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[Yu Yamashita](https://orcid.org/0000-0001-7966-3197), Harumi Hayakawa, Pushi Wang, Tatsuyuki Makita, Shohei Kumagai, Shun Watanabe, [Jun Takeya](https://orcid.org/0000-0002-7003-1350)

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[Ion sensors based on organic semiconductors acting as quasi-reference electrodes](https://mdr.nims.go.jp/datasets/c10438f9-25d7-4004-99b4-6795b50e8d57)

## Fulltext

DRAFT123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124Ion sensors based on organic semiconductorsacting as quasi-reference electrodesYu Yamashitaa,b,1, Harumi Hayakawaa, Pushi Wanga, Tatsuyuki Makitaa, Shohei Kumagaia,1,2, Shun Watanabea, and JunTakeyaa,bThin-film devices that transduce the chemical activity of ions into electronic signals areessential components in various applications, including healthcare diagnostics and envi-ronmental monitoring. Combinations of organic semiconductors (OSCs) and ion-selectivematerials have been explored for developing solution-processable ion sensors. However,the necessity of reference electrodes and operational stability in ion-permeable OSCs haveposed questions regarding whether reliable measurements with thin-film components areattainable with OSCs. Herein, we report electric double-layer transistors (EDLTs) withOSCs in single-crystal forms for ion sensing. Our EDLTs demonstrated high operationalstability, with a one-to-one relationship between the source electrode potential and deviceresistance, and served as quasi-reference electrodes. When our EDLT is served as quasi-reference electrode, its drift was as small as 0.5 mV/h and comparable to that of commonlyemployed reference electrodes. In our system, the semiconductor-electrolyte interfaceis self-passivated by the alkyl chains of OSCs in single-crystal structures, with the two-dimensional transport layer appearing unaltered upon gating. EDLT arrays with ion-selectiveand non-selective liquid junctions enable ion concentration sensing without a conventionalreference electrode. These findings provide opportunities to develop thin-film devices basedon OSCs for easy integration and reliable measurements.organic semiconductor | ion sensor | electric double layer transistorIntroduction High-resolution, real-time, and easy-to-use measurements of ionactivity in analytes are crucial in various biochemical applications, ranging fromhealthcare diagnostics to environmental monitoring(1, 2). Potentiometric ionsensing(3), which converts the difference in ionic activity to a difference in electricpotential using an ion-selective membrane (ISM), provides a portable sensingplatform. Potentiometric ion sensors commonly employ a reference electrode(RE) that is bulky, expensive, and requires a large sample volume, which doesnot meet the requirements of looming thin-film sensor applications. The demandfor miniaturized ion sensors has led to the development of solid-state ion sensorsrepresented by ion-sensitive field-effect transistors (ISFETs)(4, 5), in which anISM is fabricated adjacent to a semiconductor. However, REs are commonlyrequired to precisely control the gate voltages of ISFETs. Attempts to fabricateREs in thin-film configurations have been reported; however, their accuracy andlifetime still need to be improved(6). Replacing REs with reference transistorsis another proposed approach for fabricating miniaturized sensors in which dif-ferential measurements between the reference and ion-sensing transistors havebeen conducted(7–10). Reference transistors that demonstrate stable resistanceto changes in ion activities have been sought by passivating ISFET surfaces withion-insensitive materials such as insulating polymers(11, 12). Thus far, issuessuch as ion diffusion inside insulating polymers have hindered mV-scale reliablemeasurements in these systems, making fabrication and application of referencetransistors has remained a longstanding challenge.To fabricate low-drift transistors that may serve as a reliable reference, a stablesolid-liquid interface must be