The control of acidity happens in a customized platform presented in f ig 1. The electrochemical microelectrodes are f abricated in a multistep process combining spin coating of photoresist, optical lithography, metal evaporation and then f unctionalization with the redox molecules 4-ATP, as shown in f ig 1(a). The chips bearing four microreactors, each one composed of three electrodes connected through diff usion barriers (WE, CE, RE ), are mounted in the platf orm (f ig 1(b)) having a sealable gasket and a microfluidic system for the f illing of the system with a pneumatic piston for the opening and closure of the cells. The whole setup holds an optical setup on the top to follow the f luorescence in the cell (f ig 1(d)). The f abrication steps of the devices and the platform are detailed brief ly in the experimental section extended in SI-1. Af ter their polymerization, the 4-A TP molecules dimerize to a redox molecular state that at low applied voltages exchange two protons with the electrolyte for each dimer in the electrode (quasi)-reversibly, and each proton is compensated by the current of one electron exchanged in the WE . The CE had the same composition; thus, it balanced the reactions in the WE by the reverse proton exchange reactions. However, since the separation by the diff usive barriers keeps both partial cells apart, there are no possibilities f or cross proton exchanges. The Faradaic currents were f itted considering the contribution of the reversible proton exchange peaks af ter subtraction of the contributions f rom capacitance, the linear currents attributed to mass transport and the f rom the non-reversible residual polymerization. The resulting Faradaic contributions of the proton exchange reactions were used to calculate the proton concentration generated at the oxidation and re-adsorbed by the electrodes during the reduction cycle (details of the calculations shown in SI-3). The calculations showed similar concentration in the two cases (in the case of f ig.1(c), f or instance, 3,6.10 -4 M and 3,52.10 -4 M were f ound for the oxidation and reduction peaks, respectively). This behavior was observed for repetitive cycles (up to hundreds of cycles that we monitored the currents in some of our experiments) validating the quasi-reversibility of the system. During different experiments using diff erent chips we observed a variety of surf ace f unctionalization in the electrodes that resulted in diff erent dynamic ranges of pH obtained during the CV cycles, with typical maximum acidities reaching pH between 3 and 1.