A single carbon fiber (GoodFellow, Pittsburgh, PA, USA) of 7 μm diameter was aspirated into a 0.5 mm inner diameter (1.00 mm outer) borosilicate glass capillary (A-M Systems, Inc., Carlsberg, WA, USA) using a vacuum pump (Gast). These filled capillary tubes were subsequently pulled using a vertical puller (PC-100 or PE-22, Narishige Group, Japan) to create a tapered seal insulating the carbon fiber. While observing the electrode under a microscope, the protruding carbon fiber was then cut to a length between 130 μm to 160 μm. All the results below are reported as current densities, in which the measured current is normalized to the geometric surface area of the electrode and reported in unit of pA/µm 2 . A nichrome wire with a soldered pin (Phoenix Enterprises/PE Connectors, Chatsworth, CA, USA) was then coated with silver epoxy paint (GC Electronics, Rockford, IL, USA) and inserted into the distal end of the fiber-filled capillary. A polyolefin heat shrink tubing (Mouser, Mansfield, TX, USA) then is heat-sealed around the electrode/nichrome wire junction to secure them in place.