High sensitivity, 1-10 nM detection, of biogenic amine neurotransmitters was achieved using fast-scan cyclic voltammetry at hydrogen-terminated diamond microelectrodes. This reduced to 1-10 μM after 5-10 runs. To attempt to understand the sensitivity decrease, electron transfer kinetics, baseline current, and rate-limiting step for high (μM) and low (nM) concentrations were compared. Dopamine oxidation kinetics slowed after repeated scans, ΔEp increased from 0.142 to 0.805 V. Baseline current decreased by 50%. The rate-limiting step, measured only at the conditioned surface, was diffusion limiting. The high quality of our electrodes was verified by the voltage window in 0.5 M H2SO4 and by Raman spectroscopy. Dopamine and serotonin oxidation kinetics were characterized at more sensitive, sulfophenyl-terminated diamond microelectrodes; the responses were diffusion limited. These data were qualitatively consistent with surface chemistry changes after scanning, but do not adequately explain the sensitivity decrease. Additional testing of hydrogen- terminated surfaces is needed to provide insight.
