New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 89

a polypeptide chain, the carboxylic side of lipophilic amino acids and between sequen-
tial Leu-Leu residues, respectively. The researchers used specifi cally synthesized polyi-
onic oligopeptides containing different di- and tripeptide fragments in order to recognize
activities of a specifi c protease.
Unfortunately, in the presence of detectable polyions in the solution a strong poten-
tial drift is normally observed due to the instability of the ion concentration gradients.
Moreover, the main disadvantage of polyion-selective potentiometric electrodes lies in
the intrinsic irreversibility of the underlying response mechanism. The target polyions
eventually displace the counter-ions in the membrane phase and consequently the sen-
sor loses its response.
Extracted polyions may be removed from the membrane phase by reconditioning of
the sensor in concentrated (2 M) NaCl, for instance. More recently, a pH cross-sensitive
potentiometric heparin sensor was proposed, which contained the ion exchanger and a
charged H ionophore [50]. Heparin stripping could now be accomplished by adjusting
the pH of the sample. Unfortunately, both methods require prolonged reconditioning of
the sensor (15 min) in the stripping solution after each measurement.
Another approach described above, which was employed by Meyerhoff's group [41],
requires a set of identical electrodes, in which every electrode is disposed after a sin-
gle measurement. A typical heparin-protamine titration requires on the order of 10-12
disposable electrodes.
4.3.2 Galvanostatically controlled sensors
The disadvantages described above in terms of the irreversibility of the polyion response
stimulated further research efforts in the area of polyion-selective sensors. Recently, a
new detection technique was proposed utilizing electrochemically controlled, reversible
ion extraction into polymeric membranes in an alternating galvanostatic/potentiostatic
mode [51]. The solvent polymeric membrane of this novel class of sensors contained a
highly lipophilic electrolyte and, therefore, did not possess ion exchange properties in
contrast to potentiometric polyion electrodes. Indeed, the process of ion extraction was
here induced electrochemically by applying a constant current pulse.
The experimental setup included a three-electrode electrochemical cell with a liquid
contact membrane electrode in which the internal Ag/AgCl electrode acted as a work-
ing electrode connected to a potentiostat/galvanostat. The instrument was capable of
switching rapidly between potentiostatic and galvanostatic modes [51].
Let us consider, for instance, the response mechanism of a polycation-selective gal-
vanostatically controlled sensor. The polymeric membrane is in contact with a NaCl
solution. The membrane of the sensor is formulated with a lipophilic salt, for instance,
tetradodecylammonium dinonylnaphthalenesulfonate (TDDA-DNNS), which has a
relatively high affi nity to protamine. Even though protamine is presented in the sam-
ple, spontaneous extraction does not take place due to the high lipophilicity of TDDA-
DNNS, thus the initial concentration of protamine or sodium cations in the membrane is
close to zero.
The measurement cycle of the sensor begins with a pulse of cathodic current i that
induces a net fl ux J of cations in the direction of the membrane phase. Assuming for