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Fully inkjet-printed paper-based potentiometric ion-sensing devices Nipapan Ruecha, Orawon Chailapakul, Koji Suzuki, and Daniel Citterio Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03177 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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Analytical Chemistry

Fully inkjet-printed paper-based potentiometric ion-sensing devices Nipapan Ruecha†, Orawon Chailapakul‡, Koji Suzuki†, Daniel Citterio†* † Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 314-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ‡Electrochemistry and Optical Spectroscopy Research Unit (EOSRU), Department of Chemistry, Faculty of Science, Chulalongkorn University, Patumwan, Bangkok 10330, Thailand *Corresponding author. E-mail: [email protected], Phone: +81 45 566 1568, Fax: +81 45 566 1568

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Abstract A fully inkjet printed disposable and low cost paper-based device for potentiometric Na+- or K+-ion sensing has been developed. A printed ionophore-based all-solid-state ion selective electrode

on

a

graphene/poly(3,4-ethylenedioxythiophene)

polystyrene

sulfonate

(G/PEDOT:PSS) nanocomposite solid contact, and a printed all-solid state reference electrode consisting of a pseudo-silver/silver chloride electrode coated by a lipophilic saltincorporating poly(vinyl chloride) membrane overprinted with potassium chloride, have been combined on a microfluidically patterned paper substrate. Devices are built on standard filter paper using off-the-shelf materials. Ion sensing has been achieved within 180 s by simple addition of 20 µL sample solution without electrode preconditioning. The limits of detection were 32 µM and 101 µM for Na+ and K+, respectively. The individual single-use sensing devices showed near Nernstian response of 62.5 ± 2.1 mV/decade (Na+) and 62.9 ± 1.1 mV/decade (K+) with excellent standard potential (E0) reproducibilities of 455.7 ± 5.1 mV (Na+) and 433.9 ± 2.8 mV (K+). The current work demonstrates the promising possibility to obtain low-cost and disposable paper-based potentiometric sensing devices potentially manufacturable at large scales with industrial inkjet printing technology.

Keywords: paper-based device, inkjet printing, ion sensing, potentiometric sensor, sodium, potassium, all-solid-state electrode


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Introduction Potentiometric ion sensing is one of the most important electrochemical detection methods and is widely used for selective quantitative analysis of target ions in both clinical and environmental settings.1-2 Originally developed measurement systems required bulky, maintenance-intensive macro electrodes and conventional reference electrodes to be paired with ISEs.3-5 Therefore, overall system miniaturization and the introduction of all-solid-state potentiometric sensors have become attractive approaches to low-cost and portable alternatives.6-8 Although the overall performance of a potentiometric ion sensing device also depends on the potential stability of the reference electrode, all-solid-state reference electrodes2, 9-10 have been given much less attention than the ISEs themselves. Colloid imprinted mesoporous (CIM) carbon9, 11 coupled with a hydrophobic redox buffer has recently been introduced as a solid contact for all-solid-state reference electrodes with outstanding potential stability and applied to a paper platform. Unfortunately however, the fabrication of this type of reference electrode requires significant efforts for the synthesis of the redox couple system.11-12 In another approach to paper-based all-solid-state reference electrodes, the use of traditional plasticized PVC membranes doped with a lipophilic salt13-14 resulted in good potential stability, while being easy to prepare.10 Paper substrates have been introduced as simple and cost-effective analytical platforms for quantitative colorimetric15-16 and electrochemical17-18 analyte detection in many fields including clinical19-21 and environmental monitoring.15, 22 It has been demonstrated that paper-based devices coupled with electrochemical detection show similarly high performance to conventional electrochemical sensors.23 Therefore, the development of paper-based electrochemical analytical devices has become a very attractive research area.24-25 Not



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surprisingly, paper-based approaches are recently also drawing attention for the development of potentiometric ion sensing devices.4, 10, 26-33 The first paper-based ISE was introduced by Novell et al..30 A carbon nanotube (CNT) modified conductive paper was used as both ion-toelectron transducer and electric conductor in an all-solid-state ISE. There are many reports focusing on the development of potentiometric sensors on paper platforms using various materials to create all-solid-state ISEs, including CNTs,

