Ion-selective Sensors Research Articles

Construction of all-solid-state ion-selective sensors using electrolyte-containing polymers.

All-solid-state NO3-, K+, NH4+, Na+, and Ca2+ ion-selective sensors (ISEs) were prepared using polyvinyl butyral (S-LEC®K KX-5). In the present case, polyvinyl butyral was used as a porous material to keep the internal solution of the respective ISE. All sensors exhibited near-Nernst responses in the concentration region between approximately 10-5 and 0.1moldm-3. To avoid the influence of KCl as interfering ions, MgSO4 was used as an electrolyte within the salt bridge. Although the liquid junction potential was generated, the potential difference was stabilized within about few minutes. The NO3--ISE showed high stability with no potential drift during 12h of continuous measurements and maintained high sensitivity even after 3weeks of storage in ultrapure water. Solidification of the internal solution is expected to make the sensor smaller and increase its mechanical strength. As an actual measurement, the sodium concentration in plasma samples using the Na+-ISE was measured to confirm agreement with literature values.

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.

Lab-on-a-chip paper-based electrochemically assisted solid-phase microextraction and ion selective sensor for determination of naproxen in biological fluid samples

BackgroundThe integration of paper-based analytical devices with lab-on-a-chip technology has opened up new possibilities for miniaturized, rapid, low cost and portable diagnostic testing in resource limited settings. This work introduces an integrated lab-on-a-chip platform coupling paper-based analytical devices (LOC-PADs) for dual usage: electrochemically controlled solid-phase microextraction (EC-SPME) and determination of naproxen (NAP) by potentiometric ion-selective electrode (ISE) in biological samples. ResultsConductive papers were successfully fabricated by a chemical procedure. Thereupon, a conducting molecularly imprinted polymer (CMIP) film based on PPy and an anionic model drug (NAP) as a template were applied to the conductive paper substrate via electrochemical methods. A microfluidic chip device was employed to assess the analytical performance of the fabricated paper-based CMIP. The designed chip is divided into two parts with a microfluidic channel between them. The first chamber is an extraction zone for EC-SPME of NAP by using the CMIP paper as a selective sorbent. The second chamber is the detection zone and consists of a paper-based NAP ion selective sensor for potentiometric detection of drug. The analytical process was investigated and optimized for each chamber. Remarkable selectivity towards NAP was achieved in the concentration range from 4.0 × 10−7 to 1.0 × 10−2 mol L−1 with determination coefficient (R2 = 0.99) and the limit of detection (LOD) was 2.0 × 10−7 mol L−1. Anionic Nernstian compliance was gained −58.5 ± 0.5 mV decade−1, and response time in the detection step was 7s for ISE. Satisfactory spiking recovery values within the acceptable range of 90–110 % with RSDs ≤7 % were achieved for the analysis of real samples with fully portable LOC device. SignificanceThe LOC-PADs were favorably used for selective extraction and determination of NAP in serum and saliva samples, respectively. EC-SPME caused sample clean-up, reduced or eliminated different interferences and enhanced selective response of ISE as detection zone of device. Integration of EC-SPME and ISE in a single chip enabled easy and fast determination of trace amounts of NAP in limited real sample volumes, and fully portable advantages.

High-precision micro-total analysis of sodium ions in breast milk

Measuring sodium ion concentration in breast milk can provide crucial health information for both mother and infant, including early signs of low-grade infection and reduced milk supply. Traditional sensing methods are slow, bulky, expensive, and require skilled operators. Here, we develop a coverslip-sized, high-precision lab-on-a-chip device that processes and detects sodium ions in human breast milk. The device uses micro-electrodialysis to extract sodium ions into a simple acceptor solution with 92 ± 3 % efficiency and employs a graphene ion-selective sensor for high-performance quantification. We demonstrate a straightforward calibration strategy, enabling the device to measure breast-milk sodium ion levels in 141 seconds, with accuracy comparable to inductively coupled plasma mass spectrometry. Our approach offers a promising pathway to efficient, point-of-care diagnosis of conditions associated with metal-ion levels in complex liquid-biopsy samples.

