Rapid Analysis of Multiple Sudan Dyes in Chili Flakes Using Surface-Enhanced Raman Spectroscopy Coupled with Au-Ag Core-Shell Nanospheres

Sudan dyes are often illegally added as colorants into a variety of foodstuffs and have been tied to many food safety issues. In this study, surface-enhanced Raman spectroscopy (SERS) coupled with Au-Ag core-shell nanospheres (Au@Ag) was applied to analyze standard solutions of Sudan I-IV and Sudan dyes in chili flakes. With the use of 90 +/- 5 nm Au@Ag (Au seed 20 +/- 2 nm) as SERS substrate, the lowest detectible concentrations for Sudan I and II were 0.10 mg/L, for Sudan III was 0.08 mg/L, and for Sudan IV was 0.2 mg/L. The use of principal component analysis (PCA) could successfully classify different Sudan dyes based upon the SERS spectra of their standard solutions. For chili flakes, the use of acetonitrile as extraction solvent led to an overall higher sensitivity for analysis of Sudan dyes with SERS method compared to that of methanol, ethanol, and n-hexane. The lowest detectible concentrations for Sudan I-III in chili flakes were 1 mg/kg and for Sudan IV was 2 mg/kg, which were about ten times as much as that for their standard solutions due to the interference of non-target compounds from sample matrices. Partial least squares (PLS) models developed for quantitative analyses showed relatively high linear correlation between the actual and predicted amounts of Sudan dyes in chili flakes (R (2) (cv) = 0.869-0.959). The results showed great potential of applying Au@Ag as SERS substrate for qualitative and quantitative analysis of Sudan I-IV with simplified sample preparation method.