30-32

graphene33 and gold

nanoparticles (AuNPs).10 Recently, the interest in the development of fully integrated paper-based potentiometric sensing devices including ion selective and reference electrodes to achieve “self-standing” miniaturized potentiometric ion sensing devices has grown. Conductive papers based on carbon nanotubes used as solid contacts for separated strip-type ISE and reference electrode31, as well as inkjet-printed gold electrodes on recyclable paper as solid contacts for PVC membrane-based ISE and reference electrode10 have been reported. However, both systems require customized paper substrates. Ordinary filter paper-based potentiometric ion sensing devices based on simple stencil-printed Ag/AgCl electrodes have been reported.26,

29

The unmodified Ag/AgCl electrodes have been applied for chloride

sensing, while modification with ionophore doped PVC membranes allowed the detection of other ions (e.g. sodium,26 calcium,26 potassium26, 29). In all cases however, the user is required to apply a reference electrolyte (aqueous KCl solution) besides the actual sample solution to be measured. Finally, one limiting step in terms of fabrication effort and reproducibility of all-solidstate ISEs is the modification of the solid contact with the ion selective membrane layer.2-4, 14, 26, 30, 34

Inkjet printing technology is an ideal candidate to overcome this limitation and

effectively used for potentiometric sensor fabrication,10, 35 because it allows creation of well-


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defined patterns with precise control and enables the deposition of several materials onto an identical area in a layer-by-layer structure.10, 36-37 Herein, the reproducible fabrication of fully inkjet printed single-use paper-based potentiometric ion sensing devices combining an all-solid-state ISE and a reference electrode on conventional filter paper is presented. The present devices require no reagent handling and no preconditioning before sample measurements. As a proof of principle, the developed paper-based potentiometric ion sensing devices were applied for sodium and potassium ion determination. The observed good analytical performance and the fabrication reproducibility confirms the potential of inkjet printing technology for the large-scale production of low-cost, user-friendly potentiometric ion sensing devices.

Experimental Section Chemicals: Didecalino-16-crown-5 (DD16C5) and o-nitrophenyl octyl ether (o-NPOE, >99%) were purchased from Dojindo laboratories (Kumamoto, Japan). Potassium tetrakis (4−chlorophenyl) borate (KTpClPB, >97%) and high molecular weight poly vinyl chloride (PVC) were obtained from TCI chemicals (Tokyo, Japan). Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, 0.8 wt% in water) and 30 wt% silver nanoparticle dispersion in ethylene glycol (Ag ink) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Valinomycin (potassium ionophore I), bis(2-ethylhexyl) sebacate (DOS, >97%), sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl2), calcium chloride (CaCl2), zinc chloride (ZnCl2), ferric chloride (FeCl3), tetrabutylammonium tetrabutylborate (TBA-TBB) (97%), cyclohexanone and ethylene glycol were purchased from Wako Pure Chemical Industrials, Ltd. (Osaka, Japan). Graphene (G) nanopowder with an



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average size of 6-8 nm was purchased from SkySpring Nanomaterials, Inc. (Houston, TX, USA). G/PEDOT:PSS ink preparation: G/PEDOT:PSS ink was prepared by adding the graphene nanopowder into 0.8 wt% PEDOT:PSS aqueous solution. A concentration of 2 mg/mL of graphene was selected, since it was well dispersible in the PEDOT:PSS solution. Graphene was successfully dispersed using an Iuchi ultrasonic bath (Osaka, Japan) for 4 h with 38 kHz frequency. The prepared G/PEDOT:PSS ink was filtered through a 0.45 µm polypropylene syringe filter before use. Preparation of reference and sodium and potassium ion selective membranes: The ionselective membranes for the solid contact electrode modification contained 1.0% of ionophore valinomycin for K+ or DD16C5 for Na+, 0.5% (for K+ ion sensors) or 1.0% (for Na+ ion sensors) of KTpClPB, 65.0% of NPOE for K+ ion sensors or DOS for Na+ ion sensors and 32.0-33.0% of PVC (all wt%). The reference membrane for the solid contact reference electrode contained 12.0% of TBA-TBB, 58.0 % NPOE and 30.0 % of PVC (all wt%). The membrane solutions were prepared with a solute content of 5 wt% using cyclohexanone as the solvent. Patterning of paper substrates: The fabrication procedure of paper-based potentiometric devices is schematically shown in Scheme 1. Firstly, the wax-printing technique was used to fabricate the flow channel pattern on Whatman grade 1 filter paper (GE Healthcare, Buckinghamshire, UK). The flow channel template was printed on one side of the filter paper by a wax printer (Xerox Phaser 8570, Xerox, Norwalk, CT, USA), followed by placing on a hot plate at 150 °C for 240 s to create a hydrophobic wall. Then, the paper was subjected to Ag ink printing described in the next section, before a rectangular (18 cm × 25 cm) shape was printed on the backside of the paper in a second wax printing step. The double-side wax


