Lanthanum-fluoride electrode-based methods to monitor fluoride transport in cells and reconstituted lipid vesicles.

Fluoride (F-) export proteins, including F- channels and F- transporters, are widespread in biology. They contribute to cellular resistance against fluoride ion, which has relevance as an ancient xenobiotic, and in more modern contexts like organofluorine biosynthesis and degradation or dental medicine. This chapter summarizes quantitative methods to measure fluoride transport across membranes using fluoride-specific lanthanum-fluoride electrodes. Electrode-based measurements can be used to measure unitary fluoride transport rates by membrane proteins that have been purified and reconstituted into lipid vesicles, or to monitor fluoride efflux into living microbial cells. Thus, fluoride electrode-based measurements yield quantitative mechanistic insight into one of the major determinants of fluoride resistance in microorganisms, fungi, yeasts, and plants.

Fluoride Determination in Milk, Soy, and Water-Based Products Using Ion-Selective Electrode and Direct Measurement Technique: Single-Laboratory Validation, First Action 2022.05.

BACKGROUND: The AOAC Stakeholder Panel on Infant Formula and Adult Nutritionals issued a call for methods for the determination of fluoride in infant formula. Standard Method Performance Requirement (SMPR®) 2014.016 was approved. OBJECTIVE: A single-laboratory validation (SLV) of a fluoride ion-selective electrode (F-ISE) method was completed. METHODS: Five powder samples were reconstituted, and hydrochloric acid was added to dissolve and release any bound fluoride. Samples included infant and adult nutritional products made from milk, whey, or soy, containing intact, partially hydrolyzed, or hydrolyzed proteins. Sodium citrate buffer was added to complex any interfering ions and adjust pH and ionic strength. Samples were analyzed in duplicate over 6 days using F-ISE. RESULTS: Results were calculated based on one of two least-squares (log fit) standard curves (0.02 to 0.1 µg/100 g or 0.1 to 2.0 µg/100 g). The LOQ for the method was determined to be 3.3 µg/100 g, which was far below the minimum analytical value of 30 µg/100 g required in the SMPR. The fluoride concentrations ranged from approximately 3 to 14 µg/100 g reconstituted powder and the intermediate precision, RSDr, ranged from 0.7 to 12.4%. Although all of these results are out of scope for the SMPR due to their low fluoride, it should be noted that the RSDr for formulas with fluoride concentrations ranging from approximately 9 to 14 µg/100 g ranged from 0.7 to 4.3%, and the RSDr for the two formulas with fluoride concentrations at or below the theoretical detection limit of the method were only approximately10 and 12.4%. Recoveries ranged from 94 to 98% for samples spiked at three levels with NIST-traceable standard solutions, meeting the SMPR. CONCLUSION: The F-ISE method meets the SMPR and was adopted as a First Action Official MethodSM. HIGHLIGHTS: The fluoride ion-selective electrode method presented here is affordable and easy to run. The addition of acid to the samples dissolves any minerals which may bind the fluoride.

MXene-Coated Ion-Selective Electrode Sensors for Highly Stable and Selective Lithium Dynamics Monitoring.

Lithium holds immense significance in propelling sustainable energy and environmental systems forward. However, existing sensors used for lithium monitoring encounter issues concerning their selectivity and long-term durability. Addressing these challenges is crucial to ensure accurate and reliable lithium measurements during the lithium recovery processes. In response to these concerns, this study proposes a novel approach involving the use of an MXene composite membrane with incorporated poly(sodium 4-styrenesulfonate) (PSS) as an antibiofouling layer on the Li+ ion selective electrode (ISE) sensors. The resulting MXene-PSS Li+ ISE sensor demonstrates exceptional electrochemical performance, showcasing a superior slope (59.42 mV/dec), lower detection limit (10-7.2 M), quicker response time (∼10 s), higher selectivity to Na+ (-2.37) and K+ (-2.54), and reduced impedance (106.9 kΩ) when compared to conventional Li+ ISE sensors. These improvements are attributed to the unique electronic conductivity and layered structure of the MXene-PSS nanosheet coating layer. In addition, the study exhibits the long-term accuracy and durability of the MXene-PSS Li+ ISE sensor by subjecting it to real wastewater testing for 14 days, resulting in sensor reading errors of less than 10% when compared to laboratory validation results. This research highlights the great potential of MXene nanosheet coatings in advancing sensor technology, particularly in challenging applications, such as detecting emerging contaminants and developing implantable biosensors. The findings offer promising prospects for future advancements in sensor technology, particularly in the context of sustainable energy and environmental monitoring.

