Conventional Ion-selective Electrodes Research Articles

Perspective on the coulometric transduction principle for ion-selective electrodes

A new type of coulometric transduction method for solid-contact ion-selective electrodes was introduced in 2015. The coulometric method utilized the capacitance of the solid contact to convert a potential change into a current transient that was integrated to obtain the corresponding charge. For a given potential change, the charge was increased by increasing the capacitance of the solid contact, which improved the sensitivity of solid-contact ion-selective electrodes. By introducing an electronic capacitor in series with the ion-selective electrode (ISE), the coulometric method became faster and feasible also for conventional ISEs with an internal filling solution. Alternative electrode configurations, where the ISE was connected as the reference electrode in the electrochemical cell, was used to avoid polarization of the ISE during the coulometric readout. Results published so far indicate that the coulometric readout method can significantly improve the analytical performance of ISEs, especially for detection of very small changes in ion activity. In this perspective article, the coulometric transduction method is revisited and critically evaluated.

(Digital Presentation) Construction of a Novel Phosphate Potential Sensor and Its Application

Phosphorus content is one of important indicators in the evaluation of water quality. The level of phosphorus in oilfield water systems not only affects the overall water cycle production system, but excessive phosphorus wastewater entering the environment can cause eutrophication of the water body, as can high levels of ammonia and nitrogen, and leading to rapid growth of algae and thus affecting the balance of the aquatic ecosystem. Therefore, the regulation of phosphorus is of great importance. Phosphorus exists in nature mainly in the form of phosphate, which is currently detected by the traditional spectrophotometric method. As this method requires the addition of a large number of chemical reagents, it is not only complex and time consuming, but also produces a waste solution at the end of the test that can easily cause secondary pollution to the environment. Compared to traditional methods, Ion Selective Electrodes (ISEs) are easier to use, more sensitive, less expensive and do not cause secondary contamination. In conventional ion selective electrode detection systems, liquid contact ion selective electrodes consisting of a polymeric ion selective membrane, an internal filling fluid and an internal reference electrode are susceptible to common variables such as pressure and temperature. There are problems with complex maintenance, leakage and signal calibration. In contrast, all-solid ion-selective electrodes that eliminate the internal filling fluid and internal reference have become a hot research topic for such potential-based sensors. In this thesis, the following research was carried out with the aim of developing a phosphate potentiometric sensor with practical detection capabilities:(1) PANI/ITO electrodes were produced by electrodeposition of polyaniline on ITO conductive glass by constant potential deposition and by ultrasonic peeling. Three oxidation states of PANI/ITO electrodes were prepared at different voltages using phosphoric acid as the doping solution. LBPANI/ITO electrode in the fully reduced state, ESPANI/ITO electrode in the semi-oxidized state and PBPANI/ITO electrode in the fully oxidized state. The response sensitivity and stability of the three electrodes were investigated to determine the optimum doping potential; the response performance of the electrodes was tested for different doping times to determine the optimum preparation time. It is shown that the PBPANI/ITO electrode in the fully oxidized state has the optimum stability and response sensitivity at a doping time of 80 s. The electrode showed good linear response in the range of 1.00x10-5 ~ 1.00x10-1 M, with a response sensitivity of 37.30 dec/mV (R2 = 0.9966) and a detection limit of 10-5.6 M. The response time of the electrode to the solution was around 4 s, and it had good acid-base adaptability (pH 4.0 ~ 8.0). The performance of the prepared PBPANI/ITO electrode as a potentiometric phosphate sensor was also investigated in terms of stability (n = 10, RSD 1.76 %) and lifetime (6 weeks at room temperature), and was successfully used for phosphate detection and analysis in oilfield re-injection water, lake water and tap water samples in real environments with satisfactory results. (2) To further prepare a phosphate potential sensor with a high sensitivity response. We pretreated the home-made cobalt electrode with high pressure anodic oxidation in sodium hydroxide solution to produce a uniform cobalt oxide passivation film on the electrode surface. The sensitivity and stability of the response of the electrode at different pretreatment times were investigated to determine the optimum preparation conditions. The results showed that the pretreated cobalt electrode had a good response in the range of 1.00x10-5 ~ 1.00x10-1 M. The response sensitivity was 67.22 mV dec, R2 = 0.9939 and the detection limit was 10-5.5 M. It was much higher than that of the untreated cobalt electrode. This far exceeded that of the untreated polished cobalt electrode (36 ~ 40 mV/dec). The performance of the pretreated cobalt electrode as a potentiometric phosphate sensor was also investigated in terms of lifetime (7 weeks at room temperature), response rate (less 40 s), stability (n = 10, RSD 1.07 %) and acid-base adaptability (pH 4.0 ~ 8.0), and the sensor was successfully used for phosphate in oilfield re-injection water, lake water and tap water samples with satisfactory results.