Calibration-free and ready-to-use wearable electroanalytical reporting system (r-WEAR) for long-term remote monitoring of electrolytes markers

A major bottleneck in the development of wearable ion-selective sensors is the inherent conditioning and calibration procedures at the user's end due to the signal's instability and non-uniformity. To address this challenge, we developed a strategy that integrates three interdependent materials and device engineering approaches to realize a Ready-to-use Wearable ElectroAnalytical Reporting system (r-WEAR) for reliable electrolytes monitoring. The strategy collectively utilized (1) finely-configured diffusion-limiting polymers to stabilize the electromotive force in the electrodes, (2) a uniform electrical induction in electrochemical cells to normalize the open-circuit potential (OCP), and (3) an electrical shunt to maintain the OCP across the entire sensor in the r-WEAR. The approaches jointly enable fabrication of homogeneously stable and uniform ion-selective sensors, eliminating common conditioning and calibration practices. As a result, the r-WEAR demonstrated a signal's variation down to ±1.99 mV with a signal drift of 0.5 % per hour (0.12 mV h−1) during a 12-h continuous measurement of 10 sensors and a signal drift as low as 13.3 μV h−1 during storage. On-body evaluations of the r-WEAR for four days without conditioning and re-/calibration further validated the sensor's performance in realistic settings, indicating its remarkable potential for practical usage in a user operation-free manner in wearable healthcare applications.

Micro-droplet printed ion-selective membrane sensors for in situ monitoring of marine heavy metal ions

Fast, accurate, and reliable techniques for marine toxic heavy metal ions (HMI) detection are critical for the ecological environment and human health. One of the fatal drawbacks of traditional ion selective electrochemical sensors is that the modification of electrode cannot be accurately quantified, resulting in poor repeatability of the detection electrode and large error between the multi-electrode detection results. In order to tackle this challenge, this study presents ultra-fine micro-droplet printed electrodes for the in-situ detection of Cd2+, a carcinogenic and toxic HMI commonly found in the ocean. The ion selective membrane casting liquid was dispersed into tiny droplets with a diameter of micron through microfluidic technology, and the microdroplets were precisely arranged on the electrode surface. As a result, the modification error of electrode was reduced to pL level (accurate to 10 pL), which greatly improved the repeatability between electrodes prepared in different batches. The results of experiments with pure electrolyte, interference ions and artificial seawater indicated that the micro-droplet printed sensors possessed excellent properties of accuracy, precision, repeatability, and anti-interference. This novel micro-droplet printed sensor has the potential to capture an accurate picture of nearshore HMI in heterogeneous environments under shock conditions.

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.

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.

Electrochemical and statistical study of Nickel ion assessment in daily children intake samples relying on magnesium aluminate spinel nanoparticles

Lately, children's daily consumption of some products, such as cereals and candies, has been rising, which provides a compelling rationale for determining any metallic substances that may be present. Monitoring the concentration of certain metals, like nickel, in these products is necessary due to medical issues in humans when consumed regularly. So, in this work, a novel and highly selective carbon paste as a Ni(II) ion-selective sensor was prepared and investigated using ceramic magnesium aluminum spinel nanoparticles as the ionophore and tritolyl phosphate (TOCP) as a plasticizer. A modified co-precipitation method was used to synthesize the spinel nanoparticles. X-ray diffraction, scanning electron microscope with EDAX, transmission electron microscope, and BET surface area were used to determine the phase composition, microstructure, pores size, particle size, and surface area of the synthesized nanoparticles. The spinel nanoparticle was found to have a nano crystallite size with a cubic crystal system, a particle size ranging from 17.2 to 51.52 nm, mesoporous nature (average pore size = 8.72 nm), and a large surface area (61.75 m2/g). The composition ratio of graphite carbon as a base: TOCP as binder: spinal as ionophore was 67.3:30.0:2.7 (wt%) based on potentiometric detections over concentrations from 5.0 × 10−8 to 1.0 × 10−2 mol L−1 with LOD of 5.0 × 10−8 mol L−1. A measurement of 29.22 ± 0.12 mV decade−1 over pH 2.0–7.0 was made for the Nernstian slope. This sensor demonstrated good repeatability over nine weeks and a rapid response of 8 s. A good selectivity was shown for Ni(II) ions across many interferents, tri-, di-, and monovalent cations. The Ni(II) content in spiked real samples, including cocaine, sweets, coca, chocolate, carbonated drinks, cereals, and packages, were measured. The results obtained indicated no significant difference between the proposed potentiometric method and the officially reported ICP method according to the F- and t-test data. In addition to utilizing ANOVA statistical analysis, validation procedures have been implemented, and the results exceed the ICP-MS methodology.

