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High sensitive reduced graphene oxide-based room temperature ionic liquid electrochemical gas sensor with carbon-gold nanocomposites amplification
Sensors & Actuators: B. Chemical 299 (2019) 126952
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
High sensitive reduced graphene oxide-based room temperature ionic liquid electrochemical gas sensor with carbon-gold nanocomposites amplification ⁎
Hao Wana,b, , Ying Gana, Jiadi Suna, Tao Lianga, Shuqi Zhoua, Ping Wanga,b,
T
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a
Key Laboratory for Biomedical Engineering of Education Ministry, Biosensor National Special Laboratory, Department of Biomedical Engineering, Zhejiang University, Hangzhou, 310027, China b State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Shanghai, 200050, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon-gold nanocomposite Reduced graphene oxide Room temperature ionic liquid Electrochemical gas sensor
Gas sensors have received extensive attractions due to their critical roles in environmental monitoring, industry manufacture and human safety. This paper for the first time introduces carbon-gold nanocomposites on a reduced graphene oxide based electrochemical gas sensor for high sensitive gas detection. Carbon-gold nanocomposites (CGNs) were synthesized by glucose carbonization and gold nanoparticles deposition using the hydrothermal method. Reduced graphene oxide (RGO) was electrochemically deposited on a screen-printed gold electrode with subsequent modification of CGNs. To achieve long lifetime and good stability, thin-film room temperature ionic liquid (RTIL) was utilized as the electrolyte featuring negligible evaporation and large potential window, thus implementing the high sensitive RTIL-based electrochemical gas sensor. The amplification effect of RGO and CGNs modification was investigated using cyclic voltammetry, chronoamperometry and transient double potential amperometry (DPA), which reveals the significant enhancement of sensor performance by synergic application of RGO and CGNs. The sensor was calibrated for oxygen detection from 0.42% to 21% with good sensitivity and linearity. The reproducibility of the sensor using chronoamperometry and transient DPA was also studied with excellent reproducibility. The study paves a new way to implementing high sensitive electrochemical gas sensors for rapid monitoring of gas exposure.
1. Introduction Gas detection has received extensive attractions due their critical roles in the fields of environmental monitoring, industry manufacture and human safety [1]. A variety of gas sensing approaches has been developed to implement gas detection with good performance including gas chromatography, optical, metal oxide semiconductor and electrochemical gas sensing, etc. Among these approaches, electrochemical gas sensor exhibits outstanding superiority in high sensitivity, good selectivity, low power consumption and easy miniaturization [2,3], which reveals great potential for practical gas detection. Despite the aforementioned virtues, electrochemical gas sensors suffer from intrinsic drawbacks that supporting electrolytes require frequent maintenance due to the evaporation of the electrolyte [4,5]. In consequence, short sensor lifetime or continual human intervention impairs potential applications of electrochemical sensors. By utilizing conventional liquid supporting electrolytes such as sulfuric acid, commercial electrochemical gas sensors, that have been widely available and applied in
industry, also have the limitations in electrolyte lifetime, sensor packaging and miniaturization. In order to resolve this issue, various electrolytes such as solid electrolytes and ionic liquids have been explored in electrochemical gas sensors [6,7]. Room temperature ionic liquids (RTILs) have attracted much interest considering their remarkable benefits in negligible evaporation, high thermal stability and large working potential window [8,9], which are considered as an ideal electrolyte for electrochemical gas sensing. Nevertheless, the high viscosity of RTILs leads to increased response time and long recovery time due to the low diffusion rate of gases in RTILs [2]. Two strategies are generally adopted to address the issues, decreasing the thickness of RTIL film and avoiding the direct diffusion of gases through RTIL, and many related studies have been reported in the design of electrochemical gas sensors [10–12]. Diverse fabrication approaches have been developed for electrochemical gas sensor implementation. For instance, microfabrication is widely applied in micro/nano electrode fabrication with good reproducibility and high resolution [13]. However, microfabrication
⁎ Corresponding authors at: Key Laboratory for Biomedical Engineering of Education Ministry, Biosensor National Special Laboratory, Department of Biomedical Engineering, Zhejiang University, Hangzhou, 310027, China. E-mail addresses: [email protected] (H. Wan), [email protected] (P. Wang).
https://doi.org/10.1016/j.snb.2019.126952 Received 19 April 2019; Received in revised form 11 July 2019; Accepted 5 August 2019 Available online 06 August 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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tests. Screen printed gold electrodes (C223BT, DropSens) were used for surface modification and gas sensing. By applying a certain potential on the working electrode, oxygen absorbed on the electrode surface would be reduced, thus generating the current that can be recorded by the potentiostat and used for quantitative gas analysis. A gas chamber was designed and fabricated using 3D printing, and the SPGEs were mounted in the gas chamber for convenient gas tests. In our previous study, we presented a new method named transient double potential amperometry (DPA) for reversible gas analysis that can reduce by-products and shorten the measurement time [31,32]. Herein, this method was utilized to validate the sensor performance applying with both reduction potential at −1.2 V and oxidation potential at 0.2 V, and the principle was briefly introduced in Fig. S1. Cyclic voltammetry (CV) and chronoamperometry were also used to characterize the sensor for oxygen sensing.