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patterned paper was then inserted between two sheets of lamination film (refer also to next section), and passed through a hot laminator (QHE325, Meikoshokai Co., Ltd., Tokyo, Japan). Inkjet printing of potentiometric devices: The two-electrode system for potentiometric detection was fabricated using a piezoelectric DimatixTM material printer (DMP 2800, FUJIFILM Dimatix, Inc., Santa Clara, CA, USA). A cartridge (DMC-11610) with a nominal droplet volume of 10 pL and 16 nozzles was used for all inkjet printing experiments. For all printing steps, 7 nozzles were selected. The printing parameters for all inks deposited are shown in Table S-1 of the supporting information. Silver (Ag) nanoparticle ink for the reference electrode and for the ion selective electrode connector was printed on the single side wax-patterned paper and sintered at 120° C for 10 min. This step was followed by backside wax printing and hot lamination between two sheets of lamination film, with a rectangular area (18 cm × 25 cm) cut out of the top sheet to allow further inkjet modification. After that, freshly prepared 10 mM FeCl3 solution and the reference membrane (REM) ink were subsequently printed on top of the Ag reference electrode structure, to obtain the REMcoated pseudo silver/silver chloride electrode (REM-coated p-Ag/AgCl). Finally, a 3 M KCl solution was printed on top of the REM-coated p-Ag/AgCl electrode. For the ion selective electrode fabrication, the G/PEDOT:PSS ink (or only PEDOT:PSS solution for comparative purposes) was printed as the solid-state contact and finally overprinted with the ion selective membrane ink for sodium or potassium, respectively. The resistance of printed solid contacts for the ion-selective (G/PEDOT:PSS) and reference (Ag/AgCl) electrodes was measured over a distance of 9 mm for 10 individually printed electrodes using a digital multimeter (KEW1012, Kyoritsu, Japan). Electroanalytical measurements: All potentiometric and cyclic voltammetric measurements were performed using a potentiostat (CHI 660A, CH Instruments, Austin, TX). For cyclic



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voltammetry, a three-electrode system was used. The printed PEDOT:PSS and G/PEDOT:PSS electrodes were used as working electrodes. A stencil-screen printed carbon electrode on a PVC substrate and a conventional double junction-type Ag/AgCl electrode were used as the counter and reference electrodes, respectively. For potentiometric measurement, the printed silver electrodes acting as electrical contacts for ion selective and reference electrodes were connected to the potentiostat by copper wire. 20.0 µL of sample solution was dropped on the sample reservoir (center square area of the device shown in Scheme 1) and potentiometric measurements initiated 20 s after sample introduction. Potentials were constantly monitored and recorded over 180 s. All experiments with sodium and potassium were performed using NaCl and KCl standard solutions, respectively. To obtain a calibration curve, three individual paper-based potentiometric sensing devices were used for each concentration of standard solution. Therefore, 21 individual single-use devices were used for each calibration curve for sodium and potassium ions in the concentration range of 10−6 to 1 M. Single ion activity coefficients were estimated according to a DebyeHückel approximation.38 Potentiometric selectivity coefficients for Na+, K+, NH4+, Ca2+ and Mg2+ were estimated according to the separate solution method (SSM) using 0.1 M chloride salt solutions of the respective cations. Urine sample preparation:

Diluted human urine samples were prepared by mixing

collected urine with deionized water in a 1:1 ratio. Samples were analyzed on individual single-use paper-based potentiometric ion sensing devices, and results were compared to those obtained with compact ISE-based sodium and potassium meters (LAQUAtwin compact B-722 and B-731 ion meters; Horiba, Kyoto, Japan).

