Potassium Ion-selective Electrodes Research Articles

Comparative Study of Potassium Ion-Selective Electrodes with Solid Contact: Impact of Intermediate Layer Material on Temperature Resistance

This paper presents a comparative study on the temperature resistance of solid-contact ion-selective electrodes, depending on the type of solid-contact material. Five types of potassium electrodes, with a valinomycin-based model membrane, were developed using different types of mediation layers, namely a conductive polymer (poly(3-octylthiophene-2,5-diyl) and a perinone polymer), multi-walled carbon nanotubes, copper(II) oxide nanoparticles, and a nanocomposite consisting of multi-walled carbon nanotubes and copper(II) oxide. We examined how the measurement temperature (10 °C, 23 °C, and 36 °C) affects the sensitivity, measurement range, detection limit, selectivity, as well as the stability and reversibility of the electrode potential. Electrodes modified with a nanocomposite (GCE/NC/ISM) and a perinone polymer (GCE/PPer/ISM) showed the best resistance to temperature changes. An almost Nernst response and a stable measurement range and the lowest detection limit values for each temperature were obtained for them. The introduction of mediation layers significantly improved the stability and potential reversibility of all the modified electrodes relative to the unmodified electrode (GCE/ISM). Still, it was the GCE/PPer/ISM and GCE/NC/ISM that stood out from the others, with stability of 0.11 and 0.12 µV/s for 10 °C, 0.05 and 0.08 µV/s for 23 °C, and 0.06 and 0.09 µV/s for 36 °C, respectively.

Development of a hydroponic device using potassium Ion-Selective electrode and neural network technology

The rapid development of modern agricultural biotechnology and information technology has become a key driver of modern agricultural growth. The demand for ion detection is prevalent in all aspects of agriculture. This study details the research on an ion sensor agricultural system using potassium (K) ion-selective electrodes (ISEs). The main emphasis is on the application of artificial neural networks (ANNs) for driving ISE development of accuracy. ISEs based on solid contact with metal oxides, specifically manganese dioxide, are studied. Moreover, the design details of an ion signal acquisition device using the NI-9205 acquisition card and LabVIEW are presented. Finally, based on experimental results of nutrient solution samples, ANNs show better performance than the multiple linear regression method. The mean relative error of K-ISE detection was maintained within 4%. This study thoroughly discusses ISE technology to promote its adoption in agriculture.

Novel usage of perinone polymer as solid contact in ion-selective electrodes

The paper presents the use of a new perinone polymer (PPer) as an intermediate layer and the development of a new type of potassium ion-selective electrodes with solid contact. The PPer layer was applied to the surface of a glassy carbon electrode using potentiodynamic polymerization from an electrolyte solution containing perinone precursors (Per). Subsequently, the surface of the electrode after application of the polymer was examined using a microscope and an optical profilometer. To check the quality of the prepared ion-selective electrodes, a series of tests were carried out and the analytical and electrical parameters of the electrodes were determined. Electrode modifications using PPer improved the electrode parameters, especially the stability and reversibility of the potential, compared to the control electrode covered only with an ion-sensitive membrane. The best electrode was the electrode modified with PPer applied in 10 cycles, which, as a result of the modification, allowed better parameters to be obtained – slope 58.86 mV/decade, LOD 1.2 × 10−6 M, linearity range 5 × 10−6 – 1 × 10−1 M. A very low short-term potential drift value of 2.7 × 10−4 mV/s was obtained as well as an extremely satisfactory SD value of 1.1 mV from E0 measurements. The modification also improved electrical properties and gave a negative result of the water layer test.

Potassium Ion-Selective Electrodes with BME-44 Ionophores Covalently Attached to Condensation-Cured Silicone Membranes.

