Plasma-Enabled Amorphous TiO₂ Nanotubes as Hydrophobic Support for Molecular Sensing by SERS

We devise a unique heteronanostructure array to overcome a persistent issue of simultaneously utilizing the surface-enhanced Raman scattering, inexpensive, Earth-abundant materials, large surface areas, and multifunctionality to demonstrate near single-molecule detection. Room-temperature plasma-enhanced chemical vapor deposition and thermal evaporation provide high-density arrays of vertical TiO₂ nanotubes decorated with Ag nanoparticles. The role of the TiO₂ nanotubes is 3-fold: (i) providing a high surface area for the homogeneous distribution of supported Ag nanoparticles, (ii) increasing the water contact angle to achieve superhydrophobic limits, and (iii) enhancing the Raman signal by synergizing the localized electromagnetic field enhancement (Ag plasmons) and charge transfer chemical enhancement mechanisms (amorphous TiO₂) and by increasing the light scattering because of the formation of vertically aligned nanoarchitectures. As a result, we reach a Raman enhancement factor of up to 9.4 × 10⁷, satisfying the key practical device requirements. The enhancement mechanism is optimized through the interplay of the optimum microstructure, nanotube/shell thickness, Ag nanoparticles size distribution, and density. Vertically aligned amorphous TiO₂ nanotubes decorated with Ag nanoparticles with a mean diameter of 10-12 nm provide enough sensitivity for near-instant concentration analysis with an ultralow few-molecule detection limit of 10^-12 M (Rh6G in water) and the possibility to scale up device fabrication.