Ion-selective-membrane-free high-pressure potentiometric ammonium ion sensing

The state-of-the-art solid-contact ion-selective electrodes (SC-ISEs) for NH4+ primarily utilize organic carrier-based ion-selective membranes (ISM). However, they face challenges such as the water-layer effect at the SC/ISM interface and the weak mechanical strength of the ISM. In this work, we present an ISM-free, high-pressure potentiometric NH4+ sensor based on a bifunctional transducer, specifically a framework of copper hexacyanoferrate (CuHCF). CuHCF serves as both an ion-to-electron transducer and an NH4+ recognition element. The sensing mechanism involves electron transfer from the Fe redox center coupled with the ion transfer of NH4+ within its framework channels. To further develop an all-solid-state sensor, we integrated a solid-contact reference electrode of silver/silver tetraphenylborate electrode. This all-solid-state NH4+ sensor demonstrates Nernstian response sensitivity and comparable selectivity under 1 MPa pressure. Importantly, it avoids the generation of a water layer and exhibits long-term stability. This work highlights a concept for ISM-free high-pressure potentiometric NH4+ sensing.

Optimized detection of calcium ion in serum using constant potential coulometry with metastable liquid-liquid contact doping enhanced PEDOT: PSS ink

Highly stable calcium ion selective electrodes (Ca2+-ISEs) were developed by drop-casting a layer of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) as an ion-to-electron transfer layer onto Au electrode. The conductive PEDOT: PSS ink was prepared using a metastable liquid-liquid contact (MLLC) doping method, which induced phase separation, removed excess PSS, and significantly enhanced charge transfer kinetics and conductivity. The resulting Ca2+-ISEs exhibited excellent electrochemical performance. Potentiometric studies revealed a significant sensitivity of 33.1 ± 0.98 mV/decade (N = 3) with a high potential stability of 3.16 ± 2.53 μV/h. Importantly, Ca2+-ISEs combined with the constant potential coulometry method, the lower detection limit was optimized to 8.527 × 10-8 M (LOD = 3σ/s, N = 3). The performance of the Ca2+-ISE system was evaluated in inactivated fetal bovine serum using constant potential coulometry, demonstrating the highest measurement accuracy compared to potentiometric and chronoamperometric. The enhanced PEDOT: PSS-MLLC based Ca2+-ISEs combined with the constant potential coulometry method developed in this research demonstrate considerable potential for clinical applications in blood ion analysis.

A microneedle sensor for in-vivo sodium ion detection in plants

This study introduces a novel microneedle-type potentiometric sensor designed for the in-vivo detection of sodium ions (Na+) in plant tissues. The development of this sensor is crucial for advancing our understanding of plant responses to salinity stress. The microneedle sensor employs a highly selective Na+ ion carrier and integrates a solid-contact layer made of poly(3,4-ethylenedioxythiophene)-poly (sodium 4-styrenesulfonate) (PEDOT: PSS) prepared by electropolymerization. Due to its excellent conductivity and high chemical stability, PEDOT:PSS significantly reduces the surface impedance of the electrode, enhances charge transfer efficiency, and thereby improves the sensor's response sensitivity and stability. The sensor achieves a linear detection range of 1 × 10-2 to 1 × 10-5 M, with a slope of 56.55 ± 0.25 mV/decade and a detection limit of 1.94 × 10-6 M. The fabrication process was optimized by refining the membrane formulation, ensuring precise control over membrane thickness, and determining the optimal conditioning time, all essential for large-scale production and agricultural applications. In addition, we evaluated the sensor's ability to detect Na+ concentration changes in both artificial culture media and actual plant tissue samples. The sensor's performance was assessed through its capability to monitor Na+ concentration changes in both artificial culture media and real plant tissue samples, with results benchmarked against the standard method (ICP-OES), confirming its accuracy and reliability. Moreover, application trials involving rice seedlings validated the microneedle sensor's efficacy for in vivo detection of Na+, providing a robust tool for understanding plant physiological responses to salt stress. These findings not only offer new insights into plant adaptation mechanisms but also establish a practical platform for selecting salt-tolerant cultivars and enabling rapid salt-level assessment in agricultural practices.

New trends in potentiometric sensors: From design to clinical and biomedical applications

Potentiometry, a well-established electrochemical technique, provides a powerful and versatile method for the sensitive and selective measurement of a variety of analytes by measuring the potential difference between two electrodes, allowing for a direct and rapid readout of ion concentrations. This makes it a valuable tool in a variety of applications including industry, agriculture, forensics, medical, environmental assessment, and pharmaceutical drug analysis, therefore it has received significant attention from the scientific community. Their broad implementation in sensing applications arises through their many benefits, including ease of design, fabrication, and modification; rapid response time; high selectivity; suitability for use with colored and/or turbid solutions; and potential for integration into embedded systems interfaces. Owing to these advantages and diverse applicability, sustained research and development in the field has resulted in the emergence of several notable trends in the field. 3D printing is the most recent technique used in potentiometry which offers many benefits such as improved flexibility and precision in the manufacturing of ion-selective electrodes and rapid prototyping decreases the time needed during optimization of important electrochemical parameters. Additionally, paper-based sensors are cost-effective and versatile platforms for in-field (point-of-care, POC) analysis, permitting rapid determination of a variety of analytes. One of the most interesting applications of potentiometry are wearable sensors which allow for the continuous monitoring of biomarkers, electrolytes and even pharmaceuticals, especially those with a narrow therapeutic index. Herein this review, we discuss several recent trends in potentiometric sensors since 2010, including 3D printing, paper-based devices, and other emerging techniques and the translation of potentiometric systems to wearable devices for the determination of ionic species or pharmaceuticals in biological fluids paving the way to various clinical and biomedical uses.

Ion-Selective Electrodes: Selectivity Coefficients for Interfering Ions of the Opposite Charge Sign

The way upper limits of detection (LODs) are typically reported in the ion-selective electrode (ISE) literature is unfortunately outdated. It is well understood that the upper LOD of a polymeric-membrane ISE is limited by Donnan failure, that is, the transfer of primary ions along with interfering ions of the opposite charge sign (commonly referred to as counterions) from the sample into the sensing membrane. However, it is often difficult to compare upper LODs for ISEs from different sources. The majority of publications on ISEs describe Donnan failure for one type of counterion only, making it impossible for end users to predict the interference for other counterions. Moreover, linear ranges for ISEs based on different ionophores cannot be compared to one another when Donnan failure was reported for different counterions. To this end, we introduce here selectivity coefficients, KI,XpotX, for interfering counterions. Using this new concept, the primary ion activity at which Donnan failure occurs can be readily predicted from measured KI,XpotX values by the use of the uncomplicated expression aXzI/zx/KI,XpotX. Consistent with the intuition that many ISE users have for conventional selectivity coefficients, large KI,XpotX values are characteristic for counterions that interfere strongly. We show experimentally that trends as predicted by the phase boundary model for Donnan failure, such as the effects of counterion hydrophobicity and ionophore complex stability, are often accurately predicted with the KI,XpotX approach. However, there are notable exceptions when the underlying assumptions made by users do not apply, such as when counterions unexpectedly form aggregates with other species in the sensing membranes. The empirically measured KI,XpotX coefficients enable the discovery of such phenomena, opening a rational path to improving upper LODs and, thereby, linear response ranges.

