Multilayer Ag-Au-BP-TIBr-FASnI₃ Kretschmann surface plasmon resonance biosensor for high sensitivity refractive index...

Detection of refractive-index variations is fundamental for the development of next-generation optical sensing platforms capable of supporting future biomedical and chemical monitoring applications. Surface plasmon resonance (SPR) biosensing enables label-free, real-time detection but often faces limits in sensitivity, specificity, and stability. This work introduces a Kretschmann-based SPR biosensor that combines a multilayer stack of silver (Ag), gold (Au), and advanced functional materials including black phosphorus (BP), thallium bromide (TIBr) and formamidinium tin iodide (FASnI₃). Hemoglobin is employed only as a model analyte to evaluate resonance shifts arising from bulk refractive-index variations within the range of 1.33-1.40 RIU. Numerical modeling using COMSOL Multiphysics and transfer matrix analysis showed performance metrics with sensitivities from 145 to 375°/RIU, figure of merit values up to 34.404, and quality factors between 6.450 and 7.917 within a refractive index range of 1.33-1.40 RIU. Optimization demonstrated that Ag thickness of 30-35 nm and functional layer thickness of 1.3-3.5 nm supports deep resonance dips with minimized ohmic losses. Electric-field distribution analysis demonstrates strong plasmon-induced field confinement near the sensing interface, with peak |E| intensities of approximately 2.4 × 10⁵ V/m. Machine learning models predicted sensor behavior across different thicknesses and environmental conditions with coefficients of determination (R²) above 0.979 for layer thickness and 0.985 for refractive index. The resulting design offers high sensitivity refractive-index sensing, broad response range, and provide a foundation for future integration of biochemical functionalization toward selective biomarker detection.