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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)
Applied Chemistry for Engineering Vol.35 No.6 pp.513-521
DOI : https://doi.org/10.14478/ace.2024.1076

All-Solid-State Ion-selective Electrodes for Wearable Healthcare Devices

Hong Jun Park, Sung Tae Jang, Bong Gill Choi†
Department of Energy Resources and Chemical Engineering, Kangwon National University, Samcheok 25913, Republic of Korea

1 Co-first authors


† Corresponding Author: Kangwon National University Department of Energy Resources and Chemical Engineering, Samcheok 25913, Republic of Korea Tel: +82-33-570-6545 e-mail: bgchoi@kangwon.ac.kr
October 20, 2024 ; November 8, 2024 ; November 11, 2024

Abstract


Wearable potentiometric sensors have attracted significant attention in advancement in personal healthcare devices and diagnostics due to their remote, non-invasive, and continuous monitoring of health-related signals during daily activities. All-solid- state ion-selective electrodes have promoted practical applications, offering ease of operation, rapid measurement, and low energy consumption. Here, this review describes the principles, structure, and characterization of a potentiometric sensor based on the guidelines of the International Union of Pure and Applied Chemistry (IUPAC). Wearable sensor applications to detect various electrolytes in sweat samples were also discussed, along with their challenges and future opportunities to improve the reliability and accuracy of sensor readings.


Wearable sensor , Healthcare , Ion-selective electrode , Solid contact , Sweat sensor

초록

    1. Introduction

    Flexible and wearable devices enable real-time monitoring of various biophysical and biochemical information in a noninvasive manner, thereby implementing a new pathway in clinical diagnostics[1-3]. These devices can be worn in the form of clothing, fitness bands, and skin patches[4-6], allowing continuous recording of a range of biomarkers, including real-time monitoring of non-invasive body fluids such as sweat, tears, saliva, and urine[7-9]. Wearable sensors enable individuals to monitor their body condition without the need for expensive equipment or trained professionals[10]. However, improving the reliability, user-friendliness, and durability of these devices for long-term use remains the main challenge in collecting meaningful health metrics[11].

    Ion-selective electrodes (ISEs) are a crucial subgroup of electrochemical sensors that convert the activity of specific ions into electrical signals[12]. The potentiometric technique used in ISEs allows for nondestructive and low-energy analysis of electrode potentials at near-zero currents[13,14]. ISEs have been widely adopted for rapid ion detection in clinical, agricultural, environmental, food, and industrial applications owing to their simplicity, cost-effectiveness, small size, timeliness, and reliability[15-18]. Since the commercial development of a thin glass membrane for detecting hydronium ions in the late 1920s, ISEs have been developed for over 60 analytes, including K+, Na+, Ca2+, Cl−, F−, Zn2+, and NH4+[19,20].

    Conventional ISEs contain liquid-contact electrodes with inner filling solutions and separate reference electrodes that are used commercially in routine detection. Glass-based ISEs require careful use and maintenance owing to the evaporation of the inner filling solution and changes in the temperature and pressure of the samples. Additionally, the difference in osmotic pressure between the ionic strength of the sample and the inner filling solution causes water transport into or out of the inner filling solutions, resulting in a significant volume change and delamination of the membranes[21]. Reducing the size of liquid contact electrodes, which is essential to the miniaturization of sensors that is required for detecting small sample volumes, remains challenging. Glass-based microelectrodes with micro-sized sensor parts of less than 100 nm have been employed for a long time; however, their fragility and delicate fabrication process make the removal of the liquid contacts and replacement of conventional electrodes with all-solid- state (AS) electrodes highly desirable[22]. Cattrall et al. proposed the first AS Ca2+ ISE on Pt wire electrodes in 1971[23]. However, the resulting Ca2+ ISE exhibited an unstable potential reading because of the formation of water layers and the blocked interfaces between the ionic and electronic-conducting substrates. For a stable and reliable response, AS ISEs should fulfill three conditions: 1) reversible ion-toelectron conduction, 2) nonpolarizable interfaces with high capacitance, and 3) the absence of side reactions.

    This review outlines the principles and structure of AS potentiometric sensors and recent developments in the field of wearable devices for the noninvasive monitoring of bodily fluids. We provide recommendations for ensuring a more comprehensive adoption of wearable sensor applications.

