Ionophore-Based Voltammetric Ion Activity Sensing with Thin Layer Membranes

Abstract

As shown in recent work, thin layer ion-selective multi-ionophore membranes can be interrogated by cyclic voltammetry to detect the ion activity of multiple species simultaneously and selectively. Additional fundamental evidence is put forward on ion discrimination with thin multi-ionophore-based membranes with thicknesses of 200 ± 25 nm and backside contacted with poly-3-octylthiophene (POT). An anodic potential scan partially oxidizes the POT film (to POT(+)), thereby initiating the release of hydrophilic cations from the membrane phase to the sample solution at a characteristic potential. Varying concentration of added cation-exchanger demonstrates that it limits the ion transfer charge and not the deposited POT film. Voltammograms with multiple peaks are observed with each associated with the transfer of one type of ion (lithium, potassium, and sodium). Experimental conditions (thickness and composition of the membrane and concentration of the sample) are chosen that allow one to describe the system by a thermodynamic rather than kinetic model. As a consequence, apparent stability constants for sodium, potassium, and lithium (assuming 1:1 stoichiometry) with their respective ionophores are calculated and agree well with the values obtained by the potentiometric sandwich membrane technique. As an analytical application, a membrane containing three ionophores was used to determine lithium, sodium, and potassium in artificial samples at the same location and within a single voltammetric scan. Lithium and potassium were also determined in undiluted human plasma in the therapeutic concentration range.

Electrochemical Ion Transfer with Thin Films of Poly(3-octylthiophene)

“…20,21 By applying an anodic potential, the POT underlayer was partially oxidized to POT + accompanied by p-doping from the lipophilic anion (TFPB − ), which is present in the membrane. Therefore, Na + is exhaustively expelled from the membrane to maintain the electroneutrality of the film (vice versa during the cathodic wave; see Figure S2) resulting in a Gaussian shaped and broad peak (>90 mV).…”

Voltammetric Mechanism of Multiion Detection with Thin Ionophore-Based Polymeric Membrane

“…Apparently super-Nernstian slopes have been reported for zero-current potentiometry and explained on the basis of thermodynamic equilibria, 21-23 but not reported for voltammetry. 8-11 …”

“…Recently, Bakker and co-workers doped a thin polymeric membrane with multiple ionophores 10 to volt-ammetrically detect multiple analyte ions (e.g., Li + and K + ) at millimolar concentrations in undiluted human plasma. 11 With their approach, ionophores were initially present as their analyte complexes in the membrane to obtain separate voltammetric peaks by successively stripping up to three ions. 11 This approach, however, required the addition of a carefully controlled amount of an ionic site to a membrane to obtain voltammetric responses to multiions.…”

“…11 With their approach, ionophores were initially present as their analyte complexes in the membrane to obtain separate voltammetric peaks by successively stripping up to three ions. 11 This approach, however, required the addition of a carefully controlled amount of an ionic site to a membrane to obtain voltammetric responses to multiions.…”

“…Interestingly, this voltammetric IE reaction gives a current response when the net charge transferred by each ionophore molecule is not zero, i.e., $$\mid z_{\mathrm{J}}\mid ∕n_{\mathrm{normalJ}}-\mid z_{\mathrm{I}}\mid ∕n_{\mathrm{normalI}}>0$$ Alternatively, the IT mechanism utilizes the second ionophore, L′, to independently transfer the secondary ion without ion exchange $$n_{\mathrm{normalJ}}{\mathrm{normalL}}^{\prime }\left(\mathrm{normalm}\right)+{\mathrm{normalJ}}^{z_{\mathrm{normalJ}}}\left(\mathrm{normalw}\right)\rightleftharpoons {\mathrm{normalL}}_{n_{\mathrm{normalJ}}}^{\prime }{\mathrm{normalJ}}^{z_{\mathrm{normalJ}}}\left(\mathrm{normalm}\right)$$ thereby eliminating the requirement of eq 3. Advantageously, our approach does not require the addition of an ionic site to a membrane in contrast to the Bakker's approach, 10,11 where an analyte ion must be initially present in the membrane phase.…”