Investigation of Sensitivities and Drift Effects of the Arrayed Flexible Chloride Sensor Based on RuO2/GO at Different Temperatures

We investigate the temperature effect on sensing characteristics and drift effect of an arrayed flexible ruthenium dioxide (RuO2)/graphene oxide (GO) chloride sensor at different solution temperatures between 10 °C and 50 °C. The average sensor sensitivities according to our experimental results were 28.2 ± 1.4 mV/pCl (10 °C), 42.5 ± 2.0 mV/pCl (20 °C), 47.1 ± 1.8 mV/pCl (30 °C), 54.1 ± 2.01 mV/pCl (40 °C) and 46.6 ± 2.1 mV/pCl (50 °C). We found the drift effects of an arrayed flexible RuO2/GO chloride sensor in a 1 M NaCl solution to be between 8.2 mV/h and 2.5 mV/h with solution temperatures from 10 °C to 50 °C.


Introduction
Many researchers have investigated the sensitivity, response time and drift rate of chloride ion sensing devices, but few researchers have studied the effect of temperature effect on their chloride ion sensing devices. However, the sensitivities and longer period detecting chloride ion concentrations of the chloride ion sensing devices are interesting subjects for study at different solution temperatures. Temperature affects the sensitivity of pH sensors, and many researchers have investigated sensitivity variation with pH solution temperatures from 25 • C to 65 • C [1][2][3][4]. They found that pH sensitivity increased as solution temperature increased. They calculated the temperature coefficient of sensitivity (TCS) for the pH sensors, and investigated the relationship between TCS and pH sensors. Many researchers used a radio frequency (RF) sputtering system [5][6][7] and screen printing technology [8] to fabricate the RuO 2 sensing electrode. They have investigated and applied the physical characteristic of ruthenium.
Our research group used the arrayed flexible RuO2 chloride sensor to investigate real applications for tap water and swimming pool water [14]. The response potentials of the arrayed flexible RuO2 chloride sensor were −245.711 ± 1.410 mV (0.248 mg/L chloride concentration) and −256.058 ± 2.097 mV (0.998 mg/L chloride concentration), respectively, for tap water and swimming pool water. Mahajan et al. [15] employed Cu(II) complexes to develop highly sensitive and selective chloride sensors, sensing across chloride concentrations ranging from 2.5×10 −5 M to 1.0×10 −1 M. Garrido et al. [16] used screen printing to fabricate the three electrodes of a wearable electrochemical sensor. The sensing detection limit was 2.0 × 10 −4 M for chloride ions. Montemor et al. [17] fabricated a multi-probe chloride sensor and used it to measure response potentials of a mortar and concrete specimen. Trnkova et al. [18] prepared a carbon paste electrode (CPE) and a CPE modified with different preparations of AgNO3 and/or solid silver particles. The chloride ion sensing characteristics were investigated. The CPE modified with silver particles promoted the sensitivity for chloride ions. Patil et al. [19] integrated pH, turbidity and temperature-sensing devices, in addition to global system for mobile communications (GSM), to investigate sensing characteristics and applications at different temperature conditions. Graphenes are 2-D structures, providing large surface area, zero band gap, extremely high intrinsic charge carrier mobility and high chemical stability [20][21][22][23][24][25]. Many researchers have studied the physical characteristics of graphene [26,27]. Graphene was used to modify the ion sensors, with significant improvement of the sensing characteristics. Recently, Ali et al. [28] used graphene oxide (GO) nanosheets and poly(3,4-ethylenedioxythiophene) nanofibers (PEDOT-NFs) as electrochemical sensing interfaces to prepare microfluidic impedimetric nitrate sensors. They used electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to investigate the sensing characteristics of sensing film.
Our research group [29] investigated the sensitivity variation at the different weight ratios of the GO solution that were used to modify the arrayed flexible RuO2 chloride sensor. By adding GO, the sensitivity was enhanced, and this is attributed to the increased area of the sensing windows. In this study, temperature affected chloride sensors, therefore we investigated the sensitivities, drift effects and electrochemical impedance analysis of the arrayed flexible RuO2/GO chloride sensor by varying temperatures of the NaCl solution from 10 °C to 50 °C. Our research group used the arrayed flexible RuO 2 chloride sensor to investigate real applications for tap water and swimming pool water [14]. The response potentials of the arrayed flexible RuO 2 chloride sensor were −245.711 ± 1.410 mV (0.248 mg/L chloride concentration) and −256.058 ± 2.097 mV (0.998 mg/L chloride concentration), respectively, for tap water and swimming pool water. Mahajan et al. [15] employed Cu(II) complexes to develop highly sensitive and selective chloride sensors, sensing across chloride concentrations ranging from 2.5×10 −5 M to 1.0×10 −1 M. Garrido et al. [16] used screen printing to fabricate the three electrodes of a wearable electrochemical sensor. The sensing detection limit was 2.0 × 10 −4 M for chloride ions. Montemor et al. [17] fabricated a multi-probe chloride sensor and used it to measure response potentials of a mortar and concrete specimen. Trnkova et al. [18] prepared a carbon paste electrode (CPE) and a CPE modified with different preparations of AgNO 3 and/or solid silver particles. The chloride ion sensing characteristics were investigated. The CPE modified with silver particles promoted the sensitivity for chloride ions. Patil et al. [19] integrated pH, turbidity and temperature-sensing devices, in addition to global system for mobile communications (GSM), to investigate sensing characteristics and applications at different temperature conditions.
Our research group [29] investigated the sensitivity variation at the different weight ratios of the GO solution that were used to modify the arrayed flexible RuO 2 chloride sensor. By adding GO, the sensitivity was enhanced, and this is attributed to the increased area of the sensing windows. In this study, temperature affected chloride sensors, therefore we investigated the sensitivities, drift effects and electrochemical impedance analysis of the arrayed flexible RuO 2 /GO chloride sensor by varying temperatures of the NaCl solution from 10 • C to 50 • C.

