Potassium Ion-selective Electrodes Research Articles

A Printed LC Resonator-Based Flexible RFID for Remote Potassium Ion Detection

This article presents a flexible printed radio-frequency identification (RFID) sensor based on a printed inductive&#x2013;capacitive (LC) resonator circuit and a potassium ion-selective electrode (ISE) for remote potassium ion sensing. The potassium ion concentration of the contact solution can be monitored by measuring the change of the resonant frequency of the RFID sensor. The resonant frequency of the sensor can be directly detected by measuring the induced change in the reflection coefficient (<inline-formula> <tex-math notation="LaTeX">$S_{11}$ </tex-math></inline-formula>) of an external interrogator coil that is inductively coupled to the RFID sensor. Results obtained for the RFID sensor exhibited a second-order exponential relationship between the resonant frequency of the sensor and the K^&#x002B; concentration of the solution over 0.001&#x2013;2 mol/L dynamic range values. Effects of varying separation distance between the sensor and the interrogator coil and the effect of temperature variations on sensor&#x2019;s measurement are shown. With less than 2-s response time and the long-term stability, the wireless passive printed sensor has potential for low-cost K^&#x002B; monitoring applications such as K^&#x002B; monitoring in food packages.

All-solid-state potassium-selective sensor based on carbon black modified thermoplastic electrode

Thermoplastic carbon composite electrodes have been introduced by our group as novel electrodes with improved electrochemical properties and previously applied for accurate detection of biomarkers using voltammetric methods. Because of their exceptional properties, we hypothesized they would work well as potentiometric ion-selective electrodes as well. In this work, potassium selective thermoplastic electrodes (TPEs) were developed for potentiometric detection for the first time and applied in biofluids. TPEs were fabricated using mixture of polystyrene and polycaprolactone and two types of graphite (MG1599 and Nano19) for comparison, and modified with carbon black via drop-casting. Carbon black modification was optimized in the presence of various numbers of carbon black layers, while ion selective membrane components were optimized at different component ratios. Under optimized conditions, TPEs exhibited Nernstian response to K+ with sensitivity of 59.3 ± 1.01 mV decade−1 and near-Nernstian response with sensitivity of 56.8 ± 1.7 mV decade−1 within a linear range of 1.0 × 10−4 M-1.0 × 10−1 M with a LOD of 1.0 × 10−4- M for Nano19- and MG1599-based TPEs, respectively. The K+-selective TPEs demonstrated rapid response (4 s) and excellent stability, repeatability and reproducibility. The applicability of electrodes were shown in human pooled urine and artificial interstitial fluid samples with potassium ion concentrations ranging from healthy and unhealthy levels. Therefore, the electrodes could be used in healthcare applications for accurate detection of K+ exists in complex matrix and can be easily adapted to simultaneously monitor other ions for diagnosis of diseases due to miniaturization of TPEs.

Fabrication of laser-induced graphene-based multifunctional sensing platform for sweat ion and human motion monitoring

Human sweat monitoring, which unlike blood monitoring can be easily performed on the skin, can provide valuable personal healthcare data, as well as being suitable for use in diagnosis of disease and other conditions. However, despite the tremendous interest in sweat sensing, the development of low-cost, high-efficiency, multi-biometric sensing-capable platforms is still required for the large-scale utilization of wearable biosensors for extensive monitoring with other physiological signals. Here, we propose a wearable multifunctional LIG-based sensor, complete with Arduino-based readout electronics, which simultaneously monitors concentrations of sodium and potassium ions in human sweat as well as strain generated by vital signals (human motion). Our elastomer substrates were fabricated according to the weight of polydimethylsiloxane (PDMS) and lignin. A wearable multifunctional sensor was developed via the formation of laser-induced graphene (LIG) on PDMS/lignin composite substrates using Laser Direct writing (LDW); PDMS/Lignin transforms into the porous structure after laser irradiation. The fabricated sodium and potassium ion-selective electrodes (ISE), and Ag/AgCl electrode on PDMS/lignin composite were used to measure the potential difference at various different concentrations of both Na+(10−1–10−7) and K+ ions (10−1–10−8), and our sensor showed high sensitivity values of 63.6 mV/dec (Na+, n = 6) and 59.2 mV/dec (K+, n = 7), respectively; these results almost completely follow Nernstian behavior (which predicts values of R2 =0.99988 and 0.99855, respectively) and indicate cyclic stability (5000 cycles, Gauge Factor of ~20). This simple fabrication of a multifunctional and cost-effective sensor platform indicates real-time changes of sodium and potassium ions in sweat and strain generated by human body motion.

