Charge-transfer-activated SERS detection of methylene blue using an ultrastable, reliable, and highly sensitive semiconductor Fe3O4@C@TiO2 nano-platform

Abstract

Surface-enhanced Raman spectroscopy (SERS) is among the most sensitive analytical tools for molecular identification, yet its strong reliance on noble-metal supports (Au, Ag, Cu) still constrains stability, cost, and practical applicability. Recent advances show that certain semiconductors can provide Raman enhancement, offering a metal-free route toward more stable SERS platforms. However, the sensitivity of semiconductor-based substrates remains far inferior to noble-metal systems, with limited enhancement factors (EFs) that restrict real-world applicability. Therefore, it is a formidable yet highly desirable goal to develop an efficient semiconductor SERS substrate with enhancement levels comparable to noble metals. Here, we report Fe₃O₄@C@TiO₂ semiconductor nanostructures as a metal-free SERS substrate capable of activating and amplifying the Raman signal of methylene blue through interfacial states that facilitate efficient semiconductor-molecule charge transfer, resulting in ultrastable, reliable, and highly sensitive sensing performance. Owing to the excellent chemical stability of this platform, the SERS signals of MB remain highly stable over long storage periods, and the substrate exhibits strong reliability with relative standard deviations (RSD) below 5% for both repeatability and reproducibility. Moreover, the presence of transition energy levels within the Fe₃O₄@C@TiO₂ heterostructure substantially enhances the Raman signal, enabling a limit of detection of 76 nM and a maximum EF of 5.4 × 10⁵ – approaching those of noble-metal substrates and significantly higher than most semiconductor-based counterparts. The system further enables accurate MB detection in tap water, yielding recovery values of 93–106%. These results highlight the strong potential of semiconductor heterostructures for stable and practical SERS sensing platforms.