Ultrasensitive Ag-coated TiO2 nanotube arrays for flexible SERS-based optofluidic devices

Abstract

In this study, a novel SERS sensor has been developed for repeatable detection of organic molecules and biological assays. Vertically oriented titania nanotube (TiO₂ NT) arrays were grown by ultra-fast anodic oxidation of flexible titanium foils and then decorated with Ag nanoparticles (NPs) through d.c. sputtering deposition at room temperature. A parametric study was carried out taking into account the effect of sputtering parameters on the Ag NP arrangements on the NT surface. The structure morphology was investigated by means of scanning and transmission electron microscopy, evidencing the formation of hexagonal close-packed TiO₂ NTs coated with Ag nanoparticles showing tunable diameter and distribution. The substrates were employed in a SERS optofluidic device, consisting of a polydimethylsiloxane cover irreversibly sealed to the silver-coated TiO₂ NTs, able to detect Rhodamine molecules in ethanol over a wide range of concentrations down to 10⁻¹⁴ M, taking advantage of both electromagnetic and chemical enhancements. In order to evaluate the performances of the SERS substrates in terms of biosensing, an optimized protocol for the immobilization of oligonucleotide probes on the metal-dielectric surfaces was developed for verifying the hybridization events.