sensors

Article Wireless, Flexible, Ion-Selective Electrode System for Selective and Repeatable Detection of Sodium

Hyo-Ryoung Lim 1 , Yun-Soung Kim 1 , Shinjae Kwon 1, Musa Mahmood 1, Young-Tae Kwon 1, Yongkuk Lee 2 , Soon Min Lee 3 and Woon-Hong Yeo 1,4,5,*

1 George W. Woodruff School of Mechanical Engineering, Institute for Electronics and Nanotechnology, Georgia Institute of Technology, Atlanta, GA 30332, USA; [email protected] (H.-R.L.); [email protected] (Y.-S.K.); [email protected] (S.K.); [email protected] (M.M.); [email protected] (Y.-T.K.) 2 Department of Biomedical Engineering, Wichita State University, Wichita, KS 67260, USA; [email protected] 3 Department of Pediatrics, Gangnam Severance Hospital, Yonsei University College of Medicine, Seoul 06273, Korea; [email protected] 4 Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University School of Medicine, Atlanta, GA 30322, USA 5 Parker H. Petit Institute for Bioengineering and Biosciences, Institute for Materials, Neural Engineering Center, Institute for Robotics and Intelligent Machines, Georgia Institute of Technology, Atlanta, GA 30332, USA * Correspondence: [email protected]; Tel.: +1-404-385-5710; Fax: +1-404-894-1658



Received: 12 May 2020; Accepted: 8 June 2020; Published: 10 June 2020


Abstract: Wireless, flexible, ion-selective electrodes (ISEs) are of great interest in the development of wearable health monitors and clinical systems. Existing film-based electrochemical sensors, however, still have practical limitations due to poor electrical contact and material–interfacial leakage. Here, we introduce a wireless, flexible film-based system with a highly selective, stable, and reliable sodium sensor. A flexible and hydrophobic composite with carbon black and soft elastomer serves as an ion-to-electron transducer offering cost efficiency, design simplicity, and long-term stability. The sensor package demonstrates repeatable analysis of selective sodium detection in saliva with good sensitivity (56.1 mV/decade), stability (0.53 mV/h), and selectivity coefficient of sodium against potassium ( 3.0). The film ISEs have an additional membrane coating that provides reinforced − stability for the sensor upon mechanical bending. Collectively, the comprehensive study of materials, surface chemistry, and sensor design in this work shows the potential of the wireless flexible sensor system for low-profile wearable applications.

Keywords: wireless flexible sensor system; ion-selective electrode; sodium detection; carbon–polymer composite transducer

1. Introduction Wireless, flexible film-based ion-selective electrodes (ISEs) have great potential for replacing existing bulky equipment with a miniaturized and mobile system for many clinical applications based on noninvasive measurements [1–3]. Selective detection of sodium in sweat and saliva, correlated to its level in blood [4], has been used for disease diagnosis (e.g., cystic fibrosis and cardiovascular risk) [5,6]. However, it is still very challenging for flexible film ISEs to offer repeatable and accurate measurements over an extended period since they are vulnerable to chemical and mechanical deformation. For instance, flexible film ISEs often lose their sensing capabilities after conditioning, which is an essential processing step that ensures sensor stability, owing to leakages from the softening of the surrounding materials [7–9].

Sensors 2020, 20, 3297; doi:10.3390/s20113297 www.mdpi.com/journal/sensors Sensors 2020, 20, 3297 2 of 11

There are recent studies enabling disposable wireless film systems [10,11], whereas the long-term stability of flexible ISEs requires complicated microfabrication processes and facilities that are highly expensive and time-consuming [12]. Structural deterioration is the root cause that prevents flexible ISEs from repeatable and accurate measurements even during a short period of electrochemical testing. In order to maximize sensor lifetime, many studies focused on the proper wetting between hydrophobic ion-selective membranes (ISMs) and bottom transducers to suppress water-layer formation between the two layers [13,14]. However, poor insulation at the interface with shrouding materials has not been fully considered. Given that most of ISEs use tetrahydrofuran as a solvent, which dissolves a wide range of nonpolar and polar chemical compounds, shrinkage and swelling of the insulating structure upon drying may lead to the decreased lifetime of the film ISEs. Structural deformation is highly important when a polymer composite transducer is involved. Recent studies have introduced carbon-based materials (i.e., carbon black, carbon nanotube, and graphene) mixed with various polymers. The significance of using such composite mixture is easy handling and processing on the flexible curved platforms (e.g., flexible circuits, wearables, and fabrics); however, to the best of our knowledge, none of the carbon-based composite transducers has been applied to flexible electronics [7,14–24]. In this case, a silicone elastomer (Ecoflex) is promising as a polymer matrix because of its high stretchability and biocompatibility, of which a composite with carbon was extensively studied as a strain sensor in flexible wearables [25–29]. Considering that silicone materials swell significantly in contact with a tetrahydrofuran (THF) solvent [30,31], proper insulation that endures polymeric shrinkage should be achieved. Here, this paper demonstrates the applicability and reusability of a wireless, flexible, film ISE system for sodium detection in saliva. A sodium sensor is integrated on the microstructured contact pads of a flexible circuit. A working electrode in the sodium sensor is formed by a hydrophobic carbon black (CB)/silicone elastomer composite and highly selective ISM, along with robust chemical insulation. The CB/polymer, in its novel implementation as an ion-to-electron transducer, provides cost efficiency, design flexibility, and shelf life up to several months. The optimal membrane ingredients in the ISM enable the flexible ISE to achieve good selectivity and accuracy in detecting sodium among monovalent ions [32]. Moreover, we show that a subsequent membrane coating to a film reference electrode (RE) compensates a decrease in linear activity from repetitive mechanical bending. As a result, the flexible sensor system enables the highly accurate and selective detection of sodium in saliva (6.5 to 11.8 mM) for over 3 weeks.

2. Materials and Methods

2.1. Materials 4-tert-Butylcalix [4] arene-tetraacetic acid tetraethyl ester (sodium ionophore X), bis(2-ethylhexyl) sebacate (DOS), poly(vinyl chloride) (PVC), tetrahydrofuran (THF), potassium tetrakis(p-chlorophenyl)borate (KTClPB), hydrochloric acid (HCl), and polyvinyl butyral (PVB) were purchased from Sigma Aldrich. Sodium tetrakis-[3,5-bis(trifluoromethyl)phenyl] borate (NaTFPB) was purchased from Alfa Aesar. Sodium chloride was purchased from Acros Organics, and potassium chloride, calcium chloride dihydrate, and magnesium chloride hexahydrate were from Fisher Chemical. Ecoflex 00-30 was purchased from Smooth-On, and carbon black (CB, Vulcan XC 72R) was obtained from FuelCellStore. Noncyanide silver solution (RioGrande) was used to electroplate a thin Ag film for the solid-state RE.

2.2. Flexible-Circuit Design A pair of electrodes for sodium sensing were integrated onto a flexible circuit. The circuit design followed our prior works [33,34], including a Bluetooth low-energy chip, 2.45 GHz chip antenna, and a rechargeable battery. The flexible circuit was utilized for reading potential differences between Sensors 2020, 20, 3297 3 of 11 electrodes changed in different sodium concentrations and wireless transmission of sensing data (details of the circuit design and list of components in Figure S1 and Table S1).