High-Reproducibility and -Stability All-Solid-Contact Nitrate Ion-Selective Electrode with CoWSe2 as Solid Contact for Nitrate Monitoring in Wetland Soil

The monitoring of nitrate ions is of great significance for human health, agricultural development, and environmental protection. All-solid-state nitrate ion-selective electrodes (ASS-NO3--ISEs), as an important NO3- analysis method, still have two challenges of poor stability and reproducibility due to the ill-defined phase boundary between the solid-contact (SC) layer and the ion-selective membrane (ISM). In this work, a novel strategy for constructing the ASS-NO3--ISEs based on CoWO4, CoWS4, CoWSe2, or CoSe2 as SC layers was reported for improving the stability and reproducibility. The result shows that the developed CoWSe2-based NO3--ISE exhibits a good Nernstian response slope of -61.9 ± 0.4 mV dec-1 in the activity range from 1.0 × 10-6 to 7.5 × 10-2 M and a detection limit of 1.0 × 10-6 M. A good long-term stability (as low as 2.3 ± 0.4 μV h-1) of the CoWSe2-based NO3--ISE is the primary reason for the high redox capacitance of the ternary selenide. Experimental results show a surprisingly good reproducibility of approximately 0.5 mV for five individual ASS-NO3--ISEs. Notably, electrochemical experiments and scanning electron microscopy mapping tests are used to predict the ion-electron transduction mechanism in which the lipophilic anion (tetrakis(4-chlorophenyl)borate) participates in the transduction process at the SC/ISM interface to stabilize the electrode potential and provide high reproducibility. It was further proved that the introduction of CoWSe2 as the SC layer maintains an excellent anti-interference to water layers, light, and gas. Hence, the CoWSe2-based ASS-NO3--ISEs achieve accurate detection for free NO3- in wetland soil and the estuary of the Yellow River delta.

Detection of CO2 Locally Generated by Formate Dehydrogenase Using Carbonate Ion-Selective Micropipette Electrodes

Many technologies involve immobilizing catalysts such as enzymes on surfaces, and the catalytic activities or functional efficiencies of these surface-bound catalysts can vary depending on orientations, localized binding sites, active sites, and intrinsic molecular nature. Accurate and rapid quantification of reaction products from surface-immobilized catalysts is crucial for understanding the selectivity, mechanisms, and reaction dynamics of catalytic systems and for revealing heterogeneous catalytic activities and reaction sites for applications such as biosensors and energy conversion/generation systems. Here, we demonstrate the feasibility of localized enzymatic activity measurements using microscale carbon dioxide (CO2)-sensitive ion-selective electrode (ISE) pipettes (0.5-2.5 μm tip radius) as a probe, with in situ potentiometric scanning electrochemical microscopy (SECM). We develop carbonate (CO32-) ionophore-incorporated ISEs exhibiting a Nernstian response (26.7 mV/decade) with a detection limit of 1.72 μM and explore surface-immobilized formate dehydrogenase (FDH) activity by detecting CO2 generated by the enzymatic reaction via potentiometric measurements. SECM is used for real-time spatial/temporal investigation of FDH immobilized onto the surface at a micrometer-scale resolution. Moreover, unlike voltammetric techniques based on faradaic reactions, the potentiometric measurements using ISEs allow highly sensitive and selective detection of CO32-, rendering efficient quantification of CO2 without interference from solution composition changes arising from faradaic processes. The total amount of CO2 generated at an FDH-immobilized Au ultramicroelectrode is quantified as a function of coenzyme, i.e., NAD+, and substrate, i.e., formate, concentrations both in constant tip-sample distance mode and variable depth mode. Finally, we demonstrate the use of the ISE to quantify CO2 levels in blood serum.

Solution-processable all-solid-state chloride-selective electrode: Enhanced sensitivity from anion dopant exchange

Ion-selective electrodes (ISEs) are widely used in many industries, with recent research focusing on their miniaturization by replacing the liquid filling solutions with solid-contacts. Solution-processing is the preferred method for preparing solid-contact ISEs (SC-ISEs) due to its ease and scalability. However, while there are many solution-processable cationic SC-ISEs, this remains a challenge for anionic SC-ISEs due to poorer compatibility with the solid-contacts. Many anionic SC-ISEs are still prepared by complex techniques, such as electropolymerisation of the solid-contact. Thus, strategies for solution-processable solid-contacts which can interface well with anionic ion-selective membrane (ISMs) are required. Here, we report the fabrication of a fully-solution-processable chloride (Cl-) SC-ISE by anion exchange of poly(3,4-ethylenedioxythiophene)-polyethylene glycol (PEDOT-PEG) solid-contact before drop-casting the ISM. Significant improvement in sensitivity was observed after PEDOT-PEG anion exchange, with the optimal SC-ISE exhibiting near-Nernstian response (-53.3 ± 0.5 mV/decade, versus -33.4 ± 1.8 mV/decade for the unexchanged SC-ISE) across a wide dynamic range (0.05 M-6.03 μM). Our SC-ISE also exhibited excellent selectivity against phosphate (H2PO4-), bicarbonate (HCO3-) and acetate (CH3CO2-) and could be utilized with minimal conditioning time and for prolonged usage. Finally, given the importance of Cl- sensing in healthcare, we also demonstrated the potential of our Cl- SC-ISE in sensing multiple synthetic biological samples such as sweat, urine and blood, and real human sweat (forearm samples). This work not only demonstrates the versatility of our anion exchange protocol, but also furthers the understanding of the different enhancement mechanisms - sensitivity or selectivity - depending on whether an ionophore was present in the ISM. We showed that regardless of the mechanism, our simple and efficient protocol could mitigate the issue of the original underperformance and can thus be readily extended to the scalable preparation of multiple types of anion-selective SC-ISEs.

Advancing multiplexed ion monitoring techniques: The development of integrated thermally drawn polymer fiber-based ion probes

The monitoring of ion homeostasis in vivo is of paramount importance due to its critical functions in biological systems. However, current leading technologies for creating ion-selective electrodes often fall short of the requirements for in vivo applications in terms of multiplexity, miniaturization, and flexibility. To address this gap, we introduce an integrated multiplexed ion monitoring probe created from thermally drawn multi-electrode polymer fiber, aimed at enhancing in vivo ion homeostasis studies. This probe employs a carbon nanofiber (CNF)/graphene composite as the sensing material, utilizing a thermal drawing process, laser machining, and material functionalization to fabricate multiplexed ion probes. Our design incorporates electrodes on micron-scale fibers for sensing Na+, K+ and Cl- ions, alongside an electrode for electrophysiology recording, achieving excellent sensitivity, stability, selectivity, and reversibility in distilled water and artificial cerebrospinal fluid solutions (aCSF). These results demonstrate the potential of the probe for future in vivo applications.

Candle soot-smoked electrodes as a natural superhydrophobic material for potentiometric sensors

The application of carbon soot as a solid-contact layer in potentiometric sensor is presented. The preparation method of carbon layer from the candle is inexpensive and as short as 10 s and was optimized and described in the scope of this paper. With the use of the proposed procedure, it is possible to cover not only the glassy carbon disc electrodes, but all surfaces of various shapes and types, like foil or paper. Obtained soot layer was casted with potassium-selective membrane, for the comparison of results with other solutions presented in literature, however by changing the composition of ion-selective membrane it is possible to achieve electrodes sensitive to various cations and anions. By introducing the candle soot into the sensor, the electrical and analytical properties of electrode were significantly improved. Designed candle soot-based sensor exhibit repeatable and reversible Nernstian response towards potassium ions, low detection limit (10-6.7 M K+ ions), remarkable potential stability (drift equals to 15 μV/h) enhanced selectivity (in comparison with coated-disc electrode) and short response time. Designed sensors owe its great analytical properties to high electrical capacity (C = 343 μF) and superhydrophobic properties (proven by the high value of contact angle - 165°). Obtained sensors are insensitive to redox conditions and exhibit long lifetime. Performed tests proved that carbon layer obtained directly from the candle is appropriate material for solid-contact layer in potentiometric sensor and allows to obtain robust potentiometric sensor of competitive parameters using simple and fast procedure.

