Biochemical sensor using ZnO thin film and polymer electrolyte as a gate dielectric
Dr. Rishi Ram Ghimire1, 2, 3*
(1. Nepal Police School Sanga, 2. Visiting Professor Kathmandu University 3. Visiting Professor Patan Multiple Campus, Tribhuvan University)
*This article is based on the introductory part of mini-research project report submitted by author to Nepal Academy of Science in year 2020.
1. Introduction and motivation
Nanostructure materials are promising candidates for electronics and opto-electronic applications. Nowadays, field effect devices based on oxide material are highly desired for sensors and detectors. These devices based on oxide material grown on silica and quartz substrate are cost-effective and favourable for grown at low temperature vacuum free environment1–6. This article gives the information how to fabricate thin film transistors (TFT) on a glass substrate at low temperature using oxide semiconductor as channel materials and electric double layer (EDL) as a gate dielectric. Thus, fabricated field effect device is used for biomolecule sensing application. The semiconducting oxide such as zinc oxide (ZnO), indium zinc oxide (IZO), indium gallium zinc oxide (IGZO), and tin oxide (SnO2) are used as channel materials for thin film transistor1,7–10.The oxide materials in amorphous or crystalline have moderately high value of carrier mobility in comparison to the amorphous silicon and organic semiconductors 11. Transistors with high performance index are required for the microelectronic circuits. Most of the conventional oxide gate dielectrics used in transistor have a low value of specific capacitance (nF) which is insufficient to modulate the ultra-high electronic charge density in oxide based materials 1,11–16. Conventional oxide like SiO2 and Al2O3 require high temperature and high vacuum to deposit it on the channel as a gate dielectric. Oxide semiconductors have high value of native volume charge density (≥1019/cm3) therefore large gate voltage is required to switch on this large charge carrier density while using the conventional oxide as a gate dielectric11. An alternative way is to use the EDL gate dielectric, which is formed at the semi-conductor dielectric interface that can be tuned the large charge density from the oxide channel. These electrolytes can be deposited at room temperature and no need vacuum. The dream of fabrication of field effect devices using electrolyte as gate dielectric is to change the practical knowledge into application domain i.e. using field effect device of nanostructure material for blood serum sensor.
Estimation of urea in blood serum is an important diagnosis for liver, kidney disease, including gastrointestinal bleeding, dehydration, shock and burn etc. In humans body, synthesis of urea is a major route to dispose waste nitrogen, which is generated by protein and amino acid metabolism in liver17–21. Urea is removed from the blood and excreted in the urine via the kidney. The normal range of urea in human blood is 2.6 to 6 mM 22. The Abnormal urea level indicates various metabolic diseases; therefore it is essential to check the urea level time to time and keep it under control is a primary issue for good health. The Literature shows the different types of sensors are used for estimation and determination of urea in blood. Most of urea biosensor based on catalytic conversion of urea to hydrogen bicarbonate and ammonium. The commonly used calorimetric and spectroscopic methods are laborious and not suitable for line monitoring system4–6,17–19,23–25. The limitation of these methods is overcome by using an electrochemical technique in which bio molecules convert a biological event to an electrical signal. This method has the advantages over other methods because of its robustness, easy miniaturization, excellent detection limit and small analyte volume. Human blood serum mainly contains protein and urea. The solubility of protein relatively lowers than the urea in alcohol this helps to separate the urea and protein from blood serum. This article gives an idea to fabricate two terminals and three terminals field effect sensing devices using ZnO film on glass substrate using chemical route. ZnO nanostructures have paid great attention because of their unique properties such as high catalytic efficiency, strong absorption ability, biocompatibility and high electron communication features. In field effect device, electric double layer (EDL) formed on semiconductor and gate electrolyte interface is used as a dielectric material that can induce high value of charge carriers on the surface of the channel. The value of dielectric constant of electrolyte is modified when it is exposed to the external agent like alcohol and urea etc. The change in dielectric value of polymer electrolyte helps to control the drain current in field effect device. This is the key mechanism for the sensing urea in blood serum. Though this work based on ZnO nanostuctured film, it equally validated to other oxide based nanostructure like SnO2, ZrO2 and TiO2.