process requires precise yet complicated fabrication procedures, expensive instruments and well-trained personnel. Screen printed electrodes (SPEs) have drawn much attention as an alternative for electrode fabrication aiming at various applications [14–16]. Compared to the microfabrication, SPEs can be conveniently fabricated with easily accessible masks and inks, which is beneficial for fast and massive production [17]. Also, the SPEs feature high flexibility, easy operation and low cost [18,19] that are widely used in disposable applications. It is worth stressing that SPEs generally integrate working electrode (WE), reference electrode (RE) and counter electrode (CE) with high miniaturization that are extremely suitable for practical applications. Due to the rapid development of nanotechnology, nanomaterials have been integrated into the gas sensor design to further enhance the performance of the sensors considering their merits in fast electron mobility, high catalysis and large active surface area [20–22]. Graphene and its derivatives draw special attentions with large two-dimensional active surface area and wide electrochemical potential window [23,24]. Compared to graphene that is chemically inert, reduced graphene oxide (RGO) can be electrochemically deposited on electrode surface with high uniformity and abundant active sites [25], which exhibits the great potential in electrochemical sensing. Moreover, nanocomposites can further enhance the performance of sensors by exhibiting the synergetic effects with the integration of multiple nanomaterials [26]. Carbon nanospheres (CNs) feature large surface area, excellent electrocatalytic activity and abundant reactive functional groups that can be used as efficient carriers to form nanocomposites [27]. By immobilizing gold nanoparticles (AuNPs), carbon gold nanocomposite (CGN) has been synthesized on carbon nanospheres and utilized for biomarkers assay [28,29]. Therefore, by utilizing CGNs in electrochemical gas sensor, it is also expected to achieve high sensitive electrochemical gas sensing with the combination of merits in both CNs and AuNPs, which further implements high sensitivity and low detection limit. As far as we know, no study has been reported using CGN in electrochemical gas sensing. In this work, CGNs were, for the first time, introduced on an RGO based electrochemical gas sensor for high sensitive gas detection. CGNs were synthesized by glucose carbonization and gold nanoparticles deposition using the hydrothermal method. RGO was electrochemically deposited on a screen-printed gold electrode (SPGE), and CGNs were drop-casted on the WE surface. RTIL 1butyl-3-methylimidazolium hexafluorophosphate was used as the electrolyte, forming a thin film on the surface of the SPGE. The performance of the RGO-CGN modified sensor was systematically explored for oxygen detection to verify the superiority of the sensor.
2.2. CGN synthesis CGN synthesis is comprised of two steps, CN synthesis and AuNPs deposition. Briefly, 40 mL 0.5 M glucose solution was added into 50 mL Teflon-sealed autoclave and maintained at 180 °C for 6 h in oven. After the heating, the Teflon-sealed autoclave was naturally cooled to room temperature. The synthesized CNs solution was centrifuged at 8000 rpm for 10 min, and the supernatant was removed. Ultrapure water and alcohol were sequentially used to wash the precipitate for three times. Finally, the volume of the CNs solution was diluted to 10 mL. 1 mL freshly prepared CNs solution was mixed with 5 mL 1% trisodium citrate in a flask. The mixture was heated to boiling and remained under reflux for 15 min. Then 50 μL 1% HAuCl4 was added to the flask under vigorous stirring, and the solution was refluxed for 20 min. In order to remove the free AuNPs, the solution was centrifuged at 6000 rpm for 5 min for three times. After removing the supernatant, the CGNs was diluted to 2 mL. 2.3. The RGO-CGN modified sensor preparation As aforementioned, two strategies were mainly adopted in RTIL electrochemical gas sensor design. In our previous study, a porous PTFE substrate was used for sensor fabrication, and gases can directly diffuse through the porous substrate, thus implementing the rapid gas sensing [31]. In this study, we explored the other strategy by depositing thin RTIL film on SPGEs. The preparation of the RGO-CGN modified sensor is shown in Fig. 1. CGNs were synthesized at first as described in Section 2.2. Graphene oxide was electrochemically reduced on the SPGE by applying a potential of -0.95 V, thereby forming RGO on the WE surface. 5 μL synthesized CGN solution was drop-casted on the WE and dried at room temperature. The process was repeated for three times to ensure abundant deposition of CGNs on the WE surface. Subsequently, RTIL was also drop-casted on the sensor surface to implement thin electrolyte film, and the gas sensor was accomplished for further gas sensing. It is worth stressing that water should be eliminated during the sensor preparation because water can affect the reversible reaction of oxygen by reacting with the superoxide radical and the physical and chemical properties of RTILs such as viscosity, electrical conductivity as well as solvation and solubility that can further influence the sensor performance [33].
2. Experimental 2.1. Methods and apparatus High-purity nitrogen and compressed air (21% O2) were used as the balance gas and the targeted gas, respectively. RTIL 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) was purchased from Acros Organics and used as the electrolyte for oxygen detection considering its wide application in gas sensors [30]. All chemicals were used as received without further treatment. 0.5 mg/mL graphene oxide (XFNANO, China) in 50 mM NaCl was used for electrochemical deposition of RGO. Glucose (MACKLIN, China) and chloroauric acid (Sinopharm Chemical Reagent, China) were used for CN and CGN synthesis, respectively. A gas blender (MCQ 6000 s, Italy) was used for flow control and automatic gas mixing between the balance gas and the targeted gas, which implements different concentrations of oxygen. The overall flow rate of gases was set constant at 200 sccm. In electrochemical tests, the mixed gas 0.021% O2 in N2, rather than 100% N2 was used as the background gas to ensure the blender in the same running status. A multichannel potentiostat CHI1030-c (CH Instrument, China) was used for recording redox currents during electrochemical
3. Results and discussion 3.1. Nanomaterials characterization Since CGNs play the vital role in enhancing the performance of the gas sensor, the morphology of CNs and CGNs was characterized using transmission electron microscope (TEM). As shown in Fig. 2(a), the synthesized CNs present typical spheroidal structure with uniform size of ˜180 nm in diameter. CNs dispersed well in solutions with some CNs 2
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Fig. 1. The preparation procedures of the RGO-CGN modified electrochemical gas sensor.
to the formation of RGO. Raman spectroscopy was performed to validate the formation of RGO. In Fig. 3(c), the intensity ratio of D band to G band is 0.88 in GO, and the intensity ratio is 1 after the electrochemical reduction. The results indicate that GO was successfully reduced and their conjugated structures were partly restored, which is in good accordance with the literature [35].