Lipophilicity Criteria for Ionic Components Used in Liquid-Junction-Free Reference Electrodes

Quantitative criteria for the lipophilicity of ionic components (ICs) used in PVC membranes of liquid-junction-free reference electrodes (LJFREs) providing a stable potential, independent of the electrolyte of the sample solution, have been formulated and theoretically substantiated. It is shown that the lipophilicity parameters of an ionic liquid (IL) or lipophilic salt (LS) as a whole, expressed in terms of partition constants, as well as the lipophilicities of individual ions, expressed in terms of single-ion partition coefficients, have both upper and lower limits. Therefore, a lipophilicity window for individual ions of IL or LS exists, and its width depends on the lipophilicity and concentration of the ions present in the sample solution. Besides that, upper lipophilicity limits for both an electrolyte as a whole and for its individual ions exist, beyond which any application of LJFREs is principally impossible under specified experimental conditions. Equations have been obtained that explicitly describe the upper (ULL) and lower limits of lipophilicity (LLL) of ionic components of LJFRE membranes as functions of the single-ion partition coefficients of electrolyte ions of the sample solution and their concentration, as well as the concentration of ionic components in the membrane phase and diffusion parameters. Predicted ULL and LLL values agree well with the corresponding values numerically calculated using a dynamic diffusion model and with experimental data. Some guides for the rational choice of IL or LS components satisfying specified experimental conditions are formulated.

A novel type of planar reference electrodes based on ionic liquids

Although electrochemical sensors gained a lot of popularity through recent years, there is very little research on sensors with IL-based reference electrodes. This type of reference electrodes might be the ultimate solution for problem of RE miniaturization. In this paper a novel type of printed reference electrodes based on ionic liquids are presented. The potential stability of electrodes with membranes containing two new ILs with promising properties, namely 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMIM+FAP-) and 1-(2-methoxyethyl)-1-methylpyrrolidin-1-ium tris(pentafluoroethyl)trifluorophosphate (PYR(2o1,1)+FAP-), was investigated. Reference membranes were implemented in classic electrodes with internal electrolyte, as well as deposited on planar transducers with electrodes fabricated using screen printing or aerosol jet printing. Membranes were deposited via drop-casting or by using aerosol jet printer, to form fully printed reference electrodes. It was found that while both tested ionic liquids performed similarly, the use of (PYR(2o1,1)+FAP-) resulted in better potential stability. Planar IL-based electrode was finally used as a reference electrode in a simple pH sensor, enabling the detection of pH changes.

All-Solid-State Ion-Selective Electrode Inspired from All-Solid-State Li-Ion Batteries

Solid electrolytes employed in all-solid-state Li-ion batteries (ASSBs) electronically isolate the positive and negative electrodes, while allowing the carrier ions, Li+, to pass through. Inorganic solid-state electrolytes, which typically exhibit a Li+-transference number of 1, are theoretically applicable as ion-sensitive membranes of potentiometric ion-selective electrodes (ISEs). Inspired by the ASSB architecture, an all-solid-state Li ISE was developed in a two-layer stacking configuration using a redox-active material (LiFePO4) and a solid electrolyte (Li1+x+yAlx(Ti, Ge)2-xSiyP3-yO12) as inner and outer layers, respectively, on the substrate (i.e., current collector). The solid electrolyte acts as an ion-selective membrane because the Donnan membrane potential obeys a Nernstian response to Li+ activity in the analyte solution. The fabricated ASSB-inspired ISE selectively responds to Li ions, exhibiting a Nernstian slope of 60.8 ± 0.5 mV dec-1, limit of detection of 10-4.9±0.4, and minimal potential variation (-3 to +6 mV over 17 d). Using a two-phase LiFePO4/FePO4 layer with a highly stable potential as the inner reference electrode significantly minimizes the deviations in the response potential. Moreover, applying Li1+x+yAlx(Ti, Ge)2-xSiyP3-yO12 as a durable and highly ion-conductive inorganic solid electrolyte enables remarkable long-term stability.

Advancements in Mercury-Free Electrochemical Sensors for Iron Detection: A Decade of Progress in Electrode Materials and Modifications

Efforts to quantify iron ion concentrations across fields such as environmental, chemical, health, and food sciences have intensified over the past decade, which drives advancements in analytical methods, particularly electrochemical sensors known for their simplicity, portability, and reliability. The development of electrochemical methods using non-mercury electrodes is increasing as alternatives to environmentally unsafe mercury-based electrodes. However, detecting iron species such as Fe(II) and Fe(III) remains challenging due to their distinct chemical properties, continuous oxidation-state interconversion, presence of interfering species, and complex behavior in diverse environments and matrixes. Selective trace detection demands careful optimization of electrochemical methods, including proper electrode materials selection, electrode surface modifications, operating conditions, and sample pretreatments. This review critically evaluates advancements over the past decade in mercury-free electrode materials and surface modification strategies for iron detection. Strategies include incorporating a variety of nanomaterials, composites, conducting polymers, membranes, and iron-selective ligands to improve sensitivity, selectivity, and performance. Despite advancements, achieving ultra-low detection limits in real-world samples with minimal interference remains challenging and emphasizes the need for enhanced sample pretreatment. This review identifies challenges, knowledge gaps, and future directions and paves the way for advanced iron electrochemical sensors for environmental monitoring, health diagnostics, and analytical precision.

Integrated Headband for Monitoring Chloride Anions in Sweat Using Developed Flexible Patches

Flexible wearable potentiometric ion sensors for continuous monitoring of electrolyte cations have made significant advances in bioanalysis for personal healthcare and diagnostics. However, less attention is paid to the most abundant extracellular anion, chloride ion (Cl-) as a mark of electrolyte imbalance and an important diagnostic indicator of cystic fibrosis, which has important significance for accurate monitoring in complex biological fluids. An all-solid-state Cl--selective electrode is constructed utilizing oxygen vacancies reinforced vanadium oxide with a nitrogen-doped carbon shield as the solid contact (V2O3-x@NC/Cl--ISE). The prepared V2O3-x@NC/Cl--ISE exhibits a low detection limit of 10-5.45 M without an interfacial water layer and shows a highly stable potential with 7.24 μV/h during 24 h, which is attributed to the rapid interfacial electron transfer of the conductive carbon layers and the valence state transition of the polyvalent vanadium center in charge storage processes. Additionally, the custom flexible sensing patch presents an excellent sensitivity retention rate under bending (95%) and twisting (93%) strains and possesses good anti-interference performance (ΔE < 8 mV) against common interfering ions and organic substances in sweat. Real-time monitoring of the Cl- concentration in sweat aligns with ion chromatography analysis results. This study presents a compact wearable Cl- monitoring platform for the easy tracking of exercise-induced dehydration and cystic fibrosis screening with promising applications in smart healthcare.