2. Methodology
From the past decades, ZnO based biosensors have become the prevalent topic in thin film technology. Nanostructures ZnO are promising materials with a number of analytical detection methods such as in mass-based biosensors as well as electrochemical and optical methods. Among them, ZnO based electrochemical biosensor has attracted much attention in health care application because of its enhanced and fast response time.
Fig. 1 Components of electrochemical transducer
There are other parameters that signify the performance of the biosensors such as detection limit, sensitivity, selectivity, response time and recovery time. A novel idea is required to improve the sensing device that enhanced the performance parameters. In electrochemical method, biological events directly convert into electrical signal with higher stability. In this method, the stoichiometry of anlyte molecule does not change. Therefore, this method has advantages over other methods. Figure (1) illustrates the major parts of the electrochemical biosensor. Here, the analyte is the target biomolecules such as blood urea, glucose and protein. Second part is bioreceptor that receives the bio molecules and immobilized on active material (ZnO). In this work, polymer electrolyte synthesized using polyethylene oxide (PEO) and lithium per chlorate (LiClO4) is used as bioreceptor as well as gate dielectric material that has the affinity to interact with analyte molecules and modify the value of dielectric constant.
3. Electric Double Layer Gate Dielectric
Charge control by using electric double layer (EDL) gate dielectric has recently emerged as a strong tool to control the Fermi energy (EF) in solid. The EDL acts as a nano gap capacitor with a huge capacitance and can accumulate and deplete charge carriers over a wide range. Some interesting physical phenomena such as super conductivity, metal insulator transition and thermoelectric behaviour have been modulated by using EDL gate dielectric in transistor configuration.
Fig. 2 Electric double layer capacitor15
An electrolyte (liquid or solid) with mobile ions (cations and anions) is filled in between two electrodes (Figure 2) can be used as an EDL capacitor. Double layers are formed between the electrodes and electrolyte which consist of the space charge at electrodes and an ion space charge in the electrolyte under external electric field. Cations and anions are moved to the surface of EDL during its charging process. The EDL capacitor (EDL-C) is discharged by removing the external voltage. The operation of EDL-C is based on ion absorption and desorption to the EDL during its charging and discharging. In this work, the The EDL-C has nanogap at ZnO electrolyte interface that has high value of capacitance (˃1µF) and able modulate the ultrahigh charge carriers in the channel. When analyte molecules interact with polymer electrolyte the covalent binding between them may change the dielectric value and capacitance of this material.
4. Experimental details
4.1 Film deposition
For this investigation, ZnO thin films are grown on glass substrate by spin coating technique using acetate route. Zinc acetate dihydrate (Zn (CH3COO)2 2H2O) and propanol are used as a precursor and solvent. Diethanolamine (DEA) is used as sol stabilizing agent. It also reduces the surface tension and spreads the precursor uniformly throughout the substrate to make film with uniform thickness. Zn (CH3COO)2 2H2O in propanol is stirrered using the magnetic stirrer at 65OC for 2h which gives curdy white thick solution. DEA is added to this solution drop wise until the solution becomes clear. The solution is then filtered using WHATMANN filter paper and it allows resting for 48 hrs. The spin coating is carried out at 3000 rpm for 30s, followed by heating at 350OC for 30 min to evaporate of the unwanted solvent and deposition of the film before the next layer is applied. Thus prepared ZnO films were heated at 450OC for 6hrs in muffle furnace.
4.2 Device fabrication
Fig. 3 ZnO based sensing device
Two terminal sensors are fabricated by the thermal evaporation of the Au/Cr using the hard mask.
For field effect, device as shown in figure 3 the source, drain electrodes are made by silver pest and gate electrodes are made by Cu wire. The polymer electrolyte which is used as a gate insulator is prepared by mixing the polyethylene oxide and Lithium per chlorate (LiClO4) in 10:1 ration. The mixture of a PEO and LiClO4 is dissolved in methanol by constant stirring and evaporation till the solution becomes a gel. Li+ and ClO4- ions are suspended on PEO matrix. This gel is employed on the exposed area of the channel as a top and side gate.
