Ion-ionophore interactions in polymeric membranes studied by thin layer voltammetry

In ion sensing applications, selective ion-receptor complexation is the molecular basis for endowing the sensing material with selectivity. In this work, thin polymeric membrane-based ion transfer voltammetry is used to investigate ion-receptor complexation, using a range of electrically neutral ionophores and surfactants as examples. Previous studies lacked a convincing approach to eliminate the influence from transducing layer, resulting in deviations of the observed binding constants compared to potentiometric methods. A recently developed method allows for subtracting the potential changes of the transducing layer, thereby overcoming this challenge. Using this approach, a range of ionophores are assessed. Valinomycin for the detection of potassium gave a logarithmic complex formation constant in the membrane of 9.69 +/- 0.25 with a 1:1 stoichiometry. Lithium ionophore VI for lithium gave a logarithmic stability constant of 5.97 +/- 0.06 with 1:2 complexes; while sodium ionophore X for sodium (7.57 +/- 0.03, 1:1) and calcium ionophore IV for calcium (21.57 +/- 0.25, 1:3) were also characterized, in addition to their complexes with potential interfering ions. The complex formation of three surfactants with potassium are also explored in membranes containing valinomycin, with Brij-35 (4.88 +/- 0.08, 1:1), Triton X-100 (5.63 +/- 0.10, 1:1), F-127 (4.63 +/- 0.49, 1:1). Limitations of the approach are discussed, which includes the need for electrochemical reversibility and a sufficiently high lipophilicity to adequately retain the components in the membrane