Solid-contact Ion-selective Electrodes Research Articles

Graphene Oxide-Poly(vinyl alcohol) Hydrogel-Coated Solid-Contact Ion-Selective Electrodes for Wearable Sweat Potassium Ion Sensing.

Solid-contact ion-selective electrodes (SC-ISEs) with ionophore-based polymer-sensitive membranes have been the major devices in wearable sweat sensors toward electrolyte analysis. However, the toxicity of ionophores in ion-selective membranes (ISMs), for example, valinomycin (K+ ion carrier), is a significant challenge, since the ISM directly contacts the skin during the tests. Herein, we report coating a hydrogel of graphene oxide-poly(vinyl alcohol) (GO-PVA) on the ISM to fabricate hydrogel-based SC-ISEs. The hydrogen bond interaction between GO sheets and PVA chains could enhance the mechanical strength through the formation of a cross-linking network. Comprehensive electrochemical tests have demonstrated that hydrogel-coated K+-SC-ISE maintains Nernstian response sensitivity, high selectivity, and anti-interference ability compared with uncoated K+-SC-ISE. A flexible hydrogel-based K+ sensing device was further fabricated with the integration of a solid-contact reference electrode, which has realized the monitoring of sweat K+ in real time. This work highlights the possibility of hydrogel coating for fabricating biocompatible wearable potentiometric sweat electrolyte sensors.

Functionalizing Carbon Substrates with a Covalently Attached Cobalt Redox Buffer for Calibration-Free Solid-Contact Ion-Selective Electrodes.

With a view to potentiometric sensing with minimal calibration requirements and high long-term stability, colloid-imprinted mesoporous (CIM) carbon was functionalized by the covalent attachment of a cobalt redox buffer and used as a new solid contact for ion-selective electrodes (ISEs). The CIM carbon surface was first modified by electroless grafting of a terpyridine ligand (Tpy-ph) using diazonium chemistry, followed by stepwise binding of Co(II) and an additional Tpy ligand to the grafted ligand, forming a bis(terpyridine) Co(II) complex, CIM-ph-Tpy-Co(II)-Tpy. Half a molar equivalent of ferrocenium tetrakis(3-chlorophenyl)borate was then used to partially oxidize the Co(II) complex. Electrodes prepared with this surface-attached CIM-ph-Tpy-Co(III/II)-Tpy redox buffer as a solid contact were tested as K+ sensors in combination with valinomycin as the ionophore and Dow 3140 silicone or plasticized poly(vinyl chloride) (PVC) as the matrixes for the ion-selective membrane (ISM). This solid contact is characterized by a redox capacitance of 3.26 F/g, ensuring a well-defined interfacial potential that underpins the transduction mechanism. By use of a redox couple as an internal reference element to control the phase boundary potential at the interface of the ISM and the CIM carbon solid contact, solid-contact ion-selective electrodes (SC-ISEs) with a standard deviation of E° as low as 0.3 mV for plasticized PVC ISMs and 3.5 mV for Dow 3140 silicone ISMs were obtained. Over 100 h, these SC-ISEs exhibit an emf drift of 20 μV/h for plasticized PVC ISMs and 62 μV/h for silicone ISMs. The differences in long-term stability and reproducibility between electrodes with ISMs comprising either a plasticized PVC or silicone matrix offer valuable insights into the effect of the polymeric matrix on sensor performance.

Nonequilibrium Anion Detection in Solid-Contact Ion-Selective Electrodes.

Low-cost and portable nitrate and phosphate sensors are needed to improve farming efficiency and reduce environmental and economic impact arising from the release of these nutrients into waterways. Ion selective electrodes (ISEs) could provide a convenient platform for detecting nitrate and phosphate, but existing ionophore-based nitrate and phosphate selective membrane layers used in ISEs are high cost, and ISEs using these membrane layers suffer from long equilibration time, reference potential drift, and poor selectivity. In this work, we demonstrate that constant current operation overcomes these shortcomings for ionophore-based anion-selective ISEs through a qualitatively different response mechanism arising from differences in ion mobility rather than differences in ion binding thermodynamics. We develop a theoretical treatment of phase boundary potential and ion diffusion that allows for quantitative prediction of electrode response under applied current. We also demonstrate that under pulsed current operation, we can create functional solid-contact ISEs using lower-cost molecularly imprinted polymers (MIPs). MIP-based nitrate sensors provide comparable selectivity against chloride to costlier ionophore-based sensors and exhibit >100,000 times higher selectivity against perchlorate. Likewise, MIP-based solid contact ion-selective electrode phosphate sensors operated under pulsed current provide competitive selectivity against chloride, nitrate, perchlorate, and carbonate anions. The theoretical treatment and conceptual demonstration of pulsed-current ISE operation we report will inform the development of new materials for membrane layers in ISEs based on differences in ion mobility and will allow for improved ISE sensor designs.