4.3 Measurements
Fig. 4 Bio molecules sensing measurement setup
Electrical measurement ,as well as, sensing measurement is performed inside the metallic box as shown in figure 4. The sensing device was attached on the top surface of the box. There are 3 holes with closed system was arranged for gas inlet, gas out let and for electrical connection. At the bottom of the box, heating system was also attached which change the analyte in gases form. The electrical measurement is performed with Keithley source meter model no. 2400. IV measurement, static and dynamic current measurement of thin film transistor are performed under the influence of target analyte molecules.
Fig. 5 Human blood before centrifuged and after centrifuged
Figure (5) shows the human blood without centrifuged and after centrifuged. Centrifuged was employed to separate the blood serum from other dissolved solid particles. Blood serum mainly contains water, protein and urea. After centrifuged, the surfactant was dissolved in ethanol (serum to ethanol ratio 10:1). The solubility of urea in ethanol is more than the protein (Protein is less soluble or almost insoluble in alcohol). In this way analyte is prepared for sensing.
5. Expected outcomes:
This articles outlines the successful development and application of ZnO thin film-based biochemical sensors with polymer electrolyte gate dielectrics, highlighting their potential for enhanced sensitivity and practical use in medical diagnostics. Followings are the expected outcomes according to the above mentioned theoretical and experimental explanation of ZnO nanostructured based serum sensor.
Successful fabrication of two-terminal and three-terminal blood serum sensors using ZnO thin films and polymer electrolyte.
Direct interaction of blood serum with ZnO surface increases the conductivity in two-terminal devices.
Enhanced performance of three-terminal devices due to gate-controlled channel current.
Significant increment in the current ON/OFF ratio.
Increment in field effect mobility of carriers in the ZnO channel when exposed to blood serum.
Observation of a negative shift in the threshold voltage when the sensor is exposed to serum vapor dissolved in ethanol.
Effective control of drain current (Id) by gate bias, demonstrating the potential for electrochemical sensing of blood serum.
Enhanced electrical signals due to the interaction of polymer dielectric with HCO₃⁻ and NH₄⁺ ions from blood urea.
Successful use of polymer electrolyte to form an electric double layer at the ZnO surface, inducing ultrahigh charge carriers.
Demonstration of the polymer dielectric as an analyte immobilizing agent with covalent links to ions, enhancing the electric signal of the sensor.
Bibliography
1 R.R. Ghimire, S. Mondal, and A.K. Raychaudhuri, J. Appl. Phys. 117, (2015).
2 Y. Xie, L. Wei, G. Wei, Q. Li, D. Wang, Y. Chen, S. Yan, G. Liu, L. Mei, and J. Jiao, Nanoscale Res. Lett. 8, 1 (2013).
3 A. Bedia, F.Z. Bedia, B. Benyoucef, and S. Hamzaoui, Phys. Procedia 55, 53 (2014).
4 R. Ahmad, N. Tripathy, M. Ahn, K.S. Bhat, Y. Wang, J. Yoo, D. Kwon, and H. Yang, Sci. Rep. 1 (2017).
5 R. Rahmanian and S.A. Mozaffari, Sensors Actuators B. Chem. 207, 772 (2015).
6 G.S. Mei and P.S. Menon, Mater. Res. Express 7, 12003 (2020).
7 A. Liu, G. X. Liu,H. H. Zhu, F. Xu, E. Fortunato, R. Martin, and F. K. Shan, ACS Appl. Mater. Interfaces 6, 17364 (2014).