developed(13–15). Thus, inorganic insulators, suchas silicon oxides and silicon nitrites, are not ideal because of their susceptibilityto chemical modification of surfaces in aqueous solutions(16). Compared to mostinorganic materials that are susceptible to chemisorption processes, closed-shellmolecules without dangling bonds may provide more stable solid–liquid interfaces.Using organic semiconductors (OSCs) as the active layers of transistors(17–20) is afacile way to fabricate devices without materials that are susceptible to chemisorp-tion processes in aqueous environments. However, while the chemical structures ofOSCs are stable even in aqueous environments(21–23), most OSC thin films are ion-permeable, and thin-film structures are irreversibly altered..Significance StatementThin-film ion sensors are de-manded for a wide range ofbiochemical-sensing applications.While transistors based on inor-ganic and organic semiconduc-tors have been studied for thispurpose, conventional ion-sensingtransistors require the use of abulky reference electrode such asAg/AgCl. Therefore, the centralproblem of ion sensor miniaturiza-tion has not yet been addressed.In this study, single crystals of anorganic semiconductor were usedin electric double-layer transistorsto develop ion sensors. Owingto their unique operational stability,the fabricated transistors realizedone-to-one conversion of currentlevels and electrode potentials, thatis, they served as quasi-referenceelectrodes. Differential measure-ments between the thin-film sens-ing and reference transistors en-abled facile and reliable ion sens-ing, which eliminated conventionalreference electrodes...Author affiliations: aMaterial Innovation Research Cen-ter (MIRC) and Department of Advanced MaterialScience, Graduate School of Frontier Science, TheUniversity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,Chiba 277-8561, Japan; bResearch Center for Ma-terials Nanoarchitectonics (MANA), National Institutefor Materials Science (NIMS), 1-1 Namiki, Tsukuba,Ibaraki 305-0044, JapanY.Y. conceived of measurement principles. H. H.,T. M., and S. K. conducted device fabrications andmeasurements. P. W. evaluated stability of devices.Y.Y. and S. K. wrote the manuscript. S.W. and J.T.supervised the work.The authors declare no competing financial interests.1To whom correspondence may beaddressed. Email: YAMASHITA.Yu@nims.go.jp,kumagai.s.am@m.titech.ac.jp 2Current address:Department of Chemical Science and Engineering,School of Materials and Chemical Technology,Tokyo Institute of Technology, 4259-G1-7 Nagatsuta,Midori-ku, Yokohama 226-8502, Japan.PNAS — December 2, 2024 — vol. XXX — no. XX — 1–8DRAFT125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248through electrochemical transistor operations(24, 25). Instructurally disordered OSCs, the Fermi energy is pinnedto localized states(26), which limits the operational stabilityof electrochemical transistors based on OSCs.In this study, we demonstrate thin-film ion sensors basedon organic electric double-layer transistors (EDLTs) with-out using an RE. The concept of the proposed system isillustrated in Fig. 1a. In conventional systems, an RE isemployed with a sensor transistor that bears ion-selectivemembranes. In our system, the RE was replaced with areference EDLT composed of the same single-crystal form ofsolution-processed small-molecule OSC employed for sensingEDLTs. A stable resistance value at a given source-electrodepotential is observed in the reference EDLT. Such a one-to-one conversion of the Fermi energy and resistance isestablished with a system close to the degenerate limit,where a sharp increase in transistor operation suggests a lowdensity of trap states. Owing to the operational reliabilityof EDLT, a resistance measure provides an accurate sourceelectrode potential without using an RE, which allows thisEDLT to serve as a quasi-reference electrode (qRE). Throughdifferential measurements between a reference EDLT and anEDLT bearing a potassium ion-selective membrane, selec-tive sensing of potassium ions is realized with