Radially Aligned Carbon Nanotube Glass Fiber Composites as Ion-Selective Microelectrodes

Detection of ions is challenging due to their small size, rapid diffusion, and high mobility, especially for assaying in samples of low volumes. Among the traditional analytical methods, potentiometric ion-selective electrodes (ISE) have become a popular choice for detecting ions as they are cost-effective, user-friendly and can be miniaturized, making them useful for on-site analysis. In this context, radially aligned carbon nanotubes (RACNT) directly grown on glass fibers (GF) via the chemical vapor deposition method is investigated as a solid contact material for the fabrication of ion-selective microelectrodes (μISE) upon incorporating specific ionophores within a polymeric encapsulation membrane. As an illustration, sensitive detection of ammonium ions is accomplished by the fabricated μISE (plasticized PVC membrane containing nonactin ionophores), which yielded a LOD and a linear response range between 7.5 × 10-6 and 1.0 × 10-5 to 1.0 × 10-1 M, respectively. The μISE fabricated with RACNT-GF as an interface material exhibited improvements in LOD and enhanced the detection selectivity as compared to a conventional ISE fabricated using planar solid contact materials such as graphite. We hypothesize that the fabricated μISE with a high surface area and mechanical durability maximize the accommodation of ionophores in the barrier membrane for yielding improved potentiometric responses. Experimental results illustrate that the μISE possesses the potential to be utilized for the fabrication of selective and sensitive ISE upon incorporation of specific ionophores with RACNT-GF composites.

Next-Generation Potentiometric Sensors: A Review of Flexible and Wearable Technologies

In recent years, the field of wearable sensors has undergone significant evolution, emerging as a pivotal topic of research due to the capacity of such sensors to gather physiological data during various human activities. Transitioning from basic fitness trackers, these sensors are continuously being improved, with the ultimate objective to make compact, sophisticated, highly integrated, and adaptable multi-functional devices that seamlessly connect to clothing or the body, and continuously monitor bodily signals without impeding the wearer's comfort or well-being. Potentiometric sensors, leveraging a range of different solid contact materials, have emerged as a preferred choice for wearable chemical or biological sensors. Nanomaterials play a pivotal role, offering unique properties, such as high conductivity and surface-to-volume ratios. This article provides a review of recent advancements in wearable potentiometric sensors utilizing various solid contacts, with a particular emphasis on nanomaterials. These sensors are employed for precise ion concentration determinations, notably sodium, potassium, calcium, magnesium, ammonium, and chloride, in human biological fluids. This review highlights two primary applications, that is, (1) the enhancement of athletic performance by continuous monitoring of ion levels in sweat to gauge the athlete's health status, and (2) the facilitation of clinical diagnosis and preventive healthcare by monitoring the health status of patients, in particular to detect early signs of dehydration, fatigue, and muscle spasms.

Fabrication of promising competitive graphene nanocomposite transducer to determine Prucalopride succinate in pharmaceutical formulation and in spiked human biological fluids

The development of a newly fabricated ion-selective electrode (ISE) solid-contacted type for the determination of prucalopride succinate represents a significant advancement in analytical chemistry, particularly in the context of green chemistry principles. The optimization process involved numerous trials to ensure the selection of a cation exchanger and ionophore that offer high sensitivity and selectivity for prucalopride succinate. Through these optimization trials, sodium tetrakis was identified as the most suitable cation exchanger, while calix [8] arene demonstrated the highest affinity towards prucalopride succinate as the ionophore. This careful selection of components ensures accurate and specific detection of prucalopride succinate. To enhance the electroanalytical performance of the ISE, a graphene nanocomposite layer was developed as an ion-electron transducer between the carbon and synthetic polymeric membrane. This graphene-nanocomposite layer improves the overall performance of the ISE, providing a Nernstian slope of 57.249 mV per decade, which aligns with the recommendations of the International Union of Pure and Applied Chemistry (IUPAC). The integration of these components and the utilization of green chemistry principles in the design of the fabricated ISE enable rapid and accurate determination of prucalopride succinate. This innovative approach holds great potential for applications in pharmaceutical analysis and quality control, providing a more sustainable and efficient method for the analysis of prucalopride succinate.

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

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

Application of a Screen-Printed Ion-Selective Electrode Based on Hydrophobic Ti3C2/AuNPs for K+ Determination Across Variable Temperatures

Monitoring potassium ion (K+) concentration is essential in veterinary medicine, particularly for preventing hypokalemia in dairy cows, which can severely impact their health and productivity. While traditional laboratory methods like atomic absorption spectrometry are accurate, they are also time-consuming and require complex sample preparation. Ion-selective electrodes (ISEs) provide an alternative that is faster and more suitable for field measurements, but their performance is often compromised under variable temperature conditions, leading to inaccuracies. To address this, we developed a novel screen-printed ion-selective electrode (SPE) with hydrophobic Ti3C2 Mxene and gold nanoparticles (AuNPs), integrated with a temperature sensor. This design improves stability and accuracy across fluctuating temperatures by preventing water layer formation and enhancing conductivity. The sensor was validated across temperatures from 5 °C to 45 °C, achieving a linear detection range of 10-⁵ to 10-1 M and a response time of approximately 15 s. It also demonstrated excellent repeatability, selectivity, and stability, making it a robust tool for K+ monitoring in complex environments. This advancement could lead to broader applications in other temperature-sensitive analytical fields.

Printed Potentiometric Ammonium Sensors for Agriculture Applications

Ammonium (NH4 +) concentration is critical to both nutrient availability and nitrogen (N) loss in soil ecosystems but can be highly variable across spatial and temporal scales. For this reason, effectively informing agricultural practices such as fertilizer management and understanding of mechanisms of soil N loss require sensor technologies to monitor ammonium concentrations in real time. Our work investigates the performance of fully printed ammonium ion-selective sensors used in diverse soil environments. Ammonium sensors consisting of a printed ammonium ion-selective electrode and a printed Ag/AgCl reference were fabricated and characterized in aqueous solutions and three different soil types (sand, peat, and clay) under the range of ion concentrations likely to be present in soil (0.01-100 mM). The response of ammonium sensors was further evaluated under variable gravimetric moisture content in the soil to reflect their reliability under field conditions. Ammonium sensors demonstrated a sensitivity of 53.6 ± 5.1 mV/decade when tested in aqueous solution, and a sensitivity of 55.7 ± 11 mV/dec, 57.5 ± 4.1 mV/dec, and 43.7 ± 4 mV/dec was measured in sand, clay, and peat soils, respectively.

Poly(3,4-ethylenedioxythiophene) and Poly(3-octylthiophene-2,5-diyl) Molecules as Composite Transducers in Potentiometric Sensors-Synthesis and Application

The aim of this paper is to investigate the influence of the molecules of conducting polymers on the properties of potentiometric sensors. Two conducting polymers, poly(3-octylthiophene-2,5-diyl) and poly(3,4-ethylene-1,4-dioxythiophene), were compared in the context of the design of ion-selective electrodes. This study offers a comparison of the most popular conducting polymers in the context of the design of potentiometric sensors. Firstly, the properties of both materials, such as their microstructure, electrical performance, wettability, and thermic properties, were examined. Subsequently, conducting polymers were applied as transducer layers in potassium-selective sensors. The properties of both groups of sensors were evaluated using the potentiometry method. Research has shown that the presence of poly(3-octylthiophene-2,5-diyl) (POT) in the transducer layer makes it superhydrophobic, leading to a long lifetime of sensors. On the other hand, the addition of poly(3,4-ethylene-1,4-dioxythiophene) polystyrene sulfonate (PEDOT:PSS) allows for the enhancement of electrical capacitance parameter values, which beneficially influence the stability of the potentiometric response of sensors. Both examined conducting polymers turned out to be perfect materials for transducer layers in potentiometric sensors, each being responsible for enhancing different properties of electrodes.

