Defect-assisted charge transfer and plasmonic enhancement in ZnO nanotube/Au hybrids for ultrasensitive malathion...

Defect engineering in semiconductor substrates provides an effective strategy to strengthen the charge transfer contribution in surface-enhanced Raman scattering. In this work, oxygen vacancies are intentionally introduced into ZnO nanostructures through KCl-mediated chemical etching, which simultaneously converts ZnO nanorods into hollow nanotubes with enlarged effective surface area. The combined structural transformation and defect generation enhance molecule-substrate interactions and promote interfacial charge transfer. Using methylene blue as a probe molecule, the ZnO nanotube substrate achieves detection down to 10^-8 M. An analysis based on the degree of charge transfer is used to discuss the qualitative trend in charge-transfer-related enhancement, showing that the optimized oxygen-vacancy-rich ZnO nanotube substrate exhibits the strongest charge-transfer-related response among the ZnO-only substrates. Subsequent decoration with Au nanoparticles forms a ZnO/Au heterostructure where localized plasmonic fields interact with vacancy-assisted charge transfer. This cooperative effect enables ultrasensitive detection of methylene blue with a limit of detection of 10^-12 M and an analytical enhancement factor of 2.5 × 10⁹. The optimized substrate further enables proof-of-concept detection of malathion, achieving a detection limit of 0.01 mg L^-1 with an analytical enhancement factor of 2.8 × 10⁶. These results highlight a vacancy-plasmon coupling strategy for constructing high-performance semiconductor-metal hybrid SERS platforms.