    2. Theory of transduction mechanism of a potentiometric sensor (structure)

    Figure 1a shows a schematic of a typical potentiometric cell consisting of an AS ISE, an AS reference electrode, and a voltmeter measuring the electromotive force (EMF) under near-zero current conditions. The EMF is the difference in electrical potential between the connecting leads of the ISE and the reference electrode. The sum of all interfacial potentials in the circuit equals the measured EMF, as illustrated in Figure 1a, depending on the Nernst equation[24]. The following equation predicts the linear dependence of the potentiometric response (E) based on the activity function of the ion:

    E = E 0 + R T n F ln [ a ]
    (1)

    where E is the measured EMF, E0 is the standard potential, R is the gas constant, F is the Faraday constant, n is the electron count, and [a] is the activity of the primary ion. The configuration of both electrodes was an ion-selective membrane (ISM)/reference membrane with conductive and support substrates. The ISM consists of an ionophore, a counter ion, and a polymer matrix (Figure 1b). The primary electrolyte interacted with the cavity of the ionophore, which matched the size of the target ions. The ions bonded to the ionophore diffused through the polymeric ISM to the surface of the conductive substrate, resulting in a change in the electrical potential[25]. A typical AS reference electrode consists of an Ag/AgCl half-cell and a porous membrane containing a saturated chloride ion solution[26]. The polymeric reference membrane releases ions into the sample slowly, producing an interfacial potential at the membrane-sample interface that remains constant over a range of sample concentrations, which is attributed to the redox-active reaction in the saturated salt environment.

    2.1. Ion-selective membrane

    The ISM critically determines the selectivity and sensitivity of ISEs. The ISM was mixed with three main components, as shown in Figure 1b: an ionophore, a polymer matrix, and additives such as plasticizers and counter ions. The ionophore binds to and transports specific ions through molecular-sized cavities[19]. The binding strength between the ionophore and ion complex directly influences the selectivity of ISEs. The ionophore can bind ions by coordinating with oxygen atoms. Figure 2a illustrates different types of ionophores, including valinomycin (K+), crown ethers (Na+ and Li+), and non-actin (NH4+). The polymer matrix provides a structural framework for the ISM and enhances the mechanical stability[27]. Poly(vinyl chloride) (PVC) is typically used because of its versatility and compatibility with various additives (e.g., counter ions and plasticizers). Other polymers, such as polyurethane (PU) and silicone rubber, have also been explored to improve the biocompatibility and flexibility of wearable devices[28,29]. Parrilla et al. developed a textile-based potentiometric sensor using a PU-based ISM for noninvasive real-time monitoring (Figure 2b)[30]. The wearable sensor delivered consistent electrochemical performance under various deformations, such as bending, twisting, and stretching (Figure 2c). The high porosity of the polymer matrix further enhances the ionic conductivity of the membrane. Plasticizers, including 2-nitrophenyl octyl ether and dioctyl sebacate, were added to the matrix to increase the mobility of the ionophore and other components[31,32]. The hydrophobic counter ions stabilized the ion-ionophore complex, resulting in stable potential readings[33]. The appropriate combination of ionophores, polymer matrix, and counter ions enables the ISM to selectively interact with the target ions, thereby producing a reliable electrical signal.

    2.2. Solid contact

    Figure 1b shows an AS ISE consisting of an ISM, solid contact, and conductive substrate. Solid contact, or an ion-to-electron transducer, facilitates the interaction between the ion-ionophore complex and the conductive substrate via reduction/oxidation (redox) or electrical double- layer (EDL) capacitance[22]. Transducer layers affect the properties of ISEs, such as their potential stability, long-term lifetime, response time, and reproducibility[34]. Ion-to-electron transducers are classified into two main categories, based on their redox and EDL mechanisms. Redox-based transducers include redox polymers, metal oxides, and electroactive compounds such as couple-doped gold particles. The ion-to-electrode mechanism is as follows.

    M + A −   +   e −   →   M + A −

    where M+ represents a metal ion or oxidized polymer, M is a metal or neutral polymer, and A− is an anion. Conducting polymers such as polyaniline, polypyrrole, and PEDOT:PSS are used as ion-to-electron transducers because of their simplicity, ease of processability, and high redox capacitance[35-37]. Figure 3a shows the PEDOT:PSS-based wearable sensor for the real-time monitoring of K+ and Na+ concentrations in human sweat[37]. The flexibility of the polymer allowed the detection of biomarkers on human skin during physical activity (Figure 3b). The redox reaction involves the reversible doping and dedoping of the conjugated polymer backbone with anions and cations. However, the formation of water layers and high sensitivity to oxygen and light can result in unreliable and inaccurate measurements of the target materials.

    The interfacial potential was stabilized using a transducer with a large EDL capacitance at the ISM/solid contact interface[18]. The primary ions were asymmetrically distributed on either side of the ISM/solid contact interface, where the negative charges in the transducer attracted ions to the ISM, forming an EDL capacitance. The quantity of charge in the EDL determines the interfacial potential, and increasing the EDL capacitance improves the potential stability of ISEs. A key approach to increasing the EDL capacitance is to maximize the interfacial contact area between the ISM and the transducer. Porous carbon materials such as porous graphite rods and compressed charcoal provide a large contact area[38,39]. Furthermore, well-ordered nanostructured carbon materials, including three-dimensionally ordered microporous (3DOM) carbon, carbon nanotubes (CNTs), and graphene nanosheets (GNs), have been employed as advanced solid contacts[ 40-43]. Figure 3c shows defect-free GNs with a hydrophobic surface used for detecting potassium ions (K+)[42]. The sensor exhibited outstanding potential stability and reproducibility, with no formation of water layers (Figures 3d and e). Furthermore, Yoon et al. prepared surface- modified GNs using hydrophobic ionic liquids (GN-IL)[43]. The GN-IL-based sensor exhibited stable electrochemical performance during long-term measurements. For excellent ion-to-electron transduction, solid contacts should possess the following properties: 1) a hydrophobic surface, 2) high redox or EDL capacitance, and 3) minimal side effects.