For ion-selective electrodes (ISEs) to be employed in wearable and implantable applications, the ion-selective membrane components should be biocompatible, and leaching of components, such as plasticizer or ionophore, out of the sensing membrane should be inhibited. To achieve this, we employed a plasticizer-free silicone as the membrane matrix and synthesized as the ionophore a derivative of the bis-crown ether based potassium ionophore BME-44, incorporating a triethoxysilyl functional group that covalently attaches to condensation-cured silicones during the curing process. Soxhlet extraction of these membranes with dichloromethane shows that up to 96% of the ionophore is attached to the silicone membrane during curing. We found that the covalently attachable BME-44 derivative can inadvertently adsorb onto high surface area carbon solid contacts before attaching to the silicone matrix if the curing of the silicone is performed in the presence of the high surface area carbon, resulting in depletion of ionophore from the membrane and yielding solid-contact ISEs with poor selectivity. In contrast, we observed Nernstian responses to K+ in plasticizer-free silicone-based K+ ISMs with either mobile BME-44 or the covalently attachable BME-44 derivative when the membranes were prepared on octane-thiol coated gold electrodes, where ionophore adsorption does not occur to a noticeable extent. As compared with ISMs doped with the mobile BME-44, ISMs prepared with the covalently attachable BME-44 derivative have better selectivity for K+ vs Na+ ( values of -3.54 and <- 4.05 for mobile and covalently attachable BME-44, respectively) and lower resistance. This can be explained by a more homogeneous incorporation of the covalently attachable BME-44 derivative into the silicone matrix than is the case for the mobile BME-44.

Improved performance of Prussian blue solid contact allowing flow injection amperometric detection of potassium ions in the excess of sodium

Catalytically synthesized Prussian blue nanoparticles are a versatile solid contact for potassium and sodium ion-selective screen-printed electrodes. The cathodic peak current proportional to cation concentration was registered in flow injection amperometry regime. The ability of K+ to penetrate Prussian blue lattice resulted in 2–4-fold higher response to potassium ions relatively to sodium ions. Such intrinsic selectivity of Prussian blue to potassium provided an order of magnitude higher sensitivity of K+-selective electrodes in comparison to Na+-sensors, based on nanoparticles covered with the corresponding ion-selective membranes. In contrast to other known solid contacts, Prussian blue provides the highest sensitivity of potassium-selective electrodes in an amperometric regime (20–50 mA·cm−2·M−1). Moreover, 1 × 10-6 M is the lowest limit of detection ever achieved for Prussian blue-based ion-selective electrodes. The improved sensitivity and selectivity allowed flow injection amperometric detection of potassium ions in the presence of interfering ions. Despite K+ concentration is approximately 40-fold lower than Na+, sensors are applicable for detection of pathological and normal potassium and sodium ions in blood serum. The sensors designed in this work have opened a new opportunity for analyzing biological fluids with ion-selective electrodes in an amperometric regime.

Planar, low-cost, flexible, and fully laminated graphene paper pseudo-reference and potassium-selective electrodes

We report for the first time a one-step fabrication by lamination of inexpensive graphene paper (GP) based Ag/AgCl pseudo-reference (RE) and K+-selective coated-film electrodes forming the novelty of this work. Contrary to our previous work about laminated GP solid-contact electrodes, the electrode fabrication in this work does not require manual drop casting steps after the lamination, simplifying the fabrication and making it suitable for mass production. We also eliminated the need of the solid contact and simplified the electrode construction by placing the plasticized potassium-selective PVC membrane (ISM) directly on the GP functioning as combined electrode substrate and ion-to-electron transducer. Both Ag/AgCl and the ISMs were pre-deposited on rectangular GP sheets (10 ×100 mm2) before placing them inside laminating pouches and feeding them through an office laminator after which the electrodes were ready for use. Five batches of 32 pseudo-REs had a very good E0 reproducibility of ± 2.8 mV. The K+-electrodes had a slope and detection limit of 57.4 ± 0.3 mV/pK (n = 4) and 6 × 10−7 M, and the E0 reproducibility was in the best case only ± 1.2 mV matching the performance of non-laminated K+-electrodes. This work demonstrates that the ISMs are compatible with the lamination where the temperature can momentarily rise to ca. 130 °C.

Fast and sensitive coulometric signal transduction for ion-selective electrodes by utilizing a two-compartment cell

Here, we propose a fast and sensitive coulometric signal transduction method for ion-selective electrodes (ISEs) by utilizing a two-compartment cell. A potassium ion-selective electrode (K+-ISE) was connected as reference electrode (RE) and placed in the sample compartment. A glassy carbon (GC) electrode coated with poly(3,4-ethylenedioxythiophene) (GC/PEDOT), or reduced graphene oxide (GC/RGO), was connected as working electrode (WE) and placed in the detection compartment together with a counter electrode (CE). The two compartments were connected with an Ag/AgCl wire. The measured cumulated charge was amplified by increasing the capacitance of the WE. The observed slope of the cumulated charge with respect to the change of the logarithm of the K+ ion activity was linearly proportional to the capacitance of the GC/PEDOT and GC/RGO, estimated from impedance spectra. Furthermore, the sensitivity of the coulometric signal transduction using a commercial K+-ISE with internal filling solution as RE and GC/RGO as WE allowed to decrease the response time while still being able to detect a 0.2% change in K+ concentration. The coulometric method utilizing a two-compartment cell was found to be feasible for the determination of K+ concentrations in serum. The advantage of this two-compartment approach, compared to the coulometric transduction described earlier, was that no current passed through the K+-ISE that was connected as RE. Therefore, current-induced polarization of the K+-ISE was avoided. Furthermore, since the GCE/PEDOT and GCE/RGO (used as WE) had a low impedance, the response time of the coulometric response decreased from minutes to seconds.