Potentiometric detection of apomorphine in human plasma using a 3D printed sensor.

Apomorphine is a dopamine agonist that is used for the management of Parkinson's disease and has been proven to effectively decrease the off-time duration, where the symptoms recur, in Parkinson's disease patients. This paper describes the design and fabrication of the first potentiometric sensor for the determination of apomorphine in bulk and human plasma samples. The fabrication protocol involves stereolithographic 3D printing, which is a unique tool for the rapid fabrication of low-cost sensors. The solid-contact apomorphine ion-selective electrode combines a carbon-mesh/thermoplastic composite as the ion-to-electron transducer and a 3D printed ion-selective membrane, doped with the ionophore calix[6]arene. The sensor selectively measures apomorphine in the presence of other biologically present cations - sodium, potassium, magnesium, and calcium - as well as the commonly prescribed Parkinson's pharmaceutical, levodopa (L-Dopa). The sensor demonstrated a linear, Nernstian response, with a slope of 58.8 mV/decade over the range of 5.0 mM-9.8 µM, which covers the biologically (and pharmaceutically) relevant ranges, with a limit of detection of 2.51 µM. Moreover, the apomorphine sensor exhibited good stability (minimal drift of just 188 µV/hour over 10 h) and a shelf-life of almost 4 weeks. Experiments performed in the presence of albumin, the main plasma protein to which apomorphine binds, demonstrate that the sensor responds selectively to free-apomorphine (i.e., not bound or complexed forms). The utility of the sensor was confirmed through the successful determination of apomorphine in spiked human plasma samples.

Ion-Electron Transduction Layer of the SnS2-MoS2 Heterojunction to Elevate Superior Interface Stability for All-Solid Sodium-Ion Selective Electrode.

The high selectivity and fast ion response of all-solid sodium ion selective electrodes were widely applied in human sweat analysis. However, the potential drift due to insufficient interfacial capacitance leads to the deterioration of its stability and ultimately affects the potential accuracy of ion analysis. Designing a novel ion-electron transduction layer between the electrode and the ion selective membrane is an effective method to stabilize the interfacial potential. Herein, the SnS2-MoS2 heterojunction material was constructed by doping Sn in MoS2 nanosheets and used as the ion electron transduction layers of an all-solid sodium ion selective electrode for the first time, achieving the stable and efficient detection of Na+ ions. The proposed electrode exhibited a Nernst slope of 57.86 mV/dec for the detection of Na+ ions with a detection limit of 10-5.7 M in the activity range of 10-6-10-1 M. Via the electronic interaction at the heterojunction interfaces between SnS2 and MoS2 materials, the micro-nanostructure of the SnS2-MoS2 heterojunction was changed and SnS2-MoS2 as the ion-electron transduction layer acquired excellent capacitance (699 µF) and hydrophobicity (132°), resulting in a long-term potential stability of 1.37 µV/h. It was further proved that the large capacitance and high hydrophobicity of the ion-electron transduction layer are primary reasons for the excellent stability of the all-solid sodium ion selective electrode toward Na+ ions.

The Influence of the Nature of the Polymer Incorporating the Same A3B Multifunctional Porphyrin on the Optical or Electrical Capacity to Recognize Procaine.

The multifunctionality of an A3B mixed-substituted porphyrin, namely 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl)porphyrin (5-COOH-3MPP), was proven due to its capacity to detect procaine by different methods, depending on the polymer matrix in which it is incorporated. The hybrid nanomaterial containing k-carrageenan and AuNPs (5-COOH-3MPP-k-carrageenan-AuNPs) was able to optically detect procaine in the concentration range from 5.76 × 10-6 M to 2.75 × 10-7 M, with a limit of detection (LOD) of 1.33 × 10-7 M. This method for the detection of procaine gave complementary results to the potentiometric one, which uses 5-COOH-3MPP as an electroactive material incorporated in a polyvinylchloride (PVC) membrane plasticized with o-NPOE. The detected concentration range by this ion-selective membrane electrode is wider (enlarged in the field of higher concentrations from 10-2 to 10-6 M), linearly dependent with a 53.88 mV/decade slope, possesses a detection limit of 7 × 10-7 M, a response time of 60 s, and has a certified stability for a working period of six weeks.