Cu1.4Mn1.6O4 as a bifunctional transducer for potentiometric Cu2+ solid-contact ion-selective electrode

Current potentiometric Cu2+ sensors mostly rely on polymer-membrane-based solid-contact ion-selective electrodes (SC-ISEs) that constitute ion-selective membranes (ISM) and solid contact (SC) for respective ion recognition and ion-to-electron transduction. Herein, we report an ISM-free Cu2+-SC-ISE based on Cu–Mn oxide (Cu1.4Mn1.6O4) as a bifunctional SC layer. The starting point is simplifying complex multi-interfaces for Cu2+-SC-ISEs. Specifically, ion recognition and signal transduction have been achieved synchronously by an ion-coupled-electron transfer of crystal ion transport and electron transfer of Mn4+/3+ in Cu1.4Mn1.6O4. The proposed Cu1.4Mn1.6O4 electrode discloses comparable sensitivity, response time, high selectivity and stability compared with present ISM-based potentiometric Cu2+ sensors. In addition, the Cu1.4Mn1.6O4 electrode also exhibits near Nernstian responses toward Cu2+ in natural water background. This work emphasizes an ISM-free concept and presents a scheme for the development of potentiometric Cu2+ sensors.

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.

Achieving preeminent stability and reproducibility in an ion-selective electrode for copper ion via introducing a hydrophobic NiCo2S4/PFOA solid-contact

In this work, an ion-selective electrode (ISE) for copper ion with excellent stability and reproducibility is prepared by introducing a solid-contact (SC) layer between the screen-printed electrode (SPE) and ion-selective membrane (ISM). The SC layer is fabricated by the electrodeposition of nano-arrayed NiCo2S4 on SPE, and thereafter is surface-modified by perfluorooctanoic acid (PFOA) to realize superior double layer capacitance (CDL) and hydrophobicity. The results show the water contact angle of the SC is 134°. The CDL of the modified ISE, named as Cu2+-SC(25)ISE, is 48.53 μF·cm−2, about 6.5 times of the ISE without SC. The detection limit of the Cu2+-SC(25)ISE is 4.62 μM. The SC with nano-structured NiCo2S4 frameworks and hydrophobic PFOA chains provides outstanding capacitance and high potential stability simultaneously, and further endows Cu2+-SC(25)ISE with excellent detection accuracy. For the detection of Cu2+ in soil leachate, the average detection errors of the Cu2+-SC(25)ISE and ICP-OES are 4.14 % and 3.02 % respectively, revealing the potential application prospect of this technic. This work puts forward a novel concept to achieve the accurate measurement and flexibility of ISE.

A rational study of transduction mechanisms of different materials for all solid contact-ISEs

The new era of solid contact ion selective electrodes (SC-ISEs) miniaturized design has received an extensive amount of concern. Because it eliminated the requirement for ongoing internal solution composition optimization and created a two-phase system with stronger detection limitations. Herein, the determination of venlafaxine HCl is based on a comparison study between different ion- to electron transduction materials (such as; multiwalled carbon nanotubes (MWCNTs), polyaniline (PANi), and ferrocene) and illustrating their mechanisms in their applied sensors. Their different electrochemical features (such as bulk resistance (Rb**), double-layer capacitance (Cdl), geometric capacitance (Cg), and specific capacitance (Cp)) were evaluated and discussed by using the Electrochemical Impedance Spectroscopy (EIS), Chronopotentiometry (CP), and Cyclic Voltammetry (CV) experiments. The results indicated that each transducer's influence on the proposed sensor's electrochemical characteristics is determined by their unique chemical and physical properties. The electrochemical features vary for different solid contact materials used in transduction mechanisms. The results confirm that the MWCNT sensor revealed the best electrochemical behavior with the potentiometric response of a near-Nernestian slope of 56.1 ± 0.8 mV/decade with detection limits of 3.8 × 10−6 mol/L (r2 = 0.999) and a low potential drift (∆E/∆t) of 34.6 µV/s. Also, the selectivity study was performed in the presence of different interfering species either in single or complex matrices. This demonstrates excellent selectivity, stability, conductivity, and reliability as a VEN-TPB ion pair sensor for accurately measuring VEN in its various formulations. The proposed method was compared to HPLC reported technique and confirmed no significant difference between them. So, the proposed sensors fulfill their solutions' demand features for VEN appraisal.