Results and Discussion


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Electrode characterization Inkjet printing allows to directly and precisely deposit ink materials with high speed, low cost and high reproducibility, without the need for specifically designed masks or screens. The fabrication process for conductive electrodes on filter paper is simpler than on other flat substrates such as transparency films and glass,27, 30 since the paper substrate readily absorbs the hydrophilic printing ink resulting in high shape fidelity, while droplet aggregation and surface de-wetting frequently occur on non-ink absorbing materials. Filter paper is inherently non-conductive, but the printing of a conductive ink onto the porous structure produces large active electrode surface areas. The poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) conducting polymer was selected as the base material for the ion selective electrode, due to its known compatibility with the inkjet printing process.39 It has been widely used as conducting polymer material to obtain an electron-conducting substrate10, 40 by techniques such as electrochemical polymerization, drop-casting and printing,41 since it exhibits extraordinary electrical conductivity. However, the hydrophilicity of PEDOT:PSS potentially promotes the formation of a water layer at the electrode substrate/sensing membrane interface, which is known to decrease the sensitivity of the resulting ISEs.24, 42-43 Therefore, the use of nanocomposites of PEDOT:PSS with hydrophobic graphene is a good strategy for avoiding the accumulation of water at the interface of solid-contact ISEs. The presence of graphene not only increases the hydrophobicity of the electrode surface, but also provides a high surface area, excellent conductivity and electrocatalytic activity. Moreover, it has been reported that graphene/PEDOT:PSS nanocomposites (G/PEDOT:PSS) show higher sensitivity and conductivity than PEDOT:PSS alone, due to electron hopping at the interface of graphene and PEDOT:PSS.39,

44

Therefore, the electrochemical behavior of printed

electrodes obtained from PEDOT:PSS ink or from a G/PEDOT:PSS nanocomposite ink were investigated. The effect of graphene addition could be observed by cyclic voltammetry and



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electrochemical impedance spectroscopy. Figure S-1a shows cyclic voltammograms recorded with the resulting filter paper-based electrodes in the absence (PEDOT:PSS) and presence (G/PEDOT:PSS) of graphene in 2 mM ferri/ferrocyanide solution. The use of the graphene/polymer composite material resulted in increased anodic and cathodic peak currents compared to the conducting polymer alone. The presence of graphene is assumed to accelerate the electron transfer kinetics on the electrode. The peak-to-peak separation (∆Ep) observed with the G/PEDOT:PSS paper electrode significantly decreased compared to the PEDOT:PSS system. The Nyquist plots of impedance data for G/PEDOT:PSS and PEDOT:PSS solid contacts in the 100 kHz to 10 mHz frequency range are shown in Figure S-1b. In contrast to the data obtained for the solid contact consisting of PEDOT:PSS alone, the impedance spectrum of inkjet deposited G/PEDOT:PSS has the shape of a near vertical line due to the bulk redox capacitance of G/PEDOT:PSS. This implies a very low transfer resistance, which is in accordance with the results obtained through cyclic voltammetry. Therefore, the G/PEDOT:PSS electrode was chosen as electric contact and ion-to-electron transducer on which to build the ion selective electrode. The electrical resistance measured over a 9 mm electrode trace in dry state was found to be 13.7 ± 0.8 kΩ (n=10). For specific ion measurements, the G/PEDOT:PSS electrode was subsequently inkjet modified by a polyvinyl chloride (PVC)-based ion selective membrane (ISM). The surface of the G/PEDOT:PSS modified paper and of the additionally ISM coated electrodes was observed by SEM shown in Figure 1. In all cases the cellulosic fiber structure of the underlying paper remains visible, indicating the penetration of the electrode ink into the paper substrate. Among all reference electrode systems, the silver/silver chloride (Ag/AgCl) electrode is one of the most commonly used in miniaturization approaches.27 Recently, all-solid-state reference electrodes without liquid junction have been fabricated by inkjet printing of silver nanoparticle ink, followed by the formation of a silver chloride layer using chemical