Flexible ammonium ion-selective electrode based on inkjet-printed graphene solid contact

Miniaturization and mass-production of potentiometric sensor systems is paving the way towards distributed environmental sensing, on-body measurements and industrial process monitoring. Inkjet printing is gaining popularity as a highly adaptable and scalable production technique. Presented here is a scalable and low-cost route for flexible solid-contact ammonium ion-selective electrode fabrication by inkjet printing. Utilization of inkjet-printed melamine-intercalated graphene nanosheets as the solid-contact material significantly improved charge transport, while evading the detrimental water-layer formation. External polarization was investigated as a means of improving the inter-electrode reproducibility: the standard deviations of E0 values were reduced after electrode polarization, the linear region of the response was extended to the range 10-1-10-6 M of NH4Cl and LODs reduced to 0.88 ± 0.17 μM. Finally, we have shown that the electrodes are adequate for measurements in a complex real sample: ammonium concentration was determined in landfill leachate water, with less than 4 % deviation from the reference method.

A smartphone-based sensor for detection of iron and potassium in food and beverage samples

A novel approach for simultaneous detection of iron and potassium via a smartphone-based potentiometric method is proposed in this study. The screen printed electrodes were modified with carbon black nanomaterial and ion selective membrane including zinc (II) phtalocyanine as the ionophore. The developed Fe3+-selective electrode and K+-selective electrode exhibited detection limits of 1.0 × 10-6 M and 1.0 × 10-5 M for Fe3+ and K+ ions, respectively. The electrodes were used to simultaneously detect Fe3+ and K+ ions in apple juice, skim milk, soybean and coconut water samples with recovery values between 90%-100.5%, and validated against inductively coupled plasma-optical emission spectrometry. Due to the advantageous characteristics of the sensors and the portability of Near Field Communication potentiometer supported with a smartphone application, the proposed method offers sensitive and selective detection of iron and potassium ions in food and beverage samples at the point of need.

Electrochemical Acid-Base Transport Limitation Principle for Low Electroactive Analyte Sensing in Wastewater Monitoring

Electrochemical sensing (ES) is crucial for improving data acquisition in wastewater treatment, but obtaining the signal for a low electroactive analyte is challenging. Here, we propose an electrochemical acid-base transport limitation (eABTL) principle for inertness-based sensing, offering a new insight into generating ES signals from an interfacial transport process rather than electron transfer. This principle enables potential ES application for various weak acids and bases (WABs) without reactions themselves. We established an eABTLP method for detecting orthophosphate in solutions as a proof of concept, demonstrating commendable accuracy and precision, and a wide detection range from 10 μM to over 300 mM. Endogenous interferences were identified using 23 weak acids, indicating no significant endogenous interfering factors in typical wastewaters. Of them, volatile fatty acids are the main interference, but their effect can be eliminated by adjusting pH above 6.0. Exogenous factors like anions, cations, ion strength, temperature, organic load, and dissolved oxygen were examined, and most of their effects can be ignored by maintaining consistent analytical procedures between calibration curve and sample. Furthermore, measurement of wastewater samples confirmed the applicability toward domestic wastewater and demonstrated its wide applicability when combined with digestion pretreatment. Given the merits of inertness-based sensing, the eABTL-based methods have the potential to be a crucial part of ES techniques for environmental and industrial monitoring.

Simplistic approach to formulate an ionophore-based membrane and its study for nitrite ion sensing

A polymeric membrane based on a N,N'-bis(salicylidene)ethylenediaminocobalt(ii) complex as a cobalt ionophore (CI) was fabricated and optimized for nitrite ion sensing application. The membrane contained CI, 2-nitrophenyl octyl ether (2-NPOE) as a plasticizer and hexadecyl trimethyl ammonium bromide (HTAB) as a cationic additive in a polyvinyl chloride (PVC) matrix. The Nernstian slope (-0.020 mV per decade), detection limit (1 × 10-7 M to 3 M), and response (107 milliseconds) and recovery (22 milliseconds) times were recorded for optimum membrane composition. The ionophore functionality in the polymer matrix and their interaction were studied using Fourier-transform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), environmental scanning electron microscopy (ESEM), energy-dispersive X-ray spectroscopy (EDS), and optical microscopy analyses.

Insights into solid-contact ion-selective electrodes based on laser-induced graphene: Key performance parameters for long-term and continuous measurements

This work aims to serve as a comprehensive guide to properly characterize solid-contact ion-selective electrodes (SC-ISEs) for long-term use as they advance toward calibration-free sensors. The lack of well-defined SC-ISE performance criteria limits the ability to compare results and track progress in the field. Laser-induced graphene (LIG) is a rapid and scalable method that, by adjusting the CO2 laser parameters, can create LIG substrates with tunable surface properties, including wettability, surface chemistry, and morphology. Herein, we fabricate laser-induced graphene (LIG) solid-contact electrodes using different laser settings and subsequently convert them into ion-selective sensors using a potassium-selective membrane. We measure the aforementioned tunable surface properties and correlate them with resultant low-frequency capacitance and water layer formation in an effort to pinpoint their effects on the sensitivity (Nernstian response), reproducibility (E°' variation), and potential stability of the LIG-based SC-ISEs. More specifically, we demonstrate that the surface wettability of the LIG substrate, which can be tuned by controlling the lasing parameters, can be modified to exhibit hydrophobic (contact angle > 90°) and even highly hydrophobic surfaces (contact angle ≈ 130°) to help reduce sensor drift. Recommendations are also provided to ensure proper and robust characterization of SC-ISEs for long-term and continuous measurements. Ultimately, we believe that a comprehensive understanding of the correlation between LIG tunable surface properties and SC-ISE performance can be used to improve the electrochemical behavior and stability of SC-ISEs designed with a wide range of materials beyond LIG.

Eco-friendly electrochemical assay of oxytetracycline and flunixin in their veterinary injections and spiked milk samples

Two solid-contact electrochemical sensors were developed for detection of each of oxytetracycline HCl (OXY), and the co-formulated non-steroidal anti-inflammatory drug flunixin meglumine (FLU) in veterinary formulations and animal-derived food products. The designed sensors were based on a glassy carbon electrode as the substrate material and high molecular weight polyvinyl chloride (PVC) polymeric ion-sensing membranes doped with multiwalled carbon nanotubes (MWCNTs) to improve the potential stability and minimize signal drift. For determination of OXY, the sensing membrane was modified with potassium tetrakis (4-chlorophenyl) borate (K-TCPB), which was employed as a cation exchanger, and 2-hydroxypropyl-β-cyclodextrin (HP-ßCD), which was used as an ionophore. A linear response within a concentration range of 1 × 10- 6-1 × 10- 2 M with a slope of 59.47 mV/decade over a pH range of 1-5 was recorded. For the first time, two potentiometric electrodes were developed for determination of FLU, where the sensing membrane was modified with tetra dodecyl ammonium chloride (TDDAC) as an anion exchanger. A linear response within a concentration range of 1 × 10- 5-1 × 10- 2 M and a slope of -58.21 mV/decade over a pH range of 6-11 was observed. The suggested sensors were utilized for the selective determination of each drug in pure powder form, in veterinary formulations, and in spiked milk samples, with mean recoveries ranging from 98.50 to 102.10, and without any observed interference. The results acquired by the proposed sensors were statistically analyzed and compared with those acquired by the official methods, and the results showed no significant difference.

