Numerical Analysis of an Optimized SPR Sensor for Stress Hormone Sensing

Cortisol monitoring is relevant for stress assessment and endocrine-related disorders, but conventional assays often require labeled reagents, multistep protocols, or laboratory instrumentation. This work numerically evaluates a black phosphorus (BP)-based surface plasmon resonance (SPR) platform for label-free optical detection of cortisol-related refractive-index changes using angular interrogation at λ = 633 nm. The proposed structure consists of a SiO2 prism, an aluminum plasmonic layer, a TiO2 dielectric layer, a BP monolayer, and the sensing medium. The optical response was calculated using the transfer matrix method under TM polarization, and the platform was assessed through resonance-angle shift, sensitivity, full width at half maximum, detection accuracy, quality factor, figure of merit, theoretical refractive-index detection limit, and combined sensitivity factor. Sequential optimization identified SiO2 as the prism material, 70 nm Al as the plasmonic layer, and 21 nm TiO2 as the dielectric layer. BP was retained as the interfacial 2D material within the intended SiO2/Al/TiO2/BP architecture and exhibited the lowest imaginary refractive-index component among the evaluated 2D materials at 633 nm. For cortisol-related refractive-index changes, the resonance angle shifted from 87.08° to 87.42° as the concentration increased from 0.72 to 4.5 ng/mL. The maximum sensitivity was 480.00/RIU at 0.72 ng/mL, whereas 1.8 ng/mL yielded the highest composite performance according to the adopted CSF definition. These results support SiO2/Al/TiO2/BP as a sensitivity-oriented SPR transduction platform, with future validation requiring fabrication, calibration, and cortisol-selective surface functionalization.