The nosepiece of the optical microscope was equipped with 2.5× (numerical aperture (NA) = 0.075), 10× (NA = 0.3), 20× (NA = 0.5), and 60× (NA = 0.7) objectives. The stage was moved with a manual x-y positioner until the carbon fibre disc was in the centre of the brightfield image. The image of the CF was then focussed by moving the height of the nosepiece with a ES10ZE/D motor controller from Prior Scientific Instruments Ltd. This process was repeated on every objective (from lower to higher magnification) until reaching the objective which was to be used. Unless specified, all measurements herein were recorded using the 60× objective. During this process, the electrode was illuminated using light (intensity varied) produced by a pE-300lite LED purchased from CoolLED Ltd. During focusing, the exposure time and lookup tables (LUTs) of the camera were set to automatic. When the image was in focus, a region of interest (ROI) was drawn around the carbon disc.