Artificial drinking water (ADW) samples were prepared to simulate real drinking water conditions but without any residual chlorine present. ADW was prepared by dissolving 1 g of sodium bicarbonate, 0.0654 g of magnesium sulphate (Sigma Aldrich, 99.5% anhydrous), 0.3414 g calcium sulphate dehydrate (Honeywell, 99%), 0.007 g potassium phosphate dibasic (Fluka, 98%), potassium phosphate monobasic (Sigma Aldrich, 99%) and 0.01 g sodium nitrate (Sigma Aldrich, 99%) in 10 L of deionised water. For localised pH control, voltammograms were performed in ADW samples by scanning WE1 from -0.2 V to 1.2 V at 50 mV/s with WE2 biased at ~1.6 V to protonate (acidify) the local environment of the sensing electrodes via hydrolysis of water. The local pH can then be readily controlled by adjusting the applied potential to drive the appropriate current, as shown by O'Sullivan et al. In acidic conditions, the pH can be changed through the water splitting reactions of Eqs. ( ) and (2):