A comparative study of green solid contact ion selective electrodes for the potentiometric determination of Letrozole in dosage form and human plasma.

Analysis of drugs clinically and their identification in biological samples are of utmost importance in the process of therapeutic drug monitoring, also in pharmacokinetic investigations and tracking of illicit medications. These investigations are carried out using a variety of analytical methods, including potentiometric electrodes. Potentiometric electrodes are a wonderful solution for researchers because they outperform other methods in terms of sustainability, greenness, and cost effectiveness. In the current study, ion-selective potentiometric sensors were assembled for the aim of quantification of the anticancer drug Letrozole (LTZ). The first step was fabrication of a conventional sensor based on the formation of stable host-guest inclusion complex between the cationic drug and 4-tert-butylcalix-8-arene (TBCAX-8). Two additional sensors were prepared through membrane modification with graphene nanocomposite (GNC) and polyaniline (PANI) nanoparticles. Linear responses of 1.00 × 10-5-1.00 × 10-2, 1.00 × 10-6-1.00 × 10-2 and 1.00 × 10-8-1.00 × 10-3 with sub-Nernstian slopes of 19.90, 20.10 and 20.30 mV/decade were obtained for TBCAX-8, GNC, and PANI sensors; respectively. The developed sensors were successful in determining the drug LTZ in bulk powder and dosage form. PANI modified sensor was used to determine LTZ in human plasma with recoveries ranging from 88.00 to 96.30%. IUPAC recommendations were followed during the evaluation of the electrical performance of the developed sensors. Experimental conditions as temperature and pH were studied and optimized. Analytical Eco-scale and Analytical GREEness metric were adopted as the method greenness assessment tools.

A Floating Capsule Electrochemical System for In Situ and Multichannel Ion-Selective Sensing.

Free-floating electrochemical sensors are promising for in situ bioprocess monitoring with the advantages of movability, a lowered risk of contamination, and a simplified structure of the bioreactor. Although floating sensors were developed for the measurement of physical and chemical indicators such as temperature, velocity of flow, pH, and dissolved oxygen, it is the lack of available electrochemical sensors for the determination of the inorganic ions in bioreactors that has a significant influence on cell culture. In this study, a capsule-shaped electrochemical system (iCapsuleEC) is developed to monitor ions including K+, NH4+, Na+, Ca2+, and Mg2+ based on solid-contact ion-selective electrodes (SC-ISEs). It consists of a disposable electrochemical sensor and signal-processing device with features including multichannel measurement, self-calibration, and wireless data transmission. The capacities of the iCapsuleEC were demonstrated not only for in situ measurement of ion concentrations but also for the optimization of the sensing electrodes. We also explored the possibility of the system for use in detection in simulated cell culture media.

Molecular imprinted polymer-based potentiometric approach for the assay of the co-formulated tetracycline HCl, metronidazole and bismuth subcitrate in capsules and spiked human plasma.

BACKGROUND: An anti-H-pylori co-formulated mixture of tetracycline HCl (TET), metronidazole (MET), and bismuth subcitrate (BSC) is recently available. Only two chromatographic and spectrophotometric methods are reported for determining those drugs simultaneously where the effect of impurities that could be present as well as the biological fluids matrix influence do not be taken into consideration. There is a need to develop an easy-to-use potentiometric technique for analysis of TET, MET, and BSC in their co-formulated capsules, in presence of some official impurities and in spiked human plasma. RESULTS: Three carbon paste electrodes (CPEs) were fabricated for this purpose. Being a solid contact ion-selective electrode, CPE suffers from the creation of a water layer affecting its stability and reproducibility. Besides, it has a common problem in differentiation between two drugs carrying the same charge (positively charged TET and MET). Water layer formation was prevented through inserting polyaniline nanoparticles (≈10.0 nm diameter) between solid contact and ion-sensing membrane in the three proposed sensors. TET and MET interference was overcome by synthesizing a corresponding molecular imprinted polymer (MIP) for each drug. The synthesized MIPs were inserted in equivalent sensing membranes and characterized using several techniques. The suggested MIPs have a noticeable enhanced sensitivity in potentiometric determination. The obtained LODs were 5.88 × 10-8, 5.19 × 10-7, and 1.73 × 10-6 M for TET, MET and BSC proposed CPEs, respectively, with corresponding slopes of 57.37, 56.20, and -57.40 mV decade-1. SIGNIFICANCE: The proposed potentiometric method makes the detection of the three cited drugs simple, fast, and feasible. This approach is the first for determining three drugs potentiometrically in one combined formulation. The obtained results were compared favorably with previously reported potentiometric methods.