Unveiling the fast adsorption and desorption of heavy metals on/off nanoplastics by real-time in-situ potentiometric sensing

Nanoplastics (<1 μm) can serve as a transport vector of environmental pollutants (e.g., heavy metals) and change their toxicities and bioavailabilities. Up to date the behaviors of adsorption and desorption heavy metals on/off nanoplastics are largely unknown. Herein, polymeric membrane potentiometric ion sensors are proposed for in-situ assessment of the real-time kinetics of heavy metal adsorption and desorption on/off nanoplastics. Results show that nanoplastics can adsorb and release heavy metals in a fast manner, indicating their superior ability in transferring heavy metals. The adsorption behaviors are closely related to the characteristics of nanoplastics and background electrolytes. Particle aggregation and increases in salinity and acidity suppress the adsorption of heavy metals on nanoplastics. The desorption efficiencies of different heavy metals are Pb2+ (31 %) < Cu2+ (40 %) < Cd2+ (97 %). Our proposed method is applicable for the detection of the plastic pollutants with size <100 nm and of the samples with high salinities (e.g., seawater). This work would provide new insights into the assessment of environmental risks posed by nanoplastics and heavy metals.

A Biodegradable Fiber Calcium Ion Sensor by Covalently Bonding Ionophores on Bioinert Nanoparticles

Implantable sensors, especially ion sensors, facilitate the progress of scientific research and personalized healthcare. However, the permanent retention of implants induces health risks after sensors fulfill their mission of chronic sensing. Biodegradation is highly anticipated; while; biodegradable chemical sensors are rare due to concerns about the leakage of harmful active molecules after degradation, such as ionophores. Here, a novel biodegradable fiber calcium ion sensor is introduced, wherein ionophores are covalently bonded with bioinert nanoparticles to replace the classical ion-selective membrane. The fiber sensor demonstrates comparable sensing performance to classical ion sensors and good flexibility. It can monitor the fluctuations of Ca2+ in a 4-day lifespan in vivo and biodegrade in 4 weeks. Benefiting from the stable bonding between ionophores and nanoparticles, the biodegradable sensor exhibits a good biocompatibility after degradation. Moreover, this approach of bonding active molecules on bioinert nanoparticles can serve as an effective methodology for minimizing health concerns about biodegradable chemical sensors.

Advances in the determination of trace amounts of iron cations through electrochemical methods: A comprehensive review of principles and their limits of detection

Quantification of trace amounts of iron is of great importance in various fields. In the industrial sector, it is crucial to monitor the release of iron out of corrosion, pickling treatment, and steel manufacturing to address potential environmental and economic challenges. In biological systems, despite its indispensability, it is essential to maintain iron concentration below a specific threshold. Electrochemical (EC) methods provide significant analytical capabilities due to their simplicity, ease of use, and cost-effectiveness. This review focuses on the fundamental principles of EC methods for iron detection, including potentiometry, amperometry, coulometry, voltammetry, and electrochemical impedance spectroscopy (EIS). It further explains the process of obtaining calibration curves, and subsequently, determining the concentration of unknown ions. Additionally, technical notes are presented on selecting the initial signal value, reducing the duration of tests, excluding non-faradaic signals, and extending the linear region with the lowest detection limit. These notes are supported by key findings from relevant case studies.

Nanoporous Carbon Materials as Solid Contacts for Microneedle Ion-Selective Sensors

Continuous sensing of biomarkers, such as potassium ions or pH, in wearable patches requires miniaturization of ion-selective sensor electrodes. Such miniaturization can be achieved by using nanostructured carbon materials as solid contacts in microneedle-based ion-selective and reference electrodes. Here we compare three carbon materials as solid contacts: colloid-imprinted mesoporous (CIM) carbon microparticles with ∼24-28 nm mesopores, mesoporous carbon nanospheres with 3-9 nm mesopores, and Super P carbon black nanoparticles without internal porosity but with textural mesoporosity in particle aggregates. We compare the effects of carbon architecture and composition on specific capacitance of the material, on the ability to incorporate ion-selective membrane components in the pores, and on sensor performance. Functioning K+ and H+ ion-selective electrodes and reference electrodes were obtained with gold-coated stainless-steel microneedles using all three types of carbon. The sensors gave near-Nernstian responses in clinically relevant concentration ranges, were free of potentially detrimental water layers, and showed no response to O2. They all exhibited sufficiently low long-term potential drift values to permit calibration-free, continuous operation for close to 1 day. In spite of the different specific capacitances and pore architecture of the three types of carbon, no significant difference in potential stability for K+ ion sensing was observed between electrodes that used each material. In the observed drift values, factors other than the carbon solid contact are likely to play a role, too. However, for pH sensing, electrodes with CIM as a carbon solid contact, which had the highest specific capacitance and best access to the pores, exhibited better long-term stability than electrodes with the other carbon materials.

An endo-functionalized molecular cage for selective potentiometric determination of creatinine

Potentiometric ion-selective electrodes (ISEs), which rely on selective and lipophilic ionophores, are commonly employed in clinical diagnostics. However, there are very limited specific ionophores for the detection of creatinine, a critical biomarker for renal function assessment. In the present research, we designed and synthesized an endo-functionalized cage, which is able to selectively bind the creatininium cation (K a = 8.6 × 105 M-1) through the formation of multiple C-H⋯O and N-H⋯N hydrogen bonds and cation⋯π interactions. ISEs prepared with this host show a Nernstian response to creatinine and exhibit excellent selectivity and a low detection limit of 0.95 μM. In addition, the creatinine levels in urine or plasma samples determined by our sensor are consistent with those analyzed using enzymatic assay on a Cobas c702. The method is simple, fast and accurate, and amenable to clinical detection of creatinine levels.

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

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

Eco-conscious potentiometric sensing: a multiwalled carbon nanotube-based platform for tulathromycin monitoring in livestock products

Tulathromycin (TUL) is a widely used veterinary antibiotic for treating bovine and porcine respiratory infections. Consuming animal-derived food contaminated with this medication may jeopardize human health. This work adopted the first portable potentiometric platform for direct TUL sensing in pharmaceutical and food products. The sensor employed a plasticized PVC membrane on a glassy carbon electrode doped with calix[6]arene and multi-walled carbon nanotubes (MWCNT) in a single solid contact layer for selective binding and signal stability. Characterization via scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) confirmed the material's integrity. The MWCNT-based sensor produced a stable Nernstian response (1.0 × 10-7 to 1.0 × 10-3 M) and a limit of detection (LOD) of 9.76 × 10-8 M with instantaneous response (8 ± 2 s). IUPAC validation revealed high selectivity for TUL against interfering ions, minimal drift (0.6 mV/h), and functionality over a broad pH range (2.0-7.0), allowing direct application to dosage form, spiked milk, and liver samples. Eco-Scale, AGREE, and Whiteness assessment proved the method's ecological sustainability, economic viability, and practical feasibility, surpassing traditional approaches.

