Solution processed ZnInxOy sensing membranes on flexible PEN for extended-gate field-effect transistor pH sensors

Journal of Alloys and Compounds 822 (2020) 153630

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Solution processed ZnInxOy sensing membranes on flexible PEN for extended-gate field-effect transistor pH sensors Tung-Ming Pan a, b, *, Yen-Hsiang Huang b, Jim-Long Her c, Bih-Show Lou d, e, See-Tong Pang b a

Department of Electronics Engineering, Chang Gung University, Taoyuan, 33302, Taiwan, ROC Division of Urology, Chang Gung Memorial Hospital, Taoyuan, 33305, Taiwan, ROC Division of Natural Science, Center for General Education, Chang Gung University, Taoyuan, 33302, Taiwan, ROC d Chemistry Division, Center for General Education, Chang Gung University, Taoyuan, 33302, Taiwan, ROC e Department of Nuclear Medicine and Molecular Imaging Center, Chang Gung Memorial Hospital, Taoyuan, 33305, Taiwan, ROC b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 September 2019 Received in revised form 31 December 2019 Accepted 31 December 2019 Available online 3 January 2020

This work describes the impact of indium content of the structural properties and sensing characteristics of ZnInxOy sensing films for extended-gate field-effect transistor (EGFET) sensors. The synthesis of the ZnInxOy sensing films was made by a solution processing technique directly deposited on flexible polyethylene naphthalate (PEN) through a simple spin-coating method. The synthesized ZnInxOy films at three different indium concentrations (10%, 20% and 30% moles) were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and atomic force microscopy to examine their chemical features, film structures and surface morphologies, respectively. Experiments were performed to explore the pH sensitivity, drift rate, and hysteresis voltage in the ZnInxOy EGFET sensors. Compare with these conditions, a higher pH sensitivity of 61.76 mV/pH, a lower drift rate of 1.08 mV/h, and a smaller hysteresis voltage of ~1 mV were obtained in the ZnInxOy EGFET sensor fabricated at the 20% condition. Moreover, after 500 bending cycles, the ZnInxOy EGFET sensor fabricated at the 20% condition showed good mechanical flexibility. The ZnInxOy EGFET sensor also demonstrated a high selective response towards Hþ. This result indicates that the ZnInxOy sensing film with a rougher surface and a higher ZneOeIn content to generate lower crystal deformities, imperfections and oxygen vacancies. © 2020 Elsevier B.V. All rights reserved.

Keywords: Extended-gate field-effect transistor (EGFET) ZnInxOy Polyethylene naphthalate (PEN) Sensor

1. Introduction pH has a great important in chemical and biological processes, being able to predict the reaction rates at various pH values. Due to the size limitation, fragile glass, and lack of flexibility, a traditional glass-type electrode is very difficult to be employed for food monitoring, clinical, or in vivo biomedical applications. A simple, rapid, and robust solid-state sensor is developed for determining the success of pH sensing in many fields, e.g. environmental, industrial, and biomedical analyses [1e3]. Various solid-state sensors, such as ion-sensitive field-effect transistors (ISFET) [4], electrolyteinsulator-semiconductor (EIS) [5], extended-gate field-effect transistor (EGFET) [6], light-addressable potentiometric sensor (LAPS)

* Corresponding author. Department of Electronics Engineering, Chang Gung University, Taoyuan, 33302, Taiwan, ROC. E-mail address: [email protected] (T.-M. Pan). https://doi.org/10.1016/j.jallcom.2019.153630 0925-8388/© 2020 Elsevier B.V. All rights reserved.

[7], were used to measure the pH of the solutions. An ISFET sensor is analogous to a metal-oxide-semiconductor field-effect transistor (MOSFET) device, where a sensitive membrane and a reference electrode are substituted for the metallic gate [4]. However, the stability problem of this sensor occurred when it was immersed in an electrolyte solution. An EGFET device is that a chemically sensitive membrane is connected to the gate of the MOSFET and put in the electrolyte, and a commercial MOSFET device is separated from the chemical surroundings. With this configuration, the EGFET sensor can be employed many times without causing any stability issues relative to the ISFET one, because it isn’t directly immersed in the electrolyte [8,9]. Several conducting oxide or metal oxide materials, including SnO2 [8], ZnO [10], RuO2 [11], V2O5 [12], TiO2 [13], Al2O3 [14], were conducted as a sensing membrane for EGFET sensors. In addition, porous semiconductor or thin films, e.g. porous Si [15,16], TiN [17], CuS [18], have been investigated for use in sensitive membranes. Nevertheless, some metal oxide or semiconductor films

