Carrier-Based Ion-Selective Electrodes and Bulk Optodes. 1. General Characteristics

Towards sustainable and humane dairy farming: A low-cost electrochemical sensor for on-site diagnosis of milk fever

Milk fever is a metabolic disorder that predominantly affects dairy animals during the periparturient period and within four weeks of calving. Milk fever is primarily attributed to a decrease in the animal's serum Ca2+ levels. Clinical milk fever occurs when Ca2+ concentration drops below 1.5 mM (6 mg/dL). Without prompt intervention, clinical milk fever leads to noticeable physical symptoms and health complications including coma and fatality. Subclinical milk fever is characterized by Ca2+ levels between 1.5 and 2.12 mM (6-8.48 mg/dL). Approximately 50% of multiparous dairy cows suffer from subclinical milk fever during the transition to lactation. The economic impact of milk fever, both direct and indirect, is substantial, posing challenges for farmers. To address this issue, we developed a low-cost electrochemical sensor that can measure bovine serum calcium levels on-site, providing an opportunity for early detection of subclinical and clinical milk fever and early intervention. This calcium sensor is a scalable solid contact ion sensing platform that incorporates a polymeric calcium-selective membrane and ionic liquid-based reference membrane into laser-induced graphene (LIG) electrodes. Our sensing platform demonstrates a sensitivity close to the theoretical Nernstian value (29.6 mV/dec) with a limit of detection of 15.6 μM and selectivity against the species in bovine serum. Moreover, our sensor can detect Ca2+ in bovine serum with 91% recovery.

Persistent Luminescence Nanosensors: A Generalized Optode-Based Platform for Autofluorescence-Free Sensing in Biological Systems

Fluorescent nanosensors have revolutionized diagnostics and our ability to monitor cellular dynamics. Yet, distinguishing sensor signals from autofluorescence remains a challenge. Here, we merged optode-based sensing with near-infrared-emitting ZnGa2O4:Cr3+ persistent luminescence nanoparticles (PLNPs) to create nanocomposites for autofluorescence-free "glow-in-the-dark" sensing. Hydrophobic modification and incorporation of the persistent luminescence nanoparticles into an optode-based nanoparticle core yielded persistent luminescence nanosensors (PLNs) for five analytes (K+, Na+, Ca2+, pH, and O2) via two distinct mechanisms. We demonstrated the viability of the PLNs by quantifying K+ in fetal bovine serum, calibrating the pH PLNs in the same, and ratiometrically monitoring O2 metabolism in cultures of Saccharomyces cerevisiae, all the while overcoming their respective autofluorescence signatures. This highly modular platform allows for facile tuning of the sensing functionality, optical properties, and surface chemistry and promises high signal-to-noise ratios in complex optical environments.

Improvement of the Upper Detection Limit of Ionophore-Based H+-Selective Electrodes: Explanation and Elimination of Apparently Super-Nernstian Responses

The response range of an ion-selective electrode (ISE) has been described by counterion interference at the lower and Donnan failure at the upper detection limit. This approach fails when the potentiometric response at the upper detection limit exhibits an apparently super-Nernstian response, as has been reported repeatedly for H+-selective electrodes. While also observed when samples contain other anions, super-Nernstian responses at low pH are a problem in particular for samples that contain phthalate, a common component of commercial pH calibration solutions. This work shows that coextraction of H+ and a sample anion into the sensing membrane alone does not explain these super-Nernstian responses, even when membrane-internal diffusion potentials are taken into account. Instead, these super-Nernstian responses are explained by the formation of complexes between that anion and at least two protonated ionophore molecules. As demonstrated by experiments and explained with quantitative phase boundary models, the apparently super-Nernstian responses at low pH can be eliminated by restricting the molecular ratio of ionophore and ionic sites. Notably, this conclusion results in recommendations for the optimization of sensing membranes that, in some instances, will conflict with previously reported recommendations from the ionic site theory for the optimization of the lower detection limit. This mechanistic insight is key to maximizing the response range of these ionophore-based ISEs.

Highly Stable Solid Contact Calcium Ion-Selective Electrodes: Rapid Ion-Electron Transduction Triggered by Lipophilic Anions Participating in Redox Reactions of CunS Nanoflowers

Solid contact (SC) calcium ion-selective electrodes (Ca2+-ISEs) have been widely applied in the analysis of water quality and body fluids by virtue of the unique advantages of easy operation and rapid response. However, the potential drift during the long-term stability test hinders their further practical applications. Designing novel redox SC layers with large capacitance and high hydrophobicity is a promising approach to stabilize the potential stability, meanwhile, exploring the transduction mechanism is also of great guiding significance for the precise design of SC layer materials. Herein, flower-like copper sulfide (CunS-50) composed of nanosheets is meticulously designed as the redox SC layer by modification with the surfactant (CTAB). The CunS-50-based Ca2+-ISE (CunS-50/Ca2+-ISE) demonstrates a near-Nernstian slope of 28.23 mV/dec for Ca2+ in a wide activity linear range of 10-7 to 10-1 M, with a low detection limit of 3.16 × 10-8 M. CunS-50/Ca2+-ISE possesses an extremely low potential drift of only 1.23 ± 0.13 μV/h in the long-term potential stability test. Notably, X-ray absorption fine-structure (XAFS) spectra and electrochemical experiments are adopted to elucidate the transduction mechanism that the lipophilic anion (TFPB-) participates in the redox reaction of CunS-50 at the solid-solid interface of ion-selective membrane (ISM) and redox inorganic SC layer (CunS-50), thereby promoting the generation of free electrons to accelerate ion-electron transduction. This work provides an in-depth comprehension of the transduction mechanism of the potentiometric response and an effective strategy for designing redox materials of ion-electron transduction triggered by lipophilic anions.

Chronopotentiometric sensors for antimicrobial peptide-based biosensing of Staphylococcus aureus

Charged antimicrobial peptides can be used for direct potentiometric biosensing, but have never been explored. We report here a galvanostatically-controlled potentiometric sensor for antimicrobial peptide-based biosensing. Solid-state pulsed galvanostatic sensors that showed excellent stability under continuous galvanostatic polarization were prepared by utilizing reduced graphene oxide/poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (rGO/PEDOT: PSS) as a solid contact. More importantly, the chronopotentiometric sensor can be made sensitive to antimicrobial peptides with intrinsic charge on demand via a current pulse. In this study, a positively charged antimicrobial peptide that can bind to Staphylococcus aureus with high affinity and good selectivity was designed as a model. Two arginine residues with positive charges were linked to the C-terminal of the peptide sequence to increase its potentiometric responses on the electrode. The bacteria binding-induced charge or charge density change of the antimicrobial peptide enables the direct chronopotentiometric detection of the target. Under the optimized conditions, the concentration of Staphylococcus aureus can be determined in the linear range 10-1.0 × 105 CFU mL-1 with a detection limit of 10 CFU mL-1. It is anticipated that such a chronopotentiometric sensing platform is readily adaptable to detect other bacteria by choosing the peptides.

