Furthermore, in order to perform electrolyte flow measurements, the cell is equipped with inlet and outlet ports connected to a syringe pump with adjustable flow rate. The electrolyte used in this work is 0.25 M Na2SO4 with the co-addition of 10 µM HPTS (8-hydroxypyrene-1,3,6trisulfonicacid) (Sigma, 6358-69-6). Dilute potassium hydroxide and dilute hydrochloric acid was used to make solutions with different pHs, using an Oakton pH analyser. As a control, 0.25 M phosphate buffer, with the co-addition of 10 µM HPTS, is also tested. Further, the optoelectrochemcail detection of chloride oxidation was perfomed using 50 µM N-(4-butanoic acid) dansylsulfonamide (BADS) fluorophore by directly addid in the electrolyte (0.5 M NACl in phosphate buffer). N-(4-butanoic acid) dansylsulfonamide 99%, CAS: 102783-77-7 was purchased Henan Bao Enluo Trading Co. ltd, China. The operational opto-electrochemical imaging studies were performed using an inverted fluorescence Thunder microscope (LEICA MD4000M Fluorescence microscope with LEICA DFC450 camera), using the 395 nm and 440 nm as excitation wavelengths. The opto-electrochemical fluorescence images and movies were analysed using the Leica image analysis software (LAS X). The opto-electrochemical imaging was also performed at different electrode distances, and under varying electrolyte flow rates.