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demonstrated poor sensing performance and required some preprocessing treatments for enhancement in pH sensitivity [19,20]. Because of easy control of film composition, a low temperature synthesis, a simple and fast technique, the sol-gel derived film is possible to make the preferable in sensing and biosensing applications [21,22]. Amorphous oxide semiconductor (AOS) films, including indiumtin oxide (InSnO, ITO), zinc-tin oxide (ZnSnO), indium-gallium-zinc oxide (InGaZnO), indium-zinc oxide (InZnO), on plastic substrate have been widely used for transparent electrodes, organic lightemitting diode (OLED) devices, solar cells, liquid crystal displays, and flat panel displays due to their large carrier mobilities and high electrical conductivities [23e26]. Among these films, ITO electrode was modified by colloidal Au or Au nanoparticles to detect H2O2 concentration [27,28]. Moreover, InGaZnO film was extensively investigated as a channel layer for use in an ISFET or EGFET sensor [29,30]. However, some AOS films showed poor sensing performance in terms of a low pH sensitivity (41.43 mV/pH) and a high hysteresis voltage (~10 mV) for pH sensing [31,32]. The structural property correlated to the sensing performance for the ZnInO film on plastic substrate is rarely studied for an EGFET sensor. In this work, the influence of indium content on structural and sensing properties of solution processed ZnInxOy sensing films on flexible polyethylene naphthalate (PEN) was investigated for EGFET sensors. In order to realize the structural properties, the ZnInxOy sensing films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscopy (AFM), to examine the elemental composition, crystal structure, and film morphology, respectively. We evaluated that the effect of indium content on structural properties of ZnInxOy sensing films on flexible PEN was then related to their sensing performances (pH sensitivity, drift rate and hysteresis voltage) of EGFET pH sensors. Finally, the mechanical bendability of ZnInxOy sensing film on flexible PEN was evaluated for three different In concentrations. 1.1. Experimental For the ZnInxOy chemical solution preparation, at first, two separate zinc oxide and indium oxide solutions were prepared by dissolving zinc chloride (ZnCl2) and indium nitrate (In(NO3)3$H2O) in 0.2 M distilled water at 25  C, respectively, stirred separately for 30 min. Then, each of three zinc oxide (10%, 20% and 30% mole) chemical solutions and indium oxide chemical solution were mixed to produce 0.1 M solution concentration, subsequently stirred for 1 h. These solutions were kept aging for 24 h at 25  C to achieve a colourless and homogenous solution. In this study, a commercial flexible ITO/PEN with a resistivity of 30 U/, was used as a substrate because it has the conductive layer to send the signal to a MOSFET device during EGFET measurement. Before the deposition of the sensing film, the PEN substrate was cleaned by using alcohol and acetone for 10 s. After that, the flexible PEN was rinsed by de-ionized water, subsequently dried with N2 gas. The surface of flexible substrate was modified by oxygen plasma for 5 min to enhance the ZnInxOy film adhesion on its substrate. Various three ZnInxOy films (~20 nm) were deposited on a flexible ITO/PEN through a simple spin-coating method under the rotating speed of 3000 rpm for 30 s. These ZnInxOy films were baked at 110 С for 25 min. The thickness of ZnInxOy films treated at 10%, 20% and 30% mole conditions was determined to be 19.8, 20.1, and 20.2 nm by using spectroscopic ellipsometry, whereas the resistivity was estimated to be 50.2, 48.1 and 49.3 U/, by using fourprobe method, respectively. The sensing area was defined to be a circle, with a radius of 1.5 mm, by an automatic dispenser system (Teaching Box Series, Ganbow Technology Co., Ltd., Taiwan)