2.3. Electrode Fabrication The CB/Ecoflex composite was prepared by mixing 9 wt% CB and 91% Ecoflex 00-30 in 15 g of isopropyl alcohol (IPA) by stirring for 30 min at 600 rpm. After complete mixing, the composite paste was simply glued or printed on any substrates during several months of storage at room temperature. Three different substrates were used as electron conductors: Teflon body Au disk (BASi, 3 mm in diameter), Au e-beam evaporated Si wafer, and a Cu pad with a polyimide (PI) shroud on the flexible circuit. The CB/Ecoflex composite transducer was glued on different substrates and cured at 150 ◦C for 1 h. On the flexible circuit, we electrochemically formed silver/silver chloride (Ag/AgCl) RE on the other Cu pad of the circuit. Galvanostatic deposition of Ag was performed at 10 mA/cm2 for 3 min with a stainless-steel CE/RE. After cleaning with DI water, the Ag surface was chlorinated in a 0.1 M KCl and 0.01 M HCl solution at 1 mA/cm2 for 1 min.

2.4. Membrane-Cocktail Fabrication The CB/Ecoflex electrodes were coated with sodium ISM after complete drying. Two types of sodium ISM were used: (i) sodium ionophore X (2.67 mg), DOS (174.53 mg), PVC (88 mg), NaTFPB (1.47 mg) in 2 mL of THF; and (ii) sodium ionophore X (2.0 mg), DOS (200 mg), PVC (100 mg), and KTClPB (0.50 mg) in 2 mL of THF. The molar ratio of ionophore/NaTFPB and ionophore/KTClPB was 1.62:1 and 2:1, respectively. The mixtures were vortexed for 1 h to make a homogeneous solution. The 2 µL ISM was dropped on the electrodes. The Ag/AgCl film RE, formed on a Cu pad of the circuit, was coated with a membrane cocktail composed of 78.1 mg PVB, 50 mg sodium chloride, and 1 mL methanol [35]. The resulting ISEs and RE were dried at room temperature overnight.

2.5. Measurement of Sensing Capabilities Open-circuit potential was measured from a two-electrode system that included the different types of working electrodes along with the RE. A commercial solid-state RE (NT MRX11) was used to measure the signal voltage from the Au disk and Au/Si electrodes. Sodium chloride solutions 7 with different concentrations were used to obtain sensitivity information (10− to 1 M). Considering a 4 normal level in human saliva is 4 to 37 mM, solutions with 10− to 1 M were used for repeatability and selectivity against potassium, magnesium, and calcium ions in the corresponding chloride cation solutions. For testing long-term repeatability and selectivity, we conducted overnight conditioning before initiating measurements (when the sensor was fully dried) and at least three times cleaning upon repetitive measurements to remove any ion sources from the surface. Comparison of the voltage response to the commercial sensor was performed by using an all-solid-state sodium ISE (CNT_ISE M023) and RE (NT MRX11). Saliva samples were collected during each calibration protocol and stored less than 1 h before analysis. Chronopotentiometry was conducted for 60 s at 1 nA. ± Before measurement, the surfaces were soaked for 30 min for conditioning. All sensor measurements were conducted with a Gamry potentiostat (Interface 1010E, Gamry Instruments Inc., Warminster, PA, USA).

3. Results and Discussion

3.1. Wireless Flexible Sodium Sensor for Sodium Detection in Saliva Figure1 shows an overview of a film-type sodium sensor integrated with a flexible wireless electronic system and its sensing capability for saliva. The electronic device included a pair of all-solid-state ISE and RE using Ag/AgCl, an analog-to-digital converter for data acquisition and conversion, and a wireless data-transmission unit (Figure1a). Overall, the low-profile device with 3 2 0.2 cm in size and 2 g in weight exhibited great potential to be used as a portable sensor system × × Sensors 2020, 20, x 4 of 11

Figure 1b demonstrates the good repeatability of voltage signal from the film electrodes based on calibration protocols and corresponding saliva detection, multiple times. The developed thin-film sensor also showed long-term stable data recording with minimal change in voltage (0.53 mV/h; n = 3) adopting a chemically resistant layer around the electrodes (Figure S2; sodium concentration: 10−2 M). As illustrated in Figure 1c, the resulting saliva concentration was in the range of the reported

Sensorsvalues2020 analyzed, 20, 3297 by commercial bulky devices. Specifically, normal saliva sodium level is in the range4 of 11 of 4 to 37 mM [36,37] and 6 to 35 mM [5,6] via photonic and electrochemical detection, respectively. The measured sodium concentration from our sensor ranged from 6.5 to 11.8 mM (7 times), which forwas various consistent types with of the wearable reported applications. range. The Whilefilm sensor real-time showed detection good sensitivity was enabled and by saliva-sensing deploying a Bluetoothcapability low-energywhen compared module, with a thinthose selective for the comme membranercial electrodeall-solid-state directly ISE formedand RE on(56 a mV/decade Cu contact padand on3 to the 13 flexible mM sodium circuit showedin saliva; excellent Figure S3). selectivity Unlike and the long-termcase of bulky stability devices, of sensitivity the thin-film desired ISE for in repeatablethis work use.showed The great flexible applicability sodium sensor for exhibiteda wearable, near-Nernstian portable system, sensitivity along ofwith 56 mVwireless/decade data in 4 sodiumacquisition, concentrations mechanical from flexibility, 10− to and 1 M, repeatable covering asensing typical capabilities. spectrum of saliva [5,6,36,37].

FigureFigure 1. Wireless,Wireless, flexible flexible sodium sodium ion-selective-electrode ion-selective-electrode (ISE) (ISE)system. system. (a) Photograph (a) Photograph of wireless, of wireless,flexible electronic flexible electronic system systemwith all-soli with all-solid-stated-state ISE ISEand andreference reference electrode electrode (RE). (RE). ( (bb)) RepeatedRepeated 4 2 1 measurementsmeasurements of signal voltages in in calibra calibrationtion sodium sodium solutions solutions (concentrations: (concentrations: 10 10−4,− 10,− 102, 10− −,1 10, and− , and1 M) 1 M)and and human human saliva. saliva. (c) (cComparison) Comparison of of measured measured sodium sodium levels levels between between commercial bulkybulky devicesdevices (1–4)(1–4) andand our wireless system,system, wherewhere referencereference 1:1: 8.7–24 mM (generally known) [[36],36], reference 2:2:4–37 4–37 mM mM (photometer) (photometer) [37], [37], reference reference 3:6–35 3: 6–35 mM mM (electrochemical (electrochemical luminescence) luminescence) [5], [ reference5], reference 4:10– 4: 10–1313 mM mM (electrochemical (electrochemical ISE) ISE) [6], [ 6an],d and our our wireless wireless film film ISE: ISE: 6.5–11.8 6.5–11.8 mM. mM.

