.2 Hz), and eventually spiked with heparin (0.05MINUS SIGN 5 U mL-1).
Chronopotentiometric measurements were carried out using either the Ag supported membrane with a thickness >100 μm or the GC supported membrane with a defined thickness of 2MINUS SIGN 30 μm, which was also used in impedance measurements. Remarkable features of the potentiometric response include the linear dependence of the initial slope of the potential transient on the heparin concentration in the aqueous phase and on the square root of the stirring frequency, and the absence of the effect of the membrane thickness.
Impedance measurements (0.1 HzMINUS SIGN 10 kHz) made it possible to identify and to evaluate the geometric capacitance and the capacitance of the electric double layer at the membrane/solution interface, the bulk membrane and charge-transfer resistances, and the Warburg impedance of the chloride transport. Changes in the membrane bulk and charge-transfer resistances and the Warburg impedance upon spiking the aqueous solution with heparin were found to be consistent with the steady-state response of MINUS SIGN 25 mV, indicating that the bulk chloride concentration in the membrane decreased to about half of its initial value.