Self-assembling photonic crystal film composed of TiO₂/SiO₂ hollow spheres for ultra-sensitive and reusable all-dielectric SERS substrates

Surface-enhanced Raman scattering (SERS) substrates were fabricated using an accurately designed arrangement of porous TiO₂/SiO₂ hollow spheres, notably devoid of noble metals. The synthesis of the SERS substrate involved the production of monodispersed composite materials through a hard-template technique. Subsequently, these hollow spheres were organized into an ordered photonic crystal (PC) film on glass slides using a dip-coating method. The porous SiO₂ inter shell possessed a robust structure that significantly bolstered the durability of the SERS substrate. Moreover, the high specific surface area increased analyte absorbance, enhancing interaction with incident light. Simultaneously, the TiO₂ outer shell facilitated charge transfer and heightened the reflective intensity of the substrate. The ordered arrangement of the TiO₂/SiO₂ hollow spheres, characterized by a substantial refractive-index contrast, resulted in a PC capable of augmenting the overall light reflectivity of the SERS substrate, thereby physically amplifying Raman signal intensity. Exploiting these distinctive properties, the TiO₂/SiO₂ substrate demonstrated an impressive detection limit of less than 10⁻¹⁰ M for both Methylene Blue and Rhodamine 6 G. Additionally, the enhancement factor (EF) reached a remarkable 4.7 × 10⁷. The TiO₂/SiO₂ film's reusability and the straightforward preparation procedure position it as a promising candidate for future SERS analysis applications.