Conductive Polymer Nanoparticles as Solid Contact in Ion-Selective Electrodes Sensitive to Potassium Ions.

A preparation method of nanocomposites based on poly (3-octylthiophene-2,5-diyl) (POT) and carbon black (CB) as the transducer of an all-solid potassium ion selective electrode is proposed. POT is used as the dispersant of CB, and the obtained nanocomposites have unique characteristics, including high conductivity, high capacitance and high stability. The potassium ion selective electrode based on POT and CB was characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronopotentiometry. The results showed that the detection limit of potassium ions was 10-6.2 M, and the slope was 57.6 ± 0.8 mV/façade. The water layer test and anti-interference test show that the electrode has high hydrophobicity, the static contact angle reaches 139.7° and is not easily affected by light, O2 and CO2.

Application of Potassium Ion Conducting KTiOPO4 as Effective Inner Solid-Contact Layer in All-Solid-State Potassium Ion-Selective Electrode

Solid electrolytes used in all-solid-state batteries electronically separate the positive and negative electrodes in the battery and only allow the carrier ions to pass through. KTiOPO4 (KTP), a potassium ion-conducting solid electrolyte, was first applied as the inner solid contact layer of an all-solid-state potassium ion-selective electrode (ISE) to stabilize the membrane potential. Application of the KTP layer improved the long-term potential stability of the ISE by stabilizing the membrane potential. This can be further improved by adding acetylene black (AB) to the KTP layer which reduced the electrode resistance owing to its high double-layer capacitance.

Novel Experimental Setup for Coulometric Signal Transduction of Ion-Selective Electrodes.

In this work, a novel and versatile experimental setup for coulometric signal transduction of ion-selective electrodes (ISEs) is introduced and studied. It is based on a constant potential coulometric measurement carried out using a one-compartment three-electrode electrochemical cell. In the setup, a potassium ion-selective electrode (K+- ISE) is connected as the reference electrode (RE). A poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS)-based electrode with a dummy membrane (DM) and a glassy carbon (GC) rod are connected as the working electrode (WE) and counter electrode (CE), respectively. Adding a non-selective dummy membrane to the structure of the WE facilitates the regulation of the measured signal and response time. The results from electrochemical impedance spectroscopy measurements carried out on the WE showed that the time constant is profoundly influenced by the dummy membrane thickness. In addition, the redox capacitance of the PEDOT:PSS film shows a better correlation with the electrode area than the film thickness. Sequential addition/dilution experiments showed the improvement of current and cumulated charge signals in the new setup studied in this work compared to the setup used in the original coulometric signal transduction method. Both conventional ISEs and solid-contact ISEs (SCISEs) were used in this work. The results showed that the coulometric response was independent of the type of ISE used as RE, confirming the versatility of the novel set-up.

Carbon Nanomaterials - Poly(3-octylthiophene-2,5-diyl) – Hydrous Iridium Dioxide Triple Composite Materials as Superhydrophobic Layers for Ion-Selective Electrodes

In this paper, we introduce the new triple composite material consisting of three significantly different materials: carbon nanomaterial (carbon nanotubes and carbon black), conducting polymer (poly(3-octylthiophene-2, 5-diyl) and metal oxide (hydrous iridium dioxide). Combining those three components enabled to obtain superhydrophobic materials (of contact angle value up to 180°). Both designed materials were characterized with high values of electrical capacitance parameters (1.5 and 0.9 mF) and low values of resistance (72.9 ± 0.3 kΩ and 23.5 ± 0.2 kΩ for NT-based and CB-based electrode, respectively). The new functional material was implemented into potassium-selective electrodes in order to improve their electrical and analytical parameters. This is the first presented so far potentiometric sensor with solid-contact layer composed of three significantly different materials. The superhydrophobic layer of triple composite material improved (in contrast to previous solutions) long-term stability (characterized by potential drift of 43 μV h−1 and 79 μV h−1 for the NT-based and CB-based electrode, respectively) and repeatability enabled limiting the number of necessary calibrations. Potentiometric sensors presented in the scope of this work enable potassium determination in the wide range of potassium ions (from 10−6 to 10−1 M of K+ ions). The possibility of practical application was successfully confirmed by the analysis of potassium in vegetable juices.

