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

Abstract

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.

One-reactor vacuum and plasma synthesis of transparent conducting oxide nanotubes and nanotrees: from single wire conductivity to ultra-broadband perfect absorbers in the NIR

“…This is also extended, for instance, to the direct formation of the ONWs on device architectures and pre-formed electrodes. 37,38,42–44 …”

“…In parallel, we have revealed the advantages of the combination of the one-reactor method and the use of ONWs as 1D soft-templates supported on processable and scalable substrates. In comparison with previous articles dealing with such a protocol for plasma enhanced chemical vapour depositon (PECVD) of metal oxide shells, [35][36][37][38]43,44 herein, we do a leap forward towards the application of magnetron sputtering, a different physical vapor deposition method typically characterized by a lower conformality but widely utilized in the industry. We show how the process is extendable to the fabrication of transparent conductive oxide (TCO) nanotubes which can be applied as foundations for photovoltaics, photocatalysis, optoelectronics and nanosensors.…”

“…This is also extended, for instance, to the direct formation of the ONWs on device architectures and pre-formed electrodes. 37,38,[42][43][44] In Step 3, the ONWs are coated with ITO shells of exactly 175 nm of equivalent thickness at both LP and HP conditions. The yield of the deposition is given by the estimation of the ITO-shell thicknesses, calculated as 47 ± 10 nm for the low pressure and 65.5 ± 10 nm for the high pressure samples.…”

“…33,34 Recently, the application of such low-cost nanomaterials has been reported for the formation of core@(multi)shell NWs, NTs, and 3D counterparts as well as precursors for porous metal and metal oxide thin films. [35][36][37][38][39][40] Herein, we implement, for the first time, the fabrication of the ITO supported nanostructures by combining well-established and scalable industryrelevant nanofabrication processes. Namely, our system integrates the proprietary vacuum and plasma processes carried in the same reactor, under mild vacuum conditions.…”

Plasma‐Assisted Deposition of TiO2 3D Nanomembranes: Selective Wetting, Superomniphobicity, and Self‐Cleaning

“…[33][34][35][36][37] These results open the path for the use of the TiO 2 nanomembranes as photonic microfluidic valves for the selection of liquids under light activation and as self-cleaning membranes or sensors, which can be further used for photocatalytic removal of water pollutants. [38] 2.5. Omniphobic H 2 Pc@TiO 2 3D Membranes after Fluorine Surface Functionalization…”