8 C. Samanta, R.R. Ghimire, and B. Ghosh, IEEE Trans. Electron Devices 65,2827 (2018).
9 E. Fortunato, P. Barquinha, and R. Martins,Adv. Meter.24, 2945 (2012).
10 R.E. Presley, C.L. Munsee, C. Park, D. Hong, J.F. Wager, and D.A. Keszler, 37, 2810 (2004).
11 R.S. Bisht, R.R. Ghimire, and A.K. Raychaudhuri,J. Phys. Chem. C 119, 27813 (2015).
12 K.H. Lee, M.S. Kang, S. Zhang, Y. Gu, T.P. Lodge, and C.D. Frisbie, Adv. Mater. 24, 4457 (2012).
13 J.H. Choi, Y. Gu, K. Hong, W. Xie, C.D. Frisbie, and T.P. Lodge, ACS Appl. Mater. Interfaces 6, 19275 (2014).
14 H. Wang and L. Pilon, J. Phys. Chem. C 115, 16711 (2011).
15 H. Du, X. Lin, Z. Xu, and D. Chu, Electric Double-Layer Transistors: A Review of Recent Progress (Springer US, 2015).
16 H. Yuan, H. Wang, and Y. Cui, Acc. Chem. Res. 48, 81 (2015).
17 S.G. Ansari, R. Wahb, Y.S. Kim, Z.A. Ansari, O.B. Yang, G. Khang, and H.S. Shin, Int. J. Nanomanuf. 4, 290 (2009).
18 D. Xiang, M. Liu, and G. Chen, RSC Adv. 7, 55610 (2017).
19 T. Ahuja and D. Kumar,Sensor Lett. 6, 1 (2008).
20 S. Alegret, J. Bartroli, C. Jimhez, and D. Martorell, 16, 453 (1993).
21 P.R. Solanki, A. Kaushik, A.A. Ansari, G. Sumana, and B.D. Malhotra, Appl. Phys. Lett. 93, 163903 (2008).
22 L. Liu, H. Mo, S. Wei, D. Raftery, W. Lafayette, and W. Lafayette, 137, 595 (2016).
23 F. Bibi, M. Villain, C. Guillaume, B. Sorli, and N. Gontard, Sensor 16, 1232 (2016).
24 L. Li, S. Wang, Y. Xiao, and Y. Wang, Trans. Tianjin Univ. (2020) https://doi.org/10.1007/s12209-020-00234-y.
25 P.R. Solanki, A. Kaushik, A.A. Ansari, and B.D. Malhotra, Appl. Phys. Lett. 94, 143901 (2009).
Nanostructure materials are promising candidates for electronics and opto-electronic applications. Nowadays, field effect devices based on oxide material are highly desired for sensors and detectors. These devices based on oxide material grown on silica and quartz substrate are cost-effective and favourable for grown at low temperature vacuum free environment1–6. This article gives the information how to fabricate thin film transistors (TFT) on a glass substrate at low temperature using oxide semiconductor as channel materials and electric double layer (EDL) as a gate dielectric. Thus, fabricated field effect device is used for biomolecule sensing application. The semiconducting oxide such as zinc oxide (ZnO), indium zinc oxide (IZO), indium gallium zinc oxide (IGZO), and tin oxide (SnO2) are used as channel materials for thin film transistor1,7–10.The oxide materials in amorphous or crystalline have moderately high value of carrier mobility in comparison to the amorphous silicon and organic semiconductors 11. Transistors with high performance index are required for the microelectronic circuits. Most of the conventional oxide gate dielectrics used in transistor have a low value of specific capacitance (nF) which is insufficient to modulate the ultra-high electronic charge density in oxide based materials 1,11–16. Conventional oxide like SiO2 and Al2O3 require high temperature and high vacuum to deposit it on the channel as a gate dielectric. Oxide semiconductors have high value of native volume charge density (≥1019/cm3) therefore large gate voltage is required to switch on this large charge carrier density while using the conventional oxide as a gate dielectric11. An alternative way is to use the EDL gate dielectric, which is formed at the semi-conductor dielectric interface that can be tuned the large charge density from the oxide channel. These electrolytes can be deposited at room temperature and no need vacuum. The dream of fabrication of field effect devices using electrolyte as gate dielectric is to change the practical knowledge into application domain i.e. using field effect device of nanostructure material for blood serum sensor.