Conducting polymer functionalization in search of advanced materials in ionometry: ion-selective electrodes and optodes

Functionalized conducting polymers (FCPs) have recently garnered attention as ion-selective sensor materials, surpassing their intrinsic counterparts due to synergistic effects that lead to enhanced electrochemical and analytical parameters. Following a brief introduction of the fundamental concepts, this article provides a comprehensive review of the recent developments in the application of FCPs in ion-selective electrodes (ISEs) and ion-selective optodes (ISOs), particularly as ion-to-electron transducers, optical transducers, and ion-selective membranes. Utilizing FCPs in these devices offers a promising avenue for detecting and measuring ions in various applications, regardless of the sample nature and composition. Research has focused on functionalizing different conducting polymers, such as polyaniline and polypyrrole, through strategies such as doping and derivatization to alter their hydrophobicity, conductance, redox capacitance, surface area, pH sensitivity, gas and light sensitivity, etc. These modifications aim to enhance performance outcomes, including potential stability/emission signal stability, reproducibility and low detection limits. The advancements have led to the transition of ISEs from conventional zero-current potentiometric ion sensing to innovative current-triggered sensing approaches, enabling calibration-free applications and emerging concepts such as opto-electro dual sensing systems. The intrinsic pH cross-response and instability of the optical signal of ISOs have been overcome through the novel optical signal transduction mechanisms facilitated by FCPs. In this review, the characteristics of materials, functionalization approaches, particular implementation strategies, specific performance outcomes and challenges faced are discussed. Consolidating dispersed information in the field, the in-depth analysis presented here is poised to drive further innovations by broadening the scope of ion-selective sensors in real-world scenarios.

Bimetallic MOF-based electrochemical sensor for determination of paracetamol in spiked human plasma

Metal-organic frameworks (MOFs) with their exceptional properties have the potential to revolutionize the field of electrochemistry and pave the way for new and exciting applications. MOFs is an excellent choice as an active electrocatalyst component in the fabrication of electrochemical sensors. Here, bimetallic NiCo-MOFs, monometallic Ni-MOFs, and Co-MOFs were fabricated to modify the carbon paste electrode. Moreover, the ratio between Co and Ni within the bimetallic MOFs was optimized. Our aim in this work is to synthesize different compositions from bimetallic MOFs and systematically compare their catalytic activity with mono-metallic MOFs on paracetamol. The structure and properties of the 2D NiCo-MOFs were characterized by scanning electron microscope, X-ray photoelectron spectroscopy, Fourier transform infrared, and electrochemical method. Bimetallic Ni0.75Co0.25-MOFs modified carbon paste sensor displayed the optimum sensing performance for the electrochemical detection of paracetamol. A linear response over the range 6.00 × 10- 7 to 1.00 × 10- 4 M with a detection limit of 2.10 × 10- 8 M was obtained. The proposed method was applied to detect paracetamol in spiked human plasma and to determine paracetamol in the presence of its major toxic impurity, p-aminophenol. These findings suggest the considerable potential use of the newly developed sensor as a point-of-care tool for detecting paracetamol and p-aminophenol in the future.

A cathodically polarized PANI-based lead ion-selective electrode: achieving high stability with antibiofouling capabilities

Solid-state contact ion-selective electrodes (SC-ISEs) are an efficacious means of monitoring heavy metal contamination. Instability of the electrode potential is a key factor limiting their development, with biofouling in real water samples posing a significant challenge to maintaining stability. Therefore, addressing biofouling is crucial for optimizing solid-state ion-selective electrodes. In this work, high stability and antibiofouling capability in a solid-state contact lead ion-selective electrode (SC-Pb2+-ISE) based on polyaniline (PANI) was achieved through cathodic polarization. Specifically, PANI played a dual role in the ion-selective membrane (ISM) as an ion-to-electron transducer and antifouling agent. Given the excellent electrochemical performance of PANI, the prepared electrode (GC/PANI-Pb2+-ISM) demonstrated a remarkable antibiofouling efficiency of 98.2% under a cathodic polarization of -0.2 V. Furthermore, a standard deviation of standard potential (Eθ) as low as ± 0.5 mV was realized successfully. The excellent chrono-potentiometric stability of 17.0 ± 2.9 μV/s was also demonstrated. The electrode maintained a Nernstian response slope of 30.7 ± 0.2 (R2 = 0.998) after applying a cathode potential (-0.2 V) for 30 min. The developed GC/PANI-Pb2+-ISM electrode is suitable for practical applications in real environmental water sample monitoring.

Miniaturized inkjet-printed flexible ion-selective sensing electrodes with the addition of graphene in PVC layer for fast response real-time monitoring applications

In this letter, we propose a miniaturization scheme of inkjet printed ionic sensing electrodes by adding graphene into the ion-selective PVC film not only to reduce the impedance of the ionic liquid layer of the electrode but also to increase the electrode capacitance for the reduction of the response time. Based on the scheme, we present a fully inkjet-printed electrochemical ion-selective sensor comprising a working electrode and reference electrode, which are inkjet-printed Ag NPs/PEDOT:PSS-graphene/PVC-graphene and Ag/AgCl(s)/ionic liquid PVC-graphene layer structures, respectively. The printed ion-selective working electrode has been miniaturized to a size of 22,400 μm2 equivalent to a square shape of ∼150 × 150 μm2 comparable to the size of a human cell. By adding graphene to the ion selective PVC film, more than 90 % charge transfer resistance reduction can be achieved and the shunt capacitance is increased by 3.4-fold in shunt capacitance compared to the film without graphene, thereby more than 33 % reduction of the response time required to reach equilibrium. Meanwhile, these miniaturized potassium sensors using the working electrodes with/without adding graphene have been integrated with in-lab signal-processing and wireless-transmission module to yield similar results to the one measured by commercial electrochemical workstation showing a great potential for real-time monitoring in portable clinical trials. Specifically, the proposed sensor utilizing graphene-enhanced electrodes demonstrates a linearity uncertainty of 2.9 mV, which is approximately half of the uncertainty observed in the sensors lacking graphene integration.

Well-modulated interfacial ion transport enables D-sorbitol/PEDOT:PSS fibers to sense brain electrophysiological signals in vivo

A neuroelectrode can be easily prepared using a wet-spun fiber of D-sorbitol/PEDOT:PSS. At a D-sorbitol/PEDOT:PSS weight ratio of 6, the fiber is well-modulated with suitable characters, including the morphology, crystallization, diffusion resistance (179 kΩ), and electric double-layer capacitance (2.72 μF), for sensitive recording of brain activity during somatosensory stimulation and seizures. Additionally, the fiber is highly biocompatible with the brain. This study presents a simple and controllable strategy for the chemical construction of conducting polymer-based neurosensors.

Development of Machine Learning Models for Ion-Selective Electrode Cation Sensor Design

Polyvinyl chloride (PVC) membrane-based ion-selective electrode (ISE) sensors are common tools for water assessments, but their development relies on time-consuming and costly experimental investigations. To address this challenge, this study combines machine learning (ML), Morgan fingerprint, and Bayesian optimization technologies with experimental results to develop high-performance PVC-based ISE cation sensors. By using 1745 data sets collected from 20 years of literature, appropriate ML models are trained to enable accurate prediction and a deep understanding of the relationship between ISE components and sensor performance (R 2 = 0.75). Rapid ionophore screening is achieved using the Morgan fingerprint based on atomic groups derived from ML model interpretation. Bayesian optimization is then applied to identify optimal combinations of ISE materials with the potential to deliver desirable ISE sensor performance. Na+, Mg2+, and Al3+ sensors fabricated from Bayesian optimization results exhibit excellent Nernst slopes with less than 8.2% deviation from the ideal value and superb detection limits at 10-7 M level based on experimental validation results. This approach can potentially transform sensor development into a more time-efficient, cost-effective, and rational design process, guided by ML-based techniques.

