In comparison to reference standards of major caffeine metabolites, optimal conditions were determined via inspection of HPLC traces (Figure ) of the reaction outcomes. Prior to the electrosynthesis experiment, cyclic voltammetry studies were conducted to investigate the redox species of the caffeine structure. Caffeine has three oxidation events at 1304 mV, 1616 mV, and 1952 mV, respectively (Figure ). The first oxidation event (Epa1) was identified as reversible redox species via multiple scanning analysis up to 2.3 V and (Figure (b)). This species is capable of undergoing both oxidation and reduction processes, returning to its initial form. Epa1 has been reported as the one-electron process of radical cation caffeine formation. Determining the fixed current used in electrochemical reactions is crucial to ensure successful electrosynthesis by considering the effect of current-potential distribution at the electrode. In an electrochemical system, the potential distribution across the electrode surface may exhibit less uniformity than at the terminal part of the electrode. Therefore, the actual maximum voltage applied in the electrochemical reaction is lower than the theoretical voltage in the system. The constant current reactions of 2.0 mA (1930 mV) and 2.5 mA (1950 mV) revealed that the maximum applied voltages have an incapability to oxidise the caffeine, resulting in no metabolite products detected by using HPLC (SI: HPLC of reaction runs). Based on the CV studies, the irreversibility oxidation process occurred at 1952 mV (Figure ). This means an actual applied voltage greater than 1950 mV is required in this process.