Representative cyclic voltammograms from electrodes before (dashed lines) and after modification with carbon nanotubes (solid line). Vertical columns compare types of functionalized nanotubes: amide-CNTs (panels A–C), carboxylic acid-CNTs (panels D–F) and octadecylamine-CNTs (panels G–I). Horizontal rows compare different compounds 10 μM serotonin (panels A, D, G), 10 μM dopamine (panels B, E, H), and 200 μM ascorbic acid (panels C, F, I). The insets for ascorbic acid show the reduction peaks.