3.2. DesignFigure and1b demonstrates Fabrication of Film the good ISE repeatability of voltage signal from the film electrodes based on calibration protocols and corresponding saliva detection, multiple times. The developed thin-film sensorFigure also showed 2 shows long-term fabrication stable procedures data recording for the film with ISE, minimal and verification change in voltage strategies (0.53 for mV the/h; flexiblen = 3) structural designs. First, a highly flexible and biocompatible CB/Ecoflex composite was prepared2 as adopting a chemically resistant layer around the electrodes (Figure S2; sodium concentration: 10− M). Asan illustratedion-to-electron in Figure transducer.1c, the resultingAs depicted saliva in Figu concentrationre 2a, the composite was in the transducer range of the was reported simply values mixed analyzedwith IPA by a commercial mechanical bulkystirrer devices.where all Specifically, the ingredients normal were saliva biocompatible sodium level to be is used in the in wearable range of 4sensors. to 37 mM To compare [36,37] and different 6 to 35 insu mMlation [5,6] effects, via photonic Ecoflex and 00-30, electrochemical PVC, and PI were detection, shrouded respectively. around Thethe measuredAu/Si electrode. sodium The concentration verified functionality from our sensor of the ranged composite from 6.5transducer to 11.8 mM is shown (7 times), in whichFigure was S4. consistentWhen formed with on the a reportedstandard range. Teflon-shrouded The film sensor Au disk showed electrode, good the sensitivity CB/Ecoflex/ISM and saliva-sensing showed a capabilitysensitivity when of 57 comparedmV/decade with from those 10−5 for to the1 M commercial sodium with all-solid-state a detection limit ISE and of 4 RE × 10 (56−6 mVM. /Theredecade was and a 3potential to 13 mM drift sodium (ΔE/Δ int) saliva;of 140 Figure µV/s, S3).measured Unlike by the the case chronopotentiometry of bulky devices, the technique. thin-filmISE The in low- this workfrequency showed capacitance great applicability (CL) of the for aCB/Ecoflex wearable, portablesolid contact system, was along calculated with wireless by ΔE/ dataΔt acquisition,= I/CL and mechanicalestimated as flexibility, 7.14 µF. andThis repeatablevalue was sensingrelatively capabilities. low compared to that of conducting polymers [38], but higher than that of the polymer/carbon case (polyaniline/graphene, 0.29 µF) [15]. Overall, the 3.2.sensing Design performance and Fabrication of the of Film film ISE ISE was compatible with conventional ISEs based on carbon– polymer transducers (Table S2). The prepared carbon composite was laminated on a microfabricated Figure2 shows fabrication procedures for the film ISE, and verification strategies for the flexible Au conductor (Figure 2b) and on the Cu pad of the flexible circuit (Figure 2c). Then, it was cured at structural designs. First, a highly flexible and biocompatible CB/Ecoflex composite was prepared 150 °C for 1 h, followed by the coating of a small volume of sodium ISM on top of the transducer. as an ion-to-electron transducer. As depicted in Figure2a, the composite transducer was simply Importantly, the ISE directly formed on the circuit removed contact instability, and the need for space mixed with IPA by a mechanical stirrer where all the ingredients were biocompatible to be used in and carrying capacity to use external equipment. Collectively, the fabrication method of the wearable sensors. To compare different insulation effects, Ecoflex 00-30, PVC, and PI were shrouded CB/Ecoflex composite transducer provides cost efficiency, design flexibility, and simplicity in storage around the Au/Si electrode. The verified functionality of the composite transducer is shown in and handling. Figure S4. When formed on a standard Teflon-shrouded Au disk electrode, the CB/Ecoflex/ISM showed a sensitivity of 57 mV/decade from 10 5 to 1 M sodium with a detection limit of 4 10 6 M. There was a − × − potential drift (∆E/∆t) of 140 µV/s, measured by the chronopotentiometry technique. The low-frequency capacitance (CL) of the CB/Ecoflex solid contact was calculated by ∆E/∆t = I/CL and estimated as 7.14 µF. This value was relatively low compared to that of conducting polymers [38], but higher than that of the polymer/carbon case (polyaniline/graphene, 0.29 µF) [15]. Overall, the sensing performance of the film ISE was compatible with conventional ISEs based on carbon–polymer transducers (Table S2). Sensors 2020, 20, 3297 5 of 11

The prepared carbon composite was laminated on a microfabricated Au conductor (Figure2b) and on the Cu pad of the flexible circuit (Figure2c). Then, it was cured at 150 ◦C for 1 h, followed by the coating of a small volume of sodium ISM on top of the transducer. Importantly, the ISE directly formed on the circuit removed contact instability, and the need for space and carrying capacity to use external equipment. Collectively, the fabrication method of the CB/Ecoflex composite transducer provides cost Sensors 2020, 20, x 5 of 11 efficiency, design flexibility, and simplicity in storage and handling.

Figure 2. Fabrication process of an all-solid-state film ISE. (a) Schematic illustration showing simple Figure 2. Fabrication process of an all-solid-state film ISE. (a) Schematic illustration showing simple procedure for a CB/Ecoflex composite with long-term (several months) storage at room temperature procedure for a CB/Ecoflex composite with long-term (several months) storage at room temperature (RT). (b) Illustrated structure of film ISE, including deposited ion-selective membrane (ISM), CB/Ecoflex (RT). (b) Illustrated structure of film ISE, including deposited ion-selective membrane (ISM), transducer, insulator, and Au conductor on Si wafer. (c) Direct integration of fabricated ISE and CB/Ecoflex transducer, insulator, and Au conductor on Si wafer. (c) Direct integration of fabricated Ag/AgCl reference electrode on Cu pads in the flexible circuit for wireless data acquisition. ISE and Ag/AgCl reference electrode on Cu pads in the flexible circuit for wireless data acquisition. 3.3. Characterization of CB/Ecoflex Composite ISE 3.3. Characterization of CB/Ecoflex Composite ISE 3.3.1. Effect of Insulating Layer 3.3.1. Effect of Insulating Layer The sensing capability of the film ISEs depends not only on the properties of the distinct layers, but alsoThe on sensing their interfacial capability failure. of the film Any ISEs chemical depends damage not only at the on interfacial the properties region of is hardlythe distinct recoverable, layers, butaffecting also theon repeatabilitytheir interfacial of sensors.failure. Here,Any chemical we provide damage additional at the insights interfacial into theregion proper is hardly choice recoverable,of insulating affecting materials the with repeatability respect to chemicalof sensors. resistance Here, we to provide the membrane additional solvent. insights Three into ISEs, the propercovered choice by a diofff insulatingerent insulating materials layer, with were respect used to for chemical testing resistance repeatability. to the Figure membrane3 summarizes solvent. Threethe sensing ISEs, resultscovered with by a di differentfferent insulators insulating and laye a solventr, were (THF)used for that testing is widely repeatability. used in PVC-based Figure 3 summarizesISMs. In Figure the3 sensinga, the platinum-catalyzed results with different silicone insula Ecoflextors and is showna solvent deformed (THF) that upon is thewidely membrane used in PVC-baseddrying at least ISMs. overnight. In Figure The 3a, the use platinum-catalyz of THF typicallyed causes silicone the Ecoflex swelling is ofshown silicone, deformed such as upon Ecoflex the membraneand polydimethylsiloxane drying at least overnight. (PDMS) [30 The,31 ],use which of THF was typically observed causes in a CB the/Ecoflex-only swelling of silicone, case, leading such as to Ecoflexeventual and breakdown polydimethylsiloxane due to delamination (PDMS) (Figure [30,31], S5). which Consequently, was observed the sensor in a CB/Ecoflex-only lost its initial activity case, leadingafter three to uses,eventual resulting breakdown in out-of-range due to delamination sodium levels (Figure in saliva S5). (blue Consequently, highlighted the box, sensor Figure lost3d). its initialAn activity additional after three coating uses, of resulting the core polymerin out-of-range PVC on sodium the top levels of the in saliva interface (blue showed highlighted no visible box, Figuredelamination, 3d). as a membrane was holding compartment in bulky systems (Figure3b) [ 23,39]. However, the voltageAn additional response coating varied of in the accordance core polymer with PVC leakage on the of top the analyteof the interface solution showed during no repetitive visible delamination,measurements, as causing a membrane significant was errorsholding in thecompartment determination in bulky of ion systems concentrations (Figure 3b) (Figure [23,39].3e). However,This delamination the voltage and response leakage failurevaried werein accordance triggered with by the leakage shrinkage of the of analyte the polymeric solution chains during at repetitivethe interfacial measurements, region between causing the significant PVC and errors the soft in the silicone determination upon drying. of ion concentrations A comparison (Figure of the 3e).PVC This film, delamination formed on the and Ecoflex leakage to failure a rigid were substrate, triggered showed by the significant shrinkage shrinkage, of the polymeric pore formation, chains at theand interfacial delamination. region Consequently, between the PVC it made and thethe puresoft silicone PVC inappropriate upon drying. to A be comparison a membrane-holding of the PVC film,compartment formed on in thethe flexibleEcoflex filmto a systemrigid substrate, (Figure S6). showed The use significant of a stable shrinkage, polymer PIpore in Figureformation,3c solved and delamination.this problem since Consequent PI is a goodly, it insulator made the with pure high PVC chemical inappropriate and electrical to be resistance. a membrane-holding Although the compartmentsurface of the siliconein the flexible in the compositefilm system transducer (Figure S6). may The have use beenof a stable affected polymer by the THF,PI in Figure the inorganic 3c solved CB thislikely problem inhibited since further PI is deformation a good insulator during with complete high chemical drying if and insulation electrical was resistance. ensured. TheAlthough measured the surfacedata in Figureof the 3siliconef clearly in demonstrate the composite the desiredtransducer characteristics, may have been including affected repeatable by the THF, testing, the continuous inorganic 1 CBreading likely both inhibited in calibration further solution deformation (10− M) during and saliva complete for 30 min,drying and if long-term insulation stability was ensured. of sensitivity The measured data in Figure 3f clearly demonstrate the desired characteristics, including repeatable testing, continuous reading both in calibration solution (10−1 M) and saliva for 30 min, and long-term stability of sensitivity up to three weeks when stored dried (Figure S7). Therefore, results from this work provide experiment insights into the effect of chemical insulation on the repeated use of film ISEs.