Fundamental investigation on fluorous nanoemulsion optodes: effect of matrix fluorination on selectivity

As fundamental investigation on fluorous nanoemulsion (NE) optodes for highly selective perfluorooctanesulfonate (PFOS-) sensing, the effect of matrix fluorination on selectivity was investigated. Due to the high hydrophobicity of PFOS- itself, it responded in exhaustive mode regardless of the fluorination ratio of the matrix, and the lowest detectable PFOS- concentration was on the order of 10-7 to 10-6 M. On the other hand, the response of non-fluorous interfering anions was suppressed as the fluorination ratio of the matrix increased. It was revealed that the relative selectivity of PFOS- for hydrophobic anions, ClO4-, SCN-, and 1-octanesulfonate (OS-) was improved by more than one order of magnitude, up to nearly two orders of magnitude, and that it was also improved by less than one order of magnitude for hydrophilic anions, Br-, Cl-, and SO4-, in logarithmic selectivity coefficient (log K PFOS - , j opt ).

Fully Integrated Multiplexed Wristwatch for Real-Time Monitoring of Electrolyte Ions in Sweat

Considerable progress has already been made in sweat sensors based on electrochemical methods to realize real-time monitoring of biomarkers. However, realizing long-term monitoring of multiple targets at the atomic level remains extremely challenging, in terms of designing stable solid contact (SC) interfaces and fully integrating multiple modules for large-scale applications of sweat sensors. Herein, a fully integrated wristwatch was designed using mass-manufactured sensor arrays based on hierarchical multilayer-pore cross-linked N-doped porous carbon coated by reduced graphene oxide (NPCs@rGO-950) microspheres with high hydrophobicity as core SC, and highly selective monitoring simultaneously for K+, Na+, and Ca2+ ions in human sweat was achieved, exhibiting near-Nernst responses almost without forming an interfacial water layer. Combined with computed tomography, solid-solid interface potential diffusion simulation results reveal extremely low interface diffusion potential and high interface capacitance (598 μF), ensuring the excellent potential stability, reversibility, repeatability, and selectivity of sensor arrays. The developed highly integrated-multiplexed wristwatch with multiple modules, including SC, sensor array, microfluidic chip, signal transduction, signal processing, and data visualization, achieved reliable real-time monitoring for K+, Na+, and Ca2+ ion concentrations in sweat. Ingenious material design, scalable sensor fabrication, and electrical integration of multimodule wearables lay the foundation for developing reliable sweat-sensing systems for health monitoring.

Wireless wearable potentiometric sensor for simultaneous determination of pH, sodium and potassium in human sweat

This paper reports on the development of a flexible-wearable potentiometric sensor for real-time monitoring of sodium ion (Na+), potassium ion (K+), and pH in human sweat. Na0.44MnO2, polyaniline, and K2Co[Fe(CN)6] were used as sensing materials for Na+, H+ and K+ monitoring, respectively. The simultaneous potentiometric Na+, K+, and pH sensing were carried out by the developed sensor, which enables signal collection and transmission in real-time to the smartphone via a Wi-Fi access point. Then, the potentiometric responses were evaluated by a designed android application. Na+, K+, and pH sensors illustrated high sensitivity (59.7 ± 0.8 mV/decade for Na+, 57.8 ± 0.9 mV/decade for K+, and 54.7 ± 0.6 mV/pH for pH), excellent stability, and good batch-to-batch reproducibility. The results of on-body experiments demonstrated that the proposed platform is capable of real-time monitoring of the investigated ions.

Water monitoring with an automated smart sensor supported with solar power for real-time and long range detection of ferrous iron

Low-power and smart sensing systems for iron detection are necessary for in situ monitoring of water quality. Here, a potentiometric Fe2+-selective electrode (ISE) was fabricated based on cyanomethyl N-methyl-N-phenyl dithiocarbamate for the first time as an ionophore. Under optimal conditions, the ISE showed a Nernstian slope of 29.76 ± 0.6 mV per decade for Fe2+ ions over a wide concentration range from 1.0 × 10-1 to 1.0 × 10-5 M with a lower detection limit (LOD) of 1.0 × 10-6 M. The ISE interference of various cations on the potentiometric response was also investigated. The ISE had a response time less than 3 s and the lifetime was two months. Also, an automated, long-range (LoRa), wireless enabled sampling microfluidic device powered with a solar panel as an autonomous power source was developed for a continuous sampling and sensing process. The sensing platform was employed in the determination of Fe2+ in acid mine drainage and spiked water samples with an average recovery of 100.7%. This simple, inexpensive (below $350), portable sensing platform will allow for rapid real-time monitoring of ground-, drinking-, and industrial waters contaminated with iron.

Increasing the Sensitivity of pH Glass Electrodes with Constant Potential Coulometry at Zero Current

It has recently become possible to increase the sensitivity of ion-selective electrodes (ISEs) by imposing a constant cell potential, allowing one to record current spikes with a capacitor placed in series in the circuit. The approach requires a transient current to pass through the measurement cell, which unfortunately may introduce measurement errors and additionally excludes the use of high-impedance indicator electrodes, such as pH glass electrodes. We present here an electronic circuit that overcomes these limitations, where the cell is measured at zero current in combination with a voltage follower, and the current spike and capacitor charging occur entirely within the instrument. The approach avoids the need for a counter electrode, and one may use any electrode useful in potentiometry regardless of its impedance. The characteristics of the circuit were found to approach ideality when evaluated with either an external potential source or an Ag/AgCl electrode. The current may be linearized and extrapolated to further reduce the measurement time. The circuit is further tested with the most common yet very challenging electrode, the pH glass electrode. A precision of 64 μpH was obtained for 0.01 pH change up to 0.05 from a reference solution. Similar pH changes were also measured reliably further away from the reference solution (0.5-0.55) and resulted in a precision of 377 μpH. The limitations of this experimental setup were explored by performing pH calibrations within the measuring range of the probe.

Noninvasive and Multiplex Self-Test of Kidney Disease Biomarkers with Graphene-Based Lab-on-a-Chip (G-LOC): Toward Digital Diagnostics in the Hands of Patients

Chronic kidney disease is one of the major health issues worldwide. However, diagnosis is now highly centralized in large laboratories, resulting in low access to patient monitoring and poor personalized treatments. This work reports the development of a graphene-based lab-on-a-chip (G-LOC) for the digital testing of renal function biomarkers in serum and saliva samples. G-LOC integrates multiple bioelectronic sensors with a microfluidic system that enables multiplex self-testing of urea, potassium, sodium, and chloride. The linearity, limit of detection (LOD), accuracy, and coefficient of variability (CV) were studied. Accuracy values higher than 95.5% and CV lower than 9% were obtained for all of the biomarkers. The analytical performance was compared against three reference lab benchtop analyzers by measuring healthy- and renal-failure-level samples of serum. From receiver operating characteristic (ROC) plots, sensitivities (%) of 99.7, 97.6, 99.1, and 89.0 were obtained for urea, potassium, sodium, and chloride, respectively. Then, the test was evaluated in noninvasive saliva samples and compared against reference methods. Correlation and Bland-Altman plots showed good correlation and agreement of the G-LOC with the reference methods. It is noteworthy that the precision of G-LOC was similar to better than benchtop lab analyzers, with the advantage of being highly portable. Finally, a user testing study was conducted. The analytical performance obtained with untrained volunteers was similar to that obtained with trained chemists. Additionally, based on a user experience survey, G-LOC was found to have very simple usability and would be suitable for at-home diagnostics.