connected with each other. After AuNPs deposition, particles can be apparently observed on the surface of CNs in Fig. 2(b) that are evenly distributed on the CNs. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis were also conducted to further validate the synthesis of CNs and CGNs. The XRD pattern in Fig. 2(c) shows no diffraction peak in CNs, which indicates no crystal structure exists. As a comparison, four sharp diffraction peaks are observed in CGNs at 2θ = 38.18°, 44.39°, 64.57°, 77.54°, and well match with the standard card of Au (PDF #04-0784). The XPS results of CNs and CGNs are shown in Fig. 2(d) and (e), respectively. The peaks of C1s and O1s can be observed in both CNs and CGNs, while Au4f can be only detected in CGNs. Fig. 2(f) shows two characteristic peaks of Au4f at 83.6 eV and 87.6 eV, which match with the Au4f7/2 and Au4f5/2, respectively [34]. Moreover, the crystalline structure of these particles validates the expected deposition of AuNPs on CNs shown in Fig. S2, and these results fully validate the successful synthesis of CGNs. The electrochemical reduction of GO on the WE was also observed using optical microscope since the color change can be easily seen turning from golden to black. Fig. 3(a) presents the surface morphology of the WE on the SPGE. Golden and rough surface on the WE can be seen that greatly facilities the modification of electrode surface. The surface morphology of the WE with different reduction time of RGO is shown in Fig. S3. With the deposition of RGO at −0.95 V for 200 s, additional porous structure can be observed that entirely covers the WE surface shown in Fig. 3(b). Meanwhile, the color of WE turns dark due
3.2. The effect of RTIL thickness The thickness of RTIL can significantly affect the response of sensors to targeted gases since the gases must diffuse through the RTIL and reach the electrode surface for subsequent electrochemical reactions. Due to the high viscosity of RTIL, the thickness of RTIL should be reduced to achieve fast sensor response and recovery. To validate the influence, 2 μL (thin film) and 8 μL (thick film) [BMIM][PF6] were dropcasted on SPGEs to form electrolyte film with different thickness. To ensure the uniform distribution of the ionic liquid membrane, a pipette was used to softly spread the RTIL to cover the full circular working area of the SPE. Subsequently, the electrode covered with the ionic liquid was mounted on the gas chamber and rested for 30 min. under N2 atmosphere. Due to the mobility of ionic liquids, the ionic liquid can distribute on the electrode surface in a relatively uniform manner. The sensor performance with different RTIL thickness was characterized and compared using CV, chronoamperometry and transient DPA. The CV was scanned from −1.1 V to −0.3 V based on the reduction peak
Fig. 2. The characterization of the synthesized CNs and CGNs: the TEM image of CNs (a) and CGNs (b); (c) the XRD results of CNs and CGNs; the XPS results of CNs (d) and (e) CGNs. (f) shows the two XPS characteristic peaks of Au4f in CGNs. 3
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Fig. 3. The optical image of the WE before (a) and after (b) the RGO deposition. (c) shows the Raman spectroscopy of GO and RGO.
Fig. 4. The sensor response with thick (green) and thin (blue) RTIL film using cyclic voltammetry, chronoamperometry and transient DPA for oxygen measurements.
result can be conceivably ascribed to two reasons that nanoscaled CGN modification enlarges the sensor surface area and enhances the electrocatalytic ability due to AuNPs and CNs with abundant reactive functional groups [28]. The current response of the sensors using chronoamperometry is shown in Fig. 5(b). With RGO deposition and CGN modification, the sensor exhibits the highest current response in each gas sample from 4.2% O2 to 21% O2. It is also observed that the RGO-CGN modified sensor presents the highest response in the background gas. This result may be caused by the largest surface area of the sensor, leading to the improved non-Faradaic current generated by the charging/discharging of the electrical double layer, adsorption and desorption [38]. The currents of the RGO-CGN modified sensor decline apparently in high concentration range from 12.6% to 21%. This result is conceivably caused by the accumulation of reaction products, which hinders the diffusion of oxygen entering the sensor surface. The issue can be effectively resolved by using transient DPA that can reverse the reaction products and decrease the measurement time. In transient DPA shown in Fig. 5(c), the RGO-CGN modified sensor demonstrates the highest reduction current of oxygen, and the results are in accordance with that in chronoamperometry. Therefore, the RGO deposition and CGN modification are validated to remarkably amplify the sensor performance.
potential of oxygen and the oxidation peak potential of superoxide radical. As shown in Fig. 4(a), both voltammograms exhibit well-defined reduction peak and oxidation peak, validating the reversible reaction of oxygen in RTIL [36]. It can be apparently observed that the sensor with thin film presents much higher redox current response owing to more efficient electrolysis of oxygen in thin RTIL [37]. Chronoamperometry was conducted at −1.2 V with alternate pumping of the background gas (0.021% O2 in N2) and targeted gas samples from 4.2% O2 to 21% O2. The reduction current of oxygen is significantly enhanced with thin RTIL film in each concentration shown in Fig. 4(b). In addition, due to the thick electrolyte film, the diffusion process of oxygen from ambient air to the sensor surface takes long time, thus leading to the long response time of the sensor. As shown in Fig. S4, by defining t90 as the time cost from the response initiating to reaching 90% of the highest current response in 21% O2, the t90 of the sensor with thin film is ˜48 s. In contrast, the current response of the sensor with thick film remains increasing during the gas measurement, indicating the sensor cannot reach a steady state in 5 min. Moreover, the two sensors were also studied using transient DPA, and the current responses in 21% O2 are shown in Fig. 4(c). It is also expected that the reduction current of oxygen in thin film is much larger than that in thick film. Therefore, thin electrolyte film can evidently enhance the current response of the gas sensor, especially using RTILs that feature high viscosity and low gas diffusion rate. It is worth noting that thinner film may be beneficial for further improving the sensor response. However, considering the uneven surface of the working area on SPGEs, extremely thin RTIL film cannot be achieved due to the fluidity of RTIL, and 2 μL RTIL can ensure the entire coverage of all electrodes by the RTIL.