Application of ionic liquids in ion-selective electrodes and reference electrodes: A review.

Ionic liquids (ILs) are organic chemical compounds that are composed only of ions, a large organic cation and a smaller inorganic or organic anion. These are salts whose melting point is lower than the boiling point of water. ILs have many interesting properties, thanks to which they find great practical applications in analytics, electrochemistry, separation techniques, catalysis and others. One of the many areas of application of ionic liquids is sensors especially electrochemical sensors including ion-selective electrodes. In this case, the properties of ILs that are particularly useful include very good electrical conductivity, high electrochemical stability, good extraction properties, hydrophobic character and compatibility with other materials, e. g. polyvinyl chloride plasticizers or carbon nanomaterials. ILs were used as components of ion-selective membranes, both polymeric ones based on PVC and membranes in carbon paste electrodes. ILs performed various functions in these membranes, including lipophilic ionic additive, ionophore/ion exchanger, plasticizer, transducer media and matrix. They were also used as a component of the intermediate layer in solid contact ISEs. The last chapter presents examples of the use of ILs in reference electrodes. This review discusses the use of ionic liquids in ion-selective electrodes (ISEs) and reference electrodes over the last ten years.

Design and application of supramolecular based solid-contact ion-selective electrode for selective green determination of tenofovir disoproxil fumarate

This work describes the fabrication of solid-contact potentiometric sensor to determine the antiviral drug; tenofovir disoproxil fumarate. Screen-printed carbon electrodes have been selected as being a disposable substrate material. Ion sensing membrane is based on Polyvinyl chloride (PVC) polymer that was impregnated with calix [6] arene as being an ionophore for facilitating transferring the drug of interest from the sample phase into the membrane phase through decreasing Gibbs free energy of transfer. We applied a graphene nanocomposite interlayer between the carbon substrate and the polymeric membrane for overcoming potential drift, which is commonly encountered in solid-contact potentiometric sensors. As a result, the graphene nanocomposite layer enhanced the sensor performance and stabilized the potential signal as compared to the control sensor. The sensor performance characteristics were studied according to International Union of Pure and Applied Chemistry (IUPAC) recommendations, and linear range has been 0.01 M - 10 μM with limit of detection (LOD) 7.3 μM. We utilized the sensor for inline determination of the release of tenofovir disoproxil fumarate from its tablet pharmaceutical formulation without any separation or sample pre-treatment steps. In an attempt to be eco-friendly through evaluating the suggested method regarding its greenness, the method has been found to be excellent green according to the eco scale method.

In situ derivatisation of solid contact poly(3,4-ethylenedioxythiophene) transducers for ion-selective electrodes through “click” chemistry

The ion-to-electron transducing layer is an essential part of the solid-contact ion-selective electrode, mostly responsible for providing a stable signal readout. Among available materials such as redox probes, carbon-based materials or nanomaterial, redox polymers are a transducer of choice owing to their ability to be solvent cast or electropolymerized reproducibly, thereby ensuring inter-electrode repeatability. Although the number of commercially available redox polymer is gradually increasing, those used in ion-selective electrode as transducing layer must be synthetically modified in most cases to achieve a sufficient degree of hydrophobicity to avoid the formation of a water layer at the transducer – membrane interface. We present here a versatile solution to generate a wide range of transducers using an azide modified derivative of 3,4-ethylenedioxythiophene, EDOT-N3. In this approach, the polymer is generated through dynamic electropolymerisation and then its surface is modified by taking advantage of the high yield “click” reaction between the azide group and an alkyne. We show that before subjecting the polymer to the “click” reaction the choice of supporting electrolyte is critical to generate a polymer suitable to be used as transducer in ion-selective electrodes. Alkyl chains ranging from 6 to 18 carbon atoms and a plasticiser-like moiety were grafted to the polymer backbone to demonstrate the increase in the hydrophobic properties of the surface layer and performance as transducer. Water contact angles up to 156.6° were obtained, indicating super-hydrophobic properties.