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 ≈25 nm, and the nanostructure firm binding are achieved simultaneously. The resultant flexible freestanding hydrovoltaic device exhibits an open circuit voltage up to 4.82 V in deionized water, a wide ion sensing range of 10-7 to 100 m, and ultrahigh sensitivity as high as 1.37 V 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.

Transfer of Sodium Ion across Interface between Na+-Selective Electrode Membrane and Aqueous Electrolyte Solution: Can We Use Nernst Equation If Current Flows through Electrode?

Electrochemical impedance and chronopotentiometric measurements with Na+-selective solvent polymeric (PVC) membranes containing a neutral ionophore and a cation exchanger revealed low-frequency resistance, which is ascribed to Na+ ion transfer across the interface between the membrane and aqueous solution. The attribution is based on the observed regular dependence of this resistance on the concentration of Na+ in solutions. The respective values of the exchange current densities were found to be significantly larger than the currents flowing through ion-selective electrodes (ISEs) during an analysis in non-zero-current mode. This fact suggests that the interfacial electrochemical equilibrium is not violated by the current flow and implies that the Nernst equation can be applied to interpret the data obtained in non-zero-current mode, e.g., constant potential coulometry.

Quality-by-design ecofriendly potentiometric sensor for rapid monitoring of hydroxychloroquine purity in the presence of toxic impurities

Hydroxychloroquine (HCQ) is prescribed to treat malaria and certain autoimmune diseases. Recent studies questioned its efficiency in relieving COVID-19 symptoms and improving clinical outcomes. This work presents a quality-by-design approach to develop, optimize, and validate a potentiometric sensor for the selective analysis of HCQ in the presence of its toxic impurities (key starting materials), namely 4,7-Dichloroquinoline (DCQ) and hydroxynovaldiamine (HND). The study employed a custom experimental design of 16 sensors with different ion exchangers, plasticizers, and ionophores. We observed the Nernstian slopes, correlation coefficients, quantification limit, response time, and selectivity coefficient for DCQ and HND. The computer software constructed a prediction model for each response. The predicted responses strongly correlate to the experimental ones, indicating model fitness. The optimized sensor achieved 93.8% desirability. It proved a slope of 30.57 mV/decade, a correlation coefficient of 0.9931, a quantification limit of 1.07 × 10-6 M, a detection limit of 2.18 × 10-7 M, and a fast response of 6.5 s within the pH range of 2.5-8.5. The sensor was successfully used to determine HCQ purity in its raw material. The sensor represents a potential tool for rapid, sensitive, and selective monitoring of HCQ purity during industrial production from its starting materials.

Ion-Selective Potentiometry with Plasma-Initiated Covalent Attachment of Sensing Membranes onto Inert Polymeric Substrates and Carbon Solid Contacts

The physical delamination of the sensing membrane from underlying electrode bodies and electron conductors limits sensor lifetimes and long-term monitoring with ion-selective electrodes (ISEs). To address this problem, we developed two plasma-initiated graft polymerization methods that attach ionophore-doped polymethacrylate sensing membranes covalently to high-surface-area carbons that serve as the conducting solid contact as well as to polypropylene, poly(ethylene-co-tetrafluoroethylene), and polyurethane as the inert polymeric electrode body materials. The first strategy consists of depositing the precursor solution for the preparation of the sensing membranes onto the platform substrates with the solid contact carbon, followed by exposure to an argon plasma, which results in surface-grafting of the in situ polymerized sensing membrane. Using the second strategy, the polymeric platform substrate is pretreated with argon plasma and subsequently exposed to ambient oxygen, forming hydroperoxide groups on the surface. Those functionalities are then used for the initiation of photoinitiated graft polymerization of the sensing membrane. Attenuated total reflection-Fourier transform infrared spectroscopy, water contact angle measurements, and delamination tests confirm the covalent attachment of the in situ polymerized sensing membranes onto the polymeric substrates. Using membrane precursor solutions comprising, in addition to decyl methacrylate and a cross-linker, also 2-(diisopropylamino)ethyl methacrylate as a covalently attachable H+ ionophore and tetrakis(pentafluorophenyl)borate as ionic sites, both plasma-based fabrication methods produced electrodes that responded to pH in a Nernstian fashion, with the high selectivity expected for ionophore-based ISEs.

Polymeric membrane potentiometric antibiotic sensors using computer-aided screening of supramolecular macrocyclic carriers

Carrier-based polymeric membrane potentiometric sensors are an ideal tool for detecting ionic species. However, in the fabrication of these sensors, the screening of carriers still relies on empirical trial- and error-based optimization, which requires tedious and time-consuming experimental verification. In this work, computer-aided screening of carriers is applied in the preparation of polymeric membrane potentiometric sensors. Molecular docking is used to study the host-guest interactions between receptors and targets. Binding energies are employed as the standard to screen the appropriate carrier. As a proof-of-concept experiment, the antibiotic ciprofloxacin is selected as the target model. A series of supramolecular macrocyclic receptors including cyclodextrins, cucurbiturils and calixarenes are chosen as potential receptors. The proposed sensor based on the receptor calix[4]arene screened by molecular docking shows a lower detection limit of 0.5 μmol L-1 for ciprofloxacin. It can be expected that the proposed computer-aided screening technique of carriers can provide a simple but highly efficient method for the fabrication of carrier-based electrochemical and optical sensors.

Finding a perfect match of ion-exchanger and plasticizer for ion-selective sensors

Application of neutral ionophore based ion-selective sensors requires presence of ion-exchanger in the receptor phase, silently assuming that it is not only soluble but also dissociates to ions in the applied plasticizer. Although for typically applied ion-selective membrane constituents (plasticizers - ion-exchanger pairs) dissociation of ion-exchangers to ions is proven by theoretical (or close to) performance of resulting sensors, search for alternative plasticizers or ion-exchangers requires a method allowing estimation of the match of properties of involved compounds. In this work we propose a simple optical approach allowing estimation of ion-exchanger interactions with plasticizer. The results were confirmed by conductivity studies of model plasticizers solutions. The estimated dissociation constants of model ion-exchangers in plasticizers used are in excellent agreement with the results of optical studies. It was shown that solubility coupled with poor dissociation to ions of ion-exchanger affects performance of the resulting ion-selective membrane. Rational choice of properties of ion-exchanger and plasticizer allows finding a perfect match of the two, that results in improvements in performance of sensors (e.g. detection limits). As model sensors potassium and sodium ion-selective electrodes with poly(vinyl chloride) (PVC) based membranes, plasticized with classical plasticizer bis(2-ethylhexyl sebacate) or biodegradable alternative acetyl tributyl citrate, were prepared and studied using selected ion-exchangers.

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.