Self-Calibrated Ion-Selective Electrodes.

Ion-selective electrode (ISE) potentiometry is reliable only if on-site calibration using a standard solution is performed before ion measurements. The complex device and operation required for calibration hinder the implementation of ISEs in decentralized sensing. Reported herein is a new type of ISE that is calibrated by a built-in component of the sensor without requiring any fluid handling processes. The indicator and reference electrodes are connected by a thin ionic conductor such as an aqueous phase containing the measuring ions in a capillary tube. This connection establishes a baseline electromotive force (EMF) that incorporates phase boundary potentials across multiple interfaces of the electrochemical cell and serves as a one-point calibration. Unlike conventional ISEs that rely on one EMF reading for each measurement, the proposed sensor utilizes a sample-induced EMF change relative to the baseline for each ion measurement. The variability in relative EMF is found to be <2.0 mV for multiple full potentiometric sensors consisting of plasticizer-based K+ ISEs and hydrogel-based Ag/AgCl reference electrodes. This value is significantly smaller than the variability of absolute EMF readouts in similar sensors without the self-calibration design. Moreover, when the ion-conducting calibration bridge has a low concentration of primary ions, low ion mobility, and/or a small contact area with the indicator and reference phases, it does not compromise the Nernstian response slope toward the analyte ions in the sample and therefore does not need to be removed for sample testing. The accuracy of the single-use self-calibrated K+ sensor in testing undiluted human blood samples is validated using a commercial blood gas analyzer as the reference method. Although this study focuses on disposable sensors consisting of tubes, the fluidics-free self-calibration strategy may be adapted to other sensor configurations such as planar sensors.

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.

Advances in Therapeutic Monitoring of Lithium in the Management of Bipolar Disorder.

Since the mid-20th century, lithium continues to be prescribed as a first-line mood stabilizer for the management of bipolar disorder (BD). However, lithium has a very narrow therapeutic index, and it is crucial to carefully monitor lithium plasma levels as concentrations greater than 1.2 mmol/L are potentially toxic and can be fatal. The quantification of lithium in clinical laboratories is performed by atomic absorption spectrometry, flame emission photometry, or conventional ion-selective electrodes. All these techniques are cumbersome and require frequent blood tests with consequent discomfort which results in patients evading treatment. Furthermore, the current techniques for lithium monitoring require highly qualified personnel and expensive equipment; hence, it is crucial to develop low-cost and easy-to-use devices for decentralized monitoring of lithium. The current paper seeks to review the pertinent literature rigorously and critically with a focus on different lithium-monitoring techniques which could lead towards the development of automatic and point-of-care analytical devices for lithium determination.

Zinc Oxide Nanowire Field Effect Transistor Used as a pH Sensor

An ion sensitive field effect transistor can outperform conventional ion-selective electrodes. Thus, a zinc oxide (ZnO) nanowire field effect transistor (NWFET) pH sensor was fabricated and measured. The sensor contained a channel with 1.7×1018 cm-3 donor concentration and 100 ZnO nanowires in parallel, each with the following dimensions: 10 μm×120 nm×20 nm. The active channel is passivated with an 18 nm Al2O3 layer. The device was measured under a controlled environment with and without pH solutions. The pH range was 3–9 with a sensitivity of 2.48 mV to 10.3 mV. The voltage sensitivity translates to a percentage value of 15%. The measurements obtained before and after the pH solution treatment demonstrate the possibility of re-use of the device by rinsing and brushing the sensing layer.

Solid-Contact pH Sensor with Covalent Attachment of Ionophores and Ionic Sites to a Poly(decyl methacrylate) Matrix.

