Unintended Changes of Ion-Selective Membranes Composition—Origin and Effect on Analytical Performance

Abstract

Ion-selective membranes, as used in potentiometric sensors, are mixtures of a few important constituents in a carefully balanced proportion. The changes of composition of the ion-selective membrane, both qualitative and quantitative, affect the analytical performance of sensors. Different constructions and materials applied to improve sensors result in specific conditions of membrane formation, in consequence, potentially can result in uncontrolled modification of the membrane composition. Clearly, these effects need to be considered, especially if preparation of miniaturized, potentially disposable internal-solution free sensors is considered. Furthermore, membrane composition changes can occur during the normal operation of sensors—accumulation of species as well as release need to be taken into account, regardless of the construction of sensors used. Issues related to spontaneous changes of membrane composition that can occur during sensor construction, pre-treatment and their operation, seem to be underestimated in the subject literature. The aim of this work is to summarize available data related to potentiometric sensors and highlight the effects that can potentially be important also for other sensors using ion-selective membranes, e.g., optodes or voltammetric sensors.

Integration of an Aerosol-Assisted Deposition Technique for the Deposition of Functional Biomaterials Applied to the Fabrication of Miniaturised Ion Sensors

“…The presence of such plasticized regions with striations can preserve the sensitivity of the ions sensors since the presence of plasticizers is essential for the ion and ionophore mobility [ 55 ] as well as the diffusion of ions [ 56 ]. This fact becomes crucial in the ion sensors since the diffusion coefficient of ions inside the polymeric films is significantly lower than in water samples [ 57 ]. Thus, only when an optimal concentration of 50 wt.% plasticizer inside the PVC films was employed, a good absorption of water was obtained.…”

Highly Stable Potentiometric (Bio)Sensor for Urea and Urease Activity Determination

“…More recently, Maksymiuk et al pointed out that different materials and constructions of sensors result in specific conditions of membrane formation. Consequently, an uncontrolled modification of the membrane composition can take place, which could have an influence on membrane selectivity [35].…”

Special Issue “Advances in Artificial and Biological Membranes: Mechanisms of Ionic Sensitivity, Ion-Sensor Designs, and Applications for Ion Measurement”

“…The former case does not allow for the water layer formation, and the latter has sufficient water content to stand water transport through the membrane decreased by a small gradient of chemical potential for water between the “well-wetted” solid contact and ISE membrane (the same argument can be applied for ion transmembrane fluxes and mechanical membrane–contact adhesion making a water test redundant). In this situation, as correctly commented by Maksymiuk et al [ 13 ] an optimization of the sensor with solid contact depends on a lot of factors and in the last instance the experimental theatre in which the ion-sensors are used.…”

“…Maksymiuk et al [ 13 ] review the unfavorable effects of dissolution of the solid-contact material in the plastic membrane. In a convincing way, the authors underline a possible mutual influence of CPs on the properties of plastic ISE membrane and vice-versa.…”

“…A strong theoretical argument is supported by so-called water tests which show the sluggish response if the water layer is present at the contact–membrane interface. Maksymiuk et al [ 13 ] correctly argue though that the conventional electrodes with internal solution are by far more stable: “a classic ISM-containing arrangement offers high stability of potential readings in time due to well-defined, reversible ion-electron transfer through all sensor interfaces”. On the other hand, in the paper of Lenar et al [ 14 ], we can read that “the occurrence of structural water in RuO 2 •xH 2 O, which creates a large inner surface available for ion transport and was shown to be a favourable feature in the context of designing potentiometric sensors”.…”