Toward the Ultimate Limit of Analyte Detection, in Graphene-Based Field-Effect Transistors

The ultimate sensitivity of field-effect-transistor (FET)-based devices for ionic species detection is of great interest, given that such devices are capable of monitoring single-electron-level modulations. It is shown here, from both theoretical and experimental perspectives, that for such ultimate limits to be approached the thermodynamic as well as kinetic characteristics of the (FET surface)-(linker)-(ion-receptor) ensemble must be considered. The sensitivity was probed in terms of optimal packing of the ensemble, through a minimal charge state/capacitance point of view and atomic force microscopy. Through the fine-tuning of the linker and receptor interaction with the sensing surface, a record limit of detection as well as specificity in the femtomolar range, orders of magnitude better than previously obtained and in excellent accord with prediction, was observed.

Effect of Ion Identity on Capacitance and Ion-to-Electron Transduction in Ion-Selective Electrodes with Nanographite and Carbon Nanotube Solid Contacts

The use of large surface area carbon materials as transducers in solid-contact ion-selective electrodes (ISEs) has become widespread. Desirable qualities of ISEs, such as a small long-term drift, have been associated with a high capacitance that arises from the formation of an electrical double layer at the interface of the large surface area carbon material and the ion-selective membrane. The capacitive properties of these ISEs have been observed using a variety of techniques, but the effects of the ions present in the ion-selective membrane on the measured value of the capacitance have not been studied in detail. Here, it is shown that changes in the size and concentration of the ions in the ion-selective membrane as well as the polarity of the polymeric matrix result in capacitances that can vary by up to several hundred percent. These data illustrate that the interpretation of comparatively small differences in capacitance for different types of solid contacts is not meaningful unless the composition of the ion-selective membrane is taken into account.

Expanding the library of sumanene molecular receptors for caesium-selective potentiometric sensors

This paper reports the synthesis and characterization of eight sumanene molecular receptors for the selective recognition of caesium cations (Cs+). The sumanene derivatives differed in the number (from one to nine), type (electron donating or electron withdrawing) and method of the attachment (functionalization of sumanene at the benzylic or aromatic carbons) of substituents in the sumanene skeleton. The ultimate goal of this work was to investigate the prospective use of various sumanene derivatives in the design of Cs+-selective potentiometric sensors, thus, expanding the library of sumanene receptors for such applications. Spectroscopic fluorescence titration with caesium hexafluorophosphate revealed that the formation of sandwich complexes is highly favourable, but the steric hindrance of bulky substituents can disrupt this preference. In the case of triaryl-substituted sumanene derivatives, theoretical calculations show that, indeed, sandwich complexes are energetically more advantageous by 2.3 times than 1 : 1 complexes. Furthermore, such functionalization significantly increases receptor solubility in the polymeric membrane of the potentiometric sensors, which was quantitatively evaluated with the COSMO model.

Integrated Microfluidic Chip Technology for Copper Ion Detection Using an All-Solid-State Ion-Selective Electrode

This study involved the preparation of an all-solid-state ion-selective electrode (ASS-ISE) with copper and a poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate (PEDOT/PSS) conversion layer through electrode deposition. The morphology of the PEDOT/PSS film was characterized, and the performance of the copper ion-selective film was optimized. Additionally, a microfluidic chip for the ASS-ISE with copper was designed and prepared. An integrated microfluidic chip test system with an ASS-ISE was developed using a self-constructed potential detection device. The accuracy of the system was validated through comparison testing with atomic absorption spectrophotometry (AAS). The experimental findings indicate that the relative standard deviation (RSD) of the integrated ASS-ISE with the copper microfluidic chip test system is 4.54%, as compared to the industry standard method. This value complies with the stipulated requirement of an RSD ≤ 5% in DL/T 955-2016.

Robust Potentiometric Microelectrodes for In Situ Sensing of Ion Fluxes with High Sensitivity

Simple, reproducible, and reliable preparation of robust potentiometric microelectrodes is both challenging and of great importance for noninvasive real-time ion sensing. Herein, we report a simple strategy for the large-scale synthesis of nickel cobalt sulfide (NiCo2S4) nanowire arrays grown on carbon fibers for potentiometric microelectrodes. The highly uniform NiCo2S4 nanowire array serving as a transduction layer can provide a high capillary pressure and viscous resistance for loading the ion sensing membrane and exhibit a large redox capacitance for improving the stability. An all-solid-state lead-selective microelectrode, which presents a detection limit of 2.5 × 10-8 M in the simulated soil solution, was designed as a model for noninvasive, in situ, and real-time detection of ion fluxes near the rice root surface. Importantly, the microsensor enables sensitive detection of trace-level ion-fluxes. This work provides a simple yet general strategy for designing potentiometric microelectrodes.

Modern Potentiometric Biosensing Based on Non-Equilibrium Measurement Techniques

Modern potentiometric sensors based on polymeric membrane ion-selective electrodes (ISEs) have achieved new breakthroughs in sensitivity, selectivity, and stability and have extended applications in environmental surveillance, medical diagnostics, and industrial analysis. Moreover, nonclassical potentiometry shows promise for many applications and opens up new opportunities for potentiometric biosensing. Here, we aim to provide a concept to summarize advances over the past decade in the development of potentiometric biosensors with polymeric membrane ISEs. This Concept article articulates sensing mechanisms based on non-equilibrium measurement techniques. In particular, we emphasize new trends in potentiometric biosensing based on attractive dynamic approaches. Representative examples are selected to illustrate key applications under zero-current conditions and stimulus-controlled modes. More importantly, fruitful information obtained from non-equilibrium measurements with dynamic responses can be useful for artificial intelligence (AI). The combination of ISEs with advanced AI techniques for effective data processing is also discussed. We hope that this Concept will illustrate the great possibilities offered by non-equilibrium measurement techniques and AI in potentiometric biosensing and encourage further innovations in this exciting field.

Ionophore-modified polyaniline-based optode for the determination of hydrogen sulfite levels in beverages, wastewater, and soil

Sulfite is a very important species, affecting human health, plant and animal life, and environmental sustainability. In this study, for the first time, an ionophore-based ion-selective optode was constructed for hydrogen sulfite determination in beverages, such as Birell® and Sprite®, water, and soil samples; instead of normal pH-chromoionophores, polyaniline film was precipitated on a glass slide and used for the transduction of the sensation mechanism. The ionophore-modified polyaniline-based optode incorporated thiourea derivative as an ionophore and tridodecyl methyl ammonium chloride as an ion-exchanger. The optode film was prepared in situ with a modified chemical polymerization method, and it was characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction (XRD); also, FTIR spectroscopy was performed for the film before and after interaction with hydrogen sulfite for mechanism elucidation. The optode was applied in the hydrogen sulfite concentration range of 10-1 to 10-5 M with a low detection limit of 8.0 × 10-6 M and minimum interference of other interfering species, such as salicylate, iodide, and sulphide. The response mechanism was due to the ion-exchange of hydrogen sulfite with the anion exchanger, followed by the molecular recognition between thiourea ionophore and hydrogen sulfite, with concomitant redox reaction via the protonation of the polyaniline that causes a decrease in absorbance at 685 nm. The optode was applied successfully for the determination of hydrogen sulfite in real beverages, Birell® and Sprite® without any pretreatment steps. Also, it was applied successfully for the environmental monitoring of hydrogen sulfite in real wastewater and soil samples.