sensitivityapproaching the Nernst limit. The observed operationalstability is likely to rise from the unique solid-liquid interfacein our devices, where the surface of the two-dimensionalhole transporting layer is passivated by alkyl chains inthe single-crystal structure of OSC. These results suggestthat the single crystals of OSCs are suitable materials forfabricating electrochemical sensors and biosensors in thin-film configurations.Electric Double-Layer Transistors based on OrganicThin-Film Single CrystalThis study examines EDLTs based on thin-film sin-gle crystals (TFSCs) of OSC, which can be made intoan all-in-film structure shown in Fig. 1b. One ofthe benchmarked OSC, 3,11-dinonyldinaphtho[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene (C9-DNBDT-NW), wasemployed to leverage its suitability for manufacturing largeand few molecules-thick TFSCs by solution-coating tech-nique with high reproducibility(27). Solution-grown TFSCsof C9-DNBDT-NW were patterned on a substrate withbottom-contact source and drain electrodes. To fabricatethe all-in-film device, a bank structure made of 500 µm-thickpolydimethylsiloxane (PDMS) was filled with inner fillingsolution (IFS) containing 0.1 M K2SO4 and polyvinylpyrroli-done. This IFS was covered with a non-ionselective liq-uid junction membrane in this measurement. The EDLTproperties were investigated in an aqueous K2SO4 gateelectrolyte (0.1 M) in the presence of an Ag/AgCl referenceelectrode (See Supplementary Information Section 1 fordetailed methods). Fig. 1c is an illustration of the potentialacross the device, where the electric double layers at thegate/electrolyte and OSC/electrolyte interfaces cause dropsin the electric potential Φ. The electrochemical potentialof electrons (Fermi energy) is plotted as µ̄e. As shown inFig. 1d, a typical p-channel EDLT operation was observedwhen gate voltage (Vg) was controlled using Ag/AgCl whileapplying a constant drain voltage (Vd) of −0.05 V. Here, Idwas plotted against Vref which is defined as the electrodepotential difference between the source and Ag/AgCl. In-terestingly, the subthreshold swing of 75 mV dec−1 was closeto the theoretical limit at room temperature (59 mV dec−1)and significantly smaller than that reported for organicEDLTs or electrochemical transistors based on polymericsemiconductors (>200 mV dec−1)(28, 29), which suggests thelow trap density of states in TFSCs.To evaluate the operational stability of our device, Id wasmeasured while Vref was fixed at −0.55V using the gate andAg/AgCl electrodes. The EDLT exhibited a stable Id aftera steady state was established within an hour, where theId drift was only −0.3 nA/h (Fig. 1e). Considering that sen-sors based on electrochemical transistors with ion-permeablepolymeric semiconductors are typically tested at a timescaleof 10 min, an EDLT based on a TFSC realizes significantlyimproved stability. The observed Id drift corresponds to adrift in the threshold voltage of approximately 0.5 mV/h,which is actually comparable to that of commonly employedreference electrodes(30). The observed drifts in Id may arisefrom our EDLT or the RE employed in this measurement.The observed operational stability suggested that theEDLT based on OSC TFSC served as a qRE. In an EDLT,the amount of charge accumulated on the surface of OSCdepends on the potential drop at the electrolyte/OSC inter-face, which is defined as the effective gate voltage (Vg,eff).This value was equal to Vref in the above measurements.The observed stable Id at a constant Vg,eff suggests that themobility and trap density of states did not change duringthe measurement, and the single-crystal structure should bemaintained similarly to our previous studies in non-aqueousenvironments(31, 32). In a single-crystal structure(33), thetwo-dimensional carrier transporting layer is self-passivatedby the alkyl chains from the electrolytes, which is beneficialfor stable EDLT operations in aqueous solutions. Con-sequently, Id and Vg,eff exhibit a one-to-one relationship,where the measure of Id provides an accurate value of Vg,effthat