**4. Conclusions and outlook**

PVD methods (the sputtering technique) provide a strictly controlled, stable distribution of Ag-n on nanotubular titania, thus offering suitable platforms for SERS investigations and for measurements of high-quality SERS spectra. Such spectra were obtained for pyridine, p-mercaptobenzoic acid, rhodamine and rhodamine 6G dye adsorbed on Ag-n-functionalized nanoporous layers of titania. For the SERS measurements (with R6G), excellent spectra were obtained, even for a concentration of R6G in a solution as low as 10−9 mol/l. The EF value was found to be the largest (EF = 2.8 × 106 ) for the smallest nanotube diameter (d = 38 ± 9 nm). Stable, reproducible and highly active platforms for SERS investigations were obtained using relatively simple procedures and techniques.

The SERS enhancement induced by these new materials is associated primarily with two geometric factors: the specific surface area of the nanotubes, and an appropriate size and distribution of the Ag nanoparticles. These geometric factors (clearly visible using high-resolution microscopy) affect the properties of the metallic "nanoresonators" deposited on the TiO<sup>2</sup> nanotubes. Consequently, the probe molecules sense even slight changes on the surface, and these manifest themselves in the SERS intensity. However, the spectral differences observed result not only from the specific morphology, where the PVD techniques lead to the formation of cavities and slits that operate as adsorbed molecule resonators, but also from a specific interaction between the Ag nanoparticles and the TiO2 nanotubes themselves. The further development of this type of SERS platform may involve using Ti of varying degrees of purity, and alloys of Ti, where the impurities and alloying elements will have an impact on the formation of nanoporous oxides layers by means of doping process (a change in the electron structure of the titanium oxide). As a consequence, this could lead to a significantly stronger enhancement of the intensity of the SERS spectra due to the occurrence of modified interactions between the doped titanium oxide and the metal nanoparticles, effectively supporting surface plasmons resonance (Ag, Au, Cu). All those factors should be taken into account when planning future analytical applications of new modified SERS substrates.