Estimation of urea in blood serum is an important diagnosis for liver, kidney disease, including gastrointestinal bleeding, dehydration, shock and burn etc. In humans body, synthesis of urea is a major route to dispose waste nitrogen, which is generated by protein and amino acid metabolism in liver17–21. Urea is removed from the blood and excreted in the urine via the kidney. The normal range of urea in human blood is 2.6 to 6 mM 22. The Abnormal urea level indicates various metabolic diseases; therefore it is essential to check the urea level time to time and keep it under control is a primary issue for good health. The Literature shows the different types of sensors are used for estimation and determination of urea in blood. Most of urea biosensor based on catalytic conversion of urea to hydrogen bicarbonate and ammonium. The commonly used calorimetric and spectroscopic methods are laborious and not suitable for line monitoring system4–6,17–19,23–25. The limitation of these methods is overcome by using an electrochemical technique in which bio molecules convert a biological event to an electrical signal. This method has the advantages over other methods because of its robustness, easy miniaturization, excellent detection limit and small analyte volume. Human blood serum mainly contains protein and urea. The solubility of protein relatively lowers than the urea in alcohol this helps to separate the urea and protein from blood serum. This article gives an idea to fabricate two terminals and three terminals field effect sensing devices using ZnO film on glass substrate using chemical route. ZnO nanostructures have paid great attention because of their unique properties such as high catalytic efficiency, strong absorption ability, biocompatibility and high electron communication features. In field effect device, electric double layer (EDL) formed on semiconductor and gate electrolyte interface is used as a dielectric material that can induce high value of charge carriers on the surface of the channel. The value of dielectric constant of electrolyte is modified when it is exposed to the external agent like alcohol and urea etc. The change in dielectric value of polymer electrolyte helps to control the drain current in field effect device. This is the key mechanism for the sensing urea in blood serum. Though this work based on ZnO nanostuctured film, it equally validated to other oxide based nanostructure like SnO2, ZrO2 and TiO2.
2. Methodology
From the past decades, ZnO based biosensors have become the prevalent topic in thin film technology. Nanostructures ZnO are promising materials with a number of analytical detection methods such as in mass-based biosensors as well as electrochemical and optical methods. Among them, ZnO based electrochemical biosensor has attracted much attention in health care application because of its enhanced and fast response time.
Fig. 1 Components of electrochemical transducer
There are other parameters that signify the performance of the biosensors such as detection limit, sensitivity, selectivity, response time and recovery time. A novel idea is required to improve the sensing device that enhanced the performance parameters. In electrochemical method, biological events directly convert into electrical signal with higher stability. In this method, the stoichiometry of anlyte molecule does not change. Therefore, this method has advantages over other methods. Figure (1) illustrates the major parts of the electrochemical biosensor. Here, the analyte is the target biomolecules such as blood urea, glucose and protein. Second part is bioreceptor that receives the bio molecules and immobilized on active material (ZnO). In this work, polymer electrolyte synthesized using polyethylene oxide (PEO) and lithium per chlorate (LiClO4) is used as bioreceptor as well as gate dielectric material that has the affinity to interact with analyte molecules and modify the value of dielectric constant.
3. Electric Double Layer Gate Dielectric
Charge control by using electric double layer (EDL) gate dielectric has recently emerged as a strong tool to control the Fermi energy (EF) in solid. The EDL acts as a nano gap capacitor with a huge capacitance and can accumulate and deplete charge carriers over a wide range. Some interesting physical phenomena such as super conductivity, metal insulator transition and thermoelectric behaviour have been modulated by using EDL gate dielectric in transistor configuration.
Fig. 2 Electric double layer capacitor15
An electrolyte (liquid or solid) with mobile ions (cations and anions) is filled in between two electrodes (Figure 2) can be used as an EDL capacitor. Double layers are formed between the electrodes and electrolyte which consist of the space charge at electrodes and an ion space charge in the electrolyte under external electric field. Cations and anions are moved to the surface of EDL during its charging process. The EDL capacitor (EDL-C) is discharged by removing the external voltage. The operation of EDL-C is based on ion absorption and desorption to the EDL during its charging and discharging. In this work, the The EDL-C has nanogap at ZnO electrolyte interface that has high value of capacitance (˃1µF) and able modulate the ultrahigh charge carriers in the channel. When analyte molecules interact with polymer electrolyte the covalent binding between them may change the dielectric value and capacitance of this material.