Materials
The flexible and light polyethylene terephthalate (PET) substrate was purchased from Zencatec Corporation (New Taipei, Taiwan). The silver paste and epoxy thermosetting polymer (product no. JA643) were used to prepare the conducting wires and insulation layer by a screen printing system. The silver paste and epoxy thermosetting polymer were purchased from Advanced Electronic Material Inc. (Tainan, Taiwan) and Everwide Chemical Co., Ltd. (Yunlin, Taiwan), respectively. The ruthenium target (Ru, 99.95 wt %) was used to deposit the thin ruthenium dioxide (RuO 2 ) film onto the silver paste layer using a radio frequency sputtering system. The ruthenium target was purchased from Ultimate Materials Technology Co., Ltd. (Hsinchu, Taiwan). The graphene oxide powder was purchased from Tokyo Chemical Industry Co., Ltd. (Chuo-ku, Tokyo, Japan). The ETH9033 and TDDMACl were used as chloride sensing film, and they were purchased from Sigma-Alorich Co. Ltd. (St. Louis, MO, USA). Sodium chloride (NaCl) powder was purchased from Avantor Performance Materials, Inc. (Center Valley, PA, USA), and was then used to prepare the aqueous solutions.

Fabrication of the Arrayed Flexible RuO 2 /GO Chloride Ion Sensor
The fabrication process for the arrayed flexible RuO 2 /GO chloride ion sensor was shown in Figure 2. We used radio frequency sputtering and screen printing technology to fabricate the arrayed flexible RuO 2 pH sensor [14,29,30]. The sensing area of RuO 2 electrode is 1 mm × 1 mm. The 0.01 wt % GO solution was prepared with 10 mL deionized water and 1 mg graphene oxide powder, and the GO solution was uniformly mixed by ultrasonic vibration. Then we pipetted 2 µL of the 0.01 wt % GO solution onto each of the six sensing windows of the arrayed flexible RuO 2 sensor. We then put the sensors on a table at room temperature (25 • C) for 12 h.

