To accurately calibrate the pH control, an external Ag/AgCl reference was used. A beaker was placed downstream containing the external reference and filled with the appropriate buffer. The outlet tube for the microfluidic device was submerged in these buffer solutions to ensure a closed circuit. Chemically buffered solutions were prepared by mixing various ratios of citric acid monohydrate and monosodium phosphate (Sigma Aldrich). The pH of each solution was determined using a calibrated pH probe (Thermo Scientific Orion star A211 pH meter). A phosphate buffer (PB) solution was prepared using 268.8 mg sodium phosphate monobasic and 110.2 mg sodium phosphate dibasic, then making this solution up to 1 L using deionised water. This generated a pH 7.4 10 mM solution of PB, which was used for all electrochemical pH control buffer experiments where a potential window of -0.2V to 1.6V was used, with a scan rate of 50 mV, varying a potential bias at WE2 for the electrochemical pH control.