With a view to improving the sensor lifetime, solid-contact ion-selective electrodes (ISEs) were prepared with a plasticizer-free and cross-linked poly(decyl methacrylate) matrix, to which only the ionic sites, only the ionophore, or both the ionic sites and ionophore were covalently attached. In earlier work with covalently attached ionophores or ionic sites, it was difficult to discount the presence of ionophores or ionic site impurities that were not covalently attached to the polymer backbone because the reagents used to introduce the ionophore or ionic sites had high hydrophobicities. In this work, we deliberately chose readily available hydrophilic reagents for the introduction of covalently attached H+ ionophores with tertiary amino groups and covalently attached sulfonate groups as ionic sites. This simplified the synthesis and made it possible to thoroughly remove ionophores and ionic sites not covalently attached to the polymer backbone. Our results confirm the expectation that hydrophobic ISE membranes with both covalently attached ionophores and ionic sites have impractically long response times. In contrast, ISEs with either covalently attached H+ ionophores or covalently attached ionic sites responded to pH with quick Nernstian responses and high selectivity. Both conventional plasticized poly(vinyl chloride) (PVC)-based ISEs and the new poly(decyl methacrylate) membranes were exposed to 90 °C heat for 2 h, 10% ethanol for 1 day, or undiluted blood serum for 5 days. In all three cases, the poly(decyl methacrylate) ISEs exhibited properties superior to conventional PVC-based ISEs, confirming the advantages of the covalent attachment.

Silicone-Rubber Based Multiplex Ion-Sensitive Light-Addressable Potentiometric Sensor (ISLAPS) System for Physiological Ions Detection

Silicone-Rubber Based Multiplex Ion-Sensitive Light-Addressable Potentiometric Sensor (ISLAPS) System for Physiological Ions Detection

Printed-Circuit-Board (PCB) Based Multiplex Sensor for Ion Sensing

Combining electrochemical sensing with microfluidic system presents an attractive approach for in vitro study of physiology and pathophysiology (e.g. organ-on-chip platform) [1] and for inline monitoring of environmental parameters (e.g. water quality) [2]. Ion-selective electrodes (ISE) are potentiometric sensors that measure activities of ions. They offer a simpler analytical method compare to voltammetric sensors or ISFETs. Solid-state ISEs differ from conventional ISEs in that the inner filling solution is replaced by a solid contact material. The removal of liquid compartment can make such sensor more durable and flexible, enabling it to be integrated with microfluidic device. In this work, we fabricated and evaluated a multiplex ion sensing platform by modifying the printed circuit board with solid-state ISEs. The choice of PCB platform allows straightforward multiplexing of electrodes and interfacing with downstream electronics by leveraging standardized PCB technology, which can also be cost effective in terms of fabrication. ISEs for potassium (K+) and nitrate (NO3-) approximately 1.5 mm in diameter were prepared by coating polymeric membrane and mesoporous carbon layers as the solid contact on a pre-patterned transfer tape. The electrode response was significantly stabilized by casting the membrane away from the underlying metal pads. The stabilizing effect was attributed to the increase of effective surface area of mesoporous carbon between membrane and conducting metal. The multiplex sensor was further integrated with a 3D printed microchamber for onchip analysis (Figure 1A). The K+ and NO3- electrodes displayed near-Nernstian slopes of 53 and -50 mV/dec (vs. Ag/AgCl/0.01 M Cl- reference) in KNO3 solutions, respectively. Despite some noise and baseline drift, both electrodes showed reasonable stability and reversibility upon switching solution (Figure 1B). The sensor can be stored dry between measurements. The slope of K+ electrode decayed over time, while NO3- electrode maintained sensitivity for at least one week. The slope decrease was likely due to the diffusion of ion-exchanger (tetraphenylborate) into the surrounding adhesive tape. During integration with the microchamber, the K+ electrode was also found subject to the bonding adhesive between the chamber and transfer tape, where possible reaction and/or degradation of the ion exchanger has caused complete loss of electrode response. Nevertheless, with further improvement, the PCB based multiplex ISEs provides a promising tool for ion sensing in many biomedical and environmental applications.

