Ion-Selective Electrodes With Ionophore-Doped Sensing Membranes

Ionophore-doped sensing membranes 2561
of 1 : 2 complexes and inhibiting the binding of larger
ions. The highest potentiometric selectivity was reported
for Li + -II (10 3.3 -fold over Na + ), 102 but similar selectivities
were also found for several other compounds. 103
Interestingly, a more than 10 4 -fold selectivity but a slow
response was reported for the sterically even more hindered
Li + -III. 103
A fair number of ionophore-based ISEs with selectivities
for Rb + and Cs + , the two largest alkali metal cations,
have also been described. In some of these cases, the
potentiometric selectivities suggest that the ionophore binds
the target ion selectively. However, in a number of cases
such evidence is missing. Since Cs + has the lowest free
energy of hydration of all alkali metal cations, even
an ionophore-free ion-exchanger membrane will exhibit
selectivity for Cs + over the other alkali metal cations. To
show evidence for selective recognition of Cs + by the
ionophore, the selectivity of an ionophore-based ISE for
Cs + has be higher than for an ionophore-free ion-exchanger
ISE based on a membrane containing the same membrane
components but no ionophore (but including ionic sites),
a control experiment that is unfortunately too often not
reported.
4.3.2 Ion-selective electrodes for NH 4
+
Because of the key role of NH + 4 in various biological
processes, the direct measurement of this ion is important
for clinical and environmental analyses. Ammonium concentrations
in food also provide a measure of freshness.
Moreover, various enzymes catalyze the deamination of
numerous organic compounds, which makes NH + 4 selective
ionophores also of interest for enzyme-based ISEs
(Section 3.1.7). For all these reasons, there has been a continued
interest in the development of ionophore-based ISEs
for NH + 4 .
For many years, almost all the work on NH + 4 ISEs
focused on the macrocyclic antibiotics nonactin (NH + 4 -I)
and monactin (NH + 4 -II), see Figure 3. 4, 104 While the type
of sensing matrix has some effect on the selectivities of
ISEs based on these ionophores, an approximately 10-
fold selectivity over K + and an approximately 100-fold
selectivity over Na + is characteristic for these sensors.
Only in recent years several interesting alternatives have
been proposed. Significant improvements of the selectivity
of either Na + or K + have been obtained by use of
rigid crown ethers with bulky substituents that block
complexation with larger ions (e.g.,NH + 105, 106
4 -III), a
O
O
O
O
N
H
NHO
HN
O
O
O
O
O
O
O O
NH 4 + -IV
O
O
O
O
NH
N
H
HN
O
HN
O
NH 4 + -III
NH 4 + -VI
O
O
O
O
O
O
(CH 3 ) 2 Si
O
O
O
O
O
(CH 3 ) 2 Si
Si
O
Si(CH 3 ) 2
O
Si(CH 3 ) 2
O
O
O
O
O
NH 4 + -V
O
O
O
O
O
O