Sensing Mechanism of the Chloride Sensor
We used the screen printing system and silver paste to fabricate the difference reference electrodes and silver contrast electrodes, as shown in Figure 3. The voltage-time measurement system for the arrayed flexible RuO2/GO chloride sensor was shown in Figure 4. From Equation (2), the sensing mechanism of the single working electrode, the difference reference electrodes and silver contrast electrodes [30]. VOut is the output potential of an LT 1167 amplifier, VRef is the potential of the silver reference electrode, VSen1 is the potential of the silver contrast electrode, VSen2 is the potential of the working electrode (sensing membrane), VIn1 is the potential difference between the working electrode and the reference electrode and VIn2 is the potential difference between the silver contrast electrode and the reference electrode. The Nersntian equation of the chloride sensing membrane was as shown in Equation (3). E is the electromotive force (EMF), E0 is the initial voltage, α is the activity of the ion, R is the gas constant 8.316 mol·e −1 ·°C −1 , F is Faraday coefficient 96.487 °C. The response potentials were decreased when chloride concentration increased.  Adjustable volume micropipettes (SIS-825.0020-1PAK, Socorex Isba S.A., Switzerland) were used to pipette 2 µL of 0.01 wt % GO solution and 2 µL of the chloride sensing mixture onto each of the six sensing windows of the sensors. We used the adjustable volume micropipettes to control the reproducibility of the mixture pipetting and thickness of these layers.

Sensing Mechanism of the Chloride Sensor
We used the screen printing system and silver paste to fabricate the difference reference electrodes and silver contrast electrodes, as shown in Figure 3. The voltage-time measurement system for the arrayed flexible RuO 2 /GO chloride sensor was shown in Figure 4. From Equation (2), the sensing mechanism of the single working electrode, the difference reference electrodes and silver contrast electrodes [30]. V Out is the output potential of an LT 1167 amplifier, V Ref is the potential of the silver reference electrode, V Sen1 is the potential of the silver contrast electrode, V Sen2 is the potential of the working electrode (sensing membrane), V In1 is the potential difference between the working electrode and the reference electrode and V In2 is the potential difference between the silver contrast electrode and the reference electrode. The Nersntian equation of the chloride sensing membrane was as shown in Equation (3). E is the electromotive force (EMF), E 0 is the initial voltage, α is the activity of the ion, R is the gas constant 8.316 mol·e −1 · • C −1 , F is Faraday coefficient 96.487 • C. The response potentials were decreased when chloride concentration increased.

Sensing Mechanism of the Chloride Sensor
We used the screen printing system and silver paste to fabricate the difference reference electrodes and silver contrast electrodes, as shown in Figure 3. The voltage-time measurement system for the arrayed flexible RuO2/GO chloride sensor was shown in Figure 4. From Equation (2), the sensing mechanism of the single working electrode, the difference reference electrodes and silver contrast electrodes [30]. VOut is the output potential of an LT 1167 amplifier, VRef is the potential of the silver reference electrode, VSen1 is the potential of the silver contrast electrode, VSen2 is the potential of the working electrode (sensing membrane), VIn1 is the potential difference between the working electrode and the reference electrode and VIn2 is the potential difference between the silver contrast electrode and the reference electrode. The Nersntian equation of the chloride sensing membrane was as shown in Equation (3). E is the electromotive force (EMF), E0 is the initial voltage, α is the activity of the ion, R is the gas constant 8.316 mol·e −1 ·°C −1 , F is Faraday coefficient 96.487 °C. The response potentials were decreased when chloride concentration increased.

Voltage-Time and Eelectrochemical Impedance Spectroscopy Measurement Systems
The power supply, National Instruments Data Acquisition (DAQ) card, readout circuit, arrayed flexible RuO2/GO chloride sensor and computer were integrated to compose a voltage-time (V-T) measurement system. We used the eight amplifiers (LT1167), core wires and a circuit board for the readout circuit. The voltage-time curves reflect the response potentials of different chloride concentrations from 1 ×10 −5 M to 1 M NaCl solutions.