Potentiometric ion-selective membrane sensing of microbially induced calcite precipitation

Microbially induced calcite precipitation (MICP) is an emerging biogeotechnical technique that facilitates a bio-cementation process through biomineralisation and which can bond otherwise loose soil particles, thereby increasing the shear strength and stiffness of the soil while decreasing its permeability and compressibility. However, the uniformity of calcite (calcium carbonate) precipitation may have a relatively low ‘certainty of execution’ in situ due to strict environmental conditions required for the reaction, highlighting a strong need for in situ monitoring of the subsurface during and following the treatment. This research explored an electrochemical approach to monitoring MICP treatment through the development and use of potentiometric ion-selective sensors to measure ammonium and calcium ions continuously in both aqueous batch reactors and small laboratory-scale soil test cells. Sensor calibrations indicate a strong and reliable correlation between the electrochemical potential and target ion concentration, which shows promise in sensor technology for monitoring MICP and other similar processes. Improvements to the current design are, however, needed, as fouling due to calcite precipitation, and to some extent biofouling of the sensor, was observed in the highly dynamic, biomineralisation environment, which caused data drift/interference. These effects should be considered in data analysis and eliminated/minimised for eventual adoption towards monitoring the efficacy of MICP treatment in situ.

Wearable ion-selective sensors with rapid conditioning and extended stability achieved through modulation of water and ion transport

Solid-contact (SC) ion-selective electrodes (ISEs) are often employed in wearables for electrolytes detection owing to their simplicity and ease of miniaturization. However, to mitigate their inherently unstable open circuit potential signal, ISEs require long hours of conditioning and frequent calibration prior to and during operation, limiting their practicality in wearable applications. Inspired by strategies to address water crossover and flooding in polyelectrolyte fuel cells, we demonstrated a SCISE with minimal conditioning time and long-term stability by modulating the rate-limiting step between mass transfer of water and hydrated ions and redox kinetics in the conducting polymer (CP). Our strategy comprised a wearable ISE with a superhydrophobic CP, PEDOT:TFPB, which reduced water and ion fluxes within the ISE, resulting in a stable and less-swollen CP and diminished water layer formation while maintaining CP's high capacitance. Our PEDOT:TFPB based ISEs functioned after a short conditioning time of 30 min and exhibited extended stability with a reduced signal deviation of only 0.16 % per hour (0.02 mV h−1) during 48 h of continuous measurement. Through systematic studies, we showed that ISE performance could be further tuned by tailoring the thickness of the ion-selective membrane as well as the hydrophobicity and polymerization charges of the CP. Without the need for recurrent calibration, our ISEs sustain high accuracy and prolonged stability upon integration into a wearable format for on-body perspiration analysis. Our strategy allows wearable ion-selective sensors with minimal maintenance at the user-end for long-term continuous monitoring, unveiling their potential in sports, healthcare, and diagnosis fields.