Sensors 2020, 20, 3297 6 of 11 up to three weeks when stored dried (Figure S7). Therefore, results from this work provide experiment insightsSensors 2020 into, 20, the x effect of chemical insulation on the repeated use of film ISEs. 6 of 11

Figure 3. Analysis of repeatability in sodium sensing with different insulating layers. (a–c) Schematic Figure 3. Analysis of repeatability in sodium sensing with different insulating layers. (a–c) Schematic diagrams showing interfacial chemical interactions between membrane solvent and insulating layers. diagrams showing interfacial chemical interactions between membrane solvent and insulating layers. Insulating materials: (a,d) Ecoflex, (b,e) Ecoflex/ poly(vinyl chloride) (PVC), and (c,f) polyimide (PI) Insulating materials: (a, d) Ecoflex, (b, e) Ecoflex/ poly(vinyl chloride) (PVC), and (c, f) polyimide (PI) only. (d–f) Corresponding sensing data per each case in (a–c) showing results from both calibration only. (d–f) Corresponding sensing data per each case in (a–c) showing results from both calibration solutions and saliva. Highlighted blue boxes in (d–f): normal range of sodium concentrations in saliva. solutions and saliva. Highlighted blue boxes in (d–f): normal range of sodium concentrations in saliva. 3.3.2. Effect of Membrane Ingredient 3.3.2. Effect of Membrane Ingredient Our system targeted primarily clinical applications where sodium/potassium selectivity played a crucialOur role system in disease targeted diagnosis. primarily To achieveclinical applications good accuracy, where it was sodium/potassium essential to choose selectivity an appropriate played anionica crucial site, role asin itdisease affected diagnosis. selectivity To asachieve well as good sensitivity accuracy, [32 it,40 was,41 essential]. We selected to choose two an di ffappropriateerent salts, namely,anionic site, NaTFPB as it andaffected KTClPB, selectivity widely as usedwell inas recentsensitivity works [32,40,41]. [42–46]. We Considering selected two that different the extensive salts, conditioningnamely, NaTFPB protocol and is KTClPB, often disregarded widely used in flexible in recent film works electrodes [42–46]. and wearableConsidering platforms, that the we extensive provide theconditioning experiment protocol results is on often using disregarded both salts after in flexible the same film period electrodes of conditioning. and wearable Figure platforms,4a,b shows we theprovide signal the voltage experiment plotted results against on concentrations using both salts of after sodium, the same potassium, period magnesium, of conditioning. and calciumFigure 4a,b for dishowsfferent the film signal ISEs thatvoltage include plotted KTClPB against and concentr sodiumations anionic of sites sodium, NaTFPB. potassium, The selectivity magnesium, coefficient and wascalcium calculated for different by film ISEs that include KTClPB and sodium anionic sites NaTFPB. The selectivity coefficient was calculated by pot LogK A, B = (EB EA) zAF/2.303RT + (1 zA/zB)logαA, (1) LogKpotA, B = (E− B−EA·)·zAF/2.303RT + (1−zA/zB)logαA, (1) wherewhere E, potential; A, A, target target ion; ion; B, B, interfering interfering ion; ion; z, z,valency valency of ion; of ion; F, Faraday’s F, Faraday’s constant; constant; R, gas R, gasconstant; constant; T, temperature; T, temperature; and α and, activityα, activity [47]. Both [47]. ISEs Both showed ISEs showed sodium sodiumsensitivity, sensitivity, with the withNaTFPB the NaTFPBcase being case capable being capableof the selective of the selective detection detection of sodium of sodium (56 mV/decade) (56 mV/decade) against against potassium potassium (31 pot (31mV/decade) mV/decade) with with Kpot KNa+ K+ Naof+ –3.0,K+ of which –3.0, showed which showed better selectivity better selectivity than a CNT/PVC-based than a CNT/PVC-based ISE with pot ISENaTFPB with (K NaTFPBpot Na+ K+ (K= –2.4)Na [22].+ K+ The= –2.4) ISM [ 22formed]. The with ISM the formed KTClPB with exhibited the KTClPB relatively exhibited bad selectivity relatively bad(of less selectivity than –1), (of owing less than to –1),the owinginterfacial to the dissol interfacialution of dissolution the potassium of the ions potassium from the ions membrane. from the membrane.Furthermore, Furthermore, the film ISEs the filmmaintained ISEs maintained their abil theirity ability to detect to detect sodium sodium in insaliva saliva upon upon repeated measurementsmeasurements (55.2 mVmV/decade;/decade; 7 times) and after being stored for 1 week in air (56.1 mVmV/decade)/decade) (Figure(Figure S8).S8). FigureFigure4 c4c compares compares the the detected detected sodium sodium levels levels from from both both ISEs. ISEs. While While the KTClPB the KTClPB case hadcase a had poor a performancepoor performance (1 to (1 54 to mM), 54 mM), the NaTFPBthe NaTFPB showed showed good good detection detection of of sodium sodium in in saliva saliva (6.5–11.8(6.5–11.8 mM). TableTable1 1 summarizes summarizes thethe sensitivitysensitivity andand selectivityselectivity of filmfilm ISEs on thethe basisbasis ofof didifferentfferent anionic sites. GivenGiven thatthat salivasaliva contains contains proteins proteins/e/enzymes,nzymes, as as well well as electrolytesas electrolytes including including ions ions [48], [48], future future work work will examinewill examine long-term long-term performance performance when interacting when intera withcting biomolecules. with biomolecules. Furthermore, Furthermore, other components, other includingcomponents, the ionincluding ionophore the/anionic ion ionophore/anionic site ratio, thickness, site and ratio, conditioning thickness, time and required conditioning for the flexible time filmrequired platform, for the will flexible be considered film platform, in future will work. be considered in future work.