Voltage-Time and Eelectrochemical Impedance Spectroscopy Measurement Systems
The power supply, National Instruments Data Acquisition (DAQ) card, readout circuit, arrayed flexible RuO 2 /GO chloride sensor and computer were integrated to compose a voltage-time (V-T) measurement system. We used the eight amplifiers (LT1167), core wires and a circuit board for the readout circuit. The voltage-time curves reflect the response potentials of different chloride concentrations from 1 ×10 −5 M to 1 M NaCl solutions.
Electrochemical impedance spectroscopy (EIS; BioLogic SP 150, Aurora Biotech Inc., Seyssinet-Pariset, France) was used to get the solution resistance (R s ), electron transfer resistances (R et ) and double layer capacitor (C dl ) between the sensing membrane and NaCl solution. The working electrode was an RuO 2 /GO/chloride ion sensing film, the reference electrode was an Ag/AgCl electrode and the counter electrode was a platinum (Pt) electrode. The amplitude of the voltage of the EIS measuring system was 0.7 mV, and the frequency range of the sinusoidal excitation signal was set from 100 MHz to 10 kHz in the EIS measuring system. The cooling circulating water bath and thermometer were used to control the solution temperatures from 10 • C to 50 • C, with concentrations from 1 × 10 −5 M to 1 M NaCl.
The experiments of sensitivity, EIS and drift effect of the flexible arrayed RuO 2 /GO chloride sensor were described as follows: 1.
The sensitivities were investigated from 1 × 10 −5 M to 1 M NaCl solutions at room temperature (25 • C) with the V-T measuring system.

2.
The sensitivities were investigated from 1 × 10 −5 M to 1 M NaCl solutions at different temperatures from 10 • C to 50 • C with the V-T measuring system. 3.
The electrochemical impedance analysis was used to measure and fit the values of R et , R s and C dl from 1 × 10 −5 M to 1 M NaCl solutions at room temperature (25 • C) with the EIS measuring system. 4.
The electrochemical impedance analysis was used to measure and fit the values of R et , R s and C dl in the 1 M NaCl solution at different temperatures from 10 • C to 50 • C with the EIS measuring system.

5.
The response potential variations of 1 M NaCl solution were investigated over a longer period for different solution temperatures from 10 • C to 50 • C by the V-T measurement system.
Each experiment was tested five times and the average sensitivities, results of EIS analysis and drift rates were obtained.