4. Experimental details
4.1 Film deposition
For this investigation, ZnO thin films are grown on glass substrate by spin coating technique using acetate route. Zinc acetate dihydrate (Zn (CH3COO)2 2H2O) and propanol are used as a precursor and solvent. Diethanolamine (DEA) is used as sol stabilizing agent. It also reduces the surface tension and spreads the precursor uniformly throughout the substrate to make film with uniform thickness. Zn (CH3COO)2 2H2O in propanol is stirrered using the magnetic stirrer at 65OC for 2h which gives curdy white thick solution. DEA is added to this solution drop wise until the solution becomes clear. The solution is then filtered using WHATMANN filter paper and it allows resting for 48 hrs. The spin coating is carried out at 3000 rpm for 30s, followed by heating at 350OC for 30 min to evaporate of the unwanted solvent and deposition of the film before the next layer is applied. Thus prepared ZnO films were heated at 450OC for 6hrs in muffle furnace.
4.2 Device fabrication
Fig. 3 ZnO based sensing device
Two terminal sensors are fabricated by the thermal evaporation of the Au/Cr using the hard mask.
For field effect, device as shown in figure 3 the source, drain electrodes are made by silver pest and gate electrodes are made by Cu wire. The polymer electrolyte which is used as a gate insulator is prepared by mixing the polyethylene oxide and Lithium per chlorate (LiClO4) in 10:1 ration. The mixture of a PEO and LiClO4 is dissolved in methanol by constant stirring and evaporation till the solution becomes a gel. Li+ and ClO4- ions are suspended on PEO matrix. This gel is employed on the exposed area of the channel as a top and side gate.
4.3 Measurements
Fig. 4 Bio molecules sensing measurement setup
Electrical measurement ,as well as, sensing measurement is performed inside the metallic box as shown in figure 4. The sensing device was attached on the top surface of the box. There are 3 holes with closed system was arranged for gas inlet, gas out let and for electrical connection. At the bottom of the box, heating system was also attached which change the analyte in gases form. The electrical measurement is performed with Keithley source meter model no. 2400. IV measurement, static and dynamic current measurement of thin film transistor are performed under the influence of target analyte molecules.
Fig. 5 Human blood before centrifuged and after centrifuged
Figure (5) shows the human blood without centrifuged and after centrifuged. Centrifuged was employed to separate the blood serum from other dissolved solid particles. Blood serum mainly contains water, protein and urea. After centrifuged, the surfactant was dissolved in ethanol (serum to ethanol ratio 10:1). The solubility of urea in ethanol is more than the protein (Protein is less soluble or almost insoluble in alcohol). In this way analyte is prepared for sensing.
5. Expected outcomes:
This articles outlines the successful development and application of ZnO thin film-based biochemical sensors with polymer electrolyte gate dielectrics, highlighting their potential for enhanced sensitivity and practical use in medical diagnostics. Followings are the expected outcomes according to the above mentioned theoretical and experimental explanation of ZnO nanostructured based serum sensor.
Successful fabrication of two-terminal and three-terminal blood serum sensors using ZnO thin films and polymer electrolyte.
Direct interaction of blood serum with ZnO surface increases the conductivity in two-terminal devices.
Enhanced performance of three-terminal devices due to gate-controlled channel current.
Significant increment in the current ON/OFF ratio.
Increment in field effect mobility of carriers in the ZnO channel when exposed to blood serum.
Observation of a negative shift in the threshold voltage when the sensor is exposed to serum vapor dissolved in ethanol.
Effective control of drain current (Id) by gate bias, demonstrating the potential for electrochemical sensing of blood serum.
Enhanced electrical signals due to the interaction of polymer dielectric with HCO₃⁻ and NH₄⁺ ions from blood urea.
