ac301940n_si_001.pdf (266.36 kB)
Detection Limits of Thin Layer Coulometry with Ionophore Based Ion-Selective Membranes
journal contribution
posted on 2012-09-18, 00:00 authored by Alexey Shvarev, Bastien Neel, Eric BakkerWe 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.