equals Vref. In the following sections, we demonstratereplacement of bulky REs in ion sensing devices with ourthin-film EDLTs.Aqueous ion sensing using EDLT bearing ion-selective membranesConventional potentiometric ion sensors are composed twoREs separated by an ion-selective membrane (ISM). In thefollowing, first, we demonstrate replacement of one of theREs with our EDLT. To develop ion sensing devices based onthe ELDTs, ISMs were employed to separate the analyte andIFS. Fig. 2a shows an illustration of the potential across thedevice bearing an ISM. An ISM develops a membrane poten-tial ∆ΦISM that depends on difference in concentrations ofthe target ion between the analyte and IFS, where evaluationof ∆ΦISM is required to determine the concentration of thetarget ion. Here, we preliminarily demonstrate K+ sensingusing valinomycin(34, 35) as a K+ ionophore in the ISM.In this case, the concentration of K+ was varying in theanalyte while that in the IFS was constant. Ag/AgCl wasused in the analyte and IFS for the device operation andverification of the working mechanism, for which purposethe EDLT was built in a small container filled with the IFS.See method section and supplementary information Section2 — Yamashita et al.DRAFT2492502512522532542552562572582592602612622632642652662672682692702712722732742752762772782792802812822832842852862872882892902912922932942952962972982993003013023033043053063073083093103113123133143153163173183193203213223233243253263273283293303313323333343353363373383393403413423433443453463473483493503513523533543553563573583593603613623633643653663673683693703713721 for details of device preparation. To test responses ofthe device to the ion concentrations, transfer curves wereacquired at different [K+] values in the analyte, during whichthe gate electrode was placed in the analyte (Fig. 2b). Theobtained transfer curves showed a threshold voltage (Vth)shift toward positive Vg values with decreasing [K+]. Inthis measurement, the applied Vg should be divided at thegate electrode/analyte interface, the ISM, and the IFS/OSCinterface as shown in Fig. 2a. At a smaller [K+], the potentialshift at the ISM (∆ΦISM) should have increased, resulting inan increased Vg,eff. Linear fitting of the on states (dashedlines) afforded comparable slopes, indicating an unique holemobility for varying [K+]. Indeed, the transfer curveswere almost identical when plotted against Vref,in, which isthe electrode potential difference between the source andAg/AgCl placed in the IFS (Fig. 2c). This suggests thatthe EDLT responds to Vg,eff for all the obtained conditionsin an identical manner, which is consistent with the qREoperation of OSC TFSCs.Ion sensing was demonstrated by monitoring Id of theEDLT bearing K+ ISM while changing the [K+] of theanalyte. In this measurement, we employed a gate electrode,an Ag/AgCl RE, and the EDLT bearing K+ ISM immersedin analyte solutions. Note that analyte solutions containedNa+ that serves as an interfering ion, where [Na+] + [K+]was constant for all conditions. For the case without aninterfering ion, see Supplementary Information Fig. S1. Inthe measurement shown in Fig. 2d, constant Vg of −0.4 V andVd of −0.1 V were employed. Vref,out during this measure-ment is shown in Supplementary Information Fig. S2. Here,Vref,in, which is equal to Vg,eff, can be estimated from Idand the transfer characteristics (Supplementary InformationFig. S3). [K+] was calculated from the ∆ΦISM, which can bededuced from the following equation: ∆ΦISM =Vref,out−Vg,eff(Fig. 2e). The slope in the linear region was calculatedas 61 mV dec−1, which is consistent with the Nernstianresponse. This result demonstrates selective and reliableresponses of our EDLT bearing K+ ISM to [K+] in theanalyte.Reliable ion sensing is not limited to K+ in our system.In another example, NH4+ sensing has been demonstratedby employing nonactin (36, 37) as the ionophore. Usingthe same procedure as that for K+ sensing, Id for varying[NH4+] was obtained (Fig. 3a) and employed to estimate∆ΦISM (Fig. 3b). The slope in the linear region (dashed line)was calculated to be 44 mV dec−1, which is slightly smallerthan an ideal Nernstian response. Tuning the compositionof the ISM and IFS may improve sensitivity. The abovemeasurements suggest that EDLTs based on OSC TFSCsprovide a platform for ion sensing when combined withvarious ISMs owing to the operational and environmentalstability of the material.Aqueous ion sensing by all-in-film organic TFSCEDLTsFinally, aqueous K+ sensing was demonstrated using apair of all-in-film EDLTs acting as the sensing EDLT andreference EDLT, which realizes ion sensing without bulkyREs. K+ ISM was employed as the liquid junction of thesensing EDLT, whereas a non-ion-selective liquid junctionwas employed for the reference EDLT. A planar thin-filmAu gate electrode was employed during the ion sensing mea-surement, which is fabricated on the same substrate as theEDLT. Fig. 4a shows an illustration of the potentials acrossthe device. At the interface of analyte/IFS, there should bea negligible drop in the electric potential for the referenceEDLT owing to ionic conductivity and non-selectivity of theemployed liquid junction. In this situation, the potentialdrop at ISM can be estimated based on the Vg,eff of referenceand sensing EDLTs: ∆ΦISM =V refg,eff−V Kg,eff. Fig. 4b showsmonitored Id for varying [K+]. In this experiment, Vg wascontrolled such that Id for the reference EDLT (Irefd ) becameconstant, which is expected to keep Vg,eff constant for thereference EDLT. When the reference EDLT serves as qRE,this measurement condition is equivalent to that in whichVg was controlled to produce a constant Vref by employingan RE in the analyte. In this case, measurements of IKdis enough to evaluate [K+]. In order to convert IKd intoVg,eff, the transfer curve that serves as a calibration curvewas measured before the ion sensing measurements usingan RE (Fig. 4c). Based on the IKd plotted in Fig. 4c andthe calibration curve plotted in Fig. 4d, V Kg,eff and ∆ΦISM(Fig. 4d) were evaluated. The slope in the linear region inthe plot of ∆ΦISM was 62 mV dec−1, which is close to theNerstian response and demonstrates successful ion sensingusing the pair of EDLTs. This successful evaluation owes tostability of our EDLTs during this measurements. To achievesmall drifts of EDLTs in a longer time scale, proper choiceof initial aging time and measurement conditions would beimportant, which is discussed in Fig. S5 and S6 and studiedfurther in future research.In this study, OSC TFSCs were employed as the activelayers of EDLTs, which achieved dramatically improvedoperational stability compared with conventional organicelectrochemical transistors. Our EDLT demonstrated a one-to-one conversion between the drain currents and effectivegate voltages, which allowed the device to serve as a qRE.In our TFSCs, the interface between the electrolyte andcarrier-transporting layer was self-passivated by alkyl chains,which is likely to be the key to qRE operations. Ion-selective responses were established by combining ISMs withour EDLTs. Differential measurements of the referenceand sensing EDLTs enabled ion sensing without using aconventional RE. Our findings will lead to OSC-based thin-film devices that feature reliability and easy integration forelectrochemical and biochemical sensing applications.MethodsFabrication of PET films with liquid junctions: Valinomycinwas employed as the K+ ionophore(38). Nonactin wasemployed as NH4+ ionophore(5). PET films with arounda-few-millimeter holes are prepared, to which solutions forISM or ion-nonselective liquid junction were drop casted.Fabrication of an EDLT inside a container filled withIFS :125 µm-thick PEN films with 200 nm-thick parylene(diX-SR) layers were employed as substrates. Source anddrain electrodes were thermally deposited as 20 nm-thick Auelectrodes. Solution-processed TFSCs of C9-DNBDT-NWwere transferred to the PEN substrates with the c-axis ofYamashita et al. PNAS — December 2, 2024 — vol. XXX — no. XX — 3DRAFT373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496the TFSC parallel to the source-drain