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oxidation with ferric chloride45 or bleach solution.27 In this work, a pseudo-silver/silver chloride (p-Ag/AgCl) reference electrode was fabricated by inkjet printing of ferric chloride solution on a printed silver trace on filter paper. The formation of silver chloride was visually confirmed by the color change from silver to a dark surface. Moreover, the presence of chloride on the reference electrode was characterized by SEM and energy dispersive X-ray spectroscopy (EDX). The SEM images of silver and pseudo-Ag/AgCl (p-Ag/AgCl) paper electrodes are presented in Figure 2. Upon initial inkjet printing of silver nanoparticle ink onto filter paper and 10 min drying at 120 °C, a homogeneous silver layer was formed on the cellulose fibers (Figure 2a). After chemical oxidation by inkjet deposited ferric chloride, the surface morphology change and the appearance of a solid crystalline-like precipitation on the cellulose fibers indicated the formation of an AgCl coating (Figure 2b). The electrical resistance measured over a 9 mm electrode trace in dry state was found to be 5.5 ± 0.3 kΩ (n=10). In addition, the presence of AgCl on the paper electrode was also confirmed by EDX. The Ag electrode shows only the signals relative to Ag particles, but the additional peak of chloride was observed on p-Ag/AgCl. For a potentiometric sensing device, a stable and reliable reference electrode with a sample independent potential response is required.9, 32 For this reason, the p-Ag/AgCl electrode was inkjet coated with a reference membrane (REM) composed of PVC incorporating a lipophilic salt, tetrabutylammonium tetrabutylborate (TBA-TBB)10, 14 (Figure 2c). The potential stability of the unmodified p-Ag/AgCl and the REM-coated reference electrodes was individually tested against a conventional double junction-type Ag/AgCl reference electrode by applying KCl solutions of various concentrations (Figure S-2). Although to a lesser degree than the uncoated pseudo reference electrode, the REM-modified p-Ag/AgCl still responded to changes in chloride anion concentrations. This might be caused by the high surface area to volume ratio of filter paper, making the complete coverage of the p-Ag/AgCl electrode with the REM difficult. To obtain



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a well-defined constant potential of the reference system, 3 M KCl solution was inkjet deposited on the top of the REM-modified p-Ag/AgCl. The crystalline deposit of KCl on the REM-p-Ag/AgCl electrode was clearly observed by SEM (Figure 2d). The finalized allinkjet-printed reference electrode system with KCl coverage exhibited stable potential values (40.4 ± 5.1mV) upon exposure to KCl solutions in a concentration range of 10−6 M to 1 M (Figure 3). Therefore, the fabrication of reliable all-solid-state reference electrodes for disposable paper-based potentiometric devices on filter paper using inkjet printing technology was successfully achieved. The small error bars representing the standard deviation of measurements obtained with three individual single-use electrodes clearly demonstrate the reproducibility of the inkjet-based fabrication approach and its potential suitability for high throughput electrode manufacturing.

Paper-based potentiometric sensing device design The paper-based potentiometric sensing devices were fabricated by inkjet printing of electrodes onto wax patterned filter paper. The design included a central sample inlet zone, from where the applied sample liquid (20 µL) wicks towards the separated ion selective electrode and reference electrode zones (Figure 4). This arrangement, which gives rise to a split capillary flow-driven passive sample transport from the sample inlet in opposite directions, prevents the contamination of the ISE zone with potassium chloride from the reference electrode zone during the duration of potentiometric measurements (generally 3 min). Figure S-3 shows a cross section (along the dotted line indicated in Figure 4) through a paper-based ion sensing device after application of an aqueous red dye solution to visualize the hydrophilic areas. The wax layer deposited on the backside of the paper surface serves to prevent the access of the aqueous sample liquid to uncoated areas of the p-Ag/AgCl and the


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G/PEDOT:PSS electrodes in the reference and ion selective electrode zones. The lamination film on the backside of the devices provides additional mechanical stability to the system. In addition, the hot lamination process results in vertical compression of the filter paper accompanied by increased wax penetration46 and therefore, the closure of any gaps between the backside deposited wax and the printed electrodes. The hydrophilic flow channel inside the backside wax printed and laminated paper devices had a depth of 81.2 ± 5.1 µm (n=10) (Figure S-3b and 3c). With this device design, an electrical contact between reference and ion selective electrodes was found to be established within 30 s of sample application to the central inlet zone.

