Diazo Coupling-Based Ultrasensitive SERS Detection of Capsaicin and Its Application in Identifying Gutter Oil

The use of gutter oil in cooking, which poses a serious threat to human health, has attracted increasing attention in recent years. Capsaicin, existing in chili peppers, is used to identify gutter oil. Here, in order to detect capsaicin fast and accurately for gutter oil discrimination, we introduce a hybrid method to detect capsaicin based on a diazo coupling reaction and surface-enhanced Raman scattering (SERS) technology. A diazo coupling reaction is applied to the composite of p-aminothiophenol (4-ATP) and capsaicin, and the product, azo dye, is adopted as the SERS probe. Gold nanorods (Au NRs) are prepared using the seed-mediated growth method and utilized as the SERS substrate. The Au–S bond between the azo dye and the Au NRs has a powerful ability to spontaneously capture target molecules on the sensing interface. The production of the diazo coupling reaction is proven with an ultraviolet–visible (UV–vis) absorption spectrum. As-synthesized Au NRs are characterized with a UV–vis absorption spectrum, transmission electron microscope, and SERS. Significantly improved SERS peaks were observed at 1114 and 1392 cm −1 , showing this method can be used to detect capsaicin and identify gutter oil. In a broad range of 10 −11  ~ 10 −4  M, the log-linear relationship between SERS intensity and capsaicin concentration is well established, with a detection limit of 3.24 × 10 −12  M.