Unraveling Two Distinct Spectral Features in the SERS Spectrum of Dopamine

Abstract

Dopamine, a neurotransmitter, is involved in numerous physiological and neurological processes. Surface-enhanced Raman spectroscopy has been applied to measure low concentrations of dopamine; however, previous studies have reported two noticeably different surface-enhanced Raman spectroscopy (SERS) spectra for dopamine, leading to ambiguity in its spectral characterization. In this study, in order to characterize and unravel the ambiguity of the dopamine SERS spectrum, we utilized Ag bumpy nanoshells (AgNS) functionalized with 3-mercaptopropionic acid (MPA), which is a confirmed SERS substrate for high detection sensitivity and reliable fabrication. Through coordination bonds with Fe³⁺ or Cu²⁺ ions, the diol moiety of dopamine was tethered to the terminal carboxylate group of MPA on the AgNSs. The SERS spectra of dopamine, tethered to a silver surface via different metal ions, were analyzed. Fe³⁺-tethered dopamine exhibited a characteristic SERS band at 1487 cm^–1, while Cu²⁺-tethered dopamine exhibited a distinct band at 1388 cm^–1. Even in the SERS spectrum of Fe³⁺-tethered dopamine, the 1388 cm^–1 band gradually emerged over a few hours, which was attributed to the progressive oxidation of dopamine to 5,6-dihydroxyindole (DHI). These findings clarified the ambiguity of the two distinct SERS spectra of dopamine in previous reports. By elucidating the chemical origins of these spectral patterns, this study established the reliability of SERS-based dopamine detection and contributed to the development of more accurate diagnostic tools for neurological disorders.