Silk Fibroin-Regulated Nanochannels for Flexible Hydrovoltaic Ion Sensing.

The evaporation-induced hydrovoltaic effect based on ion-selective nanochannels can theoretically be employed for high-performance ion sensing; yet, the indeterminate ion-sensing properties and the acquisition of high sensing performance are rarely explored. Herein, a controllable nanochannel regulation strategy for flexible hydrovoltaic devices with highly sensitive ion-sensing abilities is presented across a wide concentration range. By multiple dip-coating of silk fibroin (SF) on an electrospinning nylon-66 nanofiber (NNF) film, the surface polarity enhancement, the fibers size regulation with a precision of ≈25nm, and the nanostructure firm binding are achieved simultaneously. The resultant flexible freestanding hydrovoltaic device exhibits an open circuit voltage up to 4.82V in deionized water, a wide ion sensing range of 10-7 to 100m, and ultrahigh sensitivity as high as 1.37V dec-1, which is significantly higher than the sensitivity of the traditional solid-contact ion-selective electrodes (SC-ISEs). The fabricated flexible ion-sensitive hydrovoltaic device is successfully applied for wearable human sweat electrolyte sensing and for environmental trace-ion monitoring, thereby confirming the potential application of the hydrovoltaic effect for ion sensing.

Covalently attached ionophores extend the working range of potentiometric pH sensors with poly(decyl methacrylate) sensing membranes.

The pH working range of solid-contact ion-selective electrodes (ISEs) with plasticizer-free poly(decyl methacrylate) sensing membranes is shown to be expanded by covalent attachment of H+ ionophores to the polymeric membrane matrix. In situ photopolymerization not only incorporates the ionophores into the polymer backbone, but at the same time also attaches the sensing membranes covalently to the underlying inert polymer and nanographite solid contact, minimizing sensor drift and preventing failure by membrane delamination. A new pyridine-based H+ ionophore, 3-(pyridine-3-yl)propyl methacrylate, has lower basicity than trialkylamine ionophores and expands the upper detection limit. This reduces in particular the interference from hydrogen phthalate, which is a common component of commercial pH buffers. Moreover, the lower detection limit is improved by replacing the CH2CH2 spacer of previously reported dialkylaminoethyl methacrylates with a (CH2)10 spacer, which increases its basicity. Notably, for the more basic and highly cation-selective ionophore 10-(diisopropylamino)decyl methacrylate, the extent of counterion interference from hydrogen phthalate shifted the upper detection limit to lower pH by nearly one pH unit when the crosslinker concentration was decreased.

Gradient Porous Graphene-Based Ion-Selective Electrode for Real-Time Monitoring of Sweat Electrolyte

Human sweat, a biomarker-rich physiological fluid, is an ideal candidate for non-invasive and real-time health monitoring. State-of-the-art electrochemical sweat sensors, that detect concentrations of analytes, such as electrolytes, through ion-selective membranes (ISM) in ion-selective electrodes (ISEs), show great potential for monitoring bodily conditions with high selectivity and affordability. Ion detection can be accomplished by utilizing the electrochemical potential gradient across an ISM within the solid-contact ISE. The solid-contact interface, located between the sensing membrane and electron-conducting substrate, plays a crucial role in ion-to-electron transduction. This critical layer serves to minimize the formation of the water layer, enabling stable potential responses and maintaining long-term stability.This work focuses on the use of chemical vapor deposition-grown, hydrophobic, gradient porous graphene (3D-G) material as the solid-contact layer in the ISE, offering a wider detection range, enhanced sensitivity, and improved long-term stability. The highly porous structure promotes ion recognition, diffusion, and adsorption onto the enhanced electrode surface, resulting in long-term stability, rapid analyte diffusion, and enhanced ion-to-electron transduction for sweat electrolyte sensing. A three-dimensionally interconnected porous electrode design can significantly enlarge the sensing areas in comparison to a 2D planar electrode, thereby improving the overall sensing performance of the sweat sensor. Owing to their large interface area, coupled with large double-layer capacitance in all-solid-state ISEs, hydrophobic nanostructured materials have been demonstrated to facilitate the ion-to-electron transduction process. Employing the 3D-G ISEs for health assessment has been exemplified through an on-body sweat sensing experiment, wherein the obtained results exhibit notable similarity with those generated by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Figure 1