Potentiometric measurement of sodium and potassium ions The ion selective membrane (ISM) was deposited on top of the G/PEDOT:PSS paper electrodes by inkjet printing. The amount of ISM ink applied was optimized by varying the number of printing cycles between 1 and 20, while observing the potentiometric response of the resulting paper-based ISEs measured against a conventional double junction-type Ag/AgCl reference electrode (data not shown). Since inkjet printing relies on the deposition of pL-volume ink droplets, a single print cycle, even performed at low drop spacing of 25 µm (Table S-1) is insufficient to obtain full coverage of the high surface area of the cellulose fiber network in filter paper. The slopes (sensitivity) of the ion activity-dependent calibration curves recorded with the printed paper-based ISEs increased by increasing the number of ISM printing cycles up to 10 cycles for Na+ and 15 cycles for K+ (data not shown). Potentiometric ion sensing for characterization of printed electrodes was performed in the concentration range of 10−6 M to 1 M. The electrode response curves of the optimized printed ion-selective electrodes tested against a conventional double junction-type Ag/AgCl



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reference electrode are shown in Figure S-4. A separate single-use paper electrode was used for every measurement. The printed ISEs showed a slightly super-Nernstian response with electrode function slopes of 61.6 mV/decade for both the sodium- and potassium-selective sensors. The optimized printing conditions were then applied to the fabrication of the paperbased sensing devices with integrated pseudo-reference electrodes. For the fully integrated ion sensing devices, a stable potential signal was observed a short time after the sample solution reached the ion selective electrode and reference electrode zones, closing the electrical circuit. Figure S-5 shows a set of representative time-dependent potential response curves. This data demonstrates, that the single-use disposable paper-based potentiometric sensing devices do not require any sensor conditioning before sample analysis to achieve a stable potential value within a short sample contact time. The stable signal observed in Figure S-5b also clearly indicates that during the 3 min measurement time, diffusion of K+ ions from the reference electrode zone to the ISE zone is not an issue, since otherwise a significant increase of the potential value over time would be observed for the potassium sensing device. The unidirectional capillary force-driven sample liquid wicking from the central inlet zone to the respective electrode zones efficiently prevents diffusion-based mixing. The potential values measured at 3 min into the electrode response were used to draw the ion activitydependent response curves shown in Figure 5. A linear response range for sodium and potassium ions was observed in between 10−4 M and 1 M ion concentrations, with a slope of 62.5 mV/decade for sodium (Figure 5a) and 62.9 mV/decade for potassium (Figure 5b). These values are close to the theoretical slope of 59.2 mV/decade according to the Nernst equation at 25˚C, but super-Nernstian in all cases. No active temperature control was performed during measurements at room temperature. As mentioned above, the paper-based ion sensing devices are not pre-conditioned before potentiometric measurements. It has been previously reported that super-Nernstian response is a common phenomenon for


