Chemically stabilized MgF₂-Ag-Al₂O₃ SPR biosensor for label-free brain tumor detection

An advanced surface plasmon resonance (SPR) biosensor is presented for label-free identification of primary and metastatic brain tumors. The proposed multilayer structure consists of an SF66 high-index prism, an MgF₂ low-index buffer layer, a thin silver plasmonic layer, and an ultrathin Al₂O₃ protective film deposited by atomic layer deposition (ALD). After optimization, the design achieves a high figure of merit (FOM) of 42.89 RIU^-1 and an angular sensitivity of 56.55° RIU^-1. Using clinically reported refractive-index values for human brain tissues, the sensor produces clear resonance-angle shifts relative to cerebrospinal fluid, increasing from 2.34° for healthy brain tissue to 8.48° for metastatic lesions. The ALD-grown Al₂O₃ coating protects the silver layer from oxidation, improving long-term stability while preserving strong optical performance. In addition, its surface chemistry supports future biofunctionalization. Temperature analysis at 25 °C and 37 °C shows measurable changes in the SPR resonance angle due to the negative thermo-optic response of biological tissues. These findings indicate that the proposed SPR platform could serve as a rapid, sensitive, and clinically relevant tool for distinguishing healthy, primary tumor, and metastatic brain tissues.