A novel potentiometric method using an indium tin oxide (ITO) electrode, surface-treated with aminopropylsilane and disuccinimidyl suberate successively, has been developed for measuring serotonin (5-HT), 5-hydroxy-L-tryptophan (5-HTP), 5-hydroxy-indole-acetic acid (5-HIAA), and melatonin (MT) (N-acetyl-5-methoxytryptamine) in phosphate buffered saline (PBS) solution (pH 7.4). A linear concentration range of 1 nM to 100 μM was obtained with a correlation coefficient of 0.995 for all target analytes. The minimum detectable concentration obtained was 0.1 nM for 5-HT and 1.0 nM for other analytes. The relationship between the molecular structure of the indole derivatives and the enhanced potential shift behaviour was also discussed. This simple and efficient method can be used to measure other neurotransmitters, as well.
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