Ionic strength-independent potentiometric cation concentration sensing on paper using a tetrabutylammonium-based reference electrode

Ion-selective electrodes (ISEs) are known not to strictly measure the concentration of ions, but their activities. The relationship between the two varies with ionic strength, which may result in measurement errors if samples of variable ionic strength are measured and concentration is chosen as the sensor output. Traditionally, concentration readings from potentiometric signals have been possible if the ionic strength of the solution matches that of the calibrant or is made constant by an ionic strength adjusting buffer. In this work, a direct concentration output is realized for a potassium-selective electrode by using a TBA+ (tetrabutylammonium)-selective membrane as the reference electrode and preloading a TBA+ salt in the solution channel. It is shown that the activity coefficients for K+ and TBA+ change similarly to effectively cancel out the influence of ionic strength, allowing for direct potentiometric concentration measurements with deviations from ideality of < 1 mV. The approach was realized on paper-based analytical devices (PADs). The potentiometric concentration data on paper were found to be independent of ionic strength and, owing to their geometric feature, of sample volume. K+ concentration in grape juice, infused water and green tea analyzed by the newly developed PADs showed good correspondence with Atomic Emission Spectroscopy (AES).

Potentiometric Sensor with High Capacity Composite Composed of Ruthenium Dioxide and Poly(3,4-ethylenedioxythiophene) Polystyrene Sulfonate.

This work presents the first-time application of the ruthenium dioxide–poly(3,4-ethylenedioxythiophene) polystyrene sulfonate high-capacity composite material as a mediation layer in potassium selective electrodes, which turned out to significantly enhance the electrical and analytical parameters of the electrodes. The idea was to combine the properties of two different types of materials: a conducting polymer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, and a metal oxide, ruthenium dioxide, in order to obtain the material for a solid-contact layer of great electrical and physicochemical parameters. The preparation method for composite material proposed in this work is fast and easy. The mediation layer material was examined using a scanning electron microscope and chronopotentiometry in order to confirm that all requirements for mediation layers materials were fulfilled. Ruthenium dioxide–poly(3,4-ethylenedioxythiophene) polystyrene sulfonate nancomposite material turned out to exhibit remarkably high electrical capacitance (of approximately 17.5 mF), which ensured great performance of designed K+-selective sensors. Electrodes of electrical capacity equal to 7.2 mF turned out to exhibit fast and stable (with only 0.077 mV potential change per hour) potentiometric responses in the wide range of potassium ion concentrations (10−6 M to 10−1 M). The electrical capacity of ruthenium dioxide–poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-contacted electrodes characterized by electrical capacitance parameters was the highest reported so far for this type of sensor.

Potassium-Selective Solid-Contact Electrode with High-Capacitance Hydrous Iridium Dioxide in the Transduction Layer.

