Bimetallic nanoplasmonic gap-mode SERS substrate for lung normal and cancer-derived exosomes detection

Surface-enhanced Raman scattering (SERS) has received significant attention for analytical and diagnostic applications in cancer research due to its potency for detecting low abundance of biomarkers at high sensitivity. Cell-secreted exosomes play an important role is not only physiological but also pathological conditions by carrying various key biomolecules such as proteins, lipids, and nucleic acid of their cells of origin and are involved in intercellular transport. They are thus possible biomarkers for many diseases, including cancer. Herein, we fabricated a strong plasmonic gap-mode SERS substrate using Ag nanocubes (NCs) on an Au nanorod (NR) array substrate in order to investigate exosomes. A strong plasmonic cavity effect was obtained from individual Ag NCs assembled on the hot-ring diameter (HR) area on the NR surface. The SERS signal of a low concentration of Rhodamine 6 G upon NCs on NR substrate was amplified. The enhancement factor of the optimized Ag NCs on Ag NR array substrate reached 9.11 × 10⁸. The developed SERS substrate was applied to investigate exosomes derived from lung normal (from NL-20, BEAS-2B, and L929) and cancer (from PC-9, H1975, and HCC827) cell lines with limited sample input. A significant difference was found in the SERS signals from proteins, lipids, and nucleic acid not only between lung normal and cancer exosomes but also between fibroblast and epithelium exosomes. For the lung cancer exosomes, a strong protein SERS signal was observed under time-dependent SERS analysis, whereas for the normal exosomes, the SERS signal mostly corresponded to proteins, lipids, and nucleic acid. The attachment of Ag NCs on the HR area of Au NRs is presumably key for enhancing the electromagnetic effect and thus the SERS signal of exosomes.