Flexible Screen-Printed CNT Electrode on Porous PTFE Substrate with Gold Nanoparticle Modification for Rapid Electrochemical Gas Sensing
Authors: Wan Hao , Chen Yuantao , Gan Ying , Wang Ping 
Published: May 2020

Introduction
Extensive attentions have been paid to gas sensors that play critical roles in gas sensing for various applications such as industrial manufacturing, environmental monitoring and human safety. Among all gas sensing approaches, electrochemical gas sensing reveals remarkable merits in high sensitivity, low detection limit, high selectivity and easy miniaturization. However, conventional electrochemical sensors suffer from two inherent drawbacks: short lifetime due to the evaporation of electrolyte and long response time due to the low diffusion rate of gas target in electrolyte. Different strategies have been proposed to solve the two issues such as new electrolyte and new sensor structure design. In our previous study, room temperature ionic liquid (RTIL) was utilized as the electrolyte in gas sensors featuring negligible vapor pressure, high thermal stability and wide potential window, thus implementing long lifetime of the sensor [1]. To achieve short response time, porous polytetrafluoroethylene (PTFE) was used as the substrate for electrode patterning using microfabrication, and gases can directly diffuse through the substrate from the backside rather than the electrolyte from the front, which can achieve fast response [2]. However, microfabrication requires expensive equipment and professional personnel and lacks sufficient flexibility [3]. In this work, we proposed a new flexible screen-printed carbon nanotube electrode with gold nanoparticles modification to implement rapid and high sensitive gas sensing. Preliminary results were presented to validate the feasibility of the approach for gas sensor design and gas sensing.

Method

In order to achieve fast gas response, porous PTFE was used as the sensor substrate. Three electrodes including working electrode (WE), reference electrode (RE) and counter electrode (CE) were patterned on the substrate using screen printing technique. To ensure the flexibility and high sensitivity of the sensor, carbon nanotube ink was selected to pattern the working electrode and counter electrode. Silver ink was used to pattern the reference electrode acting as a quasi-reference electrode to provide a relatively stable reference potential. An insulation layer was subsequently patterned on the substrate to expose the working area. After the patterning, the patterned substrate was sintered at 80°C to finalize the sensor fabrication. The size of the electrode is 20 mm×12 mm with high flexibility. It is worth noting that the electrode can be easily tailored into different structure and size for miniaturization and wearable applications. The diameter of the WE is 1.5 mm, and the diameter of the whole working area is 4 mm. To further improve the sensor performance, gold nanoparticles were electrochemically deposited on the WE in chloroauric acid. After rinsing and desiccation, the fabricated electrode was mounted and connected to a custom printed circuit board (PCB) to enable convenient electric connection, and the backside of the working area was exposed to the hole on the PCB that enables gas flow and diffusion. The PCB was then packaged on a 3D printed air chamber for tests, and 3 μL 1-butyl-1-methylpyrrolidinium bis-(trifluoromethylsulfonyl)-imide ([C4mpy][NTf2]) was added on the working area as the electrolyte for gas sensing.

Results and Conclusions

Gold nanoparticles (AuNPs) were deposited on the WE to further improve the performance of the sensor. The deposition was set as -0.3 V with a deposition time of 60 s. Cyclic voltammetry was performed in K3[Fe(CN)6]/K4[Fe(CN)6], and the results show that both the reduction current and oxidation current increase apparently after the AuNPs deposition compared to the electrode without modification. The AuNPs modified gas sensor was tested in 21% O2 and pure nitrogen using cyclic voltammetry, and the sensor presented significant current response, indicating the reduction of oxygen and oxidation of superoxide radical. Subsequently, the sensor as preliminarily tested in O2 with different concentration from 2.1% to 21% O2 to verify the electrochemical performance with a potential of -1.0 V. The reduction current of the sensor increases with the increasing of oxygen, indicating good sensitivity of the sensor in the low concentration range from 2.1% to 12.6% O2, and the current response remained unchanged in higher oxygen concentration. The results can be ascribed to the saturation of the sensor in high oxygen concentration. The sensor was calibrated based on the peak current at each concentration with a sensitivity of 0.09651 μA/[%O2] and a linearity of 0.9814 for oxygen sensing. The response time of the sensor, defined as the period from the response initiating to reaching 90% of the highest current response, is ~12 s. The remarkably fast response is due to the unique sensor design that gases can directly diffuse through the substrate and reach the electrode-electrolyte interface for reaction. Also, it is worth noting that the sensor is cheap, disposable, flexible and easily tailorable based on the screen-printing technique. Therefore, this study provides a unique and promising approach in gas sensor fabrication with the merits of high sensitivity, fast response, high flexibility and low cost. Further optimization of the sensor will be performed to achieve the best performance of the sensor.

References

[1] H. Wan, H. Yin, A.J. Mason, Rapid measurement of room temperature ionic liquid electrochemical gas sensor using transient double potential amperometry, Sensors and Actuators B: Chemical, 242(2017) 658-66.
[2] H. Wan, H. Yin, L. Lin, X. Zeng, A.J. Mason, Miniaturized planar room temperature ionic liquid electrochemical gas sensor for rapid multiple gas pollutants monitoring, Sensors and Actuators B: Chemical, 255(2018) 638-46.
[3] J. Kobayashi, M. Yamato, K. Itoga, A. Kikuchi, T. Okano, Preparation of microfluidic devices using micropatterning of a photosensitive material by a maskless, liquid‐crystal‐display projection method, Advanced Materials, 16(2004) 1997-2001

Figure 1