This work presents new material for solid-contact layers—hydrous iridium dioxide IrO2·2H2O, characterized by high electrical capacitance value, evaluated using chronopotentiometry (1.22 mF) and electrochemical impedance spectroscopy (1.57 mF). The remarkable electrical parameters of layers resulted in great analytical parameters of IrO2·2H2O-contacted potassium-selective electrodes. Various parameters of ion-selective electrodes were examined in the scope of this work using a potentiometry method including: linear range, repeatability, stability of potentiometric response and sensitivity to varying measurement conditions. The analytical parameters obtained for solid-contact electrodes were compared with the ones obtained for coated disc electrodes to evaluate the influence of the iridium dioxide layer. The linear range of the IrO2·2H2O-contacted K+-selective electrodes covered concentrations of K+ ions from 10−6 to 10−1 M and the potential stability was estimated at 0.097 mV/h. The IrO2·2H2O-contacted electrodes turned out to be insensitive to varying light exposure and changes in the pH values of measured solutions (in the pH range of 2 to 10.5). A water layer test proved that, contrary to the coated disc electrode, the substantial water film is not formed between the ion-selective membrane and iridium dioxide layer.

High Capacity Nanocomposite Layers Based on Nanoparticles of Carbon Materials and Ruthenium Dioxide for Potassium Sensitive Electrode.

This work presents the new concept of designing ion-selective electrodes based on the use of new composite materials consisting of carbon nanomaterials and ruthenium dioxide. Using two different materials varying in microstructure and properties, we could obtain one material for the mediation layer that adopted features coming of both components. Ruthenium dioxide characterized by high electrical capacity and mixed electronic-ionic transduction and nano-metric carbon materials were reportedly proved to improve the properties of ion-selective electrodes. Initially, only the materials and then the final electrodes were tested in the scope of the presented work, using scanning and transmission electron microscope, contact angle microscope, and various electrochemical techniques, including electrochemical impedance spectroscopy and chronopotentiometry. The obtained results confirmed beneficial influence of the designed nanocomposites on the ion-selective electrodes’ properties. Nanosized structure, high capacity (characterized by the electrical capacitance value from approximately 5.5 mF for GR + RuO2 and CB + RuO2, up to 14 mF for NT + RuO2) and low hydrophilicity (represented by the contact angle from 60° for GR+RuO2, 80° for CB+RuO2, and up to 100° for NT + RuO2) of the mediation layer materials, allowed us to obtain water layer-free potassium-selective electrodes, characterized by rapid and stable potentiometric response in a wide range of concentrations-from 10−1 to 10−6 M K+.

Evaluation of Wearable Potassium Ion Sensor Fabricated By Heat Transfer Printing

Printable wearable sensors for monitoring biomarkers in sweat of people exercising has attracted attention. For example, wearable ion sensors in sweat based on tattoo paper has been reported1).A wearable textile-based sensor has advantages such as low cost, flexibility, and light weight. For example, a stretchable self-powered biosensor has been reported previously 2). On the other hand, when a sensor is formed by printing directly on a cloth, the problem is that the resistance value increases, and the accuracy of the sensor may decrease due to the irregularity of the electrode surface.To solve the problem, we recently reported a wearable potentiometric sodium ion sensor by transfer printing technique3). In case of transfer printing, a sensor is firstly formed on a smooth substrate. Then the sensor is transferred by heating with pressure on a textile with high accuracy irregularities. In this study, we newly produced and evaluated potassium ion sensors by transfer printing technique.Schematic diagram of electrodes and the potassium ion sensor is shown in Figure 1 (a). Electrodes of the sensor were formed on a PET film by laminating overcoat, resist ink, carbon ink, Ag/AgCl ink, and adhesive using screen printing. The potassium ion selective membrane was prepared on the carbon electrode surface by drop-casting 8 μL of the solution which valinomycin, PVC, DOS, and KTCIPB were dissolved in THF and drying overnight. The Ag/AgCl reference electrode was prepared by drop-casting 8 μL of the solution which PVB and NaCl were dissolved in methanol and drying overnight. The sensor on the PET film was formed on the cloth using a heat transfer machine, and the potential difference measurement between the potassium ion selective electrode and the reference electrode was measured using a potassium chloride solution of 10-7 ~ 10-1 M.Figure 1 (b) shows the relationship between EMF of the sensor formed on the cloth by thermal transfer and potassium ion concentration. The sensor shows a linear region proportional to the potassium ion concentration in the range of 0.1 ~ 100 mM. Since the concentration of potassium ions in general human sweat is 4 ~ 24 mM4), the calculated detection lower limit suggests that this sensor may be used to quantify potassium ions in human sweat. Furthermore, selectivity coefficients of sensors for sodium ions and ammonium ions abundant in human sweat were calculated by a single solution method. The selectivity coefficient of sodium ions is , and the selectivity of ammonium ions is . These results suggest selectively measure potassium ion levels in human sweat.References1) Guinovart et al., Anal. Chem., 138, 7031-7038 (2013).2) Jeerapan et al., J. Mater. Chem. A, 4, 18342 (2016).3) Watanabe et al., Chemical Sensors, 33 (Supplement A), 67-69 (2017).4) Sonner et al., Biomicrofluidics, 9, 031301-10 (2015).AcknowledgmentsThis work was partially supported by JST-ASTEP Grant Number JPMJTS1513, JSPS Grant Number 17H02162 and Private University Research Branding Project (2017-2019) from Ministry of Education, Culture, Sports, Science and Technology, and Tokyo University of Science Grant for President's Research Promotion． Figure 1