Sensors 2020, 20, 3297 7 of 11 Sensors 2020, 20, x 7 of 11

FigureFigure 4. PerformancePerformance comparison comparison of film of filmISEs with ISEs diff witherent diff anionicerent anionic sites. Sensitivity sites. Sensitivity and selectivity and selectivityresults based results on based(a) potassium on (a) potassium anionic anionicsite (KTClPB) site (KTClPB) and (b and) sodium (b) sodium anionic anionic site site(NaTFPB). (NaTFPB). (c) (Comparisonc) Comparison of ofdetected detected sodium sodium levels levels in insaliva saliva measured measured seven seven times times from from both both ISEs. ISEs.

Table 1. Sensitivity and selectivity data of film ISEs based on different anionic sites. Table 1. Sensitivity and selectivity data of film ISEs based on different anionic sites. KTClPB NaTFPB Interfering ion, J KTClPB NaTFPB Sensitivity (mV/decade) KpotNa+ J Sensitivity (mV/decade) KpotNa+ J Interfering Ion, J Sensitivity Sensitivity + Kpot Kpot K (mV/decade)47.5 Na−+0.6J (mV/decade)31.6 Na+ J −3.0 Mg2+ 21.1 −6.6 37.5 −6.9 K+ 47.5 0.6 31.6 3.0 Ca2+ 3.9 − −7.7 17.7 − −7.6 Mg2+ 21.1 6.6 37.5 6.9 + − − NaCa 2+ 3.9 52.9 7.70 17.7 56.1 7.6 0 − − Na+ 52.9 0 56.1 0 3.4. Sensor-Performance Characterization with Mechanical Bending 3.4. Sensor-PerformanceIn this work, we demonstrated Characterization sensor with Mechanicalperformance Bending with the mechanical bending of the system. The Insensor, this work,including we demonstrated the film ISE and sensor Ag/AgCl performance RE, was with directly the mechanical formed onto bending the Cu of pads the system. of the Thecircuit. sensor, As includingsummarized the filmin Figure ISE and 5, Agthe/ AgClfilm sensor’s RE, was directlyperformance formed was onto measured the Cu pads before of the bending, circuit. under 20° bending, and after relaxation. The measured voltage signals validated a linear response to As summarized in Figure5, the film sensor’s performance was measured before bending, under 20 ◦ bending,sodium ion; and however, after relaxation. sensor Thesensitivity measured (initia voltagelly 58 signals mV/decade) validated showed a linear a substantial response to decrease sodium ion;under however, and after sensor bending sensitivity (48 and (initially49 mV/decade, 58 mV respectively)./decade) showed As shown a substantial in Figure decrease 5b, the mechanical under and afterbending bending caused (48 degradation and 49 mV/decade, of the membrane respectively). stru Ascture shown in the in Figuresensor,5 b,which the mechanical deteriorated bending sensor causedperformance degradation (Figure of 5c). the We membrane observed structure that the inRE the membrane sensor, which had significant deteriorated deformation. sensor performance Thus, the (Figurefollow-up5c). study We observed attempted that the the reinforcement RE membrane of hadtwo significantadditional deformation.PVB–NaCl layers Thus, on the the follow-up deformed studyRE at attemptedrelaxed state. the reinforcementFigure 5d captures of two the additional effect of PVB–NaCl the additi layersonal membrane on the deformed coating RE by at showing relaxed state.similar Figure sensing5d performance captures the ecomparedffect of the to additionalthe case before membrane bending. coating The near-Nernstian by showing similar potential sensing from performancethe reconstructed compared electrodes to the case confirmed before bending. our hypo Thethesis near-Nernstian of membrane potential deformation, from the supported reconstructed by electrodesstrain dependence confirmed of ourkey hypothesiselectrochemical of membrane parameters deformation, [49]. Additionally, supported given by that strain the dependence small form offactor key of electrochemical the circuit leading parameters to the distance [49]. Additionally, between the given ISE and that RE the was small very form narrow factor (3 of mm), the circuitfuture leadingwork will to focus the distance on the use between of appropriate the ISE and salt RE brid wasges very on top narrow of the (3 RE, mm), which future hinders work the will increase focus on of thesodium use ofions appropriate in the vicinity salt bridges of the onRE. top Table of the 2 RE,summarizes which hinders the comparison the increase of ofdevice sodium materials, ions in thestructures, vicinity and of the sensor RE. Table performance2 summarizes of this the work comparison with prior of device reports. materials, Overall, structures, the newly and developed sensor performancewireless and offlexible this work ISE showed with prior advantages reports. Overall,in portability, the newly sensitivity, developed and wirelessselectivity. and flexible ISE showed advantages in portability, sensitivity, and selectivity.

Figure 5. Wireless sodium detection with film ISEs. Photographs of circuit (a) before bending and (b) during 20° bending. (c) Voltage responses with different sodium concentrations for three cases: before bending (black line), during bending (red line), and after bending (green line). (d) Comparison of voltage responses, showing the role of additional polyvinyl butyral (PVB) membrane on RE.

Sensors 2020, 20, x 7 of 11

Figure 4. Performance comparison of film ISEs with different anionic sites. Sensitivity and selectivity results based on (a) potassium anionic site (KTClPB) and (b) sodium anionic site (NaTFPB). (c) Comparison of detected sodium levels in saliva measured seven times from both ISEs.

Table 1. Sensitivity and selectivity data of film ISEs based on different anionic sites.