Near-Infrared Laser Irradiation-Modulated High-Temperature Solid-Contact Ion-Selective Electrodes: Potentiometric Detection of Ca2+ in Seawater.

The high-temperature potentiometry operated by nonisothermal heating is a promising way to break through the traditional potentiometric responses of ion-selective electrodes (ISEs) at room temperature. Herein, a locally heated strategy through near-infrared region (NIR) laser irradiation upon the photothermal mesoporous carbon material placed between the ion-selective membrane and the glassy carbon substrate is introduced to obtain the high-temperature potentiometric performance of a solid-contact Ca2+-ISE for detection of Ca2+ in seawater. Based on the light-to-heat conversion of the mesoporous carbon-based solid contact, the temperature of the solid-contact Ca2+-ISE upon continuous NIR laser irradiation can be increased from room temperature to 60-70 °C, and the slope of the electrode is promoted up to about 30% according to the thermodynamic steady-state potentiometric response. The pulsed potentiometric response of the solid-contact Ca2+-ISE upon a pulsed NIR laser irradiation of 5 s also shows a linear change as a function of Ca2+ activities, and the improved slope from 27.1 ± 0.6 to 38.1 ± 0.9 mV/dec can be obtained under dual control of the temperature of the electrode and the transient current induced by the pulsed NIR laser irradiation. As compared to the traditional potentiometric measurement under zero-current conditions at room temperature, the NIR laser-modulated high-temperature potentiometric response provides an alternative way for measurement of the solid-contact ISEs.

Influence of the Composition of Plasticizer-Free Silicone-Based Ion-Selective Membranes on Signal Stability in Aqueous and Blood Plasma Samples.

Solid-contact ion-selective electrodes (SC-ISEs) in direct long-term contact with physiological samples must be biocompatible and resistant to biofouling, but most wearable SC-ISEs proposed to date contain plasticized poly(vinyl chloride) (PVC) membranes, which have poor biocompatibility. Silicones are a promising alternative to plasticized PVC because of their excellent biocompatibility, but little work has been done to study the relationship between silicone composition and ISE performance. To address this, we prepared and tested K+ SC-ISEs with colloid-imprinted mesoporous (CIM) carbon as the solid contact and three different condensation-cured silicones: a custom silicone prepared in-house (Silicone 1), a commercial silicone (Dow 3140, Silicone 2), and a commercial fluorosilicone (Dow 730, Fluorosilicone 1). SC-ISEs prepared with each of these polymers and the ionophore valinomycin and added ionic sites exhibited Nernstian responses, excellent selectivities, and signal drifts as low as 3 µV/h in 1 mM KCl solution. All ISEs maintained Nernstian response slopes and had only very slightly worsened selectivities after 41 h exposure to porcine plasma (log KK,Na values of -4.56, -4.58, and -4.49, to -4.04, -4.00, and -3.90 for Silicone 1, Silicone 2, and Fluorosilicone 1, respectively), confirming that these sensors retain the high selectivity that makes them suitable for use in physiological samples. When immersed in porcine plasma, the SC-ISEs exhibited emf drifts that were still fairly low but notably larger than when measurements were performed in pure water. Interestingly, despite the very similar structures of these matrix polymers, SC-ISEs prepared with Silicone 2 showed lower drift in porcine blood plasma (-55 µV/h, over 41 h) compared to Silicone 1 (-495 µV/h) or Fluorosilicone 1 (-297 µV/h).

Ultrasensitive Ionophore-Based Liquid Sensors for Colorimetric Ion Measurements in Blood.

