Direct potentiometric bicarbonate/carbon dioxide sensing based on polymeric membranes doped with selective meso-bisubstituted calix[4]pyrrole ionophores

Abstract

We evaluated in this work the properties of a promising class of HCO₃⁻ ionophores, which have not been recognized previously. Three types of neutral or charged calix[4]pyrroles with meso-bisubstituted groups were evaluated as ionophores for polymeric membrane HCO₃⁻ selective electrodes. Optimizing membrane components, such as ionophores, lipophilic additives and plasticizers, yielded ISEs exhibiting Nernstian response to HCO₃⁻ with improved linear range and detection limits, while the selectivity sequence differs significantly from the Hofmeister series. Interference from important biological and environmental species was reduced significantly, especially that from SCN⁻, NO₃⁻, Br⁻ and Cl⁻, which are always at high concentrations in related samples. In order to provide more insights into the properties of the ionophores and performance of the proposed ISEs, the stability constants of anion–ionophore complexes in the membrane phase were determined. Studies on the influence of the sample solution pH demonstrated that the proposed ISEs can be employed in a wide pH range of 3.0–9.0 with fast response (<30 s), good reversibility and long shelf life. Moreover, the proposed ISEs were used to quantify the concentration of HCO₃⁻ and dissolved CO₂ in mineral and beverage samples with good recoveries.