Multilayer Ag-Au-BP-TIBr-FASnI3 Kretschmann surface plasmon resonance biosensor for high sensitivity refractive index analysis: A computational and machine learning approach.

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.