Preparation of transparent SERS substrates based on the stepwise anodization potential AAO-template approach for rapid detection of trace pesticide residues

Surface-enhanced Raman spectroscopy (SERS) has become one of the most important analytical tools and has been widely applied in many fields. However, the preparation of large-scale SERS substrates is still a challenge for practical applications. In this work, the approach of applying stepwise anodic aluminum oxidation (AAO) potentials was adopted to prepare the SERS substrate templates with the nanotip structure, on which the Ag/Au nanoparticles (NPs) nanoarrays with the regular hexagonal structure and adjustable gaps were prepared by sputtering Ag-NPs or Au-NPs on the nanotips. Due to the formation of "hotspots", these ordered Ag/Au nanoarrays exhibited strong SERS activity for detecting R6G with a concentration as low as 10⁻1⁴ M. Furthermore, we also prepared the transparent SERS substrates based on the as-fabricated AAO templates, and for practical application, these SERS substrates could be directly placed on the surface of food to probe the residual pesticides. For example, using the transparent SERS substrates, the residual pesticides such as carbendazim and chlorothalonil on the surface of edible mushroom Lentinus edodes could be detected with the lower detection limit below 10 μg/L. Therefore, this work has not only demonstrated a simplified approach to preparing large-scale effective SERS substrates but also provided a practical approach to rapid detection of agricultural pesticide residues useful for food safety inspection.