through an epoxy resin (S181 adhesive silicone). Finally, the ZnInxOy sensing film was fabricated on a printed circuit board (PCB) with a Cu line by using a Ag gel to link the Cu line. To avoid the leakage current, the other non-sensor area and the Cu line was encapsulated by a S181 adhesive silicone. The crystal structure of ZnInxOy films on flexible PEN was checked by a D8 Discover X-ray diffraction system (Bruker). The surface morphologies of these films deposited on flexible PEN were examined through an NT-MDT Solver P47 (AFM). The bonding structures (chemical compositions) of ZnInxOy films treated under three concentrations were analyzed by a monochromatic Al Ka (1486.7 eV) source. The drain currentegate voltage (IDSeVGS) characteristics of ZnInxOy film on flexible PEN based EGFET sensors were measured using a semiconductor parameter Agilent 4156C. A commercial CD4007CBE chip was chosen as a n-MOSFET device, and thus the gate electrode of this device was connected to the ZnInxOy sensing film. Then, only a traditional reference electrode (Ag/AgCl) and the ZnInxOy sensing film were directly to be submerged in a buffer solution, as shown in Fig. 1. The pH buffer solutions (Merck, Germany) were used to determine the pH sensitivity of ZnInxOy EGFET sensors in the range of pH 2e12. Furthermore, the hysteresis voltage (a loop of pH 7 / 4/7 / 10/7) and drift rate (in the pH 7) were estimated to realize the stability and reliability of the ZnInxOy EGFET pH sensors measured at room temperature (25  С). 2. Results and discussion 2.1. Structural properties of ZnInxOy sensing films As for the structural characterization, XRD was employed to examine the film structure of ZnInxOy sensing films. Fig. 2 depicts the XRD patterns of the ZnInxOy sensing films fabricated on flexible PEN and treated at three various In concentrations. It is found that only very strong peak at 26.4 for the three In concentrations appeared in the XRD pattern, which it is related to flexible PEN substrate [33]. In contrast, no crystalline diffraction peak associated with ZnO, In2O3, or ZnInO peaks was detected in the ZnInxOy sensing films, suggesting that these films are belong to an amorphous structure. After the relative low-temperature annealing, the film is difficult to form a crystal structure. The ZnInxOy sensing films were firstly deposited on flexible PEN to explore the surface topology using AFM. Fig. 3(a)-(c) gives the AFM images of the ZnInxOy sensing films deposited on flexible PEN and processed at the 10%, 20%, and 30% mole conditions, respectively. By finding to the surface topology achieved, the film fabricated at the 20% mole condition had the roughest surface (7.41 nm) among these conditions. This might be attributed to the property of the material itself, which changes the surface topology due to the optimal indium content in film. It can be inferred that the ZnInxOy sensing film under the 20% condition has rougher surface to improve its pH sensitivity. In addition, the Rrms (root-mean-square) roughness value of the sensing film prepared at the 10% and 30% conditions were 6.88 and 5.77 nm, respectively. It is obvious that the 30% condition exhibited a lower surface roughness value with uniform surface than the 10% condition. The film with a high In content may improve the atom rearrangements of the material, and hence affect the modification of the surface topology and roughness for ZnInxOy film. Apart from the film structure and surface topology by XRD and AFM analyses, we used XPS to characterize the composition of the ZnInxOy surface films under three In concentrations. Fig. 4(a)-(c) shows the Zn 2p, In 3d and O 1s spectra of the ZnInxOy films deposited on flexible PEN and fabricated at the three In concentrations, respectively. The Zn 2p2/3 peak is located at 1021.5 eV as

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Fig. 1. Schematic illustration of the ZnInxOy sensing film fabricated on flexible PEN and EGFET measurement unit.

Fig. 2. XRD patterns of the ZnInxOy sensing films processed at three different In concentrations.

the ZnO reference [34]. The binding energy of Zn 2p2/3 peak was observed at 1021.7, 1021.9 and 1022.1 eV for the 10%, 20% and 30% conditions, respectively. The shift in these Zn 2p2/3 peaks for the ZnInxOy film was a higher binding energy than those of reference ZnO. This is mainly due to the difference in ZneO bonding between the ZnO and ZnInxOy. It is observed that the shift in the binding energy position gradually increased as the In concentration increased, suggesting the ZnInxOy film with a high In content. In addition, the intensity of Zn 2p double peaks gradually decreased as the In concentration increased. Fig. 4(b) demonstrates the In double 3d XPS spectra of the ZnInxOy films processed at the various three In conditions. The In 3d5/2 peak positions of the ZnInxOy films were shifted to higher binding energies (0.2e0.6 eV) relative to the control In2O3 (3d5/2 peak at 444.7 eV) [35], indicating the existence of In atoms in the ZnInxOy. The shift in the position of the In 3d5/2 peak (at 444.9 eV) for the 10% condition was a lower binding energy by 0.4 eV relative to that of the 30% condition (at 445.3 eV), suggesting the low Ti content in the ZnInxOy. Furthermore, the intensity of In 3d split peak increased upon the In concentration. Fig. 4(c) shows the O 1s XPS spectra of the ZnInxOy films deposited on flexible PEN and prepared at three In concentrations, performed with appropriate peak curve-fitting lines. After Shirley

