Biomedical Engineering Reference
In-Depth Information
1
0
3
M
10
4
M
10
5
M
10
6
M
25 mV
100 mV
10
7
M
10
8
M
10
9
M
10
10
M
10 min
10
9
8
7
6
5
4
3
Time
log
a
Ca
2+
(a)
(b)
FIGURE 4.7
Calibration curves for Ca
2
-selective membrane electrodes made from a monolith of low
porosity (i.d., 200 µm, no PVC) with inner fi lling solution of 0.1 M CaCl
2
(a) and time response of the mono-
lithic Ca
2
-ISE (b). (Figures adapted from [32].)
extend the list of the available low DL sensors, covering the most important ions, includ-
ing poisonous metal cations and inorganic anions, which are still poorly detectable by
ISEs. Advances of the modern PBP model allow one to predict various infl uences on the
DL of the sensor even in the presence of ion fl uxes in the membrane. On the basis of this
powerful theory a rational design of the low DL sensors becomes possible. However, a
unifi ed and simplifi ed procedure for sensor manufacturing and a universal experimental
protocol for low activity measurements still have to be developed. Rapid response times,
high long-term stabilities and suffi cient chemical ruggedness are key requirements for
the low DL sensors. Miniaturization of such sensors in order to perform measurements
in small sample volumes is another crucial direction of current research.
4.2.3 Reference electrodes
The proper reference electrode, completing the electrochemical cell, is always required
for accurate and reproducible measurements with ISEs. In contrast to tremendous
progress in solvent polymeric membrane sensors, much less attention was paid during
the last decade to reference electrodes. The reference elements usually contain calomel
(Hg/Hg
2
Cl
2
) or more often silver/silver chloride (Ag/AgCl) electrodes in contact with
a concentrated KCl solution. The reference half-cell is connected to the sample via
an electrolyte bridge (Fig. 4.4), fi lled with an equitransferent electrolyte (LiOAc,
NH
4
NO
3
, KCl), which provides a diminished diffusion potential and helps to avoid
sample contamination. The liquid junction potential, formed at the sample-bridge
interface, can be kept reasonably small under defi ned conditions or is accessible to
calculations according to the Henderson formalism. Unfortunately, the liquid junction,
being an essential part of the reference system, is diffi cult to miniaturize and imposes
serious restrictions on microsensor and sensor array development. Although several
potentiometric methods eliminate the use of a reference electrode, the problem of solid
contact (liquid junction-free), miniature, and robust reference is of a great importance.
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