Bifunctional polymeric membrane ion selective electrodes using phenylboronic acid as a precursor of anionic sites and fluoride as an effector: A potentiometric sensor for sodium ion and an impedimetric sensor for fluoride ion

• A new concept in ion sensing was demonstrated using fluoride-effector bifunctional Na+ selective electrodes (Na-ISEs).
• Potentiometric responses of Na-ISEs were found to depend largely on membrane resistances (Rm) varied with concentrations of fluoride ions.
• A potentiometric sensor for sodium ion and an impedimetric sensor for fluoride ion can be fabricated.

A new concept in ion sensing mimicking allosteric membranes in biological systems was proposed and demonstrated using fluoride-effector Na+ selective electrodes (Na-ISEs). DOS-plasticized PVC membranes containing p-tert-butylcalix[4]arene tetraethyl ester (NaX) and 4-[(4-tert-butyl-2-methylphenoxy)methyl]phenyl boronic acid (50 mol% compared to NaX) were found to give the best characteristic of Na-ISEs upon conditioning the membranes in 10−2 M NaF: a response slope of 57.48 mV per decade and detection limit of 7.42 × 10−7 M. The selectivity coefficients (log KNa,jpot) for the optimal electrode were −2.29, −2.84, −3.16, −3.42, and −3.01 for K+, Li+, Cs+, Mg2+ and Ca2+, respectively. The optimal electrodes also gave good reversibility and could be used in a wide range of pH (pH 3.6–9.7). The fabricated Na-ISEs were found to have membrane resistances (Rm) varied with concentrations of fluoride ions used in membrane conditioning steps. Electrochemical impedance spectra showed a linear relationship between Rm and the logarithm of the concentration range of fluoride ions from 1 × 10−1 to 1 × 10−8 M. Conditioning optimal membranes with various concentrations of Bu4NF gave a non-linear relationship. The proposed membrane electrode, thus, showed permselectivity toward Na+ using F− as an effector. Therefore, the proposed new concept led to the fabrication of a bifunctional sensor: a potentiometric sensor for sodium ion and an impedimetric sensor for fluoride ion.

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