3D composite SERS substrate constructed by Au–Ag core-satellite NPs and polystyrene sphere for ultrasensitive ratiometric Raman detection of cotinine

To enhance the efficacy of Surface-enhanced Raman spectroscopy (SERS), noble metal nanoparticles (NMNPs) can be organized onto three-dimensional (3D) hierarchical nanostructures. This study involved constructing a 3D solid SERS substrate by layering Au–Ag core-satellite nanoparticle monolayer films (MF) on a polystyrene sphere (PS) array, termed Au–Ag core-satellite NPs-MF-PS. Optimizing the PS size was essential to maximize SERS substrate activity, with the 500 nm PS providing the best SERS enhancement factor of 1.03 × 10 7 a two-fold increase over Au–Ag core-satellite NPs MF alone. Additionally, cotinine detection was improved by using a ratio between the target molecules and internal Raman signals from the SERS substrate. Compared to conventional methods relying on the target molecule's Raman signal, this ratiometric SERS method expanded the detection range from 10 −8 - 10 −1  M to 10 −9 - 10 −1  M and reduced the detection limit from 3.67 × 10 −9  M to 1.68 × 10 −10  M. This approach represents a novel direction in creating ultra-sensitive SERS platforms with broad applications, suggesting that ratiometric SERS could further promote SERS technology advancements.