Enhancing flotation beneficiation efficiency of complex ores using ionometry methods

Flotation beneficiation plays a leading role in the processing most ores. The efficiency of this process is ensured by the correct selection of operating modes, which involves choosing the most selective reagents and determining their optimal consumption. Despite the significance of this issue, the classic approach to determining beneficiation parameters involves testing followed by the processing of the results obtained and the determination of the reagent consumption. However, such studies do not reveal the essence of the physicochemical processes occurring within the pulp, and the results of testing one sample may not correspond to the optimum when the properties of the sample change.The purpose of this work is to develop and implement a methodological approach to the study of ore flotation beneficiation using ionometry methods. The data obtained from ion-selective sensors significantly deepen our insight into the transformations occurring during the flotation process and allow for consideration of possible adverse factors that hinder effective process progression.To achieve this goal, a comparative analysis of two approaches to flotation beneficiation testing was performed using complex sulfide ores as examples. In the first stage, a flotation beneficiation study was conducted through D-optimal factor testing, which included 20 individual tests to determine the optimal consumption of modifying reagents, yielding qualitative indicators. In the second stage, flotation tests were conducted using electrochemical monitoring with pH, Ag2S, Pt, and membrane electrodes. A universal flowchart for flotation studies with ion-selective sensors has been developed, facilitating the application of this approach to various ores. The implementation of the results from this comparative analysis has led to a 7.8% increase in beneficiation efficiency while reducing reagent consumption. Additionally, the insights gained into the electrochemical processes occurring allowed for assumptions about the adverse factors affecting flotation outcomes. In conclusion, a model for the potential application of this approach at existing enterprises was proposed, including the implementation of an “intelligent assistant” for flotation operators based on the developed electrochemical models.

Copper (II) Ion Detection in Food and Water Harnessing Schiff Base-Enabled Electrochemical Sensor

A copper (II) ion-selective sensor was generated using a processed membrane that included 4-(2-(2,4-Dinitrophenylhydrazono) Methyl)Benzene-1,3-diol (L). The sensor’s efficacy was tested using a variety of plasticizers, comprising sodium tetraphenylborate (NaTPB), O-Nitrophenyloctyl ether (ONPOE), benzyl acetate (BA), dibutyl phthalate (DBP), and dibutyl sebacate (DBS). Membrane layers comprised of L:DBS:OA:PVC in a ratio of 5:55:10:30 (w/w,%) provided optimum sensing effectiveness. The detection system performed well in an average concentration that ranged from 5.3×10−8 to 1.0×10−1 mol L−1, with a Nernstian slope of 29.1±0.5 mV decade-1 for Cu(II) ions. The sensor’s minimal detection limit of 2.1×10−8 mol, broad pH range (3.1–8.2), quick reaction time (9 s), strong non-aqueous resistance (up to 25% v/v), and good retention time (2 months) demonstrates its value. Potentiometric selectivity coefficients revealed an exclusive exposure for Cu(II) ions under the influence of intervening ions, allowing for accurate identification of copper in a variety of materials such as food oils, tomato plant material, and river water. The proposed sensor is a promising means for accurately detecting Cu(II) ions in environmental and food specimens, with potential utilization in quality assurance and environmental surveillance.

Monitoring of the prohibited 2-phenethylamine in dietary supplements using a t-Butyl calix[8]arene/Ag/CuO composite-based potentiometric sensor

