SERS enhancement via biotemplated Ag nanostructures on empress cicada wings: Effects of sputtering and e-gun deposition on gap geometry

In this study, a biomaterial template based on empress cicada wings was utilized to fabricate hexagonally arranged metallic nanostructures for applications in SERS. Compared to artificial nanostructures that usually require costly and time-consuming fabrication processes, empress cicada wings naturally exhibit arrays of nano-bowling pin-shaped pillars with an averaged diameter of ∼74.1 ± 5.4 nm arranged in a hexagonal lattice, serving as a low-cost alternative. Silver films were deposited onto cicada wings via sputtering or e-gun evaporation, resulting in the formation of hexagonally packed periodic structures consisting of Ag-coated nanocylinders and nanocones, with a tunable inter-pillar gap distance. We systematically investigated the hotspots generated by LSP coupling between nanocylinder and nanocone structures to optimize the SERS signals at a laser wavelength of 633 nm. A critical Ag film thickness of 45 nm, resulting in a gap of ∼5 nm between the Ag-coated nanocylinders, was identified as the optimal condition, achieving an analytical SERS enhancement factor of ∼27485 from simulations considering only the electromagnetic contribution, corresponding to ∼3436-fold when compared to Ag-coated nanocones and ∼39264-fold to bare cicada wings. In experimental measurements, the cylindrical nanostructured substrate exhibited a significantly stronger Raman signal than the unmodified substrate, with an average SERS enhancement factor of ∼10⁷. Through comparative analysis of nanostructure geometries, the cylindrical substrate exhibited superior SERS performance to the conical substrate. Consequently, this study provides a low-cost strategy for fabricating SERS substrates and elucidates how deposition-induced nanogap geometry governs electromagnetic enhancement, offering valuable insight for future biomedical sensing applications.