    2.3. Conductive substrates

    Conductive substrates are essential for the formation of electrical circuits, and serve as pathways for electron transport. Therefore, while selecting a conductive substrate, a balance should be considered between conductivity, stability, and cost. Metal-based substrates such as gold, platinum, and copper have been widely used in potentiometric sensors[ 44-46]. Gold-based substrates are highly conductive, inert, and offer good reproducibility[22]. However, the complex patterning and posttreatment processes of metal substrates limit their applicability in commercial disposable sensors. Although more affordable than gold and platinum, copper exhibits corrosive properties that can reduce the selectivity and long-term use of the sensor. Graphitic solvent deposition methods, such as screen printing and roll-to-roll processes, have been employed in industrial coatings and manufacturing[47]. Printing techniques enable large-scale production of carbon-based electrodes with precise control over the pattern size and shape, ensuring reproducibility of the sensors. Park et al. fabricated carbon ink-based pH sensors for monitoring food spoilage (Figures 4a and b)[48]. The use of graphite or carbon materials is restricted because of their relatively poor conductivities compared to those of metals. To address this limitation, advanced carbon materials, such as CNs and CNTs, have been used to form conductive inks[30,49]. Figure 4c shows a screen-printed Na+ sensor using an exfoliated GN ink obtained via a fluidic process. This sensor exhibited excellent performance in terms of theoretical sensitivity (58.3 mV/dec), reproducibility (± 0.7 mV/dec), and fast response times (9.6 s), which is attributed to the high conductivity and large aspect ratio of the GNs. Bandodkar et al. developed a fully stretchable and conductive CNT ink for monitoring the Na+ and glucose levels in human sweat (Figure 4d)[30]. The electrochemical performance of the CNT-based sensor remained consistent under stretching, twisting, and bending (Figure 4e). Conductive inks require the use of toxic solvents such as toluene, tetrahydrofuran, and N-methyl-2-pyrrolidone, posing environmental and health concerns[47]. Therefore, novel formulations based on green solvents such as water should be developed to fabricate environmentally friendly and nontoxic conductive inks.

    3. Characterization

    Potentiometric sensors detect the ion activity in a sample. The potential dependence of the activity, which is described by the Nernst equation, indicates the ideal response of a cell. The performance of the sensor should be evaluated according to the guidelines for reliable sensor properties established by the International Union of Pure and Applied Chemistry (IUPAC)[50,51].

    3.1. Sensitivity

    Sensitivity refers to the slope of the calibration curve plotted using the cell EMF versus the logarithm of the primary ion activity ([a] or concentration). Based on IUPAC recommendations, the cell EMF is plotted on the vertical axis, and pa (negative logarithm of the ion activity, −log[a]) is plotted on the horizontal axis. The slope is the Nernstian slope (2.303 RT/nF), which represents the electrochemical interactions between the ISE and the ion species.

    3.2. Reproducibility

    Reproducibility, a critical factor in the practical use of wearable sensors, is obtained from the standard deviations of the Nernst slopes collected from different sensors. The Nernst slopes provides information on the drift in the measurements and the reproducibility of the electrode. Various factors, such as the formation of water layers, interference of ions and gases, and composition of the ISEs, determine performance uniformity. Hydrophobic solid contacts can prevent water layer formation and improve ion-to-electron transfer between the conductive and ionic substrates, thereby enhancing sensor reproducibility[ 22]. Additionally, printing technologies such as screen printing, roll-to-roll printing, and gravure printing enable the fabrication of uniform electrodes with high reproducibility[49,52].

    3.3. Repeatability

    The repeatability refers to the variation between the first and subsequent EMF measurements at identical ion concentrations. This value can be obtained by measuring the EMF responses by repeatedly increasing and decreasing the concentration of the primary ions. This kinetic process requires sufficient time for the sensor to return to its initial potential[50]. Combining nanostructured materials with high electrical conductivity promotes fast ion-to-electron transfer, enhancing the reversibility and stability of the ion exchange and charge transduction processes[42].

    3.4. Response time

    The response time is an important parameter for determining sensor usability in wearable applications, making accurate evaluation of the response time essential for obtaining correct calibration curves[53]. The measured EMF signals must reach equilibrium or steady-state values to obtain the calibration plots. The IUPAC defines the response time as the time required for the cell potential to reach 90% (T90) or 95% (T95) of its steady-state value. The T90 and T95 methods require steady-state E values prior to time measurements. In practical terms, response time can be evaluated using the differential quotient method (ΔE/Δt). Further, the IUPAC defines the time required for the cell potential/time slope to reach a limiting value (typically ~1 mV/min) based on the experimental conditions. Similar to repeatability, the response time is a kinetic parameter, where increased ion and electron transport leads to fast and reliable sensor readings[48].