2-phenethylamine (PEA) is a prohibited trace amine, that possesses amphetamine-like effect and toxicity. However, it is still present in many weight loss dietary supplements on which the quantities of PEA are not always listed. Therefore, monitoring of this toxic compound in these hypothetically safe products is essential. Herein, PEA was assayed using ion-selective potentiometric membrane sensors. A novel combination of silver doped copper oxide (Ag/CuO) nanoparticles with 4-tert-butylcalix[8]arene (BCX8) was investigated as a new ion-to-electron transducer layer used with solid contact glassy carbon electrodes. To study the effect of this novel combination, three sensors were proposed based on the incorporation of plain sodium tetraphenylborate (TPB) ion exchanger (sensor 1), TPB with BCX8 only (sensor 2) and TPB with Ag/CuO nanoparticles and BCX8 (sensor 3). The doped metal oxide nanoparticles were prepared using a simple wet co-precipitation synthesis procedure and were characterized for particle shape and size, elemental composition, and optical band gap energy. Near Nernstian responses were achieved with slope values of 52.28 ± 0.45, 54.64 ± 0.98 and 55.34 ± 0.35 mV/concentration decade and LOD of 9.88 × 10−5, 7.18 × 10−5, and 9.77 × 10−6 M for sensors 1, 2 and 3; respectively. Sensor 3 showed linearity range 1 × 10−5 to 1 × 10−2 M and response time of 10 s. The Ag/CuO nanoparticles incorporated within BCX8 network provided improved electron transfer kinetics, and was found to provide the best sensitivity, widest linearity range and the most stable and fastest response among the compared sensors. The proposed sensors showed excellent percent recoveries when applied for the analysis of PEA in its multi-ingredient formulation and showed no statistical difference with the reported method.

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.

PVC plasticized membranes modified with Fe3O4 nanoparticles for potentiometric sensing of sulfate

Potentiometric sensing is one of the most wide spread platforms for analysis of aqueous solutions. The development of potentiometric sensors for hydrophilic anions such as sulfate, phosphate and bicarbonate is a challenging problem, since their high hydration energy results in a low affinity for non-polar polymer membranes – a standard material for potentiometric sensor production.In this study we propose a new type of polymeric sensor membranes for ion-selective sensors with pronounced selectivity to hydrophilic anions. The membranes are based on inorganic Fe3O4 nanoparticles (NP) embedded into plasticized poly(vinyl chloride). Six different formulations of membranes with various combinations of Fe3O4 nanoparticles and other membrane-active components were synthesized and characterized with a wide range of physico-chemical methods. Contact angle measurements, FTIR spectra, SEM and AFM imaging, magnetization hysteresis curves, ZFC/FC studies were performed. The obtained results proved the formation of nanocomposites and demonstrated a structure with “cavities” in membranes with embedded nanoparticles. Potentiometric sensitivity and selectivity of the developed sensors in aqueous solutions of SO42−, HPO42−, HCO3−, H2PO4−, NO3− and Cl−anions were studied. The developed sensors showed high sensitivity to sulfate ion (-22 mV/dec in the concentration range from 10−5 to 10−3 M) and low sensitivity to the typical interfering anions – chloride and nitrate (-14 mV/dec). The optimized sensor membrane composition demonstrated sulfate selectivity higher than that of commercially available ion-selective sensor formulations. The novel sensors were tested for quantification of SO42− in real samples of natural mineral water and the obtained content of sulfate was in a good agreement with the reference data (relative error < 3 %)

Fabrication of Ion selective PVC Membrane Sensor for Potentiometric Determination of Escitalopram Oxalate in its Pure Form and Pharmaceutical Tablet

Fabrication of Ion selective PVC Membrane Sensor for Potentiometric Determination of Escitalopram Oxalate in its Pure Form and Pharmaceutical Tablet

Detection of sodium and potassium ions in human sweat using electrochemical system

Sweat contains a large number of biomarkers which can be used to indicate the healthy state of the body. Real-time detection of ion contents in sweat at trace levels has significant implications for health monitoring and medical diagnosis. In this work, an electrochemical detection system combining an electrochemical detection circuit and an electrochemical sensor with ion selectivity was developed. Based on the system sodium and potassium ions were detected at room temperature using the open circuit voltage-time (OCPT) method. The ion-selective sensors have high sensitivity (98.3 mV/decade for sodium ion detection and 99 mV/decade for potassium ion detection), and good stability. Miniaturized detection system makes it possible to detect sweat on wearable devices and holds great promise in the wearable field.

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

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

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.