A multi-ion interference decoupling model based on ion-selective electrode arrays

Compared to traditional liquid-junction ion-selective electrodes, solid-contact ion-selective electrodes (SC-ISEs) have attracted much attention and undergone rapid development due to their compactness and ease of integration. However, the application and widespread use of SC-ISEs are limited by their non-ideal selectivity and susceptibility to signal drift. Although the principles of artificial neural network (ANN) methods have shown significant progress in partially resolving the selectivity issue, they generally require extensive calibration steps and computational resources to implement. As a result, numerical computation models are more practical and economical, but existing approaches often overlook experimental phenomena and have relatively complex modeling principles. In this study, we propose a proportional factor model based on the trend of SC-ISEs affected by the multiple ions, along with a scalable dynamic correction procedure to improve its robustness. This model utilizes an estimated response surface method to solve nonlinear equations, requiring fewer calibration experiments. It accurately extracts the concentrations of multiple target ions in the presence of multi-ion interference and dynamically adjusts the model parameters for different types of ISEs. Additionally, we design a multi-channel SC-ISE array as a carrier for theoretical validation. In a case study, we demonstrate the feasibility of decoupling multiple ions using the SC-ISE array, obtaining concentrations of calcium, magnesium, sodium, and potassium ions, and verify the accuracy of the multi-ion detection system for real-world water samples.

Sensing and extraction of hazardous metals by di-phosphonates of heterocycles: a combined experimental and theoretical study

In this study, pyridine and phenanthroline diphosphonate ligands were investigated for the first time from the context of solvent extraction and potentiometric sensing of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II) cations. The extraction efficiency under the same conditions for phenanthroline-diphosphonates is considerably higher than that for pyridine ligands. At the same time, the pyridine-diphosphonates show pronounced selectivity towards lead in this metal series. The extraction systems with phenanthroline diphosphonates provided the most efficient extraction of Cd(II) and Pb(II) cations (D > 90). The newly developed pyridine and phenanthroline diphosphonate ligands have proven to be highly effective components in plasticized polymeric membranes. These ligands can be utilized to construct potentiometric ion sensors that exhibit a notable response specifically towards Pb(II) cations. Among the previously reported tetradentate ligands, the phenanthroline diphosphonate ligand, when incorporated into plasticized polymeric membranes, demonstrated the highest sensitivity towards d-metals and Pb(II). The structure of the single crystal complex of Pb(II) and Cd(II) with pyridine-diphosphonates was studied by X-ray diffraction analysis (XRD). The geometry of Cu(II), Zn(II), Cd(II) and Pb(II) complexes and the energy effect of the complex formation, including pseudo-oligomerization reactions, were determined by DFT calculations. The high sensing and extraction efficiency of diphosphonates with respect to Pb(II) is consistent with the minimum values of complex formation energies. The variation in sensory and extraction properties observed among the studied diphosphonate ligands is influenced by the ability to form polynuclear complexes with Pb(II) cations, whereas such properties are absent in the case of Cd(II) cations.

Response Patterns of Chromoionophore-Based Bulk Optodes Containing Lipophilic Electrolytes: Toward Background-Independent pH-Sensing

Despite a number of advantages of ion-selective optical sensors (optodes), their practical application is limited by their response mechanism, which leads to the dependence of the signal on the activity of two ions (analyte ion and the so-called reference ion) in the solution at the same time. Here, we show that the introduction of a lipophilic electrolyte into the polymeric optode membrane allows assessing the ionic activity of H+ cations regardless of the concentration of the background electrolyte containing a hydrophilic cation, with NaCl as an example of such an electrolyte. For the first time, the applicability of this approach is proven theoretically utilizing the numerical simulation of optode response. A correlation between the interfacial potential stability and the single-ion optical response is established. The predicted optical response is independent of background cation concentration to a significant extent. Theoretical conclusions are supported by experimental data obtained with chromoionophore-based optodes doped with various lipophilic electrolytes, including ionic liquids, by thin-film spectrophotometry and macrophotography coupled with digital color analysis. Most of the experimental sensor characteristics, such as the response range and its median, as well as its independence from the background electrolyte concentration are in quantitative agreement with the proposed theoretical description.

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+.

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).

Nanoelectrochemistry at liquid/liquid interfaces for analytical, biological, and material applications

Herein, we feature our recent efforts toward the development and application of nanoelectrochemistry at liquid/liquid interfaces, which are also known as interfaces between two immiscible electrolyte solutions (ITIES). Nanopipets, nanopores, and nanoemulsions are developed to create the nanoscale ITIES for the quantitative electrochemical measurement of ion transfer, electron transfer, and molecular transport across the interface. The nanoscale ITIES serves as an electrochemical nanosensor to enable the selective detection of various ions and molecules as well as high-resolution chemical imaging based on scanning electrochemical microscopy. The powerful nanoelectroanalytical methods will be useful for biological and material applications as illustrated by in situ studies of solid-state nanopores, nuclear pore complexes, living bacteria, and advanced nanoemulsions. These studies provide unprecedented insights into the chemical reactivity of important biological and material systems even at the single nanostructure level.

A Screen-Printed Potentiometric Sensor for Stability Indicating Assay and Real-Time Monitoring of Trospium Chloride Dissolution Profile in its Pharmaceutical Dosage Form

According to FDA guidance, a biowaiver concept declares that dissolution testing could be approved as a replacement strategy for bioequivalence studies and/or in vivo bioavailability. From the analytical chemistry standpoint, the shift from the classically developed offline methods to the highly integrated miniaturized inline analyzers is one of the pioneering ways that would modernize future of in-vitro - in-vivo correlation (IVIVC). The emergence of screen-printed electrodes (SPE) is now making the move from successive sampling steps and off-line measurements to real-time and in-line monitoring. Recently, “SPE” potentiometric sensor was presented as real-time analyzer that can offer similar analytical results as separation-based chromatographic techniques. Thus, the main objective of this paper is to design a real-time SPE for in situ monitoring of the dissolution of trospium chloride (TRO) in neutral media. Validation of the proposed sensor was performed according to the IUPAC commendations. The measurements performed with this sensor showed an accuracy of average recovery 100.50% and standard deviation of less than 1.0%, also the repeatability and intermediate electrode variabilities were less than 1.0 and 1.3%, respectively. The developed sensor was successfully used for direct observation of the dissolution profile without any need for an extraction step or sample preparation.

