Similarly, upon decreasing the pH from 7, an increase in the band intensity at ⁓400 nm is observed with a concomitant decrease in that at ⁓450 nm. This optical property can be directly explored for pH sensing, and thus fluorescence imaging as a systematic ratiometric trend in the fluorescence ratio for excitation at 440 nm:395 nm is observed upon increasing the pH (Figure ). It is very important to notice that a sudden change in the fluorescence intensity ratio (ratiometric response) occurs upon any small deviation from neutralization (pH 7), and thus can be extremely sensitive for visualizing any minute pH changes inside the electrochemical cell upon operation. Due to the exceptional fluorescence properties of HPTS molecule, it was explored as a fluorescent probe that can be directly added to the electrolyte medium. However, at this point it is very important to analyse the electrochemical properties of the HPTS molecule to investigate its applicability in the electrochemical water splitting system. For this purpose, cyclic voltammetry was performed using a 0.25 M Na2SO4 electrolyte, with and without the addition of 50 µM HPTS probe, at a low scan rate of 5 mV/sec. Interestingly, it was observed that no oxidation or reduction peaks are observed, with an insignificant change in the cyclic voltammogram, in comparison to that for pristine electrolyte (Figure ). This certainly qualifies the use of HPTS as a fluorescent probe in the opto-electrochemical water splitting cell are visualized in real-time. Before the application of any voltage, the electrolyte around the electrodes (both cathode and anode) does not show any pH changes, and thus no optical changes are observed, and exhibit a 440 nm:395 nm emission ratio of 1.2 (Figure ) at the electrodes surface as well as far from the electrodes surface (towards bulk electrolyte). As soon as the mentioned voltage is applied, due to the formation of acidic (H + ) and basic (OH -) byproducts around the cathode and the anode, respectively, a rapid change in the fluorescence intensity is observed. At the close vicinity of the cathode, a decrease/increase in the fluorescence intensity for excitation at 395 nm/440 nm is observed, due to the formation of alkaline conditions, leading to an increase in deprotonated form of the HPTS probe. As a result, an increase in the 440 nm:395 nm fluorescence ratio near the cathode is observed, with no changes observed away from the cathode's surface at 5 seconds after voltage operation. As the time for voltage operation is increased, an increase in the 440 nm:395 nm fluorescence ratio is observed (1.62 at 20 seconds), along with an increase in the space volume, or front width, where the effect of alkalinity is observed due to diffusion of OH -species towards the bulk electrolyte from the cathode's surface. Similarly, the influence of acidification at the anode is observed, with the formation of a moving acidic front with increased voltage operation times, due to the diffusion of H + species from the electrode towards the bulk (Figure ). As a result, with increasing voltage application time, a decrease in the 440 nm:395 nm fluorescence ratio is observed from 1.2 at 0 s, 1.05 at 5 s, 0.96 at 20 s and 0.66 at 100 s. The fluorescence ratios were further analyzed and calibrated to finally obtain the change in the pH around the electrodes surface, and the in-situ spatiotemporal evolution of electrolytic medium under water splitting conditions (Figure ). As expected, as soon as the potential is applied, an increase in the pH is observed around the cathode surface which further extends up to a distance of 400 µm at 5 s (Figure ). The pH further increases and saturates at ⁓pH 8 within 100 s, and extends to more than 1 mm distance from the electrode's surface, which was beyond the field of view of the objective lens. Similarly, a decrease in the pH was observed around the vicinity of the anode, which increases temporally, reaching a value of ⁓ pH 6 at 100 seconds (Figure ). As expected, the pH changes are at their maximum level at close vicinity of the electrodes surface, while the bulk electrolyte exhibit lesser deviation from neutrality, which is expected due to diffusion limitations. Thus, for the first time, direct in-situ spatiotemporal imaging of the chemical phenomena happening during water splitting processes is performed, and is in correlation with the previous literature knowledge. As a result, a reliable imagining technique for direct visualization of electrochemical water splitting reactions is successfully developed here.