Hierarchical Au nanoclusters electrodeposited on amine-terminated ITO glass as a SERS-active substrate for the reliable and sensitive detection of serotonin in a Tris-HCl buffer solution

Although serotonin (5-HT) is one of the most important neurotransmitters that are responsible for critical roles in the central nervous system, there are few reports on the reliable and sensitive detection of 5-HT by SERS technique, owing to its lower Raman cross-section and lack of a common extraction method of 5-HT. In this study, a SERS-active substrate was fabricated by electrodepositing hierarchical Au nanostructures on amine-terminated ITO (indium tin oxide) glass to achieve an enhanced Raman signal of 5-HT. The electrodeposition process was performed by applying a nucleation potential of E1 = 0.7 V for 2 s and a growth potential of E2 = -0.2 V for 1000 s for repeated cycles. As a testing analyte, 5-HT dissolved in four different solvents such as PBS buffer, Tris-HCl buffer, distilled water, and ethanol solution (20%). Among them, Tris-HCl buffer produced the most sensitive and reproducible Raman signals of 5-HT along with the lowest detection of ~10-9 M, due to the strong adsorptive interactions between the Au surface and indole amine groups of 5-HT in alkaline condition. Surprisingly, the standard curve between the Raman intensity at 751 cm-1 and log-scale concentration of 5-HT showed the linear dynamic range over five orders of magnitude in the Tris-HCl buffer solution, suggesting a possible quantification of the 5-HT level in a biological fluid of 5-HT dysfunction or deficiency.