Multiplexed all-solid-state ion-sensitive light-addressable potentiometric sensor (ISLAPS) system based on silicone-rubber for physiological ions detection

Light-addressable potentiometric sensor (LAPS) has been widely used in biomedical applications since its advent. As a member of the potentiometric sensors, ion-sensitive LAPS (ISLAPS) can be obtained by modifying ion selective sensing membrane on the sensor surface. Compared with the conventional ion-selective electrodes (ISEs) with liquid contact, the all-solid-state ISEs have more advantages such as easy maintenance, more convenient for miniaturization and practical applications. However, the commonly used ion-sensitive membrane (ISM) matrix like PVC has many limitations such as poor adhesion to silicone-based sensor and easy overflow of the plasticizer from the membrane. In this work, LAPS was combined with a variety of ionophore-doped all-solid-state silicone-rubber ISMs for the first time, to establish a program-controlled multiplexed ISLAPS system for physiological ions (Na+, K+, Ca2+ and H+) detection. The silicone-rubber ISMs have better adhesion to silicon-based sensors without containing plasticizers, which can avoid the plasticizer pollution and improve the long-term stability. A layer of poly(3-octylthiophene-2,5-diyl) (P3OT) was pre-modified on the sensor surface to inhibit the formation of an aqueous layer and improve the sensor lifetime. With the aid of a translation stage, the light spot automatically illuminated the detection sites in sequence, and the response of the four ions could be obtained in one measurement within 1 min. The proposed multiplexed ISLAPS has good sensitivity with micromolar limit of detection (LOD), good selectivity and long-term stability (more than 3 months). The results of the real Dulbecco's Modified Eagle Medium (DMEM) sample detection proved that the ISLAPS system can be used for the physiological ions detection, and is promising to realize a multi-parameter microphysiometer.

Ion-Selective Optodes Integrated Paper-Based Device for Simultaneous Colorimetric Quantification of Salivary Na&lt;sup&gt;+&lt;/sup&gt;, K&lt;sup&gt;+&lt;/sup&gt; and Ca&lt;sup&gt;2+&lt;/sup&gt;

The quantification of electrolytes in biological fluids plays an important role for evaluating the human health condition. This work proposes a disposable and simple microfluidic paper-based analytical device (μPAD) for the quantitative colorimetric detection of salivary Na+, K+ and Ca2+, relying on ion-selective optodes (ISOs). Nanoscale ISOs composed of pluronic F-127 surfactant-based micelles were used as a colorimetric indicator system, reducing the assay time compared to the previously reported bulk membrane approach. Devices consist of three functionalized paper layers assembled by lamination, with the optical signal read out from the bottom layer. Saliva was chosen as the biological specimen because it is painlessly collected and easy-to-handle, which makes it suitable for POC assays. Simultaneous Na+, K+ and Ca2+ detection by a single μPAD was successfully confirmed by using aqueous metal ion samples with various concentrations of Na+, K+ and Ca2+ within the biologically relevant range. The simultaneous detection of the three cationic species enabled the correction of the three ion quantities by eliminating the interference from other ions using the calculated selectivity coefficients. For proof-of-concept, Na+ quantification in a human saliva sample was conducted by the developed μPADs. The calibration curve for this assay was prepared by applying artificial saliva to the μPADs. The concentrations of Na+ in spiked human saliva were also measured by the μPADs. The obtained recovery rates were 78-132 %, which indicates the quantification of the added Na+ in the real sample matrix. Finally, this μPAD was successfully integrated into a smartphone-based assay, which enables salivary electrolyte quantification in resource poor settings. The developed μPADs satisfy the criteria of low-cost (< $0.3/device), rapidity (< 5 min) and simple analytical procedure compared to the conventional ion-selective electrode (ISE) method.

Thin polymeric membrane ion-selective electrodes for trace-level potentiometric detection

In this work, we describe a novel method to improve the detection limits of the non-classical polymeric membrane ion-selective electrodes (ISEs) which are conditioned with highly discriminated ions instead of primary ions. It is based on a thin-layer ISE membrane with a thickness of 5 μm, which is coated on ordered mesoporous carbon used as solid contact. The diffusion of the primary ion from the surface of the sensing membrane to the bulk of the membrane could be avoided by the proposed thin membrane configuration. Since the detection sensitivity of the non-classical ISEs depends on the accumulation of the primary ion in the interfacial layer of the sensing membrane, a lower detection limit can be obtained. By using the copper ion as a model, the present potentiometric sensor shows a significantly improved detection sensitivity compared to the conventional ISE with a membrane thickness of ca. 200 μm. Low detection limits of 0.29 and 0.53 nM can be obtained in 0.01 and 0.5 M NaCl, respectively. In addition, the proposed sensor exhibits an excellent reversibility by using a neutral proton-selective ionophore incorporated in the thin membrane.

Plasticized PVC Membrane Modified Electrodes: Voltammetry of Highly Hydrophobic Compounds.

