Label-Free Early Pregnancy Detection Using a ZBLAN Prism-Based Surface Plasmon Resonance Biosensor

Early pregnancy detection is critical for timely prenatal care and for reducing maternal fetalcomplications. We propose a highly sensitive surface plasmon resonance (SPR) biosensor inthe Kretschmann configuration using a ZBLAN prism, a Cu plasmonic film, a ZnS dielectricspacer, and a two-dimensional black phosphorus (BP) overlayer. This multilayer architecturestrengthens analyte–surface interaction and localizes the plasmonic field at resonance.Thedevice attains a sensitivity up to 600 ◦/RIU over the analyte refractive-index range 1.340–1.342.Additional performance metrics include a minimum full width at half maximum (FWHM) of4.6607◦, a maximum figure of merit (FoM) of 115.75 RIU−1, and a limit of detection (LoD)as low as 3.4 × 10−5 RIU. Field analysis further shows a maximum electric-field amplitude ofabout 401.283 kV m−1 at the BP/analyte interface and an evanescent-field penetration depthof approximately 151.5 nm. These results position the proposed biosensor as a promisingcandidate for real-time, label-free, and highly sensitive early-pregnancy detection suitable forboth clinical and home-based diagnostics.