background subtraction, a mixed Gaussian-Lorenzian product function was used to make the curve fitting. Four sets of O 1s spectra were selected as four different components: the one OI peak at 529.6 eV is the IneO bond, the second OII peak at 530.3 eV (or 530.1 eV) is the ZneOeIn bond, the third OIII peak at 531.1 eV bond is the ZneO, and the fourth OIV peak at 531.8 eV is the eOH bond [36]. Among these components, the position of O 1s peak with respect to eOH component was shifted toward the highest binding energy. The highest binding energy is usually due to dissociated oxygen, chemisorbed oxygen or OH species (e.g. adsorbed O2, adsorbed H2O, eCO3) on the surface of the ZnInxOy film [36] because the signals were collected from the surface area without Ar ion beam presputtering process to expel adventitious adsorbed molecules on the film surface. The O 1s peaks appeared at 530.5 and 530 eV can be assigned to the control ZnO and In2O3 [34,35], respectively. The position of O 1s peak relative to ZneO and IneO components shifted higher binding energies compared with those of control ZnO and In2O3. This behavior is ascribed to O 2 ions that are in the ZnInxOy matrix containing oxygen-deficient regions. Consequently, the intensity variation of these components corresponds to the concentration of the oxygen vacancies. The O 1s peak intensity related to IneO component increased clearly with increasing the indium concentration, whereas that of ZneO component decreased, accordingly. Moreover, it is found that the film performed under the 20% mole showed a higher peak intensity in connection with ZnInxOy component than other conditions. This result indicates that the optimized In content could eliminate the oxygen vacancies in the film. 2.2. Sensing performance of ZnInxOy EGFET sensors The presence of specific interaction between the species and the solid surface in solution, especially in the case of oxide films, the adsorption of Hþ and OH ions on the surface of the ZnInxOy film, which occurs in the Helmholtz layer of a semiconductor. Furthermore, the chemical reaction of the ZnInxOyeOH group takes place mostly with Hþ and OH ions in acidic and basic solution, respectively. This reaction rate is correlated to free electron or hole concentration at the surface of the ZnInxOy film. The electron transport in an aqueous electrolyte passes the near surface region of the oxide film to produce an accumulation layer of charges. For a chemical pH sensor, the site-dissociation model was commonly used to picture the surface charging mechanism for oxides [37]. The

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Fig. 3. AFM surface images of the ZnInxOy sensing films processed at (a) 10%, (b) 20% and (c) 30% mole concentrations.

equilibrium between the Hþ ions and M  OH (metal hydroxides) surface sites in an electrolyte solution is given for oxide charging. In an amphoteric oxide, the metal ions with negative oxygen ions are usually electropositive to produce acceptor protonation from a neighboring Hþ. In contrast, the metal atoms are also enough electronegative to use as donor deprotonation from a neighboring OH. The equilibrium electrochemical reactions of an oxide surface are given by the following equations [37]:

MOH
















!
MO þ Hþ S

(1)

MOH þ Hþ S
















!
MOHþ 2

(2)

Ka ¼

½MO ½Hþ S ½MOH

Fig. 4. XPS spectra of (a) Zn 2p, (b) In 3d and (c) O 1s for the ZnInxOy sensing films processed at three different In concentrations.

(3)

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Fig. 5. Transfer characteristics (IDS-VGS) of the ZnInxOy EGFET sensors processed at (a) 10%, (b) 20% and (c) 30% mole concentrations and measured in different pH buffer solutions (pH ¼ 2 to 12). Inset: Reference voltage as a function of pH for the ZnInxOy EGFET sensors processed at three different In concentrations (in linear region).