In the last 50 years, plasticized polyvinyl chloride (PVC) membranes have gained unique importance in chemical sensor development. Originally, these membranes separated two solutions in conventional ion-selective electrodes. Later, the same membranes were applied over a variety of supporting electrodes and used in both potentiometric and voltammetric measurements of ions and electrically charged molecules. The focus of this paper is to demonstrate the utility of the plasticized PVC membrane modified working electrode for the voltammetric measurement of highly lipophilic molecules. The plasticized PVC membrane prevents electrode fouling, extends the detection limit of the voltammetric methods to sub-micromolar concentrations, and minimizes interference by electrochemically active hydrophilic analytes.

Improved Sensing Capability of Hg-Ion Selective Membrane(ISM)-Coated FET Sensors with PBS Conditioning

In this work, phosphate buffered saline (PBS) conditioned PVC-based mercury ion selective membranes (Hg-ISM) were used to fabricate ion-selective extended gate FET mercury ion sensor. At pH 7.4 phosphate buffer solution, a detection limit below 10-12M and a linear response ranging from 10-12M to 10-7M were obtained. The excellent selectivity of the PBS-conditioned mercury ion selective membrane were demonstrated by the selectivity experiment of the membrane. It has been shown that PBS-conditioned mercury ion selective membrane possesses superb selectivity toward Pb and Cd. X-ray photoelectron spectroscopy were successfully applied in the surface characterization of the mercury ion selective membrane coated extended gate field effect transistor. The X-ray photoelectron spectroscopy confirmed that mercury ion were captured in the membrane structure after testing with mercury standard solution. This PBS-conditioned mercury ion selective membrane coated extended gate FET provides the detection limit lower than the conventional ion-selective electrode (10-6M), and the detection limit is comparable to the laboratory-based instrument such as ICP-MS. Our sensor can enable us to conduct facile, instant and accurate test of the mercury ion in the daily diet. Figure 1

Modified Screen-Printed Potentiometric Sensors based on Man-Tailored Biomimetics for Diquat Herbicide Determination.

Screen-printed platforms integrated with molecularly imprinted polymers (MIP) were fabricated and characterized as potentiometric sensors for diquat (DQ). The synthesized MIP beads were studied as sensory carriers in plasticized poly(vinyl chloride) membranes. The sensors were constructed by using poly(3,4-ethylenedioxythiophene) (PEDOT) as solid-contact material to diminish charge-transfer resistance and water layer potential. Conventional ion-selective electrodes (ISEs) with internal filling solution were used for comparison. The designed electrodes showed near Nernstian slopes of 28.2 ± 0.7 (r² = 0.999) over the concentration range of 1.0 × 10−6–1.0 × 10−2 M with the limit of detection 0.026 µg/mL over the pH range 4.2–9.0. The electrode exhibited good selectivity for diquat cations over a large number of organic and inorganic cations. The sensor was successfully introduced for direct measurement of diquat content in commercial pesticide preparations and different spiked potato samples. The results showed that the proposed electrode has a fast and stable response, good reproducibility, and applicability for direct assessment of diquat content. The proposed potentiometric method is simple and accurate in comparison with the reported HPLC methods. Besides, it is applicable to turbid and colored sample solutions.

Ultrasensitive micro ion selective sensor arrays for multiplex heavy metal ions detection

Trace heavy metal ions are probable carcinogens with concentrations at ppb levels. Ion selective electrode (ISE) technology is a conventional and widely used method to detect various ions with the advantages as rapid measurement, low power consumption and low cost. However, the detection limit of a conventional bulky ISE device is only able to reach ppm level so far, not sensitive enough for practically detecting heavy metal ions. A micro ion selective electrode (μISE) array which can realize multiplex detection of heavy metal ions on one chip is reported in this paper. Its detection limit for Pb2+, Cd2+, AsO2−, and Hg2+ can reach 1, 3, 10, and 1 ppb, respectively, within the permissible limits of drinking water. The micrometer scale device arrays from microfabrication processes demonstrated enhanced sensitivity, uniformity, and reduced response time (18 s) compared to conventional ISE. Our devices showed superb selectivity, and the performance dependence on temperature and pH values was investigated as well.

All-Solid-State Sodium-Selective Electrode with a Solid Contact of Chitosan/Prussian Blue Nanocomposite.