3.4. Sensor calibration with chronoamperometry and transient DPA The RGO-CGN modified gas sensor was calibrated in gas samples with different oxygen concentration from 0.42% O2 to 21% O2 using chronoamperometry at −1.2 V. The background gas and mixed gases with different oxygen concentration were alternately pumped into the gas chamber for 5 min. As shown in Fig. 6(a), the sensor exhibits apparently increased current response to gas samples with higher oxygen concentration. The highest current of the sensor in each oxygen sample was extracted for the sensor calibration, and the plot of current versus oxygen concentration is shown in Fig. 6(b). Apparently, the sensor presents higher sensitivity in low concentration range compared to that in high concentration range. To accurately calibration the sensor, the calibration was implemented individually in low concentration range from 0.42% O2 to 2.1% O2 and high concentration range from 4.2% O2 to 21% O2. In the low concentration range, the sensor presents a sensitivity of 0.289 μA/%O2 with a linearity of 0.9599. In the high
3.3. The characterization of the sensor with different surface modification Since the proposed gas sensor was implemented with RGO deposition and CGNs modification, the sensor performance should be evaluated to validate the signal enhancement by the two successive surface modification processes. Similarly, the sensors with different surface modification (bare, RGO, RGO-CGN) were characterized using CV, chronoamperometry and transient DPA. In Fig. 5(a), the sensor with RGO deposition presents enhanced reduction and oxidation current compared to the bare sensor. CGN modification further amplifies the redox current with well-defined oxidation peak and reduction peak. The 4
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Fig. 5. The sensor response with different surface modification (bare electrode, RGO modified electrode and RGO-CGN modified electrode) using (a) cyclic voltammetry, (b) chronoamperometry and (c) transient DPA for oxygen measurements.
concentration range, the sensitivity of the sensor is 0.168 μA/%O2 with a linearity of 0.9687. The sensor was also calibrated using transient DPA from the background gas to 21% O2 with an interval of 4.2% O2. The plot of current versus time in different oxygen concentration is shown in Fig. 7(a), in which the non-Faradaic current is dominant in the current response. In the reduction phase from 2 s to 4 s, the sensor also presents higher current response in gases with higher oxygen concentration shown in the inset of Fig. 7(a). The reduction current at 4 s with different concentration in five repetitive tests was extracted for sensor calibration. By using transient DPA, the sensor presents a sensitivity of 0.2062 μA/ %O2 with a linearity of 0.9963 shown in Fig. 7(b). The largest deviation is 0.0938 μA in 21% O2 during the five repetitive tests, indicating a largest variation of 0.455% O2 in the measurement.
chronoamperometry, which is in accordance with our previous results that transient DPA can effectively the reproducibility of gas sensors by reversing by-products [31].
4. Conclusion This paper presents a novel reduced graphene oxide based electrochemical gas sensor for high sensitive gas detection by integrating carbon-gold nanocomposites for the first time. CGNs were successfully synthesized and synergetically used with RGO to improve the sensor performance. The effect of RTIL thickness to the sensor response was studied, validating that thin RTIL film facilitates rapid and enhanced sensor response. The sensor performance with different surface modification (bare, RGO, RGO-CGN) were compared using different methods, which reveals the significant enhancement of the sensor response by combing RGO and CGNs. The sensor was calibrated in different oxygen concentration with chronoamperometry and transient DPA and presented good sensitivity, linearity and reproducibility. In comparison, transient DPA also exhibits the superiority in short measurement time and better reproducibility by effectively reversing byproducts with two opposite potentials. The study paves a new way to implementing high sensitive electrochemical gas sensors for rapid monitoring of gaseous hazards.
3.5. Reproducibility of the RGO-CGN modified gas sensor Reproducibility is one of the key parameters to characterize the sensor performance. In this study, the reproducibility of the sensor was evaluated using both chronoamperometry and transient DPA. The sensor was tested with alternating pumping of the background gas and 10.5% O2 for seven cycles, and each cycle lasted for 10 min. The current response is shown in Fig. 8(a), which reveals quite reproducible response in seven cycles. Similarly, transient DPA was implemented in the last 4 s before the end of each cycles, and the transient current response is shown in Fig. 8(b). The peak current of each cycle in chronoamperometry and the reduction current at 4 s in transient DPA were extracted to quantitatively evaluate the reproducibility of the sensor shown in Fig. 8(c). In chronoamperometry, the sensor presents a standard deviation of 0.0998 μA in seven cycles with a relative standard deviation (RSD) of 5.58%. In transient DPA, the standard deviation is 0.208 μA with a RSD of 3.20%. The results using the two different methods validate the good reproducibility of the sensor. Moreover, the RSD using transient DPA is smaller than that using
Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (2019FZJD005, 2018QNA5018, 2018FZA5018), National Basic Research Program (2015CB352101) and National Natural Science Foundation of China (31627801, 31661143030).
Fig. 6. (a) The current response of the RGO-CGN modified gas sensor for oxygen measurement from 0.42% O2 to 21% O2 and (b) the calibration curves in low (red) and high (green) concentration range. 5
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Fig. 7. (a) The current response using transient DPA with oxygen concentration from 0.021% O2 to 21% O2 and (b) the calibration curve. The inset shows the zoomed curves in the reduction phase.
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Fig. 8. The reproducibility test of the RGO-CGN modified electrode using (a) chronoamperometry and (b) transient DPA for oxygen measurements. (c) presents the extracted currents in seven gas cycles using the two methods.