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unconditioned membranes.47 Due to the use of KTpClPB as the anionic additive, K+electrode membranes incorporate an initial amount of primary (K+) ions, whereas Na+electrode membranes are essentially free of primary ion before contact with sample solution. In the case of the Na+-electrode system (Fig. 5a) the point of highest ion activity is slightly deviating from linearity. A similar experimental result is not observed for the K+ sensing devices (Fig. 5b). It is therefore assumed that the unconditioned Na+-electrodes are particularly prone to super-Nernstian response. The detection limits estimated from the crosssection of an extrapolated horizontal line in the non-responsive activity range and the linear section of the calibration curve (Figure 5) were found to be 32 µM and 101 µM for sodium and potassium, respectively (Table 1). The linear dynamic response range observed for the paper-based sensing devices is narrower and the detection limits are higher than for conventional ISEs based on the same ionophores. The reason for this phenomenon, which has also been reported for other paper-based ISE approaches,29 is currently unknown, but it is attributed to the presence of anionic carboxylate groups on paper cellulose fibers, originating from the paper making process.48 Nevertheless, the linear response range covers the physiologically relevant concentrations of sodium and potassium in biological samples (e.g. blood serum or urine). The individually inkjet printed devices show very good reproducibilities with electrode function slope and E0 values of 62.5 ± 2.1 mV/decade and 455.7 ± 5.1 mV for sodium sensing devices, and 62.9 ± 1.1 mV/decade and 433.9 ± 2.8 mV for potassium sensing devices. The PEDOT:PSS and G/PEDOT:PSS solid contacts were also compared in terms of analytical performance of the resulting ion selective electrodes. The relevant parameters are summarized in Table 1 and the ion activity-dependent response curves are shown in Figure S6. The use of the graphene/polymer composite material showed significant improvements in ISE sensitivity and slightly lower detection limits. The combination of graphene and



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PEDOT:PSS results in a very effective ion-to-electron transducer and electrical conductor for potentiometric sensing devices, presumably due to the high conductivity and the large surface area of the graphene nanomaterial. In addition, graphene is a highly hydrophobic material that can prevent the formation of a water layer at the interface of the ion selective membrane and the underlying contact electrode, which is important for the performance of all-solid-state ISEs. The selectivity of the paper-based potentiometric ion sensing devices was evaluated by measuring the potentiometric response in separate solutions of 6 potentially interfering cations consisting of Na+, K+, Ca2+, Cu2+, Zn2+, Mg2+, and NH4+. As summarized in Figure S7a-b, the paper-based sensing devices showed negligible response for all interfering ions investigated up to concentrations of 10 mM, while a potential increase was observed at concentrations of 100 mM and higher. Potentiometric selectivity coefficients for Na+, K+, NH4+, Ca2+ and Mg2+ were experimentally determined according to the separate solution method. The results are summarized in Table S-2. Based on these values, it must be concluded that the selectivity of the paper-based sensing device is not as high as in the case of conventional ISEs using the same ionophores. Although the reason for this decrease in selectivity can currently not be explained, it is most likely caused by the fact that cellulosic filter paper is not a completely chemically inert substrate material, but can show some general cation exchange properties caused by the presence of carboxylate groups as already mentioned earlier. The shelf storage time-dependent stability of the paper-based potentiometric sensing devices was studied over a period of two weeks (Figure S-8). Printed devices were kept at ambient condition for one or two weeks, and then used to acquire calibration curves for sodium and potassium ions. The observed changes in the slopes of the response curves were below 5 mV/decade, indicating a reasonably good storage stability of the devices.


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The fully inkjet-printed paper-based potentiometric ion-sensing devices developed in the current work show characteristics and performance comparable to previously reported ion-selective electrode sensing devices with integrated reference electrodes implemented on filter paper platforms.26, 29 In terms of practical application however, the current device is characterized by a high degree of user-friendliness. No electrolyte solution addition is required before sample analysis.

Sodium and potassium ion sensing in human urine sample The applicability of the developed paper-based ISE sensing devices was investigated by measuring the concentration of sodium and potassium ions in human urine. Samples were diluted with deionized water (1:1) before analysis with the developed devices to reduce interference by potassium (for sodium ISE) and by sodium (for potassium ISE). The results were validated against commercialized sodium and potassium ion sensing devices. The measured concentrations were not statistically different from those obtained by the validation method (Table 2), confirming the possibility of accurate measurements of ion concentrations with the paper-based potentiometric sensing device.