KTClPB NaTFPB Interfering ion, J Sensitivity (mV/decade) KpotNa+ J Sensitivity (mV/decade) KpotNa+ J K+ 47.5 −0.6 31.6 −3.0 Mg2+ 21.1 −6.6 37.5 −6.9 Ca2+ 3.9 −7.7 17.7 −7.6 Na+ 52.9 0 56.1 0

3.4. Sensor-Performance Characterization with Mechanical Bending In this work, we demonstrated sensor performance with the mechanical bending of the system. The sensor, including the film ISE and Ag/AgCl RE, was directly formed onto the Cu pads of the circuit. As summarized in Figure 5, the film sensor’s performance was measured before bending, under 20° bending, and after relaxation. The measured voltage signals validated a linear response to sodium ion; however, sensor sensitivity (initially 58 mV/decade) showed a substantial decrease under and after bending (48 and 49 mV/decade, respectively). As shown in Figure 5b, the mechanical bending caused degradation of the membrane structure in the sensor, which deteriorated sensor performance (Figure 5c). We observed that the RE membrane had significant deformation. Thus, the follow-up study attempted the reinforcement of two additional PVB–NaCl layers on the deformed RE at relaxed state. Figure 5d captures the effect of the additional membrane coating by showing similar sensing performance compared to the case before bending. The near-Nernstian potential from the reconstructed electrodes confirmed our hypothesis of membrane deformation, supported by strain dependence of key electrochemical parameters [49]. Additionally, given that the small form factor of the circuit leading to the distance between the ISE and RE was very narrow (3 mm), future work will focus on the use of appropriate salt bridges on top of the RE, which hinders the increase of sodium ions in the vicinity of the RE. Table 2 summarizes the comparison of device materials, Sensorsstructures,2020, 20 ,and 3297 sensor performance of this work with prior reports. Overall, the newly developed8 of 11 wireless and flexible ISE showed advantages in portability, sensitivity, and selectivity.

FigureFigure 5. 5.Wireless Wireless sodiumsodium detection with with film film ISEs. ISEs. Photographs Photographs of circuit of circuit (a) before (a) before bending bending and ( andb)

(bduring) during 20° 20 bending.◦ bending. (c) Voltage (c) Voltage responses responses with withdifferent diff erentsodium sodium concentrations concentrations for three for cases: three before cases: beforebending bending (black (black line), line),during during bending bending (red (redline), line), and andafter after bending bending (green (green line). line). (d) Comparison (d) Comparison of ofvoltage voltage responses, responses, showing showing the the role role of of additi additionalonal polyvinyl polyvinyl butyral butyral (PVB) (PVB) membrane membrane on onRE. RE.

Table 2. Comparison of device materials, structure, and performance.

Sensitivity Selectivity Reference Substrate Flexible/Wireless (1) pot Lifetime (mV/decade) (K Na+ K+) This work PI Yes/yes 56.1 –3.0 3 weeks [50] PET Yes/yes 0.031 nF/mM (2) Not available Not available [51] PDMS Yes/no 58 Not available <1 h [52] Tape Yes/no 56.2 Not available Not available [53] Paper Yes/no 55.7 Not available Not available [54] Fiber Yes/no 55.1 Not available 4 weeks [55] Plastic Some/no 55.9 2.2 Not available − [12] Wafer Some/no 56.6 2.5 2 months − (1) All covers sodium level in saliva. (2) Capacitor-based sensor. PI: polyimide, PET: poly(ethylene terephthalate), and PDMS: polydimethylsiloxane.

4. Conclusions This paper introduced a wireless, flexible ISE system for highly sensitive, selective, and stable detection of sodium in saliva. A hybrid composite nanostructure, made of CB and soft elastomer, showed stable operation as an ion-to-electron transducer with long-term stability up to several months when stored at room temperature. The wireless, low-profile sensor package showed good sensitivity (56.1 mV/decade), stability (0.53 mV/h), and selectivity coefficient of sodium against potassium pot (K Na+ K+: –3.0). The additional coating of the PVB membrane on the RE enhanced the stability of the wireless sensor during mechanical bending. Collectively, the presented results in this work capture the potential of the miniaturized, wireless flexible sensor system for applications in portable wearable health monitors and clinical studies.

Supplementary Materials: The following are available online at http://www.mdpi.com/1424-8220/20/11/3297/s1. Figure S1: illustration of wireless ion-selective electrode circuit, Figure S2: voltage stability, Figure S3: saliva concentration measured from a commercial solid-state sensor, Figure S4: hydrophobicity and sensor capability of CB/Ecoflex transducer, Figure S5: drying shrinkage, Figure S6: PVC delamination from Ecoflex, Figure S7: long-term stability, Figure S8: repeatable measurement of sodium in saliva, Table S1: list of components used for circuit, and Table S2: performance comparison of carbon–polymer composite-based ISEs. Author Contributions: H.-R.L., S.M.L., and W.-H.Y. conceived and designed the paper; H.-R.L., Y.-S.K., S.K., M.M., Y.-T.K., and Y.L. conducted experiments and analyzed the data; H.-R.L. and W.-H.Y. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: We acknowledge a research fund from the Marcus Foundation, the Georgia Research Alliance, and the Georgia Tech Foundation through their support of the Marcus Center for Therapeutic Cell Characterization and Manufacturing (MC3M) at Georgia Tech. This work was partially supported by the American Heart Association (grant 19IPLOI34760577) and a faculty research grant of Yonsei University College of Medicine (6-2019-0169). Electronic devices in this work were fabricated at the Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (grant ECCS-1542174). Conflicts of Interest: The authors declare no conflict of interest. Sensors 2020, 20, 3297 9 of 11