direction(39). Thefabricated EDLT device was installed into a cylinder-shapedcontainer made of polypropylene and PEN that is filled withaqueous IFS. PET films with liquid junctions were installedto this container so that IFS and analyte are separated bythe liquid junctions. Note that an RE can be inserted insideor outside of the liquid junction for this device structure.Fabrication of an all-in-film EDLT: Souce/drain elec-trodes and TFSCs of C9-DNBDT-NW were fabricated onPEN substrates in the same method as the EDLTs insidea container filled with IFS. 500 µm-thick PDMS films in abank structure were placed on the EDLT devices. This bankstructure was filled with 0.1 M K2SO4 aqueous solution with10 wt% polyvinylpyrrolidone that serves as an IFS. PETfilms with liquid junctions were placed on top to confine theIFS.See supplementary information for further details of ma-terials, device fabrications, and employed electrolytes.1. L Burton, K Jayachandran, S Bhansali, The“ Real-Time”revolution for in situ soilnutrient sensing. J. The Electrochem. Soc. 167, 037569 (2020).2. T Ozer, I Agir, CS Henry, Low-cost Internet of Things (IoT)-enabled a wireless wearabledevice for detecting potassium ions at the point of care. Sensors Actuators B: Chem. 365,131961 (2022).3. 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IEEE Electron Device Lett. 41, 159–162(2019).15. G Parish, et al., Role of GaN cap layer for reference electrode free AlGaN/GaN-based pHsensors. Sensors Actuators B: Chem. 287, 250–257 (2019).16. S Jamasb, S Collins, RL Smith, A physical model for drift in ph isfets. Sensors ActuatorsB: Chem. 49, 146–155 (1998).17. J Rivnay, et al., Organic electrochemical transistors. Nat. Rev. Mater. 3, 1–14 (2018).18. M Sessolo, J Rivnay, E Bandiello, GG Malliaras, HJ Bolink, Ion-Selective OrganicElectrochemical Transistors. Adv. Mater. 26, 4803–4807 (2014).19. F Torricelli, et al., Electrolyte-gated transistors for enhanced performance bioelectronics.Nat. Rev. Methods Primers 1, 66 (2021).20. Y Yao, et al., Flexible and Stretchable Organic Electrochemical Transistors forPhysiological Sensing Devices. Adv. Mater. p. 2209906 (2023).21. CB Nielsen, et al., Molecular design of semiconducting polymers for high-performanceorganic electrochemical transistors. J. Am. Chem. Soc. 138, 10252–10259 (2016).22. A Giovannitti, et al., N-type organic electrochemical transistors with stability in water. Nat.Commun. 7, 13066 (2016).23. LR Savagian, et al., Balancing Charge Storage and Mobility in an Oligo(Ether)Functionalized Dioxythiophene Copolymer for Organic- and Aqueous- BasedElectrochemical Devices and Transistors. Adv. Mater. 30, 1804647 (2018).24. JT Friedlein, et al., Influence of disorder on transfer characteristics of organicelectrochemical transistors. Appl. Phys. Lett. 111, 023301 (2017).25. A Savva, et al., Influence of Water on the Performance of Organic ElectrochemicalTransistors. Chem. Mater. 31, 927–937 (2019).26. S Wang, M Ha, M Manno, C Daniel Frisbie, C Leighton, Hopping transport and the halleffect near the insulator–metal transition in electrochemically gated poly(3-hexylthiophene) transistors. Nat. Commun. 3, 1210 (2012).27. S Kumagai, T Makita, S Watanabe, J Takeya, Scalable printing of two-dimensional singlecrystals of organic semiconductors towards high-end device applications. Appl. Phys.Express 15, 030101 (2022).28. L Kergoat, et al., A Water-Gate Organic Field-Effect Transistor. Adv. Mater. 22,2565–2569 (2010).29. D Wang, V Noël, B Piro, Electrolytic gated organic field-effect transistors for application inbiosensors―A review. Electronics 5, 9 (2016).30. MP Mousavi, SA Saba, EL Anderson, MA Hillmyer, P Buhlmann, Avoiding Errors inElectrochemical Measurements: Effect of Frit Material on the Performance of ReferenceElectrodes with Porous Frit Junctions. Anal. Chem. 88, 8706–8713 (2016).31. S Watanabe, et al., Surface Doping of Organic Single-Crystal Semiconductors to ProduceStrain-Sensitive Conductive Nanosheets. Adv. Sci. 8, 2002065 (2021).32. N Kasuya, J Tsurumi, T Okamoto, S Watanabe, J Takeya, Two-dimensional hole gas inorganic semiconductors. Nat. Mater. 