Non-Equilibrium Nitrate Detection in Ionophore-Based Solid-Contact Ion-Selective Electrodes

Ion-selective electrodes (ISE’s) have benefits for solution-phase detection and quantification of ions, such as ease of operation, low cost, and high throughput compared to, e.g., ion chromatography or mass spectrometry methods. ISE devices are traditionally operated under zero-current conditions, where analyte ions passively exchange across a selective membrane barrier, yielding a Nernstian response after equilibration of Coulombic and chemical driving forces across the membrane. However, this passive mode of operation can require long equilibration times and lead to drift, for example due to electrolyte leakage and pH impacting the membrane and reference electrode. In recent years, non-equilibrium detection techniques have been of growing interest, where the ISE signal is obtained under applied electrical signal. However, the majority of this work has been devoted to cation sensing, with relatively few studies examining anion ISEs under active electrical actuation. Agricultural expansion and the corresponding uptick in application of supplemental nitrogen to soil has led to an increased demand for technologies to measure anions like nitrate – both from a production and regulation standpoint. In this work, we employ galvanostatic polarization of solid-contact ISEs as a non-equilibrium nitrate detection method. Our investigation involves coating electrodes with a redox-active charged polymer to act as a transduction layer to convert ionic to electronic signal, followed by solvent casting of a polymeric ion-selective membrane. We examine the impact of repeated rapid charging and discharging ISE electrode stacks on measurement time and stability. We compare the metrics derived from this technique to conventional open-circuit measurements using the same device. We identify benefits for this mode of operation using established nitrate membrane materials (e.g. nitrate ionophore VI membrane cocktail) that address some of the typical limitations of these conventional materials and we discuss opportunities for further refinement.

(Invited) Calibration-Free Ion-Selective Electrodes and Optodes for Decentralized Electrolyte Monitoring

Ionophore-based ion-selective sensors have been used in hospital analyzers for decades to measure electrolytes. Recently, there has been growing interest in affordable and accessible electrolyte measurements via at-home sensors, wearable sensors, transdermal sensors, and implantable sensors. However, the commercial success of ion-selective sensors in these new sensing modalities is hindered by the need for technically demanding on-site calibration. In fact, all ionophore-based sensors in clinical analyzers need to be calibrated with standard solutions at the point of use. We are developing calibration-free ionophore-based sensors, including ion-selective electrodes and optodes, to enable decentralized electrolyte measurements for the management of chronic heart, renal, and parathyroid diseases.In the first approach, we have designed self-calibrating solid-contact ion-selective electrodes. Due to microheterogeneities and variabilities in multiple phases and interfaces of the indicator and reference electrodes, the absolute electromotive force (EMF) varies among identically fabricated and stored sensors. We create an EMF baseline that corrects for the EMF variabilities by connecting two electrodes using an appropriately formulated ionic conductor. Upon the introduction of a sample, such as a drop of blood, the EMF change relative to this baseline allows for ion measurements with high sensor-to-sensor consistency in the absence of any calibration protocols. Surprisingly, when the cross-sectional area, viscosity, and formulation of the ionic conductor are precisely controlled and the ionic conductor is insulated from the sample, its presence does not compromise the Nernstian response slope of the primary ion in the sample. In other words, this built-in ionic bridge serves for one-point calibration before the sample addition but does not need to be removed for the sample testing, representing a novel self-calibration strategy for electrochemical sensors.In the second approach, we have for the first time functionalized the oil stream of droplet microfluidics with sensing chemicals to convert oil segments to sensors for adjacent aqueous droplets. Ionophores, ion exchangers, and dyes are dissolved in the oil to form highly selective fluorescent ion sensors. Analyte ions from the aqueous droplets are extracted into the oil segments to alter their fluorescence and absorbance. The composition of all oil segments is highly consistent, as they are from the same homogenous solution, avoiding the need for any on-site calibration. As the optical property of the oil segment is measured in a fashion that is spatially and temporally separated from the aqueous sample, this biphasic sensing method works for complicated samples such as undiluted whole blood.In the third approach, we have designed a home-use sensor based on a calibration-free oil sensor contained in a capillary tube. Only one segment of sensing oil is used for analysis of one segment of sample of a few microliters. By continuously alternating the relative position of the oil and the sample controlled by a stepper motor, efficient mass transfer between two phases is obtained. The color of the oil segment is acquired and analyzed by a smartphone to indicate the electrolyte concentration in an aqueous sample. The accuracy and precision of an at-home calcium monitor in this format have been validated in > 50 human blood samples.

