Synthesis of a SERRS substrate based on TiO2 nanotubes films decorated with Ag nanoparticles for anthocyanins species determination in grape skin extracts

Abstract

The synthesis of TiO₂ nanotubes film decorated with Ag nanoparticles (TiO₂NTs/AgNPs) and its potential applicability as a SERRS (Surface-Enhanced Raman Resonance Scattering) substrate for detection and speciation of anthocyanins species in grape skin extracts is reported. Preliminary spectrophotometric analysis in the UV–Vis region allowed to identify the maximum absorption bands at 527 and 620 nm, related to the flavylium cation (AH⁺) and quinoidal base (A^–), respectively. The increase in the ratio AgNPs/extract concentration in colloidal solutions of AgNPs promoted the intensification of the vibrational modes of anthocyanins species, due to the SERRS effect. From deconvolutions of these SERRS spectra, a linear relationship (R² = 0.993) for the intensity ratio of the vibrational modes A^–/AH⁺ (i.e., for the ratio 1631–1640/1648–1659 cm^–1) as a function of the grape skin extract dilutions at colloidal solutions of AgNPs was found. The deposition of the extracts onto TiO₂NTs/AgNPs surface promoted an additional SERRS signal intensification for A^– in detriment to AH⁺. The deconvolution of these SERRS spectra showed again a linear relationship (R² = 0.999) between the intensity bands ratio A^–/AH⁺ and grape skin extract dilutions in the range of 0.1–1.0 %. The SERRS technique using the TiO₂NTs/AgNPs substrate was more sensitive for anthocyanins quantification compared to the spectrophotometric one. The results revealed that the TiO₂NTs/AgNPs films are versatile, efficient, and promising platforms for SERRS quantification and speciation of anthocyanin species in situ.