Conjugated redox polymer with poly(3,4-ethylenedioxythiophene) backbone and hydroquinone pendant groups as the solid contact in potassium-selective electrodes

We have used for the first time a conjugated redox polymer with hydroquinone (HQ) pendant groups covalently attached to the poly(3,4-ethylenedioxythiophene) (PEDOT) backbone as the solid contact (SC) in plasticized poly(vinyl chloride) (PVC) based K+-selective electrodes (K-SCISE). Redox couples are one of the simplest ways to precisely adjust the standard potential (E°) of the SCISEs, but usually the initially high E° reproducibility is lost quite quickly due to leaching out of non-covalently bound redox molecules from the SCISE. In PEDOT-HQ, the covalently attached HQ groups prevent the leaching and simultaneously allow additional charge storage in PEDOT-HQ that is ca. 25–30 times higher than for unsubstituted PEDOT. Before the ion-selective membrane (ISM) deposition, we controlled the potential of the SC with high reproducibility (±0.4 mV, n = 5) by pre-polarization in a mixture of acetonitrile containing potassium tetrakis(pentafluorophenyl)borate and perchloric acid as proton source. Pre-polarization of the SC close to the formal potential where the redox buffer capacity is highest gave the best potential reproducibility. However, after the ISM deposition, the K-SCISEs showed in the best case an E° reproducibility of ±2.8 mV (n = 5). Chronopotentiometric measurements reveal that only a minor fraction of the very high redox capacitance of PEDOT-HQ can be utilized for the ion-to-electron transduction beneath the ISM. The influence of this shortcoming on the E° reproducibility of the SCISEs has been underestimated for most SC materials. Modification of the commonly used PVC-ISM formulations to allow faster ion transfer at the SC/ISM interface could be one way of overcoming the disadvantage.

Characterization of nano-layered solid-contact ion selective electrodes by simultaneous potentiometry and quartz crystal microbalance with dissipation

