Five kinds of monensin derivatives were synthesized by chemical modifications of the natural carboxylic polyether antibiotic monensin. Their usefulness as active compounds (Na+-ionophores) for Na+-selective poly(vinyl chloride)(PVC)-matrix membrane electrodes was examined. The electrodes based on monensin ester derivatives exhibited high Na+-selectivity. Among them, the electrode based on the highly lipophilic monensin dodecyl ester (log K° (partition constant)=8.7) had enough Na+-selectivity to determine the Na+-concentration in serum (log KpotNa K=-1.1, log KpotNa Mg==-4.0 and log KpotNa Ca=-3.6); the electrode gave a Nernstian response (59.0 mV/pNa) ranging from 5×10-6 to 1 M Na+. The response potential of this electrode to a 0.1 MNa+ solution drifted within ±0.01 mV/h. The values of the Na+-concentration in artificial and real serum samples were successfully determined with the monensin dodecyl ester-based electrode using diisodecylphthalate as the electrode membrane solvent.
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