Design of screen-printed potentiometric platform for sensitive determination of mirabegron in spiked human plasma; molecular docking and transducer optimization

The integration of molecular modelling simulation and electrochemical sensors is of high interest. Herein, for the first time, a portable solid-contact potentiometric electrode was designed for the sensitive determination of mirabegron (MIR) in human plasma and pharmaceutical formulation. A two-step optimization protocol was investigated for the fabrication of an ion on sensing polymeric membrane. First, molecular docking was used for optimum ionophore selection. Calix[6]arene showed the highest affinity towards MIR with a better docking score (-4.35) and potential energy (-65.23) compared to other calixarene derivatives. Second, carbon nanotubes and gold nanoparticles were investigated as ion-electron transducers using a drop-casting procedure. Gold nanoparticle-based sensors showed better slope, potential stability, and rapid response compared to carbon nanotubes. The proposed solid contact sensors (V-VII) showed comparable sensitivity and ease of handling compared to liquid contact sensors (I-IV). The optimized gold nanoparticles sensor VII produced a Nernstian response over the range of 9.77 × 10-7 to 1 × 10-3 M with LOD of 2.4 × 10-7 M. It has also been used to determine MIR in its pharmaceutical formulation in the presence of a co-formulated antioxidant butylated hydroxytoluene and spiked human plasma. This would offer a feasible and economic platform for monitoring MIR in pharmaceutical preparation and biological fluids.

A Review of the Carbon-Based Solid Transducing Layer for Ion-Selective Electrodes

As one of the key components of solid-contact ion-selective electrodes (SC-ISEs), the SC layer plays a crucial role in electrode performance. Carbon materials, known for their efficient ion-electron signal conversion, chemical stability, and low cost, are considered ideal materials for solid-state transducing layers. In this review, the application of different types of carbon materials in SC-ISEs (from 2007 to 2023) has been comprehensively summarized and discussed. Representative carbon-based materials for the fabrication of SC-ISEs have been systematically outlined, and the influence of the structural characteristics of carbon materials on achieving excellent performance has been emphasized. Finally, the persistent challenges and potential opportunities are also highlighted and discussed, aiming to inspire the design and fabrication of next-generation SC-ISEs with multifunctional composite carbon materials in the future.

Repurposing Polyether Ionophores as a New-Class of Anti-SARS-Cov-2 Agents as Adjunct Therapy

The emergence of SARS-CoV-2 and its variants have posed a significant threat to humankind in tackling the viral spread. Furthermore, currently repurposed drugs and frontline antiviral agents have failed to cure severe ongoing infections effectively. This insufficiency has fuelled research for potent and safe therapeutic agents to treat COVID-19. Nonetheless, various vaccine candidates have displayed a differential efficacy and need for repetitive dosing. The FDA-approved polyether ionophore veterinary antibiotic for treating coccidiosis has been repurposed for treating SARS-CoV-2 infection (as shown by both in vitro and in vivo studies) and other deadly human viruses. Based on selectivity index values, ionophores display therapeutic effects at sub-nanomolar concentrations and exhibit selective killing ability. They act on different viral targets (structural and non-structural proteins), host-cell components leading to SARS-CoV-2 inhibition, and their activity is further enhanced by Zn²⁺ supplementation. This review summarizes the anti-SARS-CoV-2 potential and molecular viral targets of selective ionophores like monensin, salinomycin, maduramicin, CP-80,219, nanchangmycin, narasin, X-206 and valinomycin. Ionophore combinations with Zn²⁺ are a new therapeutic strategy that warrants further investigation for possible human benefits.

Mechanistic Assessment of Metabolic Interaction between Single Oral Commensal Cells by Scanning Electrochemical Microscopy

The human oral microbiome heavily influences the status of oral and systemic diseases through different microbial compositions and complex signaling between microbes. Recent evidence suggests that investigation of interactions between oral microbes can be utilized to understand how stable communities are maintained and how they may preserve health. Herein, we investigate two highly abundant species in the human supragingival plaque, Streptococcus mitis and Corynebacterium matruchotii, to elucidate their real-time chemical communication in commensal harmony. Specifically, we apply nanoscale scanning electrochemical microscopy (SECM) using a submicropipet-supported interface between two immiscible electrolyte solutions as an SECM probe not only to image the permeability of S. mitis and C. matruchotii membranes to tetraethylammonium (TEA⁺) probe ions but also to real-time visualize the metabolic interaction between two microbes via lactate production/consumption at a single-cell level. The metabolic relationship between two strains is quantitatively assessed by determining (1) the passive permeability of both bacterial membranes of 2.4 × 10^-4 cm/s to the free diffusion of TEA⁺, (2) 0.5 mM of the lactate concentration produced by a single S. mitis strain at a rate of 2.7 × 10^-4 cm/s, and (3) a lactate oxidation rate ≥5.0 × 10⁶ s^-1 by an individual C. matruchotii strain. Significantly, this study, for the first time, describes a mechanism of in situ metabolic interaction between oral commensals at the single-cell level through quantitative analysis, which supports the observed in vivo spatial arrangements of these microbes.

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.

Hofmeister Effects Shine in Nanoscience

Hofmeister effects play a crucial role in nanoscience by affecting the physicochemical and biochemical processes. Thus far, numerous wonderful applications from various aspects of nanoscience have been developed based on the mechanism of Hofmeister effects, such as hydrogel/aerogel engineering, battery design, nanosynthesis, nanomotors, ion sensors, supramolecular chemistry, colloid and interface science, nanomedicine, and transport behaviors, etc. In this review, for the first time, the progress of applying Hofmeister effects is systematically introduced and summarized in nanoscience. It is aimed to provide a comprehensive guideline for future researchers to design more useful Hofmeister effects-based nanosystems.

Detection of Interfering Ions Using Ion Flux Phenomena in Flow-Through Cl-ISEs with Ion Exchange Membranes

Ion-selective electrodes (ISEs) are among the most successful electrochemical sensors used in various applications because of their ability to measure electrolyte concentrations in liquids easily. It is common practice to suppress ion fluxes through the ion-sensitive membranes in ISEs because such fluxes worsen the lower limit of detection. In this study, we propose a method to detect interfering ions using this ion flux phenomenon. As a proof of principle, a flow-type Cl-ISE based on an ion exchange membrane loaded with the target ion chloride was used to acquire transient potential profiles during standstill after the introduction of liquids containing various ion species. When the target ion of the ion-sensitive membrane was measured, there was almost no change in potential over time. In contrast, when hydrophilic interfering ions were measured, the potential gradually decreased, and when hydrophobic interfering ions were measured, the potential gradually increased. The direction and intensity of these changes over time depended on the ion species and concentrations. The main reason for these potential changes is presumed to be the change in the local ionic composition of the sample near the sensing membrane due to ion exchange between the sample and membrane. This phenomenon could not be observed in a hydrophobic ion exchanger membrane doped with a quaternary ammonium salt and was characteristically observable using hydrophilic ion exchange membranes with a high charge density and a high ion diffusion rate. Finally, using a high-throughput flow-type system, we demonstrated the detection of interfering ions in solutions containing multiple ion species by using the ion flux phenomenon.