Conclusion The aim of this work was the development of fully inkjet-printed disposable singleuse paper-based potentiometric ion sensing devices that are suitable for future mass production. For this purpose, an all-solid-state ISE and a reference electrode were successfully constructed. Sensing devices were realized on common filter paper using commercially available and relatively low cost off-the-shelf components not requiring any inhouse synthesized materials. Despite of the simple electrode compositions relying on



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standard materials, astonishingly high reproducibility in both electrode calibration function slopes and E0 values has been achieved, which is attributed to the liquid dispensing precision and reproducibility of the inkjet printing method. For practical measurements, user interaction is limited to the addition of the sample solution alone, with no requirement of sensor preconditioning or any further liquid handling. The printed devices are applicable to the quantification of sodium and potassium in aqueous solutions over concentration ranges that are biologically and environmentally relevant (10−4−1 M). By making use of the large choice of ionophores presented in the literature until now, it is believed that the concept demonstrated here can be adapted to further low-cost and disposable paper-based potentiometric sensing devices produced at large scales with high-speed and reproducible inkjet printing technology.

Acknowledgements This work was supported by Medical Research and Development Programs Focused on Technology Transfer: Development of Advanced Measurement and Analysis Systems (SENTAN) (Japan Agency for Medical Research and Development; AMED).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website: Printing parameters for all inks; selectivity coefficients of potentiometric ion sensing devices; cyclic voltammograms and Nyquist plots of printed solid contacts; reference electrode test setup; cross section through a paper-based potentiometric ion sensing device; calibration curves of printed ISEs measured against commercial Ag/AgCl reference


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electrode; time-dependent potential response curves; calibration curves of ISEs with PEDOT:PSS solid contact; potentiometric response to primary and interfering ions; shortterm storage stability of devices.

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Table 1 Analytical performance of paper-based potentiometric sensing devices for sodium and potassium ions relying on PEDOT:PSS and G/PEDOT:PSS solid-state electrodes. PEDOT:PSS

Sensitivity (mV/dec) Linear range LOD

G/PEDOT:PSS

Na+

K+

Na+

K+

43.6

46.1

62.5

62.9

10−4 − 1 M

10−4 − 1 M

10−4 − 1 M

10−4 − 1 M

42.2×10−6 M

108.8×10−6 M

32.3×10−6 M

100.8×10−6 M



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Table 2 Determination of sodium and potassium ion concentrations in diluted human urine samples. Sample no.

Concentration of Na+

%Error

Concentration of K+

(mM)

%Error

(mM)

Paper-

PaperHoriba

Horiba

based ISE

based ISE

1

104.22±7.8

100.07

4.1

54.09±4.2

53.79

0.5

2

126.07±5.5

121.44

3.8

47.80±4.1

50.47

5.3

3

97.82±6.9

100.70

2.9

41.64±5.2

39.8

4.6


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Scheme 1 Schematic of the fabrication process of paper-based potentiometric devices by inkjet printing.



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Figure 1 SEM images of (a) G/PEDOT:PSS; (b) ISM coated G/PEDOT:PSS for sodium; and (c) potassium ion sensing (10 and 15 printed layers of ISM ink for Na+ and K+ selective electrode, respectively).


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Figure 2 SEM images of (a) filter paper with inkjet deposited Ag nanoparticle ink; (b) pAg/AgCl electrode after AgCl layer formation by printing of ferric chloride solution on Ag paper electrode; (c) reference membrane-modified p-Ag/AgCl electrode; and (d) finalized KCl-coated REM-p-Ag/AgCl reference electrode; insets show the results of EDX analysis.



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Figure 3 Potentiometric response of p-Ag/AgCl (○), REM- p-Ag/AgCl (◊), KCl coated pAg/AgCl (□) and commercial double junction-type Ag/AgCl (∆) reference electrodes in aqueous KCl solutions (10−6 M to 1 M) measured against a conventional double junction-type Ag/AgCl reference electrode; each data point has been obtained by measurements with 3 individual single-use electrodes (inkjet printed electrodes) or 3 consecutively performed measurements (conventional reference electrode); error bars indicate the standard deviations.


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Figure 4 Photograph of a single paper-based potentiometric ion sensing device and schematic of the measurement procedure; the dotted line indicates the cross-section through the device shown in Figure S-3.



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Figure 5 Calibration curves obtained with paper-based (a) sodium and (b) potassium ion sensing devices; each data point has been obtained by measurements with 3 individual singleuse devices; error bars indicate the standard deviations.


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for TOC only


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