Synthetic colorants in food can be a potential threat to human health. In this study, surface-enhanced Raman spectroscopy (SERS) coupled with gold nanorods as substrates is proposed to analyze allura red and sunset yellow in beverages. The gold nanorods with different aspect ratios were synthesized, and their long-term stability, SERS activity, and the effect of the different salts on the SERS signal were investigated. The results demonstrate that gold nanorods have a satisfactory stability (stored up to 28 days). SERS coupled with gold nanorods exhibit stronger sensitivity. MgSO was chosen to improve the SERS signal of sunset yellow, and no salts could enhance the SERS signal of allura red. The lowest concentration was 0.10 mg/L for both colorant standard solutions. The successful prediction results using SERS were much closer to those obtained by high-performance liquid chromatography for the sample in beverages. SERS combined with gold nanorods shows potential for analyzing food colorants and other food additives as a rapid, convenient, and sensitive method.
Au-Ag core-shell (Au@Ag) bimetallic nanospheres synthesized by a facile seed-growth method are proposed as a substrate for surface-enhanced Raman spectroscopy (SERS) to detect azo-group dyes including Sudan I and Sudan II. Au@Ag nanospheres with a series of particle sizes (diameter: 30–120 nm) and silver shell thicknesses (6–51 nm) were synthesized and compared for their morphological and optical properties to obtain optimum enhancement effect. Normal Raman, SERS, infrared, and ultraviolet-visible were used to investigate the optical absorption properties of Sudan I and Sudan II as well as the enhancement mechanism of Au@Ag substrates. The nanospheres with particle size of 73 ± 6 nm in diameter and silver layer of 27 ± 2 nm resulted in the highest enhancement effect and could be used to detect Sudan I and Sudan II standard solutions at levels as low as 0.4 and 0.1 mg/L, respectively. Moreover, Sudan I and Sudan II in chili powder could be detected at 0.6 and 0.4 mg/kg, respectively. Sudan I and Sudan II with similar structures in complicated food matrices could be distinguished through applying principal component analysis, indicating good selectivity of the SERS method for detection of banned additives in food stuffs at trace levels.
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