The self-monitoring of electrolytes using a small volume of capillary blood is needed for the management of many chronic diseases. Herein, we report an ionophore-based colorimetric sensor for electrolyte measurements in a few microliters of blood. The sensor is a pipet microtip preloaded with a segment of oil (plasticizer) containing a pH-sensitive chromoionophore, a cation exchanger, and an ionophore. The analyte is extracted from the sample into the oil via a mixing protocol controlled by a stepper motor. The oil with an optimized ratio of sensing chemicals shows an unprecedentedly large color response for electrolytes in a very narrow concentration range that is clinically relevant. This ultrahigh sensitivity is based on an exhaustive response mode with a novel mechanism for defining the lower and higher limits of detection. Compared to previous optodes and molecular probes for ions, the proposed platform is especially suitable for at-home blood electrolyte measurements because (1) the oil sensor is interrogated independent of the sample and therefore works for whole blood without requiring plasma separation; (2) the sensor does not need individual calibration as the consistency between liquid sensors is high compared to solid sensors, such as ion-selective electrodes and optodes; and (3) the sensing system consisting of a disposable oil sensor, a programmed stepper motor, and a smartphone is portable, cost-effective, and user-friendly. The accuracy and precision of Ca2+ sensors are validated in 51 blood samples with varying concentrations of total plasma Ca2+. Oil sensors with an ultrasensitive response can also be obtained for other ions, such as K+.

Ion-Selective Electrodes with Solid Contact Based on Composite Materials: A Review.

Potentiometric sensors are the largest and most commonly used group of electrochemical sensors. Among them, ion-selective electrodes hold a prominent place. Since the end of the last century, their re-development has been observed, which is a consequence of the introduction of solid contact constructions, i.e., electrodes without an internal electrolyte solution. Research carried out in the field of potentiometric sensors primarily focuses on developing new variants of solid contact in order to obtain devices with better analytical parameters, and at the same time cheaper and easier to use, which has been made possible thanks to the achievements of material engineering. This paper presents an overview of new materials used as a solid contact in ion-selective electrodes over the past several years. These are primarily composite and hybrid materials that are a combination of carbon nanomaterials and polymers, as well as those obtained from carbon and polymer nanomaterials in combination with others, such as metal nanoparticles, metal oxides, ionic liquids and many others. Composite materials often have better mechanical, thermal, electrical, optical and chemical properties than the original components. With regard to their use in the construction of ion-selective electrodes, it is particularly important to increase the capacitance and surface area of the material, which makes them more effective in the process of charge transfer between the polymer membrane and the substrate material. This allows to obtain sensors with better analytical and operational parameters. Brief characteristics of electrodes with solid contact, their advantages and disadvantages, as well as research methods used to assess their parameters and analytical usefulness were presented. The work was divided into chapters according to the type of composite material, while the data in the table were arranged according to the type of ion. Selected basic analytical parameters of the obtained electrodes have been collected and summarized in order to better illustrate and compare the achievements that have been described till now in this field of analytical chemistry, which is potentiometry. This comprehensive review is a compendium of knowledge in the research area of functional composite materials and state-of-the-art SC-ISE construction technologies.

Electrochemical Visualization of an Ion-Selective Membrane Using a Carbon Nanoelectrode.

Molecular and physical probes have been widely employed to investigate physicochemical properties and mechanisms of interfaces due to their ability to provide accurate measurements with temporal and spatial resolution. However, the direct measurement of electroactive species diffusion in ion-selective electrode (ISE) membranes and quantification of the water layer have been challenging due to the high impedance and optical opacity of polymer membranes. In the present work, carbon nanoelectrodes with ultrathin insulating encapsulation and good geometrical structure are reported as physical probes for direct electrochemical measurement of the water layer. The scanning electrochemical microscopy experiment exhibits positive feedback at the interface of the fresh ISE, and negative feedback after conditioning for 3 h. The thickness of the water layer was estimated to be ca. 13 nm. For the first time, we provide direct evidence that, during conditioning, the water molecules diffuse through the chloride ion selective membrane (Cl-ISM) until a water layer establishes at almost 3 h. Furthermore, the diffusion coefficient and concentration of oxygen molecules in the Cl-ISM are also directly electrochemical measured by introducing ferrocene (Fc) as a redox molecule probe. The oxygen concentration in the Cl-ISM decreases during conditioning, suggesting the diffusion of oxygen from ISM to the water layer. The proposed method can be used for the electrochemical measurement of solid contact, providing theoretical guidance and advice for the performance optimization of ISEs.

