Flexible and Transparent SERS Substrate Using Au–Ag Bimetallic Conducting Thin Films for Detection of Molecular Level R6G Dye and Vitamin B12

Abstract

Surface roughness and nanostructures of metal thin films play a major role in Surface-Enhanced Raman Scattering (SERS) study due to their intrinsic electromagnetic (EM) hot spots, multipolar resonance, and other factors. Here, we present a low-cost and scalable fabrication route for a flexible, nanoporous Au–Ag bimetallic thin film on a plastic substrate, providing a practical alternative to complex lithography-based approaches. The substrate is engineered via thermal deposition of a 4 nm Au layer onto a predeposited Ag–TiO2/SnO2 nanoparticles (NPs) matrix. This process yields a percolated, conductive nanostructure with a root-mean-square roughness of ∼3.91 nm, high optical transparency (∼80%), and low sheet resistance (∼7.26 Ω/□). The resulting 4 nm Au/Ag–TiO2/SnO2 substrate exhibits exceptional SERS sensitivity, detecting rhodamine 6G (R6G) and vitamin B12 down to 1 pM and 1 nM, respectively. Statistical analysis confirms exceptional detection sensitivity, corresponding to approximately 2 molecules for R6G and 89 molecules for vitamin B12 within the laser spot. This enhanced performance is attributed to a synergistic mechanism combining dense EM hot spots within the porous network and efficient plasmon-induced charge transfer (PICT) from the Au–Ag fermi level and trap-assisted TiO2/SnO2 conduction band to the analyte LUMO states. These findings demonstrate a robust, flexible platform for ultratrace chemical sensing.