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