Conventional ion-selective electrodes with a liquid junction have the disadvantage of potential drift. All-solid-state ion-selective electrodes with solid contact in between the metal electrode and the ion-selective membrane offer high capacitance or conductance to enhance potential stability. Solution-casted chitosan/Prussian blue nanocomposite (ChPBN) was employed as the solid contact layer for an all-solid-state sodium ion-selective electrode in a potentiometric sodium ion sensor. Morphological and chemical analyses confirmed that the ChPBN is a macroporous network of chitosan that contains abundant Prussian blue nanoparticles. Situated between a screen-printed carbon electrode and a sodium-ionophore-filled polyvinylchloride ion-selective membrane, the ChPBN layer exhibited high redox capacitance and fast charge transfer capability, which significantly enhanced the performance of the sodium ion-selective electrode. A good Nernstian response with a slope of 52.4 mV/decade in the linear range from 10−4–1 M of NaCl was observed. The stability of the electrical potential of the new solid contact was tested by chronopotentiometry, and the capacitance of the electrode was 154 ± 4 µF. The response stability in terms of potential drift was excellent (1.3 µV/h) for 20 h of continuous measurement. The ChPBN proved to be an efficient solid contact to enhance the potential stability of the all-solid-state ion-selective electrode.

Solid-Contact pH Sensor without CO2 Interference with a Superhydrophobic PEDOT-C14 as Solid Contact: The Ultimate "Water Layer" Test.

The aim of this study was to find a conducting polymer-based solid contact (SC) for ion-selective electrodes (ISEs) that could become the ultimate, generally applicable SC, which in combination with all kinds of ion-selective membranes (ISMs) would match the performance characteristics of conventional ISEs. We present data collected with electrodes utilizing PEDOT-C14, a highly hydrophobic derivative of poly(3,4-ethylenedioxythiophene), PEDOT, as SC and compare its performance characteristics with PEDOT-based SC ISEs. PEDOT-C14 has not been used in SC ISEs previously. The PEDOT-C14-based solid contact (SC) ion-selective electrodes (ISEs) (H+, K+, and Na+) have outstanding performance characteristics (theoretical response slope, short equilibration time, excellent potential stability, etc.). Most importantly, PEDOT-C14-based SC pH sensors have no CO2 interference, an essential pH sensors property when aimed for whole-blood analysis. The superhydrophobic properties (water contact angle: 136 ± 5°) of the PEDOT-C14 SC prevent the detachment of the ion-selective membrane (ISM) from its SC and the accumulation of an aqueous film between the ISM and the SC. The accumulation of an aqueous film between the ISM and its SC has a detrimental effect on the sensor performance. Although there is a test for the presence of an undesirable water layer, if the conditions for this test are not selected properly, it does not provide an unambiguous answer. On the other hand, recording the potential drifts of SC electrodes with pH-sensitive membranes in samples with different CO2 levels can effectively prove the presence or absence of a water layer in a short time period.

A Disposable Planar Paper-Based Potentiometric Ion-Sensing Platform.

Ion-selective electrodes (ISEs) are widely used tools for fast and accurate ion sensing. Herein their design is simplified by embedding a potentiometric cell into paper, complete with an ISE, a reference electrode, and a paper-based microfluidic sample zone that offer the full function of a conventional ISE setup. The disposable planar paper-based ion-sensing platform is suitable for low-cost point-of-care and in-field testing applications. The design is symmetrical and each interfacial potential within the cell is well defined and reproducible, so that the response of the device can be theoretically predicted. For a demonstration of clinical applications, paper-based Cl(-) and K(+) sensors are fabricated with highly reproducible and linear responses towards different concentrations of analyte ions in aqueous and biological samples. The single-use devices can be fabricated by a scalable method, do not need any pretreatment prior to use, and only require a sample volume of 20 μL.

Determining the Degree of Counterion Binding to Aqueous Alkylammonium Chloride Micelles by an Electrode Method.

The degree of counterion binding (β) to micelles of aqueous n-alkylammonium chlorides (CnAC; H(CH2)nNH3Cl, n = 8-12) was evaluated from the ionic activity coefficient (γCl-), which was determined by use of a conventional chloride ion selective electrode at 25°C. In the γCl- plot of C12AC, a bend indicating the critical micelle concentration (cmc) was found, at concentrations above which the Botré plot was examined, and a value of 0.80 was determined for β. Very similar β values were obtained for the other alkylammonium chloride surfactants. The standard free energy of the micellization (ΔGm) of CnAC was estimated from the β and micellar aggregation numbers.