A simple, low cost and fast sample preparation method for fluorine determination by ISE in leguminous seeds and oilseeds.

This work presents the first report of the application of microwave-induced combustion in disposable vessels (MIC-DV) as a simple and fast sample preparation step for fluoride determination by ion-selective electrode (ISE) in leguminous seeds and oilseeds samples. Several experimental conditions of MIC-DV were evaluated, such as sample mass, absorbing solution, volume of combustion igniter, purging time with oxygen, and the use of successive combustion cycles. The accuracy of MIC-DV/ISE method was evaluated by comparing the results obtained by microwave-induced combustion and determination by ion chromatography, with agreements ranged from 94 to 106%. The method allowed to achieve relatively low limits of quantification (8.3 to 16 µg g-1) comparing to AOAC method (40 µg g-1), besides the use of small amounts of reagents, less handling of digests, making the method greener and with portable features comparing to other methods.

In Situ Drift Monitoring and Calibration of Field-Deployed Potentiometric Sensors Using Temperature Supervision.

Potentiometric ion-selective electrodes (ISEs) have broad applications in personalized healthcare, smart agriculture, oil/gas exploration, and environmental monitoring. However, high-precision potentiometric sensing is difficult with field-deployed sensors due to time-dependent voltage drift and the need for frequent calibration. In the laboratory setting, these issues are resolved by repeated calibration by measuring the voltage response at multiple standard solutions at a constant temperature. For field-deployed sensors, it is difficult to frequently interrupt operation and recalibrate with standard solutions. Moreover, the constant surrounding temperature constraint imposed by the traditional calibration process makes it unsuitable for temperature-varying field use. To address the challenges of traditional calibration for field-deployed sensors, in this study, we propose a novel in situ calibration approach in which we use natural/external temperature variation in the field to obtain the time-varying calibration parameters, without having to relocate the sensors or use any complex system. We also develop a temperature-supervised monitoring method to detect the drift of the sensor during operation. Collectively, the temperature-based drift monitoring and in situ calibration methods allow us to monitor the drift of sensors and correct them periodically to achieve high-precision sensing. We demonstrate our approach in three testbeds: (1) under controlled temperature variation in the lab, (2) under natural temperature variation in a greenhouse, and (3) in the field to monitor nitrate activity of an agricultural site. In the laboratory study, we validate that the calibration parameters of printed nitrate ISEs can be reproduced by our proposed calibration process; therefore, it can serve as an alternative to traditional calibration processes. In the greenhouse, we show the use of natural temperature variation to calibrate the sensors and detect the drift in a fixed concentration nitrate solution. Finally, we demonstrate the use of the method to monitor the nitrate activity of an agricultural field within 10% of laboratory-based measurements (i.e., a sensitivity of 0.03 mM) for a period of 22 days. The findings highlight the prospect of temperature-based calibration and drift monitoring for high-precision sensing with field-deployed ISEs.

New potentiometric PVC membrane electrode for Ferric Reduction Antioxidant Power assay.

In this study, a fast, reproducible and simple-to-apply a new potentiometric determination method was developed for the evaluation of iron (III) reducing power (FRAP), one of the antioxidant capacity tests. For this purpose, an all-solid-state contact iron (III)-selective poly vinyl chloride (PVC) membrane electrode (FRAP-PME) has been developed. In the structure of the developed polymeric membrane electrode (PME), Fe(II)-quercetin compound was used as the active ingredient. It was obtained that PME exhibits a linear potentiometric behavior against the concentration change of Fe(III) ion between 1.0 × 10-1-1.0 × 10-5 mol L-1 and the detection limit is 8.0 × 10-6 molL-1. PME showed a very rapid potential response (40-45 s) and was found to have a very high selectivity towards Fe(III) ion in the presence of other species that might interfere. The proposed potentiometric method has been successfully applied to evaluate the iron (III) reducing power of plant extracts.

Large-scale fabrication of ion-selective electrodes for simultaneous detection of Na+, K+, and Ca2+ in biofluids using a smartphone-based potentiometric sensing platform.