(4)

where HþS is the Hþ ions close the oxide surface, MOH, MO and MOHþ 2 are neutral, negative and positive surface sites, respectively, and Ka and Kb are the dissociation constants in acidic and basic solution, respectively. [Hþ]S indicates the surface activity of Hþ ions, while [MOH], [MO] and [MOHþ 2 ] represent the number of these sites per surface area. For a pH electrode, the variation in the surface potential of an oxide film is obtained by the ions exchanged between an electrolyte solution and active surface sites. Furthermore, the sensing capability of the pH electrode is affected by the properties of electrolyte/oxide interface. The Nernst equation interprets the electric potential of a cell membrane with respect to ionic concentrations occurred in the electrochemical reaction. The pH electrode response follows the Nernstian equation as expressed:

E ¼ E0 þ 2:303

RT pH nF

(5)

where E is the measured potential, E0 is the standard electrolyte potential, R is the universal gas constant, T is the absolute temperature, n is the number of moles of electrons exchanged in the electrochemical reaction, and F is the Faraday constant. The 2.303RT/F item is called the Nernst slope, which it was obtained to

be 59.2 mV/pH at 25 С. Based on the classical theory of site binding, the concentration of active binding sites on the oxide film can cause the change in the surface potential voltage at the electrolyte solution-sensing membrane interface. The influence of surface potential voltage is associated with the pH value of an electrolytic solution. As mentioned above, the surface potential voltage (j) of the ZnInxOy sensing film at the electrolyte solution-oxide membrane interface can be represented as [37]:

j ¼ 2:303

b¼

 kB T b  pHpzc  pH q bþ1

pffiffiffiffiffiffiffiffiffiffiffi 2q2 Ns Ka Kb kB TCDL

(6)

 1 2 Ka pHpzc ¼  log Kb =

  MOH þ 2 Kb ¼ ½MOH½H þ S

and

(7)

where kB is Boltzmann’s constant, q is the elementary electron charge, b is the sensitivity parameter, which reflects the chemical sensitivity of an oxide film, pHpzc is the pH value at the zero charge point of an oxide film, NS is the total number of surface sites per unit area, and CDL is the electrical double layer capacitance, which it is derived from the GouyeChapmaneStern model [38]. Resembling the MOSFET, the linear relationship between the IDS and the VGS for an EGFET sensor is given by Ref. [39]:

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Fig. 6. (a) Hysteresis characteristics of the ZnInxOy EGFET sensors processed at three different In concentrations and tested in pH loop of 7 / 4/7 / 10/7. (b) Drift rate of the ZnInxOy EGFET sensors processed at three different In concentrations and measured in solution at pH 7 buffer solution. (c) pH sensing performance of the ZnInxOy EGFET sensors as a function of bending cycle for three different In concentrations. (d) Reference voltage of the ZnInxOy EGFET sensor prepared at the 20% mole for different Hþ, Naþ, Kþ, Mg2þ, and Ca2þ ion concentrations.

IDS ¼

mn Cox W L





VGS  VTH 

VDS VDS 2

(8)

where mn is the effective carrier mobility, Cox is the gate oxide capacitance per unit area, W and L are the gate width and length, respectively, VTH is the threshold voltage of a nMOSFET device, which stands for the surface potential, and VDS is the drain-source voltage. In aqueous acidic solution, more positive charge accumulating on the ZnInxOy sensing film induces a positive surface potential, thus giving rise to a smaller applied gate voltage. For an EGFET sensor, more electron flowing from source (S) to drain (D) is governed by the VGS, which is related to the number of positive (Hþ) or negative (OH) ions adsorbing on the oxide surface to change the channel conductivity. In other words, the excess voltage or less voltage with respect to Hþ or OH ions in an electrolyte solution affects the VGS value. From eq. (8), the IDS is related to the VGS. As a result, the IDS current corresponds to the pH value. The Hþ ions accumulated on the oxide surface in an acidic solution is similar to an extra positive voltage applied at the MOSFET gate. This result increases the channel conductivity as concomitant the drain current increases. On the contrary, the conductive channel is applied an extra negative voltage in a basic solution, and thereby decreasing the IDS current. Fig. 5(a)-(c) depict the IDSeVGS transfer characteristics of ZnInxOy film on flexible PEN based EGFET sensors with different three conditions and measured in buffer solutions in the pH range 2e12.