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126952.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
High sensitive reduced graphene oxide-based room temperature ionic liquid electrochemical gas sensor with carbon-gold nanocomposites amplification ⁎
Hao Wana,b, , Ying Gana, Jiadi Suna, Tao Lianga, Shuqi Zhoua, Ping Wanga,b,
T
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a
Key Laboratory for Biomedical Engineering of Education Ministry, Biosensor National Special Laboratory, Department of Biomedical Engineering, Zhejiang University, Hangzhou, 310027, China b State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Shanghai, 200050, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon-gold nanocomposite Reduced graphene oxide Room temperature ionic liquid Electrochemical gas sensor
Gas sensors have received extensive attractions due to their critical roles in environmental monitoring, industry manufacture and human safety. This paper for the first time introduces carbon-gold nanocomposites on a reduced graphene oxide based electrochemical gas sensor for high sensitive gas detection. Carbon-gold nanocomposites (CGNs) were synthesized by glucose carbonization and gold nanoparticles deposition using the hydrothermal method. Reduced graphene oxide (RGO) was electrochemically deposited on a screen-printed gold electrode with subsequent modification of CGNs. To achieve long lifetime and good stability, thin-film room temperature ionic liquid (RTIL) was utilized as the electrolyte featuring negligible evaporation and large potential window, thus implementing the high sensitive RTIL-based electrochemical gas sensor. The amplification effect of RGO and CGNs modification was investigated using cyclic voltammetry, chronoamperometry and transient double potential amperometry (DPA), which reveals the significant enhancement of sensor performance by synergic application of RGO and CGNs. The sensor was calibrated for oxygen detection from 0.42% to 21% with good sensitivity and linearity. The reproducibility of the sensor using chronoamperometry and transient DPA was also studied with excellent reproducibility. The study paves a new way to implementing high sensitive electrochemical gas sensors for rapid monitoring of gas exposure.
1. Introduction Gas detection has received extensive attractions due their critical roles in the fields of environmental monitoring, industry manufacture and human safety [1]. A variety of gas sensing approaches has been developed to implement gas detection with good performance including gas chromatography, optical, metal oxide semiconductor and electrochemical gas sensing, etc. Among these approaches, electrochemical gas sensor exhibits outstanding superiority in high sensitivity, good selectivity, low power consumption and easy miniaturization [2,3], which reveals great potential for practical gas detection. Despite the aforementioned virtues, electrochemical gas sensors suffer from intrinsic drawbacks that supporting electrolytes require frequent maintenance due to the evaporation of the electrolyte [4,5]. In consequence, short sensor lifetime or continual human intervention impairs potential applications of electrochemical sensors. By utilizing conventional liquid supporting electrolytes such as sulfuric acid, commercial electrochemical gas sensors, that have been widely available and applied in
industry, also have the limitations in electrolyte lifetime, sensor packaging and miniaturization. In order to resolve this issue, various electrolytes such as solid electrolytes and ionic liquids have been explored in electrochemical gas sensors [6,7]. Room temperature ionic liquids (RTILs) have attracted much interest considering their remarkable benefits in negligible evaporation, high thermal stability and large working potential window [8,9], which are considered as an ideal electrolyte for electrochemical gas sensing. Nevertheless, the high viscosity of RTILs leads to increased response time and long recovery time due to the low diffusion rate of gases in RTILs [2]. Two strategies are generally adopted to address the issues, decreasing the thickness of RTIL film and avoiding the direct diffusion of gases through RTIL, and many related studies have been reported in the design of electrochemical gas sensors [10–12]. Diverse fabrication approaches have been developed for electrochemical gas sensor implementation. For instance, microfabrication is widely applied in micro/nano electrode fabrication with good reproducibility and high resolution [13]. However, microfabrication
⁎ Corresponding authors at: Key Laboratory for Biomedical Engineering of Education Ministry, Biosensor National Special Laboratory, Department of Biomedical Engineering, Zhejiang University, Hangzhou, 310027, China. E-mail addresses: [email protected] (H. Wan), [email protected] (P. Wang).
https://doi.org/10.1016/j.snb.2019.126952 Received 19 April 2019; Received in revised form 11 July 2019; Accepted 5 August 2019 Available online 06 August 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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tests. Screen printed gold electrodes (C223BT, DropSens) were used for surface modification and gas sensing. By applying a certain potential on the working electrode, oxygen absorbed on the electrode surface would be reduced, thus generating the current that can be recorded by the potentiostat and used for quantitative gas analysis. A gas chamber was designed and fabricated using 3D printing, and the SPGEs were mounted in the gas chamber for convenient gas tests. In our previous study, we presented a new method named transient double potential amperometry (DPA) for reversible gas analysis that can reduce by-products and shorten the measurement time [31,32]. Herein, this method was utilized to validate the sensor performance applying with both reduction potential at −1.2 V and oxidation potential at 0.2 V, and the principle was briefly introduced in Fig. S1. Cyclic voltammetry (CV) and chronoamperometry were also used to characterize the sensor for oxygen sensing.