References

1. Matzeu, G.; O’Quigley, C.; McNamara, E.; Zuliani, C.; Fay, C.; Glennon, T.; Diamond, D. An integrated sensing and wireless communications platform for sensing sodium in sweat. Anal. Methods 2016, 8, 64–71. [CrossRef] 2. Ainla, A.; Mousavi, M.P.S.; Tsaloglou, M.-N.; Redston, J.; Bell, J.G.; Fernández-Abedul, M.T.; Whitesides, G.M. Open-source potentiostat for wireless electrochemical detection with smartphones. Anal. Chem. 2018, 90, 6240–6246. [CrossRef][PubMed] 3. Lee, Y.; Howe, C.; Mishra, S.; Lee, D.S.; Mahmood, M.; Piper, M.; Kim, Y.; Tieu, K.; Byun, H.-S.; Coffey, J.P. Wireless, intraoral hybrid electronics for real-time quantification of sodium intake toward hypertension management. Proc. Natl. Acad. Sci. USA 2018, 115, 5377–5382. [CrossRef][PubMed] 4. Yoshimura, H.; Miyoshi, M.; Matsumoto, S.; Fujimoto, T.; Yamamoto, Y. Reflection of salt concentrations of blood upon those of saliva. Jpn. J. Physiol. 1963, 13, 523–540. [CrossRef][PubMed] 5. Labat, C.; Thul, S.; Pirault, J.; Temmar, M.; Thornton, S.N.; Benetos, A.; Bäck, M. Differential associations for salivary sodium, potassium, calcium, and phosphate levels with carotid intima media thickness, heart rate, and arterial stiffness. Dis. Markers 2018, 2018, 3152146. [CrossRef][PubMed] 6. Gonçalves, A.C.; Marson, F.A.L.; Mendonça, R.M.H.; Bertuzzo, C.S.; Paschoal, I.A.; Ribeiro, J.D.; Ribeiro, A.F.; Levy, C.E. Chloride and sodium ion concentrations in saliva and sweat as a method to diagnose cystic fibrosis. J. Pediatr. 2019, 95, 443–450. [CrossRef][PubMed] 7. Wardak, C. Solid contact cadmium ion-selective electrode based on ionic liquid and carbon nanotubes. Sens. Actuators B Chem. 2015, 209, 131–137. [CrossRef] 8. Rius-Ruiz, F.X.; Crespo, G.A.; Bejarano-Nosas, D.; Blondeau, P.; Riu, J.; Rius, F.X. Potentiometric strip cell based on carbon nanotubes as transducer layer: Toward low-cost decentralized measurements. Anal. Chem. 2011, 83, 8810–8815. [CrossRef][PubMed] 9. Hu, J.; Zou, X.U.; Stein, A.; Bühlmann, P. Ion-selective electrodes with colloid-imprinted mesoporous carbon as solid contact. Anal. Chem. 2014, 86, 7111–7118. [CrossRef][PubMed] 10. Kim, T.; Bao, C.; Hausmann, M.; Siqueira, G.; Zimmermann, T.; Kim, W.S. 3D printed disposable wireless ion sensors with biocompatible cellulose composites. Adv. Electron. Mater. 2019, 5, 1800778. [CrossRef] 11. Huang, Y.; Wang, T.; Xu, Z.; Hughes, E.; Qian, F.; Lee, M.; Fan, Y.; Lei, Y.; Brückner, C.; Li, B. Real-time in situ monitoring of nitrogen dynamics in wastewater treatment processes using wireless, solid-state, and ion-selective membrane sensors. Environ. Sci. Technol. 2019, 53, 3140–3148. [CrossRef][PubMed] 12. Wang, S.; Wu, Y.; Gu, Y.; Li, T.; Luo, H.; Li, L.-H.; Bai, Y.; Li, L.; Liu, L.; Cao, Y.; et al. Wearable sweatband sensor platform based on gold nanodendrite array as efficient solid contact of ion-selective electrode. Anal. Chem. 2017, 89, 10224–10231. [CrossRef][PubMed] 13. Papp, S.; Bojtár, M.; Gyurcsányi, R.E.E.; Lindfors, T. Potential reproducibility of potassium-selective electrodes having perfluorinated alkanoate side-chain functionalized poly (3, 4-ethylenedioxytiophene) as hydrophobic solid contact. Anal. Chem. 2019, 91, 9111–9118. [CrossRef][PubMed] 14. Kału˙za, D.; Jaworska, E.; Mazur, M.; Maksymiuk, K.; Michalska, A. Multiwalled carbon nanotubes–poly(3-octylthiophene-2,5-diyl) nanocomposite transducer for ion-selective electrodes: Raman spectroscopy insight into the transducer/membrane interface. Anal. Chem. 2019, 91, 9010–9017. [CrossRef] [PubMed] 15. Boeva, Z.A.; Lindfors, T. Few-layer graphene and polyaniline composite as ion-to-electron transducer in silicone rubber solid-contact ion-selective electrodes. Sens. Actuators B Chem. 2016, 224, 624–631. [CrossRef] 16. Javanbakht, M.; Badiei, A.; Ganjali, M.R.; Norouzi, P.; Hasheminasab, A.; Abdouss, M. Use of organofunctionalized nanoporous silica gel to improve the lifetime of carbon paste electrode for determination of copper(II) ions. Anal. Chim. Acta 2007, 601, 172–182. [CrossRef][PubMed] 17. Jaworska, E.; Naitana, M.L.; Stelmach, E.; Pomarico, G.; Wojciechowski, M.; Bulska, E.; Maksymiuk, K.; Paolesse, R.; Michalska, A. Introducing cobalt(II) porphyrin/cobalt(III) corrole containing transducers for improved potential reproducibility and performance of all-solid-state ion-selective electrodes. Anal. Chem. 2017, 89, 7107–7114. [CrossRef][PubMed] 18. Khani, H.; Rofouei, M.K.; Arab, P.; Gupta, V.K.; Vafaei, Z. Multi-walled carbon nanotubes-ionic liquid-carbon paste electrode as a super selectivity sensor: Application to potentiometric monitoring of mercury ion(II). J. Hazard. Mater. 2010, 183, 402–409. [CrossRef][PubMed] Sensors 2020, 20, 3297 10 of 11

19. Liang, R.; Yin, T.; Qin, W. A simple approach for fabricating solid-contact ion-selective electrodes using nanomaterials as transducers. Anal. Chim. Acta 2015, 853, 291–296. [CrossRef][PubMed] 20. Sun, Q.; Li, W.; Su, B. Highly hydrophobic solid contact based on graphene-hybrid nanocomposites for all solid state potentiometric sensors with well-formulated phase boundary potentials. J. Electroanal. Chem. 2015, 740, 21–27. [CrossRef] 21. Topcu, C.; Lacin, G.; Yilmaz, V.; Coldur, F.; Caglar, B.; Cubuk, O.; Isildak, I. Electrochemical determination of copper(II) in water samples using a novel ion-selective electrode based on a graphite oxide–imprinted polymer composite. Anal. Lett. 2018, 51, 1890–1910. [CrossRef] 22. Zhu, J.; Li, X.; Qin, Y.; Zhang, Y. Single-piece solid-contact ion-selective electrodes with polymer–carbon nanotube composites. Sens. Actuators B Chem. 2010, 148, 166–172. [CrossRef] 23. Zhu, J.; Qin, Y.; Zhang, Y. Preparation of all solid-state potentiometric ion sensors with polymer-CNT composites. Electrochem. Commun. 2009, 11, 1684–1687. [CrossRef] 24. Ping, J.; Wang, Y.; Ying, Y.; Wu, J. Application of electrochemically reduced graphene oxide on screen-printed ion-selective electrode. Anal. Chem. 2012, 84, 3473–3479. [CrossRef][PubMed] 25. Amjadi, M.; Yoon, Y.J.; Park, I. Ultra-stretchable and skin-mountable strain sensors using carbon nanotubes–Ecoflex nanocomposites. Nanotechnology 2015, 26, 375501. [CrossRef][PubMed] 26. Cholleti, E.R.; Stringer, J.; Assadian, M.; Battmann, V.; Bowen, C.; Aw, K. Highly stretchable capacitive sensor with printed carbon black electrodes on barium titanate elastomer composite. Sensor 2018, 19, 42. [CrossRef] [PubMed] 27. Kumbay Yildiz, S.; Mutlu, R.; Alici, G. Fabrication and characterisation of highly stretchable elastomeric strain sensors for prosthetic hand applications. Sens. Actuators A Phys. 2016, 247, 514–521. [CrossRef] 28. Shintake, J.; Piskarev, E.; Jeong, S.H.; Floreano, D. Ultrastretchable strain sensors using carbon black-filled elastomer composites and comparison of capacitive versus resistive sensors. Adv. Mater. Technol. 2018, 3, 1700284. [CrossRef] 29. Zhu, Y.; Assadian, M.; Ramezani, M.; Aw, K.C. Printing of soft stretch sensor from carbon black composites. Proceedings 2018, 2, 732. [CrossRef] 30. Giffney, T.; Xie, M.; Sartelet, M.; Aw, K.C. Vapor phase polymerization of PEDOT on silicone rubber as flexible large strain sensor. Aims Mater. Sci. 2015, 2, 414–424. 31. Wu, W.; Wu, J.; Kim, J.-H.; Lee, N.Y. Instantaneous room temperature bonding of a wide range of non-silicon substrates with poly(dimethylsiloxane) (PDMS) elastomer mediated by a mercaptosilane. Lab. A Chip 2015, 15, 2819–2825. [CrossRef][PubMed] 32. Amemiya, S.; Bühlmann, P.; Pretsch, E.; Rusterholz, B.; Umezawa, Y. Cationic or anionic sites? Selectivity optimization of ion-selective electrodes based on charged ionophores. Anal. Chem. 2000, 72, 1618–1631. [CrossRef][PubMed] 33. Mishra, S.; Kim, Y.-S.; Intarasirisawat, J.; Kwon, Y.-T.; Lee, Y.; Mahmood, M.; Lim, H.-R.; Herbert, R.; Yu, K.J.; Ang, C.S. Soft, wireless periocular wearable electronics for real-time detection of eye vergence in a virtual reality toward mobile eye therapies. Sci. Adv. 2020, 6, eaay1729. [CrossRef][PubMed] 34. Mahmood, M.; Mzurikwao, D.; Kim, Y.-S.; Lee, Y.; Mishra, S.; Herbert, R.; Duarte, A.; Ang, C.S.; Yeo, W.-H. Fully portable and wireless universal brain–machine interfaces enabled by flexible scalp electronics and deep learning algorithm. Nat. Mach. Intell. 2019, 1, 412–422. [CrossRef] 35. Guinovart, T.; Crespo, G.A.; Rius, F.X.; Andrade, F.J. A reference electrode based on polyvinyl butyral (PVB) polymer for decentralized chemical measurements. Anal. Chim. Acta 2014, 821, 72–80. [CrossRef][PubMed] 36. Singh, G.; Iyer, E.M.; Malik, H. Relative changes in salivary sodium and potassium in relation to exposure to high G stress. Med. J. Armed Forces India 1994, 50, 261–265. [CrossRef] 37. White, A.G.; Entmacher, P.S.; Rubin, G.; Leiter, L. Physiological and pharmacological regulation of human salivary electrolyte concentrations; with a discussion of electrolyte concentrations of some other exocrine secretions. J. Clin. Investig. 1955, 34, 246–255. [CrossRef][PubMed] 38. Bobacka, J. Potential stability of all-solid-state ion-selective electrodes using conducting polymers as ion-to-electron transducers. Anal. Chem. 1999, 71, 4932–4937. [CrossRef][PubMed] 39. Lai, C.-Z.; Fierke, M.A.; Stein, A.; Bühlmann, P. Ion-selective electrodes with three-dimensionally ordered macroporous carbon as the solid contact. Anal. Chem. 2007, 79, 4621–4626. [CrossRef][PubMed] 40. Khaled, E.; Hassan, H.; Mohamed, G.G.; Ragab, F.A.; Seleim, A. β-Cyclodextrin-based potentiometric sensors for flow-injection determination of acetylcholines. Int. J. Electrochem. Sci. 2010, 5, 448–458. Sensors 2020, 20, 3297 11 of 11

