Microfluidic Electrochemical Sensor for Online Detection of Chemical Oxygen Demand Based on AuNPs/Au Electrodes

A microfluidic electrochemical sensor is fabricated for the measurement of chemical oxygen demand (COD) in surface waters, such as lakes and reservoirs. An Au interdigital electrode is used as a substrate, and a layer of Au nanoparticles (AuNPs) is deposited on the working electrode (WE) surface to form a functionalized WE. A polydimethylsiloxane (PDMS) microfluidic device was fabricated and integrated with the AuNPs/Au to form a PDMS-AuNPs/Au microfluidic electrochemical sensor. Water samples are transferred into and out of the sensor through a peristaltic pump and Tygon tubes, and the online COD detection was achieved using an electrochemical workstation. The linear range of the sensor is 0–400 mg/L, and the detection limit is 0.12 mg/L, which is much lower than the concentration limit of 20 mg/L specified by the Chinese national standard. In addition, five real water samples are evaluated, and the COD values are in good agreement with that of the standard method. The size of the sensor is only $30\times 10\times 5$ mm, each measurement can be obtained within 3 min, and no toxic reagents are needed. In addition, the sensor has good convenience and integration and provides a new idea for online COD detection.