A Ratiometric SERS Strategy Based on a WO₃/W₁₈O₄₉ Homojunction Substrate with Built-In Raman Internal Standard for Accurate Trace Antibiotic Detection

Accurate detection of trace antibiotics in complex matrices remains a major analytical challenge. In this study, we report a distinctive ratiometric surface-enhanced Raman scattering (SERS) strategy involving a non-noble metal WO3/W18O49 homojunction substrate for this purpose. The homojunction structure, constructed by in situ growing W18O49 nanowires on vertically aligned WO3 nanosheets, provides abundant oxygen vacancies and a built-in electric field, which synergistically promote interfacial charge transfer and enhance the chemical enhancement effect with an enhancement factor of 6.99 × 107. Notably, the inherent and stable Raman signal at 808 cm-1 from the substrate itself was employed as a built-in internal standard, enabling ratiometric calibration without external additives and substantially improving detection reproducibility (RSD reduced from 14.0% to 12.9%) and quantitative accuracy (R2 improved from 0.96 to 0.99). The proposed substrate achieved a detection limit of 5.1 × 10-10 M for methylene blue as the model probe molecule and detection limits below 1.0 × 10-9 M for representative antibiotics, including enrofloxacin and doxycycline hydrochloride, in milk and water samples. Combined with density functional theory calculations and energy band alignment analysis, the photoinduced charge transfer mechanism and enhancement pathways were systematically elucidated. This work establishes a practical, sensitive, and cost-effective SERS platform for environmental contaminant monitoring and demonstrates a versatile design strategy for engineering non-noble metal homojunction substrates with integrated ratiometric detection capability.