Evaluation of a Ru‐bipod Complex as a Redox Transducer for Membrane‐Based Voltammetry of Anions**

AbstractMolecular redox compounds show promise for the development of solid contact ion selective electrodes (SC‐ISEs) for anion determinations. In this work, a hydrophobic bis‐tridentate Ru‐bipod complex was studied as a molecular redox transducer in voltammetric SC‐ISEs. Thin film voltammetric behavior was achieved in films comprised of the Ru‐bipod complex, polyvinylchloride (PVC) and a plasticiser, nitrophenyloctylether (NPOE). Importantly, the choice of plasticiser had a significant effect on the SC‐ISE response of the Ru‐bipod complex; i. e., voltammetric responses of the Ru‐bipod complex in NPOE films were sharper and more intense than in bis(2‐ethylhexyl) sebacate (DOS) films. Moreover, addition of tetradodecylammonium tetrakis(4‐chlorophenyl)borate (ETH 500) to the membrane had an unfavorable impact on detection of anions due to oxidation of this salt in the film. Although the thin film electrode showed close‐to‐Nernstian behavior in anion responses, the detection responses in the presence of very hydrophilic anions such as sulfate ion were shifted to higher potentials (>1 V) with a subsequent loss of sensitivity. Overall, the Ru‐bipod complex operates as a simple transducer in SC‐ISEs with benefits in anion detection.

High-redox-capacity solid contact based on ferrocenyl self-assembled monolayer functionalized macroporous gold for an all-solid-state carbonate-selective electrode

Solid-contact ion-selective electrodes (ISEs) are a promising tool for direct detection of CO32– activity/concentration in seawater. Herein, a stable solid-contact CO32–-ISE was fabricated based on ferrocenyl self-assembled monolayer (SAM) functionalized macroporous gold (m-PG). Instead of the commonly used planar electrode, the m-PG film, prepared by using an electrochemical self-templating method involving the gold electrodeposition and hydrogen bubbling generation, is used as the conductive substrate. The redox capacity of the ferrocenyl SAM on the m-PG film is much larger than that on the planar Au electrode, due to the high surface area and good conductivity of m-PG. The solid-contact CO32–-ISE based on ferrocenyl SAM modified m-PG shows a Nernstian potential response in the activity range from 2.6 × 10-5 to 5.3 × 10-4 M with a slope of 28.3 ± 0.4 mV/dec and a detection limit of 1.1 × 10-5 M. The proposed electrode also exhibits a good electrode-to-electrode reproducibility with a standard variation of the standard potential (E0) of 1.5 mV (n = 7), an improved potential stability and a long lifetime up to at least 120 days. The strategy for electrode substrate transformation from planar gold substrate to porous gold substrate provides an alternative way to improve the redox capacities of ferrocenyl SAMs for preparing the stable and reliable solid-contact ISEs.

Carbon fiber-based multichannel solid-contact potentiometric ion sensors for real-time sweat electrolyte monitoring

Solid-contact ion-selective electrodes (SC-ISEs) feature miniaturization and integration that have gained extensive attention in non-invasive wearable sweat electrolyte sensors. The state-of-the-art wearable SC-ISEs mainly use polyethylene terephthalate, gold and carbon nanotube fibers as flexible substrates but suffer from uncomfortableness, high cost and biotoxicity. Herein, we report carbon fiber-based SC-ISEs to construct a four-channel wearable potentiometric sensor for sweat electrolytes monitoring (Na+/K+/pH/Cl−). The carbon fibers were extracted from commercial cloth, of which the starting point is addressing the cost and reproducibility issues for flexible SC-ISEs. The bare carbon fiber electrodes exhibited reversible voltammetric and stable impedance performances. Further fabricated SC-ISEs based on corresponding ion-selective membranes disclosed Nernstian sensitivity and anti-interface ability toward both ions and organic species in sweat. Significantly, these carbon fiber-based SC-ISEs revealed high reproducibility of standard potentials between normal and bending states. Finally, a textile-based sensor was integrated with a solid-contact reference electrode, which realized on-body sweat electrolytes analysis. The results displayed high accuracy compared with ex-situ tests by ion chromatography. This work highlights carbon fiber-based multichannel wearable potentiometric ion sensors with low cost, biocompatibility and reproducibility.

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.