Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb A real-time on-line photoelectrochemical sensor toward chemical oxygen demand determination based on feld-efect transistor using an extended gate with 3D TiO 2 nanotube arrays Hewei Si a,b , Nengqian Pan b , Xidong Zhang a,b , Jianjun Liao a,c , M.N. Rumyantseva d , A.M. Gaskov d , Shiwei Lin a,b, ⁎ a State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou, 570228, China b College of Materials and Chemical Engineering, Hainan University, Haikou, 570228, China c Institute of Tropical Agriculture and Forestry, Hainan University, Haikou, 570228, China d Chemistry Department, Moscow State University, Leninskie Gory 1-3, Moscow, 119991, Russia ARTICLEINFO Keywords: Real-time on-line detection Photoelectrochemical sensor Chemical oxygen demand (COD) Extended-gate feld-efect transistor 3D TiO 2 nanotube arrays ABSTRACT Chemical oxygen demand (COD) is one of the most widely used parameters for water quality assessment. Quantifcation and detection of COD slight fuctuations is crucial for an early detection of abnormal change in the water environment. Here we report a real-time on-line photoelectrochemical method for sensitive COD detection using an extended-gate feld-efect transistor (EGFET) sensor. The FET gate consists of a Ti mesh electrode with 3D TiO 2 nanotube arrays modifed by Pt nanoparticles. The device shows a COD detection limit down to 0.12 mg/L and a wide dynamic linear range from 1.44 mg/L to 672 mg/L at a continuous fow rate of 1.0 mL/s. The method reported also displays excellent stability, accuracy and reproducibility. More importantly, real water samples analyses with the proposed method are in a good agreement with the standard dichromate method. The results suggest that the EGFET-based photoelectrochemical sensors are potential candidates for practical COD onsite measurement. 1. Introduction Chemical oxygen demand (COD), as one of the most important and extensively used parameters for water quality assessment and pollution control, is the overall amount of oxygen equivalent in oxidative de- gradation of the organic contaminant in wastewater [1]. In view of the inherent disadvantages [2] of standard dichromate method such as encompassing a time-consuming process (2–4 h), generating highly toxic ions (Cr 6+ and Hg 2+ ), applying corrosive (concentrated H 2 SO 4 ) and expensive (Ag 2 SO 4 ) reagents, various alternative methods based on electrocatalysis [3], photocatalysis [4] and photoelectrocatalysis [5], have been proposed and applied in the COD measurement. Currently, photoelectrocatalytic COD determination method has appealed intensive attention due to its strong oxidation ability and low recombination rate of photogenerated electron-hole pairs [6–8]. How- ever, one common drawback in the conventional photoelectrochemical measurements is the requirement of a three-electrode confguration which is an impediment for the device miniaturization. And it’s also difcult to increase the transient response and improve the sensitivity because of the relatively low degradation efciency of the working electrode [9]. Besides, it is hard to realize real-time analysis and remote control for the photoelectrochemical output from TiO 2 on COD col- lected by electrochemical circuits directly. However, this will be solved when a transducer is introduced. In our previous work, we reported a miniature photoelectrochemical sensor based on organic electro- chemical transistor (OECT) as transducer for sensitive COD determi- nation in wastewaters [10]. Such previous work realized device min- iaturization by introducing of the transducer like OECT and removal of the reference and counter electrodes. However, the relatively narrow linear range and the poor stability from direct contact of the semi- conductor active layer with tested water sample, limit its applications in the real-time on-line monitoring platform and remote control. Extended-gate feld-efect transistors (EGFETs), which originated from the ion-sensitive feld-efect transistor frst reported in 1970 [11], have been applied as a powerful chemical/bio-chemical sensing tech- nique. The confguration is composed of a sensor electrode and a con- ventional metal-oxide-semiconductor feld-efect transistor (MOSFET) [12]. The chemical reaction on the sensor electrode surface would https://doi.org/10.1016/j.snb.2019.03.071 Received 6 December 2018; Received in revised form 1 March 2019; Accepted 16 March 2019 ⁎ Corresponding author at: State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou, 570228, China. E-mail address: linsw@hainu.edu.cn (S. Lin). Sensors & Actuators: B. Chemical 289 (2019) 106–113 Available online 21 March 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved. T