Determination of SO2 concentration in red and white wine samples by surface enhanced Raman spectroscopy

Sulfur-containing compounds are widely used as preservatives and antioxidants in winemaking processes. However, their content in wine is limited by specific laws in force in every country due to their potential allergenicity. Consequently, analytic controls across the entire winemaking process are imposed. Total SO2 is defined as the sum of all its forms, either in the free state or combined with other molecules in wine. The official method by the International Organisation of Vine and Wine, OIV-MA-AS323-04A, for SO2 is time-consuming and subject to possible sources of error. For these reasons, it is important to have alternative fast and reliable methods for the quantification of SO2. In recent years, Raman spectroscopy was attested as a rapid and non-destructive technique for food and wine analysis [1]. The sensitivity of this technique can be highly increased exploiting the Surface Enhanced Raman Spectroscopy (SERS) effect obtained when target molecules are absorbed onto, or microscopically close to, a plasmonically active surface, such as roughened nanostructured metal surface, or metal colloids, such as silver nanoparticles (AgNPs) [2]. In this study, an innovative and rapid method based on SERS for the quantification of sulfites in wine is presented. Suspensions of AgNPs with different sizes were compared to investigate those that produce the highest signal enhancement, and AgNPs of 4 nm diameter were selected. Moreover, the chemical affinity between S and AgNPs was investigated and the SERS signal intensities of S=O stretching and bending modes were correlated to the amount of SO2 in the samples in the concentration range 0 - 100 mg/L. The samples were pretreated in solid-phase extraction cartridges, mixed with liquid suspensions of AgNPs and directly analyzed by SERS. The standard addition method was used to quantitatively determine the concentration of SO2 in wines, thus overcoming the matrix effect. The reliability of the results of red and white wine samples obtained by SERS method was confirmed by a comparison carried out with the results obtained by the official method.