Voltammetric Ion Selectivity of Thin Ionophore-Based Polymeric Membranes: Kinetic Effect of Ion Hydrophilicity

Abstract

The high ion selectivity of potentiometric and optical sensors based on ionophore-based polymeric membranes is thermodynamically limited. Here, we report that the voltammetric selectivity of thin ionophore-based polymeric membranes can be kinetically improved by several orders of magnitude in comparison with their thermodynamic selectivity. The kinetic improvement of voltammetric selectivity is evaluated quantitatively by newly introducing a voltammetric selectivity coefficient in addition to a thermodynamic selectivity coefficient. Experimentally, both voltammetric and thermodynamic selectivity coefficients are determined from cyclic voltammograms of excess amounts of analyte and interfering ions with respect to the amount of a Na+ or Li+-selective ionophore in thin polymeric membranes. We reveal the slower ionophore-facilitated transfer of a smaller alkaline earth metal cation with higher hydrophilicity across the membrane/water interface, thereby kinetically improving voltammetric Na+ selectivity against calcium, strontium, and barium ions by 3, 2, and 1 order of magnitude, respectively, in separate solutions. Remarkably, voltammetric Na+ and Li+ selectivity against calcium and magnesium ions in mixed solutions is improved by 4 and >7 orders of magnitude, respectively, owing to both thermodynamic and kinetic effects in comparison with thermodynamic selectivity in separate solutions. Advantageously, the simultaneous detection of sodium and calcium ions are enabled voltammetrically in contrast to the potentiometric and optical counterparts. Mechanistically, we propose a new hypothetical model that the slower transfer of a more hydrophilic ion is controlled by its partial dehydration during the formation of the adduct with a “water finger” prior to complexation with an ionophore at the membrane/water interface.

Voltammetric Thin-Layer Ionophore-Based Films: Part 1. Experimental Evidence and Numerical Simulations

“…At the second interface (II), it is assumed that only Na + or Ag + may be transferred across the phase boundary. This interface was also treated with a Nernst-type equation for silver and sodium (eq 5), because of the fact that ion transfer is generally considered a fast process that quickly reaches equilibrium (exceptions exist 33 ). Δϕ Na + 0,aq→m and Δϕ Ag + 0,aq→m are the respective standard ion transfer potentials for sodium and silver.…”

Potentiometric Sensing

“…Ion transfer at the membrane-solution interface resulting from the oxidation/reduction of a redox probe in the transducing layer as an analytical tool for ion sensing [68][69][70][71][72][73][74][75][76][77]. On the right, two response regimes are identified depending on the analyte concentration range.…”

Stripping Voltammetry at the Interface between two Immiscible Electrolyte Solutions: A Review Paper

“…The dynamics and mechanism of ion transfer at polarizable liquid/liquid interfaces has been successfully clarified using voltammetry [13,126,54]. Potentiometric detection is limited by selectivity and reversibility [127].…”