    3.5. Selectivity

    Selectivity is the ability to discriminate between primary ions and other interfering ions[54]. The IUPAC defines selectivity (Kpot) based on the modified Nikolsky-Eisenman equation:

    E M F = c o n s t a n t + 2.303 R T Z A F log ( a A + K A , B P o t a B Z A / Z B + ⋯ )
    (2)

    where A is the primary ion, and B is the interfering ion. The potentiometric selectivity coefficient (Kpot) can be obtained using two methods: the separate solution method (SSM) and the fixed interference method (FIM).

    In the SSM method, the potential of a cell is measured using two separate solutions of primary and interfering ions at the same concentration[ 54]. The measured EMF values of the solutions were used to calculate KPot values using the following equation:

    log   K p o t = ( E B − E A ) z A F R T   I n 10 + ( z B − z A z B ) log   a A
    (3)

    In the FIM method, the EMF of a cell is measured for the primary ion at a constant concentration and for interfering ions at varying concentrations[ 54]. The EMF values are plotted against the logarithm of the activity of the interfering ions. Based on the intersection of the extrapolated linear portion of the plot, KPot values were calculated using the following equation:

    K P o t = a A a B Z A / Z B
    (4)

    4. Wearable sensor

    Human biomarkers in blood samples provide valuable health-related information; however, the invasive nature of blood sampling limits user-friendly sensor applications for daily activities[55]. In addition, invasive sampling requires skilled healthcare professionals and access to medical facilities[3]. However, non-invasive biofluids, such as sweat, saliva, tears, and urine, are accessible for the continuous and real-time monitoring of health-related biomarkers. In view of this, flexible potentiometric sensors, developed for noninvasive monitoring of the physiological status of individuals, can be integrated into wearable formats such as clothing, wristbands, and skin patches to track Na+, K+, Ca2+, NH4+, and pH levels in noninvasive biofluids[56,57]. As sweat contains a wealth of chemical information and locations of sweat production are convenient for continuous measurement, wearable sweat sensors have been widely fabricated for healthcare monitoring[58].

    Sodium ion (Na+) is the most prevalent electrolyte in human sweat and significantly affect human fluid balance. The increased concentration of Na+ is associated with higher sweat rates, indicating that it is a marker for electrolyte imbalance and fluid loss[37]. Kim et al. fabricated a wearable Na+ sensor via a screen-printing process using conductive ink (Figure 5a)[56]. The on-body test was successfully performed using a wearable sensor for real-time monitoring of Na+ dynamics during cycling exercise (Figure 5b). The concentration of potassium ion (K+) in sweat is proportional to its level in the blood, offering insights into dehydration, cystic fibrosis, and electrolyte imbalances. Gao et al. developed a wearable sensor array to monitor K+ and Na+ in human sweat[57]. An increase in K+ levels in sweat was observed during prolonged exercise without water intake. Abnormal levels of Ca2+ are associated with conditions such as myeloma, acid-base balance disorder, and renal failure. Nyein et al. reported a wearable platform for monitoring the Ca2+ and pH levels in human sweat (Figure 5c)[59]. An on-body test demonstrated increased pH and decreased Ca2+ levels during physical activity. Monitoring NH4+ levels directly reflects NH4+ levels in the plasma, serving as an indicator of protein metabolic breakdown[60].

    Recently, sensors have been connected to printed circuit boards (PCBs), enabling real-time data displays on mobile applications (Figure 5d)[61]. PCB-based wearable sensors allow users to monitor and analyze their body conditions in real-time. In addition, microfluidic channels can be integrated into electrochemical sensors for efficient sweat sampling. Gao et al. fabricated a microfluidic skin sensor to monitor the physiological information during daily activities[62]. Furthermore, the integration of micro-supercapacitor and solar cells enables multi- modal sensing of physiological biomarkers with untethered battery-free operation.

    5. Conclusion and outlook

    Wearable potentiometric ion sensors developed for healthcare monitoring allow the continuous and user-friendly tracking of electrolyte imbalances, dehydration, and other physiological states during daily activities. This review summarizes the fundamental principles and components of ISEs, their characterization, and the applications of wearable sensors in healthcare. The integration of ISEs into wearable devices has shown remarkable promise for the detection of potential biomarkers (Na+, K+, Ca2+, and NH4+) of an individual’s physiological status. Advancements in solid contacts and conductive substrates, including redox polymers and carbon nanomaterials, have improved the stability of sensor readings by enhancing the ion-to-electron transduction and mechanical stability. For predictive health status, robust and user-friendly systems must be developed to provide reliable sensor signals under the conditions of strain and motion. Practical aspects, such as affordability, low power consumption, and the integration of energy harvesting and storage modules, are essential considerations to ensure widespread use. Furthermore, the large-scale production of wearable sensors will facilitate the collection of valuable healthcare data, thereby helping reveal the correlation between noninvasive analytes and blood samples. This capability is crucial for disease prediction, early diagnosis, and personalized health monitoring at the individual level.

