Nanoplasmonic Au-Ag Alloy Coatings on the Surface of TiO₂ Nanotubes for Vitamin B12 Detection by Surface-Enhanced Raman Scattering Spectroscopy

The publication shows the possibility of using nano Au-Ag alloy coatings on the surface of TiO₂ nanotubes to detect vitamin B12. The coatings were prepared using two methods of direct metal deposition: magnetron sputtering and thermal evaporation in UHV conditions. Thanks to the two-stage thermal treatment in a vacuum, Au-Ag alloy layers were obtained: 300 °C/8 h + 450 °C/0.5 h. On these surface-enhanced Raman scattering (SERS) substrates, it was possible to detect vitamin B12 in aqueous solutions at a level of 10^-8 M. Moreover, the UHV thermally evaporated substrate was characterized by a much better measurement stability for vitamin B12 than the magnetron-sputtered substrate; the relative standard deviations (RSD) were 3.93 and 14.9%, respectively. Based on a less structurally complex probe molecule, 4-mercaptobenzoic acid (PMBA), the tested substrates' enhancement factors (E _F) were determined as a function of their distribution on the surface. Enhancement maps clearly showed differences in the efficiency of the plasmonic alloy structures obtained, in favor of the samples that are thermally evaporated in UHV conditions, where E _F changed from 2.3·10³ to 5.4·10⁴. The applied methods for depositing Au and Ag metals were crucial in determining the geometric nano factors related to the surface morphology of the obtained layers, which significantly impacted the generation of "hot spots", where the electromagnetic mechanism (EM) amplification effect occurs most strongly. Detailed SERS measurements and microscopic methods such as scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) were used to visualize the surfaces and cross sections of the alloy layers. Other material characterization methods, such as X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS), made it possible to gain an additional information about the structure and chemical composition of the investigated materials. This approach allowed the authors to understand the enhancement effect of the resulting plasmonic structures, in line with the current trend of looking for stable, active SERS platforms with the broadest possible range of applications.