Poly(dopamine)-coated TiO2/SiO2 resonators for the non-plasmonic SERS detection of organic analytes

Polydopamine (PDA) is a bioinspired polymer with strong adhesive properties, enabling easy surface functionalization. These abilities have already been exploited to develop advanced plasmonic and hybrid surface-enhanced Raman scattering (SERS) platforms, but the possibility of applying PDA coatings to optimize non-plasmonic SERS substrates has to be explored. Here, we applied PDA coatings to non-plasmonic SERS substrates based on core/shell SiO2/TiO2 micrometric spheres (T-rex), which act as dielectric resonators. Functionalized T-rex substrates were tested for the detection of methylene blue (MB), a conventional Raman probe, and histamine, a food contaminant. Compared to unmodified substrates, the PDA-coated T-rex exhibited a substantial increase in SERS activity and a reduction in detection limits (1 µM for MB and 0.01 µM for histamine). This enhancement arises from the insurrection of analyte–surface intermolecular interactions (H-bonds, π–π stacking and electrostatic attraction) and from charge-transfer processes mediated by PDA. These results demonstrate that PDA is an effective and versatile modifier to boost the sensitivity of non-plasmonic SERS substrates.