Successful use of polymer electrolyte to form an electric double layer at the ZnO surface, inducing ultrahigh charge carriers.
Demonstration of the polymer dielectric as an analyte immobilizing agent with covalent links to ions, enhancing the electric signal of the sensor.
Bibliography
1 R.R. Ghimire, S. Mondal, and A.K. Raychaudhuri, J. Appl. Phys. 117, (2015).
2 Y. Xie, L. Wei, G. Wei, Q. Li, D. Wang, Y. Chen, S. Yan, G. Liu, L. Mei, and J. Jiao, Nanoscale Res. Lett. 8, 1 (2013).
3 A. Bedia, F.Z. Bedia, B. Benyoucef, and S. Hamzaoui, Phys. Procedia 55, 53 (2014).
4 R. Ahmad, N. Tripathy, M. Ahn, K.S. Bhat, Y. Wang, J. Yoo, D. Kwon, and H. Yang, Sci. Rep. 1 (2017).
5 R. Rahmanian and S.A. Mozaffari, Sensors Actuators B. Chem. 207, 772 (2015).
6 G.S. Mei and P.S. Menon, Mater. Res. Express 7, 12003 (2020).
7 A. Liu, G. X. Liu,H. H. Zhu, F. Xu, E. Fortunato, R. Martin, and F. K. Shan, ACS Appl. Mater. Interfaces 6, 17364 (2014).
8 C. Samanta, R.R. Ghimire, and B. Ghosh, IEEE Trans. Electron Devices 65,2827 (2018).
9 E. Fortunato, P. Barquinha, and R. Martins,Adv. Meter.24, 2945 (2012).
10 R.E. Presley, C.L. Munsee, C. Park, D. Hong, J.F. Wager, and D.A. Keszler, 37, 2810 (2004).
11 R.S. Bisht, R.R. Ghimire, and A.K. Raychaudhuri,J. Phys. Chem. C 119, 27813 (2015).
12 K.H. Lee, M.S. Kang, S. Zhang, Y. Gu, T.P. Lodge, and C.D. Frisbie, Adv. Mater. 24, 4457 (2012).
13 J.H. Choi, Y. Gu, K. Hong, W. Xie, C.D. Frisbie, and T.P. Lodge, ACS Appl. Mater. Interfaces 6, 19275 (2014).
14 H. Wang and L. Pilon, J. Phys. Chem. C 115, 16711 (2011).
15 H. Du, X. Lin, Z. Xu, and D. Chu, Electric Double-Layer Transistors: A Review of Recent Progress (Springer US, 2015).
16 H. Yuan, H. Wang, and Y. Cui, Acc. Chem. Res. 48, 81 (2015).
17 S.G. Ansari, R. Wahb, Y.S. Kim, Z.A. Ansari, O.B. Yang, G. Khang, and H.S. Shin, Int. J. Nanomanuf. 4, 290 (2009).
18 D. Xiang, M. Liu, and G. Chen, RSC Adv. 7, 55610 (2017).
19 T. Ahuja and D. Kumar,Sensor Lett. 6, 1 (2008).
20 S. Alegret, J. Bartroli, C. Jimhez, and D. Martorell, 16, 453 (1993).
21 P.R. Solanki, A. Kaushik, A.A. Ansari, G. Sumana, and B.D. Malhotra, Appl. Phys. Lett. 93, 163903 (2008).
22 L. Liu, H. Mo, S. Wei, D. Raftery, W. Lafayette, and W. Lafayette, 137, 595 (2016).
23 F. Bibi, M. Villain, C. Guillaume, B. Sorli, and N. Gontard, Sensor 16, 1232 (2016).
24 L. Li, S. Wang, Y. Xiao, and Y. Wang, Trans. Tianjin Univ. (2020) https://doi.org/10.1007/s12209-020-00234-y.
25 P.R. Solanki, A. Kaushik, A.A. Ansari, and B.D. Malhotra, Appl. Phys. Lett. 94, 143901 (2009).
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