Fig. 4

Nanostructured area of a microprobe. a Schematic of a nanostructured area on the microprobe surface, and SEM image of the top of a microprobe showing a gold electrode. SEM images of the boxed areas in (a); b nanostructured area; c boundary between a nanostructured area and a gold electrode; d enlargement of the boxed area in (c); e AFM image (5 × 5 μm) of a nanostructured microprobe surface; and f line profile of the white arrow in (e). Nano-holes are ~ 200 nm in both diameter and depth