Investigation of the Sensitivities for Different Solution Temperatures
In Figure 5, we can see the curves with the fitted parameters of Equations (4) and (5) as follows: where Y is response potential and X is the log of chloride concentration. -OH and -COOH groups accompany the pH variations [17][18][19][20][21][22]. Melai et al. [31] and Kim et al. [32] found the oxygen-containing functional groups base on the basal plane and edges of the GO structure. The oxygen-containing functional groups have negative ions. GO has large specific surface area and GO electrochemistry characteristics [29,[31][32][33][34] improve the chloride sensitivity of RuO 2 /GO arrayed flexible chloride ion sensors. From Table 1, the average sensitivity of RuO 2 arrayed flexible chloride ion sensors was 25.1 ± 11.3 mV/pCl at room temperature [14]. Dam et al. [35] used the screen printing system and Dupont 5876 AgCl conducting paste to prepare an AgCl layer on a PET substrate, which is a potentiometric sensing device. The sensitivity of the flexible chloride sensor was 57.0 mV/decade from 1 × 10 −3 M to 3 M KCl solutions. Harris et al. [36] used the screen printing system and silver paste to prepare a silver layer on an alumina substrate, which is a potentiometric sensing device. The chloride sensors and distributed wireless network were used to detect chloride range. The sensitivity of the wireless chloride sensor was 59.2 mV/pCl from 62.5 × 10 −3 M to 1 M NaCl solutions. Trnkova et al. [18] used the 70% graphite powder, 30% mineral oil, to fabricate the carbon paste electrode, which is an amperometric sensing device. The sensitivity of the carbon paste electrode was 1.1 nA/µM form 1 × 10 −4 M to 1 × 10 −3 M NaCl solutions. The sensitivities of their sensors were higher than the arrayed flexible RuO 2 /GO chloride sensor, but they used Ag/AgCl reference electrodes. We used the screen printing system and silver paste to fabricate the differential reference electrode and silver contrast electrode. The advantages of the arrayed flexible RuO 2 /GO chloride sensor are light weight, flexibility and low cost [14].
chloride ion sensors rose with chloride ion concentration. The GO contains the hydroxyl (-OH) and carboxyl (-COOH) groups. Protonation and de-protonation of -OH and -COOH groups accompany the pH variations [17][18][19][20][21][22]. Melai et al. [31] and Kim et al. [32] found the oxygen-containing functional groups base on the basal plane and edges of the GO structure. The oxygen-containing functional groups have negative ions. GO has large specific surface area and GO electrochemistry characteristics [29,[31][32][33][34] improve the chloride sensitivity of RuO2/GO arrayed flexible chloride ion sensors. From Table 1, the average sensitivity of RuO2 arrayed flexible chloride ion sensors was 25.1 ± 11.3 mV/pCl at room temperature [14]. Dam et al. [35] used the screen printing system and Dupont 5876 AgCl conducting paste to prepare an AgCl layer on a PET substrate, which is a potentiometric sensing device. The sensitivity of the flexible chloride sensor was 57.0 mV/decade from 1 × 10 −3 M to 3 M KCl solutions. Harris et al. [36] used the screen printing system and silver paste to prepare a silver layer on an alumina substrate, which is a potentiometric sensing device. The chloride sensors and distributed wireless network were used to detect chloride range. The sensitivity of the wireless chloride sensor was 59.2 mV/pCl from 62.5 × 10 −3 M to 1 M NaCl solutions. Trnkova et al. [18] used the 70% graphite powder, 30% mineral oil, to fabricate the carbon paste electrode, which is an amperometric sensing device. The sensitivity of the carbon paste electrode was 1.1 nA/μM form 1 × 10 −4 M to 1 × 10 −3 M NaCl solutions. The sensitivities of their sensors were higher than the arrayed flexible RuO2/GO chloride sensor, but they used Ag/AgCl reference electrodes. We used the screen printing system and silver paste to fabricate the differential reference electrode and silver contrast electrode. The advantages of the arrayed flexible RuO2/GO chloride sensor are light weight, flexibility and low cost [14].   The sensing devices were used to take five measurements in NaCl solutions from 1 × 10 −5 M to 1 M. The measured results are shown in Figure 6 and Table 2, where we see that the average Sensors 2018, 18, 632 7 of 12 sensitivities (absolute value) of the arrayed flexible RuO 2 /GO chloride sensors at different solution temperatures were 28.2 ± 2.4 mV/pCl (10 • C), 42.5 ± 2.0 mV/pCl (20 • C), 47.1 ± 1.8 mV/pCl (30 • C), 54.1 ± 2.0 mV/pCl (40 • C) and 46.6 ± 2.10mV/pCl (50 • C). According to the experimental results and our previous research [37], the average sensitivities of arrayed flexible RuO 2 /GO chloride sensors were higher than flexible RuO 2 chloride sensors at different solution temperatures. GO has large specific surface area, which supported the chloride ion sensing film to obtain more chloride ions and produce the bigger response potentials than if not GO-modified. The sensing devices were used to take five measurements in NaCl solutions from 1 × 10 −5 M to 1 M. The measured results are shown in Figure 6 and Table 2, where we see that the average sensitivities (absolute value) of the arrayed flexible RuO2/GO chloride sensors at different solution temperatures were 28.2 ± 2.4 mV/pCl (10 °C), 42.5 ± 2.0 mV/pCl (20 °C), 47.1 ± 1.8 mV/pCl (30 °C), 54.1 ± 2.0 mV/pCl (40 °C) and 46.6 ± 2.10mV/pCl (50 °C). According to the experimental results and our previous research [37], the average sensitivities of arrayed flexible RuO2/GO chloride sensors were higher than flexible RuO2 chloride sensors at different solution temperatures. GO has large specific surface area, which supported the chloride ion sensing film to obtain more chloride ions and produce the bigger response potentials than if not GO-modified.

Investigation of the Electrochemical Impedance Analysis for Different Solution Temperatures
From Figure 8 and Table 3, we see that the electron transfer resistances (Ret) of the RuO2/GO arrayed flexible chloride ion sensors were decreased in NaCl solutions from 1 × 10 −5 M to 1 M. The chloride ion sensing film caught the chloride ions at the different chloride ion concentrations from 1 × 10 −5 M to 1 M NaCl solutions, which could transform to the response potentials at different chloride ion concentrations [10,13,14].