41. Al-Alamein, A.M.A.; Kamel, M.S.; El-Alamin, M.M.A.; Khaled, E. Novel pioglitazone nanomaterial based screen printed sensors. Int. J. Electrochem. Sci. 2015, 10, 2400–2412. 42. Chen, X.; Hu, Q.; Chen, S.; Netzer, N.L.; Wang, Z.; Zhang, S.-L.; Zhang, Z. Multiplexed analysis of molecular and elemental ions using nanowire transistor sensors. Sens. Actuators B Chem. 2018, 270, 89–96. [CrossRef] 43. Luboch, E.; Jeszke, M.; Szarmach, M.; Łukasik, N. New bis(azobenzocrown)s with dodecylmethylmalonyl linkers as ionophores for sodium selective potentiometric sensors. J. Incl. Phenom. Macrocycl. Chem. 2016, 86, 323–335. [CrossRef][PubMed] 44. Moya, A.; Illa, X.; Gimenez, I.; Lazo-Fernandez, Y.; Villa, R.; Errachid, A.; Gabriel, G. Miniaturized multiparametric flexible platform for the simultaneous monitoring of ionic: Application in real urine. Sens. Actuators B Chem. 2018, 255, 2861–2870. [CrossRef] 45. Paczosa-Bator, B.; Pi˛ek,M.; Piech, R. Application of nanostructured TCNQ to potentiometric ion-selective K+ and Na+ electrodes. Anal. Chem. 2015, 87, 1718–1725. [CrossRef][PubMed] 46. Totu, E.E.; Isildak, I.; Nechifor, A.C.; Cristache, C.M.; Enachescu, M. New sensor based on membranes with magnetic nano-inclusions for early diagnosis in periodontal disease. Biosens. Bioelectron. 2018, 102, 336–344. [CrossRef][PubMed] 47. Umezawa, Y.; Bühlmann, P.; Umezawa, K.; Tohda, K.; Amemiya, S. Potentiometric selectivity coefficients of ion-selective electrodes. Part I. Inorganic cations (technical report). Pure Appl. Chem. 2000, 72, 1851–2082. [CrossRef] 48. Kubala, E.; Strzelecka, P.; Grzegocka, M.; Lietz-Kijak, D.; Gronwald, H.; Skomro, P.; Kijak, E. A review of selected studies that determine the physical and chemical properties of saliva in the field of dental treatment. Biomed. Res. Int. 2018, 2018, 6572381. [CrossRef][PubMed] 49. Lopez, L.; Kim, Y.; Jierry, L.; Hemmerle, J.; Boulmedais, F.; Schaaf, P.; Pronkin, S.; Kotov, N.A. Electrochemistry on stretchable nanocomposite electrodes: Dependence on strain. ACS Nano 2018, 12, 9223–9232. [CrossRef] [PubMed] 50. Lu, Y.; Jiang, K.; Chen, D.; Shen, G. Wearable sweat monitoring system with integrated micro-supercapacitors. Nano Energy 2019, 58, 624–632. [CrossRef] 51. Roy, S.; David-Pur, M.; Hanein, Y. Carbon nanotube-based ion selective sensors for wearable applications. ACS Appl. Mater. Interfaces 2017, 9, 35169–35177. [CrossRef][PubMed] 52. Stekolshchikova, A.A.; Radaev, A.V.; Orlova, O.Y.; Nikolaev, K.G.; Skorb, E.V. Thin and flexible ion sensors based on polyelectrolyte multilayers assembled onto the carbon adhesive tape. ACS Omega 2019, 4, 15421–15427. [CrossRef][PubMed] 53. An, Q.; Gan, S.; Xu, J.; Bao, Y.; Wu, T.; Kong, H.; Zhong, L.; Ma, Y.; Song, Z.; Niu, L. A multichannel electrochemical all-solid-state wearable potentiometric sensor for real-time sweat ion monitoring. Electrochem. Commun. 2019, 107, 106553. [CrossRef] 54. Wang, S.; Bai, Y.; Yang, X.; Liu, L.; Li, L.; Lu, Q.; Li, T.; Zhang, T. Highly stretchable potentiometric ion sensor based on surface strain redistributed fiber for sweat monitoring. Talanta 2020, 214, 120869. [CrossRef] [PubMed] 55. Parrilla, M.; Ferré, J.; Guinovart, T.; Andrade, F.J. Wearable potentiometric sensors based on commercial carbon fibres for monitoring sodium in sweat. Electroanalysis 2016, 28, 1267–1275. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).