20, 1401–1406 (2021).33. T Okamoto, et al., Bent-shaped p-type small-molecule organic semiconductors: Amolecular design strategy for next-generation practical applications. J. Am. Chem. Soc.142, 9083–9096 (2020).34. Y Alifragis, et al., Potassium selective chemically modified field effect transistors based onAlGaN/GaN two-dimensional electron gas heterostructures. Biosens. Bioelectron. 22,2796–2801 (2007).35. T Ji, P Rai, S Jung, VK Varadan, In vitro evaluation of flexible pH and potassiumion-sensitive organic field effect transistor sensors. Appl. Phys. Lett. 92, 208 (2008).36. M Cuartero, N Colozza, BM Fernández-Pérez, GA Crespo, Why ammonium detection isparticularly challenging but insightful with ionophore-based potentiometric sensors–anoverview of the progress in the last 20 years. Analyst 145, 3188–3210 (2020).37. ST Keene, et al., Wearable organic electrochemical transistor patch for multiplexedsensing of calcium and ammonium ions from human perspiration. Adv. healthcarematerials 8, 1901321 (2019).38. J Bobacka, Potential stability of all-solid-state ion-selective electrodes using conductingpolymers as ion-to-electron transducers. Anal. chemistry 71, 4932–4937 (1999).39. T Makita, et al., Nano-Ground Glass as a Superhydrophilic Template for PrintingHigh-Performance Organic Single-Crystal Thin Films. Adv. Mater. Interfaces 8, 2100033(2021).AcknowledgmentsThis work was supported in part by JSPS KAKENHI grants(nos. JP20H00392, JP22H04959). This work was supportedin part by JST, CREST Grant Number JPMJCR21O3.Data availabilityThe data supporting the plots within this study are availablefrom Zenodo at https://doi.org/10.5281/zenodo.13132262.Supplementary materialsMaterials and MethodsFigs. S1 to S6References4 — Yamashita et al.DRAFT497498499500501502503504505506507508509510511512513514515516517518519520521522523524525526527528529530531532533534535536537538539540541542543544545546547548549550551552553554555556557558559560561562563564565566567568569570571572573574575576577578579580581582583584585586587588589590591592593594595596597598599600601602603604605606607608609610611612613614615616617618619620-200-150-100-500-0.6-0.4-0.210-1010-910-810-7OSCTFSCSSC9H19C9H19VAg/AgClAAgatesensor EDLT reference EDLTConventional system This workAugate analytepotentialAg/AgClI d (nA)Vref (V)IdVg,effbcad eAgatesensor EDLT-60-50-40-30-20-1001612840Time (h)I d (nA)|Id | (A)Vref~Vg,effS DOSC S DOSCGNDVgФµeS DOSCcontrol systemcontrol systemTFSCAu gatePEN substrateIFSS DPETliquid junctiongluePDMSparyleneanalyte15 mm22.6 mmliquidjunction.Fig. 1. Proposed principle of qRE based on OSC TFSCs. (a) Illustration of the conventional and our measurementsystems. S and D denotes source and drain electrodes respectively. While gate voltage is controlled using an Ag/AgCl in aconventional three-electrode system, it was controlled using a reference EDLT in our system. (b) A photo and an illustrationof the fabricated all-in-film device together with the chemical and crystal structures of C9-DNBDT-NW. (c) Illustration of apotential diagram of an EDLT with a non-ionselective liquid junction. Electric potential Φ is shown in a solid line and theelectrochemical potential of electrons µ̄e (Fermi energy) is shown in dashed lines. GND denotes the ground level of the system,which equals to the source electrode potential. (d) Transfer characteristics of an EDLT in linear (red) and log (blue) scales,where Ag/AgCl was employed during the measurement similar to the conventional system. Vd was −0.05 V and Vg sweeprate was 0.01 V / 10 sec. (e) Operational stability of an EDLT under Vref of −0.55 V and Vd of −0.05 V, where Ag/AgCl wasemployed to keep Vref constant.Yamashita et al. PNAS — December 2, 2024 — vol. XXX — no. XX — 5DRAFT621622623624625626627628629630631632633634635636637638639640641642643644645646647648649650651652653654655656657658659660661662663664665666667668669670671672673674675676677678679680681682683684685686687688689690691692693694695696697698699700701702703704705706707708709710711712713714715716717718719720721722723724725726727728729730731732733734735736737738739740741742743744∆ФISMpotential-200-150-100-500I D (nA)-0.6-0.4-0.2 10-4 M 10-3 M 10-2 MVref,in (V)Vg (V)I D (nA)-200-150-100-500 -0.4-0.20.0Vref,inAg/AgCl 10-4 M 10-3 M 10-2 MISManalyte[K+] in analyte: low, highPtgate TFSCIFS b ca-250-200-150-100-50040003000200010000I D (nA)log [K+]∆ΦISM (V)[K+] (M): 0.0 2.0×10 -61.0×10 -51.0×10 -41.0×10 -32.7×10 -35.4×10 -36.0×10 -36.7×10 -31.0×10 -21.3×10 -22.7×10 -25.0×10 -2Time (s)edVref,outAg/AgCl0.300.250.200.150.100.050.00-6 -5 -4 -3 -2 -1Vg GNDФµe[K+] in analyte [K+] in analyteFig. 