    Acknowledgement

    This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT (No. 2021R1A2C1009926).

    Figures

    ACE-35-6-513_F1.gif
    a) Schematic illustration of the structure of an all-solid-state potentiometric cell, including the potential profile at each interface. b) Components of the ionselective membrane and their behavior.
    ACE-35-6-513_F2.gif
    a) Chemical structures of various ion receptors, such as vancomycin, crown ethers, and nonactin. Reprinted with permission from American Chemical Society, Copyright (1998). b) A textile sensor based on a PU matrix for measuring of K+ and Na+ under c) various deformation conditions. Copyright © 2016 John Wiley and Sons.
    ACE-35-6-513_F3.gif
    a) A cable type wearable sensor based on PEDOT:PSS as a solid contact. b) On-body test for real-time monitoring of K+ and Na+ level during cycling exercise. Reused with permission from American Chemical Society, Copyright (2019). c) Schematic of the preparation of defect-free GNs as a solid contact to improve d) stable potential signals and e) prevent the formation of water layers. Reproduced with permission from Elsevier, Copyright (2020).
    ACE-35-6-513_F4.gif
    a) Screen-printed carbon ink-based pH sensor and b) monitoring changes in pH values for food spoilage. Reprinted with permission from Springer Nature, Copyright (2019). c) Illustration of the preparation of GN ink for fabricating printed electrodes. Reproduced with permission from John Wiley and Sons, Copyright (2021). d) CNT ink-based electrochemical sensors for detecting Na+ and glucose levels under e) different deformation states. Copyright © 2016 American Chemical Society.
    ACE-35-6-513_F5.gif
    a) Schematic of the preparation of GN ink via a fluid dynamic process and screen-printed Na+ sensors. b) Wearable device system for real-time monitoring of Na+ dynamics during physical exercise. Reprinted with permission from Springer Nature, Copyright (2021). c) Potentiometric Ca2+ and pH sensors for healthcare monitoring during physical activity. Reused with permission from American Chemical Society, Copyright (2016). d) PCB-based wearable sensor platform for detecting pH levels in food and biofluids. Reproduced with permission from Elsevier, Copyright (2020).

