As a strong carcinogen to humans, acid orange II is forbidden to be added as an additive into foodstuff. Because the foods colored with acid orange II showed stable and colorful appearance, acid orange II is broadly utilized by illegally producers for their low costing. Currently available analytical methods for acid orange II are mainly based on fluorescence and liquid chromatography, which are time-consuming and tedious as acid orange II has to be extracted from the samples before determination. For efficient detection and insurance of food safety, it is urgent to develop fast and low-cost in situ assay methods for field detection of acid orange II on foods. In this study, shell-isolated Au@SiO₂ nanoparticles is prepared for detecting acid orange II on food. Gold colloidal solution with good size distribution (50 nm) is prepared using the standard sodium citrate reduction method, followed by coating a thin layer of SiO₂ on Au nanoparticle surface. This shell-isolated Au@SiO₂ is obtained by the addition of active silica to the gold colloidal solution, with the (3-aminopropyl)trimethoxysilane (APTMS) as the coupling agent at pH 8.5. By regulating the amount of the active silica, the Au@SiO₂ with different silica shell thickness is synthesized. Ultraviolet-visible spectroscopy (UV-Vis) and transmission electron microscope (TEM) are employed to characterize the optical property and morphology of the as-synthesized Au@SiO₂ nanoparticles. The amount of the active silica added in preparation of the Au@SiO₂ is optimized through comparing the surface enhanced Raman scattering (SERS) intensity of acid orange II, using the synthetic Au@SiO₂ structure as the SERS substrate. Under the optimized experimental conditions, acid orange II on the Si wafer can be detected at concentrations below 0.17 mg/L. The feasibility of the proposed method for detecting acid orange II in real samples is verified by spreading the Au@SiO₂ nanoparticles on the surface of the watermelon seeds stained with acid orange II. Results showed that this novel method is capable of detecting 0.01 mg/g acid orange II stained on the watermelon seeds. This proposed method was applied to assay sunflower seeds and watermelon seeds purchased from local stores. It is expected that this proposed method is applicable for in situ detection of acid orange II on the surface of other food samples.

Key words: gold sol, shell-isolated, surface enhanced Raman scattering, acid orange II, food safety