A visual COD sensor based on the magnetically oriented graphite flake-enhanced photoelectrochromic effect.

A visual chemical oxygen demand (COD) sensor was developed by electrodepositing Prussian blue (PB) on indium tin oxide (ITO) conductive glass to form an electrochromic layer, followed by the vertical alignment and immobilization of graphite flakes under a magnetic field. A TiO₂/g-C₃N₄ heterojunction photocatalyst was in situ integrated with the graphite flakes to enhance the performance. Under the irradiation of visible light, reductive substances in the water sample are oxidized by photogenerated holes from TiO₂/g-C₃N₄, while photogenerated electrons are conducted through the graphite flakes to the electrochromic layer, reducing PB to Prussian white (PW) and causing a color change. The RGB values of the captured image are used to calculate the total color difference, enabling visual quantification of COD. The overlapping of the photo-oxidation layer (POL) and the electrochromic reduction layer (EDL) shortens electron transport distances. Vertically aligned graphite flakes further reduce electron transport resistance, improving photogenerated electron efficiency and lowering the COD detection limit. Additionally, the alignment increases the sensor's effective light-capture area and the photocatalyst loading capacity, expanding the measurement range to 3.2-320 mg/L. The sensor can be easily regenerated by simple cleaning and air oxidation within 1 h, allowing repeated use without interference from chloride ions. The relative standard deviation for repeated measurements and the relative error compared to standard methods are both around 10%, demonstrating good practical applicability.