A significant bottleneck exists for mass-production of ion-selective electrodes despite recent developments in manufacturing technologies. Here, we present a fully-automated system for large-scale production of ISEs. Three materials, including polyvinyl chloride, polyethylene terephthalate and polyimide, were used as substrates for fabricating ion-selective electrodes (ISEs) using stencil printing, screen-printing and laser engraving, respectively. We compared sensitivities of the ISEs to determine the best material for the fabrication process of the ISEs. The electrode surfaces were modified with various carbon nanomaterials including multi-walled carbon nanotubes, graphene, carbon black, and their mixed suspensions as the intermediate layer to enhance sensitivities of the electrodes. An automated 3D-printed robot was used for the drop-cast procedure during ISE fabrication to eliminate manual steps. The sensor array was optimized, and the detection limits were 10-5 M, 10-5 M and 10-4 M for detection of K+, Na+ and Ca2+ ions, respectively. The sensor array integrated with a portable wireless potentiometer was used to detect K+, Na+ and Ca2+ in real urine and simulated sweat samples and results obtained were in agreement with ICP-OES with good recoveries. The developed sensing platform offers low-cost detection of electrolytes for point-of-care applications.

Construction of a Zinc Oxide Nanorod-Modified Coated Wire Electrode for Potentiometric Determination of Levocetirizine Dihydrochloride in Its Pure and Combined Form.

BACKGROUND: Coated wire electrodes (CWEs) are considered the most effective and selective type of ion- selective electrodes because of the electroactive materials which are used in surface modification. OBJECTIVE: This study aims to construct the first potentiometric method for analysis of levocetirizine (LVZ) in its combination form with montelukast (MON) drug. METHODS: A novel potentiometric sensor which consists of a silver wire coated with zinc oxide (ZnO) nanorod modified with a polymeric membrane (combining ß-cyclodextrin and tetraphenyl borate, and plasticized with di-butyl phthalate) was constructed for the determination of LVZ·2HCl in its pure form and its combination dosage form. RESULTS: The fabricated sensor exhibited a linearity range of 5 × 10-6-1 × 10-2 mol/L with a Nernstian slope 57.88 mV/decade over the pH range 2-4.5. The effect of temperature on the constructed sensor was studied and it was found that the electrode worked effectively over 10-50°C. The electrode showed a fast response time and the lifetime of the electrode was found to be 72 days without significant change in the Nernstian slope value. The selectivity of the electrode toward LVZ was estimated in the presence of some obstructive ions. CONCLUSION: The method was validated according to International Council for Harmonisation of Technical Requirments for Registration of Pharmaceuticals for Human Use (ICH) rules and applied to the determination of LVZ in its pure and combined pharmaceutical dosage forms. HIGHLIGHTS: This article introduces the synthesis of the first coated wire electrode modified with zinc oxide (ZnO) nanorods for the determination of the drug levocetirizine. The results demonstrate the ability of electrochemical methods to analyze drugs in combination. The presented method excels over the other analytical methods in terms of sensitivity, selectivity, and simplicity.

Highly Stretchable and Robust Electrochemical Sensor Based on 3D Graphene Oxide-CNT Composite for Detecting Ammonium in Sweat.

Wearable electrochemical sensors have attracted tremendous attention and have been experiencing rapid growth in recent years. Sweat, one of the most suitable biological fluids for non-invasive monitoring, contains various chemical elements relating abundant information about human health conditions. In this work, a new type of non-invasive and highly stretchable potentiometric sweat sensor was developed based on all-solid-state ion-selective electrode (ISE) coupled with poly(dimethylsiloxane; PDMS) and polyurethane (PU). This highly stretchable composite of PDMS-PU allows the sensor to be robust, with the PDMS providing a flexible backbone and the PU enhancing the adhesion between the electrodes and the substrate. In addition, graphene-carbon nanotube (CNT) network 3D nanomaterials were introduced to modify the ion selective membrane (ISM) in order to increase the charge transfer activity of the ISEs, which also could minimize the formation of water layers on the electrode surface, as such nanomaterials are highly hydrophobic. As a result, the sensor demonstrated a wide detection range of NH4+ from 10-6 M to 10-1 M with high stability and sensitivity-showing a high sensitivity of 59.6 ± 1.5 mV/log [NH4+] and an LOD lower than 10-6 M. Under a strain of 40%, the sensor still showed a sensitivity of 42.7 ± 3.1 mV/log [NH4+]. The proposed highly stretchable and robust electrochemical sweat sensor provides a new choice for wearable-device-based personal daily healthcare management beyond hospital-centric healthcare monitoring.