2. K+ sensing with an EDLT. (a) Illustration of sensing mechanism. The electric potential is shown in a solid line and the electrochemical potential of electrons (Fermienergy) is shown in dashed lines. GND denotes the ground level of the electronic system, which equals to the source electrode potential. [K+] is constant in IFS and varyingin in analyte, where potentials for the cases with high and low [K+] in the analyte are illustrated. (b) Transistor characteristics with varying K+ concentrations in the analyteplotted against Vg and (c) Vref,in. Vd was −0.04 V. (d) Continuous measurements of Id for varying K+ concentrations with Vg of −0.4 V and Vd of −0.1 V. [Na+] + [K+]was constant for this measurement. (e) The Nernst plot was obtained from EDLT measurements. Dashed line shows linear fitting with 61 mV dec−1.6 — Yamashita et al.DRAFT745746747748749750751752753754755756757758759760761762763764765766767768769770771772773774775776777778779780781782783784785786787788789790791792793794795796797798799800801802803804805806807808809810811812813814815816817818819820821822823824825826827828829830831832833834835836837838839840841842843844845846847848849850851852853854855856857858859860861862863864865866867868ablog [NH4+]∆ΦISM (V)I D (nA)-500-400-300-200-10003000200010000Time (s)[NH4+] (M): 1.0×10 -61.0×10 -51.0×10 -42.0×10 -45.0×10 -41.0×10 -35.0×10 -31.0×10 -20.200.150.100.05-6 -5 -4 -3 -2 -1Fig. 3. NH4+ sensing with an EDLT. (a) Continuous measurements of Id for varying NH4+ concentrations with Vg of −0.3 V and Vd of −0.1 V. No interfering ion wasemployed in this measurement. (b) The Nernst plot was obtained from EDLT measurements. The dashed line shows linear fitting with slope of 44 mV dec−1Yamashita et al. PNAS — December 2, 2024 — vol. XXX — no. XX — 7DRAFT869870871872873874875876877878879880881882883884885886887888889890891892893894895896897898899900901902903904905906907908909910911912913914915916917918919920921922923924925926927928929930931932933934935936937938939940941942943944945946947948949950951952953954955956957958959960961962963964965966967968969970971972973974975976977978979980981982983984985986987988989990991992∆ФISMpotentialliquidjunctionanalytegate TFSCIFSbad-250-200-150-100-500I d (nA)25002000150010005000Time (s)IdKIdref-300-250-200-150-100-500 -0.8-0.6-0.4-0.2ΔΦISM (V)log[K+]I d (nA)Vref,out (V)Kc[K+] (M): 1.0×10 -61.0×10 -51.0×10 -41.0×10 -31.0×10 -25.0×10 -20.10.0-0.1-6 -5 -4 -3 -2 -1GNDVgФµesensingreferenceFig. 4. Performance of combined sensing and reference EDLTs. (a) Illustration of sensing mechanism. The electric potential is shown in a solid line and theelectrochemical potential of electrons (Fermi energy) is shown in dashed lines. Red lines show the potentials for the reference EDLT and blue lines show potentials forthe sensing EDLT. GND denotes the ground level of the system, which equals to the source electrode potential. [K+] is constant in IFS and varying in in analyte. (b)Continuous measurements of Id for varying [K+] without using an RE. Vg was controlled so that Irefd becomes constant value. Vd was −50 mV. [Na+] was 0.1 M for thismeasurement. (c) Transistor characteristics of K+ sensing EDLT plotted against electrode potential difference between source and Ag/AgCl. Vd was −50 mV. The analytecontained 0.05 M Na2SO4 and 0.1 mM K2SO4. (d) The Nernst plot was obtained from sensing and reference EDLTs without using an RE. The dashed line shows linearfitting with slope of 62 mV dec−18 — Yamashita et al.