We used a vacuum pump (Gast, Benton Harbor, MI) to aspirate a single 7 μm thick carbon fiber (GoodFellow, Pittsburgh, PA, USA) into a 0.5 mm inner diameter (1 mm outer diameter) borosilicate glass capillary (A-M Systems, Inc., Carlsberg, WA, USA). Then the borosilicate capillaries were inserted into a vertical puller (PC-100 or PE-22, Narishige Group, Japan), resulting in two electrodes with a tapered seal insulating the carbon fiber. The exposed carbon tip was manually cut to between 130 μm to 160 μm under a CX43 microscope with EP50 camera (Evident, formerly Olympus, Tokyo, Japan) . All the results in this study are reported as current densities, where the measured current was normalized to the geometric surface area of the electrode, reported in unit of nA/µm 2 . We completed the electrode fabrication by attaching a silver epoxy paint (GC Electronics, Rockford, IL, USA) coated nichrome wire into the distal end of the fiber-filled capillary, and sealing the nichrome wire using heat-sealed polyolefin shrink tubing (Mouser, Mansfield, TX, USA) at the electrode-nichrome wire interface.