    Tables

    References

    1. S. Imani, A. J. Bandodkar, A. M. V. Mohan, R. Kumar, S. Yu, J. Wang, and P. P. Mercier, A wearable chemical–electrophysiological hybrid biosensing system for real-time health and fitness monitoring, Nat. Commun., 7, 11650 (2016).
    2. A. Keshet, L. Reicher, N. Bar, and E. Segal, Wearable and digital devices to monitor and treat metabolic diseases, Nat. Metab., 5, 563-571 (2023).
    3. H. C. Ates, P. Q. Nguyen, L. Gonzalez-Macia, E. Morales-Narváez, F. Güder, J. J. Collins, and C. Dincer, End-to-end design of wearable sensors, Nat. Rev. Mater., 7, 887-907 (2022).
    4. I. Wicaksono, C. I. Tucker, T. Sun, C. A. Guerrero, C. Liu, W. M. Woo, E. J. Pence, and C. Dagdeviren, A tailored, electronic textile conformable suit for large-scale spatiotemporal physiological sensing in vivo, npj Flex. Electron., 4, 5 (2020).
    5. H. Y. Y. Nyein, M. Bariya, B. Tran, C. H. Ahn, B. J. Brown, W. Ji, N. Davis, and A. Javey, A wearable patch for continuous analysis of thermoregulatory sweat at rest, Nat. Commun., 12, 1823 (2021).
    6. S. Gyu Son, H. Jun Park, S.-M. Kim, S. Jin Kim, M. Sik Kil, J.-M. Jeong, Y. Lee, Y. Eom, S. Yeon Hwang, J. Park, and B. Gill Choi, Ultra-fast self-healable stretchable bio-based elastomer/graphene ink using fluid dynamics process for printed wearable sweat-monitoring sensor, Chem. Eng. J., 454, 140443 (2023).
    7. C. Huang, Z. Hao, Z. Wang, H. Wang, X. Zhao, and Y. Pan, An Ultraflexible and Transparent Graphene-Based Wearable Sensor for Biofluid Biomarkers Detection, Adv. Mater. Technol., 7, 2101131 (2022).
    8. Y. Bi, M. Sun, J. Wang, Z. Zhu, J. Bai, M. Y. Emran, A. Kotb, X. Bo, and M. Zhou, Universal fully integrated wearable sensor arrays for the multiple electrolyte and metabolite monitoring in raw sweat, saliva, or urine, Anal. Chem., 95, 6690-6699 (2023).
    9. R. Liu, Q. Nie, Y. Wang, Y. Wu, Y. Tu, C. Xie, X. Xiao, R. You, and Y. Lu, Diaper-based wearable SERS sensing system with a silver nano dual-structure composite hydrogel for the detection of biomarkers and pH in urine, Chem. Eng. J., 498, 155207 (2024).
    10. J. Li, H. Jia, J. Zhou, X. Huang, L. Xu, S. Jia, Z. Gao, K. Yao, D. Li, B. Zhang, Y. Liu, Y. Huang, Y. Hu, G. Zhao, Z. Xu, J. Li, C. K. Yiu, Y. Gao, M. Wu, Y. Jiao, Q. Zhang, X. Tai, R. H. Chan, Y. Zhang, X. Ma, and X. Yu, Thin, soft, wearable system for continuous wireless monitoring of artery blood pressure, Nat. Commun., 14, 5009 (2023).
    11. M. Bariya, H. Y. Y. Nyein, and A. Javey, Wearable sweat sensors, Nat. Electron., 1, 160-171 (2018).
    12. G. Dimeski, T. Badrick, and A. S. John, Ion Selective Electrodes (ISEs) and interferences—A review, Clin. Chim. Acta, 411, 309-317 (2010).
    13. J. H. Yoon, S. B. Hong, S.-O. Yun, S. J. Lee, T. J. Lee, K. G. Lee, and B. G. Choi, High performance flexible pH sensor based on polyaniline nanopillar array electrode, J. Colloid Interface Sci., 490, 53-58 (2017).
    14. J. M. Barat, L. Gil, E. García-Breijo, M. C. Aristoy, F. Toldrá, R. Martínez-Máñez, and J. Soto, Freshness monitoring of sea bream (Sparus aurata) with a potentiometric sensor, Food Chem., 108, 681-688 (2008).
    15. J. H. Yoon, K. H. Kim, N. H. Bae, G. S. Sim, Y.-J. Oh, S. J. Lee, T. J. Lee, K. G. Lee, and B. G. Choi, Fabrication of newspaper-based potentiometric platforms for flexible and disposable ion sensors, J. Colloid Interface Sci., 508, 167-173 (2017).
    16. V. I. Adamchuk, E. D. Lund, B. Sethuramasamyraja, M. T. Morgan, A. Dobermann, and D. B. Marx, Direct measurement of soil chemical properties on-the-go using ion-selective electrodes, Comput. Electron. Agr., 48, 272-294 (2005).
    17. S. Anastasova, A. Radu, G. Matzeu, C. Zuliani, U. Mattinen, J. Bobacka, and D. Diamond, Disposable solid-contact ion-selective electrodes for environmental monitoring of lead with ppb limit-of-detection, Electrochim. Acta, 73, 93-97 (2012).
    18. J. H. Yoon, H. J. Park, S. H. Park, K. G. Lee, and B. G. Choi, Electrochemical characterization of reduced graphene oxide as an ion-to-electron transducer and application of screen-printed all-solid-state potassium ion sensors, Carbon Lett., 30, 73-80 (2020).
    19. P. Bühlmann, E. Pretsch, and E. Bakker, Carrier-based ion-selective electrodes and bulk optodes. 2. Ionophores for potentiometric and optical sensors, Chem. Rev., 98, 1593-1688 (1998).
    20. E. Bakker, P. Bühlmann, and E. Pretsch, Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics, Chem. Rev., 97, 3083-3132 (1997).
    21. M. Cuartero and G. A. Crespo, All-solid-state potentiometric sensors: A new wave for in situ aquatic research, Curr. Opin. Electrochem., 10, 98-106 (2018).
