Hybrid Nanostructures Based on TiO2 Nanotubes with Ag, Au, or Bimetallic Au–Ag Deposits for Surface-Enhanced Raman Scattering (SERS) Applications

The work shows the possibility of obtaining active platforms for surface-enhanced Raman scattering (SERS) measurements based on TiO2 nanotubes decorated with Ag, Au, or Au–Ag bimetallic layers. Plasmonic metallic layers were produced under ultra-high vacuum (UHV) conditions using the thermal evaporation method and heat treatment, which enabled them to be formed with a precisely defined thickness and size of metallic nanoparticles. The two-stage thermal treatment leads to the formation of an Ag–Au bimetallic alloy on the top of the nanotubes. In all cases, the thickness of the deposited metal layer was 10 nm. Therefore, the materials prepared in the form of decorated nanoplasmonic antennas (nanobrushes) were tested for their use as active and effective substrates in SERS spectroscopy. For this purpose, the probe molecule mercaptobenzoic acid (MBA) was used. The highest SERS enhancement factor was determined for the substrate with a monometallic Ag layer (1.5̇̇ × 105), then for the Ag–Au alloy (1.7 × 104), monometallic Au layer (9.6 × 103), and gold/silver layer-by-layer systems (∼5.6 × 103). The practical usefulness of the prepared SERS substrates was determined based on vitamin B12 detection measurements. The results showed that the proposed SERS substrates have a high detection sensitivity of 10–8 M and measurement stability. Moreover, the conducted research showed that the most important factors influencing the enhancement due to the electromagnetic mechanism are geometric factors related to the distribution of plasmonic metals on the tops and walls of nanotubes, their size, and chemical composition. Moreover, the chemical composition of the examined deposits turned out to be an important factor affecting the plasmonic properties of the produced SERS platforms. Apart from SERS spectroscopy, other research methods were used to characterize the obtained materials, such as UV–vis spectroscopy, XPS spectroscopy (in situ), XRD structural analysis, and scanning electron microscopy.