Nano-layered solid-contact potassium-selective electrodes (K+-ISEs) were explored as model ion-selective electrodes for their practical use in clinical analysis. The ultra-thin ISEs ought to be manufactured in a highly reproducible manner, potentially making them suitable for mass production. Thus, their development is pivotal towards miniaturised sensors with simplified conditioning/calibration protocols for point-of-care diagnostics. To study nano-layered ISEs, the ultra-thin nature of ISEs for the first time enabled to combine potentiometry-quartz crystal microbalance with dissipation (QCM-D) to obtain value-added information on the ISE potentiometric response regarding their physical state such as mass/thickness/viscoelastic properties/structural homogeneity. Specifically, the studies were focused on real-time observations of the ISE potentiometric response in relation to changes of their physicochemical properties during the ISE preparation (conditioning) and operation (including biofouling conditions) to identify the occurring processes that may accordingly be critical for potential instability of the ISEs, impeding their practical application. The K+-ISEs were prepared on a QCM-D gold sensor by electrodepositing poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) layer serving as an ion-to-electron transducer subsequently covered by a spin-coated poly(vinyl chloride) based K+-ion selective membrane (K+-ISM). The studies demonstrated that the performance of the nano-layered design of K+-ISEs is detrimentally affected by such processes as water layer formation accordingly causing the instability of the electrode potential. The changes in the ISE physical state such mass/viscoelastic properties associated with water layer formation and origin of the potential instability was already observed at the ISE conditioning stage. The potential instability of nano-layered ISEs limits their practical applicability, indicating the need of new solutions in designing ISEs, for instance, exploiting new water-resistant materials and modifying preparation protocols.

Printed Wearable Sensor Patch for Multi-Analytes Detection in the Sweat Range

Printed Wearable Sensor Patch for Multi-Analytes Detection in the Sweat Range

Paradox of the Variation of the Bulk Resistance of Potassium Ion-Selective Electrode Membranes within Nernstian Potentiometric Response Range

Bulk resistance and other electrochemical properties of membranes of K+-selective electrodes (ISEs) containing valinomycin are measured by means of chronopotentiometry and electrochemical impedance. It is shown that the bulk resistance of the membranes, within the Nernstian potentiometric response range, increases along decrease of KCl concentration in solution. Analogous results were reported earlier for Ca2+ and NO$$_{3}^{ - }$$ ISEs. This non-constancy of the bulk resistance is in conflict with current views on the mechanism of ISEs response. Tentatively, this paradox is ascribed to heterogeneity of membranes due to water uptake from solution.

Graphdiyne oxide enhances the stability of solid contact-based ionselective electrodes for excellent in vivo analysis

Quantitively and stably tracking ion dynamics in the living brain of animals is essential to understanding many physiological and pathological processes. Solid-state ion-selective electrodes (ISEs) are powerful tools for monitoring the dynamic change of ions at physiological concentration range; however, the unintentional accumulation of an aqueous layer at the ion selective membrane/ solid contact interface compromises the electrode potential stability, limiting its in vivo application. Here, using manganese dioxide (MnO2) and potassium ISE (K+-ISE) as model solid contact and ISEs, we demonstrate for the first time that graphdiyne oxide (GDYO) can enhance the potential stability of solid contact-based ISEs. Our results suggest that the intrinsic structural and hydrophobic features of GDYO, plays a key role in impeding and stabilizing the formation of water layer. With GDYO-MnO2 acting as the solid contact, the K+-ISE displays an excellent short-term potential stability and maintains great selectivity, achieving reliable K+ sensing at the animal level. The GDYO-based strategy is generalizable to different ISEs and does not require complicated processing steps, paving an exciting opportunity for in vivo ion recognition and sensing.

Ordered mesoporous carbon sphere-based solid-contact ion-selective electrodes

All-solid-state potassium ion-selective electrodes (K+ ISEs) using ordered mesoporous carbon (OMC) sphere as the solid contact (SC) are fabricated in this work, which exhibit excellent K+-sensing capability including the stable Nernstian response in a wide linear range of 10−4.19–10−0.21 M, a low detection limit of 5.4 μM, short response time, and good stability. The excellent performances of the K+ ISEs are attributable to the OMC spheres with interconnected mesopore structure, uniform spherical morphology, high surface area, high conductivity, and high capacitance, which provide the highly efficient ion-to-electron transduction between the ion-selective membrane and conductive component in the electrodes. More importantly, the OMC sphere-based SC layer can be deposited in the flexible ISEs by solution processing, which thus enables the construction of ion-sensitive field-effect transistor-based handheld K+-sensing system with high sensitivity and reproducibility, demonstrating the potentials of the OMC spheres in practical ion detection devices.