It is found that the shift in the threshold voltage was from left direction to the right direction when the pH increased. In the triode region, the low pH is associated with high Hþ ions accumulated on the oxide surface in an acidic solution, that is, the large amount of Hþ ions induce an excess positive voltage applied to the gate electrode, thus increasing the drain current. In contrast, the large pH value represents large OH and low Hþ ions in a basic solution. Assembling, under a negative voltage, the channel conductivity becomes low, therefore causing a reduction in the drain current. Inserts of Fig. 5(a)e(c) demonstrate that the pH sensitivity of the ZnInxOy films on flexible PEN based EGFET sensors were 46.28, 61.76 and 50.11 mV/pH for 10%, 20% and 30% mole conditions, respectively, in the measurable range of pH 2e12. The sensitivity and linearity can be determined by fitting the IDS-VGS curves at the fixed VDS of 0.1 V and IDS of 100 mA. From the linear fitting, the slope was used to determine the pH sensitivity, while the linear regression was used to evaluate the linearity. The ZnInxOy EGFET sensor processed at the 20% condition exhibited the highest pH sensitivity among these conditions. This evident improvement in sensitivity can be contributed to the ZnInxOy sensing film possessing a rougher surface as well as a higher ZneOeIn content to provide larger effective sensing areas as well as more surface binding sites. The ZnInxOy film has a larger surface area to increase the additional extra Hþ and OH ions absorbed on the oxide surface and to promote the relative oxygen binding to effectively sense Hþ ions. Consequently, it can be deduced that the pH sensitivity of the ZnInxOy film fabricated at the 20% condition is relatively higher

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than other conditions. Furthermore, pH response characteristics of the ZnInxOy EGFET sensors in acid (pH 2) and alkaline (pH 12) solutions demonstrated good corrosion resistance. In comparison with other sensing membranes, such as ZnO (38 mV/pH) [10], SnO2 (56 mV/pH) [8], V2O5 (58.1 mV/pH) [12], Al2O3 (49.8 mV/pH) [14], TiN (57 mV/pH) [17], CuS/ITO (37 mV/pH) [18], a higher pH sensitivity was achieved for the ZnInxOy sensing membrane in EGFET sensors. Besides the sensitivity of an oxide film, the hysteresis and drift of ZnInxOy sensing film are very critical important parameters to reflect its stability and reliability for an EGFET sensor. The hysteresis phenomenon is mainly due to the slow electrochemical interaction between the electrolyte ions and the active surface oxide sites, which are frequently most oxygen vacancies or defects [40]. Fig. 6(a) depicts the hysteresis curves of the ZnInxOy film on flexible PEN based EGFET sensors prepared with three In concentrations and tested in a pH loop 7 / 4/7 / 10/7. The hysteresis voltage was determined by difference on average VREF value between the first and the last VREF with respect to the first pH 7 and the last pH 7. The hysteresis voltages of the ZnInxOy EGFET sensors were evaluated to be 22, ~1 and 7 mV for 10%, 20% and 30% mole conditions, respectively. It is evident that the 20% condition exhibited a relatively lower hysteresis voltage than other conditions, demonstrating that the optimized In content in the film can lessen the crystal deformities, imperfections and oxygen vacancies. Besides the hysteresis effect, the drift phenomenon is considered to examine the stability of the ZnInxOy EGFET sensor. The pH or biological molecule in an electrolyte solution is detected by using an EIS sensor, as a result, a relatively slow chemical modification come about on the oxide surface. After a specified time period, the chemical modification took place in the oxide surface is associated with the alterations in the number of surface binding sites. This behavior has a detrimental effect on the overall capacitance of an oxide film, which represents a slow and temporal change in the reference voltage [41]. Fig. 6(b) shows the drift curves of the ZnInxOy film on flexible PEN based EGFET sensors for three different In conditions and tested in pH 7. Among these In concentrations, the 20% In concentration had the best stability (1.08 mV/h), whereas the 10% In concentration featured the worst stability (6.25 mV/h). The higher instability may be ascribed to the less ZneOeIn content in the film producing more the oxygen vacancies or defects. To realize the capability of mechanically flexible and bendable pH sensor, pH sensitivity of ZnInxOy film deposited on flexible PEN was estimated after repetitive bending cycle. Fig. 6(c) depicts the sensing performance of ZnInxOy film deposited on flexible PEN and processed at three various In concentrations, performed under different bending cycles. It is worth mentioning that the pH sensitivity for three conditions remained constant after 500 bending cycles. Nevertheless, after 700 repetitive bending cycles, a sudden drop of pH sensitivity was found for these conditions. This finding may be due to the formation of porosities, grain boundaries and microcracks in the sensing film. To realize the selectivity of the ZnInxOy EGFET sensor, different ions, e.g. Naþ, Kþ, Mg2þ, Ca2þ, were employed in this study. Fig. 6(d) shows the VREF of the ZnInxOy EGFET sensor treated at the 20% mole condition tested at various Hþ, Naþ, Kþ, Mg2þ, and Ca2þ ion concentrations in pH 7. These ion concentrations ranging from 107 to 102 M were used to evaluate the response curves of an EGFET sensor. It is observed that the ZnInxOy sensing film had a high level of response to Hþ ions and also demonstrated almost the same trend response to other ions. In addition, we examined the performance of ZnInxOy film within an operation time of 120 days in order to demonstrate the sensor lifetime. After the long-term test, an average pH sensitivity of ZnInxOy EGFET sensor prepared at the 20% mole condition was 60.9 mV/pH, which is comparable to the