process requires precise yet complicated fabrication procedures, expensive instruments and well-trained personnel. Screen printed electrodes (SPEs) have drawn much attention as an alternative for electrode fabrication aiming at various applications [14–16]. Compared to the microfabrication, SPEs can be conveniently fabricated with easily accessible masks and inks, which is beneficial for fast and massive production [17]. Also, the SPEs feature high flexibility, easy operation and low cost [18,19] that are widely used in disposable applications. It is worth stressing that SPEs generally integrate working electrode (WE), reference electrode (RE) and counter electrode (CE) with high miniaturization that are extremely suitable for practical applications. Due to the rapid development of nanotechnology, nanomaterials have been integrated into the gas sensor design to further enhance the performance of the sensors considering their merits in fast electron mobility, high catalysis and large active surface area [20–22]. Graphene and its derivatives draw special attentions with large two-dimensional active surface area and wide electrochemical potential window [23,24]. Compared to graphene that is chemically inert, reduced graphene oxide (RGO) can be electrochemically deposited on electrode surface with high uniformity and abundant active sites [25], which exhibits the great potential in electrochemical sensing. Moreover, nanocomposites can further enhance the performance of sensors by exhibiting the synergetic effects with the integration of multiple nanomaterials [26]. Carbon nanospheres (CNs) feature large surface area, excellent electrocatalytic activity and abundant reactive functional groups that can be used as efficient carriers to form nanocomposites [27]. By immobilizing gold nanoparticles (AuNPs), carbon gold nanocomposite (CGN) has been synthesized on carbon nanospheres and utilized for biomarkers assay [28,29]. Therefore, by utilizing CGNs in electrochemical gas sensor, it is also expected to achieve high sensitive electrochemical gas sensing with the combination of merits in both CNs and AuNPs, which further implements high sensitivity and low detection limit. As far as we know, no study has been reported using CGN in electrochemical gas sensing. In this work, CGNs were, for the first time, introduced on an RGO based electrochemical gas sensor for high sensitive gas detection. CGNs were synthesized by glucose carbonization and gold nanoparticles deposition using the hydrothermal method. RGO was electrochemically deposited on a screen-printed gold electrode (SPGE), and CGNs were drop-casted on the WE surface. RTIL 1butyl-3-methylimidazolium hexafluorophosphate was used as the electrolyte, forming a thin film on the surface of the SPGE. The performance of the RGO-CGN modified sensor was systematically explored for oxygen detection to verify the superiority of the sensor.
2.2. CGN synthesis CGN synthesis is comprised of two steps, CN synthesis and AuNPs deposition. Briefly, 40 mL 0.5 M glucose solution was added into 50 mL Teflon-sealed autoclave and maintained at 180 °C for 6 h in oven. After the heating, the Teflon-sealed autoclave was naturally cooled to room temperature. The synthesized CNs solution was centrifuged at 8000 rpm for 10 min, and the supernatant was removed. Ultrapure water and alcohol were sequentially used to wash the precipitate for three times. Finally, the volume of the CNs solution was diluted to 10 mL. 1 mL freshly prepared CNs solution was mixed with 5 mL 1% trisodium citrate in a flask. The mixture was heated to boiling and remained under reflux for 15 min. Then 50 μL 1% HAuCl4 was added to the flask under vigorous stirring, and the solution was refluxed for 20 min. In order to remove the free AuNPs, the solution was centrifuged at 6000 rpm for 5 min for three times. After removing the supernatant, the CGNs was diluted to 2 mL. 2.3. The RGO-CGN modified sensor preparation As aforementioned, two strategies were mainly adopted in RTIL electrochemical gas sensor design. In our previous study, a porous PTFE substrate was used for sensor fabrication, and gases can directly diffuse through the porous substrate, thus implementing the rapid gas sensing [31]. In this study, we explored the other strategy by depositing thin RTIL film on SPGEs. The preparation of the RGO-CGN modified sensor is shown in Fig. 1. CGNs were synthesized at first as described in Section 2.2. Graphene oxide was electrochemically reduced on the SPGE by applying a potential of -0.95 V, thereby forming RGO on the WE surface. 5 μL synthesized CGN solution was drop-casted on the WE and dried at room temperature. The process was repeated for three times to ensure abundant deposition of CGNs on the WE surface. Subsequently, RTIL was also drop-casted on the sensor surface to implement thin electrolyte film, and the gas sensor was accomplished for further gas sensing. It is worth stressing that water should be eliminated during the sensor preparation because water can affect the reversible reaction of oxygen by reacting with the superoxide radical and the physical and chemical properties of RTILs such as viscosity, electrical conductivity as well as solvation and solubility that can further influence the sensor performance [33].
2. Experimental 2.1. Methods and apparatus High-purity nitrogen and compressed air (21% O2) were used as the balance gas and the targeted gas, respectively. RTIL 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) was purchased from Acros Organics and used as the electrolyte for oxygen detection considering its wide application in gas sensors [30]. All chemicals were used as received without further treatment. 0.5 mg/mL graphene oxide (XFNANO, China) in 50 mM NaCl was used for electrochemical deposition of RGO. Glucose (MACKLIN, China) and chloroauric acid (Sinopharm Chemical Reagent, China) were used for CN and CGN synthesis, respectively. A gas blender (MCQ 6000 s, Italy) was used for flow control and automatic gas mixing between the balance gas and the targeted gas, which implements different concentrations of oxygen. The overall flow rate of gases was set constant at 200 sccm. In electrochemical tests, the mixed gas 0.021% O2 in N2, rather than 100% N2 was used as the background gas to ensure the blender in the same running status. A multichannel potentiostat CHI1030-c (CH Instrument, China) was used for recording redox currents during electrochemical
3. Results and discussion 3.1. Nanomaterials characterization Since CGNs play the vital role in enhancing the performance of the gas sensor, the morphology of CNs and CGNs was characterized using transmission electron microscope (TEM). As shown in Fig. 2(a), the synthesized CNs present typical spheroidal structure with uniform size of ˜180 nm in diameter. CNs dispersed well in solutions with some CNs 2
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Fig. 1. The preparation procedures of the RGO-CGN modified electrochemical gas sensor.
to the formation of RGO. Raman spectroscopy was performed to validate the formation of RGO. In Fig. 3(c), the intensity ratio of D band to G band is 0.88 in GO, and the intensity ratio is 1 after the electrochemical reduction. The results indicate that GO was successfully reduced and their conjugated structures were partly restored, which is in good accordance with the literature [35].