Recent advances in nanomaterial-based solid-contact ion-selective electrodes

Solid-contact ion-selective electrodes (SC-ISEs) are advanced potentiometric sensors with great capability to detect a wide range of ions for the monitoring of industrial processes and environmental pollutants, as well as the determination of electrolytes for clinical analysis. Over the past decades, the innovative design of ion-selective electrodes (ISEs), specifically SC-ISEs, to improve potential stability and miniaturization for in situ/real-time analysis, has attracted considerable interest. Recently, the utilisation of nanomaterials was particularly prominent in SC-ISEs due to their excellent physical and chemical properties. In this article, we review the recent applications of various types of nanostructured materials that are composed of carbon, metals and polymers for the development of SC-ISEs. The challenges and opportunities in this field, along with the prospects for future applications of nanomaterials in SC-ISEs are also discussed.

Recent Developments and Challenges in Solid-Contact Ion-Selective Electrodes

Solid-contact ion-selective electrodes (SC-ISEs) have the advantages of easy miniaturization, even chip integration, easy carrying, strong stability, and more favorable detection in complex environments. They have been widely used in conjunction with portable, wearable, and intelligent detection devices, as well as in on-site analysis and timely monitoring in the fields of environment, industry, and medicine. This article provides a comprehensive review of the composition of sensors based on redox capacitive and double-layer capacitive SC-ISEs, as well as the ion-electron transduction mechanisms in the solid-contact (SC) layer, particularly focusing on strategies proposed in the past three years (since 2021) for optimizing the performance of SC-ISEs. These strategies include the construction of ion-selective membranes, SC layer, and conductive substrates. Finally, the future research direction and possibilities in this field are discussed and prospected.

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.

Determination of benzoate in cranberry and lingonberry by using a solid-contact benzoate-selective electrode

Benzoic acid is used as a preservative in processed food, and occasionally in cosmetics and pharmaceuticals, while benzoic acid occurs naturally in, e.g., cranberry and lingonberry. Therefore, the determination of benzoate is of interest for product quality assurance, food safety, and personal health. In this work, a solid-contact benzoate-selective electrode (benzoate-ISE) was developed by utilising poly(3,4-ethylenedioxythiophene) (PEDOT) as solid contact and a solvent polymeric membrane containing a 1,3-bis(carbazolyl)urea derivative as ionophore. The benzoate-ISE was characterised in parallel with an ionophore-free control-ISE by electrochemical impedance spectroscopy and potentiometry. The presence of the ionophore in the membrane improved the selectivity to benzoate. Benzoate-ISEs and control-ISEs were used further to determine the benzoate concentration in cranberry and lingonberry by standard addition. The results obtained with both types of ISEs were compared with those obtained by ion chromatography. The results obtained with benzoate-ISEs were consistent with those obtained with ion chromatography. On the contrary, the control-ISE (without ionophore) gave significantly higher benzoate concentrations, especially in the case of cranberry where the benzoate concentration was low (ca 0.2 g kg-1) compared to lingonberry (ca 1 g kg-1). Hence, the benzoate-selectivity of the ionophore was crucial to obtain a benzoate-ISE that was practically applicable for determination of benzoate concentrations in cranberry and lingonberry.

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.

Tailored Polypyrrole Nanofibers as Ion-to-Electron Transduction Membranes for Wearable K+ Sensors

Conductive polymers are recognized as ideal candidates for the development of noninvasive and wearable sensors for real-time monitoring of potassium ions (K+) in sweat to ensure the health of life. However, the low ion-to-electron transduction efficiency and limited active surface area hamper the development of high-performance sensors for low-concentration K+ detection in the sweat. Herein, a wearable K+ sensor is developed by tailoring the nanostructure of polypyrrole (PPy), serving as an ion-to-electron transduction layer, for accurately and stably tracing the K+ fluctuation in human sweat. The PPy nanostructures can be tailored from nanospheres to nanofibers by controlling the supramolecular assembly process during PPy polymerization. Resultantly, the ion-to-electron transduction efficiency (17-fold increase in conductivity) and active surface area (1.3-fold enhancement) are significantly enhanced, accompanied by minimized water layer formation. The optimal PPy nanofibers-based K+ sensor achieved a high sensitivity of 62 mV decade-1, good selectivity, and solid stability. After being integrated with a temperature sensor, the manufactured wearable sensor realized accurate monitoring of K+ fluctuation in the human sweat.

Pinecone derived hierarchical carbon nanostructure as a transducer in a solid-state ion-selective electrode for in vivo analysis of calcium ion

Monitoring extracellular calcium ion (Ca2+) chemical signals in neurons is crucial for tracking physiological and pathological changes associated with brain diseases in live animals. Potentiometry based solid-state ion-selective electrodes (ISEs) with the assist of functional carbon nanomaterials as ideal solid-contact layer could realize the potential response for in vitro and in vivo analysis. Herein, we employ a kind of biomass derived porous carbon as a transducing layer to prompt efficient ion to electron transduction while stabilizes the potential drift. The eco-friendly porous carbon after activation (APB) displays a high specific area with inherit macropores, micropores, and large specific capacitance. When employed as transducer in ISEs, a stable potential response, minimized potential drift can be obtained. Benefiting from these excellent properties, a solid-state Ca2+ selective carbon fiber electrodes (CFEs) with a sandwich structure is constructed and employed for real time sensing of Ca2+ under electrical stimulation. This study presents a new approach to develop sustainable and versatile transducers in solid-state ISEs, a crucial way for in vivo sensing.

Molecularly Imprinted Polymer Biosensor Based on Nitrogen-Doped Electrochemically Exfoliated Graphene/Ti3 CNTX MXene Nanocomposite for Metabolites Detection

Rapid and accurate quantification of metabolites in different bodily fluids is crucial for a precise health evaluation. However, conventional metabolite sensing methods, confined to centralized laboratory settings, suffer from time-consuming processes, complex procedures, and costly instrumentation. Introducing the MXene/nitrogen-doped electrochemically exfoliated graphene (MXene@N-EEG) nanocomposite as a novel biosensing platform in this work addresses the challenges associated with conventional methods, leveraging the concept of molecularly imprinted polymers (MIP) enables the highly sensitive, specific, and reliable detection of metabolites. To validate our biosensing technology, we utilize agmatine as a significant biologically active metabolite. The MIP biosensor incorporates electrodeposited Prussian blue nanoparticles as a redox probe, facilitating the direct electrical signaling of agmatine binding in the polymeric matrix. The MXene@N-EEG nanocomposite, with excellent metal conductivity and a large electroactive specific surface area, effectively stabilizes the electrodeposited Prussian blue nanoparticles. Furthermore, increasing the content of agmatine-imprinted cavities on the electrode enhances the sensitivity of the MIP biosensor. Evaluation of the designed MIP biosensor in buffer solution and plasma samples reveals a wide linear concentration range of 1.0 nM-100.0 μM (R2 = 0.9934) and a detection limit of 0.1 nM. Notably, the developed microfluidic biosensor offers low cost, rapid response time to the target molecule (10 min of sample incubation), good recovery results for detecting agmatine in plasma samples, and acceptable autonomous performance for on-chip detection. Moreover, its high reliability and sensitivity position this MIP-based biosensor as a promising candidate for miniaturized microfluidic devices with the potential for scalable production for point-of-care applications.

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.