New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 87

0

(a): Heparin

10

5 min
24 hours

20

30

40

50

60

80

80

(b): Protamine

60

50

40

30

20

5 min
24 hours

10

0

8.0 7.5 7.0 6.5 6.0 5.5

log c polyion (mol/L)

FIGURE 4.9 Potentiometric response of heparin (a) and protamine (b) selective electrodes after different
5 min and 24 h equilibration times, respectively. Sample solutions contained 0.1 M NaCl and 50 mM TRIS
(pH 7.40) [39].

protamine to the membrane, and hence on its sample concentration. The approximate
equation describing the electrode potential in the presence of protamine can be derived
on the basis of a quasi-steady-state fl ux consideration [39]:

a

EE RT
F

(11)

0 Na

ln

PB

R z ( D δ / D δ ) c

T PA aq,PA m m,PA aq PA ,bulk

where z PA is the charge of the polyion (for protamine, ca. δ m , and

δ aq are the diffusion coeffi cients of protamine in the membrane and aqueous phases
and the resulting diffusion layer thicknesses, respectively. Since

21), D m,PA , D aq,PA ,

δ m increases with time
until it reaches the membrane thickness, which may take many hours to accomplish,
sensors based on this mechanism do not operate in a reversible manner and are not
suited for continuous monitoring purposes. Nonetheless, a range of important applica-
tions have emerged with this technology.
Meyerhoff's group developed disposable heparin-selective electrodes based on
“coated wire” technology [40]. The electrodes were successfully used as end-point
indicators for the detection of heparin concentration via protamine titration in whole
blood samples. During clinical trials 44 whole blood specimens were obtained from
eight patients undergoing open heart surgery [41]. The results at different stages of
surgery were in good agreement with those detected by ACT (Hepcon HMS System).