connected with each other. After AuNPs deposition, particles can be apparently observed on the surface of CNs in Fig. 2(b) that are evenly distributed on the CNs. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis were also conducted to further validate the synthesis of CNs and CGNs. The XRD pattern in Fig. 2(c) shows no diffraction peak in CNs, which indicates no crystal structure exists. As a comparison, four sharp diffraction peaks are observed in CGNs at 2θ = 38.18°, 44.39°, 64.57°, 77.54°, and well match with the standard card of Au (PDF #04-0784). The XPS results of CNs and CGNs are shown in Fig. 2(d) and (e), respectively. The peaks of C1s and O1s can be observed in both CNs and CGNs, while Au4f can be only detected in CGNs. Fig. 2(f) shows two characteristic peaks of Au4f at 83.6 eV and 87.6 eV, which match with the Au4f7/2 and Au4f5/2, respectively [34]. Moreover, the crystalline structure of these particles validates the expected deposition of AuNPs on CNs shown in Fig. S2, and these results fully validate the successful synthesis of CGNs. The electrochemical reduction of GO on the WE was also observed using optical microscope since the color change can be easily seen turning from golden to black. Fig. 3(a) presents the surface morphology of the WE on the SPGE. Golden and rough surface on the WE can be seen that greatly facilities the modification of electrode surface. The surface morphology of the WE with different reduction time of RGO is shown in Fig. S3. With the deposition of RGO at −0.95 V for 200 s, additional porous structure can be observed that entirely covers the WE surface shown in Fig. 3(b). Meanwhile, the color of WE turns dark due
3.2. The effect of RTIL thickness The thickness of RTIL can significantly affect the response of sensors to targeted gases since the gases must diffuse through the RTIL and reach the electrode surface for subsequent electrochemical reactions. Due to the high viscosity of RTIL, the thickness of RTIL should be reduced to achieve fast sensor response and recovery. To validate the influence, 2 μL (thin film) and 8 μL (thick film) [BMIM][PF6] were dropcasted on SPGEs to form electrolyte film with different thickness. To ensure the uniform distribution of the ionic liquid membrane, a pipette was used to softly spread the RTIL to cover the full circular working area of the SPE. Subsequently, the electrode covered with the ionic liquid was mounted on the gas chamber and rested for 30 min. under N2 atmosphere. Due to the mobility of ionic liquids, the ionic liquid can distribute on the electrode surface in a relatively uniform manner. The sensor performance with different RTIL thickness was characterized and compared using CV, chronoamperometry and transient DPA. The CV was scanned from −1.1 V to −0.3 V based on the reduction peak
Fig. 2. The characterization of the synthesized CNs and CGNs: the TEM image of CNs (a) and CGNs (b); (c) the XRD results of CNs and CGNs; the XPS results of CNs (d) and (e) CGNs. (f) shows the two XPS characteristic peaks of Au4f in CGNs. 3
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Fig. 3. The optical image of the WE before (a) and after (b) the RGO deposition. (c) shows the Raman spectroscopy of GO and RGO.
Fig. 4. The sensor response with thick (green) and thin (blue) RTIL film using cyclic voltammetry, chronoamperometry and transient DPA for oxygen measurements.
result can be conceivably ascribed to two reasons that nanoscaled CGN modification enlarges the sensor surface area and enhances the electrocatalytic ability due to AuNPs and CNs with abundant reactive functional groups [28]. The current response of the sensors using chronoamperometry is shown in Fig. 5(b). With RGO deposition and CGN modification, the sensor exhibits the highest current response in each gas sample from 4.2% O2 to 21% O2. It is also observed that the RGO-CGN modified sensor presents the highest response in the background gas. This result may be caused by the largest surface area of the sensor, leading to the improved non-Faradaic current generated by the charging/discharging of the electrical double layer, adsorption and desorption [38]. The currents of the RGO-CGN modified sensor decline apparently in high concentration range from 12.6% to 21%. This result is conceivably caused by the accumulation of reaction products, which hinders the diffusion of oxygen entering the sensor surface. The issue can be effectively resolved by using transient DPA that can reverse the reaction products and decrease the measurement time. In transient DPA shown in Fig. 5(c), the RGO-CGN modified sensor demonstrates the highest reduction current of oxygen, and the results are in accordance with that in chronoamperometry. Therefore, the RGO deposition and CGN modification are validated to remarkably amplify the sensor performance.
potential of oxygen and the oxidation peak potential of superoxide radical. As shown in Fig. 4(a), both voltammograms exhibit well-defined reduction peak and oxidation peak, validating the reversible reaction of oxygen in RTIL [36]. It can be apparently observed that the sensor with thin film presents much higher redox current response owing to more efficient electrolysis of oxygen in thin RTIL [37]. Chronoamperometry was conducted at −1.2 V with alternate pumping of the background gas (0.021% O2 in N2) and targeted gas samples from 4.2% O2 to 21% O2. The reduction current of oxygen is significantly enhanced with thin RTIL film in each concentration shown in Fig. 4(b). In addition, due to the thick electrolyte film, the diffusion process of oxygen from ambient air to the sensor surface takes long time, thus leading to the long response time of the sensor. As shown in Fig. S4, by defining t90 as the time cost from the response initiating to reaching 90% of the highest current response in 21% O2, the t90 of the sensor with thin film is ˜48 s. In contrast, the current response of the sensor with thick film remains increasing during the gas measurement, indicating the sensor cannot reach a steady state in 5 min. Moreover, the two sensors were also studied using transient DPA, and the current responses in 21% O2 are shown in Fig. 4(c). It is also expected that the reduction current of oxygen in thin film is much larger than that in thick film. Therefore, thin electrolyte film can evidently enhance the current response of the gas sensor, especially using RTILs that feature high viscosity and low gas diffusion rate. It is worth noting that thinner film may be beneficial for further improving the sensor response. However, considering the uneven surface of the working area on SPGEs, extremely thin RTIL film cannot be achieved due to the fluidity of RTIL, and 2 μL RTIL can ensure the entire coverage of all electrodes by the RTIL.