Effect of non-stoichiometry of KxFe[Fe(CN)6] as inner solid-contact layer on the potential response of all-solid-state potassium ion-selective electrodes

In the current study, we synthesized Prussian blue analogues, KxFe[Fe(CN)6] (KFeHCF) having different [Fe(CN)6] vacancies, potassium contents (x), and crystal system, and investigated the effect of the difference on ion-sensor performance by using the KFeHCF as the solid-contact layer of all-solid-state ion-selective electrodes (ASS-ISEs). Cubic-KFeHCF shows slopy potential curve during galvanostatic K+ extraction/insertion because the reaction proceeds in single-phase regime, which deteriorates the long-term potential stability as internal reference electrode of ASS-ISEs. On the other hand, monoclinic-KFeHCF exhibits flat potential plateau during galvanostatic K+ extraction/insertion through two-phase reaction, providing superior potential stability. While the potential of an ISE is primarily determined by the membrane potential of the ion-sensitive membrane, it is discovered that the different crystal structure of the KFeHCF introduced into the solid-contact layer affects the long-term stability of ISE.

Serum/plasma potassium monitoring using potentiometric point-of-care microanalyzers with improved ion selective electrodes

Different causes can trigger imbalances on homeostatic mechanisms between intracellular and extracellular compartments resulting in abnormal blood potassium concentrations (hypo or hyperkalemia). This can lead to serious consequences, even a life-threatening situation. Early diagnosis, treatment and follow-up are essential to minimize critical impacts in patients. Bedside determination of blood potassium is not accessible in all health care centers or in all emergency departments and far less common in this kind of centers in emerging countries. We have therefore proposed a portable, economic and long-lifetime potentiometric point-of-care (POC) analytical microsystem to deal with this question. It is a continuous flow microfluidic platform, made of cyclic olefin copolymer (COC), which combines microfluidics and a detection system based on the potentiometric technique containing a potassium selective electrode with a novel composition of polymeric membrane, which improves lifetime. Its size is smaller than a credit card and shows a linear range of Nernst calibration equation from 1 to 26 mM K+, a detection limit of 0.16 mM K+, a satisfactory repeatability and reproducibility, and an analysis frequency of 20 samples h−1, requiring only 25 μL as sample volume. Moreover, lifetime is as long as 9 months by intensive use. All these features comply with medical requirements. Human serum samples were analyzed with the developed device and the obtained results were compared with those provided by two methods: ICP-OES and another using ion selective electrodes. No significant differences were observed, demonstrating the suitability of the developed POC microanalyzer for bedside health applications.

Development of Dual Ion-selective Electrodes in Double-Barrel Glass Pipette at One Micrometer for Simultaneous Measurement of Sodium and Potassium Ions

We have developed the method of preparing sodium and potassium ion-selective electrodes in double-barrel glass pipettes at the size of 1 μm. Stable and precise fabrication parameters were found for producing a double-barrel 1 μm pipette, and the produced double-barrel micropipettes were used in this study. The ionophores comprising of crown ethers of 12-crown-4 for sodium ions and 15-crown-5 for potassium ions were respectively doped in poly(vinyl chloride) films in the divided double-barrel glass pipette. The obtained selectivity coefficients and selectivity ratios are compared with the previous studies. The developed ion-selective dual-micropipettes can be applied for local ion-selective measurements such as living cells.

Mean activity coefficients of KCl in the KCl–K2B4O7–H2O solutions at 288.15 K determined by cell potential method and their application to the prediction of solid–liquid equilibria of the KCl–K2B4O7–H2O system