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sensitivity of a new sensor. 3. Conclusion In this work, the impact of indium content on structural property and sensing performance of chemical solution derived ZnInxOy sensing film on flexible PEN for EGFET pH sensors. The structural, morphological and compositional characteristics of the ZnInxOy sensing films were analyzed using XRD, AFM and XPS. The pH sensitivity of the ZnInxOy sensing film on flexible PEN fabricated at the 20% condition (61.76 mV/pH) was observed to be relatively higher than other conditions (46.28 mV/pH for 10% mole and 50.11 mV/pH for 30% mole). This finding is attributed to the ZnInxOy film with a rough surface and a high ZneOeIn content to generate a high number of surface sites. Moreover, the stabilities (hysteresis voltage and drift rate) of the ZnInxOy EGFET sensor processed at the 20% condition (~1 mV and 1.08 mV/h) showed better compared with that at the 10% (22 mV and 6.25 mV/h) and 30% (7 mV and 2.11 mV/h) conditions. After 500 bending cycles, the sensing performance of the ZnInxOy EGFET sensor with the 20% mole condition kept unchanged. Furthermore, the ZnInxOy sensing film showed the highest level of response to Hþ ions among other ions (Naþ, Kþ, Mg2þ, and Ca2þ). These results suggest that the ZnInxOy sensing membrane fabricated on flexible PEN and treated with the 20% mole condition is a potentially important material for an EGFET pH sensor. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Tung-Ming Pan: Supervision, Investigation, Writing - review & editing, Conceptualization, Methodology. Yen-Hsiang Huang: Validation, Formal analysis, Investigation, Data curation, Writing original draft. Jim-Long Her: Conceptualization, Methodology, Visualization, Investigation. Bih-Show Lou: Conceptualization, Methodology, Visualization, Investigation. See-Tong Pang: Visualization, Funding acquisition. Acknowledgment This work was financial supporting by the Ministry of Science and Technology (MOST) of Taiwan under contract MOST-108-2111E-182-023 and the Chang Gung Memorial Hospital of Taiwan under contract CMRPD2H0151 and CMRPD2H0152. References [1] W.D. Huang, H. Cao, S. Deb, M. Chiao, J.C. Chiao, A flexible pH sensor based on the iridium oxide sensing film, Sens. Actuators A Phys. 169 (2011) 1e11. [2] W. Lonsdale, M. Wajrak, K. Alameh, Manufacture and application of RuO2 solid-state metal-oxide pH sensor to common beverages, Talanta 180 (2018) 277e281. [3] S.Y. Oh, S.Y. Hong, Y.R. Jeong, J. Yun, H. Park, S.W. Jin, G. Lee, J.H. Oh, H. Lee, S.S. Lee, J.S. Ha, Skin-attachable, stretchable electrochemical sweat sensor for glucose and pH detection, ACS Appl. Mater. Interfaces 10 (2018) 13729e13740. [4] P. Bergveld, Development of an ion-sensitive solid-state device for neurophysiological measurements, IEEE Trans. Biomed. Eng. 17 (1970) 70e71. [5] M.J. Schoning, M.H. Abouzar, A. Poghossian, pH and ion sensitivity of a fieldeffect EIS (electrolyte-insulator-semiconductor) sensor covered with polyelectrolyte multilayers, J. Solid State Electrochem. 13 (2009) 115e122. [6] J. van der spiegel, I. Lauks, P. Chan, D. Babic, The extended gate chemically sensitive field effect transistor as multi-species microprobe, Sens. Actuators 4 (1983) 291e298.

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