Ion-Selective Electrodes With Ionophore-Doped Sensing Membranes

2554 Supramolecular devices
Ion-selective
microelectrode
Figure 17 Ion-selective microelectrode used in scanning electrochemical microscopy (SECM) to measure profiles of ion concentrations
in three-dimensional space.
the scanning probe in scanning electrochemical microscopy
(SECM, see Figure 17). 76–78
Ag/AgCl wire
3.1.7 Combining ionophore-based ISEs with
enzymes and antibodies
While the majority of ion receptors tested in recent years
for use in ISEs are the product of organic synthesis,
especially in the early history of ISEs a significant number
of ionophores were obtained from biological sources. It
is therefore not surprising that attempts have also been
made to utilize biological compounds such as enzymes and
antibodies in potentiometry.
An important advantage of enzymes is that they not
only provide selectivity but also convert electrically neutral
substrates into ions. One classical example is urease,
which converts urea into carbon dioxide and ammonium
ions, which can then be detected with an ionophore-based
ammonium-selective electrode (Figure 18). The enzyme
can be directly incorporated into an enzyme-containing film
that is integral part of the electrode. 34 While for many
years such sensors have been largely of academic interest,
both urea and creatinine selective sensors recently became
commercially available. A more recent example for this
Ammonium-selective
membrane
Urea CO 2 + 2 NH 4
+
Enzyme-containing
film
Figure 18 Urea sensor with a urease-containing film that catalyzes
the decomposition of urea to give NH + 4 , which is detected
with the ionophore-based NH + 4 ISE.
approach uses potentiometry for the detection of prostate
specific antigen (PSA) at concentrations as low as 0.1 ng
ml −1 . 79 First, the blood serum tested for PSA is incubated
with β-galactosidase-labeled PSA tracer antibody, which
permits the antibody to bind to the PSA. Upon removal
of unbound antibody, 6,8-difluoro-4-methylumbelliferyl-β-
D-galactopyranoside is added. The galactosidase bound
through the antibody to PSA now signals the presence
of PSA by hydrolysis of the galactopyranoside to give
difluoromethylumbelliferone (DiFMU), which is detected
potentiometrically in its deprotonated form. This simple
F
OH
H
CH 2 OH
O
H
OH H
H OH
O
H
F
O
O
Galactosidase
H 2 O
OH
H
CH 2 OH
O
H
OH H
H OH
OH
H
F
HO
F
O
O
DiFMU