• The unreported mixed ion interaction of Pitzer parameters in KCl–K 2 B 4 O 7 –H 2 O solutions at 288.15 K were obtained in this work. • The experimental and predicted solubilities were compared to verify the rationality of fitting parameters. After measuring the cell potential of the cell without liquid junction K-ISE | KCl ( m 1 ), K 2 B 4 O 7 ( m 2 ) | Cl-ISE with the calibrated potassium ion and chloride ion selective electrode, the mean activity coefficients of KCl in KCl–K 2 B 4 O 7 –H 2 O solutions are calculated by the Nernst equation. The single parameters β (0) , β (1) , C Φ of K 2 B 4 O 7 , and the mixed ion interaction parameters θ Cl, B 4 O 5 (OH) 4 and ψ K,Cl, B 4 O 5 (OH) 4 of the Pitzer model were determined by programming solver and mean activity coefficients in this work. Furthermore, Pitzer equation is also used to evaluate the mean activity coefficient of potassium tetraborate in the KCl–K 2 B 4 O 7 –H 2 O solutions, as well as the osmotic coefficients, water activity and excess Gibbs free energy of the mixed solutions. Additionally, the experimental measurements of phase equilibria in the ternary system KCl–K 2 B 4 O 7 –H 2 O at 288.15 K are investigated in this work. The solubilities of the salts in the ternary system at 288.15 K are predicted by using Pitzer equation and the obtained ion parameters. The comparision of experimental and calculated phase diagram is presented in detail. The prediction results show that the predicted data are in accordance with the experimental data.

Measurements of mean activity coefficients and solubility prediction in ternary system KBr-K2SO4-H2O at 308.15 K and 288.15 K

In this work, the mean activity coefficients of KBr in KBr-H2O binary system and KBr-K2SO4-H2O ternary system were determined at 308.15 K and 288.15 K by the cell potential method. The potassium ion selective electrode and bromide ion selective electrode are used to measure the cell potential value of single salt electrolyte solution,and the standard potential value E0 and electrode response slope k of the binary system are obtained. The experimental results showed that ion selective electrodes of used in this work had a good Nernst response. The mean activity coefficients KBr in KBr-K2SO4-H2O system are also determined, the total ionic strength in the mixed solution ranging from (0.0100 to 1.0000) mol·kg−1, and ionic strength fractions yb of K2SO4 with yb=(0.8, 0.6, 0.4, 0.2 and 0.0). Using mean activity coefficients obtained in this work and Pitzer model, the Pitzer mixed ion interaction parameters θBr,SO4 and ψK,Br,SO4are fitted respectively. Furthermore, those osmotic coefficient Φ, water activity aw, excess Gibbs free energy GE were also calculated by Pitzer’s equations. The solubilities of salts in the KBr-K2SO4-H2O system at 308.15 K and 288.15 K are predicted by using Pitzer equations and ion interaction parameters fitted in this work, and phase diagrams of the ternary system are also plotted according to the calculated solubilities. The results show that the ion interaction parameters of Pitzer equations by fitting the activity coefficients can be used for calculation thermodynamic parameters and prediction of phase equilibria.

TEMPO-Functionalized Carbon Nanotubes for Solid-Contact Ion-Selective Electrodes with Largely Improved Potential Reproducibility and Stability.

Solid-contact ion-selective electrodes (SCISEs) can overcome essential limitations of their counterparts based on liquid contacts. However, attaining a highly reproducible and predictable E0, especially between different fabrication batches, turned out to be difficult even with the most established solid-contact materials, i.e., conducting polymers and large-surface-area conducting materials (e.g., carbon nanotubes), that otherwise possess excellent potential stability. An appropriate batch-to-batch E0 reproducibility of SCISEs besides aiding the rapid quality control of the electrode manufacturing process is at the core of their “calibration-free” application, which is perhaps the last major challenge for their routine use as single-use “disposable” or wearable potentiometric sensors. Therefore, here, we propose a new class of solid-contact material based on the covalent functionalization of multiwalled carbon nanotubes (MWCNTs) with a chemically stable redox molecule, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). This material combines the advantages of (i) the large double-layer capacitance of MWCNT layers, (ii) the adjustable redox couple ratio provided by the TEMPO moiety, (iii) the covalent confinement of the redox couple, and (iv) the hydrophobicity of the components to achieve the potential reproducibility and stability for demanding applications. The TEMPO-MWCNT-based SC potassium ion-selective electrodes (K+-SCISEs) showed excellent analytical performance and potential stability with no sign of an aqueous layer formation beneath the ion-selective membrane nor sensitivity toward O2, CO2, and light. A major convenience of the fabrication procedure is the E0 adjustment of the K+-SCISEs by the polarization of the TEMPO-MWCNT suspension prior to its use as solid contact. While most E0 reproducibility studies are limited to a single fabrication batch of SCISEs, the use of prepolarized TEMPO-MWCNT resulted also in an outstanding batch-to-batch potential reproducibility. We were also able to overcome the hydration-related potential drifts for the use of SCISEs without prior conditioning and to feature application for accurate K+ measurements in undiluted blood serum.