Investigation of the Electrochemical Impedance Analysis for Different Solution Temperatures
From Figure 8 and Table 3, we see that the electron transfer resistances (R et ) of the RuO 2 /GO arrayed flexible chloride ion sensors were decreased in NaCl solutions from 1 × 10 −5 M to 1 M. The chloride ion sensing film caught the chloride ions at the different chloride ion concentrations from 1 × 10 −5 M to 1 M NaCl solutions, which could transform to the response potentials at different chloride ion concentrations [10,13,14].  We used 1 M NaCl solution to investigate the R et for different solution temperatures from 10 • C to 50 • C. From Figure 9 and Table 4, the R et were 274.7 ± 52.7 kΩ (10 • C), 129.9 ± 25.1 kΩ (20 • C), 83.8 ± 4.3 kΩ (30 • C), 41.5 ± 13.0 kΩ (40 • C) and 34.9 ± 11.8 kΩ (50 • C).  Figure 9 and Table 4   At higher solution temperatures, the solution viscosity is lower and the mobility of the ions in solution is higher. The dissociation of molecules increases with solution temperature, which induced the number of ions in solution to increase with the conductivity of a solution [38,39], which helped the chloride film to catch an increasing amount of chlorides as the temperature of the NaCl solution was increased from 10 °C to 40 °C. However, the adhesion between the chloride ion sensing film and RuO2/GO sensing window was lower at 50 °C than at 40 °C, and the response potentials were also lower across the chloride concentrations at 50 °C. The average sensitivity rose with solution Cdl Ret Rs  At higher solution temperatures, the solution viscosity is lower and the mobility of the ions in solution is higher. The dissociation of molecules increases with solution temperature, which induced the number of ions in solution to increase with the conductivity of a solution [38,39], which helped the chloride film to catch an increasing amount of chlorides as the temperature of the NaCl solution was increased from 10 • C to 40 • C. However, the adhesion between the chloride ion sensing film and RuO 2 /GO sensing window was lower at 50 • C than at 40 • C, and the response potentials were also lower across the chloride concentrations at 50 • C. The average sensitivity rose with solution temperature over 10-40 • C, but was lower at 50 • C. The operating temperatures of the arrayed flexible RuO 2 /GO chloride sensor were from 10 • C to 40 • C. The temperature coefficient of sensitivity (TCS) of the arrayed flexible RuO 2 /GO chloride sensor was found to be approximately 0.81 mV/(pCl· • C).

Investigation of the Drift Effect at Different Solution Temperatures
We investigated the response potentials over a longer period in NaCl solution with different solution temperatures. The V-T measuring system was used to measure response potentials for the arrayed flexible RuO 2 /GO chloride sensor in the 1 M NaCl solution over 12 h across the 10 • C to 50 • C conditions. In Table 5 we see that the maximum and minimum drift rates were 8.2 mV/h and 2.5 mV/h at 10 • C and 50 • C, respectively. The RuO 2 /GO chloride ion sensing film produced a hydrated layer during measurement over a longer period [14,40] at room temperature, which caused the response potential to increase. Some researchers [7,41] used an RF sputtering system to prepare different metal oxides for a sensing membrane on the different substrates. They investigated the drift effects of their pH sensor at different solution temperatures. The drift variations were higher when the pH solution temperatures were higher. As per to Section 2.2, we pipetted the 2 µL of the chloride sensing mixture onto the six sensing windows of each sensor. The chloride sensing mixture was similar to the colloid. The chloride sensing films and sensing windows of the arrayed flexible RuO 2 /GO chloride sensors adhered to each other. The adhesion between the chloride ion sensing film and RuO 2 /GO sensing window was lower at higher temperatures. The lower adhesion caused the drift rate to decrease at higher temperatures (from 40 • C to 50 • C). The drift variations were declined with temperature of the 1 M NaCl for the 12 h treatment.

Conclusions
The average sensitivities of the arrayed flexible RuO 2 /GO chloride sensor were 28.2 ± 1.4, 42.5± 2.0, 47.1 ± 1.8, 54.1 ± 2.0 and 46.6 ± 2.1 mV/pCl with different concentrations of chloride solution at 10, 20, 30, 40 and 50 • C. The average sensitivities rose with solution temperature from 10 • C to 40 • C. The operating temperatures of the arrayed flexible RuO 2 /GO chloride sensor were from 10 • C to 40 • C. We found the drift effects of the arrayed flexible RuO 2 /GO chloride sensor in the 1 M NaCl solution to be between 8.2 mV/h and 2.5 mV/h with solution temperatures from 10 • C to 50 • C. The temperature coefficient of sensitivity (TCS) of the arrayed flexible RuO 2 /GO chloride sensor was approximately 0.81 mV/(pCl· • C).