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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