Rapid prototyping of ion-selective electrodes using a low-cost 3D printed internet-of-things (IoT) controlled robot

We report automated fabrication of solid-contact sodium-selective (Na+-ISEs) and potassium-selective electrodes (K+-ISEs) using a 3D printed liquid handling robot controlled with Internet of Things (IoT) technology. The printing system is affordable and can be customized for the use with micropipettes for applications such as drop-casting, biological assays, sample preparation, rinsing, cell culture, and online analyte monitoring using multi-well plates. The robot is more compact (25 × 30 × 35 cm) and user-friendly than commercially available systems and does not require mechatronic experience. For fabrication of ion-selective electrodes, a carbon black intermediate layer and ion-selective membrane were successively drop-cast on the surface of stencil-printed carbon electrode using the dispensing robot. The 3D-printed robot increased ISE robustness while decreasing the modification time by eliminating manual steps. The Na+-ISEs and K+-ISEs were characterized for their potentiometric responses using a custom-made, low-cost (<$25) multi-channel smartphone-based potentiometer capable of signal processing and wireless data transmission. The electrodes showed Nernstian responses of 58.2 ± 2.6 mV decade−1 and 56.1 ± 0.7 mV decade−1 for Na+ and K+, respectively with an LOD of 1.0 × 10−5 M. We successfully applied the ISEs for multiplexed detection of Na+ and K+ in urine and artificial sweat samples at clinically relevant concentration ranges. The 3D-printed pipetting robot cost $100 and will pave the way for more accessible mass production of ISEs for those who cannot afford the expensive commercial robots.

Hydrous Cerium Dioxide-Based Materials as Solid-Contact Layers in Potassium-Selective Electrodes.

This paper introduces hydrous cerium dioxide applied for the first time as a solid-contact layer in ion-selective electrodes. Cerium dioxide belongs to the group of metal oxides that exhibit both redox activity and a large surface area and therefore was considered to be an appropriate material for the solid-contact layer in potentiometric sensors. The material was examined both standalone and as a component of composite materials (with the addition of carbon nanomaterial or conducting polymer). Three cerium dioxide-based materials were tested as solid-contact layers in potentiometric sensors in the context of their microstructure, wettability, and electrical properties. The addition of hydrous cerium dioxide was shown to enhance the properties of carbon nanotubes and poly(3-octylthiophene-2,5-diyl) by increasing the value of electrical capacitance (798 μF and 112 μF for hCeO2-NTs and hCeO2-POT material, respectively) and the value of contact angle (100° and 120° for hCeO2-NTs and hCeO2-POT material, respectively). The proposed sensor preparation method is easy, without the need to use an advanced apparatus or specific conditions, and fast; sensors can be prepared within an hour. Designed hCeO2-based electrodes exhibit competitive linear range and potential stability within the wide range of pH values (2.0–11.5). Designed electrodes are dedicated to potassium determination in environmental and clinical samples.

Anomalous potentiometric response of solid-contact ion-selective electrodes with thin-layer membranes

In this work, the potentiometric response of solid-contact ion-selective electrodes (SCISEs) with thin-layer ion-selective membranes (ISMs) was characterized. Thin-layer potassium ISMs were deposited on the solid contact by spin-coating. When calibrating the solid-contact potassium ion-selective electrodes (K-SCISEs) at a constant background concentration of 0.1 M NaCl, an anomalous increase in the potential was observed at low concentrations of the primary potassium ion. The origin of the unusual response was elucidated in this work by varying the solid-contact material based on conducting polymers with different lipophilicity and by varying the composition and thickness of the ISM as well as the timescale of the potentiometric measurements. The composition of the ISM was found to have only a minor effect, while the thickness of the ISM, the lipophilicity of the solid-contact material and the timescale of the potentiometric measurement were found to strongly influence the potentiometric behavior. Furthermore, the potentiometric response of classical K-ISEs with inner filling solution did not show any unexpected behavior. It was concluded that the anomalous potentiometric response is a characteristic feature of SCISEs with thin-layer ISMs and originates from a relatively rapid change of the composition of the water layer between the ISM and the solid contact. Thus, the reported anomalous behavior of SCISEs with thin-layer ISMs, together with the established water layer test, can be used as a diagnostic tool indicating the presence of water between the ISM and the solid contact.