Electrochemical detection of indoles at diamond electrodes

Indoles represent an important class of chemical compounds, of biological significance, and various attempts have been made to develop electrochemical sensors for their detection. In general, this has failed because of the electrode fouling which tends to occur when conventional electrode materials are employed, so we have explored the use of diamond electrodes, in view of their known resistance to electrode poisoning. High phase purity boron-doped diamond electrodes show no detectable oxidative signal when used in solutions containing indole-3-methanol or 1-methyl indole. However, indole oxidation at electrode potentials greater than 0.7 V vs. SCE is seen if lower purity diamond or nanodiamond electrodes are employed. Electrode fouling is found to be problematic, but significantly better than at pyrolytic graphite electrodes. The oxidation of tryptophan at high phase purity diamond electrodes is seen, indicating that in general the indole oxidation chemistry is dependent both on the type of diamond electrode used, and the particular indole involved. Sensitive detection of tryptophan at micromolar concentrations is observed without electrode fouling, although electrode passivation is seen in concentrated tryptophan solutions.