A borosilicate glass capillary (o.d. 1.5 mm, i.d. 0.86 mm) was first pulled with a pipette puller (Sutter Instruments, Novato, CA, USA) and then polished to obtain an inner tip diameter of 25–30 μm (RG < 5) to make an H+ ion-selective SECM microprobe. The ion-selective cocktail was made by mixing 5% proton ionophore-I, 2% potassium tetrakis(4-chlorophenyl)borate, 30% NPOE, 3% PVC, 60% Vulcan carbon powder and 500 μL tetrahydrofuran (THF). The composition was mixed thoroughly with a glass rod on a watch glass until all THF evaporated. An extra 40% of NPOE was further added to the composition to maintain consistency in a sensor paste. We then backfilled the pulled capillary with the sensor paste and pushed it to the pulled end with a Cu wire. To make electrical contact between the sensor paste and the inserted Cu wire, we added 5% Vulcan carbon in a DOS mixture to the pipet from the back opening side.29 (link) The Cu wire connection was secured by applying 10 min epoxy to the junction of the capillary end and the Cu wire. The sensor tip (Figure 1A) was polished with lens cleaning paper and cured overnight in AS (pH 7.2).