레이저 유도 그래핀 기반 나트륨, 칼륨 이온 선택성 전극 개발

레이저 유도 그래핀 기반 나트륨, 칼륨 이온 선택성 전극 개발

Electrochemical characterization of reduced graphene oxide as an ion-to-electron transducer and application of screen-printed all-solid-state potassium ion sensors

We report potentiometric performances of ion-to-electron transducer based on reduced graphene oxide (RGO) for application of all-solid-state potassium ion sensors. A large surface area and pore structure of RGO are obtained by a hydrothermal self-assembly of graphene oxide. The extensive electrochemical characterization of RGO solid contact at the interface of ion-selective membrane and gold electrode shows that the potassium ion-selective electrode based on RGO had a high sensitivity (53.34 mV/log[K+]), a low detection of limit (− 4.24 log[K+], 0.06 mM) a good potential stability, and a high resistance to light and gas interferences. The potentiometric K+-sensor device was fabricated by combining of screen-printed electrodes and a printed circuit board. The K+-sensor device accurately measures the ion concentration of real samples of commercial sports drinks, coke and orange juice, and then transfers the collected data to a mobile application through a Bluetooth module. The screen-printed ion sensors based on RGO solid contact show a great potential for real-time monitoring and point-of-care devices in human health care, water-treatment process, and environmental and chemical industries.

Potential Reproducibility of Potassium-Selective Electrodes Having Perfluorinated Alkanoate Side Chain Functionalized Poly(3,4-ethylenedioxytiophene) as a Hydrophobic Solid Contact.

The irreproducibility of the standard potential (E°) is probably the last major challenge for the commercialization of solid-contact ion-selective electrodes (SCISEs) as single-use or wearable sensors. To overcome this issue, we are introducing for the first time a perfluorinated alkanoate side chain functionalized poly(3,4-ethylenedioxythiophene) (PEDOTF) as a hydrophobic SC in potassium-selective electrodes (K-SCISEs) based on plasticized poly(vinyl chloride). The SC incorporates the tetrakis(pentafluorophenyl)borate (TFAB–) anion, which is also present as a lipophilic additive in the ion-selective membrane (ISM), thus ensuring thermodynamic reversibility at the SC/ISM interface and improving the potential reproducibility of the electrodes. We show here that the PEDOTF-TFAB solid contact, which was prepolarized prior to the ISM deposition to either its half or fully conducting form (i.e. different oxidation states) in acetonitrile containing 0.01 M KTFAB, had a very stable open-circuit potential and an outstanding potential reproducibility of only ±0.5 mV (n = 6) for 1 h in the same solution after the prepolarization. This shows that the oxidation state of the highly hydrophobic PEDOTF-TFAB film (water contact angle 133°) is stable over time and can be precisely controlled with prepolarization. The SC was also not light sensitive, which is normally a disadvantage of conducting polymer SCs. After the ISM deposition, the standard deviation of the E° of the K-SCISEs prepared on glassy carbon was ±3.0 mV (n = 5), which is the same as that for conventional liquid contact K-ISEs. This indicates that the ISM deposition is the main source for the potential irreproducibility of the K-SCISEs, which has been overlooked previously.

Wearable All-Solid-State Potentiometric Microneedle Patch for Intradermal Potassium Detection.

