Numerical Modeling and Performance Optimization of a Highly Sensitive Bismuth Silicate and Franckeite based Hybrid SPR Biosensor

In this study, a high-performance, label-free surface plasmon resonance (SPR) biosensor based on a Kretschmann configuration is proposed and numerically evaluated at an operating wavelength of 633 nm. The sensing architecture comprises a low-index calcium fluoride (CaF₂) coupling prism, a 55 nm silver (Ag) thin film, a 9 nm bismuth silicate (Bi₁₂SiO₂₀,BSO) dielectric intermediate layer, and a 0.5 nm two-dimensional (2D) Franckeite overlay. The integration of the BSO/Franckeite bilayer strategically confines the evanescent electromagnetic field at the sensing boundary while passivating the underlying silver layer against oxidation. Using the Transfer Matrix Method (TMM) to simulate angular interrogation across analyte refractive indices ranging from 1.33 to 1.34, the structural parameters were systematically optimized. Comparative analysis demonstrates that the optimized CaF₂/Ag/BSO/Franckeite heterostructure achieves a maximum angular sensitivity of 311o/RIU, an improvement over the conventional CaF₂/Ag baseline (140^o/RIU ), alongside a detection accuracy of 0.42 deg⁻¹, a full width at half maximum of 2.34^o, and a figure of merit/quality factor of 141.36 RIU⁻¹. These results highlight the synergistic potential of 2D material-enhanced plasmonic heterostructures and provide design guidelines for sensitive biochemical sensing.

This work is significant because it demonstrates, through numerical optimization, that integrating a Bi₁₂SiO_20, (BSO) interlayer and an ultrathin Franckeite overlay into a conventional CaF₂/Ag Kretschmann SPR platform can substantially improve label-free sensing performance at 633 nm while also providing oxidation protection for the silver film. Its novelty lies in the specific use of the BSO/Franckeite bilayer to enhance electromagnetic field confinement at the sensing interface and to synergistically tune the resonance response, resulting in a marked increase in angular sensitivity relative to the bare CaF₂/Ag structure. The study’s key contribution is a systematic Transfer Matrix Method-based design framework that identifies an optimized heterostructure with high sensitivity, improved detection accuracy, a narrow resonance linewidth, and a strong figure of merit, thereby establishing a clear performance benchmark for 2D material-assisted plasmonic biosensors. Scientifically, the results strengthen understanding of how dielectric/2D-material hybrid coatings can modulate plasmonic coupling and improve sensor functionality. Potential applications include high-resolution biochemical and refractive-index sensing in environmental monitoring, biomedical diagnostics, and lab-on-chip analytical platforms.

The study proposes and numerically analyzes a label-free SPR biosensor in a Kretschmann configuration operating at 633 nm. The structure uses a CaF₂ prism, 55 nm Ag film, 9 nm BSO layer, and 0.5 nm Franckeite overlayer. The BSO/Franckeite bilayer enhances field confinement at the sensing interface and protects silver from oxidation. Using TMM and angular interrogation for refractive indices from 1.33 to 1.34, the design was optimized. The optimized sensor achieved a sensitivity of 311°/RIU, compared with 140°/RIU for the bare CaF₂/Ag system, along with 0.42 deg⁻¹ detection accuracy, 2.34° FWHM, and a figure of merit of 141.36 RIU⁻¹. The results show that 2D-material-assisted plasmonic heterostructures can significantly improve biosensing performance.