Selectivity Behavior and Multianalyte Detection Capability of Voltammetric Ionophore-Based Plasticized Polymeric Membrane Sensors

Abstract

The current response features ofvoltammetric ion-selective polymeric membranes doped with neutral ionophores in view of practical sensor development are elucidated. The membranes are designed to extract ions only under applied external potentials and interrogated by normal-pulse voltammetry and pulsed amperometry. They contain two polarizable interfaces to avoid loss of lipophilic ions at the sample side and to maximize the available potential window. A simple theoretical model is developed that describes the observed current at the end of an uptake pulse to the applied membrane potential, which is the sum of both boundary potentials (at the sample and inner electrolyte side) and the membrane internal iR drop. The results describe how the selectivity of the resulting sensor must be dependent on the applied potential. Evidently, the role of the applied potential is akin to incorporating lipophilic cationic and anionic sites with potentiometric ionophore-based membranes, which are well known to considerably affect membrane selectivity and to define the charge type of the assessed ions. This has important implications for sensor design, as the applied cell potential can be used to tune sensor selectivity. Theory also explains the role of the inner electrolyte on sensor behavior. A maximum measuring range is expected with ions in the inner electrolyte that are difficult to extract into the membrane. This corresponds to Kihara's experimental results and contrasts to common ion-selective electrode practice, where a salt of the analyte ion is normally present in the inner electrolyte. Separate and mixed solution experiments with membranes containing the sodium-selective ionophore tert-butyl calix[4]arene tetramethyl ester and the lithium ionophore ETH 1810 agree very well with theoretical expectations. Multianalyte detection capability with a single sensing membrane is demonstrated in a selectivity-modifying pulsed amperometric detection mode, where each applied voltage yields a different practical selectivity of the sensor. The sensor is altered from being sodium to potassium selective as the magnitude of the applied potential is repetitively varied within the pulse sequence. The sensors show high long-term stability under continuous measuring conditions over 15 h.

Current response of ion-selective solvent polymeric membranes at controlled potential

“…This effect is in close analogy to the behavior reported for conventional membrane ISEs where the zero-current counter transport of two ionic species can be suppressed to achieve lower detection limits down to 10 À12 M in buffer-free samples [14,15]. Several investigations on ISE assemblies are based on pulsed or cyclic electrochemical measuring techniques and were meant to lead to new types of ion sensors [16][17][18][19][20][21][22][23][24]. A different group of fundamental studies focused on the electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) [25][26][27][28] in place of the complete three-phase membrane arrangement [29] used in ISE cells.…”

“…From pulsed or cyclic measurements on ion-selective PVC membranes, the current signals were claimed to be proportional to the logarithm of ion activity or concentration [8,9,[20][21][22][23][24]. In contrast, from studies on ITIES using organic phases containing ionophores, the voltammetric or amperometric responses were apparently found to depend linearly on the ion concentration [26][27][28]30].…”

All solid state chronopotentiometric ion-selective electrodes based on ferrocene functionalized PVC

“…Most experiments at the ITIES used non-permselective membranes (so-called ideally polarizable interfaces). Here, the interfacial iontransfer at the sample side must be coupled to the charge transfer at the backside of the solution in order to fulfill the electroneutrality condition, either by the use of a common ion in the membrane and inner solution, or the extraction of a counterion [25][26][27].…”

Assessing ion-exchange properties and purity of lipophilic electrolytes by potentiometry and spectrophotometry

“…For example, inert lipophilic salts of high purity are important in electrochemical sensors based on electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) [7], polymeric membrane-based potentiometric [8] and voltammetric sensors [9], where the resulting permselectivity must be carefully controlled.…”