New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 95

Detection of Li in artifi cial serum with a voltammetric Li-selective electrode in a fl ow-
through system was demonstrated [64]. Lithium salts such as lithium carbonate have
been extensively used for treatment of manic depressive and hyperthyroidism disorders.
The therapeutic range of Li concentration is generally accepted to be 0.5-1.5 mM in
blood serum. The authors used normal pulse voltammetry in which a stripping poten-
tial was applied between pulses in order to renew the membrane surface and expel
all of the extracted ions from the membrane, similar to galvanostatically controlled
potentiometric sensors described above. Unfortunately, the insuffi cient selectivity

pot

( K LiNa 24 . 10 2 ) of the Li-ionophore dibenzyl-14-crown-4 required the use of an

ion exchange column in order to separate lithium in the presence of a high (0.14 M)
concentration of sodium. A linear response function in the range of 0.2-2 mM was
obtained.
Another important bioanalytical application of voltammetric ISEs is the detection
of polyions (see also above). A technique using cyclic voltammetry on micropipette
electrodes fi lled with the organic electrolyte solutions in 1,2-dichloroethane was suc-
cessfully applied for the detection of protamine [65] in saline solution and heparin in
undiluted sheep plasma samples [66]. Protamine transport was facilitated with dino-
nylnaphthalenesulfonic acid (DNNS). As a heparin-selective component the tetrakis-
(4-chlorophenyl)borate salt of trimethyloctadecyl ammonium was used.
Further improvement of the low detection limit was achieved using stripping vol-
tammetry based on facilitated heparin adsorption and desorption [66]. Stripping vol-
tammetry yielded a detection limit of 0.13 U mL 1 in sheep blood plasma, which is
lower than therapeutic heparin concentrations ( 0.2 U mL 1 ). A linear response func-

tion in the range of 0.2-6 U mL 1 was observed. The authors also found that blood
polypeptides and lipids with a mass above 25 000 signifi cantly interfered with heparin
detection, perhaps by hindrance of a charge transfer reaction at the interface.

4.3.4 Light-addressable potentiometric sensors
The concept of light addressable potentiometric sensors (LAPS) was introduced in
1988 [67]. LAPS is a semiconductor-based sensor with either electrolyte-insulator-
semiconductor (EIS) or metal-insulator-semiconductor (MIS) structure, respectively.
Figure 4.13 illustrates a schematic representation of a typical LAPS with EIS structure.
A semiconductor substrate (silicone) is covered with an insulator (SiO 2 ). A sensing
ion-selective layer, for instance, pH-sensitive S 3 N 4 , is deposited on top of the insulator.
The whole assembly is placed in contact with the sample solution.
A constant bias potential is applied across the sensor in order to form a depletion
layer at the insulator-semiconductor interface. The depth and capacitance of the deple-
tion layer changes with the surface potential, which is a function of the ion concen-
tration in the electrolytic solution. The variation of the capacitance is read out when
the semiconductor substrate is illuminated with a modulated light and the generated
photocurrent is measured by means of an external circuit.