    22. J. Hu, A. Stein, and P. Bühlmann, Rational design of all-solid-state ion-selective electrodes and reference electrodes, TrAC, Trends Anal. Chem., 76, 102-114 (2016).
    23. R. W. Cattrall and H. Freiser, Coated wire ion-selective electrodes, Anal. Chem., 43, 1905-1906 (1971).
    24. E. Lindner and B. D. Pendley, A tutorial on the application of ion-selective electrode potentiometry: An analytical method with unique qualities, unexplored opportunities and potential pitfalls; Tutorial, Anal. Chim. Acta, 762, 1-13 (2013).
    25. M. Akieh-Pirkanniemi, G. Lisak, J. Arroyo, J. Bobacka, and A. Ivaska, Tuned ionophore-based bi-membranes for selective transport of target ions, J. Membr. Sci., 511, 76-83 (2016).
    26. T. Guinovart, G. A. Crespo, F. X. Rius, and F. J. Andrade, A reference electrode based on polyvinyl butyral (PVB) polymer for decentralized chemical measurements, Anal. Chim. Acta, 821, 72-80 (2014).
    27. B. Mahanty, A. K. Satpati, S. Kumar, A. Leoncini, J. Huskens, W. Verboom, and P. K. Mohapatra, Development of polyvinyl chloride (PVC)-based highly efficient potentiometric sensors containing two benzene-centered tripodal diglycolamides as ionophores, Sens. Actuators B Chem., 320, 127961 (2020).
    28. M. Cuartero, G. A. Crespo, and E. Bakker, Polyurethane ionophore-based thin layer membranes for voltammetric ion activity sensing, Anal. Chem., 88, 5649-5654 (2016).
    29. T. Liang, N. Jiang, S. Zhou, X. Wang, Y. Xu, C. Wu, D. Kirsanov, A. Legin, H. Wan, and P. Wang, Multiplexed all-solidstate ion-sensitive light-addressable potentiometric sensor (ISLAPS) system based on silicone-rubber for physiological ions detection, Anal. Chim. Acta, 1179, 338603 (2021).
    30. A. J. Bandodkar, I. Jeerapan, J.-M. You, R. Nuñez-Flores, and J. Wang, Highly stretchable fully-printed CNT-based electrochemical sensors and biofuel cells: Combining intrinsic and design-induced stretchability, Nano Lett., 16, 721-727 (2016).
    31. N. Siangdee, P. Supchocksoonthorn, J. Suwanboriboon, W. Meesiri, K. Chauyrod, W. Chaisriratanakul, W. Bunjongpru, N. Youngvises, and P. Paoprasert, Flow automatic system using an ion-selective field-effect transistor for the real-time/online detection of ammo- nium ions in aquaculture water, Electrochim. Acta, 499, 144721 (2024).
    32. F. Sundfors, T. Lindfors, L. Höfler, R. Bereczki, and R. E. Gyurcsányi, FTIR-ATR study of water uptake and diffusion through ion-selective membranes based on poly(acrylates) and sili- cone rubber, Anal. Chem., 81, 5925-5934 (2009).
    33. M. Telting-Diaz and E. Bakker, Effect of lipophilic ion-exchanger leaching on the detection limit of carrier-based ion-selective elec- trodes, Anal. Chem., 73, 5582-5589 (2001).
    34. Y. Shao, Y. Ying, and J. Ping, Recent advances in solid-contact ion-selective electrodes: Functional materials, transduction mecha- nisms, and development trends, Chem. Soc. Rev., 49, 4405-4465 (2020).
    35. Y. Bao, J. Yan, J. Hu, and J. Li, Superhydrophobic polyaniline solid contact for potential stability improvement of NH +-selective electrode, Sens. Actuators B Chem., 390, 133997 (2023).
    36. E. Jaworska, A. Michalska, and K. Maksymiuk, Polypyrrole nanospheres – electrochemical properties and application as a solid contact in ion-selective electrodes, Electroanalysis, 29, 123-130 (2017).
    37. J. H. Yoon, S.-M. Kim, Y. Eom, J. M. Koo, H.-W. Cho, T. J. Lee, K. G. Lee, H. J. Park, Y. K. Kim, H.-J. Yoo, S. Y. Hwang, J. Park, and B. G. Choi, Extremely fast self-healable bio-based supramolecular polymer for wearable real-time sweat-monitoring sensor, ACS Appl. Mater. Interfaces, 11, 46165-46175 (2019).
    38. C. Chen, H. Wang, Q. Xiao, M. Zhao, Y. Li, G. Zhao, Y. Xie, X. Chen, and G. Zhu, Porous carbon hollow rod for supercapacitors with high energy density, Ind. Eng. Chem. Res., 58, 22124-22132 (2019).
    39. D. A. Yanchus, D. W. Kirk, and C. Q. Jia, Investigating the effects of biochar electrode macrostructure and dimension on electrical double-layer capacitor performance, J. Electrochem. Soc., 165, A305 (2018).
    40. T. Zhang, C.-Z. Lai, M. A. Fierke, A. Stein, and P. Bühlmann, Advantages and limitations of reference electrodes with an ionic liquid junction and three-dimensionally ordered macroporous carbon as solid contact, Anal. Chem., 84, 7771-7778 (2012).
    41. G. A. Crespo, S. Macho, J. Bobacka, and F. X. Rius, Transduction mechanism of carbon nanotubes in solid-contact ion-selective electrodes, Anal. Chem., 81, 676-681 (2009).
