New trends in ion-selective electrodes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 99

ionophores consists of the metalloporphyrins [83]. The naturally occurring ionophore
Vitamin B 12 , a derivative of cobalt(III) cobyrinate, was shown to be selective towards
chloride and nitrite in the mid-1980s, which boosted further research in this area
[84]. A number of metal centers were evaluated in porphyrin structures with a differ-
ent periphery in order to optimize selectivity. Several highly selective halide and oxo-
anion sensors were proposed based on metalloporphyrins with Ga(III), In(III), Mn(III),
Zr(IV) metal centers [83]. Other important organometallic receptors include lipophilic
uranyl salophenes, which were suggested as phosphate and fl uoride ionophores; cova-
lently bound compounds of tin and mercury are used as chloride ionophores. A fruitful
“anticrown” concept was proposed, using a macrocycle reciprocal to the original crown
structure that holds Lewis acidic hosts interacting with guest anions. The [9]mercu-
racarborand-3 ionophore (Fig. 4.1, I 1) allowed one to achieve nanomolar DLs for

iodide, which is exceptionally low for anions [85].
Ionophores based on hydrogen bonding are quite multifarious. Heterocycles with
NH groups, capable of interacting with anions in the cycle cavity, were proposed for
halides. Sensors for sulfate and phosphate with enhanced selectivity were developed
based on bipodal urea and thiourea ionophores. The task is challenging due to a high
hydration and low complexation ability of the mentioned anions. Guanidinium ion-
based ionophores were suggested for sulfi te. Hydrated trifl uoroacetophenone derivatives
(in the form of a geminal diol) are also believed to be hydrogen-bonding ionophores for
carbonate. However, there is still discussion on the exact mechanism of the relevant
ion-ionophore interaction [86].
Lipophilic ion exchangers traditionally used for polymeric membrane preparation
are the anionic tetraphenylborate derivatives and the cationic tetraalkylammonium
salts. The charges on both lipophilic ions are localized on a single (boron or nitrogen)
atom, but the steric inaccessibility of the charged center, due to bulky substituents, may
inhibit ion-pair formation in the membrane and provide, when necessary, non-specifi c
interactions between ionic sites and sample ions.
For a long time tetraphenylborates comprised the only class of cation exchanger
used in ISEs, despite of their shortcomings. Simple tetraphenylborates are of low
lipophilicity and their leaching from membrane shortens sensor lifetime [87].
Moreover, their decomposition in acidic solutions limits the applicability of this type of
cation exchanger. However, incorporation of acceptor substituents into the phenyl rings
of tetraphenylborate may to some extent enhance stability and lipophilicity of these
compounds. While lipophilic sulfonic acids were tested as ion exchangers, they have
not become widespread. Very recently a new class of cation exchangers, named carbo-
ranes, was introduced. Perhalogenated closo -dodecacarboranes were shown to be not
only of higher stability than tetraphenylborates, retaining comparable lipophilicity, but
can be with ease covalently attached to the membrane polymeric backbone, leading to
the development of plasticizer-free membranes without leachable ion exchangers [88].
Related compounds, metal dicarbollides, could also be incorporated into membrane,
providing anionic sites of extremely high chemical stability.
Quaternary ammonium salts (QAS) are anion exchangers used in solvent polymeric
membranes. Variation of substituents at the nitrogen atom is an option for tuning QAS