A new analytical all-solid-state platform for intradermal potentiometric detection of potassium in interstitial fluid is presented here. Solid microneedles are modified with different coatings and polymeric membranes to prepare both the potassium-selective electrode and reference electrode needed for the potentiometric readout. These microneedle-based electrodes are fixed in an epidermal patch suitable for insertion into the skin. The analytical performances observed for the potentiometric cell (Nernstian slope, limit of detection of 10-4.9 potassium activity, linear range of 10-4.2 to 10-1.1, drift of 0.35 ± 0.28 mV h-1), together with a fast response time, adequate selectivity, and excellent reproducibility and repeatability, are appropriate for potassium analysis in interstitial fluid within both clinical and harmful levels. The potentiometric response is maintained after several insertions into animal skin, confirming the resiliency of the microneedle-based sensor. Ex vivo tests based on the intradermal detection of potassium in chicken and porcine skin demonstrate that the microneedle patch is suitable for monitoring potassium changes inside the skin. In addition, the dimensions of the microneedles modified with the corresponding layers necessary to enhance robustness and provide sensing capabilities (1000 μm length, 45° tip angle, 15 μm thickness in the tip, and 435 μm in the base) agree with the required ranges for a painless insertion into the skin. In vitro cytotoxicity experiments showed that the patch can be used for at least 24 h without any side effect for the skin cells. Overall, the developed concept constitutes important progress in the intradermal analysis of ions related to an electrolyte imbalance in humans, which is relevant for the control of certain types of diseases.

Potassium Ion Selective Electrode Using Polyaniline and Matrix-Supported Ion-Selective PVC Membrane

Solid-state potassium ion selective electrode (K+ ISE) has been the most studied chemical sensors due to its practical importance in biomedical applications. One of the major obstacles that prevented widespread use of solid-state K+ ISE has been output potential drift problem. In this paper, we developed an electrochemical sensing unit in which working, counter, and reference electrodes are integrated in a single plane as all-solid-state form. In order to mitigate the output potential drift, a polyaniline intermediate layer and salt-saturated polyvinylebutyral top coating are introduced in the working and reference electrodes, respectively. Using cyclic voltammetry (CV), uniform layers of polyaniline are deposited on carbon electrode, as confirmed by scanning electron microscope observation. Potentiometry and electrochemical impedance spectroscopy measurement on our K+ ISE show high sensitivity (60.5 mV/decade), low concentration for the limit of detection ( $10^{-5.8}$ M), and large range of linear detection ( $10^{-5}-1$ M), and superior selectivity of K+ ISE against NH4+, Na+, Mg2+, Ca2+, and Fe3+. With its high potential to be miniaturized, we foresee that our solid-state K+ ISE will motivate the future applications in microdevices for clinical analysis, agricultural, and environmental applications.

In-field determination of soil ion content using a handheld device and screen-printed solid-state ion-selective electrodes.

Small-holding farmers in the developing world suffer from sub-optimal crop yields because they lack a soil diagnostic system that is affordable, usable, and actionable. This paper details the fabrication and characterization of an integrated point-of-use soil-testing system, comprised of disposable ion-selective electrode strips and a handheld electrochemical reader. Together, the strips and reader transduce soil ion concentrations into to an alphanumeric output that can be communicated via text message to a central service provider offering immediate, customized fertilizer advisory. The solid-state ion-selective electrode (SS-ISE) strips employ a two-electrode design with screen-printable carbon nanotube ink serving as the electrical contacts for the working and reference electrodes. The working electrode comprises a plasticizer-free butyl acrylate ion-selective membrane (ISM), doped with an ion-selective ionophore and lipophilic salt. Meanwhile, the reference electrode includes a screen-printed silver-silver chloride ink and a polyvinyl-butyral membrane, which is doped with sodium chloride for stable reference potentials. As a proof of concept, potassium-selective electrodes are studied, given potassium’s essential role in plant growth and reproduction. The ISE-based system is reproducibly manufactured to yield a Nernstian response with a sub-micromolar detection limit (pK+ of 5.18 ± 0.08) and near-Nernstian sensitivity (61 mV/decade) in the presence of a 0.02 M strontium chloride extraction solution. Analysis of soil samples using the printed electrodes and reader yielded a correlation coefficient of 𝑅2 = 0.89 with respect to values measured via inductively coupled plasma atomic emission spectroscopy (ICP-AES). The reliable performance of this system is encouraging toward its deployment for soil nutrient management in resource-limited environments.