Skin-Interfaced Wearable Sweat Sensors for Precision Medicine

Wearable sensors hold great potential in empowering personalized health monitoring, predictive analytics, and timely intervention toward personalized healthcare. Advances in flexible electronics, materials science, and electrochemistry have spurred the development of wearable sweat sensors that enable the continuous and noninvasive screening of analytes indicative of health status. Existing major challenges in wearable sensors include: improving the sweat extraction and sweat sensing capabilities, improving the form factor of the wearable device for minimal discomfort and reliable measurements when worn, and understanding the clinical value of sweat analytes toward biomarker discovery. This review provides a comprehensive review of wearable sweat sensors and outlines state-of-the-art technologies and research that strive to bridge these gaps. The physiology of sweat, materials, biosensing mechanisms and advances, and approaches for sweat induction and sampling are introduced. Additionally, design considerations for the system-level development of wearable sweat sensing devices, spanning from strategies for prolonged sweat extraction to efficient powering of wearables, are discussed. Furthermore, the applications, data analytics, commercialization efforts, challenges, and prospects of wearable sweat sensors for precision medicine are discussed.

Nanoemulsion-based silver ion-selective optode based on colorimetrically silver ion-responsive ionic liquid-based dye

In this study, we describe the fast-responsive nanoemulsion (NE)-based silver ion (Ag⁺)-selective optode based on colorimetrically silver ion-responsive ionic liquid-based dye (ILD). The ILD comprises purely functional sensing molecules, a protonated cationic merocyanine dye (KD-M13-H⁺) and an anionic Ag⁺ ionophore (BDM-SO₃^-), and thus, it can be used for highly sensitive silver ion (Ag⁺) sensing due to the extremely high content of dye in the organic phase (ionic-liquid phase). However, during the Ag⁺ sensing, the cationic merocyanine dye is converted into electrically neutral form by deprotonation of the dye, which leads to the conversion of liquified dye into solid form in the organic phase, which makes the response time slower when ILD is used for poly(vinyl chloride) (PVC) membrane-based ion-selective optode, especially for sensing of high Ag⁺ concentration. To solve this problem, we focused on the use of the nano-emulsification technique. The response time of the ILD-based nanoemulsion (NE) was considerably shorter (1 s) compared to that of the ILD-based PVC membrane (a few minutes) owing to the large surface area and excellent diffusivity of the emulsion. The ILD-based NE contained a very high dye concentration (833 mmol kg^-1) and exhibited approximately 12 times higher sensitivity than that of the plasticizer-based conventional NE. In the cation measurements, the ILD-based NE responded to Ag⁺ via a cation-exchange mechanism and demonstrated a highly selective response to Ag⁺ (log [Formula: see text] = - 3.0). ILD-NE was successfully applied to the detection of spiked Ag⁺ in a tap water sample with recoveries of 98 - 103% with a relative standard deviation (RSD) of less than 5%. In comparison with NE based on non-ionic ionophores without charge, NE based on BDM-SO₃^- responded to lower Ag⁺ concentrations owing to the effect of negative charge on the binding property. The novel ILD-based NE was capable of highly sensitive, rapid, and selective Ag⁺ sensing, providing potential for analytical devices applicable to high-performance on-site analysis.

Quantification of underivatized amino acids in solid beverages using high-performance liquid chromatography and a potentiometric detector

The simultaneous quantification of amino acids (AAs) in solid beverages without prior derivatization was explored by high-performance liquid chromatography (HPLC) coupled to a potentiometric detector. Included were threonine, leucine, methionine, phenylalanine, and histidine. The potentiometric detector was made consisting of a copper(II)-selective electrode based on a polyvinyl chloride (PVC) membrane, and the potential changes in the detector were determined according to the coordination interactions between cupric copper ions released from the inner filling solution of the electrode and AAs. Conditions were optimized for effective separation and sensitive detection. Fundamental characteristics such as linearity, limits of detection, limits of quantitation, accuracy, precision, and robustness were validated experimentally. The calibration curves showed a linear relationship between peak heights and the injection concentrations of the AAs. The detection limits down to the sub-micromolar range were achieved under isocratic conditions, outperforming ultraviolet detection. The copper(II)-selective electrode had a minimum lifetime of one month. Some real samples were examined to further demonstrate the feasibility of the proposed approach. The measurement results obtained by the present method were in good agreement with those obtained by the HPLC-mass spectrometry (MS), indicating that the combined HPLC-potentiometric method is a potential option for quantifying AAs.

Development and Optimization of a Highly Sensitive Sensor to Quinine-Based Saltiness Enhancement Effect

The saltiness enhancement effect can be produced by adding specific substances to dietary salt (sodium chloride). This effect has been used in salt-reduced food to help people forge healthy eating habits. Therefore, it is necessary to objectively evaluate the saltiness of food based on this effect. In a previous study, sensor electrodes based on lipid/polymer membrane with Na⁺ ionophore have been proposed to quantify the saltiness enhanced by branched-chain amino acids (BCAAs), citric acid, and tartaric acid. In this study, we developed a new saltiness sensor with the lipid/polymer membrane to quantify the saltiness enhancement effect of quinine by replacing a lipid that caused an unexpected initial drop in the previous study with another new lipid. As a result, the concentrations of lipid and ionophore were optimized to produce an expected response. Logarithmic responses have been found on both NaCl samples and quinine-added NaCl samples. The findings indicate the usage of lipid/polymer membranes on novel taste sensors to evaluate the saltiness enhancement effect accurately.

Rapid and Accurate Measurement of the Na+/K+ Balance in Urine for Remote Patient Monitoring Using a Symmetric Electrode Architecture

The daily monitoring of the electrolyte balance in human bodies is challenging due to the lack of a low-cost, user-friendly device that can be used remotely by patients. In this study, a potentiometric sensor for the measurement of the ratio of Na⁺ and K⁺ ions has been fabricated using a solid-state carbon-based internal layer. PVC-type membranes are deposited using an autodispenser with a fabrication rate of approximately one sensor every 10 s, allowing mass production with limited investments. The symmetric architecture of the sensor unit, built without a reference electrode, permits a very fast stabilization of the signal, under 20 s, even without preconditioning. Measurements using buffer solutions in the range Na⁺/K⁺ = 1 to Na⁺/K⁺ = 10 indicate that less than 2 min, including the single-point calibration, is necessary to provide an estimation of the Na⁺/K⁺ balance with the same accuracy as that of a conventional sensing unit (±10%). Five sensors have been tested repeatedly over 30 days, and they maintained a constant level of performance regarding membrane sensitivity and response time. A remote measurement for the ratio between Na⁺ and K⁺ ions in urine samples showed results in agreement with a commercially available sensor. This sensor design could create new perspectives in remote healthcare for the quick detection of several diseases related to electrolyte balance.

Antifouling improvement in Pb2+ ion selective electrodes by using an environmentally friendly capsaicin derivative

Biofouling is a critical issue for ion selective electrodes (ISE) in complex aqueous systems, seriously compromising the analytical performance of the electrodes (i.e., stability, sensitivity, and lifetime). Herein, an antifouling solid lead ion selective electrode (GC/PANI-PFOA/Pb²⁺-PISM) was successfully prepared by adding propyl 2-(acrylamidomethyl)-3,4,5-trihydroxy benzoate (PAMTB), an environmentally friendly capsaicin derivative, into the ion-selective membrane (ISM). The presence of PAMTB caused no loss in the detection performance of GC/PANI-PFOA/Pb²⁺-PISM (e.g., detection limit (1.9 × 10^-7 M), response slope (28.5 ± 0.8 mV/decade), the response time (20 s), stability (8.6 ± 2.9 μV/s), selectivity and no water layer), whilst imparting an excellent antifouling effect with an antibacterial rate of 98.1% when the content of PAMTB in the ISM was 2.5 wt%. Further, GC/PANI-PFOA/Pb²⁺-PISM maintained stable antifouling properties, excellent potential response, and stability even after soaking in a high-concentration bacterial suspension for 7 days.