    42. H. J. Park, J.-M. Jeong, J. H. Yoon, S. G. Son, Y. K. Kim, D. H. Kim, K. G. Lee, and B. G. Choi, Preparation of ultrathin defect-free graphene sheets from graphite via fluidic delamination for solid-contact ion-to-electron transducers in potentiometric sensors, J. Colloid Interface Sci., 560, 817-824 (2020).
    43. E. S. Yoon, H. J. Park, J. H. Yoon, and B. G. Choi, Hydrophobic ionic liquid-modified graphene via fluid-dynamic process for ionto-electron transducers for all-solid-state potentiometric sensors, Carbon Lett., 33, 1561-1569 (2023).
    44. X. V. Chen and P. Bühlmann, Ion-selective potentiometric sensors with silicone sensing membranes: A review, Curr. Opin. Electrochem., 32, 100896 (2022).
    45. O. Özbek and C. Berkel, Recent advances in potentiometric analysis: Paper–based devices, Sens. Int., 3, 100189 (2022).
    46. S. A. Hassan, N. B. ElDin, H. E. Zaazaa, A. A. Moustafa, and A. M. Mahmoud, Point-of-care diagnostics for drugs of abuse in biological fluids: application of a microfabricated disposable copper potentiometric sensor, Mikrochim. Acta, 187, 491 (2020).
    47. G. Hu, J. Kang, L. W. T. Ng, X. Zhu, R. C. T. Howe, C. G. Jones, M. C. Hersam, and T. Hasan, Functional inks and printing of two-dimensional materials, Chem. Soc. Rev., 47, 3265-3300 (2018).
    48. H. J. Park, J. H. Yoon, K. G. Lee, and B. G. Choi, Potentiometric performance of flexible pH sensor based on polyaniline nanofiber arrays, Nano Converg., 6, 9 (2019).
    49. H. J. Park, J.-M. Jeong, S. G. Son, S. J. Kim, M. Lee, H. J. Kim, J. Jeong, S. Y. Hwang, J. Park, Y. Eom, and B. G. Choi, Fluid-dynamics-processed highly stretchable, conductive, and printable graphene inks for real-time monitoring sweat during stretching exercise, Adv. Funct. Mater., 31, 2011059 (2021).
    50. R. P. Buck and E. Lindner, Recommendations for nomenclature of ionselective electrodes (IUPAC Recommendations 1994), Pure Appl. Chem., 66, 2527-2536 (1994).
    51. J. M. Pingarrón, J. Labuda, J. Barek, C. M. A. Brett, M. F. Camões, M. Fojta, and D. B. Hibbert, Terminology of electrochemical methods of analysis (IUPAC Recommendations 2019), Pure Appl. Chem., 92, 641-694 (2020).
    52. H. Y. Y. Nyein, M. Bariya, L. Kivimäki, S. Uusitalo, T. S. Liaw, E. Jansson, C. H. Ahn, J. A. Hangasky, J. Zhao, Y. Lin, T. Happonen, M. Chao, C. Liedert, Y. Zhao, L.-C. Tai, J. Hiltunen, and A. Javey, Regional and correlative sweat analysis using highthroughput microfluidic sensing patches toward decoding sweat, Sci. Adv., 5, eaaw9906 (2019).
    53. C. Maccà, Response time of ion-selective electrodes: Current usage versus IUPAC recommendations, Anal. Chim. Acta, 512, 183-190 (2004).
    54. E. Bakker, E. Pretsch, and P. Bühlmann, Selectivity of potentiometric ion sensors, Anal. Chem., 72, 1127-1133 (2000).
    55. M. Parrilla, M. Cuartero, and G. A. Crespo, Wearable potentiometric ion sensors, TrAC, Trends Anal. Chem., 110, 303-320 (2019).
    56. D. S. Kim, J.-M. Jeong, H. J. Park, Y. K. Kim, K. G. Lee, and B. G. Choi, Highly concentrated, conductive, defect-free graphene ink for screen-printed sensor application, Nano-Micro Lett., 13, 87 (2021).
    57. W. Gao, S. Emaminejad, H. Y. Y. Nyein, S. Challa, K. Chen, A. Peck, H. M. Fahad, H. Ota, H. Shiraki, D. Kiriya, D.-H. Lien, G. A. Brooks, R. W. Davis, and A. Javey, Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis, Nature, 529, 509-514 (2016).
    58. M. Rovira, C. Lafaye, S. Demuru, B. P. Kunnel, J. Aymerich, J. Cuenca, F. Serra-Graells, J. M. Margarit-Taulé, R. Haque, M. Saubade, C. Fernández-Sánchez, and C. Jimenez-Jorquera, Assessing the performance of a robust multiparametric wearable patch integrating silicon-based sensors for real-time continuous monitoring of sweat biomarkers, Biosens. Bioelectron., 262, 116560 (2024).
    59. H. Y. Y. Nyein, W. Gao, Z. Shahpar, S. Emaminejad, S. Challa, K. Chen, H. M. Fahad, L.-C. Tai, H. Ota, R. W. Davis, and A. Javey, A wearable electrochemical platform for noninvasive simultaneous monitoring of Ca2+ and pH, ACS Nano, 10, 7216-7224 (2016).
    60. T. Guinovart, A. J. Bandodkar, J. R. Windmiller, F. J. Andrade, and J. Wang, A potentiometric tattoo sensor for monitoring ammonium in sweat, Analyst, 138, 7031-7038 (2013).
    61. J. H. Yoon, S.-M. Kim, H. J. Park, Y. K. Kim, D. X. Oh, H.-W. Cho, K. G. Lee, S. Y. Hwang, J. Park, and B. G. Choi, Highly self-healable and flexible cable-type pH sensors for real-time monitoring of human fluids, Biosens. Bioelectron., 150, 111946 (2020).
    62. Y. Song, R. Y. Tay, J. Li, C. Xu, J. Min, E. Shirzaei Sani, G. Kim, W. Heng, I. Kim, and W. Gao, 3D-printed epifluidic electronic skin for machine learning–powered multimodal health surveillance, Sci. Adv., 9, eadi6492 (2023).
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