3.4. Sensor calibration with chronoamperometry and transient DPA The RGO-CGN modified gas sensor was calibrated in gas samples with different oxygen concentration from 0.42% O2 to 21% O2 using chronoamperometry at −1.2 V. The background gas and mixed gases with different oxygen concentration were alternately pumped into the gas chamber for 5 min. As shown in Fig. 6(a), the sensor exhibits apparently increased current response to gas samples with higher oxygen concentration. The highest current of the sensor in each oxygen sample was extracted for the sensor calibration, and the plot of current versus oxygen concentration is shown in Fig. 6(b). Apparently, the sensor presents higher sensitivity in low concentration range compared to that in high concentration range. To accurately calibration the sensor, the calibration was implemented individually in low concentration range from 0.42% O2 to 2.1% O2 and high concentration range from 4.2% O2 to 21% O2. In the low concentration range, the sensor presents a sensitivity of 0.289 μA/%O2 with a linearity of 0.9599. In the high
3.3. The characterization of the sensor with different surface modification Since the proposed gas sensor was implemented with RGO deposition and CGNs modification, the sensor performance should be evaluated to validate the signal enhancement by the two successive surface modification processes. Similarly, the sensors with different surface modification (bare, RGO, RGO-CGN) were characterized using CV, chronoamperometry and transient DPA. In Fig. 5(a), the sensor with RGO deposition presents enhanced reduction and oxidation current compared to the bare sensor. CGN modification further amplifies the redox current with well-defined oxidation peak and reduction peak. The 4
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Fig. 5. The sensor response with different surface modification (bare electrode, RGO modified electrode and RGO-CGN modified electrode) using (a) cyclic voltammetry, (b) chronoamperometry and (c) transient DPA for oxygen measurements.
concentration range, the sensitivity of the sensor is 0.168 μA/%O2 with a linearity of 0.9687. The sensor was also calibrated using transient DPA from the background gas to 21% O2 with an interval of 4.2% O2. The plot of current versus time in different oxygen concentration is shown in Fig. 7(a), in which the non-Faradaic current is dominant in the current response. In the reduction phase from 2 s to 4 s, the sensor also presents higher current response in gases with higher oxygen concentration shown in the inset of Fig. 7(a). The reduction current at 4 s with different concentration in five repetitive tests was extracted for sensor calibration. By using transient DPA, the sensor presents a sensitivity of 0.2062 μA/ %O2 with a linearity of 0.9963 shown in Fig. 7(b). The largest deviation is 0.0938 μA in 21% O2 during the five repetitive tests, indicating a largest variation of 0.455% O2 in the measurement.
chronoamperometry, which is in accordance with our previous results that transient DPA can effectively the reproducibility of gas sensors by reversing by-products [31].
4. Conclusion This paper presents a novel reduced graphene oxide based electrochemical gas sensor for high sensitive gas detection by integrating carbon-gold nanocomposites for the first time. CGNs were successfully synthesized and synergetically used with RGO to improve the sensor performance. The effect of RTIL thickness to the sensor response was studied, validating that thin RTIL film facilitates rapid and enhanced sensor response. The sensor performance with different surface modification (bare, RGO, RGO-CGN) were compared using different methods, which reveals the significant enhancement of the sensor response by combing RGO and CGNs. The sensor was calibrated in different oxygen concentration with chronoamperometry and transient DPA and presented good sensitivity, linearity and reproducibility. In comparison, transient DPA also exhibits the superiority in short measurement time and better reproducibility by effectively reversing byproducts with two opposite potentials. The study paves a new way to implementing high sensitive electrochemical gas sensors for rapid monitoring of gaseous hazards.
3.5. Reproducibility of the RGO-CGN modified gas sensor Reproducibility is one of the key parameters to characterize the sensor performance. In this study, the reproducibility of the sensor was evaluated using both chronoamperometry and transient DPA. The sensor was tested with alternating pumping of the background gas and 10.5% O2 for seven cycles, and each cycle lasted for 10 min. The current response is shown in Fig. 8(a), which reveals quite reproducible response in seven cycles. Similarly, transient DPA was implemented in the last 4 s before the end of each cycles, and the transient current response is shown in Fig. 8(b). The peak current of each cycle in chronoamperometry and the reduction current at 4 s in transient DPA were extracted to quantitatively evaluate the reproducibility of the sensor shown in Fig. 8(c). In chronoamperometry, the sensor presents a standard deviation of 0.0998 μA in seven cycles with a relative standard deviation (RSD) of 5.58%. In transient DPA, the standard deviation is 0.208 μA with a RSD of 3.20%. The results using the two different methods validate the good reproducibility of the sensor. Moreover, the RSD using transient DPA is smaller than that using
Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (2019FZJD005, 2018QNA5018, 2018FZA5018), National Basic Research Program (2015CB352101) and National Natural Science Foundation of China (31627801, 31661143030).
Fig. 6. (a) The current response of the RGO-CGN modified gas sensor for oxygen measurement from 0.42% O2 to 21% O2 and (b) the calibration curves in low (red) and high (green) concentration range. 5
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Fig. 7. (a) The current response using transient DPA with oxygen concentration from 0.021% O2 to 21% O2 and (b) the calibration curve. The inset shows the zoomed curves in the reduction phase.
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Fig. 8. The reproducibility test of the RGO-CGN modified electrode using (a) chronoamperometry and (b) transient DPA for oxygen measurements. (c) presents the extracted currents in seven gas cycles using the two methods.
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126952.
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