The Characterization of Nanofet Sensor with Extended Sensing Electrode and Reference Electrode for Biosensor Application
Authors: Kang Hye-Lim , Lee Kook-Nyung , Yoon Sumi , Hong Dong-ki , Song Sunga , Kim YoungJoo , Kim Won-Hyo , Seong WooKyeong 
Published: May 2020

Biosensors based on nano field-effect transistor(nanoFET) have been studied to detect from pH to biomolecules and anion in the air because its potential for label-free detection, rapid response time, low-cost and miniaturization [1-3]. The nanoFET sensor detect target species with a conductance change of nanochannel that is induced by the adsorption of charged substance or the surface potential of the sensing gate electrode. However, the sensing area on top of the nanochannel area is very small, the probability of binding of target molecules on the sensing surface of the device is very low. Therefore, we employ an electrically extended sensing electrode in order to improve adsorption probability of the charged substances and detection sensitivity. Here, we analyzed the behavior of the nanoFET sensor with an extended gate electrode and a reference electrode using pH solution of difference pH levels. It is a fundamental study to understand the behavior of nanoFET sensor with extended gate as biosensors, and it is necessary before applying it to various solution environment.
Figure 1 shows the schematics of the nanoFET sensor with the extended gate electrode for pH level sensing of solution. The top gate electrode and the bottom substrate gate of the nanoFET sensor are electrically interconnected with the extended external electrode in order to increase the sensing area. The Ids-Vg characteristics of the nanoFET sensor with extended gate electrode, which is obtained by the voltage sweeping of the gate (Vg) as shown in Figure 2. The threshold voltage of the device is approximately -2.5 V. The electrode of an indium tin oxide(ITO) glass and a Ag/AgCl wire are used for the extended sensing gate electrode and the reference electrode. The connection for gate electrode and the reference electrode are compared with three cases of ITO-Ag/AgCl, Ag/AgCl-ITO and ITO-ITO respectively to compare pH response with the Ids-Vg sweep characteristics of the nanoFET sensor. In the pH sensing experiment, about 50 μL-solution having 4, 7 and 9.2 pH level are used, and it is dropped to a micro well on the extended sensing gate electrode as shown in Figure 1. Figure 3 shows Ids-Vg measurement results of varying the pH level on solution for the three connection cases of extended sensing gate electrode and reference electrode using the same device. Figure 3(a) shows Ids-Vg curve is shifted to the right side as the pH level rises in case of the reference electrode is the Ag/AgCl wire and the extended sensing gate electrode is the ITO glass. The response to pH level is shifted to left side as shown in Figure 3(b) when the electrode connection is reversed for the reference electrode and the extended sensing gate electrode with the ITO glass and the Ag/AgCl wire, respectively. In particular, it is confirmed that the nanoFET device don’t respond to the pH change of solution when both the reference electrode and the extended sensing gate electrode are the ITO glass. We finally conclude that the nanoFET sensor with the extended sensing gate electrode is affected by the surface potential in electrolyte solution induced by varying pH level. The equivalent circuit model of the nanoFET sensor is the series interconnection of two opposing electrolytic capacitors and the MOS capacitor of nanoFET device to the applied gate voltage as shown in Figure 4. Therefore, the nanoFET sensor with the extended gate electrode may not respond to the pH change or ion concentration in electrolyte solution when the interface of liquid-reference electrode is equal to interface of liquid-sensing electrode.
In this study, we demonstrated experimentally the working principles of the nanoFET sensor with the extended gate electrode with the response to pH level of liquid solution. We found that the equivalent circuit model is the series connection of two opposing electrolytic capacitors in liquid solution and the MOS capacitor of the nanoFET device. The results suggest that the proper reference electrode should be utilized to measure the pH levels in liquid solution of nanoFET sensor and the other adequate reference electrode might be used to remove the effects induced by pH noise for the biosensor application such as immunosensors.
This work was financially supported by the research funds (Grant No. 10076874 and No. 10077599) of the Ministry of trade, Industry and Energy, Korea
[1]Matti Kaisti, Detection principles of biological and chemical FET sensors, Biosensors and Bioelectronics, 98 (2017)
[2]Benjamin M. Lowe et al. Field-effect sensors-from pH sensing to biosensing: sensitivity enhancement using streptavidin-biotin as a model system, Analyst 142 (2017)
[3]K.-N. Lee et al. Chemical gating experiment of a nano-field-effect transistor sensor using the detection of negative ions in air, Sens. Actuators, B 236 (2016)

Figure 1