Detection Limits of Thin Layer Coulometry with Ionophore Based Ion-Selective Membranes

Abstract

We report here on a significant improvement in lowering the low detection limit of thin layer coulometric sensors based on liquid ion-selective membranes, using a potassium-selective system as a model example. Various possible processes that may result in an elevated residual current reading after electrolysis were eliminated. Self-dissolution of AgCl on the Ag/AgCl inner element may result in a residual ion flux that could adversely affect the lower detection limit. It was here replaced with an Ag/AgI inner pseudoreference electrode where the self-dissolution equilibrium is largely suppressed. Possible residual currents originating from a direct contact between inner element and ion-selective membranes were eliminated by introducing an inert PVDF separator of 50 μm diameter that was coiled around the inner element by a custom-made instrument. Finally, the influence of electrolyte fluxes from the outer solution across the membrane into the sample was evaluated by altering its lipophilic nature and reducing its concentration. It was found that this last effect is most likely responsible for the observed residual current for the potassium-selective membranes studied here. For the optimized conditions, the calibration curves demonstrated a near zero intercept, thereby paving the way to the coulometric calibration-free sensing of ionic species. A linear calibration curve for the coulometric cell with valinomycin potassium-selective membrane was obtained in the range of 100 nM to 10 μM potassium in the presence of a 10 μM sodium background. In the presence of a higher (100 μM) concentration of sodium, a reliable detection of 1-100 μM of potassium was achieved.

Thin Layer Coulometry of Nitrite with Ion‐Selective Membranes

“…The outer compartment of the electrochemical cell was filled with an optimum solution containing 10 mM NaCl+10 µM NaNO 2 . Increasing the concentration of NaNO 2 in the outer solution results in a deterioration of the lower limit of detection by back diffusion of nitrite into the thin layer 29. In contrast, the complete absence of nitrite in the outer solution does not permit one to observe a stable membrane potential (OCP) used as an important reference point for the subsequent applied potential increments.…”

“…Working and counter electrodes were made of 640 µm silver wire reduced to 400 µm using a commercially available jewelry drawplate and subsequently electrochemically oxidized in a solution of 10 mM HCl for 3 h. The working electrode was coiled with PVDF fish line (Ø 50 µm, pitch distance 225 µm) to avoid direct contact between the electrode and the PP hollow fiber with 600 µm inner diameter 29. An 11 cm long PP piece of porous fiber was glued with epoxy to 1/8 in.…”

Ion selective electrodes in environmental analysis

“…Another very interesting concept for achieving chemical amplification is named swtichtrodes. [110][111][112] Since ISEs are in the super-Nernstian response mode, a switchtrode expresses chemical amplification.…”

Paper-supported thin-layer ion transfer voltammetry for ion detection

“…The electrochemical response behavior of the thin-layer coulometric sensors under a controlled potential is elucidated and the membrane response model was developed by using a numerical simulation [21]. Besides the incorporation of an T inert physical separator for cell response improvement [22], a double pulse technique was utilized for background correction [23]. By using the proposed controlled-potential thin-layer coulometry, a number of different ions including potassium, calcium, protamine, as well as nitrate, and nitrite have been measured [24].…”