Real-Time on-Line Photoelectrochemical Determination of Chemical Oxygen Demand By Field-Effect Transistor Using an Extended Gate with Controllable Nanostructures
Authors: Lin Shiwei , Si Hewei , Zhang Xidong 
Published: May 2020

Introduction
Chemical oxygen demand (COD) is the oxygen equivalent on degradation of the organic compounds, which is a significant parameter of water quality assessment.[1] Currently, photoelectrochemical (PEC) COD determination method has appealed intensive attention due to its strong oxidation ability and low recombination rate of photogenerated electron-hole pairs. However, one common drawback in the conventional PEC measurements is the requirement of a three-electrode configuration which is an impediment for the device miniaturization.[2]
Herein, we developed a real-time on-line PEC sensor toward sensitive and rapid COD determination based on extended-gate field-effect transistors (EGFETs). The performance of the EGFETs-based sensors can be easily modulated by alternating the sensing elements on the extended gate electrode.[3,4]
Experimental section
In the PEC thin-layer COD detection cell, the working electrode is composed of 3D TiO2 nanotube arrays on Ti mesh (3D TNTAs/Ti) which was prepared by electrochemical anodization (Fig. 1). The 3D TNTAs/Ti working electrode was connected to the gate of MOSFET, forming an extended-gate field-effect transistor sensor. Characterization of electrodes, PEC measurements and COD determination were then conducted in details. Besides 3D TNTAs, photoelectrodes with other controllable nanostructures were also selected and the corresponding sensing performance was investigated.

Results and Conclusions

Using different sensing elements on the extended-gate electrode, EGFETs-based sensors have been developed to measure pH, DNA, protein, urea, glucose, etc. Here, self-assembled 3D TiO2 nanotube arrays decorated with Pt nanoparticles was firstly employed as the sensor electrode in the EGFET-based sensor toward COD detection. The COD detection conditions have been firstly optimized, and then the analytical principle has been proposed and validated.
The COD sensor shows a low detection limit of 0.12 mg/L COD and a wide linear range from 1.44 mg/L to 672 mg/L at the flow rate of 1.0 mL/s, suggesting its great potential application in real-time on-line COD measurement. The validity of the proposed device has been testified in both synthetic and real water samples. The sensitive and stable detection can be attributed to synergistic effects from the superior oxidation ability of the 3D TNTAs/Ti sensing electrode, the flexibility of EGFET-based device, and the unique structure of the thin-layer reactor.
In order to demonstrate the feasibility of the proposed device in practical COD determination, the actual wastewater samples were analyzed by both conventional dichromate method and the proposed sensor. The relative standard deviation (RSD) obtained from the proposed sensor were below 9% indicating that the reproducibility is good. The COD values determined by the proposed sensor show excellent correspondence with the conventional dichromate method according to the small relative error within 10%. Moreover, other EGFET-based sensors with various controllable nanostructures were also investigated, and the underlying mechanism was verified.

References

[1] H. Si, X. Zhang, S. Lin, A simple flow injection sensing system for the real-time on-line determination of chemical oxygen demand based on 3D Au-NPsTiO2 Nanotube Arrays, Frontiers in Materials. 6 (2019) 238.
[2] H. Si, N. Pan, X. Zhang, J. Liao, M.N. Rumyantseva, A.M. Gaskov, S. Lin, A real-time on-line photoelectrochemical sensor toward chemical oxygen demand determination based on field-effect transistor using an extended gate with 3D TiO2 nanotube arrays, Sensors and Actuators B: Chemical 289, 106-113 (2019)..
[3] J. Liao, H. Si, X. Zhang, and S. Lin, Functional Sensing Interfaces of PEDOT:PSS Organic Electrochemical Transistors for Chemical and Biological Sensors: A Mini Review, Sensors 19, 218 (2019).
[4] J. Liao, H. Si, X. Zhang, C-Z Wang and S. Lin, The crystal facet-dependent electrochemical performance of TiO2 nanocrystals for heavy metal detection: theoretical prediction and experimental proof, Sensors & Actuators: B-Chemical, 271(2018) 195-202.

Figure 1