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.
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