Nanoscale Carbonate Ion-Selective Amperometric/Voltammetric Probes Based on Ion-Ionophore Recognition at the Organic/Water Interface: Hidden Pieces of the Puzzle in the Nanoscale Phase

Here, we report on the successful demonstration and application of carbonate (CO₃^2-) ion-selective amperometric/voltammetric nanoprobes based on facilitated ion transfer (IT) at the nanoscale interface between two immiscible electrolyte solutions. This electrochemical study reveals critical factors to govern CO₃^2--selective nanoprobes using broadly available Simon-type ionophores forming a covalent bond with CO₃^2-, i.e., slow dissolution of lipophilic ionophores in the organic phase, activation of hydrated ionophores, peculiar solubility of a hydrated ion-ionophore complex near the interface, and cleanness at the nanoscale interface. These factors are experimentally confirmed by nanopipet voltammetry, where a facilitated CO₃^2- IT is studied with a nanopipet filled with an organic phase containing the trifluoroacetophenone derivative CO₃^2-ionophore (CO₃^2-ionophore VII) by voltammetrically and amperometrically sensing CO₃^2- in water. Theoretical assessments of reproducible voltammetric data confirm that the dynamics of CO₃^2- ionophore VII-facilitated ITs (FITs) follows the one-step electrochemical (E) mechanism controlled by both water-finger formation/dissociation and ion-ionophore complexation/dissociation during interfacial ITs. The yielded rate constant, k⁰ = 0.048 cm/s, is very similar to the reported values of other FIT reactions using ionophores forming non-covalent bonds with ions, implying that a weak binding between CO₃^2- ion-ionophore enables us to observe FITs by fast nanopipet voltammetry regardless of the nature of bondings between the ion and ionophore. The analytical utility of CO₃^2--selective amperometric nanoprobes is further demonstrated by measuring the CO₃^2- concentration produced by metal-reducing bacteria Shewanella oneidensis MR-1 as a result of organic fuel oxidation in bacterial growth media in the presence of various interferents such as H₂PO₄^-, Cl^-, and SO₄^2-.

Novel solid-contact ion-selective electrode based on a polyaniline transducer layer for determination of alcaftadine in biological fluid

Fabrication of a novel ion selective electrode for determining alcaftadine was achieved. The glassy carbon electrode (GCE) was utilized as a substrate in fabrication of an electrochemical sensor containing polyaniline (PANI) as an ion-to-electron transducer layer. A PVC polymeric matrix and nitrophenyl-octyl-ether were employed in designing the ion-sensing membrane (ISM). Potential stability was improved and minimization of electrical signal drift was achieved for inhibition of water layer formation at the electrode interface. Potential stability was achieved by inclusion of PANI between the electronic substrate and the ion-sensing membrane. The sensor's performance was evaluated following IUPAC recommendations. The sensor dynamic linear range was from 1.0 × 10^-2 to 1.0 × 10^-6 mol L^-1 and it had a 6.3 × 10^-7 mol L^-1 detection limit. The selectivity and capabilities of the formed alcaftadine sensor were tested in the presence of its pharmaceutical formulation excipients as well as its degradation products. Additionally, the sensor was capable of quantifying the studied drug in a rabbit aqueous humor. Method's greenness profile was evaluated by the means of Analytical Greenness (AGREE) metric assessment tool.

A sprayed graphene transistor platform for rapid and low-cost chemical sensing

We demonstrate a novel and versatile sensing platform, based on electrolyte-gated graphene field-effect transistors, for easy, low-cost and scalable production of chemical sensor test strips. The Lab-on-PCB platform is enabled by low-boiling, low-surface-tension sprayable graphene ink deposited on a substrate manufactured using a commercial printed circuit board process. We demonstrate the versatility of the platform by sensing pH and Na⁺ concentrations in an aqueous solution, achieving a sensitivity of 143 ± 4 μA per pH and 131 ± 5 μA per log₁₀Na⁺, respectively, in line with state-of-the-art graphene chemical sensing performance.

Home-Based Electrochemical Rapid Sensor (HERS): A Diagnostic Tool for Bacterial Vaginosis

Bacterial vaginosis (BV) is the most frequently occurring vaginal infection worldwide, yet it remains significantly underdiagnosed as a majority of patients are asymptomatic. Untreated BV poses a serious threat as it increases one's risk of STI acquisition, pregnancy complications, and infertility. We aim to minimize these risks by creating a low-cost disposable sensor for at-home BV diagnosis. A clinical diagnosis of BV is most commonly made according to the Amsel criteria. In this method, a fish-like odor, caused by increased levels of trimethylamine (TMA) in vaginal fluid, is used as a key diagnostic. This paper outlines the development of a Home-Based Electrochemical Rapid Sensor (HERS), capable of detecting TMA in simulated vaginal fluid (sVF). Instead of odor-based detection of volatilized TMA, we identify TMA in trimethylammonium form by utilizing HERS and a potentiometric readout. We fabricated the ion selective electrode using a carbon-black-coated cotton string and a TMA-selective membrane consisting of calix[4]arene and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. When paired with a standard reference electrode, our device was able to quantify TMA concentration in deionized (DI) water, as well as sVF samples at multiple pH levels with a clinically relevant limit of detection (8.66 µM, and theoretically expected Nernstian slope of 55.14 mV/decade).

Novel (E)−2-((1-(thiophen-2-yl)ethylidene)-amino) phenol Manganese(II) as an Ionophore Based on Thiocyanate-Selective Electrodes and Its Applications

Novel highly selective potentiometric sensor based on aquadichloro(E)−2-((1-(thiophen-2-yl)ethylidene)-amino)phenol manganese(II)trihydrate as an anion carrier. The thiocayanate electrode displayed a very high selectivity compared with others inorganic anions. Different sensors with plasticized PVC membranes have been investigated. The sensors construction containing different amounts of ionophore with and without additives. The pH over 3.5–6.5 range has been studied. Optimized membrane electrode included 66 mg PVC, 132 mg o-nitrophenyloctylether, 10 mol % tetrakis(trifluoromethyl)phenyl borate and 2% [Mn(C12H11NOS)(Cl2)(H2O)]· 3H2O. The optimized sensors exhibit Nernstian response for thiocyanate through a linear concentration ranging from (5 × 10−8 to 9.06 × 10−1 M) with a detection limit of 3 × 10−8 M and a slope of −57.7 mV decade−1, the measurement carried out in acetate buffer pH 4.7. The response time of electrode <10 s and the lifetime of the sensor more than 6 weeks. The proposed electrode was effectively utilized to estimation of thiocyanate in saliva sample, the results revealed a valid agreement with reference colorimetric method.