Insights on Charge-Transfer Mechanisms in ZnO and ZnO–GO Substrates for the SERS Analysis of 4-Mercaptopyridine

Abstract Conventional Raman spectroscopy is a powerful analytical technique but suffers from an inherently low sensitivity. This limitation can be overcome through surface-enhanced Raman scattering (SERS), a phenomenon traditionally achieved by using plasmonic substrates such as gold and silver nanostructures. Nonplasmonic materials, including nanostructures based on binary inorganic semiconductors, have emerged as promising alternatives because of their low cost, chemical stability, and tunability for specific applications. However, semiconductor substrates typically provide a lower Raman enhancement than plasmonic substrates, motivating the development of new substrate designs. In this work, we investigate the SERS activity of zinc oxide (ZnO), graphene oxide (GO), and a ZnO–GO hybrid material using 4-mercaptopyridine (4-MPy) as a molecular probe, combining experimental Raman measurements with computational modeling. Our results demonstrate that ZnO amplifies the Raman signal of 4-MPy, whereas GO does not contribute significantly to the enhancement mechanism. Computational analysis reveals that the observed Raman signal originates from a charge-transfer (CT) process, preferentially mediated by N–Zn and S–Zn bond formation. These findings confirm ZnO as a promising nonplasmonic SERS substrate and provide mechanistic insight into charge-transfer-induced enhancement observed in Raman experiments.